Titanium(II) oxide (TiO) is an inorganicchemical compound of titanium and oxygen. It can be prepared from titanium dioxide and titanium metal at 1500 °C. It is non-stoichiometric in a range TiO0.7 to TiO1.3 and this is caused by vacancies of either Ti or O in the defect rock salt structure. In pure TiO 15% of both Ti and O sites are vacant. Careful annealing can cause ordering of the vacancies producing a monoclinic form which has 5 TiO units in the primitive cell that exhibits higher resistivity. A high temperature form with titanium atoms with trigonal prismatic coordination is also known. Acid solutions of TiO are stable for a short time then decompose to give hydrogen:
Ti2+ + H+ → Ti3+ + ½ H2
Evidence has been obtained for the presence of the diatomic molecule TiO in the interstellar medium. TiO shows strong bands in the optical spectra of cool (M-type) stars.