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In contrast to cubic ZrW2O8 and ZrMo2O8, which have isotropic negative thermal expansion, the compounds Sc2(WO4)3 and Sc2(MoO4)3 with the orthorhombic structure show anisotropic thermal expansion that is negative along the b and c axes. Using ab initio density functional theory calculations of lattice dynamics, we identify that the anisotropic expansion is not due to anisotropic elasticity or negative linear compressibility, which is the case in several compounds; instead, it is due to anisotropic Grüneisen parameters of specific phonons, mostly of energy around 6 meV. Such a behavior not involving anomalous elasticity is unusual. The phonon eigenvectors show that these phonons have a rather complex vibrational character of MoO4/WO4 and ScO6 polyhedral units that involve translational, librational, and internal distortion dynamics. [ABSTRACT FROM AUTHOR]

In contrast to cubic ZrW2O8 and ZrMo2O8, which have isotropic negative thermal expansion, the compounds Sc2(WO4)3 and Sc2(MoO4)3 with the orthorhombic structure show anisotropic thermal expansion that is negative along the b and c axes. Using ab initio density functional theory calculations of lattice dynamics, we identify that the anisotropic expansion is not due to anisotropic elasticity or negative linear compressibility, which is the case in several compounds; instead, it is due to anisotropic Grüneisen parameters of specific phonons, mostly of energy around 6 meV. Such a behavior not involving anomalous elasticity is unusual. The phonon eigenvectors show that these phonons have a rather complex vibrational character of MoO4/WO4 and ScO6 polyhedral units that involve translational, librational, and internal distortion dynamics. [ABSTRACT FROM AUTHOR]

We have carried out an extensive phonon study on multiferroic GaFeO3 to elucidate its dynamical behavior. Inelastic neutron scattering measurements are performed over a wide temperature range, 150 to 1198 K. First principles lattice dynamical calculations are done for the sake of the analysis and interpretation of the observations. The comparison of the phonon spectra from magnetic and nonmagnetic calculations highlights pronounced differences. The energy range of the vibrational atomistic contributions of the Fe and O ions are found to differ significantly in the two calculation types. Therefore, magnetism induced by the active spin degrees of freedom of Fe cations plays a key role in stabilizing the structure and dynamics of GaFeO3. Moreover, the computed enthalpy in various phases of GaFeO3 is used to gain deeper insights into the high-pressure phase stability of this material. Further, the volume dependence of the phonon spectra is used to determine its thermal expansion behavior. [ABSTRACT FROM AUTHOR]

We have investigated the anisotropic thermal expansion of graphite using ab-initio calculations of the implicit and explicit anharmonicity of phonons, which occur due to change in volume and increase in the thermal amplitudes, respectively. We find that the negative thermal expansion (NTE) in the a-b plane below 600 K and very large positive thermal expansion along the c-axis up to high temperatures arise due to various phonons polarized along the c-axis. While the NTE arises from the anharmonicity of transverse phonons over a broad energy range up to 60 meV, the large positive expansion along the c-axis occurs largely due to the longitudinal optic phonon modes around 16 meV. We find that these phonons have large implicit anharmonicity, but very little explicit anharmonicity. We also find very significant increase in the linear compressibility along the c-axis with increase in volume, and this has important role to quantitatively explain the thermal expansion behavior along the c-axis. The hugely anisotropic bonding in graphite is found to be responsible for wide difference in the energy range of the transverse and longitudinal phonon modes polarized along the c-axis, which are responsible for the anomalous thermal expansion behavior. This behaviour is in contrast to other nearly isotropic hexagonal structures like water-ice, which show anomalous thermal expansion in a small temperature range arising from a narrow energy range of phonons. • Ab-initio anharmonic phonon calculations in graphite including van der Waals interactions. • Negative thermal expansion (NTE) in a-b plane due to transverse phonons up to 60 meV. • Shear waves in the a-b plane result in NTE in a-b plane and expansion along c-axis. • Hugely anisotropic bonding responsible for anomalous thermal expansion behavior. [ABSTRACT FROM AUTHOR]

We present temperature-dependent inelastic-neutron-scattering measurements, accompanied by ab initio calculations of the phonon spectra and elastic properties as a function of pressure to quantitatively explain an unusual combination of negative thermal expansion and negative linear compressibility behavior of ZnAu2(CN)4. The mechanism of the negative thermal expansion is identified in terms of specific anharmonic phonon modes that involve bending of the -Zn-NC-Au-CN-Zn- linkage. The soft phonon at the L point at the Brillouin zone boundary quantitatively relates to the high-pressure phase transition at about 2 GPa. The ambient pressure structure is also found to be close to an elastic instability that leads to a weakly first-order transition. [ABSTRACT FROM AUTHOR]