Synthesis and Crystal Structure of the Nitridosilicates Ca5[Si2N6] and Ca7[NbSi2N9]

Ca₅Si₂N₆] and Ca₇NbSi₂N₉] were obtained by reaction of Ca₃N₂, Ca₂N and Si₃N₄ (with addition of niobium powder in case of Ca₇NbSi₂N₉]) in closed tantalum ampoules at temperatures at 1060 °C and 1000 °C, respectively. Ca₅Si₂N₆] is monoclinic C2/c with a = 983.6(2) pm, b = 605.2(1) pm, c = 1275.7(3), β = 100.20(3)° and Z = 4 crystallising homotypically to Ba₅Si₂N₆]. The crystal structure contains pairs of edgesharing SiN₄ tetrahedra forming isolated nitridosilicate anions of Si₂N₆]^10?. Ca₇NbSi₂N₉] is monoclinic P2₁/m with a = 605.1(1), b = 994.6(2), c = 899.7(2), β = 92.10(1)°, Z = 2 and crystallises in an hitherto unknown structure type. Ca₇NbSi₂N₉] contains isolated anions NbSi₂N₉]^14? which are composed of two edgesharing SiN₄ tetrahedra and an edge‐sharing NbN₅ pyramid. So far, such a pseudotrisilicate unit has not been observed in the family of silicates.