Synopsis

The structural compression mechanism of Ce4[Si4O3+xN7-x]Cl1-xOx, x ≈ 0.2, was investigated by in situ single-crystal synchrotron X-ray diffraction at pressures of 3.0, 8.5 and 8.6 GPa using the diamond-anvil cell technique. On increasing pressure the low-pressure cubic structure first undergoes only minor structural changes. Between 8.5 and 8.6 GPa a first-order phase transition occurs, accompanied by a change of the single-crystal colour from light orange to dark red. Unit cell dimensions of a = 10.824(9) Å, b = 10.479(3) Å and c = 8.967(3) Å were found for the high-pressure phase at 8.6 GPa and the structure was solved in space group P212121. The main structural mechanisms, leading to a volume reduction of about 5 % at the phase transition, are an increase in and a rearrangement of the Ce coordination, the loss of the Ce2-Ce3 split position, and a bending of some of the inter-polyhedral Si-N-Si angles in the arrangement of the corner-sharing Si-tetrahedra. The latter is responsible for the short c axis of the orthorhombic high-pressure structure compared to the cell parameter of the cubic low-pressure structure.