High-temperature neutron diffraction and the temperature-dependent crystal structures of the MAX phases Ti3SiC2 and Ti3GeC2

Herein, we report on the crystal structures of the isostructural Ti3SiC2 and Ti3GeC2 phases determined by Rietveld analysis of neutron diffraction data in the 100 to 1100 degrees C temperature range. The results show that the Si and Ge atoms vibrate anisotropically with the highest amplitudes and within the basal planes. The equivalent isotropic thermal motion behavior does not differ significantly between the two phases; the anisotropic thermal motion, interatomic distances, and bond angles, however, show strikingly different behavior. Furthermore, while the Ti- Si bonds increase linearly with increasing temperature, the Ti- Ge bonds apparently do not. The anisotropic motion of the Ge atoms in the basal plane with the correlated motion between the Ti and the Ge atoms is invoked as a possible explanation. The volume expansions are 9.0(+/- 0.1) X 10(-6) K-1 and 8.7(+/- 0.1) X 10(-6) K-1 for Ti3SiC2 and Ti3GeC2, respectively; the expansions along the a and c axes are alpha(a) = 8.9 (+/- 0.1) X 10(-6) K-1 and alpha(c)= 9.4 (+/- 0.1) X 10(-6) K-1 for Ti3SiC2 and alpha(a) = 8.5(+/- 0.1) X 10(-6) K-1 and alpha(c) = 9.2(+/- 0.1) X 10(-6) K-1 for Ti3GeC2. A dramatic increase in error bars and a discontinuity in thermal motion parameters of the Ti-II atoms in Ti3GeC2 were also observed between 300 and 500 C during both heating and cooling. This discontinuity may in turn explain why the internal friction rises dramatically in that temperature range.