Synthesis and compressibility of $Ti3(Al,Sn0.2)C2$ and $Ti3Al(C0.5,N0.5)2$

In this paper we report on the synthesis of a composition, $Ti3Al(C0.5,N0.5)2$⁠, belonging to the $Mn+1AXn$ family of ternary layered carbides and nitrides. X-ray and selected area diffraction confirm that this compound is isostructural with $Ti3SiC2$⁠; its $a$ and $c$-lattice parameters are $3.0404(5)$ and $18.414(6)Å$⁠, respectively. Chemical analysis performed by electron dispersive and electron energy loss spectroscopy confirmed the $Ti3AlCN$ chemistry. Using a synchrotron radiation source and a diamond anvil cell, we also measured the pressure dependencies of the lattice parameters. Up to a pressure of $≈50GPa$⁠, no phase transformations were observed. The bulk modulus is $219±4GPa$⁠, with a pressure derivative, $Ko′$⁠, of $3.7±0.3$⁠. We also fabricated the ternary $Ti3AlC2$⁠, with some Sn [nominal composition $Ti3(AlSn0.2)C2$]. Its $a$ and $c$-lattice parameters are $3.0804(7)$ and $18.5426(7)Å$⁠, respectively. Its bulk modulus is $226±3GPa$⁠, with a pressure derivative, $Ko′$⁠, of $≈4$⁠. In both cases, the compressibility was greater along the $c$ than along the $a$ axes. We also show that in the case of $Tin+1AlCn$ MAX-phases, replacing C by $N$ results in a decrease in both lattice parameters and bulk moduli. The apparent contradiction inherent in this observation can be reconciled by assuming that the addition of $N$ results in the formation of vacancies on the Al and/or X-sites.