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

The thermal expansion for the perovskite $(La,Sr)(Al,Ta)O3,$ 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 $(La0.29(5)Sr0.71(5))A site(Al0.65(1)Ta0.35(1))B siteO3.$ 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.