Compression behaviour of nitridocarbidosilicates M2[Si4N6C]M2[Si4N6C]M=Y,Ho,ErM=Y,Ho,Er studied with X-ray diffraction and ab initio calculations

The compressibilities of the nitridocarbidosilicates Ln2[Si4N6C]Ln2[Si4N6C], Ln=HoLn=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–Vp–V data resulted in the bulk modulus B0=162(2)GPa and its pressure derivative B′=5.1(3)B′=5.1(3) at V0=605.1(2)A˚3 for Ho2[Si4N6C]Ho2[Si4N6C]. For Er2[Si4N6C]Er2[Si4N6C] the values are B0=163(5)GPa and B′=4.5(6)B′=4.5(6) at V0=602.8(2)A˚3. Complementary data of the pressure dependence of this family of compounds were obtained by density functional theory based model calculations for Y2[Si4N6C]Y2[Si4N6C]. An anomalous compression behaviour is predicted to occur between 8 and 12 GPa for Y2[Si4N6C]Y2[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 Er2[Si4N6C]Er2[Si4N6C] and Ho2[Si4N6C]Ho2[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)N3CSi(3)N3C tetrahedron.