(Cu_xZn_1-x)_0.456In_1.084Ge_0.46O₃ (0 ≤ x ≤ 1): A complex, ordered, anion-deficient fluorite with unusual site-specific cation mixing

The anion-deficient fluorite-related family of materials exhibits a number of commercially useful properties arising from the specific arrangement of anion vacancies in each structure. One recently reported member, Zn_0.456In_1.084Ge_0.46O₃, is the only known example with its particular complex structure in which cation coordinations span one 4-coordinate (4b), two 6-coordinate (8e and 16f), and one 8-coordinate (4a) site. A new, complete, solid solution (Cu_xZn_1-x)_0.456In_1.084Ge_0.46O₃, (0 ≤ x ≤ 1) has been discovered via the isovalent substitution of Cu²⁺ for Zn²⁺, significantly expanding the known phase space of this structure. Synchrotron X-ray data confirm the ZIGO structure over the entire composition range. Inclusion of Cu in the structure is found to occur exclusively at the 16f site, increasing the number of cations mixed on that site from three to four, while all others remain singly occupied, including the other 6-coordinate (8e) position. Furthermore, transmission electron microscopy investigations show no evidence of long-range cation ordering. Thus, disorder on the 16f site appears remarkably favorable in this otherwise highly ordered structure. Nonideal trends in the lattice parameters and diffuse reflectance spectra as a function of Cu content are examined. Further implications of the mixed order and disorder in the solid solution for materials discovery and possible properties of interest are briefly discussed.