Why is it so difficult to realize Dy4+ in as-synthesized BaZrO3?

Rare-earth doped barium zirconate (BaZrO3) ceramics are of interest as proton-conducting and luminescent materials. Here, we report a study of dysprosium (Dy) and other relevant point defects in BaZrO3 using hybrid density-functional defect calculations. The tetravalent Dy4+ is found to be structurally and electronically stable at the Zr lattice site (i.e., as Dy-Zr(0)), but most often energetically less favorable than the trivalent Dy3+ (i.e., Dy-Zr(-)) in as-synthesized BaZrO3, due to the formation of low-energy, positively charged oxygen vacancies and the mixed-site occupancy of Dy in the host lattice. The Dy4+/Dy3+ ratio can, in principle, be increased by preparing the material under highly oxidizing and Ba-rich conditions and co-doping with acceptor-like impurities; however, a post-synthesis treatment may still be needed to realize a nonnegligible Dy4+ concentration. We also find that certain unoccupied Dy 4f states and the O 2p states in the Dy-Zr(0) configuration are strongly hybridized, a feature not often seen in rare-earth-containing materials, and that the isolated Dy-Zr defect might be the source of a broad blue emission in band-to-defect ("charge-transfer") luminescence.