Raman shifts of c-BN as an ideal P-T sensor for studying water-rock interactions in a diamond-anvil cell

Cubic boron nitride (c-BN) has the same structure as diamond, and it shows very inert reaction activity in different chemical environments, even under high-pressure (P) and high-temperature (T) conditions. Furthermore, the P-and T-dependent Raman shift of c-BN (e.g., TO mode) can be distinguished from that of the diamond anvil (c-BN at similar to 1054 cm(-1) vs. diamond at similar to 1331 cm(-1) at ambient conditions), making c-BN a potential P-Tsensor for diamond-anvil cell (DAC) experiments. However, the Raman shift of c-BN has not been well studied at high P -T conditions, especially at temperatures above 700 K. In this study, we systematically calibrated the Raman shift of the TO mode (nu(TO)) for synthetic c-BN grains at high -P and high -T conditions up to 15 GPa and 1300 K. Both ruby (Mao et al. 1986) and Sm2+:SrB4O7 (Datchi et al. 2007) were used as internally consistent standards for calibration of c-BN P -T sensor. Our results show that the Raman shift of c-BN is negatively correlated with temperature [partial derivative nu(TO)/partial derivative(T) = -0.02206(71)] but positively correlated with pressure [partial derivative nu(TO)/partial derivative(P) = -3.35(2)]. More importantly, we found that the P-T cross derivative for the Raman shift of c-BN [partial derivative(2)nu(TO)/partial derivative(P)partial derivative(T) = 0.00105(7)] cannot be ignored, as it was assumed in previous studies. Finally, we calibrated a Raman shift P-T sensor of c-BN up to 15 GPa and 1300 K as follows: P = A(T)-root A(T)(2) +0.2194B(T, Delta nu)/0.1097 where A(T) = 3.47(6) + 0.00105(7)T, B(T,Delta nu(TO)) = 2.81(51) - 0.0053(16)T - 1.78(11) x 10(-5) T-2 -Delta nu(TO). The c-BN Raman shift P-T sensor in this study fills the P-T gap ranging from previously performed externally resistance-heated to laser-heated DAC experiments. The effect of c-BN grain size and Raman system laser power on the calibration were also tested for the P-T sensor. In addition, we conducted three sets of high-P-T experiments to test the practicability of c-BN P-T sensor for water-rock interaction experiments in DAC. Testing experiments showed c-BN has very stable chemical activity in water and clear Raman signal at high-P-T conditions in comparison with other P-T sensors (e.g., ruby, Sm2+:SrB4O7, and quartz). Hence, the Raman shifts of c-BN may serve as an ideal P-T sensor for studying water-rock interactions in a DAC, especially at high-P and high-T conditions relevant to subduction zones.

Automated summary

We calibrated the Raman shift of synthetic c-BN grains as a P-T sensor for diamond-anvil cell experiments under high-pressure and high-temperature conditions. We found a negative correlation between the Raman shift of c-BN and temperature, and a positive correlation with pressure. We also established a Raman shift P-T sensor for c-BN and tested its stability and signal clarity in water-rock interaction experiments.