Molecular Motions and Phase Transitions in Solid CH₃NH₃PbX₃ (X = C1, Br, I) as Studied by NMR and NQR

Abstract

Abstract The temperature dependence of 35 C1, 81 Br, and 127 I NQR frequencies and 1 Hspin-lattice relaxation times (T 1 ) for CH 3 NH 3 PbX 3 (X = Cl, Br, I) was measured through the successive phase transitions in these solids. The isotropic reorientation of the CH 3 NH 3 ions takes place in the higher-temperature phases (tetragonal [I4/mcm] and cubic) of the three salts (E a = 11 kJ mol -1 ). T 1 's in the lowest-temperature phases (orthorhombic) indicate that the cations undergo correlated C 3 -reorientation in the chloride (E a = 5.45 kJ mol -1 )and in the iodide (E a = 5.80 kJ mol -1 , whereas correlated (E a = 2.40 kJ mol -1 ) and uncorrected (E a = 7.50 kJ mol -1 ) C 3 - reorientations are excited in the bromide. It is also revealed that the rotational tunneling of the cations governs T 1 at lowtemperature region in the orthorhombic phases of these salts