  Ab-initio density functional theory (DFT) calculations of the relative
stability of anatase and rutile polymorphs of TiO2 were carried using
all-electron atomic orbitals methods with local density approximation (LDA).
The rutile phase exhibited a moderate margin of stability of ~ 3 meV relative
to the anatase phase in pristine material. From computational analysis of the
formation energies of Si, Al, Fe and F dopants of various charge states across
different Fermi level energies in anatase and in rutile, it was found that the
cationic dopants are most stable in Ti substitutional lattice positions while
formation energy is minimised for F- doping in interstitial positions. All
dopants were found to considerably stabilise anatase relative to the rutile
phase, suggesting the anatase to rutile phase transformation is inhibited in
such systems with the dopants ranked F>Si>Fe>Al in order of anatase
stabilisation strength. Al and Fe dopants were found to act as shallow
acceptors with charge compensation achieved through the formation of mobile
carriers rather than the formation of anion vacancies.
