We calculated in this study the Raman frequencies of some lattice modes in the melting region of ammonia solid I. The Raman frequencies of those phonon modes were obtained through the Grüneisen relation using the volume data for the ammonia solid I from the literature. Our calculated Raman frequencies require for comparison the experimental Raman frequencies measured as a function of pressure for some fixed temperatures in the ammonia solid I.
The displacive phase transformations can be considered as composed of two processes, namely, pure displacements, shuffling or shearing of atomic planes, and supplementary homogeneous lattice deformation changing also the dimensions of the moving planes. Such deformation causes shape memory effect when the structural transformation is reversed. General displacements of atomic planes will be examined, i.e. γ-surface type calculations will be reported for single plane shuffling, alternate shuffling of every other bcc atomic plane and successive displacements of parallel atomic planes producing in combination with homogeneous deformation the close packed structures. The results of calculations using the many-body potentials of the Finnis-Sinclair type will be compared with ab initio calculations that indicate in which way the phase transformation can be initiated.
Discussion of the austenite-single-variant martensite interfaces in Cu-Al-Ni alloy is performed in the frame of a topological model of martensite interfaces. This model takes into account admissible defects lying in the interface. The results are compared with the experimental data obtained on the foils of Cu-Al-Ni alloys deformed in situ in a transmission electron microscope.
The pressure dependence of the Raman frequencies for the ν_{21} mode is studied by using the volume data at room temperature close to the transitions among the phases of III, IV and V in cyclohexane. By determining the pressure dependence of the mode Grüneisen parameter γ_{T} in the phases and taking the average values, the Raman frequencies of those modes associated with the phase transitions are predicted through the volume data in cyclohexane. Our predicted Raman frequencies agree with those observed experimentally in this organic molecule. The Raman frequencies of the other modes can be calculated in cyclohexane and also in some molecular crystals using the method given here.
Motivated by the properties of one-dimensional lattice systems with asymmetric on-site potential, one can formulate a hypothesis of an asymmetry driven phase transformation. Characteristic feature of one-dimensional systems exhibiting asymmetry driven phase transformation is a sequence of the two phase conversions. In particular class of such systems with a triple-well potential, phase conversions of one-dimensional systems would evolve into a sequence of two phase transitions in three-dimensional models. We propose here a model of three-dimensional system exhibiting a sequence of two first order asymmetry driven phase transitions.
The aim of this paper is to calculate the equilibrium displacements of the ^4T_{2g} potential surface minimum from the ground state along the a_{1g} and e_g Jahn-Teller active modes for the octahedral [MnF_6]^{2-} cluster obtained by doping Mn^{4+} ions in Cs_2SiF_6 host crystal. The equilibrium displacements in normal and Cartesian coordinates were estimated from force constants of [MnF_6]^{2-} cluster and Huang-Rhys factors associated with the Jahn-Teller stabilization energy. A net equatorial expansion and an axial stretching of the geometry of the [MnF_6]_{2-} cluster in first ^4T_{2g} excited state, as combined effect of the a_{1g} and e_g displacements, were demonstrated.
The electronic structure of the ternary YNi_4B compound, crystallizing in the hexagonal CeCo_4B structure (P6/ mmm space group), was studied by X-ray photoelectron spectroscopy and ab initio calculations. Core levels and the valence band were investigated. The X-ray photoelectron spectroscopy valence band is compared with that obtained from ab initio calculations. The valence band spectrum at the Fermi level exhibits the domination of the Ni(3d) states, which are hybridized with 4d states of Y and 2p states of B. The theoretical electronic specific heat coefficientγ derived from N(E_F) is about 11.33 mJ/(mol K^2) for experimental lattice parameters. The calculated bulk modulus is B_0=1.61632 Mbar.
Crystal structures of two yttrium aluminium oxides, namely YAlO_3 and Y_3Al_5O_{12}, were investigated in the temperature range 3.4-300 K by high-resolution neutron powder diffraction. Neither traces of phase transformations nor discontinuous changes of physical properties were observed. Thermal expansion of yttrium aluminium oxides was evaluated in terms of 1st order Grüneisen approximation, where the Debye temperatures and the Grüneisen parameters have been estimated for both compositions. Anomalies in the thermal expansion of yttrium aluminium perovskite have been observed and modelled using the Einstein oscillator with negative Grüneisen parameter. Extended bond length analysis revealed significant thermally-driven modifications of the aluminium-oxygen framework.
A short-range force constant model has been applied using normal coordinate analysis to investigate the Raman and the infrared modes in multiferroic MnWO_4 having space group P2/c. The calculation of zone centre phonons has been made with eight stretching and seven bending force constants. The calculated Raman and infrared wave numbers are in good agreement with the observed ones. The potential energy distribution has also been investigated for determining the significance of contribution from each force constant toward the Raman and the infrared wave numbers.
The diagonal and non-diagonal parts for the Debye-Waller factor have been established using equation of motion technique of quantum dynamics and the Dyson equation approach. The double time temperature dependent phonon Green function has been taken to find the phonon linewidth and phonon shift. Renormalized mode frequency has been investigated in terms of electron-phonon coupling constant and temperature. The effect of electron-phonon interaction on the Debye-Waller factor has been studied in low temperature limit in low impurity concentration in semiconductor crystals.
Novel techniques have been created for studying phonon kinetics. A supersonic conduction electron flux is used to produce a phonon flux. Transverse electron focusing (TEF) is applied for checking the electron gas affected by phonons. A turbulent regime of phonon flow-phonon flux step broadening-reveals itself as step broadening of non-equilibrium electron flux.
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