Ab-initio study of different structures of CaC: Magnetism, Bonding, and Lattice Dynamics

TL;DR

This study uses ab-initio calculations to explore various structural, magnetic, and dynamical properties of hypothetical CaC compounds, revealing stability trends, magnetic behavior linked to ionicity, and phonon-related instabilities across different crystal structures.

Contribution

It provides a comprehensive ab-initio analysis of multiple CaC structures, identifying the most stable form and elucidating the relationship between bonding type and magnetic properties.

Findings

B33 structure is most stable across temperature and pressure.

Ionic structures exhibit ferromagnetism with high spin polarization.

Dynamical instabilities are linked to covalent bonding in certain structures.

Abstract

On the basis of ab-initio pseudopotential calculations, we study structural, magnetic, dynamical, and mechanical properties of the hypothetical CaC ionic compound in the rock-salt (RS), B2, zinc-blende (ZB), wurtzite (WZ), NiAs (NA), anti-NiAs (NA*), and CrB (B33) structures. It is argued that the ZB, WZ, NA, and RS structures are more ionic while the NA*, B2, and B33 structures are more covalent systems. As a result of that, the nonmagnetic B33-CaC is the energetically preferred system, while the more ionic structures prefer a ferromagnetic ground state with high Fermi level spin polarization. The observed ferromagnetism in the more ionic systems is attributed to the sharp partially filled $p$ states of carbon atom in the system. In the framework of density functional perturbation theory, the phonon spectra of these systems are computed and the observed dynamical instabilities of the NA* and B2 structures are explained in terms of the covalent bonds between carbon atoms. The calculated Helmholtz and Enthalpy free energies indicate the highest stability of the B33 structure in a wide range of temperatures and pressures. Among the ferromagnetic structures, RS-CaC and ZB-CaC are reported, respectively, to be the most and the least metastable systems in various thermodynamics conditions. Several mechanical properties of the dynamically stable structures of CaC are determined from their phonon spectra.