The paper presents a study of two full-core, edge dislocations of opposite Burgers vectors in 4H-... more The paper presents a study of two full-core, edge dislocations of opposite Burgers vectors in 4H-SiC, conducted using the first-principles density functional theory methods. We have determined the creation energy of the dislocations as a function of distance between their cores. The radial distribution function has been applied to examine strong impact of the dislocations on the local crystal structure. The analysis of the electronic structure reveals mid-gap levels induced by broken atomic bonds in the dislocation core. The maps of charge distribution and electrostatic potential have been calculated and the significant decrease of the electrostatic barriers in the vicinity of the dislocation cores has been quantified. The obtained results have been discussed in the light of available experimental data.
The structure, magnetic properties, and lattice dynamics of ordered Fe-Pt alloys with three stoic... more The structure, magnetic properties, and lattice dynamics of ordered Fe-Pt alloys with three stoichiometric compositions, Fe3Pt, FePt and FePt3, have been investigated using the density functional theory. Additionally, the existing experimental data have been complemented by new measurements of the Fe projected phonon density of states performed for the Fe3Pt and FePt3 thin films using the nuclear inelastic scattering technique. The calculated phonon dispersion relations and phonon density of states have been compared with the experimental data. The dispersion curves are very well reproduced by the calculations, although, the softening of the transversal acoustic mode TA1 leads to some discrepancy between the theory and experiment in Fe3Pt. A very goood agreement between the measured spectra and calculations performed for the tetragonal structure derived from the soft mode may signal that the tetragonal phase with the space group P 4/mbm plays an important role in the martensitic transformation observed in Fe3Pt. For FePt3, the antiferromagnetic order appearing with decreasing temperature has been also investigated. The studies showed that the phonon density of states of FePt3 very weakly depends on the magnetic configuration. arXiv:1501.05550v1 [cond-mat.mtrl-sci]
ABSTRACT Investigation of well-defined nanostructures often requires that the preparation, charac... more ABSTRACT Investigation of well-defined nanostructures often requires that the preparation, characterization, and maintenance of the samples during experiments are performed under controlled, in most cases ultrahigh vacuum (UHV), conditions. This chapter reports recent advances in the in situ Mössbauer spectroscopy with synchrotron radiation on thin films that became possible due to the instrumentation developments at the nuclear resonance beamline ID18 of the European synchrotron radiation facility (ESRF). After a detailed description of the beamline and of the ultrahigh vacuum (UHV) system for in situ experiments, the chapter provides a brief introduction into the basic nuclear resonant scattering (NRS) techniques. Finally, the application of these techniques to investigate magnetic, diffusion, and lattice dynamics phenomena in ultrathin epitaxial 57Fe films deposited on a W(110) substrate is presented and discussed.
Density functional theory was used to study structural and dynamical changes related to the magne... more Density functional theory was used to study structural and dynamical changes related to the magnetostructural phase transition in MnAs. The soft mode inducing the transition from the high-symmetry hexagonal to the low-symmetry orthorhombic phase was revealed. A giant coupling between the soft mode and magnetic moments was found and its crucial role in the magnetostructural transition was established. The estimated phonon contribution to the total entropy change has the opposite sign to the magnetic entropy change.
ABSTRACT The phonon dispersion relations of HgSe, which is a cubic polar semiconductor with a zer... more ABSTRACT The phonon dispersion relations of HgSe, which is a cubic polar semiconductor with a zero energy gap, has been investigated by means of the coherent inelastic neutron scattering. The complete set of phonon branches has been determined along the [001], [110], and [111] high-symmetry directions. Using the ab initio local-density approximation with ultrasoft pseudopotentials, we have calculated the Hellmann-Feynman forces for 2x2x2 supercell. Applying the direct method, the phonon dispersion relations and the phonon density of states have been derived. They are in good agreement with the results of the neutron-scattering data. The 1x1x10 elongated supercell has been used to find the LO/TO splitting of the optical phonon branches for two different concentrations of free carriers. Higher free-carriers density diminish the LO/TO splitting.
ABSTRACT Based on first-principles calculations, the structural, magnetic, dynamic properties, an... more ABSTRACT Based on first-principles calculations, the structural, magnetic, dynamic properties, and the alpha-hexagonal -> beta-orthorhombic -> gamma-hexagonal phase transitions of ferromagnetic, antiferromagnetic, and paramagnetic MnAs are studied. The phonon-dispersion curves for some of the phases were derived using the direct method. Soft modes were found in ferromagnetic and antiferromagnetic structures in a wide range of pressures. A strong dependence the soft-mode energy on the magnetic moment and magnetic order was revealed. The double-well potential was found as a function of the soft-mode amplitude at the reduced crystal volume. In the beta-orthorhombic phase, a new antiferromagnetic configuration consisting of linear chains of alternating spins was found. Therefore, the mechanism of the magnetostructural phase transitions confirms the antiferromagnetic or paramagnetic (with antiferromagnetic fluctuations) state of the beta-orthorhombic phase. The paramagnetic gamma-hexagonal phase is stabilized at high temperature in the displacive second-order phase transition owing to the disappearance of the soft mode.
Using the density functional theory and the direct method, phonon dispersion relations and phonon... more Using the density functional theory and the direct method, phonon dispersion relations and phonon density spectra have been calculated for two slabs with Fe͑100͒ and Fe͑110͒ surfaces. A drastic difference between phonon state distributions for both surfaces has been found. The magnetic moments of surface Fe atoms are enhanced by 30% and 14% for ͑100͒ and ͑110͒ surfaces, respectively.
The crystal structure and vibrational spectrum of kanemite ͑NaHSi 2 O 5 ·3H 2 O͒ were studied usi... more The crystal structure and vibrational spectrum of kanemite ͑NaHSi 2 O 5 ·3H 2 O͒ were studied using the density-functional theory. The calculations were carried out in a lower space group ͑Pna2 1 ͒ in order to eliminate the finite-temperature disordered configurations of protons in hydrogen bonds. It was found that the SiO 4 tetrahedra are linked by a short strong O-H¯O hydrogen bond with a single asymmetric potential well at low temperatures. Two energetically equivalent sites were obtained for the position of the H atom. Apart from the hydrogen bonds reported in earlier studies, additional moderate O-H¯O hydrogen bond was found to stabilize the hydrated Na layers within the silicon layers at low temperatures. The vibrational spectrum was analyzed in detail in the entire energy range ͑0-500 meV͒, focusing on the vibrations of hydrogen atoms. A perfect correlation appears to exist between the OH ͑stretching and torsional͒ frequencies and donor-acceptor distances of the respective hydrogen bonds.
ABSTRACT First-principles calculations based on density functional theory are used to determine t... more ABSTRACT First-principles calculations based on density functional theory are used to determine the structural, lattice dynamical, elastic, and thermodynamical properties of magnetic CuFeS2. The theoretical results are compared with available experimental data and are used as a basis to critically discuss existing discrepancies between theory and experiment as well as of inconsistencies between experimental data.
ABSTRACT The lattice dynamics of samarium is determined by in situ low-temperature nuclear inelas... more ABSTRACT The lattice dynamics of samarium is determined by in situ low-temperature nuclear inelastic scattering on a single crystalline (0001) Sm film, a polycrystalline Sm foil, and by first-principles theory. The ab initio calculated phonon dispersion relations and phonon density of states for the Sm-type structure and the double hexagonal-close-packed (dhcp) lattice, characteristic for light lanthanides, are compared. The dhcp unit cell, which is a factor of 2.24 smaller in height, exhibits more pronounced vibrational anisotropy in comparison to the Sm-type structure. The analysis reveals a minor influence of the spin-orbit coupling in the Sm atom on the lattice dynamics. A broadening of the longitudinal peak, not found in the calculations, suggests the influence of electron correlations on lattice dynamics in metallic samarium.
ABSTRACT The lattice dynamics of chalcopyrite crystals AgGaS2, AgGaSe2, AgGaTe2, CdGeAs2 and CdGe... more ABSTRACT The lattice dynamics of chalcopyrite crystals AgGaS2, AgGaSe2, AgGaTe2, CdGeAs2 and CdGeP2 have been studied within the force field approach. The lattice constants are derived from minimization of the unit cell potential and they agree within 1.5% with the experimental ones. The calculated interatomic forces were used by the direct method, which allowed to find the phonon dispersion curves and phonon density of states fur these crystals. The calculated phonon frequencies for AgGaSe2 fit quite well to the inelastic neutron scattering data.
The structure, lattice dynamics and mechanical properties of magnesium hydroxide have been invest... more The structure, lattice dynamics and mechanical properties of magnesium hydroxide have been investigated by static density functional theory calculations as well as ab initio molecular dynamics. The hypothesis of a superstructure existing in the lattice formed by the hydrogen atoms has been tested. The elastic constants of the material have been calculated with a static deformations approach and are in fair agreement with the experimental data. The hydrogen subsystem structure exhibits signs of disordered behaviour while maintaining correlations between the angular positions of neighbouring atoms. We establish that the essential angular correlations between hydrogen positions are maintained to a temperature of at least 150 K and that they are well described by a physically motivated probabilistic model. The rotational degree of freedom appears to be decoupled from the lattice directions above 30 K.
Lattice dynamics and thermodynamic properties of antiferromagnetic Fe 2 SiO 4 -spinel have been s... more Lattice dynamics and thermodynamic properties of antiferromagnetic Fe 2 SiO 4 -spinel have been studied using density functional theory. Phonon dispersions are obtained for several hydrostatic pressures up to 20 GPa. They are used to calculate thermodynamic properties within the quasiharmonic approximation. Comparison with ab initio results obtained for Mg 2 SiO 4 -spinel is made in order to study the effect of the cation exchange on the dynamic and thermodynamic properties of (Mg, Fe) 2 SiO 4 -spinel. The obtained results have been compared with the available experimental data.
ABSTRACT Total energy calculations within the density functional theory (DFT), generalized gradie... more ABSTRACT Total energy calculations within the density functional theory (DFT), generalized gradient approximation and ultrasoft pseudopotentials have been used to investigate structural, dynamical and thermodynamical properties of the high-pressure structure of the magnesium orthosilicate (Mg2SiO4, gamma -spinel, ringwoodite). The phonon dispersion curves and phonon density of states have been calculated using the direct method. The results show agreement with the available experimental Raman data. (C) 2001 Elsevier Science B.V. All rights reserved.
ABSTRACT Self-consistent density-functional calculations are reported for chalcopyrite structure ... more ABSTRACT Self-consistent density-functional calculations are reported for chalcopyrite structure semiconductor ZnSnP2. Results include lattice parameters obtained from total energy minimization as well as phonon dispersion relations and phonon density of states derivated from the Hellmann-Feynman forces using the direct method. Comparison with experimental data and previous calculations based on phenomenological models shows overall agreement.
The lattice dynamics of the antiferromagnetic FeBO3 crystal has been calculated by ab initio dens... more The lattice dynamics of the antiferromagnetic FeBO3 crystal has been calculated by ab initio density-functional theory and measured by nuclear inelastic absorption spectroscopy. The calculations for the antiferromagnetic phase reproduce the experimental lattice parameters of the unit cell and provide phonon dispersion relations which agree well with the measured partial density of phonon states for the Fe atoms. Calculations for the nonmagnetic configuration lead to a smaller crystal volume and drastically higher phonon frequencies for the Fe atoms.
ABSTRACT The lattice vibrational properties of CuInSe2 with CuAu-type ordering of the cation subl... more ABSTRACT The lattice vibrational properties of CuInSe2 with CuAu-type ordering of the cation sublattice are investigated theoretically by a first-principles calculations of the structure and the lattice dynamical characteristics including zone-center optical mode frequencies, phonon dispersion and density of states, and elastic constants. The results obtained for CuAu-ordered CuInSe2 are compared with related experimental data and comparative theoretical calculations for the chalcopyrite phase of the compound. A critical analysis is given of the partly contradictory experimental lattice vibration data reported for chalcopyrite CuInSe2.
Using the ab initio density-functional generalized gradient approximation with varying on-site Co... more Using the ab initio density-functional generalized gradient approximation with varying on-site Coulomb interaction U, the electronic and crystal structures of Fe 2 SiO 4 spinel were optimized at high pressure. It was found that the ground state changes from the metallic ferromagnetic state at U = 0 into the antiferromagnetic state, stable in the regime of Mott insulating state for U Ͼ 2 eV. We found that Fe 2 SiO 4 spinel is an antiferromagnetic Mott insulator at the realistic value of U ϳ 4.5 eV, with a distinct gap which opens in the minority-Fe͑3d͒ states. We point out that the low value of the Néel temperature T N ϳ 12 K follows from the geometrical frustration of magnetic interactions, while the exchange constant J ϳ 1.7 meV estimated from the energies of ordered magnetic phases gives a rather high value of the Curie-Weiss temperature CW Ӎ 340 K. The phonon-dispersion curves and phonon density of states were derived for both the metallic and insulating antiferromagnetic phase of Fe 2 SiO 4 spinel, and significant changes of energy and dispersion of certain phonon modes, particularly with large Fe character, caused by local electron correlations were detected and analyzed. We point out some similarities with the electronic and dynamical properties of magnetite.
The paper presents a study of two full-core, edge dislocations of opposite Burgers vectors in 4H-... more The paper presents a study of two full-core, edge dislocations of opposite Burgers vectors in 4H-SiC, conducted using the first-principles density functional theory methods. We have determined the creation energy of the dislocations as a function of distance between their cores. The radial distribution function has been applied to examine strong impact of the dislocations on the local crystal structure. The analysis of the electronic structure reveals mid-gap levels induced by broken atomic bonds in the dislocation core. The maps of charge distribution and electrostatic potential have been calculated and the significant decrease of the electrostatic barriers in the vicinity of the dislocation cores has been quantified. The obtained results have been discussed in the light of available experimental data.
The structure, magnetic properties, and lattice dynamics of ordered Fe-Pt alloys with three stoic... more The structure, magnetic properties, and lattice dynamics of ordered Fe-Pt alloys with three stoichiometric compositions, Fe3Pt, FePt and FePt3, have been investigated using the density functional theory. Additionally, the existing experimental data have been complemented by new measurements of the Fe projected phonon density of states performed for the Fe3Pt and FePt3 thin films using the nuclear inelastic scattering technique. The calculated phonon dispersion relations and phonon density of states have been compared with the experimental data. The dispersion curves are very well reproduced by the calculations, although, the softening of the transversal acoustic mode TA1 leads to some discrepancy between the theory and experiment in Fe3Pt. A very goood agreement between the measured spectra and calculations performed for the tetragonal structure derived from the soft mode may signal that the tetragonal phase with the space group P 4/mbm plays an important role in the martensitic transformation observed in Fe3Pt. For FePt3, the antiferromagnetic order appearing with decreasing temperature has been also investigated. The studies showed that the phonon density of states of FePt3 very weakly depends on the magnetic configuration. arXiv:1501.05550v1 [cond-mat.mtrl-sci]
ABSTRACT Investigation of well-defined nanostructures often requires that the preparation, charac... more ABSTRACT Investigation of well-defined nanostructures often requires that the preparation, characterization, and maintenance of the samples during experiments are performed under controlled, in most cases ultrahigh vacuum (UHV), conditions. This chapter reports recent advances in the in situ Mössbauer spectroscopy with synchrotron radiation on thin films that became possible due to the instrumentation developments at the nuclear resonance beamline ID18 of the European synchrotron radiation facility (ESRF). After a detailed description of the beamline and of the ultrahigh vacuum (UHV) system for in situ experiments, the chapter provides a brief introduction into the basic nuclear resonant scattering (NRS) techniques. Finally, the application of these techniques to investigate magnetic, diffusion, and lattice dynamics phenomena in ultrathin epitaxial 57Fe films deposited on a W(110) substrate is presented and discussed.
Density functional theory was used to study structural and dynamical changes related to the magne... more Density functional theory was used to study structural and dynamical changes related to the magnetostructural phase transition in MnAs. The soft mode inducing the transition from the high-symmetry hexagonal to the low-symmetry orthorhombic phase was revealed. A giant coupling between the soft mode and magnetic moments was found and its crucial role in the magnetostructural transition was established. The estimated phonon contribution to the total entropy change has the opposite sign to the magnetic entropy change.
ABSTRACT The phonon dispersion relations of HgSe, which is a cubic polar semiconductor with a zer... more ABSTRACT The phonon dispersion relations of HgSe, which is a cubic polar semiconductor with a zero energy gap, has been investigated by means of the coherent inelastic neutron scattering. The complete set of phonon branches has been determined along the [001], [110], and [111] high-symmetry directions. Using the ab initio local-density approximation with ultrasoft pseudopotentials, we have calculated the Hellmann-Feynman forces for 2x2x2 supercell. Applying the direct method, the phonon dispersion relations and the phonon density of states have been derived. They are in good agreement with the results of the neutron-scattering data. The 1x1x10 elongated supercell has been used to find the LO/TO splitting of the optical phonon branches for two different concentrations of free carriers. Higher free-carriers density diminish the LO/TO splitting.
ABSTRACT Based on first-principles calculations, the structural, magnetic, dynamic properties, an... more ABSTRACT Based on first-principles calculations, the structural, magnetic, dynamic properties, and the alpha-hexagonal -> beta-orthorhombic -> gamma-hexagonal phase transitions of ferromagnetic, antiferromagnetic, and paramagnetic MnAs are studied. The phonon-dispersion curves for some of the phases were derived using the direct method. Soft modes were found in ferromagnetic and antiferromagnetic structures in a wide range of pressures. A strong dependence the soft-mode energy on the magnetic moment and magnetic order was revealed. The double-well potential was found as a function of the soft-mode amplitude at the reduced crystal volume. In the beta-orthorhombic phase, a new antiferromagnetic configuration consisting of linear chains of alternating spins was found. Therefore, the mechanism of the magnetostructural phase transitions confirms the antiferromagnetic or paramagnetic (with antiferromagnetic fluctuations) state of the beta-orthorhombic phase. The paramagnetic gamma-hexagonal phase is stabilized at high temperature in the displacive second-order phase transition owing to the disappearance of the soft mode.
Using the density functional theory and the direct method, phonon dispersion relations and phonon... more Using the density functional theory and the direct method, phonon dispersion relations and phonon density spectra have been calculated for two slabs with Fe͑100͒ and Fe͑110͒ surfaces. A drastic difference between phonon state distributions for both surfaces has been found. The magnetic moments of surface Fe atoms are enhanced by 30% and 14% for ͑100͒ and ͑110͒ surfaces, respectively.
The crystal structure and vibrational spectrum of kanemite ͑NaHSi 2 O 5 ·3H 2 O͒ were studied usi... more The crystal structure and vibrational spectrum of kanemite ͑NaHSi 2 O 5 ·3H 2 O͒ were studied using the density-functional theory. The calculations were carried out in a lower space group ͑Pna2 1 ͒ in order to eliminate the finite-temperature disordered configurations of protons in hydrogen bonds. It was found that the SiO 4 tetrahedra are linked by a short strong O-H¯O hydrogen bond with a single asymmetric potential well at low temperatures. Two energetically equivalent sites were obtained for the position of the H atom. Apart from the hydrogen bonds reported in earlier studies, additional moderate O-H¯O hydrogen bond was found to stabilize the hydrated Na layers within the silicon layers at low temperatures. The vibrational spectrum was analyzed in detail in the entire energy range ͑0-500 meV͒, focusing on the vibrations of hydrogen atoms. A perfect correlation appears to exist between the OH ͑stretching and torsional͒ frequencies and donor-acceptor distances of the respective hydrogen bonds.
ABSTRACT First-principles calculations based on density functional theory are used to determine t... more ABSTRACT First-principles calculations based on density functional theory are used to determine the structural, lattice dynamical, elastic, and thermodynamical properties of magnetic CuFeS2. The theoretical results are compared with available experimental data and are used as a basis to critically discuss existing discrepancies between theory and experiment as well as of inconsistencies between experimental data.
ABSTRACT The lattice dynamics of samarium is determined by in situ low-temperature nuclear inelas... more ABSTRACT The lattice dynamics of samarium is determined by in situ low-temperature nuclear inelastic scattering on a single crystalline (0001) Sm film, a polycrystalline Sm foil, and by first-principles theory. The ab initio calculated phonon dispersion relations and phonon density of states for the Sm-type structure and the double hexagonal-close-packed (dhcp) lattice, characteristic for light lanthanides, are compared. The dhcp unit cell, which is a factor of 2.24 smaller in height, exhibits more pronounced vibrational anisotropy in comparison to the Sm-type structure. The analysis reveals a minor influence of the spin-orbit coupling in the Sm atom on the lattice dynamics. A broadening of the longitudinal peak, not found in the calculations, suggests the influence of electron correlations on lattice dynamics in metallic samarium.
ABSTRACT The lattice dynamics of chalcopyrite crystals AgGaS2, AgGaSe2, AgGaTe2, CdGeAs2 and CdGe... more ABSTRACT The lattice dynamics of chalcopyrite crystals AgGaS2, AgGaSe2, AgGaTe2, CdGeAs2 and CdGeP2 have been studied within the force field approach. The lattice constants are derived from minimization of the unit cell potential and they agree within 1.5% with the experimental ones. The calculated interatomic forces were used by the direct method, which allowed to find the phonon dispersion curves and phonon density of states fur these crystals. The calculated phonon frequencies for AgGaSe2 fit quite well to the inelastic neutron scattering data.
The structure, lattice dynamics and mechanical properties of magnesium hydroxide have been invest... more The structure, lattice dynamics and mechanical properties of magnesium hydroxide have been investigated by static density functional theory calculations as well as ab initio molecular dynamics. The hypothesis of a superstructure existing in the lattice formed by the hydrogen atoms has been tested. The elastic constants of the material have been calculated with a static deformations approach and are in fair agreement with the experimental data. The hydrogen subsystem structure exhibits signs of disordered behaviour while maintaining correlations between the angular positions of neighbouring atoms. We establish that the essential angular correlations between hydrogen positions are maintained to a temperature of at least 150 K and that they are well described by a physically motivated probabilistic model. The rotational degree of freedom appears to be decoupled from the lattice directions above 30 K.
Lattice dynamics and thermodynamic properties of antiferromagnetic Fe 2 SiO 4 -spinel have been s... more Lattice dynamics and thermodynamic properties of antiferromagnetic Fe 2 SiO 4 -spinel have been studied using density functional theory. Phonon dispersions are obtained for several hydrostatic pressures up to 20 GPa. They are used to calculate thermodynamic properties within the quasiharmonic approximation. Comparison with ab initio results obtained for Mg 2 SiO 4 -spinel is made in order to study the effect of the cation exchange on the dynamic and thermodynamic properties of (Mg, Fe) 2 SiO 4 -spinel. The obtained results have been compared with the available experimental data.
ABSTRACT Total energy calculations within the density functional theory (DFT), generalized gradie... more ABSTRACT Total energy calculations within the density functional theory (DFT), generalized gradient approximation and ultrasoft pseudopotentials have been used to investigate structural, dynamical and thermodynamical properties of the high-pressure structure of the magnesium orthosilicate (Mg2SiO4, gamma -spinel, ringwoodite). The phonon dispersion curves and phonon density of states have been calculated using the direct method. The results show agreement with the available experimental Raman data. (C) 2001 Elsevier Science B.V. All rights reserved.
ABSTRACT Self-consistent density-functional calculations are reported for chalcopyrite structure ... more ABSTRACT Self-consistent density-functional calculations are reported for chalcopyrite structure semiconductor ZnSnP2. Results include lattice parameters obtained from total energy minimization as well as phonon dispersion relations and phonon density of states derivated from the Hellmann-Feynman forces using the direct method. Comparison with experimental data and previous calculations based on phenomenological models shows overall agreement.
The lattice dynamics of the antiferromagnetic FeBO3 crystal has been calculated by ab initio dens... more The lattice dynamics of the antiferromagnetic FeBO3 crystal has been calculated by ab initio density-functional theory and measured by nuclear inelastic absorption spectroscopy. The calculations for the antiferromagnetic phase reproduce the experimental lattice parameters of the unit cell and provide phonon dispersion relations which agree well with the measured partial density of phonon states for the Fe atoms. Calculations for the nonmagnetic configuration lead to a smaller crystal volume and drastically higher phonon frequencies for the Fe atoms.
ABSTRACT The lattice vibrational properties of CuInSe2 with CuAu-type ordering of the cation subl... more ABSTRACT The lattice vibrational properties of CuInSe2 with CuAu-type ordering of the cation sublattice are investigated theoretically by a first-principles calculations of the structure and the lattice dynamical characteristics including zone-center optical mode frequencies, phonon dispersion and density of states, and elastic constants. The results obtained for CuAu-ordered CuInSe2 are compared with related experimental data and comparative theoretical calculations for the chalcopyrite phase of the compound. A critical analysis is given of the partly contradictory experimental lattice vibration data reported for chalcopyrite CuInSe2.
Using the ab initio density-functional generalized gradient approximation with varying on-site Co... more Using the ab initio density-functional generalized gradient approximation with varying on-site Coulomb interaction U, the electronic and crystal structures of Fe 2 SiO 4 spinel were optimized at high pressure. It was found that the ground state changes from the metallic ferromagnetic state at U = 0 into the antiferromagnetic state, stable in the regime of Mott insulating state for U Ͼ 2 eV. We found that Fe 2 SiO 4 spinel is an antiferromagnetic Mott insulator at the realistic value of U ϳ 4.5 eV, with a distinct gap which opens in the minority-Fe͑3d͒ states. We point out that the low value of the Néel temperature T N ϳ 12 K follows from the geometrical frustration of magnetic interactions, while the exchange constant J ϳ 1.7 meV estimated from the energies of ordered magnetic phases gives a rather high value of the Curie-Weiss temperature CW Ӎ 340 K. The phonon-dispersion curves and phonon density of states were derived for both the metallic and insulating antiferromagnetic phase of Fe 2 SiO 4 spinel, and significant changes of energy and dispersion of certain phonon modes, particularly with large Fe character, caused by local electron correlations were detected and analyzed. We point out some similarities with the electronic and dynamical properties of magnetite.
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Papers by Jan Łażewski