Structures and phase transitions of (BrCH2CH2NH3)2CdBr4 and (BrCH2CH2CH2NH3)2CdBr4 as investigated by 81Br NQR, DTA, and single-crystal X-ray diffraction

The crystal structures of (BrCH2CH2NH3)2CdBr4 (1) and (BrCH2CH2CH2NH3)2CdBr4 (2) have been determined at T = 113 K: orthorhombic, Pbcn, a = 843.8(4), b = 775.4(4), c = 2339.6(11) pm, Z = 4 for 1; orthorhombic, Pbcn, a = 858.7(3), b = 783.6(2), c = 2519.4(7) pm, Z = 4 for 2. Both crystals are isomorphic, showing distinctive layered perovskite-related structures, in which the cations orient their aliphatic parts toward each other in their cation layers and connect to the infinite anion layers with their ammonium parts through N–H · · · Br hydrogen bonds. 81Br NQR and differential thermal analysis measurements for both compounds revealed the existence of successive phase transitions as seen in n-(CH3CH2CH2NH3)2CdBr4 (3). These phase transitions appear to be induced by the activated motions of the cations. The changes in the hydrogen bonding schemes resulting from the motions of the cations lead to the different phases of the respective crystals.