Comparative investigation of the crystal structure of LnCuSe(2) compounds (Ln = Tb, Dy, Ho, Er, Tm, Yb and Lu)

LnCuSe(2) compounds crystallize either in the monoclinic LaCuS2 (space group P2(1)/c) or the trigonal CaLi2Si2 (space group P (3) over bar m1) type of structure. The first group is formed by the light rare-earth elements (Ln = La, Ce, Pr, Nd, Sm, Gd and Tb), the second by the heavier rare-earth elements (Ln = Tb, Dy, Ho, Er, Tm, Yb, Lu). Accordingly, the formation of the structure type critically depends on the ionic radius of Ln. This dependence is clearly reflected by the spatial separation of the two structure types in a plot V-at(r) listing the average atomic volume versus the ionic radius of the rare earth element in its trivalent state. The crystal structures presented here exhibit trigonal symmetry. Seven crystal structures were determined by means of the single crystal X-ray diffraction. The Se atoms form a hexagonal closest packing of spheres with AB stacking sequence of the atomic layers. The Ln atoms occupy 1/2 of the octahedral interstices. In each of the seven trigonal structures the Cu atoms are statistically distributed over 1/2 of the tetrahedral interstices with 50 % occupation probability.