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Zirconium dioxide (ZrO2) or zirconia is a common and widely used zirconium compound naturally occurring in the mineral baddeleyite[1][2]. It is a wide band-gap metal oxide semiconductor with different polymorphs[1]. At atmospheric pressure, pure ZrO2 exists in three thermodynamically stable crystal phases: monoclinic below 1273 K, tetragonal below 2640 K, and cubic below the melting point at 2950 K[2][3].


The tetragonal ZrO2 has a distorted cubic fluorite type of structure (space group P42/nmc)[4]. The unit cell consists of formula units. Each Zr4+ cation is bonded to eight O2- ions (Figure 1). Four of the oxygen atoms are located at a distance of 2.079 Å from the zirconium atom and form a flattened tetrahedron while the Zr-O distance of the other four is 2.373 Å and they form an elongated tetrahedron[4]. The elongated tetrahedrons combined with the displaced oxygen atoms from the ideal positions contributes in the distortion of the fluorite type structure[5].  

Figure 1. Visualization of the structure of t-ZrO2, where zirconium and oxygen are coloured green and red, respectively. (Data source ICSD 66784) (Figure: Laura Kangas-Riikonen).


Tetragonal zirconia is a metastable phase of ZrO2 but can be stabilized by doping it with certain trivalent or tetravalent dopants[6]. Undoped t-ZrO2 can be stabilized in room temperature by rapid quenching[2] or by a nanocrystal size effect[6].


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G. Lamas, A.M. Rosso, M.S. Anzorena, A. Fernández, M.G. Bellino, M.D. Cabezas, N.E. Walsöe de Reca, A.F. Craievich, Crystal structure of pure ZrO2 nanopowders. Scr. Mater. 2006, 55, 553–556. doi:10.1016/j.scriptamat.2006.05.035

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P. Bouvier, E. Djurado, G. Lucazeau, T. Bihan, High-Pressure Structural Evolution of Undoped Tetragonal Nanocrystalline Zirconia, Phys. Rev. B Condens. Matter. 2000, 62, 8731-8737. (DOI:10.1103/PhysRevB.62.8731)

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