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Phase Identification of γ- and β-Ca₂SiO₄ via the Rear-Earth Fluorescence Probe

Lu, Xiaolei, Wang, Shuxian, Liu, Shuxin, Du, Peng, Ye, Zhengmao, Geng, Xuefei, Cheng, Xin
Journal of physical chemistry 2019 v.123 no.22 pp. 13877-13884
calcium, calcium silicate, emissions, europium, fluorescent dyes, ions, magnetism, phase transition, prediction, wavelengths
Ca₂SiO₄ is a phase-change material, and γ- and β-Ca₂SiO₄ usually coexist in the sintered product. The overlap of the diffraction data between the two polymorphs makes the relevant phase identification difficult. In this work, the Eu³⁺ rear-earth fluorescence probe is introduced in the preparation of Ca₂SiO₄, and the obtained result indicates that the introduction of Eu³⁺ ions facilitates the increase in the content of β-Ca₂SiO₄ in the final product. In addition, the differences of the local coordination environments between the two Ca₂SiO₄ phases obviously influence the crystal-field splittings of Eu³⁺ ions, especially in the ⁵D₀ → ⁷F₀ nondegenerate and ⁵D₀ → ⁷F₁ magnetic dipole transition regions. On the basis of the excitation from the Eu³⁺–O²– charge-transfer band, the low-temperature spectral changes are systematically discussed. The transition area ratios of ⁵D₀ → ⁷F₀/⁵D₀ → ⁷F₂ and ⁵D₀ → ⁷F₁/⁵D₀ → ⁷F₂ present a positive and negative correlation with the increase of the excitation wavelength, respectively. The four nonequivalent ⁵D₀ → ⁷F₀ emission peaks with low to high wavelengths are identified as Eu³⁺ ion-occupied Ca(β-2) (571.8 nm), Ca(β-1) (573.8 nm), Ca(γ-2) (577.6 nm), and Ca(γ-1) (578.9 nm) sites. Besides, the relationship between the phase content of γ-Ca₂SiO₄ and the spectral change for the ⁵D₀ → ⁷F₀ emissions of the Eu³⁺ ions is also built. It is fitted by an exponential correlation function and is quite suitable for the prediction of the phase content of γ-Ca₂SiO₄ based on the measured spectra. The findings in this study manifest the promising application of the Eu³⁺ fluorescent probe in phase identification and enrich the analysis method in inorganic phase-change materials.