Unusual Behaviour of (Np,Pu)B2C

TL;DR

This study synthesizes and characterizes the crystal structures and magnetic properties of transuranium boron carbides NpB2C and PuB2C, revealing antiferromagnetism in NpB2C and Pauli paramagnetism in PuB2C, with insights into electron hybridization effects.

Contribution

The paper provides the first detailed structural and magnetic analysis of NpB2C and PuB2C, highlighting their isotypic crystal structures and contrasting magnetic behaviors.

Findings

NpB2C exhibits antiferromagnetic order with T_N = 68 K.

PuB2C is a Pauli paramagnet with strong electron hybridization.

Both compounds show evidence of a pseudo-gap due to hybridization.

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 R3m, 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 K 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 hybridisation 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 hybridisation. The magnetic behaviour of {Np,Pu}B2C is consistent with the criterion of Hill.