Br NQR relaxation and successive phase transitions of CH3NH3HgBr3

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.