Simulation of normalRb2ZnCl4near the incommensurate transition

Abstract

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 rubidiuminduced instabilities in the chloride sublattices of the average experimental structure. The zincdestabilized 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 rubidiumdestabilized 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.

Cd+(5s1) Centres in Rb2ZnCl4 Single Crystals

“…The smaller variation of the FA shift from the CdC(Ij to the Cd+(II) centres in Rb2ZnC1,, compared to the larger variation observed from the Cd,f to the Cd;v, centres in alkali chlorides, is to be expected if one considers the already large 6A shift of the Cd' (I) centre. This large 6A shift is explained by a stronger covalent character of the chemical bond in the ZnC1, tetrahedra compared to alkali chlorides, in agreement with earlier observations [22]. Indeed, the Cd; and Cd,fv, centres in alkali chlorides exhibit a much smaller 6 A shift (= 15%), as expected for such strongly ionic compounds.…”

A Study of the Incommensurate Phase of Rb2ZnCl4by ESR

“…Above 303 K, Rb 2 ZnCl 4 exists in the high temperature normal (N ) phase, which is orthorhombic, belonging to space group Pnma-D 16 2h [2] and containing four formula units per unit cell. At 303 K there is a transition to the incommensurate (I ) phase, and at 194 K the structure 'locks in' to a commensurate phase (C) that is also orthorhombic, belonging to the space group P2cn-C 9 2v , in which the unit cell is tripled along the a-axis and contains 12 formula units [4]. Finally, below 92 K there is a fourth structural transition to a monoclinic structure.…”

“…Rb 2 ZnCl 4 belongs to the family of A 2 BX 4 crystals with the β-K 2 SO 4 structure that exhibits several successive phase transitions that have been experimentally measured [1][2][3][4][5][6] and theoretically predicted [7][8][9][10][11]. Above 303 K, Rb 2 ZnCl 4 exists in the high temperature normal (N ) phase, which is orthorhombic, belonging to space group Pnma-D 16 2h [2] and containing four formula units per unit cell.…”

Electronic band structure and related properties of Rb 2 ZnCl 4 crystals at different hydrostatic pressures

“…Below the temperature 74 K, the crystal symmetry becomes monoclinic. Structural and dielectric properties of the crystal were extensively studied in the temperature range of the commensurate-incommensurate transition [2][3][4][5][6].…”