Acta Crystallographica Section B

Structural Science

Volume 65, Part 3 (June 2009)

feature articles

Acta Cryst. (2009). B65, 269-290 [ doi:10.1107/S0108768109011355 ]

Complex ceramic structures. I. Weberites

L. Cai and J. C. Nino

Abstract: The weberite structure (A₂B₂X₇) is an anion-deficient fluorite-related superstructure. Compared with fluorites, the reduction in the number of anions leads to a decrease in the coordination number of the B cations (VI coordination) with respect to the A cations (VIII coordination), thus allowing the accommodation of diverse cations. As a result, weberite compounds have a broad range of chemical and physical properties and great technological potential. This article summarizes the structural features of weberite and describes the structure in several different ways. This is the first time that the stacking vector and stacking angle are used to represent the weberite structure. This paper also discusses the crystallographic relationship between weberite, fluorite and pyrochlore (another fluorite-related structure). The cation sublattices of weberite and pyrochlore are correlated by an axial transformation. It has been shown that the different coordination environment of anions is due to the alternating layering of the AB₃ and A₃B close-packed cation layers. A stability field of weberite oxides is proposed in terms of the ratio of ionic radius of cations and relative bond ionicity. In addition, a selection of weberite compounds with interesting properties is discussed.

Keywords: weberite structure; ceramics; coordination environment; relative bond ionicity.


	Quicktime video file (3299.7 kbytes) [ doi:10.1107/S0108768109011355/kd5026sup1.mov ] Interactive slideshow version of Fig. 8(a) showing the cationic network on (111) of fluorite (black dashed lines: fluorite unit cell)

	Quicktime video file (3707.4 kbytes) [ doi:10.1107/S0108768109011355/kd5026sup2.mov ] Interactive slideshow version of Fig. 8(b) showing the AB3 layer parallel to (011) of weberite (red dotted lines: weberite unit cell; blue spheres: A cations; yellow spheres: B cations)

	Quicktime video file (4334.1 kbytes) [ doi:10.1107/S0108768109011355/kd5026sup3.mov ] Interactive slideshow version of Fig. 8(c) showing the AB3 layer parallel to (011) of weberite (red dotted lines: weberite unit cell; blue spheres: A cations; yellow spheres: B cations)

	Quicktime video file (7244.6 kbytes) [ doi:10.1107/S0108768109011355/kd5026sup4.mov ] Axial transformation of pyrochlore to weberite (black dashed lines: fluorite cell; blue lines: double pyrochlore cells; red dotted lines: weberite lattice: grenn shadow (011)w and (111)p

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