Thermal expansion of diamond

Abstract

Using the X-ray method, the change in the lattice spacing of diamond with temperature bas been measured over the range of temperature from 28°C. to 605°C. The technique employed is rather different from that usually adopted, being designed to enable the rather small expansion of diamond to be measured accurately. The increase in the lattice constant at ordinary temperatures is small, while it is very rapid at higher temperatures. From the observed changes in lattice constant, the mean coefficients of volume expansion for different ranges of temperature have been deduced. The coefficient of volume expansion of diamond varies from 4.36 × 10^-6 in the range 301°-378° T. to 11. 76 × 10^-6 in the range 778° -878° T. Using the observed values of the coefficient of thermal expansion and the existing data on the atomic heat, the Griineisen's constant γ which is the ratio αV₀/χ₀C_ν has been evaluated. V₀ is the atomic volume and χ₀ the com pressibility of diamond. It is shown that y is not a constant, but increases from about 1.1 at low temperatures to about 1. 3 at 600° T. and falls again slowly as the temperature is raised.