AN APPROACH TO THE LOCAL ARRANGEMENT OF THE FLUORINE-ATOMS IN THE ANIONIC CONDUCTORS WITH THE FLUORITE STRUCTURE NA0.5-XY0.5+XF2+2X

The average fluorine arrangement in the Na//0//. //5 minus //xY//0//. //5// plus //xF//2// plus //2//x solid solutions has been determined at room temperature (RT) and also in a partially-ordered Na//1//. //5Y//2//. //5F//9 compound. From neutron diffuse scattering measurements at RT and 760 C on the Na//0//. //4//1Y//0//. //5//9F//2//. //1//8 phase, the existence of coherent domains could be retained. The crystallographic investigations, in order to approach the local fluorine arrangement in the disordered fluorite Na//0//. //5 minus //xY//0//. //5// plus //xF//2// plus //2//x solid solutions, agree with the assumption that the anion excess is trapped near the yttrium atoms to form thermally-stable fluorine cuboctahedra.