Terbium(III,IV) oxide explained

Terbium(III,IV) oxide, occasionally called tetraterbium heptaoxide, has the formula Tb₄O₇, though some texts refer to it as TbO_1.75. There is some debate as to whether it is a discrete compound, or simply one phase in an interstitial oxide system. Tb₄O₇ is one of the main commercial terbium compounds, and the only such product containing at least some Tb(IV) (terbium in the +4 oxidation state), along with the more stable Tb(III). It is produced by heating the metal oxalate, and it is used in the preparation of other terbium compounds. Terbium forms three other major oxides: Tb₂O₃, TbO₂, and Tb₆O₁₁.

Synthesis

Tb₄O₇ is most often produced by ignition of the oxalate or the sulfate in air.^[1] The oxalate (at 1000 °C) is generally preferred, since the sulfate requires a higher temperature, and it produces an almost black product contaminated with Tb₆O₁₁ or other oxygen-rich oxides.

Chemical properties

Terbium(III,IV) oxide loses O₂ when heated at high temperatures; at more moderate temperatures (ca. 350 °C) it reversibly loses oxygen, as shown by exchange with¹⁸O₂. This property, also seen in Pr₆O₁₁ and V₂O₅, allows it to work like V₂O₅ as a redox catalyst in reactions involving oxygen. It was found as early as 1916 that hot Tb₄O₇ catalyses the reaction of coal gas (CO + H₂) with air, leading to incandescence and often ignition.^[2]

Tb₄O₇ reacts with atomic oxygen to produce TbO₂, but more convenient preparations are available.^[3]

(s) + 6 HCl (aq) → 2 (s) + 2 (aq) + 3 (l).Tb₄O₇ reacts with other hot concentrated acids to produce terbium(III) salts. For example, reaction with sulfuric acid gives terbium(III) sulfate. Terbium oxide reacts slowly with hydrochloric acid to form terbium(III) chloride solution, and elemental chlorine. At ambient temperature, complete dissolution might require a month; in a hot water bath, about a week.

Anhydrous terbium(III) chloride can be produced by the ammonium chloride route^{[4] [5] [6]} In the first step, terbium oxide is heated with ammonium chloride to produce the ammonium salt of the pentachloride:

Tb₄O₇ + 22NH₄Cl → 4(NH₄)₂TbCl₅ + 7H₂O + 14NH₃In the second step, the ammonium chloride salt is converted to the trichlorides by heating in a vacuum at 350-400 °C:

(NH₄)₂TbCl₅ → TbCl₃ + 2HCl + 2NH₃

Further reading

• Book: CRC Handbook of Chemistry and Physics. registration. 71st. 1990. CRC Press. Ann Arbor, Michigan. 978-0-8493-0471-2.
• Book: Mellor, J.W.. A Comprehensive Treatise on Inorganic and Theoretical Chemistry. Longmans, Green & Co.. London. 692–696.

Notes and References

1. Book: Hartmut Bergmann, Leopold Gmelin . Gmelin Handbook of Inorganic Chemistry, System Number 39 . 1986 . Springer-Verlag . 9783540935254 . 397 .
2. Bissell . D. W. . James . C. . Gadolinium Sodium Sulfate . Journal of the American Chemical Society . 38 . 4 . 873–875 . 1916. 10.1021/ja02261a012.
3. Book: Synthetic Methods of Organometallic and Inorganic Chemistry. Edelmann. F.T.. Poremba. P.. W.A.. Herrmann. 6. 1967. Georg Thieme Verlag. Stuttgart. 3-13-103071-2.
4. Book: Handbook of Preparative Inorganic Chemistry. 2nd. Brauer, G. . Academic Press. 1963. New York.
5. Book: Meyer , G. . The Ammonium Chloride Route to Anhydrous Rare Earth Chlorides-The Example of YCl₃. The Ammonium Chloride Route to Anhydrous Rare Earth Chlorides—The Example of Ycl ₃. Inorganic Syntheses. 25. 1989. 146–150. 10.1002/9780470132562.ch35. 978-0-470-13256-2.
6. Book: Edelmann, F. T. . Synthetic Methods of Organometallic and Inorganic Chemistry . VI . Poremba, P. . Herrmann, W. A. . 1997 . Georg Thieme Verlag . Stuttgart . 978-3-13-103021-4 .