Proton Spin-Lattice Relaxation in Solid Würster’s Blue Perchlorate

Abstract

Temperature dependence of the proton spin-lattice relaxation time in powdered Würster’s blue perchlorate was measured between 56 and 300 K by using the 90°–τ–90° pulse NMR method at the frequencies 8.0, 12.0 and 23.4 MHz. In the paramagnetic state above the phase transition temperature of 190 K, the relaxation rate was independent of temperature and of frequency and could be explained by the hyperfine interaction modulated by an exchange interaction. Below T_c the relaxation time increased with decreasing temperature and depended on frequency in accordance with BPP theory. The relaxation rate is proportional to the product of the triplet population and the effective exchange frequency both of which vary with temperature. Between 70 and 56 K, the relaxation rate is governed by paramagnetic impurities or monomer radicals.