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Using First-Principles Calculations in CALPHAD Models to Determine Carrier Concentration of the Binary PbSe Semiconductor
摘要： PbSe is a promising thermoelectric that can be further improved by nanostructuring, band engineering, and carrier concentration tuning; therefore, a firm understanding of the defects in PbSe is necessary. The formation energies of point defects in PbSe are computed via first-principles calculations under the dilute-limit approximation. We find that under Pb-rich conditions, PbSe is an n-type semiconductor dominated by doubly-charged Se vacancies. Conversely, under Se-rich conditions, PbSe is a p-type semiconductor dominated by doubly-charged Pb vacancies. Both of these results agree with previously performed experiments. Temperature- and chemical potential-dependent Fermi levels and carrier concentrations are found by enforcing the condition of charge neutrality across all charged atomic and electronic states in the system. The first-principles-predicted charge-carrier concentration is in qualitative agreement with experiment, but slightly varies in the magnitude of carriers. To better describe the experimental data, a CALPHAD assessment of PbSe is performed. Parameters determined via first-principles calculations are used as inputs to a five-sublattice CALPHAD model that was developed explicitly for binary semiconductors. This five sublattice model is in contrast to previous work which treated PbSe as a stoichiometric compound. The current treatment allows for experimental carrier concentrations to be accurately described within the CALPHAD formalism. In addition to the five-sublattice model, a two-sublattice model is also developed for use in multicomponent databases. Both models show excellent agreement with the experimental data and close agreement with first-principles calculations. These CALPHAD models can be used to determine processing parameters that will result in an optimized carrier concentration and peak zT value.
关键词： CALPHAD,first-principles,DFT,defect chemistry,Thermoelectrics
更新于2025-09-23 15:23:52
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On the Theoretical and Experimental Control of Defect Chemistry, Electrical and Photoelectrochemical Properties of Hematite Nanostructures
摘要： Hematite (α-Fe2O3) is regarded as one of the most promising cost-effective and stable anode materials in photoelectrochemical applications, and its performance, like other transition metal oxides, depends strongly on its electrical and defect properties. In this work, the electrical and thermo-mechanical properties of undoped and Sn-doped α-Fe2O3 nanoscale powders were characterized in-situ under controlled temperatures (T = 250 to 400oC) and atmospheres (pO2 = 10-4 to 1 atm O2) to investigate their transport and defect properties. Frequency-dependent complex impedance spectra show that interfacial resistance between particles is negligible in comparison with particle resistance. Detailed defect models predicting the dependence of electron, hole, iron and oxygen vacancy concentrations on temperature and oxygen partial pressures for undoped and doped α-Fe2O3 were derived. Using these defect equilibria models, the operative defect regimes were established and the bandgap energy of undoped α-Fe2O3 and oxidation enthalpy of Sn-doped α-Fe2O3 were obtained from the analysis of the temperature and pO2 dependence of the electrical conductivity. Based on these results, we are able to explain the surprisingly weak impact of donor doping on the electrical conductivity of α-Fe2O3. Furthermore, experimental means based on the results of this study are given for successfully tuning hematite to enhance its photocatalytic activity for the water oxidation reaction.
关键词： dilatometry,α-Fe2O3,defect chemistry,photoelectrochemistry,electrical conductivity
更新于2025-09-23 15:19:57
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Nd3+ Cluster Adjustment in Nd3+:CaNb2O6 by Co-doping La3+ Buffers for Improvement of Fundamental and Self-stimulated Raman Scattering Laser Operation: A Study Case from the Perspective of Defect Chemistry
摘要： The fluorescence quenching caused by Nd3+ clusters in Nd3+ doped crystal even at low Nd3+ concentration has restrained the performance of Nd3+ laser. In this work, Nd3+:CaNb2O6 and Nd3+:La3+:CaNb2O6 single crystals have been grown by the Czochralski method. RE3+ (RE3+=Nd3+, La3+) ion incorporation mechanisms, the formation of Nd3+ clusters, and the feasibility of La3+ ions as buffers in RE3+:CaNb2O6 crystal have been demonstrated and assessed by atomistic simulation methods. The spectral and laser properties of the Nd3+:CaNb2O6 and Nd3+:La3+:CaNb2O6 have been measured. All the results indicate the Nd3+ dimers with distances between Nd3+ close to the critical interaction distance have formed in the Nd3+:CaNb2O6 and the La3+ ions as buffers can alleviate the interaction of Nd3+ effectively in the Nd3+:La3+:CaNb2O6 as expected. Benefitting from the introducing of La3+ buffers, about 2.6 W fundament laser with slope efficiency of 36.5% and about 310 mW self-stimulated Raman scattering laser with conversion and slope efficiencies of 8.3% and 9.5%, respectively, which are superior to those of the Nd3+:CaNb2O6, have been achieved in the Nd3+:La3+:CaNb2O6.
关键词： laser,buffers ions,cluster adjustment,defect chemistry
更新于2025-09-11 14:15:04
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Conductivity of Iron-doped Strontium Titanate in the Quenched and Degraded States
摘要： The electrical behavior of iron-doped strontium titanate (Fe:SrTiO3) single crystals equilibrated at 900 °C and quenched below 400 °C at various oxygen partial pressures ( ) was investigated via impedance spectroscopy and compared to defect chemistry models. Fe:SrTiO3 annealed and quenched between and Pa exhibits a conduction activation energy (EA) around 0.6 eV, consistent with ionic conduction of oxygen vacancies. However, sudden changes in EA are found to either side of this range; a transition from 0.6 to 1 eV is found in more oxidizing conditions, while a sudden transition to 1.1 and then 0.23 eV is found in reducing .These transitions, not described by the widely used canonical model, are consistent with predictions of transitions from ionic to electronic conductivity, based on first principles point defect chemistry simulations. These models demonstrate that activation energies in mixed conductors may not correlate to specific conduction mechanisms, but are determined by the cumulative response of all operative conduction processes and are very sensitive to impurities. A comparison to electrically degraded Fe:SrTiO3 provides insight into the origins of the conductivity activation energies observed in those samples.
关键词： defect chemistry,Strontium titanate,conductivity,resistance degradation
更新于2025-09-09 09:28:46
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Point defect chemistry of donor-doped bismuth titanate ceramic
摘要： This communication reports on the defect chemistry at room temperature of barium (Ba) doped bismuth titanate (Bi4Ti3O12), emphasizing the influence of the point defects on its electrical properties. Pure and Ba doped Bi4Ti3O12 were prepared by a conventional solid-phase reaction technique. The addition of Ba into the crystal structure of Bi4Ti3O12 was monitored by X-ray diffraction measurements combined with Rietveld refinement studies where it was determined that Ba occupies the bismuth (Bi) lattice sites as well as the presence of oxygen vacancies (V??O). The characterization of the point defects was carried out using impedance and electron spin resonance spectroscopies where the results support the models of compensation mechanisms dominated by electrical positive charges (h?) and oxygen vacancies (V??O).
关键词： defect chemistry,X-ray diffraction,Rietveld refinement,impedance spectroscopy,solid-phase reaction,barium doped bismuth titanate,electron spin resonance spectroscopy,electrical properties,oxygen vacancies
更新于2025-09-04 15:30:14