Dynamical studies of CsH₂AsO₄ using EPR lineshapes, ENDOR results and theoretical calculations

The EPR spectra of the AsO₄^4- centre in CsH₂AsO₄ are overwhelmed by inhomogeneous broadening due to superhyperfine (SHF) interactions predominantly with one caesium and two protons but with small contributions from many other nuclei. The temperature dependence of these spectra shows marked changes in the temperature range 250-320 K and thus far has not been explained. Using a model which involves SHF coupling of the AsO₄^4- centre with two caesiums, two close protons and two far protons we have performed calculations based on the modified Bloch equations to simulate the EPR spectra. The ENDOR of ¹³³Cs and ¹H was also performed to obtain the necessary ingredients for computer simulations. Motional correlation times for the fluctuations involving relative motions of ¹³³Cs, ⁷⁵As and ¹H nuclei have thus been obtained in the temperature range 260-320 K. The near equality of these motional times which bears strong similarity to the soft-mode motions but on a different time scale is suggested and discussed.