Editing Aluminium oxide (section)

==Structure==
[[Image:Corindon azulEZ.jpg|thumb|180px|left|Corundum from [[Brazil]], size about 2×3&nbsp;cm.]]

The most common form of crystalline aluminium oxide is known as [[corundum]], which is the thermodynamically stable form.<ref name=Levin>{{cite journal |author=I. Levin |author2=D. Brandon |year=1999
|title=Metastable Alumina Polymorphs: Crystal Structures and Transition Sequences |journal=Journal of the American Ceramic Society |volume=81 |issue=8 |pages=1995–2012 |doi=10.1111/j.1151-2916.1998.tb02581.x }}</ref> The oxygen ions form a nearly [[Close-packing of equal spheres|hexagonal close-packed]] structure with the aluminium ions filling two-thirds of the octahedral interstices. Each Al<sup>3+</sup> center is [[octahedral molecular geometry|octahedral]]. In terms of its [[crystallography]], corundum adopts a [[trigonal]] [[Bravais lattice]] with a [[space group]] of [[Hexagonal crystal family|R{{overline|3}}c]] (number 167 in the International Tables). The [[primitive cell]] contains two formula units of aluminium oxide.

Aluminium oxide also exists in other metastable phases, including the cubic γ and η phases, the monoclinic θ phase, the hexagonal χ phase, the orthorhombic κ phase and the δ phase that can be tetragonal or orthorhombic.<ref name=Levin /><ref name=Paglia>{{cite news| author = Paglia, G. | title =Determination of the Structure of γ-Alumina using Empirical and First Principles Calculations Combined with Supporting Experiments| publisher = Curtin University of Technology, Perth| year = 2004| url = http://espace.library.curtin.edu.au/R?func=search-simple-go&ADJACENT=Y&REQUEST=adt-WCU20040621.123301|format=free download|access-date = 2009-05-05}}</ref> Each has a unique crystal structure and properties.  Cubic γ-Al<sub>2</sub>O<sub>3</sub> has important technical applications. The so-called β-Al<sub>2</sub>O<sub>3</sub> proved to be NaAl<sub>11</sub>O<sub>17</sub>.<ref name = "Wiberg&Holleman">{{cite book|author1=Wiberg, E.  |author2=Holleman, A. F. |year = 2001| title = Inorganic Chemistry| publisher = Elsevier| isbn = 978-0-12-352651-9}}</ref>

Molten aluminium oxide near the melting temperature is roughly 2/3 [[tetrahedral]] (i.e. 2/3 of the Al are surrounded by 4 oxygen neighbors), and 1/3 5-coordinated, with very little (<5%) [[octahedral]] Al-O present.<ref name="Skinner2013">{{cite journal|title=Joint diffraction and modeling approach to the structure of liquid alumina|doi=10.1103/PhysRevB.87.024201|year=2013|author=Skinner, L.B.|journal=Phys. Rev. B|volume=87|issue=2|page=024201|bibcode=2013PhRvB..87b4201S|display-authors=etal|doi-access=free}}</ref> Around 80% of the oxygen atoms are shared among three or more Al-O polyhedra, and the majority of inter-polyhedral connections are corner-sharing, with the remaining 10–20% being edge-sharing.<ref name = "Skinner2013"/> The breakdown of octahedra upon melting is accompanied by a relatively large volume increase (~33%), the density of the liquid close to its melting point is 2.93 g/cm<sup>3</sup>.<ref>{{cite journal|title=Non-Contact Thermophysical Property Measurements of Liquid and Undercooled Alumina |doi=10.1143/JJAP.43.1496|year=2004|author=Paradis, P.-F.|journal= Jpn. J. Appl. Phys. |volume=43|issue=4| pages=1496–1500|bibcode = 2004JaJAP..43.1496P |s2cid=250779901 |display-authors=etal}}</ref> The structure of molten alumina is temperature dependent and the fraction of 5- and 6-fold aluminium increases during cooling (and supercooling), at the expense of tetrahedral AlO<sub>4</sub> units, approaching the local structural arrangements found in amorphous alumina.<ref>{{cite journal |last1=Shi |first1=C |last2=Alderman |first2=O L G |last3=Berman |first3=D |last4=Du |first4=J |last5=Neuefeind |first5=J |last6=Tamalonis |first6=A |last7=Weber |first7=R |last8=You |first8=J |last9=Benmore |first9=C J |title=The structure of amorphous and deeply supercooled liquid alumina |journal=Frontiers in Materials |date=2019 |volume=6 |issue=38 |pages=38 |doi=10.3389/fmats.2019.00038 |bibcode=2019FrMat...6...38S |doi-access=free }}</ref>