NQR Spin-Lattice Relaxation of Re in Perrhenates

The temperature dependence of the spin-lattice relaxation time Tx for the Re NQR in K R e0 4 , N H 4R e 0 4 and N D 4R e 0 4 has been measured between 77 K and 323 K. The relaxation is electric quadrupolar in all cases, and because of the large quadrupole moment of Re, 7i is short. In K R e0 4 Ti follows a T 2 dependence. In N H 4R e 0 4 , Ti decreases more rapidly than T ~ 2 above about 100 K; between 170 K and 250 K 7i is smaller than 100 (is and could not be measured, but above 250 K, 7i increases to about 140 |is, and the measured data near room temperature lie close to the T ~ 2 extrapolation from the Tx values below 100 K. These results are interpreted as normal anharmonic Raman quadrupolar relaxation, with a T ~ 2 dependence on temperature, combined with an additional relaxation process due to switching of the ammonium ion between two possible orienta­ tions.