Synthesis of Cr2AuC via thermal substitution reaction in Au-covered Cr2GaC and Cr2GeC thin films

Abstract

Intercalation of noble metals into non-van der Waals solids provides a new avenue to synthesize novel nanolaminated compounds with distinct material properties. Herein, we use solid-state reaction at 400 °C to prepare Cr2AuC from two Cr-based Mn+1AXn phase precursors and demonstrate the formation of Cr2AuC upon full replacement of Ga layers with Au in Cr2GaC thin films via thermal substitution reaction. The resulting Cr2AuC exhibits 2.7% lattice expansion relative to the original Cr2GaC, whereas Ge in a Cr2GeC thin film was sparsely replaced by Au, as revealed by electron microscopy and x-ray diffraction analysis. To explain the observed differences, using ab initio calculations, we consider the bonding properties of Cr2GaC and Cr2GeC, and the energetic driving forces for substitution by evaluating the mixing free energy of Au on both A-sites of the MAX phases, and of both A-elements in the surrounding Au lattice. The results suggest that it is somewhat easier to mix Au on the A-site in Cr2GaC than in Cr2GeC, and substantially easier to mix Ga into the Au-lattice than Ge. Finally, we discuss how the gained insights can be consulted for exploring a wider class of Mn+1AXn phases with intercalated noble metals.