Effect of substitutional impurities (Al,Co,Fe,Ga) on the orthorhombic phase of ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{7\mathrm{-}\mathrm{\delta }}$

Structural changes in ${\mathrm{YBa}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_{7\mathrm{-}\mathrm{\delta }}$ resulting from alloying with Al, Co, Fe, and Ga are studied theoretically. A bond model for the various interatomic interactions yields results indicating that the tetragonal phase seemingly observed by x-ray diffraction experiments for ${\mathrm{YBa}}_2$(${\mathrm{Cu}}_{1-x}$ $M_x$ ${)}_3$ $O_7$+y (M=Al,Co,Fe; x>0.03) consists of differently oriented orthorhombic domains. Doping with Al,Co,Fe and domain formation causes uptake of excess oxygen.