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

Ca5[Si2N6] and Ca7[NbSi2N9] were obtained by reaction of Ca3N2, Ca2N and Si3N4 (with addition of niobium powder in case of Ca7[NbSi2N9]) in closed tantalum ampoules at temperatures at 1060 °C and 1000 °C, respectively. Ca5[Si2N6] 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 Ba5[Si2N6]. The crystal structure contains pairs of edgesharing SiN4 tetrahedra forming isolated nitridosilicate anions of [Si2N6]10−. Ca7[NbSi2N9] 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. Ca7[NbSi2N9] 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.