Investigating intrinsic and extrinsic defects in an inorganic hybrid system (BaTiO₃-CaF₂) via vacuum ultraviolet-excited low-temperature luminescence

Investigations into the luminescence of undoped and Eu-doped BaTiO3@CaF2 hybrid compound, as well as its individual components, were performed at both room and low temperatures, and the findings were discussed in the context of intrinsic and extrinsic defects present in both matrices. The synthesis process employed an ecofriendly method, capable of producing crystalline phases in just 1 min of plateau time at temperatures as low as 140 degrees C. The microwave-hydrothermal method can provide free oxygen ions at the surface, allowing them to interact with the ions from the precursor solution of CaF2. For Eu-doped samples, while the isolated CaF2 showed the characteristic 4f65d1-4f7 Eu2+ transition, BaTiO3@CaF2 hybrid compound showed the association of two defects, resulting in emissions related to defects from oxygen ions as well as Eu2+. The evidence of intrinsic defects was confirmed by comparing the samples measured at 60 K, where the influence of lattice vibrations was reduced, leading to a decrease in non-radiative processes. Another difference observed was the presence of the 4f-4f transitions associated with the presence of Eu3+ in this sample. Discussions were further supported by X-ray Absorption Spectroscopy (XAS) results, performed at the Ti K-edge and Eu L3-edge. EXAFS parameters were estimated based on the incorporation of Eu into the CaF2 structure and replacing Ca sites. For the BaTiO3@CaF2: Eu system, both EuBa and EuTi had to be included in the fitting process, indicating that some of the Eu dopant is either in the lattice of CaF2 or in BaTiO3.