Electron spin–lattice relaxation rates of octahedrally coordinated Yb3+ in Cs2NaYCl6

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Abstract
We have measured the electron spin–lattice relaxation rate of Yb3+ in the cubic host lattice Cs2NaYCl6 in the temperature interval between 1.75 and 4.5 K using a microwave frequency of 16.532 GHz. The T9 Raman relaxation rate was isotropic with a coefficient of 0.0203±0.0014 s−1K−9. The direct relaxation rate exhibited the expected angular anisotropy, varying with temperature and angle as [(3.20±1.0)+(50.6±4.0)u]coth(0.3967/T) s−1, where u=l2m2+m2n2+n2l2, and l, m, n are direction cosines of the magnetic field with respect to the cubic axes. The data were analyzed in terms of five models: the Newman superposition model, Buisson and Borg’s approach, and effective point charge models including, respectively, 1st, 1st, and 2nd, and 1st, 2nd, and 3rd nearest neighbors. The relaxation data are most consistent for the three effective point charge models, which are based upon the two known static crystal field parameters and two remaining variables: the effective charge for second order spherical harmonics, ‖Zeff(k=2) ‖=0.334±0.065 and vt, the transverse sound velocity, calculated from the data to be (1.97±0.12) ×105 cm/s.