Structural Diversity and Incompatibility Induced Complex Phase Formation Behavior in the Stuffed Tridymites Ca1- xSrxGa2O4

Abstract

Stuffed tridymites AM₂O₄ composed of a condensed MO₄-tetrahedra-based framework have been widely investigated due to their structural diversity and rich physical properties. Herein, the strategy of stuffing mixed Ca²⁺ and Sr²⁺ cations into the [Ga₂O₄]^2- framework in (Ca_1-xSr_x)Ga₂O₄ (CSGO, 0 ≤ x ≤ 1) is utilized to manipulate the phase formation behavior with different structure types at particular annealing temperatures. Five derivatives, including α- and β-CaGa₂O₄, β- and γ-SrGa₂O₄, and new CSGO-type structures, were observed. The distinctive feature of the CSGO-structure is the coexistence of UUDDUD- and UDUDUD-type six-membered rings, where U (up) and D (down) denote the orientations of GaO₄-tetrahedra with respect to the plane grids, in a ratio of 2:1. Single-phase α-Ca_1-xSr_xGa₂O₄ (x < 0.2) and γ-Ca_1-xSr_xGa₂O₄ (x > 0.67) could be obtained at low temperatures. Biphasic regions, including α-Ca_1-xSr_xGa₂O₄/CSGO (0.2 ≤ x ≤ 0.67), γ-Ca_1-xSr_xGa₂O₄/CSGO (0.67 < x ≤ 0.8), and β-Ca_1-xSr_xGa₂O₄/CSGO (0.8 < x < 1), were observed at the intermediate temperature region and evolve irreversibly into the CSGO single-phase region upon elevating the temperature. Moreover, the structure-property relationship of the new CSGO-phase was further studied by doping coordination-sensitive Bi³⁺ activators to advance the development and applications of stuffed tridymites.