Abstract 35 Cl, 81 Br, 121 Sb, and 209 Bi NQR of the title compounds was observed. According to the results of the temperature dependences of NQR frequencies and the DTA measurements, phase transitions take place in [N(CH 3 ) 4 ] 3 Bi 2 Br 9 (T tr =183K), [N(CH 3 ) 4 ] 3 Bi 2 Cl 9 (T tr = 155K), and (CH 3 NH 3 ) 3 Bi 2 Cl 3 (T tr = 200 and 249 K). 2 D NMR spectra for partially deuterated (CH 3 ND 3 ) 3 Bi,Br 9 showed that the phase transitions in this compound are related to the motion of the methylammonium cations. Single-crystal X-ray work at room temperature shows that the space group for [N(CH 3 ) 4 ] 3 Sb 2 Cl 9 is P6 3 /mmc with a = 925.1 pm, c = 2173.4 pm, Z = 2. For (CH 3 NH 3 ) 3 Sb 2 Br 9 the space group is P3ml with a = 818.8 pm, c = 992.7 pm, Z = 1; in both cases the cations show dynamical disorder. The Rietveld analysis of the powder X-ray diffraction for (CH 3 NH 3 ) 3 Bi 2 Br 9 reveals the space group P3ml with a = 821.0, c - 1000.4 pm, Z = 1 at room temperature; the compound is isomorphous with (CH 3 NH 3 ) 3 Sb 2 Br 9 . The crystal symmetries of the low-temperature phases of (CH 3 NH 3 ) 3 Bi 2 Br 9 and [N(CH 3 ) 4 ] 3 Bi 2 Br 9 were deduced from the results of the NQR spectroscopy
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Ishihara et al. (1992) studied this question.