Influence of Y–Gd ratio on phase formation and spectroscopic properties of NaGd0.8−xYxYb0.17Er0.03F4 solid solutions

Abstract

The work is devoted to the study of phase formation in the NaGd0.8Yb0.17Er0.03F4–NaY0.8Yb0.17Er0.03F4 system. The effect of the Gd3+ и Y3+ ions concentrations ratio on the phase composition and lattice parameters of the resulting solid solutions, the spectral-luminescent properties, and the heating temperature under laser irradiation were studied. It is shown that an increase of the molar fraction of Gd3+ ions in the composition leads to the NaGdF4 hexagonal phase formation and to increase in the lattice parameter values. The measured upconversion efficiency for NaGd0.8Yb0.17Er0.03F4 is significantly lower than for NaY0.8Yb0.17Er0.03F4. In order to interpret the obtained result, a theoretical model on the basis of rate equations system was used, taking into account the experimentally determined lattice parameters. It was shown that as the lattice parameters increase, the distance between the active ions increases and the probability of energy transfer decreases. Shorter lifetimes of the excited states for NaGdF4 cause an increase in the back energy transfer probability. Additionally, the shorter lifetime of intermediate metastable states in the NaGdF4 matrix negatively affects the efficiency of the upconversion processes. All these processes result in emitting states populations decrease and a decrease in upconversion efficiency, respectively. According to the calculations, the efficiency for NaGd0.8Yb0.17Er0.03F4 is reduced by 40%. Experimentally observed decrease was much higher. We propose that the additional decrease is due to the influence of factors not considered in the model. The size reduction for NaGdF4 particles was observed (350 nm in comparison to 1 µm for NaYF4 particles). The second additional factor, which leads to experimental efficiency decrease in comparison with theoretical value, is the heating. The study of the samples heating temperature under laser radiation showed that the increase in the gadolinium ions fraction in the composition results in the sample heating temperature increase. The NaGd0.8Yb0.17Er0.03F4 sample was heated by laser radiation by 24.7 °C, while the sample NaY0.8Yb0.17Er0.03F4 was heated by 5.6 °C.