Trioxide

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.