Introduction

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].

Structure

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 O^2- 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).

Properties

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].

References