Abstract
A Raman-scattering study of the optical coupled soft mode in ${R}_{b}{\mathrm{H}}_{2}{\mathrm{AsO}}_{4}$ is reported. It was found that in the temperature range 110--220 K the soft mode frequency has the behavior ${\ensuremath{\omega}}_{1}^{2}=29.5{\mathrm{cm}}^{\mathrm{\ensuremath{-}}2}/\mathrm{K}(T\ensuremath{-}91$ K). Therefore, there is a large gap between the temperature at which \ensuremath{\omega} extrapolates to zero (91 K) and the transition temperature of the mechanically free crystal ${(T}_{c}=110$ K). We predict that 91 K is the transition temperature of the mechanically clamped crystal, not yet measured, and that the large gap between these temperatures is the result of unusually large coupling between the optical soft mode and the shear $\mathrm{xy}$ acoustic mode.
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