Decomposition of silicon carbide at high pressures and temperatures

Abstract

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.

High-Pressure, High-Temperature Behavior of Silicon Carbide: A Review

“…The temperature of decomposition and the nature of the decomposition phase boundary appear to be strongly influenced by the pressure-induced phase transitions to higher density structures in SiC, silicon and carbon, as is discussed further in [18]. However, additional work is necessary to understand the melting characteristics of the rocksalt structure at pressures above 60 GPa.…”

“…These findings were supported by observations in [17] at pressures below 3 GPa. The phase boundary marking the transition to 6H likely crosses the melting/decomposition line at high pressures [18,19], however, implying that 3C may be the more stable solid phase at higher-pressure conditions. Shock studies on both α-SiC and β-SiC compare the proportion of polytypes present in samples after being shocked to P-T conditions of 5-25 GPa and 600-1500 K [16].…”

“…DAC studies find that 15R remains under compression to at least 35 GPa at 300 K [22], 3C remains stable to 105 GPa at 300 K and 6H remains stable to 95 GPa at 300 K before transition to a dense, high-pressure rocksalt structure [25]. LHDAC experiments find that both 6H and 3C remain in experiments covering conditions of~10-80 GPa and~3000 K (shown in blue) before transition to the rocksalt structure [18,23,24,27,28]. Shock experiments up to 25 GPa and~2000 K find an increase in the proportion of rhombohedral and cubic phases at high P-T and a decrease in hexagonal phases [16].…”

“…The ambient pressure decomposition of SiC into solid C plus liquid Si begins at~2840 K in experiments [19] but is predicted to occur at higher temperatures of 3100 K in computations [104,105]. Prior to recent diamond-anvil cell work up to~80 GPa [18], explorations of high-pressure melting and decomposition have gone up to~10 GPa while heating to temperatures as high as 3500 K [17,[106][107][108][109][110]. Although confusion has arisen as to the nature of SiC melting at lower pressures, higher pressure studies indicate that 3C-SiC continues to decompose at high temperature, at least up to the transition to the rocksalt structure at~60 GPa.…”

“…Recent diamond-anvil cell work [18] finds that 3C-SiC continues to decompose at high pressures and high temperatures, following a phase boundary with a negative slope. The high-pressure decomposition temperatures measured are considerably lower than the decomposition temperature at ambient, with the measurements indicating that SiC begins to decompose at~2000 K at 60 GPa as compared to~2800 K at ambient pressure.…”