Natural diamonds that have been partially replaced by graphite have been observed to occur in natural rocks. While the graphite-to-diamond phase transition has been extensively studied the opposite of this (diamond to graphite) remains poorly understood. We performed high-pressure and temperature hydrous and anhydrous experiments up to 1.0 GPa and 1300 °C using Amplex premium virgin synthetic diamonds (20–40 μm size) as the starting material mixed with Mg(OH)2 as a source of H2O for the hydrous experiments. The experiments revealed that the diamond-to-graphite transformation at P = 1.0 GPa and T = 1300 °C was triggered by the presence of H2O and was accomplished through a three-stage process. Stage 1: diamond reacts with a supercritical H2O producing an intermediate 200–500 nm size “globular carbon” phase. This phase is a linear carbon chain; i.e. a polyyne or carbyne. Stage 2: the linear carbon chains are unstable and highly reactive, and they decompose by zigzagging and cross-linking to form sp2-hybridized structures. Stage 3: normal, disordered, and onion-like graphite is produced by the decomposition of the sp-hybridized carbon chains which are re-organized into sp2 bonds. Our experiments show that there is no direct transformation from sp3 C-bonds into sp2 C-bonds. Our hydrous high-pressure and high-temperature experiments show that the diamond-to-graphite transformation requires an intermediate metastable phase of a linear hydrocarbon. This process also provides a simple mechanism for the substitution of other elements into the graphite structure (e.g. H, S, O).
O'Bannon, E. F., et al. "The transformation of diamond to graphite: Experiments reveal the presence of an intermediate linear carbon phase." Diamond and Related Materials, vol. 108, no. C, May. 2020. https://doi.org/10.1016/j.diamond.2020.107876
O'Bannon, E. F., Xia, G., Shi, F., Wirth, R., King, R. A., & Dobrzhinetskaya, L. (2020). The transformation of diamond to graphite: Experiments reveal the presence of an intermediate linear carbon phase. Diamond and Related Materials, 108(C). https://doi.org/10.1016/j.diamond.2020.107876
O'Bannon, E. F., Xia, G., Shi, F., et al., "The transformation of diamond to graphite: Experiments reveal the presence of an intermediate linear carbon phase," Diamond and Related Materials 108, no. C (2020), https://doi.org/10.1016/j.diamond.2020.107876
@article{osti_1631913,
author = {O'Bannon, E. F. and Xia, G. and Shi, F. and Wirth, R. and King, R. A. and Dobrzhinetskaya, L.},
title = {The transformation of diamond to graphite: Experiments reveal the presence of an intermediate linear carbon phase},
annote = {Natural diamonds that have been partially replaced by graphite have been observed to occur in natural rocks. While the graphite-to-diamond phase transition has been extensively studied the opposite of this (diamond to graphite) remains poorly understood. We performed high-pressure and temperature hydrous and anhydrous experiments up to 1.0 GPa and 1300 °C using Amplex premium virgin synthetic diamonds (20–40 μm size) as the starting material mixed with Mg(OH)2 as a source of H2O for the hydrous experiments. The experiments revealed that the diamond-to-graphite transformation at P = 1.0 GPa and T = 1300 °C was triggered by the presence of H2O and was accomplished through a three-stage process. Stage 1: diamond reacts with a supercritical H2O producing an intermediate 200–500 nm size “globular carbon” phase. This phase is a linear carbon chain; i.e. a polyyne or carbyne. Stage 2: the linear carbon chains are unstable and highly reactive, and they decompose by zigzagging and cross-linking to form sp2-hybridized structures. Stage 3: normal, disordered, and onion-like graphite is produced by the decomposition of the sp-hybridized carbon chains which are re-organized into sp2 bonds. Our experiments show that there is no direct transformation from sp3 C-bonds into sp2 C-bonds. Our hydrous high-pressure and high-temperature experiments show that the diamond-to-graphite transformation requires an intermediate metastable phase of a linear hydrocarbon. This process also provides a simple mechanism for the substitution of other elements into the graphite structure (e.g. H, S, O).},
doi = {10.1016/j.diamond.2020.107876},
url = {https://www.osti.gov/biblio/1631913},
journal = {Diamond and Related Materials},
issn = {ISSN 0925-9635},
number = {C},
volume = {108},
place = {United States},
publisher = {Elsevier},
year = {2020},
month = {05}}
Philosophical Transactions of the Royal Society of London. Series A: Mathematical, Physical and Engineering Sciences, Vol. 362, Issue 1824https://doi.org/10.1098/rsta.2004.1454