Electron Paramagnetic Resonance Studies of X-Irradiated KH2AsO4, KD2AsO4, RbH2AsO4, RbD2AsO4, CsH2AsO4, NH4H2AsO4, and ND4D2AsO4 (Ferroelectrics and Antiferroelectrics)

Electron paramagnetic resonance (EPR) studies of defect centers produced by x or γ irradiation of the hydrogen‐bonded ferroelectrics KH₂AsO₄, KD₂AsO₄, RbH₂AsO₄, RbD₂AsO₄, and CsH₂AsO₄ and of the antiferroelectrics NH₄H₂AsO₄ and ND₄D₂AsO₄, are presented. The spectra, observed over 300–4.2°K, are characterized by a very large hyperfine interaction of the unpaired electron with the ${}^{75}\mathrm{As}\mathrm{\hspace{0.17em}(I=}\frac{3}{2})$ nucleus, and exhibit orthorhombic symmetry in general. The analysis thus required an exact diagonalization of the spin Hamiltonian. Such a procedure, coupled with the use of the method of least‐squares adjustment, has resulted in accurate values for the hyperfine parameters describing the observed spectra, both for single crystal and powder samples. A comparison of the hyperfine parameters with those reported previously for the ${\mathrm{AsO}}_4^{\text{4−}}$ in KH₂AsO₄ has enabled us to identify lines from this center in all the above crystals. The parameters are found to be sensitive to change of the cation and also to a variation of the sample temperature. This indicates that EPR can be used as a microscopic probe for studying hydrogen bonding and the proton‐lattice motion in these compounds. An observed hydrogen‐deuterium isotope effect on the hyperfine parameters has also been discussed. Absence of proton superhyperfine for ${\mathrm{AsO}}_4^{\text{4−}}$ in RbH₂AsO₄, and CsH₂AsO₄ has been discussed in the light of preliminary ENDOR experiments on RbH₂AsO₄. Accurate values have also been obtained for the spin Hamiltonian parameters of several other centers which show fairly similar spectra, for both the high and low temperature phases. Here also the ⁷⁵As hyperfine parameters show isotope effects, both for the change of the cation and for the substitution of D in place of H. All these centers are assigned to a family of ${\mathrm{AsO}}_3^{\text{2−}}$ centers. A previously reported proton superhyperfine structure, which cast strong doubt on the identification of these centers, was not observed. This lends further support to our assignment. Possible mechanisms for the formation of these centers are discussed, but no definite answer could be obtained through these studies alone.