Selection rules for superstructures in thiourea, Rb2ZnBr4 and Rb2ZnCl4

Hogervorst, A.C.R.; Wolff, Philippe

doi:10.1016/0038-1098(82)90106-5

Cited by 13 publications

(6 citation statements)

References 12 publications

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“…The value of y is constant at 5/17 (0.294) within experimental error in the large temperature interval between T~ and T~ + 10 K [see de Pater (1979) and Iizumi & Gesi (1983)]. Thus, in this range the structure of the so-called incommensurate phase is commensurate (Hogervorst & de Wolff, 1982).…”

Section: Introductionmentioning

confidence: 64%

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Structural study of three modulated phases in Rb₂ZnBr₄

Hogervorst¹,

Helmholdt²

1988

Acta Crystallogr Sect B

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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.

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Section: Introductionmentioning

confidence: 64%

“…Hogervorst & de Wolff (1982)], it shows systematic extinctions which strongly suggest a (3 + 1)-dimensional space group as the best description of its symmetry. The reflection conditions which we observed on X-ray precession photographs are shown in Table 2.…”

Section: Symmetrymentioning

confidence: 99%

Structural study of three modulated phases in Rb₂ZnBr₄

Hogervorst¹,

Helmholdt²

1988

Acta Crystallogr Sect B

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“…Close to the incommensuratecommensurate transition successive jumps in the wave vector were observed in this compound but the various « stairs » are not specified by an exact fraction and moreover seem to coexist in a certain temperature range [6,7].…”

mentioning

confidence: 87%

“…The situation is somewhat different in the high T part of the I phase in Rb2ZnBr4 where a « commensurate » value of the modulation wave vector 5/17 has been observed [6,7]. The 1/2 -+ -1/2 87Rb NMR transitions are here significantly broader than in Rb2ZnCl4 and the resolution of the 8' Rb 1/2 -+ -1/2 NMR spectrum is not good enough [9] to discriminate between a true I phase and a phase with N = 17.…”

mentioning

confidence: 90%

“…Long period C phases have been observed in thiourea [4] where there is a phase with a ninefold period and in [N(CH3)412ZnC'4 where there is a C phase with a period five times that of the high temperature phase [5]. A peculiar case is that of Rb2ZnBr4 where the modulation wave vector does not change at all for more than 140 K and equals 5/17, so that the unit cell should be 17 times that of the high T phase [6,7]. This result is rather unusual since phases with a large superstructure should exist over a narrower range of temperatures than phases with a small superstructure.…”

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confidence: 99%

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