研究目的
To investigate the crystal structures, physical properties, and phase transformations of calcium pyroselenites CaSe2O5, specifically comparing the orthorhombic α-phase and monoclinic β-phase, including their synthesis, optical properties, and thermal behavior.
研究成果
β-CaSe2O5 is a new structure type with 1D calcium oxide chains, differing from the 2D layers in α-CaSe2O5. This structural difference leads to distinct physical properties: α-CaSe2O5 is a direct bandgap semiconductor (4.74 eV), while β-CaSe2O5 is indirect (4.12 eV), consistent with theoretical predictions. Phase transformation between the phases occurs reversibly with temperature, as confirmed by thermal analysis and XRD. The research provides insights into the synthesis and properties of calcium selenites, with potential applications in materials science.
研究不足
The study uses powder samples which may have disorder affecting optical properties (e.g., Urbach tail). DFT calculations underestimate bandgap values. The phase transformation is gradual without a clear mutational point on heating. Synthesis of pure β-phase is challenging, with only small crystals obtained manually.
1：Experimental Design and Method Selection:
The study involved synthesizing α-CaSe2O5 via microwave-assisted solid-state reaction and β-CaSe2O5 via hydrothermal reaction. Crystal structures were analyzed using single-crystal X-ray diffraction and Rietveld refinement. Optical properties were studied using UV-Vis-NIR diffuse reflectance spectroscopy with Kubelka-Munk fit. Thermal analysis included TGA and DSC, and phase transformations were investigated using variable temperature powder XRD. Electronic structures were calculated using DFT methods.
2：Sample Selection and Data Sources:
Samples were prepared from raw materials CaCO3 and SeO2 in stoichiometric proportions. α-CaSe2O5 was synthesized from a mixture ground and heated in a microwave oven; β-CaSe2O5 was obtained from a hydrothermal reaction with additional H3BO
3：List of Experimental Equipment and Materials:
Equipment included a microwave oven, agate mortar, corundum crucible, autoclave with Teflon liner, optical microscope, X-ray powder diffractometer, UV-Vis-NIR spectrometer, TGA instrument, DSC instrument, and software for data analysis (GSAS-II, Igor Pro). Materials included CaCO3, SeO2, H3BO3, and H2O.
4：2O. Experimental Procedures and Operational Workflow:
4. Experimental Procedures and Operational Workflow: For α-CaSe2O5, mix and grind CaCO3 and SeO2, heat in microwave oven for 10 min, repeat six times. For β-CaSe2O5, mix CaCO3, H3BO3, and SeO2 in water, seal in autoclave, heat at 230°C for four days. Characterize using XRD, optical spectroscopy, thermal analysis, and theoretical calculations.
5：Data Analysis Methods:
Rietveld refinement for phase quantification, Kubelka-Munk fit for optical bandgap determination, Urbach tail analysis, Tauc's function for transition type, and DFT calculations for electronic structures.
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microwave oven
Used for heating the mixture in solid-state synthesis of α-CaSe2O5.
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agate mortar
Used for grinding raw materials.
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corundum crucible
Container for heating samples in the microwave oven.
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autoclave
Used for hydrothermal synthesis of β-CaSe2O5.
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optical microscope
Used to manually pick up β-CaSe2O5 crystals from the mixture.
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X-ray powder diffractometer
Used for phase identification and Rietveld refinement.
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UV-Vis-NIR spectrometer
Used for optical diffuse reflectance spectroscopy.
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TGA instrument
Used for thermogravimetric analysis to study weight loss.
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DSC instrument
Used for differential scanning calorimetry to study thermal events.
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GSAS-II software
Used for Rietveld refinement of XRD data.
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Igor Pro software
WaveMetrics, Inc.
Used for data analysis of optical spectra.
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