Synthesis and structure of alkaline earth silicon nitrides:: BaSiN2, SrSiN2, and CaSiN2

The alkaline earth silicon nitrides AESiN(2) (AE = Ca, Sr, Ba) are reported, synthesized as clear, colorless, single crystals from molten sodium at 900-1100 degreesC or, in the cases of BaSiN2 and SrSiN2, as white powders by reacting powdered intermetallics AESi with flowing anhydrous ammonia at 550-1000 degreesC. Structures were determined from single-crystal X-ray diffraction measurements at 150 K: BaSiN2 crystallizes in space group Cmca (No. 64) with a = 5.6046(1) Angstrom, b = 11.3605(3) Angstrom, c = 7.5851(2) Angstrom, and Z = 8. The structure consists of pairs of SiN4 tetrahedra edge-linked to form bow-tie-shaped Si2N6 dimers which share vertexes to form layers and has no analogue in oxide chemistry. SrSiN2 has a distorted form of this structure (SrSiN2: space group P2(1)/c (No. 14), a = 5.9750(5) Angstrom, b = 7.2826(7) Angstrom, c = 5.4969(4) Angstrom, beta = 113.496(4)degrees, Z = 4). The structure of CaSiN2 contains only vertex-sharing SiN4 tetrahedra, linked to form a three-dimensional stuffed-cristobalite type framework isostructural with KGaO2 (CaSiN2: space group Pbca (No. 61), a = 5.1229(3) Angstrom, b = 10.2074(6) Angstrom, c = 14.8233(9) Angstrom, z = 16).