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

Abstract

Abstract 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 ~1054 cm–1 vs. diamond at ~1331 cm–1 at ambient conditions), making c-BN a potential P-T sensor 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 (νTO) 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 [∂νTO/∂T = –0.02206(71)] but positively correlated with pressure [∂νTO/∂P = –3.35(2)]. More importantly, we found that the P-T cross derivative for the Raman shift of c-BN [∂2νTO/∂P∂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 ) − A ( T ) 2 + 0.2194 B ( T , Δ v ) 0.1097 where A(T) = 3.47(6) + 0.00105(7)T, B(T, ΔνTO) = 2.81(51) – 0.0053(16)T – 1.78(11) × 10–5T2 – ΔνTO. 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.