Molecular reorientation in solid (CH3NH3)2CdBR4 as studied by 79Br and 81Br nuclear quadrupole resonance and 1H nuclear magnetic resonance

Abstract

The temperature dependence of ^79,81Br NQR frequencies, and spin–lattice relaxation times of ¹H NMR at 20.4 MHz and ^79,81Br NQR have been measured in (CH₃NH₃)₂CdBr₄ crystals. There was no evidence of any phase transition between 4.2 and 300 K. T, minima of ¹H NMR observed at 35 and 100 K are governed by the reorientations of CH₃NH⁺₃ groups at site 2 (E_a= 2.5 kJ mol^–1) and at site 1 (E_a= 9.2 kJ mol^–1), respectively. Four NQR lines were observed in the whole temperature region. The frequencies of the ν₃ line increase to a maximum value at T= 120 K, and then decrease. The frequencies of the ν₄ line increase monotonically on heating the sample. This ‘anomalous’ temperature dependence is probably due to anisotropic thermal expansion of the crystalline lattice. ⁷⁹Br and ⁸¹Br T₁ measurements revealed that the overall reorientation of [CdBr₄]^2–(E_a= 20 ± 3 kJ mol^–1) is activated above 200 K.