Temperature-Dependent Crystal Structures of Ti2AlN and Cr2GeC as Determined from High Temperature Neutron Diffraction

In this work, we report on the temperature-dependent crystal structures of the isostructural, layered hexagonal phases Ti2AlN and Cr2GeC determined by Rietveld analysis of high temperature neutron powder diffraction data of fully dense, polycrystalline, bulk samples in the 100 degrees to 1100 degrees C temperature range. For both phases, the A-group atoms, Al and Ge, vibrate with the highest amplitude and do so anisotropically within the basal plane. All bonds expand linearly with temperature, with the highest relative thermal expansion occurring in the Ti-Al and Cr-Ge bonds. The thermal expansion coefficients in the a-and c-direction are, respectively, 10.3(+/- 0.2) x 10(-6) and 9.3(+/- 0.2) x 10(-6) K-1 for Ti2AlN and 12.8(+/- 0.3) x 10(-6) and 14.6(+/- 0.3) x 10(-6) K-1 for Cr2GeC. The unit cell volume expansions observed by HTND are 10.0(+/- 0.2) x 10(-6) K-1 for Ti2AlN and 13.4(+/- 0.3) x 10(-6) K-1 for Cr2GeC.