Rb87electric-field-gradient tensors in incommensurateRb2ZnCl4

From the angular dependence of the edge singularities in the second-order quadrupole-shift spectra of the ${}_{}{}^{87}{}_{}{}^{}\mathrm{Rb}$ NMR ½ → -½ transitions, the distribution of the electric-field-gradient (EFG) tensors at the Rb sites has been determined in the incommensurate ($I$) phase of ${\mathrm{Rb}}_2$Zh${\mathrm{Cl}}_4$. The EFG tensor elements $V_{\mathrm{ab}}$ and $V_{\mathrm{bz}}$—which are zero in the paraelectric phase in view of the $a-c$ mirror symmetry—become nonzero in the $I$ phase and increase with decreasing temperature linearly with the amplitude of the incommensurate distortion wave. The displacement amplitudes are larger at the Rb(2) than at the Rb(1) sites. The plane-wave-modulation model provides a rather good description of the distribution of the Rb EFG tensors in the high-temperature part of the $I$ phase in ${\mathrm{Rb}}_2$Zn${\mathrm{Cl}}_4$.