OPTICAL PROPERTIES OF Ag3AsS3 CRYSTAL DOPED WITH Ho
DOI:
https://doi.org/10.32782/pet-2025-2-9Keywords:
Ag₃AsS₃ crystal, Ho doping, electronic structure, Density Functional Theory (DFT), optical absorption, band gap, Varshni modelAbstract
This paper presents a comprehensive study of the effect of rare-earth holmium (Ho) doping on the electronic structure, crystal lattice, and optical properties of the semiconductor proustite crystal (Ag3AsS3). The investigated samples with impurity concentrations of 0.3, 0.6, and 0.9 wt% were synthesized by melting high-purity components followed by homogenizing annealing. X-ray diffraction analysis confirmed the single-phase nature of the obtained materials and the correspondence of their structure to the R3c space group symmetry. Theoretical calculations of the electronic energy structure were performed within the framework of Density Functional Theory (DFT) using the CASTEP code. The Generalized Gradient Approximation (GGA) with Perdew-Burke-Ernzerhof (PBE) parameterization and Vanderbilt ultrasoft pseudopotentials were applied to describe the exchange-correlation interaction. Geometric optimization of the crystal lattice was carried out using the Broyden-Fletcher-Goldfarb-Shanno (BFGS) algorithm. Supercell calculations revealed that the incorporation of Ho atoms, substituting Ag sites, leads to the appearance of narrow bands of localized f-electrons near the top of the valence band formed by sulfur p-states. It was established that doping does not alter the indirect nature of the Ag3AsS3 matrix band gap. The spectral distribution of the absorption coefficient was experimentally investigated in the temperature range of 100–300 K. The optical band gap (Eg) was determined using the Tauc method for indirect transitions. It was found that increasing the Ho concentration from 0.3 to 0.9 wt% results in a monotonic decrease of Eg at room temperature from 2.09 eV to 2.06 eV. This narrowing is attributed to local lattice distortions arising from the difference in ionic radii between Ag+ and Ho3+. The temperature dependence of the absorption edge was analyzed within the Varshni model. The parameter related to the Debye temperature remained stable for all concentrations (~180 K), indicating the preservation of the vibrational properties of the host lattice. The obtained results demonstrate that holmium doping is an effective tool for the controlled modification of the optical parameters of Ag3AsS3 crystals for prospective applications in optoelectronics and nonlinear optics.
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