Fig. 4.

Crystal structure of Ca₂Al₂SiO₇ viewed a from the side and b from above layers and Al–O–Al bonds energy plot (b). a, b Tetragonal cell with spacegroup $\text{P}\overline{4}{2}_{1\mathrm{m}}$ (113) and a = b = 7.716 Å and c = 5.089 Å. Wyckoff site 2a labelled T1 (yellow tetrahedra) is fully occupied by Al atoms and Wyckoff site 4e, labelled T2 (blue tetrahedra) is filled with a mix of Al (50 %) and Si (50 %) atoms. c Energy of configurations for cell $1\times 1\times 2$ vs the number of Al–O–Al bonds in isolated groups. The slope of linear regression (0.48 eV) agrees well with previously reported value of 0.52 eV [21]