Nitridogermanate Nitrides Sr₇[GeN₄]N₂ and Ca₇[GeN₄]N₂: Synthesis Employing Sodium Melts, Crystal Structure, and Density-Functional Theory Calculations

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.