Neutron powder diffraction study of rhombohedral rare-earth aluminates and the rhombohedral to cubic phase transition

Neutron powder diffraction has been used to examine the structural changes of the rare-earth aluminates LaAlO3, PrAlO3 and NdAlO3 over a wide range of temperatures. At room temperature, all three aluminates adopt the rhombohedral perovskite structure in space group R (3) over bar c (a = 5.3647(1) Angstrom, c = 13.1114(3) Angstrom for LaAlO3, a = 5.3337(2) Angstrom, c = 12.9842(4) Angstrom for PrAlO3, a = 5.3223(2) Angstrom, c = 12.9292(5) Angstrom for NdAlO3). The rhombohedral structure is characterized by rotation of the oxygen atom octahedra about the threefold axis, and compression of these octahedra parallel to the same axis. As the temperature is increased, the rotation angle and the compression decrease, indicative of an approach to the cubic symmetry of the ideal perovskite. Only for LaAlO3, however. was the transition at a low enough temperature to unequivocally obtain the cubic phase. For PrAlO3 the transition was closely approached before the sample can failed, but for NdAlO3 the transition appeared to be inaccessible within the available temperature range. The rotation angle is taken to represent the order parameter, and its temperature variation is well described by a generalized mean field approach. Such a description suggests the transitions are continuous, being at 820 K and second order for the transition in LaAlO3, and at 1768 K and tricritical for the transition in PrAlO3. In the proximity of the phase transition, the octahedral compression varies with the square of the rotation angle, though this description is inadequate remote from the transition, and the constant of proportionality is different for the different compounds.