Raman spin-lattice relaxation, Debye temperature and disorder effects studied with electron spin echo of Cu2+ in Tutton salt crystals

Spin-lattice relaxation time T-1 was determined by the electron spin echo (ESE) method in the temperature range 4-60 K in a series of Tutton salt crystals (M2MII)-M-I(SO4)(2). 6X(2)O (M-I = NH4, K; M-II = Zn, Mg; X = H, D) weakly doped (less than or equal to 10(18) ions cm(-3)) with the Cu-63(2+) isotope. The ESE signal was undetectable at higher temperatures. The relaxation rate increases over the six decades in the studied temperature range with T-1 equal to 1 s at 4 K and 0.5 tts at 50 K. Various possible relaxation mechanisms are discussed with the conclusion that the relaxation is governed by two-phonon Raman processes without a noticeable contribution from the reorientations of Cu(H2O)(6) octahedra between Jahn-Teller distorted configurations. Deuteration of the crystal has no effect in spin-lattice relaxation. For a few crystals, having the largest Cu2+ concentration among the studied crystals, a strong and linear in temperature contribution to the relaxation rate was found below 15 K. Possible explanations are discussed with the final conclusion that this effect is due to a non-uniform Cu2+ distribution in the host lattice producing effective relaxation via pairs and triads of the Cu2+ ions. From the T-1 (T) dependence the Debye temperature OD was determined for the all crystals studied. This varies from Theta (D) = 166 K for K2Zn(SO4)(2). 6H(2)O to Theta (D) = 238 K for (NH4)(2)Mg(SO4)(2). 6H(2)O. The Theta (D) values are discussed and used for calculation of the sound velocity which was found to be similar in all crystals and equal to nu = 4150(+/- 150) m s(-1).