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 \(\hbox {P}\bar{4}{2}_\mathrm{1m}\) (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]