Structure, Atomistic Simulations, and Phase Transition of Stoichiometric Yeelimite

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.