A study of electron spin dynamics and phase transition in [Co(H2O)6][SiF6] by single crystal 19F NMR spin-lattice relaxation time measurements

The temperature dependence of the F-19 NMR spin-lattice relaxation time, T-1, was measured for a [Co(H2O)(6)][SiF6] single crystal in the temperature range 170-320 K. A large hysteresis for the transition was observed: T-c approximate to 265 K with increasing temperature, T-c approximate to 245 K with decreasing temperature. The spin-lattice relaxation time of the unpaired electron spin in the Co2+ ion (T-1e) was estimated from F-19 NMR T-1, which is dominated by a dipole interaction between the F-19 nuclei and the electron spins of the Co2+ ions. The energy differences between the electronic ground and excited states in the Co2+ ion involving the Orbach process were estimated from the temperature dependence of T-1e as epsilon = 180 and 330 cm(-1) above and below T-c, respectively. The investigations of the angular variations of F-19 NMR T-1 were performed at 291 and 193 K. Although T-1 for the rotation about the c axis was invariant, the variation of T-1 was observed in a plane containing the c axis at both temperatures. An anisotropy of T-1e was not seen at either temperature.