Al2O3
Alumina · aluminium oxide, aluminum oxide, corundum, alpha-alumina, dialuminium trioxide, aluminium sesquioxide
Al2O3, commonly known as alumina or aluminium oxide, is a thermodynamically stable, wide-band-gap insulating binary oxide of aluminium and oxygen. Most alumina produced goes into making aluminium metal. It is also one of the most widely used ceramic materials, known for its exceptional hardness and chemical resistance, and is used as a refractory material, abrasive, and catalyst support.
About Alumina
What is Al2O3 and what structure does it adopt?
Al2O3 is aluminium oxide, an inorganic binary compound of aluminium and oxygen. Its thermodynamically stable phase at ambient pressure is alpha-alumina. This phase adopts the trigonal corundum crystal structure, space group R-3c. In this structure the oxygen anions form an approximately hexagonal close-packed array, and aluminium cations fill two-thirds of the octahedral sites. Aluminium oxide also forms several metastable transition polymorphs, such as the gamma, delta and theta phases. Many of these have defective spinel-like cation arrangements, and they convert to the stable alpha phase at high temperatures.
What is Al2O3 used for?
By volume, the dominant use of alumina is as the feedstock for electrolytic smelting of aluminium metal. In chemical processing, high-surface-area transition aluminas serve as catalysts or as robust catalyst supports. Alpha-alumina has a high melting point and resists wear, so it is made into refractory linings, abrasives and cutting tools. It is also used as an electrical insulator and as a substrate in microelectronics. Single-crystal corundum, including synthetic sapphire and its chromium-doped variety ruby, is used for optical windows and as a gemstone.
How stable and synthesizable is Al2O3?
Alpha-Al2O3 is a highly stable material. It is the thermodynamic ground state of aluminium oxide under ambient conditions. It is readily made by calcining aluminium hydroxides and oxyhydroxides (such as gibbsite or boehmite) or aluminium salts. It can also be made by chemical vapor deposition and by solution combustion methods. Heating to high temperatures irreversibly converts the transition aluminas into dense, chemically inert alpha-alumina.
What is the electronic character of Al2O3?
Al2O3 is a classic wide-band-gap electrical insulator. Its valence band is made up mainly of filled oxygen 2p states. Its conduction band is made up mainly of aluminium-derived states, separated from the valence band by a wide gap. Its intrinsic electrical conductivity is negligible, so alumina has high dielectric strength, and pure crystals are transparent across the visible spectrum. These properties make it useful for dielectric barriers, electrical insulation and optical windows.
How does Al2O3 compare to other oxides like MgAl2O4, CuO, and NiO?
Compared with many functional oxides, Al2O3 stands out for its chemical inertness, its wide insulating band gap and its structural stability. MgAl2O4 is a ternary spinel that shares alumina's refractory and insulating character. CuO and NiO are p-type transition-metal oxides with narrower band gaps than alumina. In catalysis they are typically used as active redox components rather than inert supports. Alumina is mainly valued as an unreactive, high-temperature support and refractory ceramic.
Key Properties
Cross-validated computational properties for Alumina, aggregated across 5 databases.
Band GapEnergy needed to move an electron from the valence band to the conduction band. Lower or zero values tend to behave more metallic; larger gaps are more insulating or semiconducting.
Energy Above HullThermodynamic distance from the most stable set of competing phases. 0 eV/atom is on the convex hull; small positive values may still be experimentally accessible.
StabilityA plain-language summary of the best reported energy-above-hull result. It reflects whether the lowest-energy structure is on, near, or far from the stability hull.
StructuresCount of reported calculated crystal structures for this formula, including alternate polymorphs, source databases, and observed space groups.
Cross-Source DFT Agreement
How well independent DFT databases agree on the thermodynamics of Al2O3. Tight agreement means computed properties can be trusted without re-running calculations.
Agreement ScoreA normalized confidence score summarizing how closely independent DFT databases agree. Higher scores mean tighter cross-source agreement.
Hull SpreadDifference between the highest and lowest energy-above-hull values reported by comparable sources. Smaller spread means less thermodynamic disagreement.
Sources ComparedNumber and names of computational sources with comparable entries for this formula.
Space Group ConsensusWhether independent sources predict the same crystal symmetry for the lowest-energy structure.
Reported Structures
Lowest-energy structures reported for Al2O3, ranked by energy above hull.
| Space GroupSymmetry classification of the crystal arrangement. The number is the international space-group index. | Crystal SystemBroad lattice family, such as cubic, tetragonal, monoclinic, or triclinic, derived from unit-cell symmetry. | Band Gap (eV)Electronic gap calculated for this specific reported structure, measured in electronvolts. | E above hull (eV/atom)Thermodynamic distance from the convex hull for this structure, normalized per atom. Lower is generally more stable. | E/atom (eV)Computed total energy normalized per atom. Use energy above hull, not this value alone, when comparing stability. | Density (g/cm³)Mass per relaxed crystal volume, reported in grams per cubic centimeter. |
|---|---|---|---|---|---|
| R-3c (No. 167) | trigonal | 6.43 | 0.0000 | -3.313 | — |
| — | — | 6.31 | 0.0000 | -3.265 | — |
| R-3c (No. 167) | trigonal | 5.87 | 0.0000 | -7.894 | 3.87 |
| — | — | 6.30 | 0.0001 | -3.265 | — |
| — | — | — | 0.0015 | -3.264 | — |
| — | — | 6.21 | 0.0019 | -3.263 | — |
| — | — | 4.84 | 0.0115 | -3.254 | — |
| C2/m (No. 12) | monoclinic | 4.94 | 0.0165 | -3.297 | — |
| C2/m (No. 12) | monoclinic | 4.46 | 0.0172 | -7.876 | 3.63 |
| — | — | 5.29 | 0.0184 | -3.247 | — |
| Pna21 (No. 33) | orthorhombic | 4.83 | 0.0208 | -7.873 | 3.78 |
| — | — | 5.63 | 0.0296 | -3.236 | — |
Synthesis Routes
Literature-extracted synthesis procedures targeting Al2O3.
Applications
Where Alumina is used.
Patent Landscape
58 patents reference Al2O3 or close compositional variants.
| Patent | Title | Assignee | Granted |
|---|---|---|---|
| 7763346 | Surface coated cutting tool made of cermet having property-modified α type Al2O3 layer of hard coating layer | — | — |
| 8834833 | Process for preparing an aluminium oxide powder having a high alpha-Al2O3 content | — | — |
| 8551319 | Solid polycrystalline potassium ion conductor having a β″-Al2O3 structure, its production and the preparation | — | — |
| 6592640 | Fused Al2O3-Y2O3 eutectic abrasive particles, abrasive articles, and methods of making and using the same | — | — |
| 6673132 | SiO2/Al2O3 composite abrasive and method for producing the same | — | — |
| 6372679 | Cr2O3-Al2O3 sintered body and processes for producing same | — | — |
| 9902654 | ZrO2-Al2O3-based ceramic sintered compact and production method thereof | — | — |
| 7459408 | Al2O3 dispersion-strengthened Ti2AlN composites and a method for producing the same | — | — |
| 7169198 | Microcrystalline α-Al2O3 shaped body, method for the production and use thereof | — | — |
| 7141523 | Ceramics comprising Al2O3, REO, ZrO2 and/or HfO2, and Nb2O5 and/or Ta2O5 and methods of making the same | — | — |
Frequently Asked Questions
Common questions about Alumina, answered from cross-validated data and the published literature.
Is alumina the same thing as sapphire?
Sapphire is single-crystal alpha-Al2O3, a gem or synthetic form of corundum. Most technical alumina, by contrast, is a polycrystalline ceramic or a powder. Pure sapphire is colorless. Gem varieties get their color from trace impurities: iron and titanium make blue sapphire, and chromium makes red ruby.
Why is Al2O3 widely used as a catalyst support?
Is Al2O3 an electrical conductor?
What is the difference between alpha and gamma alumina?
How is Al2O3 produced industrially?
What is Al2O3?
What is Al2O3 used for?
What is the band gap of Al2O3?
Is Al2O3 a metal, semiconductor, or insulator?
Is Al2O3 thermodynamically stable?
What is the crystal structure of Al2O3?
How many polymorphs of Al2O3 are known?
How is Al2O3 synthesized?
What elements does Al2O3 contain?
Where does the data for Al2O3 come from?
Related Research
Data sources & attribution
- jarvis — Data from JARVIS (jarvis.nist.gov), NIST. Cite: Choudhary et al., npj Comp. Mater. 6, 173 (2020). (LicenseRef-US-Gov-PD)
- oqmd — Data from the OQMD (oqmd.org). Cite: Saal et al., JOM 65, 1501 (2013); Kirklin et al., npj Comp. Mater. 1, 15010 (2015). (CC-BY-4.0)
- materials_project — Data from the Materials Project (materialsproject.org). Cite: Jain et al., APL Materials 1, 011002 (2013). (CC-BY-4.0)
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