High-pressure phase transition of the oxonitridosilicate chloride Ce₄[Si₄O_3+xN_7–x]Cl_1–xO_x with x = 0.12 and 0.18

Abstract

The high-pressure behaviour of the oxonitridosilicate chlorides Ce₄[Si₄O_3+xN_7–x]Cl_1–xO_x, 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 P2₁3) is reduced to orthorhombic (space group P2₁2₁2₁) following a translationengleiche group-subgroup relationship of index 3. A fit of a third-order Birch-Murnaghan equation of state to the p–V data results in a bulk modulus B₀ = 124(5) GPa with its pressure derivative B′ = 5(1) at V₀ = 1134.3(4) ų for the low-pressure phase and in B₀ = 153(10) GPa with B′ = 3.0(6) at V₀ = 1071(3) ų 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) 1/GPa). The experimental results confirm an earlier prediction of the pressure-induced instability of isotypic Ce₄[Si₄O₄N₆]O, and also show that the bulk modulus was predicted reasonably well.