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The electronic transport behaviour of CsAgO has been discussed using the theory beyond relaxation time approximation from room temperature to 800 K. Different scattering mechanisms such as acoustic deformation potential scattering, impurity phonon scattering, and polar optical phonon scattering are considered for calculating carrier scattering rates to predict the absolute values of thermoelectric coefficients. The scattering lifetime is of the order of 10 -14 s. The lattice thermal transport properties like lattice thermal conductivity and phonon-lifetime have been evaluated. The calculated lattice thermal conductivity equals 0.12 and 0.18 W mK -1 along ' a ' and ' c ' axes, respectively, at room temperature, which is very low compared to state-of-the-art thermoelectric materials. The anisotropy in the electrical conductivity indicates that the holes are favourable for the out-of-plane thermoelectrics while the electrons for in-plane thermoelectrics. The thermoelectric figure of merit for holes and electrons is nearly same with a value higher than 1 at 800 K for different doping concentrations. The value of the thermoelectric figure of merit is significantly higher than the existing oxide materials, which might be appealing for future applications in CsAgO., (© 2022 IOP Publishing Ltd.)

Extension of the topological concepts to the bosonic systems has led to the prediction of topological phonons in materials. Here we discuss the topological phonons and electronic structure of Li 2 BaX (X = Si, Ge, Sn, and Pb) materials using first-principles theoretical modelling. A careful analysis of the phonon spectrum of Li 2 BaX reveals an optical mode inversion with the formation of nodal line states in the Brillouin zone. Our electronic structure results reveal a double band inversion at the Γ point with the formation of inner nodal-chain states in the absence of spin-orbit coupling (SOC). Inclusion of the SOC opens a materials-dependent gap at the band crossing points and transitions the system into a trivial insulator state. We also discuss the lattice thermal conductivity and transport properties of Li 2 BaX materials. Our results show that coexisting phonon and electron nontrivial topology with robust transport properties would make Li 2 BaX materials appealing for device applications., (© 2022 IOP Publishing Ltd.)

Ab initio molecular dynamics simulations are used to elucidate the mechanism of the phase transition in shock experiments from hexagonal graphite (HG) to hexagonal diamond (HD) or to cubic diamond (CD). The transition from HG to HD is found to occur swiftly in very small time of 0.2 ps, with large cooperative displacements of all the atoms. We observe that alternate layers of atoms in HG slide in opposite directions by 1/6 along the ±[2, 1, 0], which is about 0.7 Å, while simultaneously puckering by about ±0.25 Å perpendicular to the a - b plane. The transition from HG to CD occurred with more complex cooperative displacements. In this case, six successive HG layers slide in pairs by 1/3 along [0, 1, 0], [-1, -1, 0] and [1, 0, 0], respectively along with the puckering as above. We have also performed calculations of the phonon spectrum in HG at high pressure, which reveal soft phonon modes that may facilitate the phase transition involving the sliding and puckering of the HG layers. We have further calculated the Gibbs free energy, including the vibrational energy and entropy, and derived the phase diagram between HG and CD phases., (© 2021 IOP Publishing Ltd.)

Direct engineering of material properties through exploitation of spin, phonon, and charge-coupled degrees of freedom is an active area of development in materials science. However, the relative contribution of the competing orders to controlling the desired behavior is challenging to decipher. In particular, the independent role of phonons, magnons, and electrons, quasiparticle coupling, and relative contributions to the phase transition free energy largely remain unexplored, especially for magnetic phase transitions. Here, we study the lattice and magnetic dynamics of biferroic yttrium orthochromite using Raman, infrared, and inelastic neutron spectroscopy techniques, supporting our experimental results with first-principles lattice dynamics and spin-wave simulations across the antiferromagnetic transition at T N ∼ 138 K. Spectroscopy data and simulations together with the heat capacity ( C p ) measurements, allow us to quantify individual entropic contributions from phonons (0.01 ± 0.01 k B atom -1 ), dilational (0.03 ± 0.01 k B atom -1 ), and magnons (0.11 ± 0.01 k B atom -1 ) across T N . High-resolution phonon measurements conducted in a magnetic field show that anomalous T -dependence of phonon energies across T N originates from magnetoelastic coupling. Phonon scattering is primarily governed by the phonon-phonon coupling, with little contribution from magnon-phonon coupling, short-range spin correlations, or magnetostriction effects; a conclusion further supported by our thermal conductivity measurements conducted up to 14 T, and phenomenological modeling., (© 2021 IOP Publishing Ltd.)

We report detailed temperature-dependent inelastic neutron scattering and ab initio lattice dynamics investigation of magnetic perovskites YCrO 3 and LaCrO 3 . The magnetic neutron scattering from the Cr ions exhibits significant changes with temperature and dominates at low momentum transfer regime. Ab initio calculations performed including magnetic interactions show that the effect of magnetic interactions is very significant on the low- as well as high-energy phonon modes. We have also shown that the inelastic neutron spectrum of YCrO 3 mimics the magnon spectrum from a G-type antiferromagnetic system, which is consistent with previously reported magnetic structure in the compound. The pressure-dependent ab initio lattice dynamics calculations are used to calculate the anisotropic thermal expansion behaviour in orthorhombic YCrO 3 , which is in excellent agreement with the available experimental data in the paramagnetic phase. We identify that the low energy anharmonic phonon modes involving Y vibrations contribute maximum to the thermal expansion behaviour., (© 2020 IOP Publishing Ltd.)

We present a thorough density functional theory based computational study of crystalline properties of cubane caged potential energetic material octanitrocubane (ONC). As expected for a layered molecular solid, van der Waals corrections are inevitable and the same has been incorporated to capture the ground state properties more accurately. Study of Born effective charge and zone centered phonon frequencies using density functional perturbation theory reveals the important role of N2, N4 type nitrogen and associated oxygen atoms in contributing to the high intensity infrared modes. From the calculated electronic band structure we can conclude that ONC is an insulator with a band gap of 5.31 eV. The optical properties of ONC are found to be nearly isotropic in low energy region in spite of strong anisotropic crystal structure., (© 2020 IOP Publishing Ltd.)

Nickel cyanide is a layered material showing markedly anisotropic behaviour. High-pressure neutron diffraction measurements show that at pressures up to 20.1 kbar, compressibility is much higher in the direction perpendicular to the layers, c, than in the plane of the strongly chemically bonded metal-cyanide sheets. Detailed examination of the behaviour of the tetragonal lattice parameters, a and c, as a function of pressure reveal regions in which large changes in slope occur, for example, in c(P) at 1 kbar. The experimental pressure dependence of the volume data is fitted to a bulk modulus, B0, of 1050 (20) kbar over the pressure range 0-1 kbar, and to 124 (2) kbar over the range 1-20.1 kbar. Raman spectroscopy measurements yield additional information on how the structure and bonding in the Ni(CN)2 layers change with pressure and show that a phase change occurs at about 1 kbar. The new high-pressure phase, (Phase PII), has ordered cyanide groups with sheets of D4h symmetry containing Ni(CN)4 and Ni(NC)4 groups. The Raman spectrum of phase PII closely resembles that of the related layered compound, Cu1/2Ni1/2(CN)2, which has previously been shown to contain ordered C≡N groups. The phase change, PI to PII, is also observed in inelastic neutron scattering studies which show significant changes occurring in the phonon spectra as the pressure is raised from 0.3 to 1.5 kbar. These changes reflect the large reduction in the interlayer spacing which occurs as Phase PI transforms to Phase PII and the consequent increase in difficulty for out-of-plane atomic motions. Unlike other cyanide materials e.g. Zn(CN)2 and Ag3Co(CN)6, which show an amorphization and/or a decomposition at much lower pressures (~100 kbar), Ni(CN)2 can be recovered after pressurising to 200 kbar, albeit in a more ordered form.

To shed light on the role of magnetism on the superconducting mechanism of the oxygen-free FeAs pnictides, we investigate the effect of magnetic ordering on phonon dynamics in the low-temperature orthorhombic parent compounds, which present a spin density wave. The study covers both the 122 (AFe(2)As(2); A = Ca, Sr, Ba) and 1111 (AFeAsF; A = Ca, Sr) phases. We extend our recent work on the Ca (122 and 1111) and Ba (122) cases by treating, computationally and experimentally, the 122 and 1111 Sr compounds. The effect of magnetic ordering is investigated through detailed non-magnetic and magnetic lattice dynamical calculations. The comparison of the experimental and calculated phonon spectra shows that the magnetic interactions/ordering have to be included in order to reproduce well the measured density of states. This highlights a spin-correlated phonon behavior which is more pronounced than the apparently weak electron-phonon coupling estimated in these materials. Furthermore, there is no noticeable difference between phonon spectra of the 122 Ba and Sr, whereas there are substantial differences when comparing these to CaFe(2)As(2) originating from different aspects of structure and bonding.