Stabilization of the Trigonal Langasite Structure in Ca₃Ga_2–2xZn_xGe_4+xO₁₄ (0 ≤ x ≤ 1) with Partial Ordering of Three Isoelectronic Cations Characterized by a Multitechnique Approach

Crystallization of oxide glasses rich in Zn²⁺, Ga³⁺, and Ge⁴⁺ 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 Ca₃Ga_2–2xZn_xGe_4+xO₁₄ (0 ≤ x ≤ 1). These compounds adopt the trigonal langasite structure type, offering three possible crystallographic sites for the coordination of isoelectronic Zn²⁺, Ga³⁺, and Ge⁴⁺. We used neutron diffraction to determine distributions of Ga³⁺/Ge⁴⁺ and Zn²⁺/Ge⁴⁺ in the simpler end members Ca₃Ga₂Ge₄O₁₄ and Ca₃ZnGe₅O₁₄, while for the complex intermediate member Ca₃GaZn_0.5Ge_4.5O₁₄, 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 Ge⁴⁺, while Zn²⁺, Ga³⁺, and the remaining Ge⁴⁺ 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 Ca₃ZnGe₅O₁₄, a compound that was previously observed only in a distorted monoclinic langasite polymorph, is attributed to substantial disorder between Zn²⁺ and Ge⁴⁺ 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.