Stuffed Tridymites A′A″Ga₄O₈ (A′ = Ba, Sr, Ca; A″ = Pb): Integrating Stereochemically Active Lone Pairs with A-Site Ordering to Design a Polar Structure Featuring Distinctive Ring Topology

Archetype stuffed tridymites AM₂O₄ typically exhibit a polar P622-structure, characterized by MO₄-tetrahedra arranged in a UDUDUD ring topology, where U (up) and D (down) denote the orientation of the MO₄-tetrahedra with respect to the plane grids. However, most stuffed tridymites adopt centrosymmetric structures due to the size mismatch between the A-cation and the cavity of the [M₂O₄]ⁿ⁻ anionic framework, which alters the ring topology and results in centrosymmetry. Therefore, innovative strategies are pressingly needed to design polar stuffed tridymites with alternative ring topologies to facilitate fine manipulation of structure-driven functionalities. To this end, we rationally designed A′A″Ga₄O₈ (A′ = Ba, Sr, Ca; A″ = Pb) by employing A-site ordering and a stereochemically active Pb²⁺ lone pair to achieve polar structures. Crystal structure solution revealed that A′PbGa₄O₈ possesses 2-, 4-, and 6-fold stuffed tridymite-type superstructures for A′ = Ba, Ca, and Sr, respectively. We found that the differences in ionic radius and chemical bonding of the A-site cations, together with the critical size of A′, significantly influence the crystal structures. Specifically, A′PbGa₄O₈ (A′ = Ba and Ca), which exhibit substantial differences in both chemical bonding and ionic radius, possess A-site 1:1 layered ordering and a UUUDDD ring topology. In contrast, SrPbGa₄O₈, which has only a difference in chemical bonding, shows a partial 2:1 columnar ordering and features 2:1 ordered UUDDUD/UDUDUD rings. Notably, BaPbGa₄O₈ is the first polar Pmc2₁-superstructure with a distinctive UUUDDD ring topology, attributed to the larger A′-cation size that inhibits the GaO₄-tetrahedral orientational reversal and thus breaking the centrosymmetry, as compared to the centrosymmetric Pbcm-CaPbGa₄O₈. Moreover, the optical properties, second harmonic generation (SHG) properties, and chemical bonding characteristics of BaPbGa₄O₈ were also investigated thoroughly. This work enriched the structural diversity of acentric stuffed tridymites and broadened avenues for designing polar stuffed tridymite variants by manipulating the ring topology.