Br NQR relaxation of the mixed-valence compound (NH4)4SbIIISbVBr12

Abstract

81 Br NQR measurements of the mixed-valence compound (NH 4 ) 4 Sb III Sb V Br 12 , which has Sb V Br − 6 and Sb III Br 3− 6 octahedra in different oxidation states in the tetragonal pseudo-K 2 PtCl 6 structure, were carried out by pulse method in the temperature range between 80 and 300 K. The phase transition temperature of T c = 212 K was redetermined. The T 1 values are quite different between Sb V Br − 6 and Sb III Br 3− 6 octahedra. For all resonance lines T 1 minima were observed at T c . The T 1 behavior at T c was explained by a softening of the rotary lattice mode around a principal axis of each octahedron along the c axis of the crystal. The distinctive feature in the temperature dependence of both 81 Br NQR frequencies and T 1 values for each anion indicates that the static rotation may occur for Sb V Br − 6 but not for Sb III Br 3− 6 at T c in the low temperature phase.

Br NQR relaxation and successive phase transitions of CH3NH3HgBr3

“…All the resonance lines seem almost continuously to change at the I↔II phase transition of T 239 as seen in Fig. 2, showing a displacive-type nature of this phase transition [7][8][9]. The very small discontinuity is however seen between the related curves at T 239 , which may show that the phase transition is almost a second-order but has a little first-order nature as well.…”

“…The T −1 1 vs. T curves in the vicinity of the I↔II phase transition at 239 K can be described by the following equation [7][8][9],…”