87Rb electric field gradient tensors and the symmetry of the incommensurate phase in Rb2ZnBr4and Rb2ZnCl4

Abstract

The form of the Rb electric field gradient tensors observed in the incommensurate phases of Rb₂ZnBr₄ and Rb₂ZnCl₄ suggests that the superspace symmetry group of the incommensurate phase in these systems is P_ss1^Pcmn. The incommensurate phase thus represents a modulation of the structure of the high-temperature phase and not a modulation of the structure of the low-temperature commensurate ferroelectric phase. The angular dependence of the ⁸⁷Rb¹/₂ to -¹/₂ NMR lineshape also suggests the above conclusion.