Modulation of two ye'elimite phases via Ga3+ cation substitution

Abstract

The space groups of the crystal structure of ye'elimite (Ca₄Al₆SO₁₆) include the space groups of orthogonal Pcc2 and cubic I3m, and these structures are room-temperature stable and high-temperature metastable phases, respectively. In this study, the phase transition process from Pcc2 to I3m is adjusted by Ga³⁺ cation substitution, and the change in Al/GaO₄ tetrahedra and the derived influence for adjacent Ca²⁺ ions are surveyed by structural refinements. The obtained results indicate that when the Ga³⁺-doped concentration (x) is below and above 10 at%, the introduction of Ga³⁺ ions mainly influences the crystal structures of the Pcc2 and I3m phases, respectively. Moreover, when x is above 14%, the pure unstable I3m phase can be stabilized at room temperature, and this is verified by its all-extinction phenomenon observed under a polarizing microscope, the absence of structural changes in the temperature-dependent XRD survey and the lack of endothermic effect in the DSC curves. In addition, due to the different coordination environments, the Eu³⁺ fluorescence probe also demonstrates pronounced difference in the crystal field splitting between the two phases of Ca₄Al₆SO₁₆. The research discussed in this study provides a useful platform for adjusting the phase transformation of Ca₄Al₆SO₁₆, and it also shows some guidelines for analyzing the phase transition between the two polymorphs in ye'elimite-containing materials by combining structural refinement and common optical characterizations such as polarizing microscopy and photoluminescence.