Copper-indium ordering in RECu6In6 (RE=Y, Ce, Pr, Nd, Gd, Tb, Dy)

The rare earth metal-copper-indides RECu₆In₆ (RE=Y, Ce, Pr, Nd, Gd, Tb, Dy) were synthesized from the elements by arc-melting. Well-crystallized samples were obtained by slowly cooling the melted buttons from 1320 to 670 K in sealed silica tubes in a muffle furnace. They were investigated by X-ray diffraction on powders and single crystals: ThMn₁₂ type, space group I4/mmm, Z=2, a=916.3(2), c=535.8(2) pm, wR₂=0.063, 216 F² values, 15 variables for YCu₆In₆, a=926.5(4), c=543.5(3) pm, wR₂=0.064, 314 F² values, 15 variables for CeCu₆In₆, a=925.7(4), c=540.1(3) pm, wR₂=0.075, 219 F² values, 15 variables for PrCu₆In₆, a=923.1(4), c=540.3(3) pm, wR₂=0.071, 218 F² values, 15 variables for NdCu₆In₆, a=917.7(4), c=540.2(3) pm, wR₂=0.076, 207 F² values, 15 variables for GdCu₆In₆, a=917.0(5), c=540.5(4) pm, wR₂=0.062, 215 F² values, 15 variables for TbCu₆In₆, a=915.2(8), c=540.7(7) pm, wR₂=0.108, 218 F² values, 15 variables for DyCu₆In₆. The structures have been refined with a split position (50% Cu+50% In) for the 8j site. They can be explained by a tetragonal body-centered packing of CN 20 polyhedra (10Cu+10In) around the rare earth atoms. The ordering models of the copper and indium atoms and the limitations/resolution of X-ray diffraction for this topic are discussed.