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<p>The relativistic GGA + U+SOC framework, implemented within the WIEN2k full-potential linearized augmented plane-wave code, is applied to systematically investigate the structural, electronic, optical, and thermoelectric properties of pristine, Eu-doped, and Tm-doped CaS phosphors at a 3.12% substitutional doping concentration. Hubbard-U corrections of 7.07&#xa0;eV for Eu and 8.16&#xa0;eV for Tm, combined with spin-orbit coupling, are applied to treat the strong 4f-electron correlations and relativistic effects inherent to lanthanide dopants. The calculated lattice parameter of pristine CaS (a = 5.74 Å) agrees closely with the experimental XRD value of 5.68 Å, and negative formation energies (-3.18, -2.50, and − 2.12&#xa0;eV/atom for pristine, Eu-CaS, and Tm-CaS, respectively) confirm thermodynamic stability. Pristine CaS exhibits an indirect band gap of 2.49&#xa0;eV within GGA. Application of GGA + U+SOC yields effective band gaps of 1.391&#xa0;eV (Eu-CaS) and 0.979&#xa0;eV (Tm-CaS), and eliminates the artificial metallicity of Tm-doped CaS obtained without these corrections, demonstrating the necessity of the relativistic treatment for lanthanide-doped systems. Eu doping introduces nearly dispersionless 4f-derived localized states in the − 4 to 0&#xa0;eV energy range, consistent with the experimentally observed 4f⁷ → 4f⁶5d¹ absorption band at 420–600&#xa0;nm. Tm³⁺ doping introduces stronger 5d-derived coupling near the conduction band edge, while 4f-host hybridization is weaker than in Eu-CaS owing to greater 4f orbital contraction, and generates n-type donor character consistent with experimental electron-donation behavior. Optical analysis yields static dielectric constants of 4.92 (pristine), 4.88 (Eu-CaS), and 4.97 (Tm-CaS), with visible-range refractive indices of 2.8–3.0 and reflectivity of 20–30%, supporting efficient light coupling. Thermoelectric calculations via BoltzTraP identify complementary carrier types: Eu-CaS is p-type (S = + 8.67 × 10⁻⁵ V/K at 300&#xa0;K) and Tm-CaS is n-type (S = − 1.51 × 10⁻⁴ V/K at 300&#xa0;K), directly reflecting the contrasting defect chemistry of each dopant. Peak ZT values of 1.80 × 10⁻² (Tm-CaS) and 1.36 × 10⁻² (Eu-CaS) are obtained at 50&#xa0;K under the constant relaxation time approximation; representing upper-bound estimates as lattice thermal conductivity is not included. The computed electronic structure, optical response, and Seebeck behavior are validated against independent experimental photoluminescence, diffuse reflectance, Tauc-plot band gap, and time-resolved fluorescence measurements on CaS: Eu, Tm phosphors.</p>

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Defect-chemistry-guided design of Eu/Tm-doped CaS phosphors: GGA+ U+SOC insights into electronic structure, optical response, and thermoelectric transport for advanced LED applications

  • 期刊论文

  • 发表日期：

    2026年7月28日

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  • Habib Arshad,
  • Salman Ahmad,
  • Amin Ur Rahman,
  • Sikander Azam,
  • Faisal Alresheedi,
  • Muhammad Asif Hasham,
  • Asiya Zaman Khan

摘要

The relativistic GGA + U+SOC framework, implemented within the WIEN2k full-potential linearized augmented plane-wave code, is applied to systematically investigate the structural, electronic, optical, and thermoelectric properties of pristine, Eu-doped, and Tm-doped CaS phosphors at a 3.12% substitutional doping concentration. Hubbard-U corrections of 7.07 eV for Eu and 8.16 eV for Tm, combined with spin-orbit coupling, are applied to treat the strong 4f-electron correlations and relativistic effects inherent to lanthanide dopants. The calculated lattice parameter of pristine CaS (a = 5.74 Å) agrees closely with the experimental XRD value of 5.68 Å, and negative formation energies (-3.18, -2.50, and − 2.12 eV/atom for pristine, Eu-CaS, and Tm-CaS, respectively) confirm thermodynamic stability. Pristine CaS exhibits an indirect band gap of 2.49 eV within GGA. Application of GGA + U+SOC yields effective band gaps of 1.391 eV (Eu-CaS) and 0.979 eV (Tm-CaS), and eliminates the artificial metallicity of Tm-doped CaS obtained without these corrections, demonstrating the necessity of the relativistic treatment for lanthanide-doped systems. Eu doping introduces nearly dispersionless 4f-derived localized states in the − 4 to 0 eV energy range, consistent with the experimentally observed 4f⁷ → 4f⁶5d¹ absorption band at 420–600 nm. Tm³⁺ doping introduces stronger 5d-derived coupling near the conduction band edge, while 4f-host hybridization is weaker than in Eu-CaS owing to greater 4f orbital contraction, and generates n-type donor character consistent with experimental electron-donation behavior. Optical analysis yields static dielectric constants of 4.92 (pristine), 4.88 (Eu-CaS), and 4.97 (Tm-CaS), with visible-range refractive indices of 2.8–3.0 and reflectivity of 20–30%, supporting efficient light coupling. Thermoelectric calculations via BoltzTraP identify complementary carrier types: Eu-CaS is p-type (S = + 8.67 × 10⁻⁵ V/K at 300 K) and Tm-CaS is n-type (S = − 1.51 × 10⁻⁴ V/K at 300 K), directly reflecting the contrasting defect chemistry of each dopant. Peak ZT values of 1.80 × 10⁻² (Tm-CaS) and 1.36 × 10⁻² (Eu-CaS) are obtained at 50 K under the constant relaxation time approximation; representing upper-bound estimates as lattice thermal conductivity is not included. The computed electronic structure, optical response, and Seebeck behavior are validated against independent experimental photoluminescence, diffuse reflectance, Tauc-plot band gap, and time-resolved fluorescence measurements on CaS: Eu, Tm phosphors.