Cross relaxation of the proton magnetization in ammonium compounds

Expressions are derived for the time constants T_1D and T_SD of the NH₄ protons in tunneling ammonium compounds below the line-width transition temperature. T_1D characterizes the speed of the spin-lattice relaxation of the dipolar energy and T_SD the speed of the cross relaxation between the A and T symmetry species. The expressions should be valid if all the tunnel splittings between the T species levels are larger than the magnetic dipolar interaction. Predictions are compared with new experimental results on T_SD in (NH₄)₂PbCl₆ and with some earlier results on T_SD and T_1D in (NH₄)₂ SnBr₆ and NH₄ClO₄. They support the conclusion that for T_1D>T_SD the T levels are nondegenerate, while the condition T_1D< T_SD refers to at least a partial degeneracy.