EPR studies of x-ray irradiated KH2AsO4 and KH2PO4-type ferroelectrics and antiferroelectrics

EPR has been used to characterize the hyperfine couplings and electronic structure of a new paramagnetic center (H2AsO4)2− in x-irradiated ferroelectric crystals of pure KH2AsO4 and mixed KH2PO4–KH2AsO4, and antiferroelectric mixed crystals of NH4H2PO4–NH4H2AsO4, NH4H2PO4–KH2AsO4, and ND4D2PO4–KD2AsO4. This center is formed by the capture of an electron by proton deficient AsO3−4 units in these crystals. The assignment of the center is supported by EPR studies on the temperature dependence of the proton superhyperfine structure of these crystals. The present study also corrects an earlier identification of this center in NH4H2PO4–KH2AsO4 as a hole-type ionic defect center AsO2−4. Our results also provide a direct experimental basis for the currently accepted model of electrical (protonic) conduction in these crystals and help to resolve an earlier controversy regarding conductivity measurements on sulfate-doped ammonium and potassium salts. In conjunction with x-ray photoelectron spectroscopic studies, the EPR results on doped crystals also provide interesting results on electron-capture processes in single crystals. The present work also provides an explanation of some anomalous trends of the hyperfine coupling variation in other XO4-type radicals.