Nanostructured crystals of fluorite phases Sr1 − x R x F2 + x (R are rare-earth elements) and their ordering. I. Crystal growth of Sr1 − x R x F2 + x (R = Y, La, Ce, Pr, Nd, Sm, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu)

Abstract

Crystals of nonstoichiometric phases Sr1 − x R x F2 + x (R are 14 rare-earth elements) and the ordered phase Sr4Lu3F17 with a trigonally distorted fluorite lattice were grown by the Bridgman method. Ten of 26 Sr1 − x R x F2 + x crystals, where R = La-Ho or Y, melt congruently. The isoconcentration series Sr0.90 R 0.10F2.10 includes four crystals with R = Er-Lu. The compositions corresponding to the maxima for the latter crystals were not determined. The concentration series, in which the mole fraction of RF3 varies from 10 to 50 mol %, were obtained for the crystals with R = La, Nd, and Gd. Most of the crystals are of good optical quality. To evaluate the composition changes in the course of crystal growth, the cubic unit-cell parameters were determined by X-ray powder diffraction. The line-broadening analysis revealed a nonmonotonic change of microdistortions as regards both the rare-earth content and rare-earth series. The changes in the lattice parameters and the congruent-melting points of the Sr1 − x R x F2 + x phases in the rare-earth series reflect the morphotropic transitions in the series of pure RF3 despite the fact that SrF2 dominates in nonstoichiometric fluorite crystals.