Regular Article Oxygen vacancy stabilized zirconia (OVSZ); a joint experimental and theoretical study Mohsin Raza a, ⁎, David Cornil b , Jérôme Cornil b , Stéphane Lucas c , Rony Snyders a,d , Stéphanos Konstantinidis a a Chimie des Interactions Plasma-Surface (ChIPS), University of Mons, 23 Place du Parc, 7000 Mons, Belgium b Service de Chimie des Matériaux Nouveaux, University of Mons, 23 Place du Parc, 7000 Mons, Belgium c Research center for the Physics of Matter and Radiation (PMR-LARN), University of Namur, B-5000 Namur, Belgium d Materia Nova Research Center, Parc Initialis, B-7000 Mons, Belgium abstract article info Article history: Received 3 May 2016 Received in revised form 16 June 2016 Accepted 20 June 2016 Available online 5 July 2016 Understanding the phase formation in zirconia (ZrO 2 ) has triggered a great debate over the last couple of de- cades, with several mechanisms proposed so far. In the present letter, we demonstrate by well-optimized exper- imental measurements supported by Density Functional Theory (DFT) calculations that only O vacancies allow for the stabilization of the cubic (c) phase at room temperature. These vacancies distort the zirconia lattice, forc- ing the crystal to arrange itself in a high symmetric c structure. © 2016 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. Keywords: DFT calculations Zirconia (ZrO 2 ) Oxygen vacancy Phase transformation Phase stabilization The specific crystal structure of any material is a decisive factor for controlling its properties [1,2] and leads sometime to the discovery of new functionalities [3,4]. In this respect, zirconia (ZrO 2 ) is a material which exists in three crystallographic phases under atmospheric pres- sure: (i) the monoclinic phase (m, space group P2 1 /c) stable up to ~1205 °C; (ii) the tetragonal phase (t, space group P4 2 /nmc) appears from ~1205 °C to 2377 °C; and finally (iii) the cubic phase (c, space group Fm-3m) from 2377 °C to 2710 °C (melting temperature) [5]. Be- cause of ZrO 2 superior chemical stability, high hardness [6], high dielec- tric constant [7] and prominent optical properties [8], ZrO 2 films have been exploited for a broad range of applications e.g., medical applica- tions [9,10], wear resistant coatings [11], oxygen detectors [12,13] and for thermal barrier coatings (TBC) [14,15]. However, for pure zirconia, it is not possible to exploit most of the above mentioned applications as this is restricted by the change in volume of the zirconia-based com- ponents (~5 vol.%) due to the phase transformation upon heating and cooling of the device, which ultimately leads to the deterioration of the device components [16,17]. Therefore, the stabilization of the high temperature c-phase at room temperature is of paramount importance. This has been achieved for decades by doping of cations of lower valence than Zr in the zirconia lattice (e.g. Y or Mg) [18,19]. By adding around 12 mol% of yttria (Y 2 O 3 ), the c-phase of zirconia is found to be stabilized at room temperature and is known as yttria-stabilized zirco- nia (YSZ) [19]. In YSZ, zirconium (Zr 4+ ) is replaced by yttrium (Y 3+ ) so that to maintain the charge neutrality, for two substituting yttrium cations, one oxygen vacancy is created. This makes YSZ not only useful for TBC but also as an electrolyte in solid oxide fuel cells (SOFC) [20– 22] and in oxygen sensors [13] because of its very good ionic conductiv- ity [23]. However, it has also been found that the doping by aliovalent cations leads to the perturbation of the periodic potential of the oxide- ion array, which results in higher energy barrier for O 2– ions during their diffusion to a vacant site in the solid as compared to intrinsic va- cancy-doped oxides [24]. Therefore, to stabilize high temperature c- phases of zirconia at room temperature without any doping of yttria, an intense research has been developed during the last one and a half decade using various synthesis techniques. The stabilization procedure has been related to the grain size, energy input during growth, stresses in the film and O vacancies/N atom incorporation in the zirconia lattice [16,25–34]. However, a consensus over what drives the phase formation in zirconia has not been reached so far. In this letter, we demonstrate that O vacancies are the sole responsi- ble for the stabilization of the high temperature c-phase of zirconia at room temperature. To achieve this, we coupled cold plasma-based reac- tive magnetron sputtering experiments to Ab Initio Density Functional Scripta Materialia 124 (2016) 26–29 ⁎ Corresponding author. E-mail addresses: mohsin.raza@umons.ac.be (M. Raza), Stephanos.KONSTANTINIDIS@umons.ac.be (S. Konstantinidis). http://dx.doi.org/10.1016/j.scriptamat.2016.06.025 1359-6462/© 2016 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. Contents lists available at ScienceDirect Scripta Materialia journal homepage: www.elsevier.com/locate/scriptamat