Chemical and structural variability in cubic spinel oxides

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Marco E. Ciriotti
Messaggi: 25649
Iscritto il: ven 25 giu, 2004 11:31
Località: via San Pietro, 55 I-10073 Devesi/Cirié TO - Italy
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Chemical and structural variability in cubic spinel oxides

Messaggio da Marco E. Ciriotti » dom 17 mar, 2019 12:06

Referenza:
▪ Bosi, F. (2919): Chemical and structural variability in cubic spinel oxides. Acta Crystallographica, B75, 279-285.

Synapsis:
The empirical relations between cubic spinel oxides with different compositions were investigated on the basis of data from 349 refined crystal structures. The results show that the spinel structure is able to tolerate many constituents and the structure distortion can be explained in terms of variation in ionic potential (= charge/size) at the cation sites.
Marco E. Ciriotti

«Things are interesting only in so far as they relate themselves to other things»

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Marco E. Ciriotti
Messaggi: 25649
Iscritto il: ven 25 giu, 2004 11:31
Località: via San Pietro, 55 I-10073 Devesi/Cirié TO - Italy
Contatta:

Re: Chemical and structural variability in cubic spinel oxides

Messaggio da Marco E. Ciriotti » mer 10 apr, 2019 15:31

Referenza:
▪ Bosi, F. (2919): Chemical and structural variability in cubic spinel oxides. Acta Crystallographica, B75, 279-285.

Abstract:
The empirical relations between cubic spinel oxides of different compositions were investigated using data from 349 refined crystal structures. The results show that the spinel structure is able to tolerate many constituents (at least 36) by enlarging and decreasing the tetrahedra and octahedra. This is reflected in a large variation in tetrahedral and octahedral bond distances. The oxygen positional parameter (u) may be regarded as a measure of the distortion of the spinel structure from cubic close packing or of the angular distortion of the octahedron. The distortion can best be explained in terms of ionic potential (IP), which merges the size and charge properties of an ion. Sterically induced distortion depends on ion size, whereas electrostatically induced distortion is caused by cation–cation repulsion across faces of tetrahedra and shared edges of octahedra. The strong correlations between the u parameter and the IP at the T and M sites are consistent with the main role played by the both charge and size. Large distortions (u ≫ 0.27) result in oxygen–oxygen distances of the octahedron shorter than 2.50 Å, which would lead to structural instability because of increased non‐bonded repulsion forces between the oxygen atoms.
Marco E. Ciriotti

«Things are interesting only in so far as they relate themselves to other things»

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Marco E. Ciriotti
Messaggi: 25649
Iscritto il: ven 25 giu, 2004 11:31
Località: via San Pietro, 55 I-10073 Devesi/Cirié TO - Italy
Contatta:

Re: Chemical and structural variability in cubic spinel oxides

Messaggio da Marco E. Ciriotti » gio 11 apr, 2019 12:17

Conclusions:

The spinel structure can be defined as rigid because only two variables (u and a) are necessary to describe its geometry, but the interplay between tetrahedra and octahedra allows remarkable spinel chemical flexibility. The site-charge variations are larger at T (from 0 to +6) than at M (from +1 to +4) because of the ratio of T to M sites, which must be equal to 1:2, in accordance with the stoichiometry of the spinel structure: T(A1–iBi)M(AiB2–i)O4. The structure of the spinel oxides is able to tolerate many constituents (at least36) by expansion or contraction of the tetrahedra and octahedra. This is reflected in the large variation in T—O (1.64–2.25 Å) and M—O (1.91–2.53 Å) as well as in u (0.23–0.27). The latter varies with respect to the ideal c.c.p. (u = 0.25), giving a bond angular distortion of the octahedron and hence of the whole spinel structure. With respect to u = 0.2625 (at which, T—O = M—O), T—O may be ∼0.20 Å larger than M—O, whereas M—O may be about ∼0.70 Å larger than T—O. In general, distortion of the spinel structure can be best explained in terms of variation in IP, which merges the steric and electronic properties of an ion: sterically induced distortion is obviously dependent on ion size, whereas electrostatically induced distortion is caused by cation–cation repulsion across faces of tetrahedra and shared edges of octahedra. About 90% of the observed structure distortion can be described by linear regression models between u and either TIP or MIP. Values of u ≫ 0.27 result in M(O—O) distances shorter than 2.50 Å, which would lead to structural instability because of increased non-bonded repulsion between the oxygen atoms.
Marco E. Ciriotti

«Things are interesting only in so far as they relate themselves to other things»

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