We report on the Raman spectra of Ti 3 SiC 2 ͑312͒, M 2 AlC ͑211͒ ͑M = Ti, V, Cr, and Nb͒ and Ti 4 AlN 3 ͑413͒, as representative compounds from the family of M n+1 AX n phases. Intense and narrow first-order Raman peaks are observed, and we present an analysis of the spectra based on symmetry considerations and from results of first-principles calculations of phonon frequencies. The agreement between experimental and calculated mode energies is excellent. The identification of the modes enables application of Raman scattering as a diagnostic tool for the detailed study of the structural and physical properties of this family of compounds and their engineered solid solutions.

Herein we report on a systematic investigation of the thermal expansion of select M n+1 AX n phases. The bulk dilatometric thermal expansion coefficient ␣ dil was measured in the 25-1200°C temperature range and the thermal expansion of more than 15 of these phases was studied by x-ray diffraction in the 25-800°C temperature range. The coefficient of thermal expansion for the a axis ␣ a ranged between ͑2.9Ϯ 0.1͒ ϫ 10 −6°C−1 ͑Nb 2 AsC͒ and ͑12.9Ϯ 0.1͒ ϫ 10 −6°C−1 ͑Cr 2 GeC͒ while the coefficient for the c axis ͑␣ c ͒ ranged between ͑6.4Ϯ 0.2͒ ϫ 10 −6°C−1 ͑Ta 2 AlC͒ and ͑17.6Ϯ 0.2͒ ϫ 10 −6°C−1 ͑Cr 2 GeC͒. Weak anisotropy in the thermal expansion was seen in most phases, with the largest value of ␣ c / ␣ a belonging to Nb 2 AsC. The Grüneisen parameters along the a and c directions were calculated from ab initio values for the elastic compliances and were relatively isotropic. A good correlation was found between the thermal expansion anisotropy and the elastic constant c 13 and we conclude that the anisotropy in thermal expansion is related to the bonding between the M − A elements.

In this letter we report on the low-temperature specific heat of several M2AX phases: Ti2AlC, V2AlC, V2AsC, Nb2SnC, Ti2AlN, Hf2InC, Nb2AlC, and Cr2AlC. The Debye temperatures are quite high. The density of states at the Fermi level, N(EF) varies from ≈1.4 (eV formula unit)−1 to 6 (eV formula unit)−1. Ab initio calculations show that N(EF) is dictated by the transition metal d–d bands; the A-group element has little effect. We also measured the velocity of sound in V2AlC, V2AsC, Ti2AlC, and Ti2AlN. The average bulk modulus of these materials is over 100 GPa, with a high of ≈140 GPa for Ti2AlN. Our theoretical calculations correctly predict the trend in both the density of states and the bulk modulus, although there is some disagreement in the actual values.