Molten Salt Synthesis of a Mixed-Valent Lanthanide(III/IV) Oxychloride with an Unprecedented Sillen X₂⁴ Structure: Ce_1.3Nd_0.7O₃Cl

A new cerium neodymium oxychloride, Ce_1.3Nd_0.7O₃Cl, has been synthesized by precipitation in a LiCl–CaCl₂ molten salt by humid argon sparging. Chemical and structure characterization have been undertaken by powder X-ray diffraction, scanning electron microscopy, high-temperature X-ray diffraction, thermogravimetric analysis, and X-ray photoelectron scattering. This oxychloride crystallizes in space group P4/nmm, a = 3.9848(3) Å and c = 12.467(2) Å, in a new Sillen-type phase represented by the symbol X₂⁴ where “quadruple” fluorite-type layers [M₄O₆], containing Ce^IV in “inner” sublayers and both Ce^III and Nd^III in “outer” sublayers, alternate with double-halide ion sheets. The structure is also described as a stacking of LnOCl and fluorite-type blocks and constitutes the term n = 2 of a possible series (MO₂)_n(NdOCl)₂.