SYNTHESIS AND CHARACTERIZATION OF SrF2:Yb:Tm POWDERS

Abstract

This paper discloses the results of synthesis of Sr1-x-yYbxTmyF2+x+y (x = 0.010–0.300; y = 0.001–0.060) solid solutions by various precipitation techniques, including so-called direct method, reverse method and simultaneous precipitation with different fluorinating agents (ammonium fluoride or hydrofluoric acid). Use of ammonium fluoride as the fluoride-ion source for precipitation in direct or reverse method protocols resulted in the phase separation in the formed solid solutions with general concentration of rare-earth elements (REE) varied within the following range: 0.10<x+y<0.24. Such solid solution precipitates possessed similar structures, but their lattice unit cell parameters were slightly different within the aforementioned concentration range. Heat treatment of samples at 600 ºC led to the sintered solid solutions with homogeneously distributed chemical composition. Precipitation by so-called simultaneous method did not produce multiple phase mixtures for any composition ratio. Use of hydrofluoric acid as fluorinating agent produced multiphase precipitates having much larger differences for the different phase crystal lattice parameter values than the differences for the specimens synthesized with the use of NH4F. In contrast with NH4F-precipitated fluoride samples, annealing the HF-precipitated specimens at 600 ºC did not result in formation of sintered solid solutions with homogeneous distribution of chemical composition. Analysis of the change of the crystal lattice parameters vs. REE concentration for various thermal treatment conditions has shown that increase of REE concentration was accompanied by the simultaneous increase of NH4+-ion concentrations in the strontium fluoride lattice. Lattice constants of strontium fluoride-based phases went up with the increase of NH4+ ion concentration and decreased with the growth of Yb and Tm concentrations, respectively. The present paper also discusses thermal treatment of the formed precipitates, which resulted in their thermolysis, as well as the microstructure of the samples prepared by the different synthetic techniques.

ACKNOWLEDGEMENTS

The authors thank A. E. Baranchikov and V. K. Ivanov for the characterization of the samples by scanning electron microscopy. The work was supported by grant Russian Foundation for Basic Research no. 16-32-00654-mol_a.