Structural properties of defective (CH₃NH₃)₂Cu(Cl_1-xBr_x)₄ compounds

The crystal structures of (CH₃NH₃)₂Cu(Cl_1-xBr_x)₄ compounds have been investigated by means of synchrotron powder X-ray diffraction and pair distribution function analysis at room temperature. As a result, new insights are gained about the structural properties of these compounds, suggesting a monoclinic symmetry (space group No. 14: P2₁/c - C_{2h}^{5}) induced by the co-operative orbital ordering produced by the Jahn-Teller distortion characterizing the 3d⁹ Cu²⁺ ion. In contrast to previous studies, a significant amount of vacancies is found at halogen positions, a feature that can be likely ascribed to the synthesis technique adopted in the present study. Br atoms preferentially occupy axial positions, likely on account of reduced steric hindrance at these sites.