Elastic, electronic and optical properties of cotunnite TiO2 from first principles calculations

The ultrasoft pseudopotential technique is used to explore the elastic, electronic and optical properties of cotunnite TiO2 using LDA and GGA proposed by Perdew Wang (PW91), Perdew-Burke-Ernzerhof (PBE) functional as defined by Wu and Cohen (PBEWC) and PBE functional for solids (PBESOL). The calculated elastic constants bulk modulus, shear modulus and Young's modulus are in agreement with the previous theoretical reports. From our investigated shear anisotropy factors (A1, A2, and A3), we infer that cotunnite TiO2 is strong anisotropy in case of A1 and A2 and less anisotropy in case of A3. The value of mean sound speed and Debye temperature are calculated using the obtained values of elastic moduli. The calculated structural parameters are in accord with the reported experiment and theoretical results. Our obtained values of direct bandgaps show an improvement over the other previous theoretical reports. The values of the dielectric constant (ε1(ω)) of cotunnite TiO2 calculated within LDA and GGA approximations are 7.655 (LDA (CA-PZ)), 7.578 (GGA (PW91)), 7.685 (GGA (WC)) and 7.655 (GGA (PBESOL)), which are slightly higher than the experimental values of rutile (6.69) and anatase (6.55) polymorphs. The obtained values of the refractive index are consistent with rutile TiO2 and higher than anatase phase. The investigated imaginary part of dielectric constant and absorption spectrum reflect that the cotunnite TiO2 is a weak photocatalytic material as compared to anatase and similar to rutile phases.