Impact of Disorder on Properties of Vacancies: A Case Study of B2 and A2 Polymorphs of Non-Stoichiometric Fe2CoAl

Martin Friák,Mojmír Šob,Jana Pavlů

doi:10.3390/cryst11101207

Abstract

We have performed an ab initio study of vacancy-induced changes in thermodynamic, structural and magnetic properties of single-phase ferromagnetic Fe2CoAl with a chemically disordered (i) two-sublattice B2 phase or (ii) single-sublattice A2 phase. The two polymorphs of slightly non-stoichiometric Fe2CoAl (Fe27Co14Al13) were modeled by two different 54-atom supercells with atoms distributed according to the special quasi-random structure (SQS) concept. Both the lower-energy B2 phase and a higher-energy A2 phase possess elastic constants that correspond to an auxetic material that is mechanically stable. The properties of vacancies were computed by systematically removing different atoms (one at a time) from the supercells and quite wide ranges of values of vacancy-related characteristics were obtained. The increase in the level of disorder (when changing from the B2 to the A2 phase) results in an increase in the scatter of calculated values. The Fe and Co vacancies have lower vacancy formation energies than the Al ones. The total magnetic moment of the supercell decreases when introducing Fe and Co vacancies but it increases due to Al ones. The latter findings can be partly explained by an increase of the local magnetic moment of Fe atoms when the number of Al atoms in the first neighbor shell of Fe atoms is reduced, such as due to Al vacancies.

Highlights

We have performed a quantum-mechanical study of vacancy-induced changes in thermodynamic, structural and magnetic properties of single-phase ferromagnetic slightly non-stoichiometric Fe2 CoAl with a chemically disordered either two-sublattice B2 or singlesublattice A2 phase
The two polymorphs of Fe2 CoAl were modeled by two different 54atom supercells with atoms on either two B2 sublattices or a single A2 sublattice distributed according to the special quasi-random structure (SQS) concept
The Fe and Co vacancies have lower vacancy formation energies than the Al ones

Summary

Introduction

Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations. Our study is focused on the Fe2 CoAl intermetallic compound belonging to a very large class of ternary X2 YZ materials with Heusler-type crystal lattice [1]. This family of compounds covers numerous combinations of different chemical elements, see, e.g., highthroughput theoretical studies in refs. This compositional variability provides a wide range of properties [4,5], including magnetic ones [6,7,8], half-metallic properties that are interesting for spintronic applications [9,10,11,12,13], magneto-optical functionalities [14], topological quantum features [15,16] or, e.g., shape-memory behavior [17,18,19]

Methods

Results

Conclusion