Substitutional Cu Doping at Ca and Nb Sites in Ba₃CaNb₂O₉ Toward Improved Photoactivity–A First-Principles HSE06 Study

Seeking tenable and sustainable alternatives for clean energy harvesting, artificial photosynthesis has emerged as a realm of research. We have investigated the effects of Cu doping at Ca and Nb sites in Ba3CaNb2O9 (BCN) on magneto-optoelectronic properties through Density Functional Theory studies, employing the HSE06 hybrid functional to guide rational syntheses. Our calculations show that while Cu replacement of a Ca atom requires lower formation energy than replacing an Nb atom, oxygen vacancies (OV) gradually reduce this to a large extent within the thermodynamic formability window when placed at the nearest O site, promoting Nb replacement. Cu doping at the Nb site substantially narrows the band gap, inducing the absorption of visible photons as the dopant concentration increases, while the placement of OV at CuO6 causes gradual blue shifts. Cu doping induces weak electron polaron formation in the CuO6 unit due to net spin polarization in Cu d states, while the placement of the OV annihilates it, albeit without reducing the oxidation state of Cu due to strong charge delocalization. Cu doping at the Nb site significantly improves the optical activity vis-à-vis pure BCN in the visible light region, especially ∼2–3 eV, resulting in promising candidates for photoelectrochemical catalysts.