Crystal structure and formation mechanism of (Cr2/3Ti1/3)3A1C2 MAX phase

The crystal structure of the newly synthesized quaternary MAX phase (Cr2/3Ti1/3)(3)A1C(2) was systematically characterized by various techniques. The space group of (Cr2/3Ti1/3)(3)A1C(2) is determined to be P6(3)/mmc by a combination of selected-area and convergent-beam electron diffraction techniques. Rietveld refinements of the neutron diffraction and X-ray diffraction data show that in (Cr2/3Ti1/3)(3)A1C(2), Ti and Cr are ordered with Ti in the 2a and Cr in the 4f Wyckoff sites of a M(3)AX(2) lattice. It is interesting to find that when the order of the magnetic moment of Cr atoms is considered, the ferromagnetic configuration of (Cr2/3Ti1/3)(3)A1C(2) becomes the ground state. Meanwhile, the Raman-active mode wavenumbers of (Cr2/3Ti1/3)(3)A1C(2) were calculated, and the theoretical data are quite consistent with the experimental data, further proving the ordered crystal structure of this phase. The formation of (Cr2/3Ti1/3)(3)A1C(2) with a unique crystal structure may be related to the distinctly different electronegativities and covalent radii of Cr and Ti atoms. (C) 2014 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.