Structural stability of TiO2 at high pressure in density-functional theory based calculations

Xiang Wu1,2,3 , Eva Holbig1 and Gerd Steinle-Neumann1

1 Bayerisches Geoinstitut, Universit¨at Bayreuth, Bayreuth D-95440, Germany

2 Key Laboratory of Orogenic Belts and Crustal Evolution, MOE and School of Earth and Space Sciences, Peking University, Beijing 100871, People’s Republic of China

E-mail: xiang.wu@pku.edu.cn

Received 5 May 2010, in final form 30 May 2010 Published 28 June 2010


Abstract:

A new study on the pressure-induced phase transitions of TiO2 has been performed using all-electron density-functional theory based computations with the projector augmented wave and the linearized augmented plane wave methods considering five experimentally observed structures. The static results yield a picture that is consistent with experiments, i.e., phase transitions with pressure are predicted as rutile → monoclinic baddeleyite (MI) → orthorhombic I (OI) → cotunnite (OII) on compression, and OII → OI → MI → columbite (TiO2II) on decompression. The elasticities of these five polymorphs are compared. Except for the baddeleyite structure, which is considerably softer than the other polymorphs, all phases show a zero pressure bulk modulus in the range of 200–240 GPa, consistent with compression results and the single crystal elastic constant; on the basis of these results we can say that the cotunnite phase is not a superhard TiO2 polymorph as has been suggested previously. We further find that the rutile and columbite structures are energetically very similar, with the columbite structure favored slightly. All polymorphs are predicted as insulating with comparable band gaps (∼1.7–2.3 eV). Crystal field splitting for the Ti 3d electronic states leads to two distinct conduction bands in rutile and TiO2II for energies smaller than 8 eV, while there is a single conduction band for the other high pressure structures.

To download the article click on the link below:

http://www.staff.uni-bayreuth.de/~bt220235/refs/JPCM_22_295501.pdf