Papers by Keyword: Chalcogenides

Abstract: Machine Learning (ML) approach seeks to open new frontiers in the search for novel thermoelectric materials that convert heat waste into useful electrical energy. Five regression-type ML algorithms Linear Regression, Random Forest, XGBoost, Bagging Regressor, and Gradient Boosting Regressor were employed in this study to forecast the thermoelectric figure of merit (ZT) of doped chalcogenide compounds. Gradient Boosting Regressor achieved the best baseline performance (R² = 94.5%, MAE = 0.073, RMSE = 0.128), further improved with hyperparameter tuning to R² = 95.8%, MAE = 0.065, and RMSE = 0.112. Compared to the baseline, tuning reduced RMSE by 12.6% and MAE by 10.8%. The optimized model reliably reproduced experimental ZT trends in doped Bi₂Te₂Se and Ag₂Te, validating its predictive capacity. Our findings show that hyperparameter tuning is greatly recommended for high-fidelity predictions in thermoelectrics.

Abstract: The work is devoted to preparation and optical study of thin solid films of pseudo-binary AsS₃ - GeS₄ glasses, which are remarkable as topologically belonging to isostatic materials from intermediate phase (IP) region. The glasses from IP are stress free due to their structural self-organization, consequently have to be more stable and viable. To maintain the composition of the bulk source material, the films were grown via a very fast thermal vacuum evaporation of powder of relevant glasses onto optical quartz substrates. The optical properties of (GeS₄)_x(AsS₃)_1-x films (x = 0 ÷ 1) were studied by spectroscopic ellipsometry in the spectral range 0.35 ÷ 1.77 µm. It was pointed out that the films under investigation are entirely transparent in the visible and IR spectral region λ = 0.45 ÷ 1.77 µm. The refractive index follows the usual curves of a normal dispersion, reaching the maximal value around n = 3.0 at λ = 0.35 µm for AsS₃ and the minimal one around n = 1.97 at λ = 1,77 µm for Ge_17.2As_3.5S_79.3. The glass composition strongly influences the refractive index in the visible spectral range but this influence becomes must more moderate in the IR one. It was pointed out that independent on wavelength, both the extinction coefficient and refractive index nearly linearly falls with germanium concentration increase, with an exception related to ternary Ge_17.2As_3.5S_79.3, in which the extinction coefficient shifts toward higher values but the refractive index toward lower values than those expected from aforementioned linearities. The Raman spectra analysis of the studied thin films allowed explanation of this feature in terms of inhomogeneity of the Ge_17.2As_3.5S_79.3 films, caused by a great quantity of the nanoscale - separated stoichiometric As₂S₃ forming in Ge_17.2As_3.5S_79.3 films during their growing.

Abstract: The scientific basis for the production of a new composite material (1-x)PbSexPbSeO₃, where x=0-1, by oxidation with oxygen at temperatures of 500-550 °C and oxidation times of 0.5-4 h from the initial phase of PbSe in the form of powder, film or compact material, having a ferroelectric phase transition in disordered crystals is developed. On the X-ray spectra of the original PbSe samples oxidized at 500°C (oxidation time of 0.5 h) it has been found that the PbSe phase reflexes are predominately present, including the X-ray spectra of the original PbSe samples oxidized at 500 °C (oxidation time of 4 h) - PbSeO₃ monoclinic phase reflexes. For all other PbSe oxygen-oxidized samples at temperatures of 500-550 °C and within the time range of 0.5-4 h, X-ray spectra show the simultaneous presence of X-ray reflexes of both phases with the trend of increasing the PbSeO₃ phase as the oxidation time increases. Temperature measurements of the DC resistance of the PbSe samples revealed an abnormal change in electrical resistance at the initial oxidation stage for both the film and the compact material, and further oxidation contributed to the capsulation of PbSe grains by the dielectric casing PbSeO₃ and the gradual increase in the resistance of the material.

Abstract: The influence of reactivity of different sheath metals such as Ag, Fe, Cu, Ni and SS on the superconducting properties of NdFeAsO_0.6F_0.4 wires has been investigated. The observed structural and superconducting properties of the metal sheathed wires conclude that among the five candidates Ag and Fe are technically compatible to be used as sheath materials in terms of reactivity. However, the best superconducting properties such as T_C and transport J_C were exhibited by Ag sheathed NdFeAsO_0.6F_0.4 wires.

Abstract: We have studied the impact of simultaneous Ba²⁺ doping at Sm³⁺ site (hole doping) and F^- doping at O^2- site (electron doping) on the structural and superconducting properties of Sm1111 system using a single source, BaF₂. With BaF₂ doping the extent of layering of the grains and the grain size have been improved considerably. High transport current of 875 A/cm² at 12 K has been achieved for the doped sample. The binary doped sample shows higher T_C of 51 K than the corresponding F-alone doped sample. In addition, substantially high values of H_C2 and better J_C-H behavior are observed for the BaF₂ doped samples. These results strongly indicate the scope for SmFeAsO based conductors with higher current carrying capacity.

Abstract: The transport, resonance and optical properties of anion-substituted manganese chalcogenides MnSe_1-xTe_x in the 77-300 K temperature range in magnetic fields up to 1T are studied. The magnetoresistance effect with the maximum value in the vicinity of the Neel temperature for the composition x = 0.1 is revealed. EPR data indirectly indicate the type of the current carriers – lattice polarons. The changes in electron structure occurring due to the anionic substitution are studied using the optical methods.

Abstract: The high pressure induced structural phase transition and elastic properties of three Europium chalcogenides (EuX; X = S, Se, Te) have been studied using a two body potential approach. The calculated compression curves of EuS, EuSe and EuTe obtained so has been compared with recently measured three body potential data. The calculated transition pressures are in good agreement with the experimental data. The phase transition pressure for EuS, EuSe and EuTe going from the NaCl phase to CsCl phase have been observed are 22 GPa, 15 GPa, 10 GPa respectively, close the theoretical and experimental data. We have also calculated bulk modulas and second order elastic constants at high pressure which show partly ionic nature of theses compounds. The B₁ (NaCl) phase is found to be higher in energy than the B₂ (CsCl) phase and more stable at zero pressure.

Abstract: Several research groups have reported that nanocrystalline II-VI semiconductors show enhanced luminescence, increased oscillator strength and shorter response time. Nanocrystalline powder samples of CdS, CdSe, ZnS and ZnSe nanocrystals and their composites with PVA and PVK have been prepared by chemical route. SEM. TEM and AFM images indicate agglomeration of particles. XRD reveal the crystal structure and size in nanometer range and absorption spectra show increased band gap due to quantum confinement.The EL studies on nanocrystalline powder samples and nanocrystal/polymer composites have shown that the light emission starts at certain threshold voltage, different for different specimens and then increases with increasing voltage. It is found that smaller nanocrystals have lower threshold voltage and higher EL brightness. It is observed that nanocomposite give much higher electroluminescence starting at lower voltage and increasing very fast with the voltage as compared to nanocrystalline powder. The emission spectra are found to depend on the material, crystalline size and doping. Electroluminescence in undoped and doped chalcogenide nanocrystals and nanocomposites is reviewed in this paper. In nanosize regime, electroluminescence (EL) is governed by the size quantization effect. Contents of Paper

Abstract: Abstract. In this research experimental investigation of the influence of copper introduction on some relevant parameters in As-S-Se amorphous thin films is performed. Copper is introduced into As₂(S_0.5Se_0.5)₃ amorphous thin film in concentration of 3 at.%. Samples of As₂(S_0.5Se_0.5)₃ and Cu₃(As₂(S_0.5Se_0.5)₃)₉₇ amorphous thin films are prepared by the vacuum thermal evaporation technique from previously synthesized bulk samples. Envelope method is applied for the determination of the optical constants, using the transmission and reflection spectra. The dispersion of the refractive index is discussed in terms of the single oscillator model proposed by Wemple–DiDomenico. Values of absorption coefficients in the high absorption region are discussed according to Tauc's law.Instrumented indentation testing is performed, using the Berkovich geometry indenter, for obtaining the value of nano-hardness.All the determined parameters have shown the increase with introduction of copper into amorphous thin film.

Abstract: This work is devoted to development of synthesis methods of nano- and macrofunctional materials for industry, electronics, and modern science and technology. The authors have developed advanced methods of obtaining various carbon, silicate and chalcogenide materials by melt and hydrothermal methods with using of optimized chemical and thermodynamic parameters of synthesis. As a result a new super hard material - carbonate-synthetic diamond polycrystals were obtained. Nanodiamonds were synthesized by direct conversion of graphite into diamond. New species of macro- and fine crystalline piezoelectric tourmaline containing 3d-elements were produced by hydrothermal method. Using a flux technique the single crystals of different chalcogenide phase with unique superconducting properties were grown.