Section 3.1.5.2.4. Lanthanum aluminate, LaAlO₃

A structural distortion related to that in strontium titanate is exhibited in lanthanum aluminate at approximately 840 K. As in strontium titanate, the distortion consists primarily of a nearly rigid rotation of oxygen octahedra. However, in the lanthanide aluminates (including NdAlO₃ and PrAlO₃) the rotation is about the [111] body diagonal(s) of the prototype cubic structure. The rotation, shown in Fig. 3.1.5.8, is out-of-phase in adjacent cubic unit cells, analogous to that in strontium titanate.

Figure 3.1.5.8 | top | pdf | (a) Structure of lanthanum aluminate above (undistorted) and below (arrows) its cubic–rhombohedral phase transition near 840 K. As in strontium titanate (Figs. 3.1.5.4–3.1.5.7), there is a nearly rigid rotation of oxygen octahedra (the oxygen ions actually remain on the cube faces); however, in the lanthanide aluminates (Ln = La, Pr, Nd) the rotation is about a cube [111] body diagonal, so that the resulting structure is rhombohedral, rather than tetragonal. The primitive unit cell doubles along the cubic [111] axis; domains will form with the unique axis along all originally equivalent body diagonals of the cubic lattice. (b) Optical phonon frequences versus temperature in lanthanum aluminate.

Historically, this phase transition and indeed the structure of lanthanum aluminate were incorrectly characterized by X-ray crystallography (Geller & Bala, 1956) and correctly assigned by Scott (1969) and Scott & Remeika (1970) via Raman spectroscopy. The causes were as in the case of strontium titanate, namely that it is difficult to assess small, nearly rigid rotations of light ions in twinned specimens. In the case of lanthanum aluminate, Geller and Bala incorrectly determined the space group to be (), rather than the correct () shown in Fig. 3.1.5.8, and they had the size of the primitive unit cell as one formula group rather than two.