fl-RbCrC13 (at 295 K), monoclinic, C2/m C 3 (2n), a = 12.224 (4), b = 7-040 (3), c = 6.250 (3) A, fl = 93.34 (5) ° , V = 537.68 A 3, Z = 4, D x = 3.008, D m : 3.01 gcm -3 (Seifert & Klatyk (1964). Z. Anorg. Allg. Chem. 334, 113-124). This phase has a slightly distorted hexagonal perovskite structure and consists of chains of alternating elongated and compressed face- sharing octahedra. Below 170 K CsCrCI 3 is iso- morphous with fl-RbCrC13. Introduction. RbCrC13 and CsCrC13 were prepared by melting equimolar amounts of RbC1 (CsCI) and CrCI 2 in an evacuated quartz tube and crystallized by the Bridgman method. Single-crystal measurements were performed on a three-circle diffractometer with graphite-mono- chromated Mo K~ radiation. 1512 reflections from a crystal of RbCrC13 (0.07 × 0.16 × 0.36 mm) were measured. All data were corrected for absorption. Powder neutron diffraction experiments on CsCrCl 3 at 5 and 60 K were carried out at the HFR reactor at Petten. AGe double monochromator (2 = 2.403 A) was used to avoid second-order contamination. Col- limators of 15' were applied in front of the monochro- mator and the detector. From systematic absences (hkl, h + k = 2n + 1) the space group appeared to be C2/m, Cm or C2. The cell constants are given in Table 1, together with the results of X-ray powder diffraction for RbCrBr 3. Starting with small deviations from the CsNiCl 3 structure (Tishenko, 1955) the positional and thermal parameters of RbCrC! 3 were refined for all three possible space groups. The final discrepancy indices are Table 1. Cell constants of RbCrCI 3, CsCrCI 3 and RbCrBr 3
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Crama et al. (1978) studied this question.