Ab Initio Study on the Effect of Au Layers on Phase Stability, Chemical Bonding, Elastic, and Thermal Properties of Ti₄Au_nC₃ (n = 1–3)

The recently synthesized Ti₄Au₃C₃ with a unique gold trilayer stacking offers a promising model to explore the influence of noble metal layers on phase stability, chemical bonding, and elastic and thermal properties. In this work, these properties are investigated by first-principles simulations for Ti₄Au₃C₃ as well as hypothetical Ti₄AuC₃ and Ti₄Au₂C₃, thereby clarifying their MAX-phase characteristics and enabling assessment of potential future applications. Increasing the Au-layer count from a monolayer (Ti₄AuC₃) through a bilayer (Ti₄Au₂C₃) to a trilayer (Ti₄Au₃C₃) significantly affects both the elastic behavior and finite-temperature thermal properties. Analyses of bond stiffness and related mechanical indicators suggest lower damage tolerance for Ti₄Au₃C₃ than for Ti₄Au₂C₃ and Ti₄AuC₃. Also including the contribution from electronic excitation, their heat capacity and thermal expansion are predicted by quasi-harmonic approximations, with near-room-temperature heat capacities of approximately 154, 179, and 206 J·mol⁻¹·K⁻¹, and average linear thermal expansion coefficients of 10.10 × 10⁻⁶, 12.45 × 10⁻⁶, and 18.02 × 10⁻⁶ K⁻¹ for Ti₄AuC₃, Ti₄Au₂C₃, and Ti₄Au₃C₃ over 300–1000 K, respectively. This work offers quantitative guidance for A-layer engineering of Au-containing MAX phases and a method to tune the properties of MAX phases for potential engineering applications.