High-pressure phase transition of the oxonitridosilicate chloride Ce4[Si4O3+xN7-x]Cl1-xOx with x=0.12 and 0.18

The high-pressure behaviour of the oxonitridosilicate chlorides Ce-4[Si4O3+xN7-x]Cl1-xOx, x = 0.12 and 0.18, is investigated by in situ powder synchrotron X-ray diffraction. Pressures up to 28 GPa are generated using the diamond-anvil cell technique. A reversible phase transition of first order occurs at pressures between 8 and 10 GPa. Within this pressure range the high- and the low-pressure phases are observed concomitantly. At the phase transition the unit cell volume is reduced by about 5%, and the cubic symmetry (space group P2(1)3) is reduced to orthorhombic (space group P2(1)2(1)2(1)) following a translationengleiche group-subgroup relationship of index 3. A fit of a third-order Birch-Mumaghan equation of state to the p - V data results in a bulk modulus B-0 = 124(5) GPa with its pressure derivative B' = 5(1) at V-0 = 1134.3(4) angstrom(3) for the low-pressure phase and in B-0 = 153(10) GPa with B' = 3.0(6) at V-0 = 1071(3) angstrom(3) for the high-pressure phase. The orthorhombic phase shows an anisotropic axial compression with the a axis (which is the shortest axis) being more compressible (k(a) = 0.0143(4) 1/GPa) than the b and c axes (k(b) = 0.0045(2), k(c) = 0.0058(2) l/GPa). The experimental results confirm an earlier prediction of the pressure induced instability of isotypic Ce-4[Si4O4N6]O, and also show that the bulk modulus was predicted reasonably well.