Structural phase transitions in Pr_(1-x)La_(x)AlO₃: Heat capacity and x-ray scattering studies

Structural phase transitions in Pr$_{1-x}$La$_x$AlO$_3$ (0 $\leq x\leq$ 1) single crystals have been studied through heat capacity and high-resolution x-ray scattering measurements. For PrAlO$_3$, the heat capacity shows a sharp first-order peak at the rhombohedral to orthorhombic transition, while a classical mean-field anomaly is observed at the orthorhombic to monoclinic transition at lower temperatures. The transition temperature and the heat capacity anomaly of the two transitions diminish with increasing $x$, and only a single rhombohedral to monoclinic transition is observed for $x = 0.8$. Although this latter transition is required by group theory to be first order, no such evidence is found in the heat capacity and x-ray scattering measurements. Instead, the results are consistent with mean-field criticality, with a small distribution of transition temperature originating from weak disorder.