Compression behavior of M2AlC (M=Ti, V, Cr, Nb, and Ta) phases to above 50 GPa -: art. no. 024110

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 K-0 is 201 +/- 3 GPa. As V is substituted by Nb, K-0 increases by 4%. The K-0 of Ta2AlC, 251 GPa, is the highest reported for any MAX phase to date. For Ti2AlC, K-0 is 186 +/- 2 GPa. As Ti is substituted by V, K-0 increases by 8%. Surprisingly, the substitution of Ti or V by Cr leads to a reduction in K-0 to 165 +/- 2 GPa. With the notable exception of Cr2AlC, the agreement between experimental and calculated K-0 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.