Tunable emission of Li4SrCaSi2O4−yN2y/3:Eu2+ phosphors based on anion substitution induction for WLEDs and optical thermometry

Abstract

Polychromatic emission can be achieved by controlling the distribution of the rare earth activator in multi-cation lattices, which can be used in the fields of white light LED and fluorescence temperature sensing. However, it is still a challenge to control their distribution and location of the target site in a given host material because the distribution of the rare earth activator is uncertain. In this paper, we have chosen Li₄SrCa(SiO₄)₂ as the multi-cation site host and induced the distribution of Eu²⁺ ions between different cation sites through anion substitution, for the first time, to regulate the luminescence characteristics of a series of Li₄SrCaSi₂O_8−yN_2y/3:Eu²⁺ phosphors. In Li₄SrCa(SiO₄)₂:Eu²⁺ phosphors, the substitution of O²⁻ by N³⁻ triggered a distinct ordered to disordered structure transition of the SiO₄ tetrahedron and induced the remote distribution of the Eu²⁺ activator, which was verified through the analysis of the XRD, EPR, FT-IR and fluorescence spectra. Due to the location of Eu²⁺ ions in different cation sites (Eu²⁺_Sr and Eu²⁺_Ca), two distinguishable emission peaks with tunable color emissions and different responses to temperature were realized. The white LED that utilized blue-orange-emitting Li₄SrCaSi₂O₄N_8/3:Eu²⁺ and green-emitting BaSi₂O₂N₂:Eu²⁺ (500 nm) displayed an outstanding color rendering index (R_a) of 85.1. Based on the fluorescence intensity ratio (FIR) technique, an optical temperature measurement mechanism was hypothesized and studied in the temperature range of 293–473 K. The highest S_a of the material was 0.086 K⁻¹, and S_r was 1.76% K⁻¹ based on the FIR detection technology, revealing obviously better than most inorganic optical temperature-measuring materials reported before. Our work indicates that Li₄SrCaSi₂O_8−2yN_4y/3:Eu²⁺ is a promising material for application in White LEDs and optical thermometers.