The Ba1-x R x F2 + x (R= Gd-Lu) phases with the distorted fluorite structure as products of crystallization of incongruent melts in BaF2-RF3 systems. II. Crystal structures of two new forms of Ba1-x YbxF2 +x fluorite phases and some features of the anionic packing and the cation distribution

Abstract

Two fluorite-type phases (P and F) possessing different defect structures were isolated from products of double recrystallization of a melt of the nominal composition (with respect to the charge)Ba0.8Yb0.2F2.2. The P phase crystallized in the space group Pm3m with a = 6.096(1) A. The structure was refined to R = 2.19% and Rw = 2.14% (142 independent reflections; λAgKα radiation, sinθ /2≤1.2A−1; 20 parameters were refined). In the primitive cubic unit cell of theP phase, two crystallographic positions are differentiated by the type of the cations occupying these positions. Thus, the position 1 a (0, 0, 0) is occupied predominantly by Yb3+ cations (0.8Yb + 0.2Ba), whereas the position 3c (0, 1/2, 1/2) is completely occupied by Ba2+ cations. The Fphase crystallized in the space group Fm3m with a = 6.075(1) A; the structure was refined to R= 0.69% and Rw = 0.85% (72 independent reflections; λMoKα radiation, sinθ/λ≤1.0A−1; 15 parameters were refined). In the F phase, all Yb 3+ ions are shifted with respect to Ba2+ ions by 0.34 A in the [111] direction.