Inelastic Neutron Scattering Studies of Phonon Spectra and Simulations in Tungstates, AWO4 (A = Ba, Sr, Ca and Pb)

Lattice dynamics and high pressure phase transitions in AWO4 (A = Ba, Sr, Ca and Pb) have been investigated using inelastic neutron scattering experiments, ab-initio density functional theory calculations and extensive molecular dynamics simulations. The vibrational modes that are internal to WO4 tetrahedra occur at the highest energies consistent with the relative stability of WO4 tetrahedra. The neutron data and the ab-initio calculations are found to be in excellent agreement. The neutron and structural data are used to develop and validate an interatomic potential model. The model is used for classical molecular dynamics simulations to study their response to high pressure. We have calculated the enthalpies of the scheelite and fergusonite phases as a function of pressure, which confirms that the scheelite to fergusonite transition is second order in nature. With increase in pressure, there is a gradual change in the AO8 polyhedra, while there is no apparent change in the WO4 tetrahedra. We found that that all the four tungstates amorphize at high pressure. This is in good agreement with available experimental observations which show amorphization at around 45 GPa in BaWO4 and 40 GPa in CaWO4. On amorphization, there is an abrupt increase in the coordination of the W atom while the bisdisphenoids around A atom are considerably distorted. The pair correlation functions of the various atom pairs corroborate these observations. Our observations aid in predicting the pressure of amorphization in SrWO4 and PbWO4, which have not been experimentally reported.

Spin-Phonon Coupling, High Pressure Phase Transitions and Thermal Expansion of Multiferroic GaFeO3: A Combined First Principles and Inelastic Neutron Scattering Study

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 non-magnetic 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.

Phonon Dynamics and Inelastic Neutron Scattering of Sodium Niobate

Sodium niobate (NaNbO3) exhibits most complex sequence of structural phase transitions in perovskite family and therefore provides as excellent model system for understanding the mechanism of structural phase transitions. We report temperature dependence of inelastic neutron scattering measurements of phonon densities of states in sodium niobate. The measurements are carried out in various crystallographic phases of this material at various temperatures from 300 K to 1048 K. The phonon spectra exhibit peaks centered around 19, 37, 51, 70 and 105 meV. Interestingly, the peak around 70 meV shifts significantly towards lower energy with increasing temperature, while the other peaks do not exhibit an appreciable change. The phonon spectra at 783 K show prominent change and become more diffusive as compared to those at 303 K. In order to better analyze these features, we have performed first principles lattice dynamics calculations based on the density functional theory. The computed phonon density of states is found to be in good agreement with the experimental data. Based on our calculation we are able to assign the characteristic Raman modes in the antiferroelectric phase to the A1g symmetry, which are due to the folding of the T (w=95 cm-1) and delta(w=129 cm-1) points of the cubic Brillouin zone.

Spin-Lattice Coupling in K0.8Fe1.6Se2 and KFe2Se2: Inelastic Neutron Scattering and ab-initio Phonon Calculations

We report measurements of the temperature dependence of phonon densities of states in K0.8Fe1.6Se2 using inelastic neutron scattering technique. While cooling down to 150 K, a phonon peak splitting around 25 meV is observed and a new peak appears at 31 meV. The measurements support the recent Raman and infra-red measurements indicating a lowering of symmetry of K0.8Fe1.6Se2 upon cooling below 250 K. Ab-initio phonon calculations have been carried out for K0.8Fe1.6Se2 and KFe2Se2. The comparison of the phonon spectra as obtained from the magnetic as well as non magnetic calculations show pronounced differences. We show that in the two calculations the energy range of the vibrational contribution from both Fe and Se are quite different. We conclude that Fe magnetism is correlated to the phonon dynamics and it plays an important role in stabilizing the structure of K0.8Fe1.6Se2 as well as that of KFe2Se2. The calculations highlight the presence of low energy librational modes in K0.8Fe1.6Se2 as compared to KFe2Se2.

Behavior of Lithium Oxide at Superionic Transition: First Principles and Molecular Dynamics Studies

We report studies on the vibrational and elastic behavior of lithium oxide, Li2O around its superionic transition temperature. Phonon frequencies calculated using the ab-initio and empirical potential model are in excellent agreement with the reported experimental data. Further, volume dependence of phonon dispersion relation has been calculated, which indicates softening of zone boundary transverse acoustic phonon mode along [110] at volume corresponding to the superionic transition in Li2O. The instability of phonon mode could be a precursor leading to the dynamical disorder of the lithium sub lattice. Empirical potential model calculations have been carried out to deduce the probable direction of lithium diffusion by constructing a super cell consisting of 12000 atoms. The barrier energy for lithium ion diffusion from one lattice site to another at ambient and elevated temperature has been computed. Barrier energy considerations along various symmetry directions indicate that [001] is the most favourable direction for lithium diffusion in the fast ion phase. This result corroborates our observation of dynamical instability in the transverse mode along (110) wave vector. Using molecular dynamics simulations we have studied the temperature variation of elastic constants, which are important to the high-temperature stability of lithium oxide.