Na2Ca3Si2O8 or γ‐Na2Ca6Si4O15? A hybrid approach combining 3D single‐crystal electron and powder X‐ray diffraction

Phase assemblages of mixtures containing Na2CO3, CaCO3, and SiO2 in the molar ratio 1:3:2 have been studied at elevated temperatures. Synthesis experiments have been performed at 1000, 1100, and 1200°C within a differential thermal analysis (DTA‐TGA) apparatus. Mass losses during heating and annealing periods of the high‐temperature treatment have been investigated in situ using thermogravimetry. For the run at 1200°C, the solid‐state reactions resulted in almost phase pure polycrystalline material of a previously unknown high‐temperature polymorph of Na2Ca6Si4O15, whose formation was triggered by significant Na2O losses at the reaction temperature. The new so‐called γ‐phase has been structurally characterized by a combination of 3D single‐crystal electron and powder X‐ray diffraction. Basic crystallographic data at ambient conditions are as follows: monoclinic symmetry, space group C 2, a = 17.2066(1) Å, b = 5.47863(3) Å, c = 7.32583(4) Å, β = 91.435(4)°, V = 690.38(1) Å3, Z = 2. Structure solution was accomplished by electron diffraction, whereas the subsequent refinement calculations were based on the Rietveld method using high‐resolution data from a laboratory powder diffractometer. Similar to the other two already known Na2Ca6Si4O15 modifications, the crystal structure of the γ‐phase contains both [Si2O7] dimers and insular [SiO4] moieties. Tetrahedra and [CaO6] octahedra form a three‐dimensional framework whose topological characteristics have been studied. The remaining Ca and Na cations are located on five symmetrically independent positions located in cavities of the network. There are sufficiently strong arguments that previously described “triclinic Na2Ca3Si2O8” is actually misinterpreted γ‐Na2Ca6Si4O15 and that a sodium calcium silicate with a molar ratio of Na2O:CaO:SiO2 = 1:3:2 does probably not exist. Our investigation is an excellent example that 3D electron diffraction has transformed from an exotic technique for crystal‐structure determination into an indispensable method for problems where small sizes of the crystallites is an issue.