Structural effects of hydrostatic pressure in orthorhombic ${\mathrm{La}}_{2\mathrm{-}\mathit{x}}$${\mathrm{Sr}}_{\mathit{x}}$${\mathrm{CuO}}_4$

We have investigated the effect of hydrostatic pressure on the structure of orthorhombic ${\mathrm{La}}_{2\mathrm{-}\mathit{x}}$ ${\mathrm{Sr}}_{\mathit{x}}$ ${\mathrm{CuO}}_4$ between 1 atm and 0.6 GPa, using neutron powder diffraction. Increasing pressure causes the tilt angle of ${\mathrm{CuO}}_6$ octahedra to decrease, leading to a transition to tetragonal symmetry. There is a strong compressibility anisotropy for the orthorhombic structure. Order parameters for the transition versus pressure are consistent with Landau theory for a second-order transition. It is shown that this transition can be universally scaled over (x,T,P) space in the context of Landau theory. We conclude that ${\mathit{T}}_{\mathit{c}}$ varies inversely with the tilt angle and is maximum for the tetragonal structure, i.e., for flat and square ${\mathrm{CuO}}_2$ planes, for this compound.