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Abstract

The techniques of EPR, ENDOR and double ENDOR were employed with a view to obtaining detailed information on hydrogen-bonding and on the role of protons and heavier nuclei in the phase transitions in the X-irradiated ferroelectrics KH₂AsO₄, KD₂AsO₄, RbH₂AsO₄, mixed KH₂PO₄₋ KH₂AsO₄ and the antiferroelectric compound NH₄H₂AsO₄. The paramagnetic Centre AsO[sup 4-/sub 4], formed by the capture of an electron by an AsO[sup 3-/sub 4] ion, was used as a microscopic probe. EPR investigations, combined with the use of electric fields, have resulted in an accurate interpretation of the As⁷⁵ hyperfine structure in the EPR spectra of AsO[sup 4-/sub 4]. The use of EPR to obtain the dielectric hysterisis loop has been demonstrated for the first time. ENDOR studies of the AsO[sup 4-/sub 4] centre in KH₂AsO₄ at 4.2°K have yielded accurate superhyperfine parameters. The results provide evidence for covalent character in both the O-H….and O….H- parts of the O-H….O bond in such systems. A new method has been proposed for determining the signs of hyperfine couplings for S=1/2 systems and has been illustrated by application to the AsO[sup 4-/sub 4] centre in KH₂AsO₄. Analysis of the temperature dependence of the EPR spectra indicates that both the As⁷⁵ and the protons perform jump-type motions. At lower temperatures, however, the correlation times for the As⁷⁵ and the proton motions are found to be essentially the same. This provides experimental evidence for the recently postulated coupled proton-lattice motion in these systems. A method for performing Electron-Nuclear Triple Resonance experiments has been demonstrated. This development may extend the range of applicability of the ENDOR technique.