Neutron powder diffraction study of rhombohedral rare-earth aluminates andthe 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 rhombohe dral perovskite structure in space group R (3) over bar c (a = 5.3647(1) An gstrom, 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) Angstro m for NdAlO3). The rhombohedral structure is characterized by rotation of t he 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 t he cubic symmetry of the ideal perovskite. Only for LaAlO3, however. was th e transition at a low enough temperature to unequivocally obtain the cubic phase. For PrAlO3 the transition was closely approached before the sample c an 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 g eneralized mean field approach. Such a description suggests the transitions are continuous, being at 820 K and second order for the transition in LaAl O3, and at 1768 K and tricritical for the transition in PrAlO3. In the prox imity of the phase transition, the octahedral compression varies with the s quare of the rotation angle, though this description is inadequate remote f rom the transition, and the constant of proportionality is different for th e different compounds.