A trioxide is a compound with three oxygen atoms. For metals with the M2O3 formula there are several common structures. Al2O3, Cr2O3, Fe2O3, and V2O3 adopt the corundum structure. Many rare earth oxides adopt the "A-type rare earth structure" which is hexagonal. Several others plus indium oxide adopt the "C-type rare earth structure", also called "bixbyite", which is cubic and related to the fluorite structure.
List of trioxides
MO3
Carbon trioxide, CO3
Chromium trioxide, CrO3
Molybdenum trioxide, MoO3
Rhenium trioxide, ReO3
Selenium trioxide, SeO3
Sulfur trioxide, SO3
Tellurium trioxide, TeO3
Tungsten trioxide, WO3
Uranium trioxide, UO3
Xenon trioxide, XeO3
M2O3
Antimony trioxide, Sb2O3
Arsenic trioxide, As2O3
Bismuth(III) oxide, Bi2O3
Boron trioxide, B2O3
Cobalt(III) oxide, Co2O3
Dichlorine trioxide, Cl2O3
Dinitrogen trioxide, N2O3
Gadolinium oxide, Gd2O3
Gallium(III) oxide, Ga2O3
Gold trioxide, Au2O3
Indium(III) oxide, In2O3
Iron(III) oxide, Fe2O3
Manganese(III) oxide, Mn2O3
Nickel(III) oxide, Ni2O3
Phosphorus trioxide, P4O6 (named before the true formula known) Thallium(III) oxide, Tl2O3 Terbium(III) oxide, Tb2O3 Trioxidane, H2O3 Vanadium trioxide, V2O3 Ytterbium(III) oxide, Yb2O3 Yttrium(III) oxide, Y2O3
Other trioxides
Mineral trioxide aggregate
Sulfur trioxide pyridine complex, SO3(py)