Phase stability regions in the Na 2CO 3–YbF 3–H 2O system at 190 °C. Crystal structures of two new fluoride carbonates, Na 2Yb(CO 3) 2F and Na 3Yb(CO 3) 2F 2

Abstract

Phase stability regions in the Na 2CO 3–YbF 3–H 2O system at T=190 °C, P=11×10 5 Pa, t=1 h, microwave heating, and the crystal structures of two new carbonate fluorides Na 2Yb(CO 3) 2F and Na 3Yb(CO 3) 2F 2 are presented. Five phases, obtained for [YbF 3]⩽0.75 M and [Na]/[Yb]<30, are characterized by powder or single crystal X-ray diffraction, thermogravimetric or chemical analyses: NaYb(CO 3)F 2, Na 2Yb(CO 3) 2F, Na 3Yb(CO 3) 2F 2, Na 5Yb(CO 3) 4·2H 2O, and Yb(CO 3)(OH,F)· xH 2O. NaYb(CO 3)F 2 ( a=6.897(7) Å, b=9.118(9) Å, c=6.219(6) Å) and Na 5Yb(CO 3) 4·2H 2O are isostructural with NaY(CO 3)F 2 (horvathite) and Na 5Sc(CO 3) 4·2H 2O, respectively; the other phases were still unknown. Na 2Yb(CO 3) 2F is monoclinic, C2/ c, with a=17.440(6) Å, b=6.100(2) Å, c=11.237(1) Å, β=95.64(3)°; V=1189.7(4) Å 3; Z=8 (1729 unique reflections, R=0.034, R w=0.087). Na 2Yb(CO 3) 2F is described from infinite [001] chains [YbO 5F] ∞ built up from [YbO 6F] 2 dimers. Na 3Yb(CO 3) 2F 2 is noncentrosymmetric, Cc, with a=7.127(2) Å, b=29.916(9) Å, c=6.928(4) Å, β=112.56(3)°; V=1359(4) Å 3; Z=8 (1838 unique reflections, R=0.074, R w=0.235). The structure is built up from [NaYb(CO 3) 2F 2] ∞ layers; Yb 3+ and Na + cations are alternatively ordered and disordered in successive (010) layers which are separated by other Na + cations.