First-principles characterisation of structural and electronic properties of some RuO₂ crystals

Density functional theory at the level of LDA, GGA, LDA + U, GGA + U and hybrid functionals is applied to investigate structural and electronic properties of three RuO₂ crystals. The rutile structure, and the pyrite and flourite modifications of RuO₂ are undertaken. The structural properties, enthalpy-pressure curves, electronic states, and Fermi surfaces are presented. The enthalpy-pressure curves show that pressure causes the rutile-RuO₂ to transform into pyrite and flourite phases. The pyrtie phase transforms in the fluorite phase. All calculations point out pressure induced rutile → pyrite phase transition in confirmation with the experimental studies. The pyrite → fluorite transition is pointed out by current calculations. The rutile and pyrite crystals are metals while hypothetical fluorite is a semiconductor. All calculations show s that the fluorite has an indirect bandgap in the 0.57–2.96 eV range. The Fermi surface of metallic rutile structure using GGA + U shows improvement over GGA on comparison with the measurement. The GGA + U calculations suggest that rutile → fluorite and pyrite → fluorite metal-insulator transitions are accompanied by orbital ordering. (paper)