Thermal expansion of select M>n+1>AX>n (M=early transition metal, A=A group element, X=C or N) phases measured by high temperature x-ray diffraction and dilatometry

Herein we report on a systematic investigation of the thermal expansion of select M(n+1)AX(n) phases. The bulk dilatometric thermal expansion coefficient alpha(dil) was measured in the 25-1200 degrees C temperature range and the thermal expansion of more than 15 of these phases was studied by x- ray diffraction in the 25- 800 degrees C temperature range. The coefficient of thermal expansion for the a axis alpha(a) ranged between (2.9 +/- 0.1) X 10(-6) degrees C-1 (Nb2AsC) and (12.9 +/- 0.1) X 10(-6) degrees C-1 (Cr2GeC) while the coefficient for the c axis (alpha(c)) ranged between (6.4 +/- 0.2) X 10(-6) degrees C-1 (Ta2AlC) and (17.6 +/- 0.2) X 10(-6) degrees C-1 (Cr2GeC). Weak anisotropy in the thermal expansion was seen in most phases, with the largest value of alpha(c)/alpha(a) belonging to Nb2AsC. The Greneisen 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 c(13) and we conclude that the anisotropy in thermal expansion is related to the bonding between the M-A elements. (C) 2009 American Institute of Physics.