Static quadrupolar perturbed NMR in structurally incommensurate systems. II. 87Rb satellite transitions in Rb2ZnBr4.

Abstract

The $^{87}\mathrm{Rb}$ NMR m=\ifmmode\pm\else\textpm\fi{}(1/2\ensuremath{\leftrightarrows}m=\ifmmode\pm\else\textpm\fi{} 3) / 2 satellite transitions are studied in a detailed manner in the normal, incommensurate, and commensurate phases of high-quality ${\mathrm{Rb}}_{2}$${\mathrm{ZnBr}}_{4}$ single crystals. By these means, an experimental method is realized for investigating local phenomena related to structural phase transitions which is considerably more direct and sensitive than the m=+(1/2\ensuremath{\leftrightarrows}m=- 1) / 2 central transition, which, so far, has been used nearly exclusively in NMR studies of ${\mathrm{Rb}}_{2}$${\mathrm{ZnBr}}_{4}$ and its isomorphous. In the normal phase the electric-field-gradient tensor (EFG) at the Rb sites is determined by the well-known Volkoff method. The widths of the frequency distributions appearing in the incommensurate phase, which generally show some centers of intensity and, particularly, edge singularities, depend strongly on both the temperature and crystal orientation. The temperature dependence is measured for special crystal orientations, where one crystallographic axis is parallel to the direction of the static magnetic field, while for special temperatures, for which the plane-wave limit holds, the orientational dependence is investigated in detail. On passing to the low-temperature commensurate phase, the spectra of the incommensurate phase merge into the discrete lines corresponding to the structure of that phase. The degeneracies and symmetries of the NMR rotation patterns in the incommensurate phase are discussed without any special model on the basis of the general symmetry properties of that phase as described in the preceding work. The data can be fitted quantitatively by a general Fourier series for the EFG. On the contrary, the ``local'' model, which previously has often been applied, fails to describe both the orientational and temperature dependences of the NMR spectra. The observed temperature dependences are related to the temperature dependence of the order parameter of the incommensurate phase. The present results exclude the strong floating of the modulation waves reported previously in another work.