Quaternary rare earth transition metal arsenide oxides RTAsO (T=Fe, Ru, Co) with ZrCuSiAs type structure

The equiatomic quaternary compounds RFeAsO (R = La-Nd, Sm, Gd), RRuAsO (R = La-Nd, Sm, Gd-Dy) and RCoAsO (R = La-Nd) crystallize with the tetragonal ZrCuSiAs type structure, which was refined from single-crystal X-ray diffractometer data of PrFeAsO: P4/nmm, Z = 2, a = 398.5(1) pm, c = 859.5(3) pm, R = 0.058 for 167 structure factors and 12 variable parameters. All atomic positions are fully occupied. Chemical bonding in PrFeAsO can be rationalized with oxidation numbers corresponding to the formula Pr+3Fe+2As-3O-2. An electron count of 18 for the iron atoms can only be achieved if Fe-Fe interactions of 281.8 pm are considered as bonding. (C) 2000 Elsevier Science S.A. All rights reserved.