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Abstract
The recent achievement of hydrostatic pressures of the order of 1 Mbar within the Diamond Anvil Cell suggests that it may be possible to approach in the laboratory both the pressure and temperature conditions prevailing in the earth's interior. Thus it would be desirable to know if diamond retains its superhard characteristics at high temperatures. The elastic stiffness constants of a crystalline material are related to its stability and hardness. If the material is transparent, as is the case for diamond, Brillouin scattering is a convenient method for the determination of its elastic constants at high temperatures. The upper temperature limit (1600 K) of our measurements was set by the softening and surface deterioration of the quartz crucibles in which the diamond samples were held under vacuum, and not by the diamond samples themselves. Additionally, the measurement of a particular phonon branch in two different scattering geometries allowed us to measure the index of refraction of diamond up to 1600 K.