Electron paramagnetic resonance and electron nuclear double resonance of the AsO<SUB>4</SUB><SUP>4-</SUP> centre in X-irradiated KH<SUB>2</SUB>AsO<SUB>4</SUB>

Electron paramagnetic resonance studies of the AsO44- centre in X-irradiated crystals of KH2AsO4 in the ferroelectric phase at 77 and 4·2 K are reported. The symmetry of the spin-hamiltonian has been found to be orthorhombic, and the g tensor and the A tensor describing the interaction of the unpaired electron with the arsenic nucleus (I=3/2) have been obtained. Domain splitting has been observed in he spectra recorded in the ab plane of the crystal. By studying these spectra in the presence of an applied electric field, it has been possible to plot the hysteresis curve of ferroelectric KH2AsO4. Electron-nuclear double resonance (ENDOR) of protons surrounding the AsO44- units has been studied at 4·2 K. Two sets of ENDOR lines have been found arising from the protons in the two equilibrium positions (labelled 'close' and 'far') along the hydrogen bonds linking the AsO4 tetrahedra. The angular variation of the ENDOR lines from both 'close' and 'far' protons has been plotted in the crystal symmetry planes. The observed ENDOR frequencies have been fitted to those calculated from the numerical diagonalization of the Hamiltonian. The superhyperfine parameters for the 'close' and 'far' protons thus obtained are found to be quite anisotropic. The ENDOR results are shown to explain all details of the partially-resolved proton superhyperfine structure at room temperature as well as at low temperatures. An isotropic contact hyperfine coupling of -2·875 MHz of the unpaired electron to the proton in the 'far' position of the hydrogen bond has been determined, providing direct evidence for covalency in the hydrogen bonding in KH2AsO4 crystals