Ba 4 YZr 3 F 23 and Ba 4 LnZr 3 F 23 : a new noncentrosymmetric potential host structure type for optical devices.

The first ternary BaF2 -Ln(Y)F3 -ZrF4 phases crystallizing near the composition range of fluoride glasses, namely Ba4 YZr3 F23 (barium yttrium fluoridozirconate) and isotypic Ba4 LnZr3 F23 (Ln = Yb, Er and Nd), have been synthesized. The crystal structure of Ba4 YZr3 F23 cannot be correctly determined in the centrosymmetric space group Pnma (R1 = 0.09); it is preferably described in the noncentrosymmetric space group Pn21 a (R1 = 0.028). All cations are located at y = 0.25-0.29 and 0.75-0.79. This new structure type is formed by infinite {[YZr3 F23 ]8- }n columns associating, along [001], YF9 tricapped trigonal prisms, ZrF8 dodecahedra and ZrF7 polyhedra. These columns are interconnected via BaF10+1 and BaF12 irregular polyhedra.