Effect of the pH on the formation of NaYF4:Yb:Er nanopowders by co-crystallization in presence of polyethyleneimine

•Precipitation of Na–Y–Yb–Er fluorides in polyethyleneimine (PEI) presence was studied formation of both cubic and hexagonal NaYF4-type phases has been observed.•Cubic Na–Y–Yb–Er fluoride phase formed at pH = 10 when nitrates added to aq. NaF–PEI.•Hexagonal phase formed at pH = 12 when NaF added to metal nitrates – PEI solutions.

Study of nanopowder precipitation from aqueous sodium fluoride and yttrium and/or other rare earth nitrate solutions in the presence of polyethyleneimine (PEI) has revealed the strong dependence of the solid phase composition upon the solutions’ pH values and the order of reagent addition. The conditions for single phase formation have been determined for both cubic and hexagonal NaYF4-type phases. Cubic single-phase samples were obtained by adding rare earth element nitrate solutions to aqueous NaF–PEI mixtures at pH = 10; while hexagonal single-phase precipitates were synthesized by the addition of aqueous NaF to aqueous rare earth element nitrate solutions + PEI mixtures at pH = 12.

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