ELECTRON SPIN-LATTICE RELAXATION OF ASO44- AND SEO43- CENTERS IN KH2PO4 TYPE CRYSTALS - CRYSTAL-FIELD VERSUS HYPERFINE EFFECTS

The temperature dependence of electron spin-lattice relaxation time (T -1) has been investigated for the AsO44- and SeO43- centers in gamma-i rradiated KH2AsO4, KH2PO4 and KD2PO4. Experimental studies included th e measurement of T-1 for these systems by utilizing the double modulat ion ESR technique. The T-1 data obtained in this study and additional data from earlier electron spin echo, ESR power saturation and electro n nuclear double resonance measurements were analyzed using a modified model. This model assumes that the T-1 process for the AsO44- and SeO 43- type centers involves phonon modulation of hyperfine interactions, and not of the crystalline electric held as assumed in some earlier m odels. The Debye temperatures for all three lattices, as derived from the analysis of the T-1 data utilizing this model, agree with specific heat derived values, which was not the case with the earlier models. The modified model utilizes two modes, one with energy around 90 K and the other around 2000 K. Both of these modes exhibit an H --> D isoto pe effect and the proposed model provides an explanation for this obse rvation too. For KH2PO4 the 90 K mode can be identified with an Einste in mode of energy 86 K, as estimated from low temperature specific hea t data. The 2000 K mode can be identified with a temperature-independe nt OH (or OD) mode of energy 1868 K, following some recent inelastic n eutron scattering data and electronic structure calculations. The resu lts underscore the utility of these probes for studying the local prot on-lattice dynamics and the O-H ... O potential surfaces for the KH2PO 4-type lattices.