Structural and Electronic Properties of Monoclinic TiO$_2$ (B) Polymorph

Three major polymorphs of TiO$_2$ are known: rutile, anatase, and brookite. A further phase, TiO$_2$ (B), which coexists with, and derives from natural anatase has recently been identified\footnote{J.F.Banfield \textit{et al.}, Am. Mineral. \textbf{76}, 343 (1991).}. It is monoclinic with $C^3_{2h}$ space group, and its conventional cell contains 8 TiO$_2$ formula units. Using the DMol$^3$ approach\footnote{B. Delley, J. Chem. Phys. \textbf{113}, 7756 (2000).}, we study the structural and electronic properties of this polymorph in terms of the OTi$_3$ complex, that we have recently shown\footnote{M. Posternak \textit{et al.}, Phys. Rev. B \textbf{74}, 125113 (2006).} to be the relevant building block for describing the electronic properties of the three major polymorphs. At variance with these latter cases, the 16 O atoms in TiO$_2$ (B) are \textit{not} all threefold coordinated: indeed, 12 O atoms belong to anataselike OTi$_3$ structural units, and the remaining 4 O atoms are twofold coordinated. The outcome of structural differences on the electronic properties of the TiO$_2$ phases is analyzed.