TEMPERATURE DEPENDENCES OF THE K-39 AND RB-85 QUADRUPOLE COUPLING-CONSTANTS IN PARAELECTRIC KH2PO4, KH2ASO4, RBH2PO4, AND RBH2ASO4

Temperature dependences of K-39 Rb-85 nuclear-quadrupole resonance frequencies have been measured in tetragonal paraelectric phases of KH2PO4, KH2AsO4, RbH2PO4, and RbH2AsO4. The measured quadrupole coupling constants are compared to the quadrupole coupling constants calculated in a point-charge model in which a cation bears a positive charge e whereas the negative charge -e of a PO4- or AsO4- ion is, in a time average, equally shared by the four oxygen atoms. The influence of the anisotropic thermal expansion of the crystal lattices on the temperature dependence of the quadrupole coupling constants is calculated in the same model. Both the calculated and measured quadrupole coupling constants as well as the calculated and measured temperature coefficients of the quadrupole coupling constants agree within the experimental error. Thus the ''antiferroelectric'' Slater configurations of the H2PO4 and of the H2AsO4 groups which seem to have a strong influence on the As-75 NQR frequencies in the tetragonal paraelectric phase are of a local nature and they have nearly no influence on the potassium and rubidium NQR frequencies.