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.

Recently two families of hexagonal ternary carbides and nitrides, namely, M nϩ1 AX n , where n ϭ 1 to 3, M is an early transition metal, A is a group A element (mostly IIIA and IVA) and X is C or N, were identified and shown to possess an unusual combination of properties. [1][2][3][4] Most of these phases were first synthesized by Nowotny and co-workers 5-8 in the sixties and have since mostly been ignored. They are layered, with a c/a ratio between 4 and 6, wherein the transition metal carbide or nitride layers are interleaved with hexagonal nets of pure group A element. With Vickers hardness values in the range of 3-5 GPa, they are anomalously soft when compared with most other carbides or nitrides. They are readily machinable by a process that is more reminiscent of the machining of glass-ceramics or graphite than that of typical metals. 1-3 Their room temperature thermal and electrical conductivities are good, and fall in the range of 20-40 W/m K, and (0.5-14) ϫ 10 6 (⍀ m) Ϫ1 , respectively. 3,4 At this time, there is little doubt that the many of these unusual properties are directly traceable to the relative weakness in shear of the bonds between the transition metal and A group elements; relative, that is, to the transition metal/carbon or nitrogen bonds. For example, basal plane dislocations multiply and are mobile at room temperatures in Ti 3 SiC 2 . 9,10 Rietveld analysis of neutron diffraction data in the 25-1000ЊC temperature range showed that, at all temperatures, the amplitudes of the vibrations of the Si atoms are substantially higher than those of the Ti or C atoms. 11 A recent Raman spectroscopy study has shown that all the first-order Raman disorder-induced modes active in TiC 0.67 are also active in Ti 3 SiC 2 but shifted to different energies. 12 Two additional peaks at 150 and 372 cm Ϫ1 are observed in Ti 3 SiC 2 ; the former was ascribed to a soft shear mode between the Si and Ti planes.Consistent with these notions is the fact that these ternaries do not melt congruently, but rather decompose into the A group element and the corresponding transition metal carbide or nitride. 4,13 The relative weakness of the transition metal/A group element bond is also reflected in the anisotropy of the hardness measurements; in Ti 3 SiC 2 the hardness normal to the basal planes is roughly four times higher than parallel to them. 14 Ti 3 SiC 2 is between two and three orders of magnitude more conductive than the graphite used as electrodes in the Hall-Heroult cells. This fact, together with a report in the literature, claiming that Ti 3 SiC 2 was chemically stable in molten cryolite, 15 instigated this work. As discussed below, and contrary to published results, Ti 3 SiC 2 is not stable in molten cryolite. The aim of this paper is to report on the transformation of Ti 3 SiC 2 into a partially ordered cubic phase of approximate chemistry Ti(C 0.67 , Si 0.06 ). The transformation is chemically induced by immersion of Ti 3 SiC 2 in molten cryolite and occurs topotactically by the diffusion of Si out of the b...

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In this paper, the Raman spectrum of Ti3SiC2 is reported and compared with that of TiC0.67. All the TiC0.67 first order Raman disorder-induced modes are active, but shifted, in Ti3SiC2. Two additional peaks at 150 and 372 cm−1 are observed in Ti3SiC2. The former is ascribed to a shear mode between the Si and Ti planes; the origin of the latter is unknown. No second order Raman bands are detected. Micro-Raman spectroscopy also reveal the presence of ≈50 Å graphite crystallites in samples hot pressed in graphite dies—these crystallites are not detected in samples processed by hot isostatic pressing in molten glass containers.

We have investigated the behavior of single-walled carbon nanotubes and nanospheres (C(60)) under high hydrostatic pressure using Raman spectroscopy over the pressure range 0.2-10 GPa using a diamond anvil cell. Different liquid mixtures were used as pressure transmission fluids (PTF). Comparing the pressure dependence of the Raman peak positions for the nanotubes and the nanospheres in different PTF leads to the observation of a number of new phenomena. The observed shift in Raman peak position of both radial and tangential modes as a function of applied pressure and their dependence on the PTF chemical composition can be rationalized in terms of adsorption of molecular species from the of PTF on to the surface of the carbon nanotubes and/or nanospheres. The peak shifts are fully reversible and take place at a comparatively modest pressure (2-3 GPa) that is far below pressures that might be required to collapse the nanoparticles. Surface adsorption of molecular species on the nanotube or nanospheres provides a far more plausible rational for the observed phenomena than ideas based on the notion of tube collapse that have been put forward in the recent literature.