La3F3[Si3O9]: The first fluoride silicate in the ternary system LaF3/La2O3/SiO2

By reacting La2O3 with LaF3 and SiO2 (silica gel) using CsCl as a flux (mol ar ratio 1:1:3:6; 700 degrees C, 21 d) it was possible to obtain single cry stals of La3F3[Si3O9] (hexagonal, space group: P (6) over bar 2c (no. 190); a = 708.32(3), c = 1089.48(6) pm; Z = 2) as colourless, hexagonal platelet s, The crystal structure comprises discrete cyclic [Si3O9](6-) anions of th ree corner-linked [SiO4] tetrahedra along with a graphite-like network of t he composition (2)(infinity){[LaF3/3](2+)} Shorter reaction times even prod uced single crystals of tysonite-type LaF3 (trigonal, space group: P (3) ov er bar c1 (no.165); a = 718.80(6), c = 735.94(6) pm; Z = 6) on which a X-ra y structure analysis was achieved, too. The structures of both compounds, e ach of which show an elevenfold anionic coordination (CN = 9 + 2) for the L a3+ cations, are compared. The influence of the reactivity of the educts an d the temperature on the reaction as well as the difficulties in the X-ray differentiation of fluorine and oxygen will be discussed.