Diffuse x-ray scattering study of an oxygen-disordered tetragonal YBa_{2}(Cu_{0.955}Al_{0.045})_{3}O_{7} crystal

X-ray scattering from a tetragonal single crystal of ${\mathrm{YBa}}_2$(${\mathrm{Cu}}_{0.955}$ ${\mathrm{Al}}_{0.045}$ ${)}_3$ ${\mathrm{O}}_7$ shows diffuse streaking in [110] directions about the Bragg peaks. Through a quantitative calculation using a coupled concentration wave, static displacement wave approach, we show that this is essentially attributable to the shear displacement field produced by a disordered oxygen array on the Cu(1)-O ‘‘chain’’ plane. We observe pronounced short-range chain order (local orthorhombic fluctuations) whose correlation range is considerably greater than the superconducting coherence length. This suggests that on a scale relevant for superconductivity the (local) orthorhombicity remains important in this tetragonal structure.