Structure and magnetic properties of the phosphides CaCo 2P 2 and LnT 2P 2 with ThCr 2Si 2 structure and LnTP with PbFCl structure (Ln = Lanthanoids, T = Fe, Co, Ni)†

Abstract

The structures of the ThCr 2Si 2-type phosphides PrFe 2P 2 (a new compound) and EuFe 2P 2 were refined from single-crystal X-ray data to residuals of R = 0.011 (246 F-values) and R = 0.027 (215 Fs). As was previously reported for the Ni atoms of the ThCr 2Si 2-type phosphides LnNi 2P 2, the Fe atoms in the series LnFe 2P 2 do not carry magnetic moments. In contrast, the magnetic behaviour of the corresponding Co-containing phosphides is frequently complex, due to the possibility for magnetic order in both the Ln and the Co sublattices. Antiferromagnetic order can be inferred for the Co sublattices of CeCo 2P 2 ( T N = 440 K), PrCo 2P 2 ( T N = 304 K), NdCo 2P 2 ( T N = 309 K) and SmCo 2P 2 ( T N = 302 K). At about 15–30 K the 1 χ vs T plots of PrFe 2P 2, PrCo 2P 2 and NdCo 2P 2 suggest a variety of different ordering phenomena for the magnetic moments of the lanthanoid atoms. The magnetic properties of CeFe 2P 2 and CeCo 2P 2 indicate valence fluctuations of the Ce atoms. At lower temperatures the magnetic susceptibility of SmCo 2P 2 reflects the Van Vleck paramagnetism of the Sm 3+ species, while the magnetic behaviour of CaCo 2P 2 is suggestive of the itinerant exchange of a Fermi liquid. In the closely related (anti-) PbFCl-type structure of the new compound SmCoP the Co sublattice also shows antiferromagnetic order with a magnetic moment of μ exp = 1.36 μ B per Co atom. The isotypic compounds SmFeP, PrCoP and NdCoP are reported for the first time. The compounds LaFe 2P 2, CeFe 2P 2, CaNi 2P 2, LaNi 2P 2 and CeNi 2P 2 do not become superconducting down to 1.8 K.