Mn+1AXnphases in theTi−Si−Csystem studied by thin-film synthesis andab initiocalculations

Abstract

Thin films of $M_{n+1}A{X}_n$ layered compounds in the $\mathrm{Ti}\text{−}\mathrm{Si}\text{−}\mathrm{C}$ system were deposited on $\mathrm{MgO}\left(111\right)$ and ${\mathrm{Al}}_2{\mathrm{O}}_3\left(0001\right)$ substrates held at $900°\mathrm{C}$ using dc magnetron sputtering from elemental targets of $\mathrm{Ti}$, $\mathrm{Si}$, and $\mathrm{C}$. We report on single-crystal and epitaxial deposition of ${\mathrm{Ti}}_3{\mathrm{SiC}}_2$ (the previously reported $MAX$ phase in the $\mathrm{Ti}\text{−}\mathrm{Si}\text{−}\mathrm{C}$ system), a previously unknown $MAX$ phase ${\mathrm{Ti}}_4{\mathrm{SiC}}_3$ and another type of structure having the stoichiometry of ${\mathrm{Ti}}_5{\mathrm{Si}}_2{\mathrm{C}}_3$ and ${\mathrm{Ti}}_7{\mathrm{Si}}_2{\mathrm{C}}_5$. The latter two structures can be viewed as an intergrowth of 2 and 3 or 3 and 4 $M$ layers between each $A$ layer. In addition, epitaxial films of ${\mathrm{Ti}}_5{\mathrm{Si}}_3{\mathrm{C}}_{\mathrm{x}}$ were deposited and ${\mathrm{Ti}}_5{\mathrm{Si}}_4$ is also observed. First-principles calculations, based on density functional theory (DFT) of ${\mathrm{Ti}}_{n+1}{\mathrm{SiC}}_n$ for $n=1$,2,3,4 and the observed intergrown ${\mathrm{Ti}}_5{\mathrm{Si}}_2{\mathrm{C}}_3$ and ${\mathrm{Ti}}_7{\mathrm{Si}}_2{\mathrm{C}}_5$ structures show that the calculated difference in cohesive energy between the $MAX$ phases reported here and competing phases ($\mathrm{TiC}$, ${\mathrm{Ti}}_3{\mathrm{SiC}}_2$, ${\mathrm{TiSi}}_2$, and ${\mathrm{Ti}}_5{\mathrm{Si}}_3$) are very...