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The Mn+1AXn (n=1-3) phases have been subject for considerable activities lately. The interest can be seen from the combined ceramic and metallic properties of the archetype Ti3SiC2, e.g. high oxidation and decomposition temperature with thermal and electrical conductivity values atypical for a ceramic material. These properties stem from a layered hexagonal crystal structure consisting of layers of group 13-15 elements (A) that are inserted to an early transition metal carbide and/or nitride skeleton (MX). Substituting elements in the structure makes it possible to synthesize a multitude of phases, and approximately 50 different phases are known today. However, relatively little attention has been paid to the Ti-Ge-C and Ti-Sn-C systems with the known phases Ti3GeC2, Ti2GeC, and Ti2SnC. Using d.c. magnetron sputtering of elemental sources we have shown that these phases can be deposited as epitaxial films on Al2O3(0001) substrates at 700-1000 oC. In addition, we have synthesized the new phases Ti4GeC3 and Ti3SnC2 as well as the intergrown structures Ti5Ge2C3 and Ti7Ge2C5. The growth conditions for these MAX phases depend on the properties of the A-element. For the Ti-Sn-C system we find a pronounced segregation of Sn, resulting in preferential growth of TiC and Sn. A similar growth behavior, however much less pronounced, is also present in the Ti-Ge-C system as seen by a surface segregation of elemental Ge and the formation of Ti5Ge3Cx. Four-point probe measurements show that the i films are good conductors, but that the growth conditions affect the resistivity. The trend obtained from the measurements is that the Ti2GeC films show the lowest values of ~20 µΩcm compared with ~50 µΩcm for Ti3GeC2. Ti-Sn-C films show a relatively large spread due to the lower stability of Ti2SnC, but with a value of ~50 µΩcm for an effectively phase-pure Ti2SnC film. |