Correlation between local lattice environment and upconversion emission in Ho3+-doped CaHfO3 in terms of substitution site dependence

Abstract

The upconversion (UC) emission properties of Ho3+ and Yb3+ co-doped CaHfO3 ceramics were investigated in terms of the site substitutions, i.e., Ho3+ and Yb3+ ions occupying the Ca sites (A-site) and Hf sites (B-site). We performed the Rietveld refinement on their X-ray diffraction patterns to obtain the crystal structural parameters. Upon excitation of a 980-nm near-infrared light, the Ho3+ activators in the samples exhibited sizable green emissions with assistance of Yb3+ ions as sensitizers, along with relatively small red emissions. The intensity of the green UC emission was found to decrease with an increasing Ho3+ concentration commonly in both, A-site and B-site doped samples. Interestingly, the A-site doped samples presented significantly stronger UC emission intensities than the B-site doped samples. This finding was explained in terms of the distortion index, which could be a measure of the local structural asymmetry degree around the Ho3+/Yb3+ ions. The value of the distortion index was found to be bigger for the A-sites than for the B-sites, which indicated that the A-sites should have lower crystalline symmetry than the B-sites in CaHfO3. The results showed that the UC emissions were closely related to the local structural properties and could be finely tuned by site-selective doping in Ho3+ and Yb3+ co-doped CaHfO3. PACS numbers: 78.20.-e, 78.55.Hx.