MOLECULAR MOTIONS AND PHASE-TRANSITIONS IN SOLID CH3NH3PBCL3, CH3NH3PBBR3, CH3NH3PBI3, AS STUDIED BY NMR AND NQR

The temperature dependence of Cl-35, Br-81, and I-127 NQR frequencies and H-1 spin-lattice relaxation times (T1) for CH3NH3PbX3 (X = Cl, Br, I) was measured through the successive phase transitions in these solids. The isotropic reorientation of the CH3NH3+ ions takes place in the higher-temperature phases (tetragonal [14/mcm] and cubic) of the three salts (E(a) = 11 kJ mol-1). T1's in the lowest-temperature phases (orthorhombic) indicate that the cations undergo correlated C3-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 uncorrelated (E(a) = 7.50 kJ mol-1) C3-reorientations are excited in the bromide. It is also revealed that the rotational tunneling of the cations governs T1 at low-temperature region in the orthorhombic phases of these salts.