Abstract
In this paper, we report the mild sonochemical synthesis of hexagonal (β) NaYF4:Yb,Er mesocrystals at low temperature of 40 °C. The nucleation and transformation of crystal structures is investigated in the course of time, and it is shown that the pure β phase is obtained after 2 h of pulsed sonication. The crystallization of orthorhombic YF3:Yb,Er and cubic (α) NaYF4:Yb,Er precedes the appearance of a thermodynamically stable β NaYF4:Yb,Er phase. Based on the evolution of the nanoparticle morphology, it is concluded that the transition from α to β 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 β 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 β phase mesocrystals.
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