Effect of Uniaxial Stress on Unstable A1(To) Phonon in Ferroelectric Gadolinium Molybdate

The effect of uniaxial stress along [100] on the unstable ${A}_{1}(\mathrm{TO})$ phonon, 47 ${\mathrm{cm}}^{\ensuremath{-}1}$ at room temperature, in ferroelectric (159\ifmmode^\circ\else\textdegree\fi{}C) gadolinium molybdate has been studied by Raman scattering at 138, 141, 146, and 155\ifmmode^\circ\else\textdegree\fi{}C. An increase in peak frequency and narrowing of the line shape is observed with increasing stress. The data are analyzed by fitting to a Lorentzian line shape. The force-constant parameter is insensitive to both stress and temperature, whereas the damping constant decreases nonlinearly with stress, approaching an asymptotic value at high stress.