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-irradiated KH2AsO4, KH2PO4 and KD2PO4. Experimental studies included the measurement of T-1 for these systems by utilizing the double modulation ESR technique. The T-1 data obtained in this study and additional data from earlier electron spin echo, ESR power saturation and electron nuclear double resonance measurements were analyzed using a modified model. This model assumes that the T-1 process for the AsO44- and SeO43- type centers involves phonon modulation of hyperfine interactions, and not of the crystalline electric field as assumed in some earlier models. 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 isotope effect and the proposed model provides an explanation for this observation too. For KH2PO4 the 90 K mode can be identified with an Einstein mode of energy 86 K, as estimated from low temperature specific heat data. The 2000 K mode can be identified with a temperature-independent OH (or OD) mode of energy 1868 K, following some recent inelastic neutron scattering data and electronic structure calculations. The results underscore the utility of these probes for studying the local proton-lattice dynamics and the O-H ... O potential surfaces for the KH2PO4-type lattices.