Erni2.23al2.77 And Ybni2.31al2.69 - I3 Superstructures Of The Cacu5 Type

The title compounds have been synthesized by reaction of the elements in sealed tantalum crucibles in a muffle furnace using special annealing sequences. The crystal structures of YbNi2.31Al2.69 (R1 = 0.0100 for 212 F-2 values and 18 variables) and for ErNi2.23Al2.77 (R1 = 0.0154 for 255 F-2 values and 18 variables) were refined from single crystal X-ray data. They belong to the YNi2Al3 type (i3 superstructure of CaCu5) with the following crystallographic parameters: space group P6/mmm, Pearson symbol hP18, Z = 3, a = 8.2723(12), c = 4.0672(8) angstrom, V = 241.03(8) angstrom(3) for YbNi2.31Al2.69 and a = 8.9109(13), c = 4.0669(8) angstrom, V = 279.66(8) angstrom(3) for ErNi2.23Al2.77. The crystal chemical discussion is supported by electronic structure calculations.