Scandium oxide explained

Scandium(III) oxide or scandia is a inorganic compound with formula Sc₂O₃. It is one of several oxides of rare earth elements with a high melting point. It is used in the preparation of other scandium compounds as well as in high-temperature systems (for its resistance to heat and thermal shock), electronic ceramics, and glass composition (as a helper material).

Structure and physical properties

Scandium(III) oxide adopts a cubic crystal structure (point group: tetrahedral (T_h), space group: Ia) containing 6-coordinate metal centres.^[1] Powder diffraction analysis shows Sc-O bond distances of 2.159 - 2.071 Å.^[2]

Scandium oxide is an insulator with a band gap of 6.0 eV.^[3]

Production

Scandium oxide is the primary form of refined scandium produced by the mining industry. Scandium-rich ores, such as thortveitite (Sc,Y)₂(Si₂O₇) and kolbeckite ScPO₄·2H₂O are rare, however trace amounts of scandium are present in many other minerals. Scandium oxide is therefore predominantly produced as a by-product from the extraction of other elements.

Reactions

Scandium oxide is the primary form of refined scandium produced by the mining industry, making it the start point for all scandium chemistry.

Scandium oxide reacts with most acids upon heating, to produce the expected hydrated product. For example, heating in excess aqueous HCl produces hydrated ScCl₃·nH₂O. This can be rendered anhydrous by evaporation to dryness in the presence of NH₄Cl, with the mixture then being purified by removal of NH₄Cl by sublimation at 300-500 °C.^[4] The presence of NH₄Cl is required, as the hydrated ScCl₃·nH₂O would otherwise form a mixed oxychloride upon drying.

Sc₂O₃ + 6 HCl + x H₂O → 2 ScCl₃·nH₂O + 3 H₂O

ScCl₃·nH₂O + n NH₄Cl → ScCl₃ + n H₂O + n NH₄Cl

Likewise, it is converted into hydrated scandium(III) triflate (Sc(OTf)₃·nH₂O) by a reaction with triflic acid.^[5]

Metallic scandium is produced industrially by the reduction of scandium oxide; this proceeds via conversion to scandium fluoride followed by a reduction with metallic calcium. This process is in some ways similar to the Kroll process for the production of metallic titanium.

Scandium oxide forms scandate salts with alkalis, unlike its higher homologues yttrium oxide and lanthanum oxide (but like lutetium oxide), for example forming K₃Sc(OH)₆ with KOH. In this, scandium oxide shows more similarity with aluminium oxide.

Natural occurrence

Natural scandia, although impure, occurs as mineral kangite.^[6]

Notes and References

1. Wells A.F. (1984) Structural Inorganic Chemistry 5th edition Oxford Science Publications
2. Knop. Osvald. Hartley, Jean M. . Refinement of the crystal structure of scandium oxide. Canadian Journal of Chemistry. 15 April 1968. 46. 8. 1446–1450. 10.1139/v68-236.
3. Emeline. A. V.. Kataeva, G. V. . Ryabchuk, V. K. . Serpone, N. . Photostimulated Generation of Defects and Surface Reactions on a Series of Wide Band Gap Metal-Oxide Solids. The Journal of Physical Chemistry B. 1 October 1999. 103. 43. 9190–9199. 10.1021/jp990664z.
4. Stotz. Robert W.. Melson, Gordon A. . Preparation and mechanism of formation of anhydrous scandium(III) chloride and bromide. Inorganic Chemistry. 1 July 1972. 11. 7. 1720–1721. 10.1021/ic50113a058.
5. McCleverty, J.A. and Meyer, T.J., Comprehensive Coordination Chemistry II, 2003, Elsevier Science,, Vol. 3, p. 99 ["Refluxing scandium oxide with triflic acid leads to the isolation of hydrated scandium triflate"]
6. Mindat, http://www.mindat.org/min-42879.html