XL. Thermal expansion of diamond

X-ray measurements are reported of the thermal expansion of industrial and gem-quality diamond from −150°c to +950°c. It is found that the thermal expansion of industrial diamond shows an anomaly at low temperatures. No such anomaly is found for gem-quality diamond, for which Grüneisen's law holds in general throughout the range. The values of the unit-cell dimension at 20°c are : industrial diamond, a 0=3·5662, Å, gem-quality diamond, a 0=3·56684 Å.