Pseudocubic Crystal Structure and Phase Transition in Doped Ye'elimite

Sodalites are tridimensional alumino-silicate materials containing cages where loosely bonded anions are located. Yeelimite, Ca-4[Al6O12]SO4, is outstanding as an aluminate sodalite with a flexible framework accepting several type of dopants with important structural consequences. Moreover, yeelimite is also important from an applied perspective as it is the most relevant phase in calcium sulfoaluminate cements. The crystal structure of stoichiometric yeelimite has recently been unraveled, but the structure of dopant-containing yeelimite, which is present in cements, is not well studied. Here, we report the pseudocubic crystal structure of doped yeelimite, Ca3.8Na0.2Al5.6Fe0.2Si0.2O12SO4, from high-resolution synchrotron powder diffraction data. The powder pattern is indexed with a cubic cell, and a structural model is reported based on the I (4) over bar (3)m space group. However, this compound displays diffraction peak narrowing on heating. Furthermore, some high-angle split peaks become a single peak on heating, and a phase transition is measured at 525 degrees C. Therefore, it is concluded that the crystal structure at room temperature has a lower symmetry, although it can be described as cubic. The structural study at 800 degrees C suggests a truly cubic structure, and we speculate that this phase transition, on heating, is likely related to the dynamical disordering of the sulfate anions. Finally it is concluded that the high temperature cubic state was not quenchable to ambient, even when the tested chemical substituents are introduced into the structure.