Determination of the Electric-Field-Gradient Tensor at the Arsenic Site in KH2AsO4by Proton Relaxation Measurements

Crossover contact of protons and arsenic in K${\mathrm{H}}_2$As${\mathrm{O}}_4$ below the ferroelectric transition temperature $T_C=96\mathrm{}$°K makes a significant contribution to the proton spin-lattice relaxation rate. The decrease in proton relaxation time occurs at those values of the applied magnetic field and crystal orientation at which the proton transition frequency is equal to one of the arsenic transition frequencies. It is found that the observed crossover pattern can be reproduced with considerable precision by a calculation based on a model of the electric-field-gradient tensor at the arsenic site which is suggested by the crystal structure in the ordered state. The comparison with experimental data yields a value of the electric quadrupole coupling constant equal to 34.8 Mc/sec at 77°K. These results are difficult to determine by direct observation of the arsenic resonance because only the ½ to -½ transition is observable. The orientation of the principal axes and the value of the asymmetry parameter are described.