Phase stability regions in the Na2CO3-YbF3-H2O system at 190°C.: Crystal structures of two new fluoride carbonates, Na2Yb(CO3)2F and Na3Yb(CO)3F2

Phase stability regions in the Na2CO3-YbF3-H2O system at T=190degreesC, P=11x10(5) Pa, t=1 h, microwave heating, and the crystal structures of two new carbonate fluorides Na2Yb(CO3)(2)F and Na3Yb(CO3)(2)F-2 are presented. Five phases, obtained for [YbF3]less than or equal to0.75 M and [Na]/[Yb]<30, are characterized by powder or single crystal X-ray diffraction, thermogravimetric or chemical analyses: NaYb(CO3)F-2, Na2Yb(CO3)(2)F, Na3Yb(CO3)(2)F-2, Na5Yb(CO3)(4)•2H(2)O, and Yb(CO3)(OH,F)•xH(2)O. NaYb(CO3)F-2 (a=6.897(7) &ANGS;, b=9.118(9) &ANGS;, c=6.219(6) &ANGS;) and Na5Yb(CO3)(4)•2H(2)O are isostructural with NaY(CO3)F-2 (horvathite) and Na5Sc(CO3)(4)•2H(2)O, respectively; the other phases were still unknown. Na2Yb(CO3)(2)F is monoclinic, C2/c, with a=17.440(6) &ANGS;, b=6.100(2) &ANGS;, c=11.237(1) &ANGS;, β=95.64(3)°; V=1189.7(4) &ANGS;(3); Z=8 (1729 unique reflections, R=0.034, R-W=0.087). Na2Yb(CO3)(2)F is described from infinite [001] chains [YbO5F](∞) built up from [YbO6F](2) dimers. Na3Yb(CO3)(2)F-2 is noncentrosymmetric, Cc, with a=7.127(2) &ANGS;, b=29.916(9) &ANGS;, c=6.928(4) &ANGS;, β=112.56(3)°; V=1359(4) &ANGS;(3); Z=8 (1838 unique reflections, R=6.074, R-W=0.235). The structure is built up from [NaYb(CO3)(2)F-2](∞), layers; Yb3+ and Na+ cations are alternatively ordered and disordered in successive (010) layers which are separated by other Na+ cations. (C)2002 Editions scientifiques et medicales Elsevier SAS. All rights reserved.