Antimony(III) oxide occurs as a natural compound in the form of the minerals senarmontite and valentinite.
Antimony(III) oxide is a white, crystalline powder that is insoluble in water. It dissolves in concentrated acids and alkalis. Antimony(III) oxide occurs in nature like arsenic(III) oxide in two mineral modifications: Cubic sénarmontite is transformed into orthorhombic valentinite when heated to 570 °C. This mineral is also known as “white-spit luster”. When heated openly in air, antimony (III) oxide oxidizes to antimony tetraoxide Sb2O4 under oxygen uptake. When heated with reducing agents such as hydrogen or coal, it is reduced to the elemental antimony in a redox reaction.
Antimony(III) oxide exhibits thermochromic properties. When heated to temperatures well above 600 °C, the compound turns yellow. Reversibly, the compound turns white again on cooling. The cause of the colour change is a polymorphic, enantiotropic transformation from the white cubic crystal form (Senarmontite) to a yellow orthorhombic crystal form (Valentinite) at 606 °C. The formation enthalpy of the cubic form is ΔfH = -720.5 kJ-mol-1, that of the orthorhombic form ΔfH = -708.5 kJ-mol-1, so that for the polymorphic transformation a transformation enthalpy of ΔtH = 12 kJ-mol-1 results.
One production possibility is the heating of antimony (III) sulfide in the oxygen stream. This produces antimony(III) oxide and sulphur dioxide.
Another possibility is the burning of powdered antimony in oxygen. It burns with a white-blue flame to form antimony(III) oxide:
4 Sb + 3 O2 → 2 Sb2O3
The hydrolysis of antimony(III) chloride produces valentinite, which is metastable at room temperature and gradually changes to senarmontite when treated with alkalis.
Antimony(III) oxide is used as a flame retardant in plastics and paper. Occasionally it is also contained for this purpose in mattress covers. In enamel production, it serves as an opacifier for colourless enamel. In the production of PET, antimony (III) oxide is needed as a catalyst. “Antimony white” was formerly used as a white pigment.
Together with a doping of tin as ATO (Antimony-Tin-Oxide) for transparent conductive coatings of glasses, e.g. for display technology and pigments for bright and transparent antistatic coatings. In flame retardants such as decabromodiphenyl ether, antimony trioxide is used as a synergist.
The International Agency for Research on Cancer (IARC) classified antimony trioxide as a potentially carcinogenic substance.