Effect of uniaxial stress on the unstableA1(TO)phonon in ferroelectric gadolinium molybdate

The effect of uniaxial stress along [100] on the unstable $A_1\left(\mathrm{TO}\right)$ phonon, 47 ${\mathrm{cm}}^{-1\mathrm{}}$ at room temperature, in ferroelectric (<159°C) gadolinium molybdate has been studied by Raman scattering at 138, 141, 146, and 155°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.