Synthesis, crystallographic structure and thermodynamic properties of T2-Al2MgC2

Abstract

T2-Al2MgC2 was synthesized from the elements in a Mg-Al melt at 1000°C using sealed Ta crucibles. Single crystals of T2-Al2MgC2 were extracted by evaporating the Mg-Al matrix. The crystal structure of T2-Al2MgC2 was refined for the first time on the basis of single-crystal X-ray diffraction. The crystal is trigonal (space group P-3m1, Z=1) with lattice parameters of a=3.3767(11) Å, c=5.807(2) Å and V=57.34(5) Å 3. Based on the refined crystal structure, DFT calculations were conducted to evaluate the thermodynamic properties and the electronic structure of the phase. The heat of formation of T2-Al2MgC2 was calculated to be-23.6 kJ/moles of atoms at 298K. The heat capacity of T2-Al2MgC2 was measured by DSC from 300 to 871K and calculated by DFT from 0 to 1000K. Based on the calculated heat capacity, the entropy of formation of the phase at 298K was determined to be 70.0 J/mol/K. The band structure and the electronic density of state of T2-Al2MgC2 was calculated leading to an indirect band gap value of 1.73 eV.

Thermal stability of Al2MgC2 and thermodynamic modeling of the Al–C–Mg system - Application to grain refinement of Mg–Al alloys

“…On the one hand, there is the prospect of an application of Al-C-Mg carbides in electronic. Al4C3 and Al2MgC2 are semiconductors with an indirect band gap calculated by DFT at respectively 1.34 [1] and 1.73 eV [2], although it is to be noted that those values are very likely to be underestimated. Plus, Al4C3 can contain a significant amount of Mg in solid solution [2,3] pointing to the possibility of a p-type doping.…”

“…Al4C3 and Al2MgC2 are semiconductors with an indirect band gap calculated by DFT at respectively 1.34 [1] and 1.73 eV [2], although it is to be noted that those values are very likely to be underestimated. Plus, Al4C3 can contain a significant amount of Mg in solid solution [2,3] pointing to the possibility of a p-type doping. Finally, the hexagonal structure of Al4C3 and Al2MgC2 suggests a good compatibility with the 4H polytype of SiC in the scope of making heterojunctions by epitaxial growth.…”

“…Finally, the hexagonal structure of Al4C3 and Al2MgC2 suggests a good compatibility with the 4H polytype of SiC in the scope of making heterojunctions by epitaxial growth. On the other hand, the crystal structure of Al2MgC2 is made of Al-C layers separated by weakly bonded Mg layers [2]. As a result, the delamination of the structure to make 2D materials similarly to the synthesis of MXenes from MAX phases [4,5] can be considered.…”

“…In spite of the major industrial interest of the Al-C-Mg ternary system, a complete thermodynamic assessment of Al-C-Mg have yet to be proposed as neither Al2MgC2 nor the solubility of Mg in Al4C3 [2,3] is currently described in commercial databases [34][35][36]. This shortage surely fueled the debate on the grain refinement of Mg-Al alloys by carbon inoculation as one cannot rely on thermodynamic calculations to support very delicate experimental work.…”

“…In the scope of the thermodynamic modeling of the Al-C-Mg system, experimental investigations coupled with DFT calculations on the thermodynamic properties of Al4C3 [1] and Al2MgC2 [2] were recently performed. Notably, the thermodynamic description of the Al-C system was revised [39] as conflicting results could be found regarding the enthalpy of formation of Al4C3 [1] and phase equilibria [39].…”