HP‐CaSiN 2 – A New High‐Pressure Modification

HP‐CaSiN 2 was obtained by HP/HT (high pressure / high temperature) synthesis utilizing the multianvil technique (4.5 to 18 GPa, 900 to 1200 °C) starting from the ambient pressure phase CaSiN 2 . Structure solution and Rietveld refinement were started with the cell parameters of HP‐CaSiN 2 determined by selected area electron diffraction (SAED). The crystal structure of the high‐pressure phase was refined in space group Pbca (no. 61) with a = 5.1296(2), b = 10.2993(2), c = 14.5442(3) Å, V = 768.39 Å 3 , Z = 16, R Bragg = 0.0484, R w = 0.0256 and GoF = 1.0. HP‐CaSiN 2 is built up by a three‐dimensional nitridosilicate network of vertex‐sharing SiN 4 ‐tetrahedra with N[2][W. A. Groen, 1994] bridging. The arrangement of the vertex‐linked tetrahedra in HP‐CaSiN 2 , being isostructural with KGaO 2 , is a tilting distortion of the stuffed β‐cristobalite type framework. Due to the high pressure the tilting angle of the SiN 4 ‐tetrahedra has become much smaller so that the latter appear edge‐sharing sharing which is only a consequence of the chosen view along [100]. The Ca ions are located within these sechser ‐ring channels and are coordinated by 6 and 7 N‐atoms, respectively. HP‐CaSiN 2 is similar to CaSiN 2 , but differs clearly in the rotation of the SiN 4 ‐tetrahedra and in the conformation of the sechser ‐rings. Because of their coexistence HP‐CaSiN 2 and CaSiN 2 represent a very good example of an isosymmetric polymorphism.