Competing rhombohedral and monoclinic crystal structures in Mn$Pn_2Ch_4$ compounds: an {\em ab-initio} study

TL;DR

This study uses ab-initio calculations to explore the crystal, electronic, and magnetic properties of MnPn2Ch4 compounds, revealing a preference for rhombohedral or monoclinic structures based on element heaviness and detailing their magnetic coupling and bandgap differences.

Contribution

It provides the first detailed comparison of rhombohedral and monoclinic structures in MnPn2Ch4 compounds, linking structural preferences to element types and magnetic properties.

Findings

Heavier elements favor rhombohedral structure

Lighter elements tend to form monoclinic structure

Monoclinic compounds have wider bandgaps

Abstract

Based on the relativistic spin-polarized density functional theory calculations we investigate the crystal structure, electronic and magnetic properties of a family MnPn2Ch4 compounds, where pnictogen metal atoms (Pn) are Sb and Bi; chalcogens (Ch) are Se, Te. We show that in the series the compounds of this family with heavier elements prefer to adopt rhombohedral crystal structure composed of weakly bonded septuple monoatomic layers while those with lighter elements tend to be in the monoclinic structure. Irrespective of the crystal structure all compounds of the MnPn2Ch4 series demonstrate a weak energy gain (of a few meV per formula unit or even smaller than meV) for antiferromagnetic (AFM) coupling for magnetic moments on Mn atoms with respect to their ferromagnetic (FM) state. For rhombohedral structures the interlayer AFM coupling is preferable while in monoclinic phases intralayer AFM configuration with ferromagnetic ordering along the Mn chain and antiferromagnetic ordering between the chains has a minimum energy. Over the series the monoclinic compounds are characterized by substantially wider bandgap than compounds with rhombohedral structure.