Unusual Coordination of Sr 2+ in Strontium Nitridogermanate Nitride Sr 7 [GeN 4 ]N 2

The alkaline earth nitridogermanate nitrides AE7[GeN4]N2 (AE = Ca, Sr) have been synthesized using a Na flux technique in sealed Ta tubes. According to single-crystal X-ray diffraction the isotypic compounds crystallize in space group Pbcn (No. 60) with Z = 4, (Sr7[GeN4]N2: a = 1152.6(2), b = 658.66(13), c = 1383.6(3) pm, V = 1050.5(4) × 106 pm3, R1 = 0.049; Ca7[GeN4]N2: a = 1082.6(2), b = 619.40(12), c = 1312.1(3) pm, V = 879.8(3) × 106 pm3, R1 = 0.016). Owing to the high N/Ge ratio, the compounds contain discrete N3− ions coordinated by six AE2+ besides discrete [GeN4]8− tetrahedrons. One of the AE2+ ion is coordinated by only four N3− ions, which is rather an unusual low coordination number for Sr2+. Together with the isolated [GeN4]8− tetrahedrons, these Sr2+ ions form chains of alternating cation centered edge sharing tetrahedrons. The electronic structure and chemical bonding in Sr7[GeN4]N2 has been analyzed employing linear muffin-tin orbital (LMTO) band structure calculations.