The critical temperature dependence of birefringence near the normal-incommensurate phase transition in Rb₂ZnBr₄

Abstract

The purposes of the reported computer simulation of the normal (high-temperature) phase of rubi-dium 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 incommensu-rate 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.

Optical studies of order parameter fluctuations in solid solutions based on lead germanate crystals

“…This correlates well with the known fact that the PT in is more essentially smeared than that in (6). Unlike the method adopted in [16,19] though similar to that used in the studies of critical fluctuations for deuterated triglycine sulphate [20,21], we have put the fitting parameters Table 1.…”

“…On the other hand, one can roughly estimate G from the experiment itself, using the dependences ) (τ ζ g (the details see in Ref. [16]). …”

“…Nonetheless, the analysis of fluctuations after the optical activity possesses an evident advantage in this respect, when compare with the analysis based on the optical birefringence or thermal expansion data (see [16,[19][20][21]). Namely, the derivatives of the latter quantities include extra fitting parameters.…”

“…Following the technique adopted in [16] and using the data represented in Fig 1, one can estimate the Ginzburg number G (see Table 1 On the contrary, in case of the substitution of Ge with Si, one can pay attention to a tendency to decreasing G, with the opposite conclusions. However, to make our consideration more universal, let us stress that the Ginzburg number would define, in the strict sense, only intrinsic (thermal) fluctuations but not extrinsic ones caused by structural defects.…”

Optical activity and critical exponent of the order parameter in lead germanate crystals.1.The case of diffused phase transition in Pb5Ge3O11 doped with Cu, Ba and Si ions

“…For example, the electrogyration coefficient 333 γ in Pb 5 Ge 3 O 11 :Cr 3+ [16,17] 6 3 ↔ . Moreover, it has been shown in the recent papers that the temperature behaviour of electrogyration effect induced by spontaneous polarisation may be used for obtaining the data concerned with critical fluctuations occurred in the vicinity of phase transitions [18], in particular for the case of lead germanate crystals [19,20]. The other known fact is that the temperature dependence of optical activity in the lead germanate may be used for deriving the critical exponent of temperature dependence of the order parameter.…”