Unraveling the effect of Al-doping on the local structure and the photoluminescence of CaTiO3:Pr nanophosphor

Abstract

Phosphors are inorganic materials that produce visible light when stimulated by external energy sources and have found applications in a wide range of technologies. CaTiO₃:Pr³⁺ emerges as a promising phosphor material with an intense emission due to ¹D₂ - ³H₄ transition of the rare-earth ion very close to the ideal red. Various strategies have been employed to enhance this emission, such as heterovalent substitution of Ti⁴⁺ ions for Al³⁺ ions. However, the mechanism for charge compensation in this case is not well described in the literature. In this study, Ca_0.998Pr_0.002Ti_1-xAl_xO₃ (CPTA) nanophosphor samples were synthesized by the polymeric precursor method with Pbnm space group without spurious phases. X-ray absorption near edge structure (XANES) spectra at Ti K- and L_II,III-edges do not show disorder inside TiO₆ octahedra as Al atoms are incorporated into lattice, which is corroborated by extended X-ray absorption fine structure (EXAFS) analysis at K-edge of the same element. These XANES spectra also indicates the absence of Ti³⁺ ions, confirmed by electron paramagnetic resonance (EPR) measurements. On the other hand, XANES spectra at O K-edge and calculated projected density of states, as well as Raman spectra, show that Al incorporation cause a symmetry breaking at the local structure due to the formation of O vacancies. These vacancies originate a reduction of intensity values in photoluminescent red emission because of the quenching in Pr ions with addition of Al atoms at CaTiO₃ structure.