Gallium(III) oxide

Preparation

Gallium oxide is precipitated in hydrated form upon neutralization of acidic or basic solution of gallium salt. Also, it is formed on heating gallium in air or by thermally decomposing gallium nitrate at 200–250˚C. It can occur in five different modifications, α, β, γ, δ, and ε. Of these modifications β-Ga₂O₃ is the most stable form.

β-Ga₂O₃ is prepared by heating nitrate, acetate, oxalate or other organic derivatives above 1000 °C.

α-Ga₂O₃ can be obtained by heating β-Ga₂O₃ at 65 kbars and 1100 °C. The hydrated form can be prepared by decomposing precipitated and "aged" gallium hydroxide at 500 °C.

γ-Ga₂O₃ is prepared by rapidly heating the hydroxide gel at 400 - 500 °C. A more crystalline form of this polymorph can be prepared directly from gallium metal by a solvothermal synthesis.

δ-Ga₂O₃ is obtained by heating Ga(NO₃)₃ at 250 °C.

ε-Ga₂O₃ is prepared by heating δ-Ga₂O₃ at 550 °C .

Reactions

Gallium(III) oxide is amphoteric. It reacts with alkali metal oxides at high temperature to form e.g. NaGaO₂, and with Mg, Zn, Co, Ni, Cu oxides to form spinels, e.g. MgGa₂O₄. It dissolves in strong alkali to form a solution of the gallate ion, Ga(OH)−
4.

With HCl, it forms gallium trichloride GaCl₃.

Ga₂O₃ + 6 HCl → 2 GaCl₃ + 3 H₂O

It can be reduced to gallium suboxide (gallium(I) oxide) Ga₂O by H₂. or by reaction with gallium metal

Ga₂O₃ + 2 H₂ → Ga₂O + 2 H₂O Ga₂O₃ + 4 Ga → 3 Ga₂O

Structure

β-Ga₂O₃, with a melting point of 1900˚C, is the most stable crystalline modification. The oxide ions are in a distorted cubic closest packing arrangement, and the gallium (III) ions occupy distorted tetrahedral and octahedral sites, with Ga-O bond distances of 1.83 and 2.00 Å respectively.

α-Ga₂O₃ has the same structure (corundum) as α-Al₂O₃, wherein Ga ions are 6-coordinate. γ-Ga₂O₃ has a defect spinel structure similar to that of γ-Al₂O₃.

Potential applications

Gallium(III) oxide has been studied in the use of lasers, phosphors, and luminescent materials. It has also been used as an insulating barrier in tight junctions. Monoclinic β-Ga₂O₃ is used in gas sensors and luminescent phosphors and can be applied to dielectric coatings for solar cells. This stable oxide has also shown potential for deep-ultraviolet transparent conductive oxides, and transistor applications.

Thin Ga₂O₃ films are of commercial interest as gas sensitive materials and Ga₂O₃.. Ellipsometry is a procedure that can be used to determine optical functions of the β-Ga₂O₃.

β-Gallium(III) oxide used in the production of Ga₂O₃-Al₂O₃ catalyst.