Lattice Site Engineering and Photoluminescence Tuning in Yb3+ and Ho3+ codoped NaY(MoO4)2 Single Crystal via Alkali-Ion Substitution

Yb 3+ and Ho 3+ co-doped AB(MoO 4 ) 2 (A = Na, Li, K; B = Y or Bi) single crystals were successfully grown by the Czochralski method, and the effects of Li + and K + doping on the structural and optical properties of NaY(MoO 4 ) 2 (NYM) crystals were systematically investigated. The crystals confirmed with zircon-type tetragonal structure (space group 4). First-principle calculations revealed the band structure features of NYM crystals and elucidated the differences in bandgap and oxygen defect formation compared with NaBi(MoO 4 ) 2 (NBM). Judd-Ofelt calculations were conducted and the spectral transition properties of Ho 3+ were revealed. Photoluminescence studies demonstrated that the crystals exhibited high-purity red down-conversion emission under 450 nm excitation and typical two-photon up-conversion green emission under 980 nm excitation. Li + /K + doping effectively suppressed the green emission and enhanced the red component, with a more pronounced effect for K + . Temperature-dependent spectra (300-540 K) indicated good repeatability and thermal stability, with the K + -doped crystal retaining 55% of its room-temperature emission intensity at 540 K. The activation energies derived from temperature-dependent spectra suggest that Li + and K + incorporation significantly improved thermal stability. Furthermore, optical thermometry analysis revealed that Li + and K + doping significantly enhanced the optical sensing sensitivity. Fluorescence decay curves suggested that Li + and K + doping increased lattice disorder, suppressed Ho 3+ -Ho 3+ cross-relaxation, and consequently modified the red-to-green emission ratio. These results indicate that Yb,Ho:NYM crystals and their Li + /K + -doped counterparts exhibit outstanding performance in optical anti-counterfeiting, near-infrared solid-state lasers and optical thermometry applications.