The layered structure of tetragonal Ni(CN)2, consisting of square-planar Ni(CN)4 units linked in the a-b plane, with no true periodicity along the c-axis, is expected to show anisotropic compression on the application of pressure. High-pressure neutr
on diffraction (elastic) and inelastic neutron scattering experiments have been performed on polycrystalline Ni(CN)2 to investigate its compressibility and stability. The intralayer a lattice parameter does not show any appreciable variation with increase of pressure up to 2.7 kbar. Above this pressure value, a decrease in a is observed. The c lattice parameter decreases slowly up to 1 kbar, then decreases sharply up to 20 kbar. It does not show any significant variation with further pressure increase up to 50 kbar. The response of the lattice parameters to the applied pressure is strongly anisotropic as the interlayer spacing (along the c-axis) shows a significantly larger contraction than the a-b plane. 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 154 (2) kbar in the range 1-50 kbar. The change in the slope of the lattice parameters at 1 kbar is also supported by high-pressure Raman measurements, which indicate a phase transition at 1 kbar. Probably arising from a change in the CN ordering within the Ni(CN)2 layers. Raman measurements, performed up to 200 kbar, highlight the possible existence of a second phase transition taking place at about 70 kbar. Our neutron inelastic scattering measurements of the pressure dependence of the phonon spectra performed up to 2.7 kbar, also support the occurrence of a phase transition at low pressure.
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.
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 calcula
tions 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.
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 depend
ence 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.
The structural properties of the CaFe2As2 have been investigated by x-ray and neutron powder diffraction techniques as a function of temperature. Unambiguous experimental evidence is shown for coexistence of tetragonal and orthorhombic phases below 1
70 K in contrast to existing literature. Detailed Rietveld analyses of thermo-diffractograms show that the sample does not transform completely in to the orthorhombic phase at the lowest temperature even though it is the majority phase. We have found that the unit cell volume of the orthorhombic phase is higher compared to that of the tetragonal phase for all the temperatures. X-ray data on CaFe2As2 shows anomalous (at) lattice parameter contraction with increasing temperature and phase co-existence behavior below 170 K unlike other members of the 122 family of compounds like SrFe2As2 and EuFe2As2. Temperature dependent magnetization of polycrystalline CaFe2As2 sample show weak anomalies below 170 K. This behavior of the polycrystalline sample is in contrast to that of a single crystal reported earlier.
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.
We report a detailed high-temperature powder neutron diffraction investigation of the structural behavior of the multiferroic GaFeO3 between 296 and 1368 K. Temperature dependent neutron diffraction patterns do not show any appreciable change either
in intensity or appearance/disappearance of the observed peaks upto 1368 K, ruling out any structural transition in the entire temperature range. The lattice parameters and volume exhibit normal thermal expansion behaviour, indicating the absence of any structural changes with increasing temperature. The origin of the magnetoelectric couplings and multiferroicity in GaFeO3 is known to be influenced by the site disorder from Ga/Fe atoms. Our analysis shows that this disorder remains nearly the same upon increase of temperature from 296 to 1368 K. The structural parameters as obtained from Rietveld refinement of neutron diffraction data are used to calculate the interatomic distances and distortions of the oxygen polyhedra around the Ga1, Ga2, Fe1 and Fe2 cations. Evolution of the distortion of the oxygen polyhedra around these sites suggests that the Ga1-O tetrahedron is least distorted and Fe1-O is most distorted. Structural features regarding the distortion of polyhedral units would be crucial to understand the temperature dependence of the microscopic origin of polarizations. The electric polarization has been estimated using a simple ionic model and its value is found to decrease with increasing temperature.
Recently colossal positive volume thermal expansion has been found in the framework compounds Ag3Co(CN)6 and Ag3Fe(CN)6. Phonon spectra have been measured using the inelastic neutron scattering technique as a function of temperature and pressure. The
data has been analyzed using ab-initio calculations. We find that the bonding is very similar in both compounds. At ambient pressure modes in the intermediate frequency part of the vibrational spectra in the Co compound are shifted to slightly higher energies as compared to the Fe compound. The temperature dependence of the phonon spectra gives evidence for large explicit anharmonic contribution to the total anharmonicity for low-energy modes below 5 meV. We found that modes are mainly affected by the change in the size of unit cell, which in turn changes the bond lengths and vibrational frequencies. Thermal expansion has been calculated via the volume dependence of phonon spectra. Our analysis indicates that Ag phonon modes in the energy range from 2 to 5 meV are strongly anharmonic and major contributors to thermal expansion in both compounds. The application of pressure hardens the low-energy part of the phonon spectra involving Ag vibrations and confirms the highly anharmonic nature of these modes.
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 reporte
d 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.
Zn(CN)2 and Ni(CN)2 are known for exhibiting anomalous thermal expansion over a wide temperature range. The volume thermal expansion coefficient for the cubic, three dimensionally connected material, Zn(CN)2, is negative ({alpha}V = -51 x 10-6 K-1) w
hile for Ni(CN)2, a tetragonal material, the thermal expansion coefficient is negative in the two dimensionally connected sheets ({alpha}a=-7 x 10-6 K-1), but the overall thermal expansion coefficient is positive ({alpha}V=48 x 10-6 K-1). We have measured the temperature dependence of phonon spectra in these compounds and analyzed them using ab-initio calculations. The spectra of the two compounds show large differences that cannot be explained by simple mass renormalization of the modes involving Zn (65.38 amu) and Ni (58.69 amu) atoms. This reflects the fact that the structure and bonding are quite different in the two compounds. The calculated pressure dependence of the phonon modes and of the thermal expansion coefficient, {alpha}V, are used to understand the anomalous behavior in these compounds. Our ab-initio calculations indicate that it is the low-energy rotational modes in Zn(CN)2, which are shifted to higher energies in Ni(CN)2, that are responsible for the large negative thermal expansion. The measured temperature dependence of the phonon spectra has been used to estimate the total anharmonicity of both compounds. For Zn(CN)2, the temperature- dependent measurements (total anharmonicity), along with our previously reported pressure dependence of the phonon spectra (quasiharmonic), is used to separate the explicit temperature effect at constant volume (intrinsic anharmonicity).
