The low-temperature sonochemical synthesis of up-converting β NaYF4: Yb,Er mesocrystals

In this paper, we report the mild sonochemical synthesis of hexagonal (beta) NaYF4:Yb,Er mesocrystals at low temperature of 40 degrees C. The nucleation and transformation of crystal structures is investigated in the course of time, and it is shown that the pure beta phase is obtained after 2 h of pulsed sonication. The crystallization of orthorhombic YF3:Yb,Er and cubic (alpha) NaYF4:Yb,Er precedes the appearance of a thermodynamically stable beta NaYF4:Yb,Er phase. Based on the evolution of the nanoparticle morphology, it is concluded that the transition from alpha to beta phase is consistent with the dissolution-recrystallization process, while the final shape of mesocrystals is a consequence of the oriented attachment growth in the newly formed beta phase nanocrystallites along two planes. The structural properties of all compounds are analyzed and correlated with their thermal and optical characteristics. The pump power dependence of green and red emissions confirms that only two-photon process is involved in up-conversion, which is superior in beta phase mesocrystals. (C) 2021 The Society of Powder Technology Japan. Published by Elsevier B.V. and The Society of Powder Technology Japan. All rights reserved.

Automated summary

This paper reports the successful synthesis of hexagonal NaYF4:Yb,Er mesocrystals using mild sonochemical methods at low temperature. The formation process of crystal structures is investigated and it is found that the pure beta phase can be obtained after 2 hours of pulsed sonication. The transition from the alpha phase to the beta phase is concluded to be a result of dissolution-recrystallization process based on the analysis of nanoparticle morphology.