We explore the stability of the ambient pressure zinc-blende polymorph (B3) structure of silicon carbide (SiC) at high pressures and temperatures where it transforms to the rock-salt (B1) structure. We find that the transition occurs ~40 GPa lower than previously measured when heated to moderately high temperatures. A lower transition pressure is consistent with the transition pressures predicted in numerous ab initio computations. We find a large volume decrease across the transition of ~17%, with the volume drop increasing at higher formation pressures, suggesting this transition is volume driven yielding a nearly pressure-independent Clapeyron slope. Such a dramatic density increase occurring at pressure is important to consider in applications where SiC is exposed to extreme conditions, such as in industrial applications or planetary interiors.

Abstract:The high-pressure behavior of silicon carbide (SiC), a hard, semi-conducting material commonly known for its many polytypic structures and refractory nature, has increasingly become the subject of current research. Through work done both experimentally and computationally, many interesting aspects of high-pressure SiC have been measured and explored. Considerable work has been done to measure the effect of pressure on the vibrational and material properties of SiC. Additionally, the transition from the low-pressure zinc-blende B3 structure to the high-pressure rocksalt B1 structure has been measured by several groups in both the diamond-anvil cell and shock communities and predicted in numerous computational studies. Finally, high-temperature studies have explored the thermal equation of state and thermal expansion of SiC, as well as the high-pressure and high-temperature melting behavior. From high-pressure phase transitions, phonon behavior, and melting characteristics, our increased knowledge of SiC is improving our understanding of its industrial uses, as well as opening up its application to other fields such as the Earth sciences.

6We measure the onset of decomposition of silicon carbide SiC to silicon and carbon (e.g., 7 diamond) at high pressures and high temperatures in a laser-heated diamond-anvil cell. We 8 identify decomposition through x-ray diffraction and multi-wavelength imaging radiometry 9 coupled with electron microscopy analyses on quenched samples. We find that B3 SiC (also 10 known as 3C or zinc-blende SiC) decomposes at high pressures and high temperatures, following 11 a phase boundary with a negative slope. The high-pressure decomposition temperatures 12 measured are considerably lower than that at ambient, with our measurements indicating that SiC 13 begins to decompose at ~2000 K at 60 GPa as compared to ~2800 K at ambient pressure. Once 14 B3 SiC transitions to the high-pressure B1 (rocksalt) structure, we no longer observe 15 decomposition, despite heating to temperatures in excess of ~3200 K. The temperature of 16 decomposition and the nature of the decomposition phase boundary appear to be strongly 17 influenced by the pressure-induced phase transitions to higher density structures in SiC, silicon 18 and carbon. The decomposition of SiC at high-pressure and temperature has implications for the 19 stability of naturally forming moissanite on Earth and in carbon-rich exoplanets. 20 21A large body of work has been performed to better understand aspects of the SiC phase 30 diagram at various pressure and temperature conditions [8][9][10][11][12][13][14][15][16][17][18][19][20][21]. The ambient pressure, high 31 temperature behavior of the Si-C system has been explored in detail [11]. Ambient temperature 32 studies have explored a wide range of pressure conditions. Zhuravlev et al., 2013 [20] proposed 33 the use of SiC as a pressure standard in the diamond-anvil cell (DAC) based on elasticity studies 34 carried out up to 80 GPa. It was found that cubic B3 SiC transforms to the B1 structure at a 35 pressure of ~100 GPa when compressed in a DAC [19]. This transition was additionally 36 observed in shock studies [13,15] and was recently reported at lower pressures of ~60 GPa when 37 heated in a laser-heated DAC [10]. Additional studies of the high pressure and temperature (high 38 P-T) behavior of SiC include measurements of the thermal equation of state (EOS) up to 39 pressures of 8.

Significance Massive carbon (C) release with abrupt warming has occurred repeatedly during greenhouse states, and these events have driven episodes of ocean deoxygenation and extinction. Records from these paleo events, coupled with biogeochemical modeling, provide clear evidence that with continued warming, the modern oceans will experience substantial deoxygenation. There are, however, few constraints from the geologic record on the effects of rapid warming under icehouse conditions. We document a C-cycle perturbation that occurred under an Earth system state experiencing recurrent glaciation. A suite of proxies suggests increased seafloor anoxia during this event in step with abrupt increase in CO 2 partial pressure and a biodiversity nadir. Warming-mediated increases in marine anoxia may be more pronounced in a glaciated versus unglaciated climate state.