High-Luminance Red Ba2CaB2Si4O14: Eu3+ with “Abnormal” Thermal Stability and Adjustable Photoluminescence from Defect Regulation and Energy Transfer via Dy3+ Codoping for “Warm” Illumination and Pressure Sensing Applications

In pursuit of illumination-grade wLEDs, innovative red-emitting phosphors with sufficient luminance and thermal stability are required. Herein, a Eu³⁺-incorporated Ba₂CaB₂Si₄O₁₄ (BCBS) phosphor, exhibiting relatively low efficiency, poor thermal stability, and red line peak photoluminescence (PL) at 614 nm, is preliminarily obtained as a result of Eu³⁺ 4f–4f transitions. The Rietveld refinements and density functional theory (DFT) calculations indicate that Eu³⁺ is energetically favored in Ca²⁺ sites. To overcome the above issues of BCBS: Eu³⁺, heterovalent Dy³⁺ is codoped into the phosphor to simultaneously realize: (i) ultrahigh thermal stability of red PL based on structure tailoring and trap regulation associated with both Dy(Eu)_Ca^• and V_O^″ defects. For comparison, by tridoping Na⁺ at the expense of Dy(Eu)_Ca^• via Dy(Eu)³⁺ + Na⁺ → 2Ca²⁺, the red PL performance is becoming very bad and even worse than that of BCBS: Eu³⁺, indicating that Dy(Eu)_Ca^• plays a major role, though the effect of V_O^″ cannot be completely ruled out, (ii) energy transfer (ET) from Dy to Eu for enhancing PL efficiency/intensity, and (iii) color-tunable PL, including “warm” white light, by combination of both autologous “cool” white PL from Dy and red emission from Eu. The assembled phosphor-converted white light-emitting diode (pc-wLED), adopting single-phase BCBS: Dy³⁺/Eu³⁺ with n near UV (nUV) LED chip via a remote “capping” packaging strategy, is a promising candidate to be applied as a white-emitting phosphor in wLEDs. Meanwhile, due to a strong mutual relationship between the intensity ratios of PL bands from magnetic/electric dipole transitions (EDT/MDT) for either Dy (Y/B) or Eu (R/O) and the crystal field environment, the obviously visual PL color change under distinct pressures in a wide range of pressure fluctuations indicates that the phosphor could also be used as a potentially optical pressure sensor with high sensitivity.