Importance of bond exchange in MnC structural stability and half-metallic ferromagnetism: a comprehensive density functional benchmark study

TL;DR

MnC, newly synthesized in zincblende form, presents a unique test case for first-principles descriptions of structure, bonding, and magnetism in late 3$d$ carbides. By benchmarking PBESol, SCAN, PBE0r, HSE06, and DFT+$U$+$V$ functionals, the study identifies bond-exchange as a key mechanism stabilizing the zincblende phase, with SCAN and PBE0r correctly predicting the experimental structure. The electronic ground state remains contentious: semi-local and Hubbard-based approaches suggest half-metallicity under certain volumes, while hybrid functionals predict metallic behavior at equilibrium, underscoring MnC’s location at the border between correlated metal and half-metal. Experimental spin-resolved and high-pressure measurements are needed to resolve this discrepancy, establishing MnC as a benchmark for covalent exchange and spintronic potential in late 3$d$ compounds.

Abstract

Recently, a first successful synthesis of the unknown bulk manganese monocarbide (MnC) has been reported. Suggested as a potential superhard material, the compound is found to crystallize in the zincblende (B3) structure. In the present work, we report an exhaustive density functional investigation on structural, electronic and magnetic properties of MnC, using different levels of exchange-correlation approximations. We show that SCAN meta-GGA and PBE0r hybrid functional outperform GGA-PBEsol, HSE06 hybrid functional and the mean field DFT+$U$+$V$; in predicting correctly the experimental zincblende structure as the groundstate one. It is demonstrated that the bond exchange plays a crucial role in this stabilization. At theoretical equilibrium, hybrid functionals (PBE0r and HSE06) lead to a metallic behavior of MnC. Semilocal ones (PBEsol and SCAN) and mean field DFT+$U$+$V$, however, predict two opposite kinds of half-metallic ferromagnetism; opening a debate on the origin and the nature this behavior. If the latter is confirmed experimentally, MnC could be for importance as a spintronic material.

Figures (7)

• Figure 1: Energy versus volume optimization of MnC in rocksalt (RS) and zincblende (ZB) structures; as obtained within QE: a) PBESol, b) SCAN, c) HSE06 and d) DFT+$U$+$V$ functionals.
• Figure 2: Energy versus volume optimization of MnC in rocksalt (RS) and zincblende (ZB) structures; as obtained within PBE0r functional: a) PBE0r (offsite=T) with the bond exchange, and b) PBE0r (offsite=F) without the bond exchange.
• Figure 3: Atom- and spin-resolved densities of states (PDOS) of ${\rm MnC}$ in RS-type (a) and ZB-type (b) structures, as arising from the PBESol spin-polarized FM calculations.
• Figure 4: Atom- and spin resolved densities of states (PDOS) of ${\rm MnC}$ in RS-type (a) and ZB-type (b) structures, as arising from the SCAN spin-polarized FM calculations.
• Figure 5: Atom- and spin resolved densities of states (PDOS) of ${\rm MnC}$ in RS-type (a) and ZB-type (b) structures, as arising from the PBE0r, with bond exchange (offsite=T), spin-polarized FM calculations.