Abstract
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
P
6
/
m
m
m
$P6/mmm$
, Pearson’s symbol hP18, Z = 3, a = 8.2723(12), c = 4.0672(8) Å, V = 241.03(8) Å3 for YbNi2.31Al2.69 and a = 8.9109(13), c = 4.0669(8) Å, V = 279.66(8) Å3 for ErNi2.23Al2.77. The crystal chemical discussion is supported by electronic structure calculations.