Synthesis and characterization of novel Ho³⁺-Doped calcium aluminates as efficient green emitters for lighting applications

A novel series of Calcium Aluminates doped with varying amounts of Ho³⁺ ions has been successfully synthesized using the solid-state reaction method. Structural analysis confirms the formation of a pure cubic phase with the I-43d space group, indicating that the doping process does not alter the crystal structure. The average crystallite sizes of Ca_2-xAl₂O₅: xHo³⁺ (x = 0–5 mol%) range from 0.2165 μm, making these materials promising candidates for lighting applications. Notably, the ⁵I₈→ ⁵F₁ transition of Ho³⁺ ions leads to a significant excitation band at 452 nm in the photoluminescence excitation (PLE) spectra for Ca₂Al₂O₅: 2 mol% Ho³⁺ phosphors. Photoluminescence (PL) emission spectra reveal a distinct green emission band around 550 nm, attributed to the ⁵I₈ ← ⁵F₄ transition of Ho³⁺ ions, with the highest intensity observed at a doping concentration of 2 mol%. The CIE coordinates indicate that these phosphors emit in the green region, with a color temperature of 6236 K and color purity of 99.5%. Optical absorption and reflectance spectra show various transitions related to Ho³⁺, with an optical energy band gap of 4.43 eV. Scanning electron microscopy (SEM) images depict agglomerated particles with irregular structures. Temperature-dependent PL tests indicate thermal stability up to 382 K, comparable to commercial phosphor-converted LEDs, with an activation energy of 0.16 eV, highlighting their potential for thermal sensing and solid-state lighting applications. These findings suggest that Ho³⁺-doped Ca₂Al₂O₅ phosphors could serve as effective green emitters in lighting technologies.