Investigations of the structural, magnetic and Ce-valence properties of quaternary CeM2B2C compounds (M = Co, Ni, Rh, Pd, Ir and Pt)

Polycrystalline samples with nominal formula CeM2B2C (M=Fe, Co, Ni, Cu, Ru, Rh, Pd, Os, Ir, Pt and Au) were prepared by arc melting. CeM2B2C essentially forms as a single phase with the LuNi2B2C-type structure only for M=Co, Ni, Rh, Pd, Ir and Pt. CeOs2B2C exists as a minority phase in the corresponding alloy. No CeM2B2C phase could be detected in the alloys melted with M=Fe, Cu, Ru, and Au. Highly accurate structural parameters were obtained by single crystal diffractometry on CeRh2B2C and CeIr2B2C. None of these compounds exhibits superconductivity or magnetic order down to 2 K. From X-ray absorption spectroscopy (XAS) measurements, we conclude that the Cc ions have a non-integer valence in CeM2B2C (M=Co, Ni, Rh, Ir), but are nearly trivalent in CePd2B2C (in contrast to an earlier report) and in CePt2B2C. The magnetic and valence properties of the CeM2B2C series show a regular evolution from delocalised to localised 4f-electron states driven by the value of the atomic radius of the M metal. For all these compounds, the Ce-Ce distances ('a' parameter) are larger than the Hill limit (approximate to3.4 Angstrom), clearly suggesting the importance of 4f-nd overlaps. (C) 2002 Elsevier Science B.V. All rights reserved.