Experimental and DFT investigation of (Cr,Ti)3AlC2 MAX phases stability

TL;DR

This study combines experimental and DFT methods to explore the stability and solubility limits of (Cr,Ti)3AlC2 MAX phases, revealing specific site occupation and bonding characteristics that influence their stability.

Contribution

It provides new insights into the solubility behavior and atomic site preferences of Cr in (Cr,Ti)3AlC2 MAX phases through combined experimental and computational analysis.

Findings

Cr can only be incorporated at very low concentrations or specific ratios.

Cr exclusively occupies 4f sites bridging layers.

Strong Cr-C bonds contribute to phase stability and volume changes.

Abstract

Using a synergistic combination of experimental and computational methods, we shed light on the unusual solubility of (Cr,Ti)3AlC2 MAX phase, showing that it may accommodate Cr only at very low concentrations (<2at%) or at the exact Cr/(Cr+Ti) ratio of 2/3, even when the ratio of reactants is far from this stoichiometry (1/2 < Cr/(Cr+Ti) < 5/6). In both phases Cr exclusively occupies the 4f sites, bridging carbide layers with the Al layer. Despite this, the peculiar stability of (Cr2/3Ti1/3)3AlC2 is attributed to the formation of strong, spin-polarised Cr-C bonds, which result in volume reduction and marked increase in c/a ratio.