Tricalcium silicate Ca₃SiO₅ superstructure analysis: A route towards the structure of the M₁ polymorph

In this paper, we present a structural model for the M₁ polymorph of tricalcium silicate Ca₃SiO₅ from Powder X-Ray Diffraction (XRD) data, including weak intensity superstructure Bragg lines. As no single crystal is available, this structural model has been deduced using the structural relationships found between the previously known triclinic and monoclinic M₃ polymorphs. We find that the better starting set of atomic positions for Rietveld refinement is the triclinic set and not the monoclinic set. A key observation is that the monoclinic M₁ structure is closer to the low temperature triclinic T₃ structure (within the Golovastikov model) than to the higher temperature monoclinic M₃ structure. The unit cell and the set of atomic positions of the two best models, called 3<M>Pc and 3<T>Pn models, are provided. We prefer the 3<M>Pc model for it better reproduces the weak characteristic Bragg lines of the superstructure.