HP-CaSiN2 - A New High-Pressure Modification

HP-CaSiN2 was obtained by HP/HT (high pressure / high temperature) synthesis utilizing the multianvil technique (4.5 to 18 GPa, 900 to 1200 degrees C) starting from the ambient pressure phase CaSiN2. Structure solution and Rietveld refinement were started with the cell parameters of HP-CaSiN2 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) angstrom, V = 768.39 angstrom(3), Z = 16, R-Bragg = 0.0484, R-w = 0.0256 and GoF = 1.0. HP-CaSiN2 is built up by a three-dimensional nitridosilicate network of vertex-sharing SiN4-tetrahedra with N bridging. The arrangement of the vertex-linked tetrahedra in HP-CaSiN2, being isostructural with KGaO2, is a tilting distortion of the stuffed -cristobalite type framework. Due to the high pressure the tilting angle of the SiN4-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-CaSiN2 is similar to CaSiN2, but differs clearly in the rotation of the SiN4-tetrahedra and in the conformation of the sechser-rings. Because of their coexistence HP-CaSiN2 and CaSiN2 represent a very good example of an isosymmetric polymorphism.