Role of NH 4 ions in successive phase transitions of perovskite type (NH 4 ) 2 ZnX 4 (X = Cl, Br) by 1 H MAS NMR and 14 N NMR.

The 1 H chemical shifts and the spin-lattice relaxation time, T 1ρ , in the rotating frame of (NH 4 ) 2 ZnX 4 (X = Cl, Br) are observed in order to investigate local phenomena related to successive phase transitions. The temperature dependence of T 1ρ values for 1 H showed a minimum, and the T 1ρ values for 1 H appeared to be governed by tumbling molecular motions at high temperatures. In addition, 14 N NMR spectra are studied in each phase of (NH 4 ) 2 ZnX 4 single crystals in the laboratory frame. The phase transition temperatures strongly affect the 14 N number of symmetry related nitrogen centers within the unit cell. The 1 H MAS NMR and 14 N NMR results are discussed to elucidate the roles of NH 4 ions during the phase transitions of (NH 4 ) 2 ZnX 4 .