Cubic pseudo-unary A1-xBx high-entropy metallic alloys and pseudo-binary A1-xBxC disordered semiconductor alloys set a benchmark to explore how physical properties are impacted by disorder. Through its diversity, the lattice dynamics offers a unique playground to assign the relevant length scales at which operate various kinds of disorders induced by alloying. (i) In high-entropy metallic alloys, the overdamping of the bond-collective (multi-bond→1-mode) acoustic modes at short wavelength originates from force-constant fluctuations. (ii) In semiconductor alloys, the lattice mismatch splits, at any wavelength, the bond-specific (1-bond→1-mode) optical modes in duos distinguishing "same" from "alien" environments, as explained by the percolation model. Zn1-xMgxS is ideal to test both univocal assignments. Its force-constant disorder is small, reducing the cause for overdamping of the acoustic modes. Its local strain is inverted, the lighter substituent being the larger one and forming the longer bond. This forecasts a dramatic inversion of the mode-duos. Further, its wurtzite structure enables (iii) to test whether/how the percolation model for the mode-duos transfers under lowering the crystal symmetry from cubic to hexagonal. The triple acoustic-(i)/optical-(ii-iii) test on Zn1-xMgxS, combining inelastic neutron scattering with first-principles simulations, is positive. This highlights a few key points behind the lattice dynamics of atomic alloys.

Vanadium metal is extensively used in modern technology, especially in the alloy and steel industry; it exhibits anomalous thermal expansion behavior across the entire temperature regime. Here, we extensively investigate the phonon anharmonicity contributed by volume change (implicit anharmonicity) and thermal amplitude (explicit anharmonicity) and their impact on thermal expansion and thermal transport in vanadium up to 2000 K, close to the melting temperature. We compared the different methods to evaluate the phonon anharmonicity, namely, the quasiharmonic approximation (QHA), temperature-dependent effective potential (TDEP) method, and machine-learned force-field-based molecular dynamics (MLMD) simulations. At 300 K, QHA overestimates the thermal expansion coefficient by 20%, while TDEP provides an excellent description of the experimental data. This reveals a significant explicit anharmonicity at room temperature. At higher temperatures, the experimental thermal expansion coefficient continues to rise up to twice the QHA estimates, indicating significant anharmonicity. However, TDEP underestimates the experimental observations, as it only includes low orders of anharmonicity. MLMD, which includes all the anharmonic effects, successfully explains the anomalous expansion behavior over 500-2000 K. It is expected that the electronic entropy and the electron-phonon interaction would influence the thermal expansion, but their effect appears to be small. We used MLMD to calculate the spectral energy density of phonons up to 2000 K, which revealed small phonon shifts but large broadening. Above 2000 K, MLMD captures the melting and reproduces the experimental volume increase on melting. We have also calculated the lattice thermal conductivity using the TDEP-based third-order-perturbation method and the MLMD-based Green-Kubo method, over 300-1500 K, which includes a higher order of anharmonicity. This brings out the important contribution from the four and higher orders of phonon anharmonicity to the lattice thermal conductivity.

A shallow potential energy surface facilitated by a specific structural topology of 32 g sites key to three dimensional Na-diffusion.

Machine-learning molecular dynamics simulations pave the way to completely treat the anharmonicity of phonons. Low-energy anharmonic modes in transition-metal dichalcogenides drive the thermal and transport properties.

Pure NaNbO3 has an antiferroelectric phase at ambient pressure. The structural behaviour of the chemically engineered ferroelectric phase of sodium niobate, NNBT05: [(0.95) NaNbO3-(0.05) BaTiO3], under high-pressure has been studied using Raman scattering and angle-dispersive synchrotron X-ray diffraction techniques. At pressure > 1 GPa, noticeable changes in the Raman spectra can be seen in the low wavenumber modes (150–300 cm−1). Large changes in the positions and intensities of the Raman bands as a function of pressure provide evidence for structural phase transition. The results indicate significant changes in the bond-lengths and the orientation of the NbO6 octahedra at ~1 GPa, and a transition to the paraelectric phase at ~5 GPa, which are at lower pressures than previously found in pure NaNbO3. The powder X-ray diffraction pattern shows an appreciable change in the peak profile in terms of position and width on increasing pressure. The pressure dependences of the structural parameters show that the response of the lattice parameters to pressure is strongly anisotropic. By fitting the pressure–volume data using the Birch–Murnaghan equation of state, the isothermal bulk modulus was estimated. The experimental results suggest that on doping BaTiO3 in NaNbO3, the bulk modulus increases. The bulk modulus of NNBT05 has been estimated to be 164.5 GPa, which is fairly close to 157.5 GPa, as previously observed in NaNbO3.

Cu2Se and Cu2S are excellent model systems of superionic conductors with large diffusion coefficients that have been reported to exhibit different solidliquid-like Cu-ion diffusion. In this paper, we clarify the atomic dynamics of these compounds with temperature-dependent ab-initio molecular dynamics (AIMD) simulations and inelastic neutron scattering (INS) experiments. Using the dynamical structure factor and Van-Hove correlation function, we interrogate the jump-time, hopping length distribution and associated diffusion coefficients. In cubic-Cu2Se at 500 K, we find solid-like diffusion with Cu-jump lengths matching well the first-neighbour Cu-Cu distance of ~3 Å in the crystal, and clearly defined optic phonons involving Cu-vibrations. Above 700 K, the jump-length distribution becomes a broad maximum cantered around 4 Å, spanning the first and second neighbour lattice distances, and a concurrent broadening of the Cu-phonon density of states. Further, above 900 K, the Cu-diffusion becomes close to liquid-like, with distributions of Cu-atoms continuously connecting crystal sites, while the vibrational modes involving Cu motions are highly damped, though still not fully over-damped as in a liquid. At low temperatures, the solid-like diffusion is consistent with previous X-ray diffraction and quasielastic neutron scattering experiments, while the higher-temperature observation of the liquid-like diffusion is in agreement with previous AIMD simulations. We also report AIMD simulations in Cu2S in the hexagonal and cubic superionic phases, and observe similar solid and liquid-like diffusion at lowand hightemperatures, respectively. The calculated ionic-conductivity is in fair agreement with reported experimental values.

${\mathrm{Cu}}_{2}\mathrm{Se}$ and ${\mathrm{Cu}}_{2}\mathrm{S}$ are excellent model systems of superionic conductors with large diffusion coefficients that have been reported to exhibit different solidlike and liquidlike Cu-ion diffusion. In this paper, we clarify the atomic dynamics of these compounds with temperature-dependent ab initio molecular dynamics (AIMD) simulations and inelastic neutron scattering experiments. Using the dynamical structure factor and Van Hove correlation function, we interrogate the jump time, hopping length distribution, and associated diffusion coefficients. In cubic ${\mathrm{Cu}}_{2}\mathrm{Se}$ at 500 K, we find solidlike diffusion with Cu jump lengths matching well the first-neighbor Cu-Cu distance of \ensuremath{\sim}3 \AA{} in the crystal, and clearly defined optic phonons involving Cu vibrations. Above 700 K, the jump-length distribution becomes a broad maximum centered around 4 \AA{}, spanning the first and second neighbor lattice distances, and a concurrent broadening of the Cu-phonon density of states. Further, above 900 K, the Cu diffusion becomes close to liquidlike, with distributions of Cu atoms continuously connecting crystal sites, while the vibrational modes involving Cu motions are highly damped, though still not fully overdamped as in a liquid. At low temperatures, the solidlike diffusion is consistent with previous x-ray diffraction and quasielastic neutron scattering experiments, while the higher-temperature observation of the liquidlike diffusion is in agreement with previous AIMD simulations. We also report AIMD simulations in ${\mathrm{Cu}}_{2}\mathrm{S}$ in the hexagonal and cubic superionic phases, and observe nearly liquidlike diffusion above \ensuremath{\sim}500 K. The calculated ionic conductivity is in fair agreement with reported experimental values.

Structural as well as magnetization studies have been carried out on graphite samples irradiated by neutrons over 50 years in the CIRUS research reactor at Trombay. Neutron diffraction studies reveal that the defects in irradiated graphite samples are not well annealed and remain significant up to high temperatures much greater than 653 K where the Wigner energy is completely released. We infer that the remnant defects may be intralayer Frenkel defects, which do not store large energy, unlike the interlayer Frenkel defects that store the Wigner energy. Magnetization studies on the irradiated graphite show ferromagnetic behavior even at 300 K and a large additional paramagnetic contribution at 5 K. Ab-initio calculations based on the spin-polarized density-functional theory show that the magnetism in defected graphite is essentially confined on to a single 2-coordinated carbon atom that is located around a vacancy in the hexagonal layer.

A class of Na-based antiperovskites Na3XY (X = F, H; Y = S, Se, Te) was recently reported with a remarkably high ionic conductivity similar to 0.1 mS/cm near room temperature. Herein, we report comprehensive atomic dynamics investigations using ab initio and large-scale machine-learned molecular dynamics (MLMD) simulation on these sodium superionic conductors. Previous studies identified the role of soft phonons involving rotations of FNa6 octahedral units in Na diffusion. In contrast, our MLMD simulations show that the Na diffusion pathways are essentially uncorrelated and do not involve the collective dynamics of octahedral rotations or reorientations. Moreover, while the soft phonons do not involve any Y atomic dynamics, the diffusion pathways are sterically hindered unless facilitated by Y displacements and Na vacancies. However, we do find that the anharmonic low-energy phonon modes of wave vectors along the M-R line at the Brillouin zone (BZ) boundary are important precursors of Na diffusion. These modes involve vibrations perpendicular to the F-Na ionic bond, along < 100 > directions, which provide the initial pathways for the Na diffusion. We have calculated the branch-resolved phonon spectral energy density (SED) in the entire BZ using the MLMD simulations. The SED results as a function of temperature reveal the large anharmonicity of the soft phonon modes, which leads to floppy dynamics of Na atoms. The volume of cubic-Na3FS is the smallest among the three antiperovskites and has the largest mean-phonon energy. Our calculated Na diffusion coefficient at 700 K in Na3FS of similar to 0.63 x 10(-6) cm(2)/s is significantly larger than similar to 0.16 x 10(-6) cm(2)/s in Na3FTe, which clearly shows contrast with the hypothesis of a softer lattice leading to faster diffusion. The estimated diffusion barrier energy for Na3FS with 2% Na vacancy is similar to 0.44 eV, which is in fair agreement with the reported value of 0.44 eV estimated from the total conductivity measurement with Na-vacant iodine-doped Na3FS. Similarly, for Na3FSe and Na3FTe, the barrier energies have been estimated to be 0.50 and 0.55 eV, respectively. Our large-scale MLMD-based theoretical study provides a comprehensive understanding of the role of soft phonons, host dynamics, and vacancies in Na diffusion in Na3FY (Y = S, Se, Te) and other materials of this class, which will be helpful in designing materials for application in solid-state batteries.

By means of diffuse x-ray scattering (DS) and inelastic x-ray scattering (IXS), we probe directly the chargeordering (CO) dynamics in the Verwey system (NaMn3)Mn4O12, where a peculiar quadruple perovskite structure with no oxygen disorder stabilizes a nearly full Mn3+/Mn4+ static charge order at TCO = 175 K concomitant to a commensurate structural modulation with propagation vector qCO = (21, 12, 0). At TCO, the IXS spectra unveil a softening of a 35.3-meV phonon at qCO. Lattice dynamical calculations enable us to attribute this soft phonon to an Ag mode whose polarization matches the Jahn-Teller-like distortion pattern of the structural modulation. This result demonstrates that the Jahn-Teller instability is the driving force of the CO Verwey transition in (NaMn3)Mn4O12, thus elucidating a long-standing controversy regarding the mechanism of this transition observed in other mixed-valence systems like magnetite.

We have performed ab-initio molecular dynamics simulations to elucidate the mechanism of the phase transition at high pressure 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 (1/3, 1/6, 0) and (-1/3, -1/6, 0), respectively, which is about 0.7 {\AA} along the pm[2, 1, 0] direction, while simultaneously puckering by about pm0.25 {\AA} 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. The zero-point vibrational energy and the vibrational entropy are found to have important role in stabilizing HG up to higher pressures (>10 GPa) and temperatures than that estimated (<6 GPa) from previous enthalpy calculations.

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.

Cu2ZnSnS4 is a p-type semiconductor that has been seen as a possible replacement for SnSe, Cu (In,Ga)Se2 etc. in thin film solar cells, due to non-toxicity and natural abundance of their constituent elements. Polycrystalline Cu2ZnSnS4 samples (kesterite-type) were synthesized by a microwave assisted method and investigated by inelastic neutron scattering. The experimentally measured neutron-weighted phonon density of states compares reasonably well with that estimated by a computation of the lattice dynamics. These studies are then extended to elucidate the understanding of the thermal, elastic and thermoelectric properties. The lattice-dynamics calculations are able to reproduce the temperature dependence of the thermal conductivity and the thermoelectric figure of merit, as observed in experiments previously.

We report detailed temperature-dependent inelastic neutron scattering andab initiolattice dynamics investigation of magnetic perovskites YCrO3and LaCrO3. The magnetic neutron scattering from the Cr ions exhibits significant changes with temperature and dominates at low momentum transfer regime.Ab initiocalculations 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 YCrO3mimics the magnon spectrum from a G-type antiferromagnetic system, which is consistent with previously reported magnetic structure in the compound. The pressure-dependentab initiolattice dynamics calculations are used to calculate the anisotropic thermal expansion behaviour in orthorhombic YCrO3, 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.

Cyanide based framework compounds are known to show large negative thermal expansion behaviour. Here we report the phonon and anomalous lattice behavior of two metal cyanide framework compounds i.e. KMnAg3(CN)6 and KNiAu3(CN)6. We have studied the role of van der Waals dispersion and magnetic interactions on structural stability of these compounds. The behavior of these compounds under isotropic compression shows the presence of negative linear compressibility. The calculated phonon spectra, validated by inelastic neutron scattering measurements and elastic constants are used to study the negative thermal expansion behavior which is found to arise from low energy phonon modes involving the folding of A-NC-B-CN-A linkage about B atoms.

Graphite has been used as a neutron moderator or reflector in many nuclear reactors. The irradiation of graphite in a nuclear reactor results in a complex population of defects. Heating of the irradiated graphite at high temperatures results in annihilation of the defects with release of an unusually large energy, called the Wigner energy. From various experiments on highly irradiated graphite samples from the CIRUS reactor at Trombay and ab initio simulations, we have identified various 2-, 3-, and 4-coordinated topological structures in defected graphite, and provided a microscopic mechanism of defect annihilation on heating and release of the Wigner energy. The annihilation process involves cascading cooperative movement of atoms in multiple steps involving an intermediate structure. Our work provides insights in understanding of the defect topologies and annihilation in graphite which is of considerable importance to wider areas of graphitic materials including graphene and carbon nanotubes.

We report investigation of phonons and oxygen diffusion in Bi2O3and (Bi0.7Y0.3)2O3. The phonon spectra have been measured in Bi2O3at high temperatures up to 1083 K using inelastic neutron scattering.Ab initiocalculations have been used to compute the individual contributions of the constituent atoms in Bi2O3and (Bi0.7Y0.3)2O3to the total phonon density of states. Our computed results indicate that as temperature is increased, there is a complete loss of sharp peak structure in the vibrational density of states.Ab initiomolecular dynamics simulations show that even at 1000 K in δ-phase Bi2O3, Bi-Bi correlations remain ordered in the crystalline lattice while the correlations between O-O show liquid like disordered behavior. In the case of (Bi0.7Y0.3)2O3, the O-O correlations broadened at around 500 K indicating that oxygen conductivity is possible at such low temperatures in (Bi0.7Y0.3)2O3although the conductivity is much less than that observed in the undoped high temperature δ-phase of Bi2O3. This result is consistent with the calculated diffusion coefficients of oxygen and observation by quasielastic neutron scattering experiments. Ourab initiomolecular dynamics calculations predict that macroscopic diffusion is attainable in (Bi0.7Y0.3)2O3at much lower temperatures, which is more suited for technological applications. Our studies elucidate the easy directions of diffusion in δ-Bi2O3and (Bi0.7Y0.3)2O3.

The alkali atoms, due to their small sizes and low charge ionic states, are most eligible to intercalate in the structural layers of V2O5. We have applied ab-initio density functional theory to study the dynamics of Li-ion in layers of α-V2O5. The calculations are performed for two compositions, namely, Li0.08V2O5 and Li0.16V2O5, and show that there are unstable phonon frequencies. The unstable modes have large amplitude of Li atom along the b-axis of the orthorhombic unit cell indicating that such unstable modes could initiate Li-ion diffusion along b-axis. The ab-initio molecular dynamics simulations are performed up to 25 ps at 1200 K, which reveal one-dimensional diffusion of Li atoms. The diffusion pathways of Li atoms from the simulations seem to follow the eigenvectors of the unstable phonon modes obtained in the intercalated structure.

We have investigated the dynamics of Na ions in amorphous Na2Si2O5, a potential solid electrolyte material for Na-battery. We have employed quasielastic neutron scattering (QENS) technique in the amorphous Na2Si2O5 from 300 to 748 K to understand the diffusion pathways and relaxation timescales of Na atom dynamics. The microscopic analysis of the QENS data has been performed using ab-initio and classical molecular dynamics simulations (MD) to understand the Na-ion diffusion in the amorphous phase. Our experimental studies show that the traditional model, such as the Hall and Ross (H-R) model, fairly well describe the diffusion in the amorphous phase giving a mean jump length of ~3 {\AA} and residence time about 9.1 picoseconds. Our MD simulations have indicated that the diffusion of Na+ ions occurs in the amorphous phase of Na2Si2O5 while that is not observed in the crystalline orthorhombic phase even up to 1100 K. The MD simulations have revealed that in the amorphous phase, due to different orientations of silicon polyhedral units, accessible pathways are opened up for Na+ diffusions. These pathways are not available in the crystalline phase of Na2Si2O5 due to rigid spatial arrangement of silicon polyhedral units.