Thermal expansion of LaAlO3 and (La,Sr)(Al,Ta)O3, substrate materials for superconducting thin-film device applications

The thermal expansion for the perovskite (La,Sr)(Al,Ta)O-3, i.e., LSAT, grown from the formulation 0.29(LaAlO3):0.35(Sr2AlTaO6), was determined by Rietveld refinement of neutron powder diffraction data over the temperature range of 15-1200 K. In comparison to LaAlO3 the relative volume thermal expansion is the same, although the cell volume of LSAT is slightly larger. Site occupation refinement for LSAT gives a structural formula of (La-0.29(5)Sr-0.71(5)) (A site)(Al-0.65(1)Ta-0.35(1))(B site)O-3. At and below 150 K, LSAT shows a small distortion from cubic symmetry. Unlike the cubic to-rhombohedral transition (800 K) observed in LaAlO3, the low temperature structural phase transition in LSAT appears to be cubic-to-tetragonal or cubic-to-orthorhombic. The rms displacement of the A site in LSAT is significantly larger than that for LaAlO3, and about half of the difference can be accounted for by a static displacement component. (C) 1998 American Institute of Physics.