Low-temperature phases in Cs 2 CdBr 4 and Cs 2 HgBr 4

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.

Section: Discussionmentioning

confidence: 92%

Structural study of three modulated phases in Rb₂ZnBr₄

Hogervorst¹,

Helmholdt²

1988

“…In both compounds the phase transformations are described by a rotation wave through YBr4 tetrahedra, which changes the environment of the Cs atoms (Altermatt et al, 1984;Speziali & Chapuis, 1989). The cavities around the two Cs positions have different sizes, as can be seen by the large difference in the Ueq values of the Cs atoms.…”

Section: Discussionmentioning

confidence: 99%

“…A projection along the a axis makes evident a pseudohexagonal aspect of the structure. In the RT phase the existence of static disorder was excluded by the unique NQR signal (Altermatt et al, 1984). Between 252 and 237 K weak satellite reflections were observed and this phase (INC) could be well described as incommensurate in the Pnma(iss) space group (Speziali & Chapuis, 1989).…”

Section: Introductionmentioning

confidence: 93%

See 1 more Smart Citation

Structural Analysis of Cs₂HgBr₄ in Normal, Incommensurate and Twinned Phases

Pinheiro¹,

Jório²,

Pimenta³

et al. 1998

X-ray diffraction and Raman spectroscopy experiments have been used to investigate different phases of CszHgBr4, dicesium mercury tetrabromide, from room temperature to 213 K. Structural analyses have shown that the crystal could be described, in the normal and in the incommensurate phases, both by ordered and disordered models, but the latter gave more consistent results. Raman results corroborate the descriptions based on X-ray analysis; the presence of an extra peak, which according to group theory should be forbidden in an ordered structure, indicates the lack of local symmetry and was associated with an orientational disorder of [HgBr4] 2-tetrahedra. In the transition from the incommensurate to the commensurate phase a multi-soliton behavior was observed. The CszHgBr4 crystal in the low-temperature commensurate phase is composed of two types of ordered pseudomerohedral twinned domains.

“…In addition, they detected a linear decrease of the q value from 0-15 to 0.20 with decreasing temperature. The superspace group of the INC phase discussed by other authors seemed to be controversial (Maeda et al, 1983;Altermatt et al, 1984).…”

Section: Introductionmentioning

confidence: 99%

Study of the incommensurately modulated structure of Cs₂CdBr₄ by single-crystal diffraction

Speziali¹,

Chapuis²

1989

Three thermotropic phase transitions have been detected in Cs2CdBr4: a second-order orthorhombic to incommensurate transition at T~ = 252 K, a first-order incommensurate to monoclinic transition at T,,~ = 237 K and a second-order monoclinic to triclinic transition at T~2 = 156 K. The incommensurate phase of Cs2CdBr 4 has been studied by single-crystal X-ray diffraction. Main reflections and first-order satellites were measured at 245 K. Based on extinction rules for the diffracted intensities, the superspace group could be determined unambiguously. The modulation vector could be precisely measured as q = 0.170 (3)a*. The incommensurately modulated structure is characterized by a rotation of the CdBr 4 tetrahedra around a in addition to a slight translational component along b. In the monoclinic phase the rotation of the tetrahedra is 'locked' and the structure analysis shows the existence of two twinning domains. Crystal data: dicaesium tetrabromocadmate, Cs2CdBr4, M r = 697.9. At 245 K: ppnma rs s, a = 10.205 (4), b = 7.887 (3), ¢ = 13.925 (7) A, 2(CuKct)= 1.5418 A, R =0.054 for 578 reflections.