Paramagnetic resonance studies of trivalent dysprosium and cerium in the cubic host dicesium sodium yttrium hexachloride

We have used X-band electron paramagnetic resonance (EPR) to study octahec1rally coordinated rare earth ions in the host lattice CSNaYC16, This host is ,ideal for EPR work, because all the trivalent rare earths will substitute into the Y3+ site which has exactly octahedral symmetry, Previous EPR studies of rare earths in insulators have concentrated on materials with low point symmetry and on hosts such as alkaline earth oxides and fluorides where a trivalent rare earth substitutes for the divalent cation, necessitating charge compensation, which in turn produces some rare earth sites of non-cubic symmetry. Spin-lattice relaxation studies in CS NaYC16 are especially attractive from both theoretical and experimental points of view. The orbit-lattice Hamiltonian is greatly simplified in cubic symmetry, making quantitative theoretical calculations tractable. Experimental determination of relaxation times is easier than in charge-compensated materials, where cross-relaxation of ions in cubic sites to ions in other site symmetries often obscures true spin-lattice relaxation processes. In order to test various models of the orbit-lattice interaction, we have measured the spin-lattice relaxation rate of Dy:CsNaYCIas Z6 a function of temperature and direction of the applied dc magnetic field. In the one-phonon direct process regime, our results agreed with theory, which predicts that the relaxation rate varies with temperature and angle as (A + Bu)coth(nw/2kT) where u = l2m2 + m2n2 + n2l2 is a cubically invariant function of the direction cosines of the applied field with respect to the cubic fourfold axes. At higher temperatures the relaxation was dominated by a two-phonon resonant Orbach process arising from the first excited state, identified as a fS quartet with measured energy (35.1 ± 0.8) K. The direct process results were examined quantitatively using the general electrostatic model of Buisson and Borg, true and effective point charge models, and the Newman nearest neighbor superposition model. None of these models provided a fit with parameters comparable to those expected on the basis of experience with other rare earth salts and with Er3+ and Y3b+ in the same host lattice. Electron nuclear double resonance (ENDOR) studies were made with Ce:CsNaYC16 to study ligand coupling to the paramagnetic impurity. Resonances with resolved structure were seen from 133Cs and 23Na. resonance showed isotropic structure arising from a Fermi contact hyper- fine interaction. A multi-line anisotropic structure was seen in the 23Na ENDOR spectrum, with splittings several times as large as that expected for a purely point dipolar coupling between Ce3+ and the nearest Na+ ion. The angular dependence and observed splittings of this hyper-fine structure suggest the possibility that the Na+ ions nearest the Ce+ impurity are in sites slightly distorted from cubic symmetry.U of I OnlyThesi