BrNQR relaxation of the mixed-valence compound (NH4)4SbIIISbvBr12

⁸¹Br NQR measurements of the mixed-valence compound (NH₄)₄Sb^III Sb_vBr_l2, which has Sb^vBr{₆/^-} and Sb^IIIBr{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 Sb^vBr{₆/^-} and Sb^IIIBr{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 Sb^IIIBr{₆/^3-} at T _c in the low temperature phase.