A High-Temperature Neutron Diffraction Study of Nb2AlC and TiNbAlC

Herein, we report on the crystal structures of Nb2AlC and TiNbAlC—actual composition (Ti0.45,Nb0.55)2AlC—compounds determined from Rietveld analysis of neutron diffraction patterns in the 300–1173 K temperature range. The average linear thermal expansion coefficients of a Nb2AlC sample in the a and c directions are, respectively, 7.9(5) × 10−6 and 7.7(5) × 10−6 K−1 on one neutron diffractometer and 7.3(3) × 10−6 and 7.0(2) × 10−6 K−1 on a second diffractometer. The respective values for the (Ti0.45,Nb0.55)2AlC composition—only tested on one diffractometer—are 8.5(3) × 10−6 and 7.5(5) × 10−6 K−1. These values are relatively low compared to other MAX phases. Like other MAX phases, however, the atomic displacement parameters (APDs) show that the Al atoms vibrate with higher amplitudes than the Ti and C atoms, and more along the basal planes than normal to them. When the predictions of the APDs obtained from density functional theory are compared to the experimental results, good quantitative agreement is found for the Al atoms. In case of the Nb and C atoms, the agreement was more qualitative.