First principles investigation of magnetic new carbon-rich layered compounds UCn (n= 2, 6, 12)

Abstract

Backing structural experimental observations of uranium dicarbide and diboride binary compounds, density functional theory DFT cohesive energy calculations let identify UC2 as only stable in three dimensional 3D structure while UB2 is stable in a 2D-assimilated structure with honeycomb-like boron network (AlB2-type). Nevertheless calculations assuming hexagonal UC2 lead to a magnetic solution with M = 0.96 μB albeit in a metastable state that we assign to the limited expansion of the carbon substructure in AlB2-type. Proof is provided through the proposition of new extended carbon honeycomb networks structures UC6 and UC12 whose cohesive energies are found similar to experimental UC2 for UC6 and larger for UC12. From energy-volume equation of state, a magnetically ordered ground state is confirmed with M(UC6) = 2.88 μB and M(UC12) = 3.27 μB. Check calculations imposing antiferromagnetic order lead to energy increase thus letting predicting long range ferromagnetic order.