Variations on the theme of closest packing: The structural chemistry of barium titanate compounds |
| |
Authors: | Ekkehart Tillmanns Wolfgang Hofmeister Werner H Baur |
| |
Institution: | Institut für Geowissenschaften, Postfach 3980, Universität Mainz, D-6500 Mainz, Bundesrepublik Deutschland;Department of Geological Sciences, Box 4348, University of Illinois at Chicago, Chicago, Illinois 60680 USA |
| |
Abstract: | The crystal structures of most barium titanates can be described as hexagonal closest packings consisting of atoms of Ba and O and of vacancies between them. The pseudohexagonal cell constants of these compounds are close to the ideal hexagonal values. The most common stacking is the six-layer sequence (hcc)2. The mean diameter of a closest packed particle in these packings is 2.85 Å, while the mean thickness of a layer is 2.33 Å. The recognition of this closest packing principle has been helpful in the solution of many of the crystal structures of this group. With the exception of BaTi4O9, all the barium titanates which are usually classified as tunnel structures or Wadsley-Andersson phases can be described as cubic closest packings. The deviations of the pseudocubic phases from the ideal cubic values are larger than in the pseudohexagonal cases. A few of the barium titanates are related to the so-called 3.0-Å phases based on the rutile-type octahedral chain. These structures are characterized by having two almost closest packed corrugated layers at right angles to each other. The structures of a number of compounds in which the Ti4+ atoms are replaced partly by Ti3+, Al, Pt4+, or Li are based on principles similar to those of the barium titanates proper. The mean TiO distances in the coordination octahedra of the barium titanates depend strongly on the distortions of the octahedra. |
| |
Keywords: | |
本文献已被 ScienceDirect 等数据库收录! |
|