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

摘要

81Br NQR measurements of the mixed-valence compound (NH4)4SbIII SbVBr12, which has SbVBr6 ? and SbIIIBr6 3? octahedra in different oxidation states in the tetragonal pseudo-K2PtCl6 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 SbVBr6 ? and SbIIIBr6 3? 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 81Br NQR frequencies and T 1 values for each anion indicates that the static rotation may occur for SbVBr6 ? but not for SbIIIBr6 3? at T c in the low temperature phase.