Structural study of three modulated phases in Rb2ZnBr4

Abstract

The crystal structures of Rb2ZnBr 4 at 293, 140 and 95 K have been studied using single-crystal X-ray diffraction or single-crystal neutron diffraction (for the structure at 95 K). The so-called incommensurate structure (293K) and the two threefold superstructures (140 and 95 K) have been determined using a computer program for modulated structures. The resulting 293 K structure has space group Pcmn(00 y) (ssl). Because of conflicting symmetry evidence in the literature, several other space groups were tried, but they yielded less satisfactory results. For the other two phases, the space group Pc2~n was used. The R values are 0.08 (for 1512 main and 2654 satellite reflections), 0.12 (1494 + 3002 reflections) and 0.05 (for 502 + 980 reflections) for the structures at 293, 140 and 95 K respectively. There is no clear structural difference between the two threefold superstructure phases. The displacive modulation is strikingly similar in the three structures. It mainly consists of rotations of the rather rigid ZnBr 4 tetrahedra. The amplitude of the modulation functions is roughly 35% smaller in the 293 K structure than in the 95 K structure. The modulation functions of the two threefold superstructures show a strong pseudo-Pcmn(OOy)(ssi) symmetry. The temperature parameters clearly reveal that librations of the tetrahedra form an important component of the thermal vibrations. Crystal data at 95 (1)K: Mr= 555.9, a= 13.184 (4), b= 7.599 (2), c = 9.623 (5) A, V = 964.1 A 3.