Quadrupolar perturbedN14NMR in the structurally commensurate and incommensurate phases of ammonium tetrachlorozincate

Quadrupolar perturbed ${}_{}{}^{14}{}_{}{}^{}\mathrm{N}$ NMR spectra were studied in the normal, incommensurate and the various commensurate phases of ammonium tetrachlorozincate (${\mathrm{NH}}_4$ ${)}_2$ ${\mathrm{ZnCl}}_4$ (AZC) single crystals, in order to investigate local phenomena related to structural phase transitions. In the high-temperature normal and the commensurate lock-in phase, whose lattice constant is quadrupled along the c direction of the high-temperature structure, the electric-field-gradient (EFG) tensors at the nitrogen sites are determined by the well-known Volkoff formalism. The results are related to the symmetries of these phases. In particular, the space group of the commensurate phase mentioned could be derived, and the distinct influence of the domain structure is seen in the NMR spectra of that phase. The frequency distributions appearing in the incommensurate (IC) phase were studied as a function of both crystal orientation and temperature. The degeneracies and symmetries of the NMR rotation patterns in the incommensurate phase are discussed on the basis of the general symmetry properties of that phase, derived by a symmetry analysis of the phase sequence of AZC. The superspace group of the IC phase is equal to that determined for other ${\mathit{A}}_2$ ${\mathit{BX}}_4$-type crystals like ${\mathrm{Rb}}_2$ ${\mathrm{ZnBr}}_4$ and ${\mathrm{Rb}}_2$ ${\mathrm{ZnCl}}_4$. The data can be fitted quantitatively by taking into account the nonvanishing terms of the lowest order in a general symmetry-adapted Fourier series for the EFG tensor. The temperature dependences of the NMR spectra observed in the incommensurate phase are related to the static critical behavior. It is in particular demonstrated that AZC and the other substances of the ${\mathit{A}}_2$ ${\mathit{BX}}_4$-type mentioned show a universal static critical behavior.