Mechanical properties and domain wall mobility of LaGaO₃ perovskite over a first-order phase transition

La_xNd_1-xGaO₃ perovskites have been studied by dynamic mechanical analysis to relate internal friction to the mechanisms controlling first-order phase transitions in both single crystal and polycrystalline samples, in the presence of an external oscillating stress. Reflected light images of the crystal during the transition provide insight into the mechanism of the transition and the corresponding attenuation and storage modulus. La_xNd_1-xGaO₃ has a disordered perovskite structure. At room temperature the structure is orthorhombic, Pbnm, with a first-order phase transition to rhombohedral, R over(3, ̄) c, occurring on heating. In both polycrystalline and single crystal samples the internal friction can be considered to arise from the motion of the phase interface across the sample. First, internal friction rises as the phase interface moves across the sample, then microstructural reorganisation and relaxation within the sample causes the internal friction to decrease back to a background level. The velocity of the phase interface can be expressed as an exponential of the form V_∞+V₀ exp(-t/C), where V_∞, V₀ and C are dependent on the sample composition, and the force and frequency of applied external stress.