Complexation and ionic arrangement in Na3ErCl6 and K3ErCl6 melts analyzed by X-ray diffraction

The short range structure of Na3ErCl6 melt at 973 K and K3ErCl6 melt at 105 3 K was investigated by X-rap diffraction. The nearest neighbor distance of Er3+-Cl- and the coordination number of Cl- around Er3+ ill Na3ErCl6 melt, for instance, were estimated to be 2.61 Angstrom and 5.7, respectively, an d the Er3+-Er3+ and Cl--Cl- distances 5.15 Angstrom and 3.90 Angstrom, resp ectively. The ratio of the anion-anion pair distance to the cation-anion pa ir distance (r(--)/r(+-)) was, therefore, 1.49, being close to 1.41, charac teristic of octahedral geometry. These results indicate that the stable ErC l63- octahedra exist in Na3ErCl6 and K3ErCl6 melts. Probably, complex ions such as Er2Cl104- and Er2Cl116- expected to be in the pure ErCl3 melt are l ittle formed at these compositions according to the geometrical calculation s of bond lengths and bond angles.