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Yeelimite, Ca4[Al6O12]SO4, is outstanding as an aluminate sodalite, being the framework of these type of materials flexible and dependent on ion sizes and anion ordering/disordering. On the other hand, yeelimite is also important from an applied perspective as it is the most important phase in calcium sulfoaluminate cements. However, its crystal structure is not well studied. Here, we characterize the room temperature crystal structure of stoichiometric yeelimite through joint Rietveld refinement using neutron and X-ray powder diffraction data coupled with chemical soft-constraints. Our structural study shows that yeelimite has a lower symmetry than that of the previously reported tetragonal system, which we establish to likely be the acentric orthorhombic space group Pcc2, with a √2a × √2a × a superstructure based on the cubic sodalite structure. Final unit cell values were a = 13.0356(7) Å, b = 13.0350(7) Å, and c = 9.1677(2) Å. We determine several structures using density functional theory calculations, with the lowest energy structure being Pcc2 in agreement with our experimental result. Yeelimite undergoes a reversible phase transition to a higher-symmetry phase which has been characterized to occur at 470 C by thermodiffractometry. The higher-symmetry phase is likely cubic or pseudocubic possessing an incommensurate superstructure, as suggested by our theoretical calculations which show a phase transition from an orthorhombic to a tetragonal structure. Our theoretical study also predicts a pressure-induced phase transition to a cubic structure of space group I43m. Finally, we show that our reported crystal structure of yeelimite enables better mineralogical phase analysis of commercial calcium sulfoaluminate cements, as shown by RF values for this phase, 6.9% and 4.8% for the previously published orthorhombic structure and for the one reported in this study, respectively. © 2013 American Chemical Society.The work at UMA was funded by MAT2010-16213 research grant (Spain) which is cofunded by FEDER. The work at the DIPC was supported by the Basque Departamento de Educación and the UPV/EHU (Grant No. IT-366-07), the Spanish Ministerio de Innovación, Ciencia y Tecnología (Grant Nos. TEC2007-68065-C03-03 and FIS2010-19609-C02-02), and the ETORTEK research program (NANO-IKER Grant No. IE11-304) funded by the Basque Departamento de Industria and the Diputación Foral de Guipuzcoa.Peer Reviewe

Structure, Atomistic Simulations, and Phase Transition of Stoichiometric Yeelimite