Thermodynamic stability and optoelectronic properties of cubic and rhombohedral ZnTiO3 crystallographic phases

ZnTiO3 material is attracting attention due to its possible application in quantum dots sensitized solar cells (QDSSCs) and dye sensitized solar cells (DSSCs) as nanostructured mesoporous electrodes for hosting absorbing... [ view full abstract ]

ZnTiO₃ material is attracting attention due to its possible application in quantum dots sensitized solar cells (QDSSCs) and dye sensitized solar cells (DSSCs) as nanostructured mesoporous electrodes for hosting absorbing quantum dots or dyes. High mobility values about 400 and 150 cm²/Vs are observed for hexagonal ZnTiO₃ along 100 and 001 directions. These mobility values are one order larger than commonly used TiO₂ photoelectrode material. However, synthesis of phase pure ZnTiO₃ is difficult and wide discrepancy is reported for cubic ZnTiO₃. The present work considered cubic and rhombohedral ZnTiO₃ phases and structural optimization is carried out using total energy and force minimization method as implemented in Wien2K. The optimized lattice parameter is 3.84 Å for cubic ZnTiO₃ and a = 5.13 Å, c =14.083 Å for rhombohedral ZnTiO₃ and are consistent with the reported literature. The optimized Ti-O bond lengths are 1.92 Å and 2.11 Å for cubic and rhombohedral ZnTiO₃, respectively. The phonon dispersion for cubic ZnTiO₃ showed large negative/imaginary frequencies, suggesting that cubic phase is dynamically unstable. Goldschmidt tolerance factor is 0.76 for ZnTiO₃, substantiating the deviation from cubic phase, consistent with the observed dynamical instability. Thus, the present studies suggest that single phase cubic ZnTiO₃ material may not be possible to synthesize, contradicting cubic ZnTiO₃ reports. Further, low Goldschmidt tolerance factor suggest rhombohedral ZnTiO₃ and is substantiated by the respective phonon dispersion. The electronic properties such as band structure and density of states are computed for both thermodynamically unstable cubic and stable hexagonal ZnTiO₃ phases. An indirect band gap (Γ - M) values ~ 2.34 eV and 3.94 eV are observed using mBJ exchange correlation potential. The work will also discuss the physical properties such as absorption, reflection and optical conductivity for these structures.