Lattice-strain versus dynamic effects in electron spin resonance studies of Rb_1-x(NH₄)_xH₂PO₄

A methodology for investigating dynamics of various spin probes in mixed/glassy systems of the KH₂PO₄ type is developed. Using this methodology, electron spin resonance (ESR) features of the AsO₄^4- centre in the paraelectric phase of Rb_1-x(NH₄)_xH₂PO₄ are shown to arise from strain-broadening and motional-broadening effects. These conclusions are in disagreement with those of a recent study, which implicated the various features observed in the existence of ferroelectric-antiferroelectric fluctuations in Rb_1-x(NH₄)_xH₂PO₄. It is additionally shown that hyperfine couplings from Rb isotopes make the dominant contribution to the ESR spectra in Rb-containing compounds.