Crystal Structure, Microstructure and Electronic Properties of a Newly Discovered Ternary Phase in the Al-Cr-Sc System

Anton Meden,Jean-Marie Dubois,Émilie Gaudry,Pascal Boulet,Sašo Šturm,Boštjan Markoli,Monika Kušter,Zoran Samardžija,Maja Vončina

doi:10.3390/cryst11121535

Abstract

This study focused on the crystal and electronic structures of a newly discovered phase in the Al-Cr-Sc system. The latter two species do not mix in a binary alloy, but can be alloyed with aluminium in the vicinity of the Al2−xCrxSc composition, where 0.3 < x < 0.5. After preparation of the pure constituents via arc melting, high-temperature annealing at 990 °C for 240 h was required to achieve full mixing of the elements. A detailed characterisation of the crystal structure, alloy microstructure and stability was obtained using single-crystal X-ray diffraction (SCXRD) and powder X-ray diffraction (PXRD), in addition to transmission electron microscopy (TEM), especially in high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) mode, scanning electron microscopy (SEM) with energy-dispersive X-ray spectroscopy (EDXS) and differential scanning calorimetry (DSC) measurements. The crystal structure was refined to a hexagonal unit cell of the MgZn2 type, space group no. 194, P63/mmc, which belongs to the Laves phases family. Special attention was paid to the occupancy of the crystallographic sites that were filled by both Cr and Al atoms. First-principles calculations based on the density functional theory (DFT) were performed to investigate the electronic structure of this ternary phase. The total density of states (DOS) exhibited a pronounced sp character, where a shallow pseudo-gap was visible 0.5 eV below the Fermi energy that brought a small but definite contribution to the thermodynamic stability of the compound.

Highlights

With the only exception of the binary Cd-Ca and Zn-Sc systems [1], all stable metallic quasicrystals discovered so far belong to a ternary system in which two of the constituents are mutually immiscible but can be combined with a third species
We can summarise that the ternary phase in the Al-Cr-Sc push–pull system with composition Al2−x Crx Sc (0.4 ≤ x ≤ 0.5) belonged to the Laves phases family with a
The Rietveld refinement and single-crystal diffraction confirmed the full occupancy of Sc on the 4f

Summary

Introduction

With the only exception of the binary Cd-Ca and Zn-Sc systems [1], all stable metallic quasicrystals discovered so far belong to a ternary (or multinary) system in which two of the constituents are mutually immiscible but can be combined with a third species. This fact has led to the concept of push–pull alloys (PPAs), which were introduced a few years ago by one of us and his late colleagues, E. Of specific relevance to the present aim is the formation, or not, of a pseudo-gap at the Fermi energy that is recognised as one of the main features of the total density of states (DOS) characteristic

Objectives

Methods

Results

Conclusion