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

81Br NQR measurements of the mixed-valence compound (NH4)4SbIII SbvBrl2, which has SbvBr 6 − and SbIIIBr 6 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=212Kwas redetermined. The T 1 values are quite different between SbvBr 6 − and SbIIIBr 6 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 SbIIIBr 6 3− at T c in the low temperature phase.