Compression behaviour of nitridocarbidosilicates M-2[Si4N6C] M = Y, Ho, Er studied with X-ray diffraction and ab initio calculations

The compressibilities of the nitridocarbidosilicates Ln(2)[Si4N6C], Ln = Ho, Er, were investigated by in situ powder synchrotron X-ray diffraction. Pressures up to 36 GPa were generated using the diamond-anvil cell technique. A fit of a third-order Birch-Murnaghan equation of state to the p-V data resulted in the bulk modulus B-0 = 162(2) GPa and its pressure derivative B' = 5.1(3) at V-0 = 605.1(2) angstrom(3) for Ho-2[Si4N6C]. For Er-2[Si4N6C] the values are B-0 = 163(5) GPa and B' = 4.5(6) at V-0 = 602.8(2) angstrom(3). Complementary data of the pressure dependence of this family of compounds were obtained by density functional theory based model calculations for Y-2[Si4N6C]. An anomalous compression behaviour is predicted to occur between 8 and 12 GPa for Y-2[Si4N6C]. The bulk modulus was obtained from an Eulerian strain versus normalized stress plot to be about 165 GPa, i.e., a value very similar to the bulk moduli obtained experimentally for Er-2[Si4N6C] and Ho-2[Si4N6C]. The anomaly in the pressure dependence of the cell parameters seems to be due to an increase in the coordination number of the two symmetrically independent Y cations. This increase of coordination is mainly achieved by a rotation of the Si(3)N3C tetrahedron. (C) 2008 Elsevier Ltd. All rights reserved.