Doubly Ordered Stuffed Tridymite CaBaZn2Ge2O8: Structural Dimensionality Reduction via A-Site-Layered Ordering

Abstract

Cationic ordering is a powerful strategy for tuning crystal structure and physical properties in complex oxides, yet its pivotal role in stuffed tridymites remains unexplored, particularly concurrent A- and B-site ordering. Here, we report the rational design of CaBaZn2Ge2O8, a doubly ordered A’A″B′2B″2O8-type-stuffed tridymite, stabilized by introducing large size mismatches at both A- and B-sites. High-resolution neutron powder diffraction (NPD), three-dimensional electron diffraction, and neutron pair distribution function (nPDF) analyses reveal a centrosymmetric P̅3-structure with layered A-site Ca2+/Ba2+ ordering and B-site alternatively ordered Zn2+/Ge4+ arrangements along the six-membered rings (SMRs). Remarkably, this structure possesses a distinctive UUUUUU/DDDDDD SMR topology, where the cooperative A- and B-site ordering transforms the conventional three-dimensional-stuffed tridymite framework into two-dimensional [Zn2Ge2O8]4– layers, confirmed at the atomic scale by aberration-corrected scanning transmission electron microscopy. Theoretical calculations reveal that CaBaZn2Ge2O8 has a direct band gap value of ∼4.5 eV; however, the reduced structural dimensionality enables efficient separation of photogenerated charge carriers, thereby yielding an enhanced photocatalytic H2 evolution rate compared to the three-dimensional tetrahedral framework-stuffed tridymite BaZnGeO4. These findings demonstrate that double cation ordering can fundamentally tailor the framework topology and dimensionality, providing a viable strategy for accessing tetrahedral framework-derived oxides with intriguing functionalities.