Piezoelectric, ferroelectric, optoelectronic phenomena in hydroxyapatite by first principles and with various defects

The results are based on DFT calculations of hydroxyapatite (HAP) structures, pristine and defective, determined by various defects formed by Oxygen vacancy dependent on its charged states. Ordered hexagonal and monoclinic HAP phases have own spontaneous polarization, while disordered do not have it. These ferroelectric properties determined by orientation of OH dipoles provide piezoelectric and pyroelectric phenomena in such HAP structures for which all corresponding values and coefficients were investigated and calculated in this paper. The optical properties of defective HAP are mainly determined by various types of Oxygen vacancy defects (as well as hydroxyl OH group vacancy) and can lead to absorption and photocatalysis under ultraviolet illumination. The hybrid density functional theory (DFT) approaches of the structural and electronic properties of oxygen vacancies in large HAP supercells within the plane-wave formalism were mainly used in this work. The vacancies were investigated in large supercells, for which formation energies and electronic transition energies were calculated. The calculations were carried out using DFT, as implemented by the (Vienna ab initio package) VASP. The exchange-correlation potential was evaluated either using the generalized gradient approximation according to PBE functional or the threeparameter hybrid B3LYP functional, which incorporates a fraction of exact exchange with local and semi-local functional. These methods are also applied to the calculation of defect levels. Several other methods, such as AIMPRO and HyperChem, were used too, for several necessary cases and comparison. As a result, the main new features of pristine and defective HAP were established and analysed from first-principles.publishe