Abstract
Single crystals of a new sodium calcium silicate with composition Na~2~Ca~2~Si~2~O~7~ have been synthesized. The title compound is monoclinic, space group I__12/a__1, a = 10.4906(5) Å, b = 10.3762(6) Å, c = 24.5349(12) Å, β = 91.217(6)°, V = 2670.1(2) Å^3^, Z = 16. The crystal used for the data collection showed twinning by pseudo-merohedry, a feature we took account of by separating the reflections from the different twin individuals. Structure determination was based on direct methods followed by difference Fourier calculations and least squares refinements (R(|F|) = 0.056 for 2414 independent observed reflections (I>2__σ(I)) and 245 parameters). Na~2~Ca~2~Si~2~O~7~ belongs to the group of mixed anion silicates comprising insular [SiO~4~]-groups and [Si~3~O~10~]-trimers. Both anion groups are located in layers perpendicular to [001]. Ten symmetrically independent M-sites occupied by Na and/or Ca crosslink between the tetrahedral anion groups. Another very interesting understanding of the structure can be obtained from the comparison with the ABO~3~ perovskite structure type. The compound can be regarded as a superstructure of perovskite (a ≈ 2√2__a
~p~, b ≈ 2√2__a__
~p~, c ≈ 6__a__
~p~) with 22.2% oxygen vacancies. The vacancies are ordered in such a way that the silicon atoms residing on the B-sites of the perovskite structure are coordinated to four oxygens. The relationship with perovskite can be expressed in the following crystal chemical formula: [(Na,Ca)][(Na,Ca)~1/4~Si~3/4~][O~7/9~∀~2/9~]~3~.