Thermal expansion of select Mn+1AXn (M=earlytransitionmetal, A=Agroupelement, X=C or N) phases measured by high temperature x-ray diffraction and dilatometry

Abstract

Herein we report on a systematic investigation of the thermal expansion of select Mn+1AXn phases. The bulk dilatometric thermal expansion coefficient αdil was measured in the 25–1200 °C temperature range and the thermal expansion of more than 15 of these phases was studied by x-ray diffraction in the 25–800 °C temperature range. The coefficient of thermal expansion for the a axis αa ranged between (2.9±0.1)×10−6 °C−1 (Nb2AsC) and (12.9±0.1)×10−6 °C−1 (Cr2GeC) while the coefficient for the c axis (αc) ranged between (6.4±0.2)×10−6 °C−1 (Ta2AlC) and (17.6±0.2)×10−6 °C−1 (Cr2GeC). Weak anisotropy in the thermal expansion was seen in most phases, with the largest value of αc/αa belonging to Nb2AsC. The Grüneisen parameters along the a and c directions were calculated from ab initio values for the elastic compliances and were relatively isotropic. A good correlation was found between the thermal expansion anisotropy and the elastic constant c13 and we conclude that the anisotropy in thermal expansion is related to the bonding between the M−A elements.