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

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