CRYSTAL-STRUCTURE AND INFRARED-SPECTRUM OF THE CYCLOSILICATE CA2ZRSI4O12

The CaO-ZxO2-SiO2 phase diagram has been the subject of several investigations; two publications have pointed out the existence of Ca2ZrSi4O12 whereas two other publications deny its existence. This compounds has been obtained by the conventional ceramic technique at 1400°C. The cell is monoclinic, space group P2/m, with a = 7.399(1), b = 13.651(2), c = 3.312(1) Å and β = 108°9(20), Z = 2. Its structure has been determined from 880 single-crystal data and refined to R = 0.016 with 94 parameters; it is based on rings of Si4O12 with four corner-sharing tetrahedra belonging to the type A classification of T. Zoltai and M. J. Buerger (Z. Kristallogr.114, 1-8,1960); these rings are interconnected by ZrO6 octahedra, outlining tunnels parallel to the c axis; calcium ions fill the 8- and 9-coordinated interstices. The low-point symmetry of the Si4O12 rings precludes any detailed interpretation of the infrared spectrum, which, however, may be compared to other cyclotetrasilicate spectra, and is tentatively subdivided into groups of lines belonging to a given type of vibration. © 1993 Academic Press, Inc.