Alling, Björn
Overview
Works:  73 works in 76 publications in 1 language and 81 library holdings 

Roles:  Author, Other, Thesis advisor 
Publication Timeline
.
Most widely held works by
Björn Alling
Theoretical study of phase stability, crystal and electronic structure of MeMgN2 (Me = Ti, Zr, Hf) compounds by
M. A Gharavi(
)
2 editions published between 2017 and 2018 in English and held by 3 WorldCat member libraries worldwide
Scandium nitride has recently gained interest as a prospective compound for thermoelectric applications due to its high Seebeck coefficient. However, ScN also has a relatively high thermal conductivity, which limits its thermoelectric efficiency and figure of merit (zT). These properties motivate a search for other semiconductor materials that share the electronic structure features of ScN, but which have a lower thermal conductivity. Thus, the focus of our study is to predict the existence and stability of such materials among inherently layered equivalent ternaries that incorporate heavier atoms for enhanced phonon scattering and to calculate their thermoelectric properties. Using density functional theory calculations, the phase stability of TiMgN2, ZrMgN2 and HfMgN2 compounds has been calculated. From the computationally predicted phase diagrams for these materials, we conclude that all three compounds are stable in these stoichiometries. The stable compounds may have one of two competing crystal structures: a monoclinic structure (LiUN2 prototype) or a trigonal superstructure (NaCrS2 prototype; RmH). The band structure for the two competing structures for each ternary is also calculated and predicts semiconducting behavior for all three compounds in the NaCrS2 crystal structure with an indirect band gap and semiconducting behavior for ZrMgN2 and HfMgN2 in the monoclinic crystal structure with a direct band gap. Seebeck coefficient and power factors are also predicted, showing that all three compounds in both the NaCrS2 and the LiUN2 structures have large Seebeck coefficients. The predicted stability of these compounds suggests that they can be synthesized by, e.g., physical vapor deposition
2 editions published between 2017 and 2018 in English and held by 3 WorldCat member libraries worldwide
Scandium nitride has recently gained interest as a prospective compound for thermoelectric applications due to its high Seebeck coefficient. However, ScN also has a relatively high thermal conductivity, which limits its thermoelectric efficiency and figure of merit (zT). These properties motivate a search for other semiconductor materials that share the electronic structure features of ScN, but which have a lower thermal conductivity. Thus, the focus of our study is to predict the existence and stability of such materials among inherently layered equivalent ternaries that incorporate heavier atoms for enhanced phonon scattering and to calculate their thermoelectric properties. Using density functional theory calculations, the phase stability of TiMgN2, ZrMgN2 and HfMgN2 compounds has been calculated. From the computationally predicted phase diagrams for these materials, we conclude that all three compounds are stable in these stoichiometries. The stable compounds may have one of two competing crystal structures: a monoclinic structure (LiUN2 prototype) or a trigonal superstructure (NaCrS2 prototype; RmH). The band structure for the two competing structures for each ternary is also calculated and predicts semiconducting behavior for all three compounds in the NaCrS2 crystal structure with an indirect band gap and semiconducting behavior for ZrMgN2 and HfMgN2 in the monoclinic crystal structure with a direct band gap. Seebeck coefficient and power factors are also predicted, showing that all three compounds in both the NaCrS2 and the LiUN2 structures have large Seebeck coefficients. The predicted stability of these compounds suggests that they can be synthesized by, e.g., physical vapor deposition
Anomalous Phonon Lifetime Shortening in Paramagnetic CrN Caused by SpinLattice Coupling: A Combined Spin and Ab Initio Molecular
Dynamics Study by
Irina Stockem(
)
2 editions published in 2018 in English and held by 3 WorldCat member libraries worldwide
We study the mutual coupling of spin fluctuations and lattice vibrations in paramagnetic CrN by combining atomistic spin dynamics and ab initio molecular dynamics. The two degrees of freedom are dynamically coupled, leading to nonadiabatic effects. Those effects suppress the phonon lifetimes at low temperature compared to an adiabatic approach. The dynamic coupling identified here provides an explanation for the experimentally observed unexpected temperature dependence of the thermal conductivity of magnetic semiconductors above the magnetic ordering temperature
2 editions published in 2018 in English and held by 3 WorldCat member libraries worldwide
We study the mutual coupling of spin fluctuations and lattice vibrations in paramagnetic CrN by combining atomistic spin dynamics and ab initio molecular dynamics. The two degrees of freedom are dynamically coupled, leading to nonadiabatic effects. Those effects suppress the phonon lifetimes at low temperature compared to an adiabatic approach. The dynamic coupling identified here provides an explanation for the experimentally observed unexpected temperature dependence of the thermal conductivity of magnetic semiconductors above the magnetic ordering temperature
Comparison of thermodynamic properties of cubic Cr 1x Al x N and Ti 1x Al x N from firstprinciples calculations by
Björn Alling(
)
1 edition published in 2007 in English and held by 2 WorldCat member libraries worldwide
In order to investigate the stability of the cubic phase of Cr 1− x Al x N at high AlN content, first principles calculations of magnetic properties, lattice parameters, electronic structure, and mixing enthalpies of the system were performed. The mixing enthalpy was calculated on a fine concentration mesh to make possible the accurate determination of its second concentration derivative. The results are compared to calculations performed for the related compound Ti 1− x Al x N and with experiments. The mixing enthalpy is discussed in the context of isostructural spinodal decomposition. It is shown that the magnetism is the key to understand the difference between the Cr and Ticontaining systems. Cr 1− x Al x N turns out to be more stable against spinodal decomposition than Ti 1− x Al x N , especially for AlNrich samples which are of interest in cutting tools applications
1 edition published in 2007 in English and held by 2 WorldCat member libraries worldwide
In order to investigate the stability of the cubic phase of Cr 1− x Al x N at high AlN content, first principles calculations of magnetic properties, lattice parameters, electronic structure, and mixing enthalpies of the system were performed. The mixing enthalpy was calculated on a fine concentration mesh to make possible the accurate determination of its second concentration derivative. The results are compared to calculations performed for the related compound Ti 1− x Al x N and with experiments. The mixing enthalpy is discussed in the context of isostructural spinodal decomposition. It is shown that the magnetism is the key to understand the difference between the Cr and Ticontaining systems. Cr 1− x Al x N turns out to be more stable against spinodal decomposition than Ti 1− x Al x N , especially for AlNrich samples which are of interest in cutting tools applications
A theoretical study of disorder and decomposition in multinary nitrides hard coatings materials by
Björn Alling(
Book
)
2 editions published in 2009 in English and held by 2 WorldCat member libraries worldwide
2 editions published in 2009 in English and held by 2 WorldCat member libraries worldwide
<> by
Daniel Eklöf(
)
1 edition published in 2019 in English and held by 2 WorldCat member libraries worldwide
1 edition published in 2019 in English and held by 2 WorldCat member libraries worldwide
Strong impact of lattice vibrations on electronic and magnetic properties of paramagnetic Fe revealed by disordered local
moments molecular dynamics by
Björn Alling(
)
1 edition published in 2016 in English and held by 2 WorldCat member libraries worldwide
1 edition published in 2016 in English and held by 2 WorldCat member libraries worldwide
Effect of temperature and configurational disorder on the electronic band gap of boron carbide from first principles by Annop Ektarawong(
)
1 edition published in 2018 in English and held by 1 WorldCat member library worldwide
The overestimation, rather than the usual underestimation, of the electronic band gap at 0 K of boron carbide with the ideally stoichiometric composition of B4C, represented by B11CP (CBC), in density functional theory calculations is one of the outstanding controversial issues in the field of icosahedral boronrich solids. Using a firstprinciples approach, we explore the effect of temperature and configurational disorder on the electronic band gap of B4C. Ab initio molecular dynamics simulations are performed to account for the effects of vibrational disorder. The results reveal that the volumetric thermal expansion as well as the thermally induced configurational disorder of icosahedral CP atoms residing in the B11CP icosahedra have a minimal impact on the band gap of B4C, while a major decrease of the band gap is caused by explicit atomic displacements, induced by lattice vibrations. At 298 K, the band gap of B4C is overestimated, as compared to the experimental value, by approximately 31%. However, configurational disorder induced by introducing a small fraction of B12 (CBC) and B12 (B4) into a matrix of B11CP (CBC) to make the composition of boron carbide approximately B4.3C, claimed to be the carbonrich limit of the material in experiment, leads to a smaller band gap due to the appearance of midgap states. These results can explain at least a part of the previous discrepancies between theory and experiments for the band gap of boron carbide
1 edition published in 2018 in English and held by 1 WorldCat member library worldwide
The overestimation, rather than the usual underestimation, of the electronic band gap at 0 K of boron carbide with the ideally stoichiometric composition of B4C, represented by B11CP (CBC), in density functional theory calculations is one of the outstanding controversial issues in the field of icosahedral boronrich solids. Using a firstprinciples approach, we explore the effect of temperature and configurational disorder on the electronic band gap of B4C. Ab initio molecular dynamics simulations are performed to account for the effects of vibrational disorder. The results reveal that the volumetric thermal expansion as well as the thermally induced configurational disorder of icosahedral CP atoms residing in the B11CP icosahedra have a minimal impact on the band gap of B4C, while a major decrease of the band gap is caused by explicit atomic displacements, induced by lattice vibrations. At 298 K, the band gap of B4C is overestimated, as compared to the experimental value, by approximately 31%. However, configurational disorder induced by introducing a small fraction of B12 (CBC) and B12 (B4) into a matrix of B11CP (CBC) to make the composition of boron carbide approximately B4.3C, claimed to be the carbonrich limit of the material in experiment, leads to a smaller band gap due to the appearance of midgap states. These results can explain at least a part of the previous discrepancies between theory and experiments for the band gap of boron carbide
Strong electron correlations stabilize paramagnetic cubic Cr1xAlxN solid solutions by
Björn Alling(
)
1 edition published in 2013 in English and held by 1 WorldCat member library worldwide
The stability of rock salt structure cubic Cr1xAlxN solid solutions at high Al content and high temperature has made it one of the most important materials systems for protective coating applications. We show that the strong electron correlations in a material with dynamic magnetic disorder is the underlying reason for the observed stability against isostructural decomposition. This is done by using the firstprinciples disordered local moments molecular dynamics technique, which allows us to simultaneously consider electronic, magnetic, and vibrational degrees of freedom
1 edition published in 2013 in English and held by 1 WorldCat member library worldwide
The stability of rock salt structure cubic Cr1xAlxN solid solutions at high Al content and high temperature has made it one of the most important materials systems for protective coating applications. We show that the strong electron correlations in a material with dynamic magnetic disorder is the underlying reason for the observed stability against isostructural decomposition. This is done by using the firstprinciples disordered local moments molecular dynamics technique, which allows us to simultaneously consider electronic, magnetic, and vibrational degrees of freedom
Firstprinciples study of the SiNx/TiN(001) interface by
Tobias Marten(
)
1 edition published in 2012 in English and held by 1 WorldCat member library worldwide
The structure of the SiNx tissue phase in superhard TiN/SiNx nanocomposites has been debated in the literature. We present a theoretical investigation of the possibility of crystalline and coherent (001) interfaces that satisfies the two necessary criteria, stability with respect to lattice vibrations as well as to variations in stoichiometry. It is found that one monolayer of Si tetrahedrally coordinated by N in a B3like geometry embedded between B1TiN(001) surfaces is both dynamically stable and thermodynamically stable with respect to vacancy formation. However, with increasing layer thickness the B3type structure becomes unstable with respect to Si vacancy formation. Instead we suggest that a tetragonal D0(22)like order of Si vacancies can stabilize the interface. These structures are in line with the experimental findings of the crystalline tissue phase which has coherent interfaces with TiN
1 edition published in 2012 in English and held by 1 WorldCat member library worldwide
The structure of the SiNx tissue phase in superhard TiN/SiNx nanocomposites has been debated in the literature. We present a theoretical investigation of the possibility of crystalline and coherent (001) interfaces that satisfies the two necessary criteria, stability with respect to lattice vibrations as well as to variations in stoichiometry. It is found that one monolayer of Si tetrahedrally coordinated by N in a B3like geometry embedded between B1TiN(001) surfaces is both dynamically stable and thermodynamically stable with respect to vacancy formation. However, with increasing layer thickness the B3type structure becomes unstable with respect to Si vacancy formation. Instead we suggest that a tetragonal D0(22)like order of Si vacancies can stabilize the interface. These structures are in line with the experimental findings of the crystalline tissue phase which has coherent interfaces with TiN
Wurtzitestructure Sc 1x Al x N solid solution films grown by reactive magnetron sputter epitaxy structural characterization
and firstprinciples calculations by
Carina Höglund(
)
1 edition published in 2010 in English and held by 1 WorldCat member library worldwide
AlN(0001) was alloyed with ScN with molar fractions up to ~22%, while retaining a singlecrystal wurtzite (w ) structure and with lattice parameters matching calculated values. Material synthesis was realized by magnetron sputter epitaxy of thin films starting from optimal conditions for the formation of wAlN onto latticematched wAlN seed layers on Al2O3(0001) and MgO(111) substrates. Films with ScN contents between 23% and ~50% exhibit phase separation into nanocrystalline ScN and AlN, while ScNrich growth conditions yield a transformation to rocksaltstructure Sc1xAlxN(111) films. The experimental results are analyzed with ion beam analysis, Xray diffraction, and transmission electron microscopy, together with abinitio calculations of mixing enthalpies and lattice parameters of solid solutions in wurtzite, rocksalt, and layered hexagonal phases
1 edition published in 2010 in English and held by 1 WorldCat member library worldwide
AlN(0001) was alloyed with ScN with molar fractions up to ~22%, while retaining a singlecrystal wurtzite (w ) structure and with lattice parameters matching calculated values. Material synthesis was realized by magnetron sputter epitaxy of thin films starting from optimal conditions for the formation of wAlN onto latticematched wAlN seed layers on Al2O3(0001) and MgO(111) substrates. Films with ScN contents between 23% and ~50% exhibit phase separation into nanocrystalline ScN and AlN, while ScNrich growth conditions yield a transformation to rocksaltstructure Sc1xAlxN(111) films. The experimental results are analyzed with ion beam analysis, Xray diffraction, and transmission electron microscopy, together with abinitio calculations of mixing enthalpies and lattice parameters of solid solutions in wurtzite, rocksalt, and layered hexagonal phases
Structural and magnetic disorder in crystalline materials a first principles study by
Davide Gambino(
Book
)
1 edition published in 2019 in English and held by 1 WorldCat member library worldwide
Disorder in crystalline materials can take different forms and originate from different sources. In particular, temperature introduces disorder in any kind of material. This can be observed as the appearance of vacant lattice sites in an otherwise perfect crystal, or as a random distribution of different elements on the same lattice in an alloy; at the same time, if the material is magnetic, temperature induces disorder also on the magnetic degrees of freedom. In this thesis, different levels of disorder associated to structure and magnetism are investigated by means of density functional theory and thermodynamic models. I start with diffusion of Ti vacancies in TiN, which is studied by means of nonequilibrium ab initio molecular dynamics using the color diffusion algorithm at different temperatures. The result is an Arrhenius behavior of Ti vacancy jump rates. A method to perform structural relaxations in magnetic materials in their hightemperature paramagnetic phase is then developed based on the disordered local moments approach in order to study vacancies, interstitial atoms, and combinations of defects in paramagnetic bcc Fe and B1 CrN, as well as the mixing enthalpy of bcc Fe1−xCrx random alloys. A correction to the energetics of every system due to the relaxation in the disordered magnetic state is observed in all cases. Not related to temperature and disorder, but very important for an accurate description of magnetic materials, is the choice of the exchange and correlation functional to be employed in the first principles calculations. We have investigated the performance of a recently developed metaGGA functional, the strongly constrained and appropriately normed (SCAN) functional, in comparison with the more commonly used LDA and PBE on the ferromagnetic elemental solids bcc Fe, fcc Ni, and hcp Co, and SCAN it is found to give negligible improvements, if not a worsening, in the description of these materials. Finally, the coupling between vibrational and magnetic degrees of freedom is discussed by reviewing the literature and proposing an investigation of the influence of vibrations on longitudinal spin fluctuations. These excitations are here studied by means of thermodynamic models based on Landau expansion of the energy in even powers of the magnitude of the local magnetic moments. We find that vibrational and magnetic disorder alter the energy landscapes as a function of moment size also in bcc Fe, which is often considered a Heisenberg system, inducing a more itinerant electron behavior
1 edition published in 2019 in English and held by 1 WorldCat member library worldwide
Disorder in crystalline materials can take different forms and originate from different sources. In particular, temperature introduces disorder in any kind of material. This can be observed as the appearance of vacant lattice sites in an otherwise perfect crystal, or as a random distribution of different elements on the same lattice in an alloy; at the same time, if the material is magnetic, temperature induces disorder also on the magnetic degrees of freedom. In this thesis, different levels of disorder associated to structure and magnetism are investigated by means of density functional theory and thermodynamic models. I start with diffusion of Ti vacancies in TiN, which is studied by means of nonequilibrium ab initio molecular dynamics using the color diffusion algorithm at different temperatures. The result is an Arrhenius behavior of Ti vacancy jump rates. A method to perform structural relaxations in magnetic materials in their hightemperature paramagnetic phase is then developed based on the disordered local moments approach in order to study vacancies, interstitial atoms, and combinations of defects in paramagnetic bcc Fe and B1 CrN, as well as the mixing enthalpy of bcc Fe1−xCrx random alloys. A correction to the energetics of every system due to the relaxation in the disordered magnetic state is observed in all cases. Not related to temperature and disorder, but very important for an accurate description of magnetic materials, is the choice of the exchange and correlation functional to be employed in the first principles calculations. We have investigated the performance of a recently developed metaGGA functional, the strongly constrained and appropriately normed (SCAN) functional, in comparison with the more commonly used LDA and PBE on the ferromagnetic elemental solids bcc Fe, fcc Ni, and hcp Co, and SCAN it is found to give negligible improvements, if not a worsening, in the description of these materials. Finally, the coupling between vibrational and magnetic degrees of freedom is discussed by reviewing the literature and proposing an investigation of the influence of vibrations on longitudinal spin fluctuations. These excitations are here studied by means of thermodynamic models based on Landau expansion of the energy in even powers of the magnitude of the local magnetic moments. We find that vibrational and magnetic disorder alter the energy landscapes as a function of moment size also in bcc Fe, which is often considered a Heisenberg system, inducing a more itinerant electron behavior
Theory of the ferromagnetism in Ti 1x Cr x N solid solutions by
Björn Alling(
)
1 edition published in 2010 in English and held by 1 WorldCat member library worldwide
Firstprinciples calculations are used to investigate the magnetic properties of Ti 1x Cr x N solid solutions. We show that the magnetic interactions between Cr spins that favor antiferromagnetism in CrN is changed upon alloying with TiN leading to the appearance of ferromagnetism in the system at approximately x≤0.50 in agreement with experimental reports. Furthermore we suggest that this effect originates in an electron density redistribution from Ti to Cr that decreases the polarization of Crd states with t 2g symmetry while it increases the polarization of Crd states with e g symmetry, both changes working in favor of ferromagnetism
1 edition published in 2010 in English and held by 1 WorldCat member library worldwide
Firstprinciples calculations are used to investigate the magnetic properties of Ti 1x Cr x N solid solutions. We show that the magnetic interactions between Cr spins that favor antiferromagnetism in CrN is changed upon alloying with TiN leading to the appearance of ferromagnetism in the system at approximately x≤0.50 in agreement with experimental reports. Furthermore we suggest that this effect originates in an electron density redistribution from Ti to Cr that decreases the polarization of Crd states with t 2g symmetry while it increases the polarization of Crd states with e g symmetry, both changes working in favor of ferromagnetism
Mysterious SiB3: Identifying the Relation between alpha and betaSiB3 by
Daniel Eklof(
)
1 edition published in 2019 in English and held by 1 WorldCat member library worldwide
Binary silicon boride SiB3 has been reported to occur in two forms, as disordered and nonstoichiometric alphaSiB3x, which relates to the alpharhombohedral phase of boron, and as strictly ordered and stoichiometric betaSiB3. Similar to other boronrich icosahedral solids, these SiB3 phases represent potentially interesting refractory materials. However, their thermal stability, formation conditions, and thermodynamic relation are poorly understood. Here, we map the formation conditions of alphaSiB3x and betaSiB3 and analyze their relative thermodynamic stabilities. alphaSiB3x is metastable (with respect to betaSiB3 and Si), and its formation is kinetically driven. Pure polycrystalline bulk samples may be obtained within hours when heating stoichiometric mixtures of elemental silicon and boron at temperatures 12001300 degrees C. At the same time, alphaSiB3x decomposes into SiB6 and Si, and optimum timetemperature synthesis conditions represent a tradeoff between rates of formation and decomposition. The formation of stable betaSiB3 was observed after prolonged treatment (days to weeks) of elemental mixtures with ratios Si/B = 1:11:4 at temperatures 11751200 degrees C. The application of high pressures greatly improves the kinetics of SiB3 formation and allows decoupling of SiB3 formation from decomposition. Quantitative formation of betaSiB3 was seen at 1100 degrees C for samples pressurized to 5.58 GPa. betaSiB3 decomposes peritectoidally at temperatures between 1250 and 1300 degrees C. The highly ordered nature of betaSiB3 is reflected in its Raman spectrum, which features narrow and distinct lines. In contrast, the Raman spectrum of alphaSiB3x is characterized by broad bands, which show a clear relation to the vibrational modes of isostructural, ordered B6P. The detailed composition and structural properties of disordered alphaSiB3x were ascertained by a combination of singlecrystal Xray diffraction and Si29 magic angle spinning NMR experiments. Notably, the compositions of polycrystalline bulk samples (obtained at T amp;lt;= 1200 degrees C) and single crystal samples (obtained from Sirich molten SiB mixtures at T amp;gt; 1400 degrees C) are different, SiB2.93(7) and SiB2.64(2), respectively. The incorporation of Si in the polar position of B12 icosahedra results in highly strained cluster units. This disorder feature was accounted for in the refined crystal structure model by splitting the polar position into three sites. The electronprecise composition of alphaSiB3x is SiB2.5 and corresponds to the incorporation of, on average, two Si atoms in each B12 icosahedron. Accordingly, alphaSiB3x constitutes a mixture of B10Si2 and B11Si clusters. The structural and phase stability of alphaSiB3x were explored using a firstprinciples cluster expansion. The most stable composition at 0 K is SiB2.5, which however is unstable with respect to the decomposition betaSiB3 + Si. Modeling of the configurational and vibrational entropies suggests that alphaSiB3x only becomes more stable than betaSiB3 at temperatures above its decomposition into SiB6 and Si. Hence, we conclude that alphaSiB3x is metastable at all temperatures. Density functional theory electronic structure calculations yield band gaps of similar size for electronprecise alphaSiB2.5 and betaSiB3, whereas alphaSiB3 represents a ptype conductor
1 edition published in 2019 in English and held by 1 WorldCat member library worldwide
Binary silicon boride SiB3 has been reported to occur in two forms, as disordered and nonstoichiometric alphaSiB3x, which relates to the alpharhombohedral phase of boron, and as strictly ordered and stoichiometric betaSiB3. Similar to other boronrich icosahedral solids, these SiB3 phases represent potentially interesting refractory materials. However, their thermal stability, formation conditions, and thermodynamic relation are poorly understood. Here, we map the formation conditions of alphaSiB3x and betaSiB3 and analyze their relative thermodynamic stabilities. alphaSiB3x is metastable (with respect to betaSiB3 and Si), and its formation is kinetically driven. Pure polycrystalline bulk samples may be obtained within hours when heating stoichiometric mixtures of elemental silicon and boron at temperatures 12001300 degrees C. At the same time, alphaSiB3x decomposes into SiB6 and Si, and optimum timetemperature synthesis conditions represent a tradeoff between rates of formation and decomposition. The formation of stable betaSiB3 was observed after prolonged treatment (days to weeks) of elemental mixtures with ratios Si/B = 1:11:4 at temperatures 11751200 degrees C. The application of high pressures greatly improves the kinetics of SiB3 formation and allows decoupling of SiB3 formation from decomposition. Quantitative formation of betaSiB3 was seen at 1100 degrees C for samples pressurized to 5.58 GPa. betaSiB3 decomposes peritectoidally at temperatures between 1250 and 1300 degrees C. The highly ordered nature of betaSiB3 is reflected in its Raman spectrum, which features narrow and distinct lines. In contrast, the Raman spectrum of alphaSiB3x is characterized by broad bands, which show a clear relation to the vibrational modes of isostructural, ordered B6P. The detailed composition and structural properties of disordered alphaSiB3x were ascertained by a combination of singlecrystal Xray diffraction and Si29 magic angle spinning NMR experiments. Notably, the compositions of polycrystalline bulk samples (obtained at T amp;lt;= 1200 degrees C) and single crystal samples (obtained from Sirich molten SiB mixtures at T amp;gt; 1400 degrees C) are different, SiB2.93(7) and SiB2.64(2), respectively. The incorporation of Si in the polar position of B12 icosahedra results in highly strained cluster units. This disorder feature was accounted for in the refined crystal structure model by splitting the polar position into three sites. The electronprecise composition of alphaSiB3x is SiB2.5 and corresponds to the incorporation of, on average, two Si atoms in each B12 icosahedron. Accordingly, alphaSiB3x constitutes a mixture of B10Si2 and B11Si clusters. The structural and phase stability of alphaSiB3x were explored using a firstprinciples cluster expansion. The most stable composition at 0 K is SiB2.5, which however is unstable with respect to the decomposition betaSiB3 + Si. Modeling of the configurational and vibrational entropies suggests that alphaSiB3x only becomes more stable than betaSiB3 at temperatures above its decomposition into SiB6 and Si. Hence, we conclude that alphaSiB3x is metastable at all temperatures. Density functional theory electronic structure calculations yield band gaps of similar size for electronprecise alphaSiB2.5 and betaSiB3, whereas alphaSiB3 represents a ptype conductor
Screw dislocation core structure in the paramagnetic state of bcc iron from firstprinciples calculations by Luis Casillas Trujillo(
)
1 edition published in 2020 in English and held by 1 WorldCat member library worldwide
Ironbased alloys are widely used as structural components in engineering applications. This calls for a fundamental understanding of their mechanical properties, including those of pure iron. Under operational temperatures the mechanical and magnetic properties will differ from those of ferromagnetic bodycenteredcubic iron at 0 K. In this theoretical work we study the effect of disordered magnetism on the screw dislocation core structure and compare with results for the ordered ferromagnetic case. Dislocation cores control some local properties such as the choice of glide plane and the associated dislocation mobility. Changes in the magnetic state can lead to modifications in the structure of the core and affect dislocation mobility. In particular, we focus on the core properties of the 1/2 < 111 > screw dislocation in the paramagnetic state. Using the noncollinear disordered local moment approximation to address paramagnetism, we perform structural relaxations within density functional theory. We obtain the dislocation core structure for the easy and hard cores in the paramagnetic state, and compare them with their ferromagnetic counterparts. By averaging the energy of several disordered magnetic configurations, we obtain an energy difference between the easy and hardcore configurations, with a lower, but statistically close, value than the one reported for the ferromagnetic case. The magnetic moment and atomic volume at the dislocation core differ between paramagnetic and ferromagnetic states, with possible consequences on the temperature dependence of defectdislocation interactions
1 edition published in 2020 in English and held by 1 WorldCat member library worldwide
Ironbased alloys are widely used as structural components in engineering applications. This calls for a fundamental understanding of their mechanical properties, including those of pure iron. Under operational temperatures the mechanical and magnetic properties will differ from those of ferromagnetic bodycenteredcubic iron at 0 K. In this theoretical work we study the effect of disordered magnetism on the screw dislocation core structure and compare with results for the ordered ferromagnetic case. Dislocation cores control some local properties such as the choice of glide plane and the associated dislocation mobility. Changes in the magnetic state can lead to modifications in the structure of the core and affect dislocation mobility. In particular, we focus on the core properties of the 1/2 < 111 > screw dislocation in the paramagnetic state. Using the noncollinear disordered local moment approximation to address paramagnetism, we perform structural relaxations within density functional theory. We obtain the dislocation core structure for the easy and hard cores in the paramagnetic state, and compare them with their ferromagnetic counterparts. By averaging the energy of several disordered magnetic configurations, we obtain an energy difference between the easy and hardcore configurations, with a lower, but statistically close, value than the one reported for the ferromagnetic case. The magnetic moment and atomic volume at the dislocation core differ between paramagnetic and ferromagnetic states, with possible consequences on the temperature dependence of defectdislocation interactions
Role of spinorbit coupling in the alloying behavior of multilayer Bi1xSbx solid solutions revealed by a firstprinciples
cluster expansion by A Ektarawong(
)
1 edition published in 2020 in English and held by 1 WorldCat member library worldwide
We employ a firstprinciples clusterexpansion method in combination with canonical Monte Carlo simulations to study the effect of spinorbit coupling on the alloying behavior of multilayer Bi1xSbx. Our simulations reveal that spinorbit coupling plays an essential role in determining the configurational thermodynamics of Bi and Sb atoms. Without the presence of spinorbit coupling, Bi1xSbx is predicted to exhibit at lowtemperature chemical ordering of Bi and Sb atoms, leading to formation of an ordered structure at x approximate to 0.5. Interestingly, the spinorbitcoupling effect intrinsically induced by the existence of Bi and Sb results in the disappearance of chemical ordering of the constituent elements within an immiscible region existing at T & lt; 370 K, and consequently Bi1xSbx displays merely a tendency toward local segregation of Bi and Sb atoms, resulting in coexistence of Birich and Sbrich Bi1xSbx solid solutions without the formation of any ordered structure of Bi1xSbx as predicted in the absence of spinorbit coupling. These findings distinctly highlight an influence of spinorbit coupling on the alloying behavior of Bi1xSbx and probably other alloys composed of heavy elements, where the spinorbitcoupling effect is supposed to be robust
1 edition published in 2020 in English and held by 1 WorldCat member library worldwide
We employ a firstprinciples clusterexpansion method in combination with canonical Monte Carlo simulations to study the effect of spinorbit coupling on the alloying behavior of multilayer Bi1xSbx. Our simulations reveal that spinorbit coupling plays an essential role in determining the configurational thermodynamics of Bi and Sb atoms. Without the presence of spinorbit coupling, Bi1xSbx is predicted to exhibit at lowtemperature chemical ordering of Bi and Sb atoms, leading to formation of an ordered structure at x approximate to 0.5. Interestingly, the spinorbitcoupling effect intrinsically induced by the existence of Bi and Sb results in the disappearance of chemical ordering of the constituent elements within an immiscible region existing at T & lt; 370 K, and consequently Bi1xSbx displays merely a tendency toward local segregation of Bi and Sb atoms, resulting in coexistence of Birich and Sbrich Bi1xSbx solid solutions without the formation of any ordered structure of Bi1xSbx as predicted in the absence of spinorbit coupling. These findings distinctly highlight an influence of spinorbit coupling on the alloying behavior of Bi1xSbx and probably other alloys composed of heavy elements, where the spinorbitcoupling effect is supposed to be robust
Magnetic nanoscale laminates with tunable exchange coupling from first principles by Martin Dahlqvist(
)
1 edition published in 2011 in English and held by 1 WorldCat member library worldwide
The M(n+1)AX(n) (MAX) phases are nanolaminated compounds with a unique combination of metallic and ceramic properties, not yet including magnetism. We carry out a systematic theoretical study of potential magnetic MAX phases and predict the existence of stable magnetic (Cr(1x)Mn(x))(2)AlC alloys. We show that in this system ferromagnetically ordered Mn layers are exchange coupled via nearly nonmagnetic Cr layers, forming an inherent structure of atomicthin magnetic multilayers, and that the degree of disorder between Cr and Mn in the alloy can be used to tune the sign and magnitude of the coupling
1 edition published in 2011 in English and held by 1 WorldCat member library worldwide
The M(n+1)AX(n) (MAX) phases are nanolaminated compounds with a unique combination of metallic and ceramic properties, not yet including magnetism. We carry out a systematic theoretical study of potential magnetic MAX phases and predict the existence of stable magnetic (Cr(1x)Mn(x))(2)AlC alloys. We show that in this system ferromagnetically ordered Mn layers are exchange coupled via nearly nonmagnetic Cr layers, forming an inherent structure of atomicthin magnetic multilayers, and that the degree of disorder between Cr and Mn in the alloy can be used to tune the sign and magnitude of the coupling
Thermodynamic stability of hexagonal and rhombohedral boron nitride under chemical vapor deposition conditions from van der
Waals corrected first principles calculations by Henrik Pedersen(
)
1 edition published in 2019 in English and held by 1 WorldCat member library worldwide
Thin films of boron nitride (BN), particularly the sp(2)hybridized polytypes hexagonal BN (hBN) and rhombohedral BN (rBN), are interesting for several electronic applications, given the bandgaps in the UV. They are typically deposited close to thermal equilibrium by chemical vapor deposition (CVD) at temperatures and pressures in the regions 14001800K and 100010000Pa, respectively. In this letter, the authors use the van der Waals corrected density functional theory and thermodynamic stability calculations to determine the stability of rBN and compare it to that of hBN as well as to cubic BN and wurtzitic BN. The authors find that rBN is the stable sp(2)hybridized phase at CVD conditions, while hBN is metastable. Thus, their calculations suggest that thin films of hBN must be deposited far from thermal equilibrium
1 edition published in 2019 in English and held by 1 WorldCat member library worldwide
Thin films of boron nitride (BN), particularly the sp(2)hybridized polytypes hexagonal BN (hBN) and rhombohedral BN (rBN), are interesting for several electronic applications, given the bandgaps in the UV. They are typically deposited close to thermal equilibrium by chemical vapor deposition (CVD) at temperatures and pressures in the regions 14001800K and 100010000Pa, respectively. In this letter, the authors use the van der Waals corrected density functional theory and thermodynamic stability calculations to determine the stability of rBN and compare it to that of hBN as well as to cubic BN and wurtzitic BN. The authors find that rBN is the stable sp(2)hybridized phase at CVD conditions, while hBN is metastable. Thus, their calculations suggest that thin films of hBN must be deposited far from thermal equilibrium
Competition between Magnetic Structures in the FeRich FCC FeNi Alloys by
Igor Abrikosov(
)
1 edition published in 2007 in English and held by 1 WorldCat member library worldwide
We report on the results of a systematic ab initio study of the magnetic structure of Fe rich fcc FeNi binary alloys for Ni concentrations up to 50 at. %. Calculations are carried out within densityfunctional theory using two complementary techniques, one based on the exact muffintin orbital theory within the coherent potential approximation and another one based on the projector augmentedwave method. We observe that the evolution of the magnetic structure of the alloy with increasing Ni concentration is determined by a competition between a large number of magnetic states, collinear as well as noncollinear, all close in energy. We emphasize a series of transitions between these magnetic structures, in particular we have investigated a competition between disordered local moment configurations, spin spiral states, the double layer antiferromagnetic state, and the ferromagnetic phase, as well as the ferrimagnetic phase with a single spin flipped with respect to all others. We show that the latter should be particularly important for the understanding of the magnetic structure of the Invar alloys
1 edition published in 2007 in English and held by 1 WorldCat member library worldwide
We report on the results of a systematic ab initio study of the magnetic structure of Fe rich fcc FeNi binary alloys for Ni concentrations up to 50 at. %. Calculations are carried out within densityfunctional theory using two complementary techniques, one based on the exact muffintin orbital theory within the coherent potential approximation and another one based on the projector augmentedwave method. We observe that the evolution of the magnetic structure of the alloy with increasing Ni concentration is determined by a competition between a large number of magnetic states, collinear as well as noncollinear, all close in energy. We emphasize a series of transitions between these magnetic structures, in particular we have investigated a competition between disordered local moment configurations, spin spiral states, the double layer antiferromagnetic state, and the ferromagnetic phase, as well as the ferrimagnetic phase with a single spin flipped with respect to all others. We show that the latter should be particularly important for the understanding of the magnetic structure of the Invar alloys
The effect of strain and pressure on the electronphonon coupling and superconductivity in MgB2Benchmark of theoretical methodologies
and outlook for nanostructure design by Erik Johansson(
)
1 edition published in 2022 in English and held by 1 WorldCat member library worldwide
Different theoretical methodologies are employed to investigate the effect of hydrostatic pressure and anisotropic stress and strain on the superconducting transition temperature ( Tc) of MgB2. This is done both by studying Kohn anomalies in the phonon dispersions alone and by explicit calculation of the electronphonon coupling. It is found that increasing pressure suppresses Tc in all cases, whereas isotropic and anisotropic strain enhances the superconductivity. In contrast to trialed epitaxial growth that is limited in the amount of achievable lattice strain, we propose a different path by codeposition with ternary diborides that thermodynamically avoid mixing with MgB2. This is suggested to promote columnar growth that can introduce strain in all directions
1 edition published in 2022 in English and held by 1 WorldCat member library worldwide
Different theoretical methodologies are employed to investigate the effect of hydrostatic pressure and anisotropic stress and strain on the superconducting transition temperature ( Tc) of MgB2. This is done both by studying Kohn anomalies in the phonon dispersions alone and by explicit calculation of the electronphonon coupling. It is found that increasing pressure suppresses Tc in all cases, whereas isotropic and anisotropic strain enhances the superconductivity. In contrast to trialed epitaxial growth that is limited in the amount of achievable lattice strain, we propose a different path by codeposition with ternary diborides that thermodynamically avoid mixing with MgB2. This is suggested to promote columnar growth that can introduce strain in all directions
Ab Initio Modeling of Magnetic Materials in the HighTemperature Paramagnetic Phase by
Davide Gambino(
)
1 edition published in 2021 in English and held by 1 WorldCat member library worldwide
The modeling of magnetic materials at finite temperatures is an ongoing challenge in the field of theoretical physics. This field has strongly benefited from the development of computational methods, which allow to predict material's properties and explain physical effects on the atomic scale, and are now employed to direct the design of new materials. However, simulations need to be as accurate as possible to give reliable insights into solidstate phenomena, which means that, most desirably, all competing effects occurring in a system at realistic conditions should be included. This task is particularly difficult in the modeling of magnetic materials from first principles, due to the quantum nature of magnetism and its interplay with other phenomena related to the atomic degrees of freedom. The aim of this thesis is therefore to develop methods that enable the inclusion of magnetic effects in finite temperature simulations based on density functional theory (DFT), while considering on the same footing vibrational and structural degrees of freedom,with a particular focus on the hightemperature paramagnetic phase. The type of couplings investigated in this thesis can be separated in two big categories: interplay between magnetism and structure, and between magnetism and vibrations. Regarding the former category, I have tried to shine some light on the effect of the paramagnetic state on atomic positions in a crystal in the presence of defects or for complicated systems, as opposed to the ordered magnetic state. To model the hightemperature paramagnetic phase of magnetic materials, the disordered local moment (DLM) approach is employed in the whole work. In this framework, I have developed a method to perform local lattice relaxations in the disordered magnetic state, which consists of a stepwise partial relaxation of the atomic positions, while changing the configuration of the magnetic moments at each step of the procedure. This method has been tested on point defects in paramagnetic bcc Fe, namely the single vacancy and, separately, the C interstitial in octahedral position, and on Fe 1x Cr x alloys, finding nonnegligible effects on formation energies. In addition, the feasibility of investigating extended defects like dislocations in the paramagnetic state with this method has also been proven by studying the screw dislocation in bcc Fe. The DLMrelaxation method has then been used to investigate intrinsic and extrinsic defects in CrN, an antiferromagnetic semiconductor studied for thermoelectric applications, found in the paramagnetic state at operating temperature, and a newly synthesized compound, Fe 3 CO 7 , which features a complicated crystal structure and unusual electronic properties, with possible important implications for the chemistry of Earth's mantle. The other focus of this thesis is the coupling between magnetism and lattice vibrations. As a prestep to perform fully coupled atomistic spin dynamicsab initio molecular dynamics (ASDAIMD) simulations, I have first investigated the effect of vibrations on the so called longitudinal spin fluctuations, a mechanism occurring at finite temperatures and important for itinerant electron magnetic systems. I have developed a framework to investigate the dependence of the local moment's energy landscapes on the instantaneous positions of the atoms, testing it on Fe at different temperature and pressure conditions. This study has laid the foundation to apply machine learning techniques to the prediction of the energy landscapes during an ASDAIMD simulation. Finally, I have investigated the phase stability of Fe at ambient pressure from the theoretical Curie temperature up to its melting point with ASDAIMD. This task is carried out by applying a pool of thermodynamic techniques to calculate free energy differences, and therefore I have defined a strategy to discern the thermodynamic equilibrium structure in magnetic materials in the high temperature paramagnetic phase based on first principles dynamical simulations. The methodologies developed and applied in this work constitute an improvement towards the simulation of magnetic materials accounting for the coupling of all effects, and the hope is to bridge a gap between theory and experiments
1 edition published in 2021 in English and held by 1 WorldCat member library worldwide
The modeling of magnetic materials at finite temperatures is an ongoing challenge in the field of theoretical physics. This field has strongly benefited from the development of computational methods, which allow to predict material's properties and explain physical effects on the atomic scale, and are now employed to direct the design of new materials. However, simulations need to be as accurate as possible to give reliable insights into solidstate phenomena, which means that, most desirably, all competing effects occurring in a system at realistic conditions should be included. This task is particularly difficult in the modeling of magnetic materials from first principles, due to the quantum nature of magnetism and its interplay with other phenomena related to the atomic degrees of freedom. The aim of this thesis is therefore to develop methods that enable the inclusion of magnetic effects in finite temperature simulations based on density functional theory (DFT), while considering on the same footing vibrational and structural degrees of freedom,with a particular focus on the hightemperature paramagnetic phase. The type of couplings investigated in this thesis can be separated in two big categories: interplay between magnetism and structure, and between magnetism and vibrations. Regarding the former category, I have tried to shine some light on the effect of the paramagnetic state on atomic positions in a crystal in the presence of defects or for complicated systems, as opposed to the ordered magnetic state. To model the hightemperature paramagnetic phase of magnetic materials, the disordered local moment (DLM) approach is employed in the whole work. In this framework, I have developed a method to perform local lattice relaxations in the disordered magnetic state, which consists of a stepwise partial relaxation of the atomic positions, while changing the configuration of the magnetic moments at each step of the procedure. This method has been tested on point defects in paramagnetic bcc Fe, namely the single vacancy and, separately, the C interstitial in octahedral position, and on Fe 1x Cr x alloys, finding nonnegligible effects on formation energies. In addition, the feasibility of investigating extended defects like dislocations in the paramagnetic state with this method has also been proven by studying the screw dislocation in bcc Fe. The DLMrelaxation method has then been used to investigate intrinsic and extrinsic defects in CrN, an antiferromagnetic semiconductor studied for thermoelectric applications, found in the paramagnetic state at operating temperature, and a newly synthesized compound, Fe 3 CO 7 , which features a complicated crystal structure and unusual electronic properties, with possible important implications for the chemistry of Earth's mantle. The other focus of this thesis is the coupling between magnetism and lattice vibrations. As a prestep to perform fully coupled atomistic spin dynamicsab initio molecular dynamics (ASDAIMD) simulations, I have first investigated the effect of vibrations on the so called longitudinal spin fluctuations, a mechanism occurring at finite temperatures and important for itinerant electron magnetic systems. I have developed a framework to investigate the dependence of the local moment's energy landscapes on the instantaneous positions of the atoms, testing it on Fe at different temperature and pressure conditions. This study has laid the foundation to apply machine learning techniques to the prediction of the energy landscapes during an ASDAIMD simulation. Finally, I have investigated the phase stability of Fe at ambient pressure from the theoretical Curie temperature up to its melting point with ASDAIMD. This task is carried out by applying a pool of thermodynamic techniques to calculate free energy differences, and therefore I have defined a strategy to discern the thermodynamic equilibrium structure in magnetic materials in the high temperature paramagnetic phase based on first principles dynamical simulations. The methodologies developed and applied in this work constitute an improvement towards the simulation of magnetic materials accounting for the coupling of all effects, and the hope is to bridge a gap between theory and experiments
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