Bistable Fluctuation of SeO₄³⁻ Centers Doped in Partially Deuterated KH₂PO₄ Observed by ESR

Abstract

Temperature dependence of ⁷⁷Se hyperfine structure of SeO₄³⁻ radicals doped in KH_2(1−x)D_2xPO₄ is measured. It is found that the spectrum-splitting temperature T_t for low-D crystal (x\lesssim0.4) is nearly constant (160 K–170 K) and T_t for high-D one (x\gtrsim0.5) coincides with the respective Curie temperature. This D-dependence of T_t is compatible approximately with the picture of D-independent bistable fluctuation of PO₄. But temperature dependence of the rate of bistable fluctuation of SeO₄³⁻ is found to depend on x. This D-dependence is explained qualitatively by introducing interactions among the PO₄ dipoles mediated by the electric dipoles on the hydrogen bonds, effective moment of which depends on dynamical behavior of the hydrogen nuclei. It is pointed out that the present result is also compatible with both the local freeze-out model and the geometrical isotope effect.