NMR determination of order parameters in the quadrupolar glasses Na(CN)xCl1−xandNaxK1−xCN

Abstract

It is shown that quadrupolar perturbed nuclear magnetic resonance (NMR) is a powerful method to investigate the quadrupolar glasses Na(CN${)}_{\mathit{x}}$${\mathrm{Cl}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$ and ${\mathrm{Na}}_{\mathit{x}}$${\mathrm{K}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$CN. In both systems at the sodium and chlorine sites, distributions of electric-field-gradient tensors occur which are restricted by the fact that the average structure of the systems under investigation is cubic. Correspondingly, inhomogeneous distributions of NMR lines result, which for I=3/2 nuclear-spin systems consist of inhomogeneously broadened central lines and broad distributions of satellite lines. Measurements of these frequency distributions and their dependences on the composition, the orientation, and the temperature of the samples are presented. The widths of the electric-field-gradient-tensor distributions are related in a general quadrupolar glass model to the quadrupolar Edwards-Anderson order parameter ${\mathit{q}}_{\mathrm{EA}}$. As a consequence, the temperature dependence of ${\mathit{q}}_{\mathrm{EA}}$ is derived, reflecting the random orientational freeze-out of the CN quadrupoles with decreasing temperature. By interpreting the results in terms of theoretical models, it is shown that in the mixed cyanides we deal with a smearing of a collective quadrupolar glass transition by weak random fields and not with a pure random-field-type freezing or a pure random-bond-type glass transition. The results are compared to those obtained from other experimental methods. In particular, the critical elastic behavior of these systems is discussed in a general context.