Hyperfine Interact (2010) 198:109–115 DOI 10.1007/s10751-010-0185-4 Br NQR relaxation and successive phase transitions of CH 3 NH 3 HgBr 3 H. Niki · K. Higa · Y. Okada · M. Oshiro · M. Yogi · H. Terao Published online: 3 November 2010 © Springer Science+Business Media B.V. 2010 Abstract The measurement of 81 Br NQR in CH 3 NH 3 HgBr 3 has been carried out in the temperature range between 80 and 300 K using a pulse NQR method. The temperature dependence of 81 Br NQR frequencies in CH 3 NH 3 HgBr 3 has revealed that it undergoes three characteristic successive phase transitions at T = 123, 184 and 239 K. The phase transition temperature at T = 239 K is the second-order type, whereas those at T = 184 and 123 K are the first-order nature of the phase transitions. Each phase transition seems to be closely related to the motions of methyl ammonium cation as a partial or whole. The enhancement of 1/ T 1 at T = 239 K indicates the onset of the molecular motion of the cation as a whole with increasing temperatures. Keywords CH 3 NH 3 HgBr 3 · Phase transition · 81 Br NQR · Spin-lattice relaxation time · Critical behavior 1 Introduction It is well known that CH 3 NH 3 HgCl 3 undergoes the ferro- to paraelectric phase transition at 335 K [1–3]. A 35 Cl NQR study revealed that another transition in the range 108 to 121 K which is closely related to the motion of CH 3 NH + 3 under the influence of the N-H··· Cl hydrogen bonding [4]. H. Niki (B ) · K. Higa · Y. Okada · M. Oshiro · M. Yogi Department of Physics, Faculty of Science, University of the Ryukyus, Nishihara, Okinawa 903-0213, Japan e-mail: niki@sci.u-ryukyu.ac.jp H. Terao Department of Chemistry, Faculty of Integrated Arts and Sciences, Tokushima University, Minamijosanjima-cho, Tokushima 770-8502, Japan