A Model Calculation of Electric Field Gradients in the Incommensurate Phases of Cs₂HgBr₄ and Cs₂CdBr₄

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Temperature dependence of ⁸¹Br NQR frequencies in the incommensurate phase was measured in Cs₂HgBr₄ and Cs₂CdBr₄. In relation to the analysis of the spectrum, the electric field gradients in the incommensurate phases of both substances were calculated on the basis of a model that rotation of [MBr₄]²⁻ tetrahedron about the a axis occurs and the rotation angle at each site is regularly modulated by incommensurate modulation wave. This model accounted for the temperature dependence of the two NQR lines, i.e., the fact that only two lines are detected in the incommensurate phase.