Cationic Dynamics in the Crystalline Phases of (CH3NH3)PbX3 (X: Cl, Br) as Studied by Proton Magnetic Resonance Techniques

Abstract

The temperature dependences of the 1H spin-lattice relaxation time T1, the linewidth parameter T2*. and the second moment M2 of 1H NMR absorption were measured for solid (CH3NH3)PbX3 (X: CI. Br). In the room-temperature cubic phases of both salts, and also in the high-temperature tetragonal phase of (CH3NH3)PbBr3, the cations undergo rapid overall rotation or reorientation. In the lowest-temperature phase of both salts the orientation of the cations is fixed but rapid C3 reorientation of the CH3 and NH+ 3 groups of the cations about their C - N bond axes takes place. From the M2 measurements, a precessional motion of the cations in the intermediate-temperature phase of both complexes is suggested. Above room temperature, 1H T1 of both salts can be explained by assuming spin-rotational relaxation operative due to the rapid rotation of the cations. An anomalous behavior of 1H T1, attributable to cross relaxation between 1H and 81Br nuclei, was detected for (CH3NH3) PbBr3 when T1 was measured at 42 MHz