Paramagnetic Structural Defects of MAX-Phases Cr–Ti–Al–C

The phase composition, surface condition and nature of paramagnetic centers in the sample of MAX-phase Cr₂TiAlC₂ obtained by sintering pressed powders of Cr, Al, TiC, C in vacuum at 1300°C were studied using X-ray phase analysis, X-ray photoelectron spectroscopy (XPS) and electron paramagnetic resonance. According to X-ray diffraction data, the main phase is MAX-phase 312 (Cr_2/3Ti_1/3)₃AlC₂ with impurities of Cr_1.5Ti_0.5AlC, Cr₂AlC, α-Al₂O₃, carbides and chromium oxides. XPS analysis revealed that aluminum oxide and oxidized states of titanium and chromium predominate on the sample surface, with the Ti⁴⁺/Ti³⁺/Ti²⁺ ratio being 1.58/1.0/1.08, and the Cr³⁺ content exceeding the Cr⁶⁺ content by more than three times. The nature of paramagnetic centers in the (Cr_2/3Ti_1/3)₃AlC₂ sample is established. Electronic defects and various types of O^– and Cr³⁺ hole centers were identified in various oxide matrices of the near-surface layers, which may be important for the electron conductivity and magnetic properties.

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