High-pressure synthesis and application of a ¹³C diamond pressure sensor for experiments in a hydrothermal diamond anvil cell

Polycrystalline, cubic ¹³C diamond was synthesized from amorphous carbon in the Kawai-type multianvil apparatus at 21 GPa and at a temperature greater than 2350°C. The polycrystalline diamond was homogeneous with a small grain size (10-20 μm) and a sharp Raman peak, and thereby was suitable as a pressure sensor for the experiments in a hydrothermal diamond anvil cell. Pressure-and temperature-dependence of the Raman shift of the synthesized ¹³C diamond was investigated in situ at simultaneous high pressures and high temperatures in the hydrothermal diamond anvil cell, using the ruby fluorescence line, quartz Raman shift and H₂O phase transitions as pressure references. It was observed that the frequency shift with pressure is independent of temperature and vice versa up to 500°C and 4.2 GPa. The present study indicates that the ¹³C diamond Raman shift can be used for pressure determination with an accuracy better than ±0.3 GPa under the conditions examined.