Stabilization of the Trigonal Langasite Structure in Ca3Ga2–2xZnxGe4+xO14 (0 ≤ x ≤ 1) with Partial Ordering of Three Isoelectronic Cations Characterized by a Multitechnique Approach

Crystallization of oxide glasses rich in Zn 2+ , Ga 3+ , and Ge 4+ is of interest for the synthesis of new transparent ceramics. In this context, we report the identification and detailed structural characterization of a new solid solution Ca 3 Ga 2-2 x Zn x Ge 4+ x O 14 (0 ≤ x ≤ 1). These compounds adopt the trigonal langasite structure type, offering three possible crystallographic sites for the coordination of isoelectronic Zn 2+ , Ga 3+ , and Ge 4+ . We used neutron diffraction to determine distributions of Ga 3+ /Ge 4+ and Zn 2+ /Ge 4+ in the simpler end members Ca 3 Ga 2 Ge 4 O 14 and Ca 3 ZnGe 5 O 14 , while for the complex intermediate member Ca 3 GaZn 0.5 Ge 4.5 O 14 , we used an original approach combining quantitative 2D analysis of atomic-resolution STEM-EDS maps with neutron diffraction. This revealed that, across the solid solution, the tetrahedral D sites remain fully occupied by Ge 4+ , while Zn 2+ , Ga 3+ , and the remaining Ge 4+ are shared between octahedral B- and tetrahedral C sites in proportions that depend upon their relative ionic radii. The adoption of the trigonal langasite structure by glass-crystallized Ca 3 ZnGe 5 O 14 , a compound that was previously observed only in a distorted monoclinic langasite polymorph, is attributed to substantial disorder between Zn 2+ and Ge 4+ over the B and C sites. The quantitative 2D refinement of atomic-resolution STEM-EDS maps is applicable to a wide range of materials where multiple cations with poor scattering contrast are distributed over different crystallographic sites in a crystal structure.