Crystal structure of the high-temperature superconductor ${\mathrm{La}}_{1.85}$${\mathrm{Sr}}_{0.15}$${\mathrm{CuO}}_4$ above and below $T_c$

The crystal structure of ${\mathrm{La}}_{1.85}$${\mathrm{Sr}}_{0.15}$${\mathrm{CuO}}_4$ has been determined at 300, 60, and 10 K by neutron diffraction powder profile analysis. The structure is of the tetragonal ${\mathrm{K}}_2$${\mathrm{NiF}}_4$ type at ambient temperature, but undergoes an orthorhombic distortion near 200 K which buckles the copper-oxygen planes. The crystal structure of the orthorhombic phase is not significantly different above and below $T_c$. The copper-oxygen coordination polyhedron changes shape in a subtle manner at the tetragonal-to-orthorhombic phase transition, but is unchanged at $T_c$.