Thermal expansion of 4H and 6H SiC from 5 K to 340 K

The first thermal expansion measurements of the 4H and 6H polytypes of SiC below room temperature are reported. The measurements were carried out on single-crystal specimens using high-resolution capacitive based dilatometry. For both polytypes, the thermal expansion coefficient is below 2.4 x 10(-6) 1/K near room temperature. No phase transitions are observed over the 5 K to 340 K temperature range of the measurements. The thermal expansion coefficient alpha of 4H SiC is slightly anisotropic for measurements parallel and perpendicular to the crystallographic c axis with alpha(||) about 2.2 x 10(-7) 1/K larger than alpha(perpendicular to) near room temperature. For 6H SiC no discernible anisotropy is observed. The differences in anisotropy can be understood by considering the ratio of hexagonal to cubic bonds of each polytype. Narrow regions with negative thermal expansion that are within the limits of our resolution (similar to 1 x 10(-8)) are observed in the vicinity of 30 K for both specimens. Tabulated data, polynomial fits, fit parameters, and comparison to data based on lattice-parameter measurements are provided.