Thermal expansion coefficient of synthetic diamond single crystal at low temperatures

The lattice parameter of synthetic diamond single crystals has been measured in the range 4.2-320 K by the X-ray diffraction method. The lattice parameter is found to be nearly constant between 4.2 and 90 K. The thermal expansion coefficient α calculated from the experimental results is very small (of the order of 10 -7 or less) and no definite evidence of the negative thermal expansion is found within our relative experimental accuracy of the order of ±1×10 -6 .