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

The thermal expansion for the perovskite ${\mathrm{(La,Sr)(Al,Ta)O}}_{\mathrm{3}},$ i.e., LSAT, grown from the formulation ${\mathrm{0.29(LaAlO}}_{\mathrm{3}}{\mathrm{):0.35(Sr}}_{\mathrm{2}}{\mathrm{AlTaO}}_{\mathrm{6}}\mathrm{)},$ was determined by Rietveld refinement of neutron powder diffraction data over the temperature range of 15–1200 K. In comparison to ${\mathrm{LaAlO}}_{\mathrm{3}}$ 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 ${\mathrm{(La}}_{\mathrm{0.29(5)}}{\mathrm{Sr}}_{\mathrm{0.71(5)}}{\mathrm{)}}_{\mathrm{A\hspace{0.5em}site}}{\mathrm{(Al}}_{\mathrm{0.65(1)}}{\mathrm{Ta}}_{\mathrm{0.35(1)}}{\mathrm{)}}_{\mathrm{B\hspace{0.5em}site}}{\mathrm{O}}_{\mathrm{3}}.$ At and below 150 K, LSAT shows a small distortion from cubic symmetry. Unlike the cubic-to-rhombohedral transition (800 K) observed in ${\mathrm{LaAlO}}_{\mathrm{3}},$ 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 ${\mathrm{LaAlO}}_{\mathrm{3}},$ and about half of the difference can be accounted for by a static displacement component.