Excitation-dependent local symmetry reversal in single host lattice Ba2A(BO3)2:Eu3+ [A = Mg and Ca] phosphors with tunable emission colours

Abstract

Eu³⁺ activated phosphors are widely used as red emitters in various display devices and light emitting diodes (LEDs). The emission characteristics of Eu³⁺ depend on the local site symmetry. The present study demonstrates the role of excitation-dependent local symmetry changes due to the structural reorganization on the emission colour tuning of Eu³⁺ from orange-red to orange in single host lattices, Ba₂Mg(BO₃)₂ and Ba₂Ca(BO₃)₂. The choice of these lattices was based on the difference in the extent of strain experienced by the oxygen atoms. The samples with Eu³⁺ at Ba or Mg (Ca) sites were synthesized using the conventional high-temperature solid-state reaction method. The samples were characterized using powder XRD, ¹¹B MAS-NMR, FT-IR, and diffuse reflectance UV-Vis spectroscopic techniques. The room temperature photoluminescence (PL) recorded using different excitation wavelengths revealed a clear difference in the PL emission features due to symmetry reversal from non-inversion to inversion symmetry around Eu³⁺. The reorganization of highly strained oxygen atoms leads to such symmetry reversal. First-principles calculations were used to deduce the optimized structures of the two borate host lattices, and local geometries and their distortions upon Eu³⁺ substitution. The outcomes of these calculations support the experimental findings.