Friedrich, A., Haussuhl, E., Morgenroth, W., Lieb, A., Winkler, B., Knorr, K. and Schnick, W. (2006) Crystal structure of the high-pressure phase of the oxonitridosilicate chloride Ce-4 Si4O3+xN7-x Cl1-xOx, x similar or equal to 0.2. Acta Crystallographica Section B-Structural Science, 62 . pp. 205-211. DOI 10.1107/s0108768106003302.

Abstract

The structural compression mechanism of Ce-4[Si4O3 N-+ x(7) (- x)]Cl-1 O-- x(x), x similar or equal to 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. 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 cornersharing Si tetrahedra. The latter is responsible for the short c axis of the orthorhombic high-pressure structure compared with the cell parameter of the cubic low-pressure structure.

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