Central-Peak-Soft-Mode, Coupling in Ferroelectric Gd\u3csub\u3e2\u3c/sub\u3e(MoO\u3csub\u3e4\u3c/sub\u3e)\u3csub\u3e3\u3c/sub\u3e

Transmission measurements on Gd2(MoO4)3 in the ( 5-50)-cm-1 region were performed with use of tunable backward-wave oscillator sources (5-30 cm\u3e-1) and a Fourier spectrometer (30-50 cm\u3e-1). The resulting dielectric spectra show an additional low-frequency dispersion which was fitted with a standard central-peak model. Its characteristic relaxation frequency is ~20 cm\u3e-1 and the coupling between the soft mode and central mode increases near the transition temperature. This model also accounts very well for the weak anomaly in the clamped permittivity Ec measured at 63 MHz. The same central mode was used to fit earlier Raman soft-mode spectra. All of these data were fitted with a three-coupled-mode model which revealed that the soft-mode spectrum consists of two strongly coupled bare modes: a higher-frequency mode which softens and carries the entire Raman strength and a lower-frequency mode which is hard (59 cm-1) and Raman inactive. Both of these modes are also coupled to the central mode and this coupling increases sharply near the transition. The relatively large width of the central mode indicates its intrinsic nature and suggests partial disorder near the transition

Lattice- and Molecular-Dynamics Studies of Phase Transitions in CsLiSO\u3csub\u3e4\u3c/sub\u3e

We report results of a simulation of the phase transitions in CsLiSO4. These are based on our previously developed method for calculating parameter-free potential-energy surface for ionic molecular crystals. Our lattice-dynamical and molecular-dynamics studies show that the roomtemperature (Pnam) phase is unstable and transforms to the observed low-temperature (P1121/n) phase over approximately 200–280 K. The unstable modes of the Pnam phase have maximum instability at the zone center, which indicates a possible phase transformation without a cell multiplication. The rotational ordering of tetrahedral SOM4 2- was found to be the driving mechanism of these phase transitions. The quality of the agreement between theoretical and experimental structural parameters and transition temperatures confirms that our potentials for Li+ containing sulfates are of comparable accuracy to those for other alkali sulfates

Lattice- and Molecular-Dynamics Studies of RbLiSO\u3csub\u3e4\u3c/sub\u3e

Using our previously developed method for calculating parameter-free potential-energy surfaces for ionic molecular crystals, specifically sulfates, we study phase transitions in RbLiSO4 by means of lattice and molecular dynamics. We found that the high-temperature phase I (Pnam) is highly unstable and transforms to the observed lower temperature phase VI (P1121/n) at about 475–525 K. Compared with isomorphous CsLiSO4, there are more branches of unstable modes in the Pnam phase for RbLiSO4. The maximum instability of these modes occurs away from the zone center, q=(0.118a*,0,0), which implies that a high-order incommensurate phase could form during the phase transformation. The driving mechanism of these phase transitions is directly related to the rotational ordering of tetrahedral SO4 2- groups. The rms values of the deviations of the S-O bonds from their orientations in the Pnam phase to those in the P1121/n phase were found to be approximately ±23–25°

First-Principles Theoretical Explanation of Incommensurate Behavior in RbZnC\u3csub\u3e4\u3c/sub\u3e

We have carried out an ab initio theoretical study of the potential energy and molecular dynamics of Rb2ZnCl4. These calculations demonstrate that the incommensurate transition in this compound is caused by the relaxation of imperfect-hexagonal spirals of highly unstable ZnCl42- ions. This leads to angstrom-size displacements with essentially zero energy change. We argue that such latent (or imperfect) symmetry could be the general cause of incommensurate transitions in insulators

Simulation of Normal Rb\u3csub\u3e2\u3c/sub\u3eZnCl\u3csub\u3e4\u3c/sub\u3e Near the Incommensurate Transition

The purposes of the reported computer simulation of the normal (high-temperature) phase of rubidium tetrachlorozincate are to understand the disordered structure in that phase and to investigate the possibility that the transition, upon cooling, from the normal phase to one with an incommensurate modulation is associated with a change from the disordered structure to an ordered one. The simulation of the dynamics of 168 ions in a periodic structure begins from a slight perturbation of a structure that is determined by minimization of the potential energy within the constraints of the experimentally determined average symmetry. Rigid ions with short-range interactions described by the electron-gas model (with a qualification) are assumed. We find both zinc-induced and rubidium-induced instabilities in the chloride sublattices of the average experimental structure. The zinc-destabilized chloride ions move to a new sublattice in the simulation; however, a crude estimate indicates that this is caused by neglect of ionic polarizability and that these chlorides should either remain at their original sites or be disordered with chains of correlated positions. The rubidium-destabilized chloride ions form two-dimensional ordered networks in the disordered structure. We suggest that the inevitable freezing-out of disorder among the chains of zinc-destabilized chloride ions and among the networks of rubidium-destabilized chloride ions is the mechanism for the transition to the incommensurate phase

Lattice- and molecular-dynamics studies of phase transitions inCsLiSO4

We report results of a simulation of the phase transitions in CsLiSO4. These are based on our previously developed method for calculating parameter-free potential-energy surface for ionic molecular crystals. Our lattice-dynamical and molecular-dynamics studies show that the roomtemperature (Pnam) phase is unstable and transforms to the observed low-temperature (P1121/n) phase over approximately 200–280 K. The unstable modes of the Pnam phase have maximum instability at the zone center, which indicates a possible phase transformation without a cell multiplication. The rotational ordering of tetrahedral SO42- was found to be the driving mechanism of these phase transitions. The quality of the agreement between theoretical and experimental structural parameters and transition temperatures confirms that our potentials for Li+ containing sulfates are of comparable accuracy to those for other alkali sulfates