Role of NH4 ions in successive phase transitions of perovskite type (NH4)2ZnX4 (X = Cl, Br) by 1H MAS NMR and 14N NMR

Abstract

The ¹H chemical shifts and the spin-lattice relaxation time, T_1ρ, in the rotating frame of (NH₄)₂ZnX₄ (X = Cl, Br) are observed in order to investigate local phenomena related to successive phase transitions. The temperature dependence of T_1ρ values for ¹H showed a minimum, and the T_1ρ values for ¹H appeared to be governed by tumbling molecular motions at high temperatures. In addition, ¹⁴N NMR spectra are studied in each phase of (NH₄)₂ZnX₄ single crystals in the laboratory frame. The phase transition temperatures strongly affect the ¹⁴N number of symmetry related nitrogen centers within the unit cell. The ¹H MAS NMR and ¹⁴N NMR results are discussed to elucidate the roles of NH₄ ions during the phase transitions of (NH₄)₂ZnX₄.