Indium sesqui-oxide, In₂O₃

Indium sesqui-oxide, In₂O₃, obtained when indium is heated in air or oxygen, is prepared by the ignition of the hydroxide, carbonate, nitrate, or sulphate. Prepared at fairly low temperatures, it forms a yellow, amorphous powder which is readily soluble in acids, producing indium salts ; but when produced at high temperatures, it forms rhombohedral crystals of the colour of chlorine, and is extremely resistant towards acids. The density is 7.18; the specific heat (0° to 100°), 0.0807. It is diamagnetic.

Indium sesqui-oxide is reduced to indium when heated with sodium, carbon, or magnesium, or when heated in either hydrogen or ammonia; and by carefully regulating the reduction in hydrogen, Winkler claims to have obtained lower oxides, including a black monoxide InO.

When heated above 850°, indium sesqui-oxide loses oxygen and becomes converted into the oxide In₃O₄, which crystallises in regular octahedra isomorphous with Fe₃O₄. The loss in weight associated with this change had previously been attributed to the volatility of the sesqui-oxide.

Neither the oxide In₃O₄ nor the (metastable) In₂O₃ fuses in the oxy-hydrogen flame

From the data supplied by Ditte the following results may be deduced: -

2[In] + 3(O) = [In₂O₃] + 239.8 Cals.
[In₂O₃] + 3H₂SO₄.aq. = In₂(SO₄)₃.aq. + 72.78 Cals.