Bromine and Antimony NQR of 4-Aminopyridinium Tetrabromoantimonate(III)

Abstract

The temperature dependences of T1 and T2 of 81Br and 121Sb NQR have been measured in order to study the phase transition at Tc = 224 K as well as the crystal dynamics in 4-aminopyridinium tetrabromoantimonate(III) (4-NH2PyHSbBr4). The temperature dependence of T1 of 81Br and 121Sb NQR follows the usual T2 law in the range from 80 to 120 K. T1 is dominated by fluctuations of the EFG at the Br and Sb nuclei due to lattice vibrations. The T1 vs. 1/T curves in the region between about 120 and 140 K can be described by exponential curves. The activation energies of motions obtained from these curves are similar to those from proton NMR. The exponential changes of T1 of Br and Sb NQR are attributable to fluctuations caused by the thermal motion of 4-NH2PyH cations. Echo signals of Br and SbNQRin the lowtemperature phase could not be detected at temperatures higher than 145 and 150 K, respectively, because the S/N ratios became poor owing to the very short T2. The echo signals could also not be detected at temperatures above Tc because of the short T2