We report the ab initio density functional theory calculation of phonons in the cubic phase of ZrW${}_{2}$O${}_{8}$ in the entire Brillouin zone and identify specific anharmonic phonons that are responsible for large negative thermal expansion (NTE) in terms of the translation, rotation, and distortion of WO${}_{4}$ and ZrO${}_{6}$. We have used density functional calculations to interpret the experimental phonon spectra as a function of pressure and temperature as reported in literature. We discover that the phonons showing anharmonicity with temperature are not necessarily the same as those showing anharmonicity with pressure although both are of similar frequencies. Only the latter phonons are associated with NTE. Therefore, the cubic and/or quadratic anharmonicity of phonons is not relevant to NTE but just the volume dependence of frequencies. The calculations are able to reproduce the observed anomalous trends, namely, the softening of the low-frequency peak at about 4 meV in the phonon spectra with pressure and its hardening with temperature, whereas, both changes involve a compression of the lattice.

In this communication we report the details of the structural and thermal properties of monoclinic layered perovskite type Pr₂Ti₂O₇ (PTO) using ambient to higher temperature XRD and Raman spectroscopic studies.

Using inelastic neutron scattering and X-ray diffraction measurements, together with ab initio and machine-learning molecular dynamics simulations, we bring out the distinct nature of anharmonicity in the phonon spectra of MoSe₂ and WSe₂ relevant to thermal transport and thermal expansion behaviour. We show that the perturbation method, including 4^th-order force constants, is insufficient to capture the temperature-dependent explicit anharmonicity. The Green–Kubo method captures the explicit anharmonicity and reproduces the thermal conductivity (κ_l) with high fidelity. Our mode-resolved calculation reveals that the major contribution (∼90%) to κ_l is attributed to a small explicit anharmonicity of low-energy phonons. In contrast, these modes exhibit large positive Grüneisen parameters (implicit anharmonicity), causing the large thermal expansion of the material.

We report a comparative study of the dynamics of Cu2O, Ag2O, and Au2O (i.e., M2O with M = Au, Ag, and Cu) using first principle calculations based on the density functional theory. Here, for the first time, we show that the nature of chemical bonding and open space in the unit cell are directly related to the magnitude of thermal expansion coefficient. A good match between the calculated phonon density of states and that derived from inelastic neutron scattering measurements is obtained for Cu2O and Ag2O. The calculated thermal expansions of Ag2O and Cu2O are negative, in agreement with available experimental data, while it is found to be positive for Au2O. We identify the low energy phonon modes responsible for this anomalous thermal expansion. We further calculate the charge density in the three compounds and find that the magnitude of the ionic character of the Ag2O, Cu2O, and Au2O crystals is in decreasing order, with an Au-O bond of covalent nature strongly rigidifying the Au4O tetrahedral units. The nature of the chemical bonding is also found to be an important ingredient to understand the large shift of the phonon frequencies of these solids with pressure and temperature. In particular, the quartic component of the anharmonic term in the crystal potential is able to account for the temperature dependence of the phonon modes.