Structure of the solid solution [N(CH3)4]2ZnCl1.8Br2.2 in the high- and in a new low-temperature phase

Abstract

Domains were encountered in the crystal and a large decrease of the temperature factor was observed. The phases studied are separated by two other phases: an incommensurate one and a monoclinic one with a threefold unit cell (Z = 12). The ZnHal 4 tetrahedra drive the transitions, rotating principally around the a axis (11 °) and shifting their centre along the b axis (0.42/~). Their small distortion is probably due to the crystal field effect (Hal-Zn-Hal angles range from 108.7 to 112.2°). The mean bond length Zn-Hal is 2.352/~ for Pnma and 2.360/~ for P12Jal. Tetrahedra in the same bc plane rotate in the same sense.

Influence of incommensurate modulation on the optical properties of the solid solution ((CH3)4<

“…At T i a phase transition into an incommensurate phase occurs. From our birefringence measurements we find the value T i ϭ294 K, which differs from the value T i ϭ290 K given by Colla et al 2 The behavior of the modulation parameter ␥ in the incommensurate phase has been investigated by Vogels et al 14,15 It was found that the modulation wave vector q ជ locks at several commensurate values of ␥ in the incommensurate phases of ((CH 3 ) 4 N) 2 ZnCl 4Ϫx Br x crystals (xϾ0). The exact temperature behavior of ␥ can, however, differ from one crystal to another.…”

“…3 Thus, if one takes the difference between Cl and Br into account, all lattice translational symmetry and point group symmetry are generally lost already in the high-temperature phase. Although this has only subtle effects on x-ray diffraction experiments, 2 we show in this paper that it has definite consequences for the optical properties of these crystals.…”

“…9 This was done with a high-accuracy universal polarimeter ͑HAUP͒ that is very well suited for this purpose. [9][10][11][12] In the case of ((CH 3 ) 4 N) 2 CuCl 4 a rotation of the optical indicatrix was detected in the incommensurate phase, which contradicts an orthorhombic symmetry. In addition, ͑small͒ optical activity was found which contradicts the presence of an inversion center.…”

“…Crystals of the solid solution ((CH 3 ) 4 N) 2 ZnCl 4Ϫx Br x , 0ϽxϽ4, can be considered to be members of the family of A 2 BX 4 -type crystals that have a ␤-K 2 SO 4 structure at high temperatures. In the neighborhood of room temperature a phase transition occurs to an incommensurately modulated structure with modulation wave vector q ជ ϭ␥c ជ *, for the crystals with xϽ3.…”

“…The crystal can, therefore, show nonzero optical activity and indicatrix rotation even in the hightemperature phase. Instead of searching for a deviation from zero in the incommensurate phase as has been done for ((CH 3 ) 4 N) 2 ZnCl 4 , it is, therefore, possible to study the deviation from a nonzero behavior in the case of ((CH 3 ) 4 N) 2 We have organized this paper as follows. First, the temperature dependence of the crystal structure of ((CH 3 ) 4 N) 2 ZnCl 2.8 Br 1.2 is described.…”