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 (La_0.29(5)Sr_0.71(5))A site (Al_0.65(1)Ta_0.35(1))_B site O₃. 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.