Hybrid HF–DFT modeling of monolayer water adsorption on (001) surface of cubic BaHfO 3 and BaZrO 3 crystals A.V. Bandura ⁎, R.A. Evarestov, D.D. Kuruch Department of Quantum Chemistry, St. Petersburg State University, 26 Universitetsky Prospect, Peterhof, St. Petersburg, 198504, Russia abstract article info Article history: Received 22 March 2010 Accepted 26 May 2010 Available online 4 June 2010 Keywords: Perovskites Low index single crystal surfaces Surface relaxation and reconstruction Water adsorption Water dissociation Hybrid HF–DFT calculations First-principles calculations have been used to study the atomic structure, preferred sites and adsorption energies for water adsorption at different terminations of the cubic phase of perovskite-structured BaHfO 3 and BaZrO 3 . By considering different initial positions of water molecules, the possibility of water dissociation has been investigated. It is demonstrated that the site selectivity and the form of adsorbed molecule can be affected by the choice of surface unit cell. Dissociative adsorption was found to be favorable for all surfaces in consideration. Hydroxylation of ZrO 2 - and HfO 2 -terminated surfaces is accomplished by a noticeable reconstruction of the surface structure of cubic phase towards the orthorhombic phase. Calculated atomic charges in bare and hydroxylated surfaces show that BaHfO 3 crystal is slightly more ionic than BaZrO 3 . © 2010 Elsevier B.V. All rights reserved. 1. Introduction The ABO 3 perovskite-type crystals, where A is a divalent cation, B is a tetravalent transition-metal atom, exhibit a variety of interesting electronic, electromechanical, and conductive properties [1,2], which are the basis for many existing and potential applications [3,4]. Perovskites can exist in different phase modifications depending on the combination of cations A and B [5,6] and their properties are often symmetry dependent. In the ABO 3 perovskite structure the smaller tetravalent B cation resides in the center of corner-sharing BO 6 octahedra. The larger bivalent A cation is located in the cavities between eight octahedra with a 12-fold oxygen coordination. In barium zirconate and barium hafnate tetravalent cation fits almost perfectly on the B site: thus, the Goldschmidt [7] tolerance factor is about 1.0 [8] and the symmetry of these crystals is cubic. Barium zirconate, BaZrO 3 , is a very important ceramic material which has many technological applications. Moreover, it has attracted great attention as a high temperature proton conductor with possible applications in fuel cells and hydrogen sensors [9,10]. The properties of BaZrO 3 have been measured during the past 40 years. Nevertheless, until now, only several calculations have been reported [11–13] on the atomic and electronic structure of bulk BaZrO 3 . The work of King- Smith and Vanderbilt [11] was the first where the lattice constants, elastic constants, zone-center phonon frequencies, Grüneisen para- meters, and band structures were calculated for series of perovskites including BaZrO 3 . Combined theoretical and experimental study of the low-temperature properties of BaZrO 3 was made by Akbarzadeh et al. [13]. At the same time, a number of works was attended to modeling of the protonic conduction in doped zirconates [2,14]. Conduction of perovskites is controlled by the oxygen vacancies generated by doping with trivalent cations. Exposure to humid atmospheres incorporates hydroxide ions into vacancies, allowing the crystal to behave as an efficient proton conductor. Barium hafnate, BaHfO 3 , apparently, is similar to BaZrO 3 on its useful properties and it is a promising material for electronic applications. It has been well known as a high melting temperature material [15,16]. However, to the best of our knowledge, there are a few works exploring its physical properties. Recently, Maekawa and co-workers [17] have reported some thermal and mechanical properties of a polycrystalline sample of this compound. Crystal structure and microwave dielectric properties of some alkaline-earth hafnates including BaHfO 3 , have been investigated by Feteira et al. [18]. The theoretical study of structural, elastic, electronic and optical properties for BaHfO 3 , using plane wave method (PW), in the framework of the density functional theory (DFT) within the local density approximation (LDA) was carried in [19]. In [20] the electronic band structure and vibrational properties of the BaHfO 3 , using DFT–LDA were investigated. Cohesive energies for a range of simple II–IV perovskites were calculated in [8] using generalized gradient approximation (GGA) exchange-correlation functionals [21] in PW DFT framework. The surface effects of ceramics are important for their applications. There has been a continuous interest in the surface properties of these materials. In all mentioned applications perovskites serve as an electrolyte in contact with catalysts and electrodes. Therefore, knowledge about BaHfO 3 and BaZrO 3 surface structures would be Surface Science 604 (2010) 1591–1597 ⁎ Corresponding author. Tel.: +7 812 348 21 19; fax: +7 812 428 69 39. E-mail address: andrei@ab1955.spb.edu (A.V. Bandura). 0039-6028/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.susc.2010.05.030 Contents lists available at ScienceDirect Surface Science journal homepage: www.elsevier.com/ locate/susc