Defect and dopant induced photoluminescence of molten salt synthesized BaZrO3 crystals

Defects in materials can induce advantageous optical, catalytic and magnetic properties. For optical properties, defects could give visible photoluminescence when photoexcited electrons trapped at oxygen vacancies are combined with hole. In this work, BaZrO3 (BZO) crystals with different sizes were synthesized using a molten salt synthesis method at four annealing temperatures. Spherical nanocrystals (NCs) are formed at 720 degrees C and cubic submicron crystals (SCs) at 1020 degrees C. These BZO crystals emit bright blue light around 437 nm upon ultraviolet excitation while the spherical NCs show higher photon output than the cubical SCs. The emission intensity of the BZO crystals has an opposite correlation with the MSS annealing temperature. Also, the emission intensity of the BZO crystals increases and decreases upon post-synthesis annealing under reducing or oxidizing atmosphere. Therefore, the photoluminescence from these BZO crystal possibly arises from oxygen vacancies. Moreover, Cr3+ doped BZO cubic NCs show bright red emission from the spin-forbidden E-2(g) -> (4)A(2g) transition of Cr3+ ions. Several optical parameters such as crystal field splitting energy and Racah Parameters are evaluated for the BZO:Cr NCs which suggest low covalency of Cr3+ ions in the BZO host. Lifetime spectroscopy revealed that Cr3+ ions are localized at both BaO12 and ZrO6 polyhedra. On the other hand, Pr3+ doped BZO cubic NCs demonstrate greenish blue emission due to the blue BZO host emission and the green/red emission from Pr3+ ions. Overall, this work demonstrates a strategy for making crystal materials with tunable luminescence by understanding the role of defects and judiciously choosing dopant ions.