Compression behavior of M₂AlC (M=Ti, V, Cr, Nb, and Ta) phases to above 50 GPa

The compression behavior of the hexagonal MAX phases M₂AlC (M=Ti, V, Cr, Nb, and Ta)—selected because they represent two series; a horizontal series in which the atomic number of the M element increases from 23 to 25 and a vertical series where the M element (V, Nb, or Ta) belongs to the VA group—were measured as a function of quasihydrostatic pressure up to 55 GPa, using a synchrotron radiation source and a diamond-anvil cell. No phase transformations were observed in any of these compounds. The contractions for the Ti- and V-containing compositions were higher along the c axis than along the a axis; the opposite was true for Cr₂AlC and Nb₂AlC. In Ta₂AlC, the shrinkages in both directions are almost identical. For V₂AlC the bulk modulus K₀ is 201±3 GPa. As V is substituted by Nb, K₀ increases by 4%. The K₀ of Ta₂AlC, 251 GPa, is the highest reported for any MAX phase to date. For Ti₂AlC, K₀ is 186±2 GPa. As Ti is substituted by V, K₀ increases by 8%. Surprisingly, the substitution of Ti or V by Cr leads to a reduction in K₀ to 165±2 GPa. With the notable exception of Cr₂AlC, the agreement between experimental and calculated K₀ values is acceptable. The presence of C in these structures appears to have a stabilizing effect on the M-Al bonds, presumably by attracting electrons to the M-X bonds.