Far-infrared spectrum and dielectric function of KCN and NaCN in the disordered cubic phase

The far-infrared reflection spectrum of KCN and NaCN in their cubic disordered phases is measured between 20 and 400 cm^?1 with dispersive Fourier-spectroscopy and the complex dielectric function ε̂(ω) is evaluated. At 295 K KCN exhibits a well defined TO mode at 148 cm^?1. The LO mode is at 234 cm^?1. The dielectric function of NaCN between 295 and 363 K is more complicated. Below 150 cm^?1 it can be approximated by an overdamped oscillator at 120 cm^?1.     

Martensitic phase transformation cubic-orthorhombic of NaCN and (NaCN)0.98 (KCN)0.02

The ferroelastic cubic-orthorhombic transition of single crystals NaCN and (NaCN)_0.98 (KCN)_0.02 has been studied by x-ray diffraction. The domain configuration obtained is consistent with the Wechsler, Lieberman, Read-model for martensitic transformations.     

Lattice dynamics,ferroelectricity and raman spectrum of BaTiQ3 in cubic phase

A dynamical model involving non linear anisotropic polarisability of oxygen is used to calculate the phonon dispersion curves and the Raman spectrum of barium titanate at 421 K near the paraelectric-ferroelectric phase transition. The temperature phase dependence of the dispersion curves is studied and the ferroelectric soft mode is described.     

Lattice dynamics of orientationally disordered crystals

A one-dimensional model is considered. It describes planar rotators coupled to the vibrations of a monoatomic chain. It is used to compare two theories of lattice dynamics, one involving the canonical transformation method, the other describing the orientationally disordered crystal by an effective system of coupled local modes. It is shown that the latter method has to be renormalized in order to take properly into account the self-interactions of the rotators. The only effect of the renormalization is to change the effective temperature of the rotators.     

Lattice dynamics and structural phase transitions

Results of lattice dynamics, or atomic motions in a solid, explain many of the thermodynamic properties of solids. Inelastic neutron scattering conveniently explores the atomic motions, quantized as phonons. Of particular interest are materials that undergo structural phase transitions. The soft mode theory has been successful in relating anomalous phonon behavior to structural changes in solids. One such example is the ferromagnetic shape memory alloy, Ni₂MnGa, which undergoes a sequence of phase transitions leading to a magnetic, incommensurate modulated, tetragonal phase as the ground state. The experiments, coupled with first principles calculations, provide evidence that strong electron–phonon coupling is the driving mechanism of the phase transformation.     

Lattice dynamics of cubic SrZrO3

Using density functional perturbation theory, the optical dielectric constant, Born effective charges and phonon dispersion curves of cubic SrZrO₃ have been calculated. The obtained dispersion curves show a soft phonon branch spreading from R to M points of the cubic Brillouin zone. An analysis based on the symmetry relationships indicates that the experimentally observed low-symmetry phases of SrZrO₃ can be considered as results of the soft mode condensation at R and M points.     

Lattice dynamics of cubic PbTiO3 by inelastic neutron scattering

High-temperature dispersion relations of the phonon modes in a cubic PbTiO₃ single crystal have been investigated along the [ξ 0 0] and [ξ ξ 0] directions by inelastic neutron scattering. Above T _c, the phonon dispersions are only temperature-dependent close to the Brillouin zone centre where the mode softening comes through. The measurements indicate large cubic anisotropy of the elastic tensor and relatively low anisotropy of the soft mode dispersion. The differences from an earlier inelastic neutron scattering study are discussed.     

Lattice dynamics and phase transition of NiTi alloy

We have performed detailed first-principles calculations to investigate the structural and lattice dynamical properties of NiTi alloy. The calculated static structures consist well with the experimental data and other theoretical results. With quasi harmonic approximation, the phase boundary between B19′ and BCO phases can be described as a five order polynomial $T=100?89.28P+296.75P^2?717.94P^3+734.62P^4?274.25P^5$. The change of vibrational entropy is $0.07k_B$/atom at the transition temperature 100 K under zero pressure.     

Lattice dynamics of a Rb2KScF6 crystal in unstable cubic and tetragonal phases and in a stable monoclinic phase

The results of a nonempirical calculation of the static and dynamic properties of a Rb₂KScF₆ crystal with elpasolite structure in cubic, tetragonal, and monoclinic phases are presented. The calculation is performed on the basis of a microscopic model of an ionic crystal that takes account of the deformability and polarizability of the ions. The deformability parameters of the ions are determined from the condition that the total energy of the crystal is minimum. The computational results for the equilibrium lattice parameters are in satisfactory agreement with experimental data. Unstable vibrational modes are found in the vibrational spectrum of the lattice in the cubic and tetragonal phases. These modes occupy the phase space throughout the entire Brillouin zone. The characteristic vectors of the most unstable mode at the center of the Brillouin zone of the cubic phase are related to the displacements of the fluorine ions and correspond to rotation of ScF₆ octahedra. Condensation of this mode leads to a tetragonal distortion of the structure. In the tetragonal phase the most unstable mode belongs to the boundary point of the Brillouin zone and condensation of this mode leads to monoclinic distortion with doubling of the unit-cell volume. In the monoclinic phase unstable modes are absent in the vibrational spectrum of the lattice. Fiz. Tverd. Tela (St. Petersburg) 41, 1297–1305 (July 1999)     

Lattice dynamics and hydrostatic-pressure-induced phase transitions in ScF3

New phase transitions induced by hydrostatic pressure in a cubic (under standard conditions) ScF₃ crystal are discovered by the methods of polarization microscopy and Raman scattering. The space groups $R\bar 3c$ for Z=2 and Pnma for Z=4 are proposed for the high-pressure phases. A nonempirical computation of the lattice dynamics of the crystal is carried out. It is shown that, under normal pressure, the cubic phase is stable down to T=0 K, while the application of a hydrostatic pressure gives rise to a phonon branch in the vibrational spectrum (between points R and M of the Brillouin zone) with negative values of squares of frequencies. The condensation of soft mode R₅ at the boundary point of the Brillouin zone leads to rhombohedral distortion of the cubic structure with the unit cell volume doubling. The calculated frequencies at q=0 of the ScF₃ lattice in the distorted rhombohedral phase are real-valued; the number and position of frequencies active in Raman scattering are in accord with the experimental values.     

Lattice dynamics and the phase transition from the cubic phase to the tetragonal phase in the LaMnO<Subscript>3</Subscript> crystal within the polarizable-ion model

The paper reports on the results of ab initio calculations of the static and dynamic properties of the LaMnO₃ crystal with a perovskite structure in the cubic, rhombohedral, and orthorhombic phases. The calculations are performed within the ionic crystal model, which takes into account the deformability and polarizability of the ions. It is revealed that the spectrum of lattice vibrations in the cubic phase contains unstable vibrational modes, which occupy the phase space in the entire Brillouin zone. The eigenvectors of the softest mode at the boundary point R of the Brillouin zone are associated with the displacements of the oxygen ions and correspond to the “rotation” of the MnO₆ octahedron. The condensation of one, two, and three components of this mode leads to the tetragonal, orthorhombic, and rhombohedral distortions of the structure. The structural phase transition is described in terms of the local mode approximation with the use of the double perovskite unit cell, in which the MnO₆ octahedron is explicitly separated. The parameters of the model Hamiltonian are determined. The static properties are investigated by the Monte Carlo method. The calculated temperature of the phase transition from the cubic phase (9800 K) is considerably higher than the melting temperature of the crystal under investigation. The calculated frequencies of long-wavelength lattice vibrations in the experimentally observed orthorhombic and rhombohedral phases are in reasonable agreement with experimental data.     

Light scattering spectrum of disordered molecular crystals: KCN

We have investigated the effect of the low temperature order-disorder phase transitions on the Raman spectrum of KCN. In the disordered cubic room temperature phase we only find a broad central component in the spectrum which sharpens near the transition temperature at 168K. From the doubling of the librational mode of the CN^--molecules we conclude that an antipolar orientation of the CN^--molecules is probable for the low temperature phase below 83 K.     

Inelastic and quasi-elastic light scattering in (NaCN)1−x(KCN)x quadrupolar glasses

The elastic constantsc ₁₁ andc ₄₄, and the attenuation of longitudinal phonons along (100)-direction in (NaCN)_1–x(KCN)_x mixed crystals have been determined by brillouin spectroscopy in the temperature range fromT=10K–300K. Minima inc ₄₄ (T) which determine the freezing tempratureT _F in these orientational glasses appear at lower temperatures compared to neutron scattering results. The coupling of molecular reorientations to longitudinal phonons in NaCN-rich smaples shows up in dispersion effects inc ₁₁ (T) and the appearence of a dynamical central peak nearT _F .U.A. 1119 is associated to the Centre National de la Recherche Scientifique     

Lattice dynamics and phase diagrams for hydrogen bonded systems

Abstract

A variational correlated ground state wave-function theory is applied to investigate the high-pressure deformation of hydrogen-bond potential described by a double Morse potential functions. The evolution of the hydrogen bond potential with increasing pressure and the effects of deuterium isotope substitution on the quantum-fluctuation-driven phenomena are discussed. The theory was employed for predicting and interpreting the pressure dependence of the phase transition temperatue for KDP-like ferroelectrics and NQR-frequency coefficients for several deuterated and undeuterated crystals.     

Interplay between lattice dynamics and the low-pressure phase of simple cubic polonium

Low-pressure structural properties of simple cubic polonium are explored through first-principles density-functional theory based relativistic total energy calculations using pseudopotentials and plane-wave basis set, as well as linear-response theory. We have found that Po undergoes structural phase transition at low pressure near 2 GPa, where the element transforms from simple cubic to a mixture of two trigonal phases namely, hR1 (α=86°) and hR2 (α=97.9°) structures. The lattice dynamics calculations provide strong support for the observed phase transition, and show the dynamical stability (instability) of the hR2 (hR1) phase.     

Lattice dynamics and phase diagram of aluminum at high temperatures

The dispersion of phonons in the fcc, hcp, and bcc phases of aluminum is calculated at ultrahigh pressures by the method of small displacements in a supercell. The stability of the phonon subsystem is studied. The thermodynamic characteristics are calculated in the quasi-harmonic approximation, and a phase diagram of aluminum is plotted. As compared to the Debye model, the use of a phonon spectrum calculated in the quasi-harmonic approximation significantly broadens the hcp phase field and strongly shifts the phase boundary between the fcc and bcc phases. The normal isentrope is calculated at megabar pressures. It is shown to intersect the fcc-hcp and hcp-bcc phase boundaries. The sound velocity along the normal isentrope is calculated. It is shown to have a nonmonotonic character.     

Lattice vibrations in ordered and disordered thiourea

Raman scattering and infrared reflection spectra of single crystals of thiourea were recorded at several temperatures between 2 K and 240 K in the spectral region (03500) cm^–1. The intention was to search for changes in frequency, linewidth, and intensity of optical phonon modes near the various phase transitions which are known to exist in this material. All observed phonon modes have been tabulated according to a group-theoretical enumeration, and they are attributed to internal motions of the molecular groups and external lattice vibrations. A survey of the temperature dependence of all observed lines is given.Some modes are in fact influenced by the phase transitions in a characteristic manner. The observed temperature dependences can be interpreted by a simple pseudospinphonon coupling model (following contribution II).