Unusual behaviour of (Np,Pu)B₂C

Abstract

Two transuranium metal boron carbides, NpB₂C and PuB₂C have been synthesized by argon arc melting. The crystal structures of the {Np,Pu}B₂C compounds were determined from single-crystal X-ray data to be isotypic with the ThB₂C-type (space group , a = 0.6532(2) nm; c = 1.0769(3) nm for NpB₂C and a = 0.6509(2) nm; c = 1.0818(3) nm for PuB₂C; Z = 9). Physical properties have been derived from polycrystalline bulk material in the temperature range from 2 to 300 K and in magnetic fields up to 9 T. Magnetic susceptibility and heat capacity data indicate the occurrence of antiferromagnetic ordering for NpB₂C with a Neel temperature T_N = 68 K. PuB₂C is a Pauli paramagnet most likely due to a strong hybridization of s(p,d) electrons with the Pu-5f states. A pseudo-gap, as concluded from the Sommerfeld value and the electronic transport, is thought to be a consequence of the hybridization. The magnetic behaviour of {Np,Pu}B₂C is consistent with the criterion of Hill.

Keywords:

• antiferromagnetism
• actinoid compounds
• crystal structure
• electrical resistivity
• specific heat

Acknowledgements

We are thankful for Dr Jean Rebizant for his help in this project. The high purity Np and Pu metals required for the fabrication of the title compounds were made available through a loan agreement between Lawrence Livermore National Laboratory and JRC-ITU, in the frame of a collaboration involving LLNL, Los Alamos National Laboratory and the US Department of Energy. We acknowledge the access to infrastructures provided by the European Commission, DG-JRC within its ‘Actinide User Laboratory’ program, and financial support to users provided by the European Commission, DG-Research Contracts No. HPRI-CT-2001-00118, and No. RITA-CT-2006-026176.