Diamond can be grown from chlorinated organic compounds reacted with concentrated NaOH under hydrothermal conditions, but the chemistry behind this striking metastable synthesis is unproven. The open problem is to establish, with direct chemical and spectroscopic evidence, how diamond nucleates and grows in this system: which reactive carbon species form and by what reaction pathway, whether the observed crystals arise by new nucleation or by reincorporation of seed material, and how the yield depends on reagent ratio, alkali concentration, pressure, temperature, time, heating rate, and substrate. Later work on mild-condition hydrothermal nanodiamond growth has made the phenomenon more credible but has not resolved the mechanism, the seed question, or the quantitative parameter map for the chlorinated-organics system.

Background

Diamond — the crystalline sp3sp^3-bonded (tetrahedrally coordinated) allotrope of carbon — is thermodynamically metastable at ambient conditions and was first synthesized deliberately by the high-pressure/high-temperature (HPHT) route in the 1950s; chemical vapor deposition (CVD) later enabled growth from hydrogen-rich carbon plasmas at low pressure. A much stranger family of routes sits between these: 'soft chemistry' synthesis in hot compressed water. Hydrothermal synthesis means carrying out a chemical reaction in liquid water above roughly 100 °C under self-generated or applied pressure, which speeds diffusion and dissolution–reprecipitation; when the solvent is not water the process is called solvothermal. In a landmark illustration of what hot water can do, Gogotsi, Welz, Ersoy and McNallan showed in 2001 (Nature 411, 283) that silicon carbide exposed to supercritical water at ambient pressure converts partially to diamond-structured carbon by selective leaching of silicon — diamond-like carbon forming far outside its stability field.

Building on this background, Korablov, Yokosawa, Korablov, Tohji and Yamasaki reported in Materials Letters 60 (2006) 3041 the hydrothermal formation of diamond from chlorinated organic compounds reacted with concentrated (10 M) sodium hydroxide at a few hundred degrees Celsius under high pressure, in the presence of diamond seeds and a metal substrate. The group's extended study (Korablov et al., Journal of Materials Science 42 (2007) 7939) confirmed the system: a liquid chlorinated organic precursor (trichloroethane, C2H3Cl3\mathrm{C_2H_3Cl_3}) with 10 M NaOH and diamond or cubic-boron-nitride seeds at 300 °C and 1 GPa yields diamond particles and (220)-oriented films, characterized by thermogravimetric–differential thermal analysis (TG-DTA), scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDX), transmission electron microscopy (TEM) and Raman spectroscopy, with film orientation confirmed by X-ray diffraction (XRD). The group proposed a stoichiometric decomposition reaction followed by condensation–polymerization of reactive carbon species, but acknowledged that the growth mechanism is unknown: in its precursor study (Korablov, Yokosawa, Tohji and Yamasaki, Transactions of the Materials Research Society of Japan 29 (2004) 2371), the authors hypothesized that trichloroethane decomposes under alkaline hydrothermal conditions to a colloidal solution of unstable, highly reactive carbon that condenses and polymerizes into an sp3sp^3-bonded network, with growth possibly proceeding by insertion of active carbon species (e.g., C2\mathrm{C_2}) into C–H bonds of hydrogen-terminated diamond surfaces. The same study noted that proving new diamond growth in the presence of diamond seeds is an open problem — initial seeds were not found on the surfaces of the large plates analyzed by micro-XRD, but seed involvement was not excluded — and yields and dependence on reagent ratio, NaOH concentration, pressure, temperature, time, heating rate and substrate role were never quantified.

Interest in this problem has grown rather than faded. A separate line of work (Alkahtani and Hemmer, Optical Materials Express 10 (2020) 1224; Alzahrani and Alkahtani, Optical Materials Express 13 (2023) 2192; Alkahtani et al., Advanced Optical Materials 14 (2026) 2503553) reports the hydrothermal growth of sub-10 nm nanodiamonds from simple organics such as glucose at temperatures as low as 220 °C at the saturated vapor pressure of water, validating that the product is genuinely cubic diamond by creating nitrogen-vacancy (NV) and silicon-vacancy (SiV) color centers — atomic defects whose optical and magnetic-resonance signatures are unique to the diamond lattice — and by high-resolution electron microscopy. Notably, the 2023 paper states explicitly that diamond growth at such mild conditions 'is still controversial', and none of this work identifies the reactive species or kinetics in the alkaline chlorinated-organics system. Understanding this chemistry matters both for a potential sustainable, low-temperature route to quantum-grade nanodiamonds and for geochemistry, since it bears on whether natural pneumatolytic–hydrothermal (hot-vapor-and-fluid) environments could precipitate diamond.

Problem Statement

For the alkaline hydrothermal system in which chlorinated organic compounds (e.g., trichloroethane) react with concentrated NaOH (on the order of 10 M) at temperatures of a few hundred degrees Celsius and high pressure, determine with direct chemical and spectroscopic evidence how diamond forms. Concretely: (1) identify the reactive carbon species and reaction intermediates in the fluid phase, and establish the reaction pathway and kinetics of nucleation and growth — including whether the source group's hypothesis is correct, namely alkaline hydrothermal decomposition of the precursor to unstable, highly reactive (colloidal) carbon species followed by condensation–polymerization into an sp3sp^3-bonded diamond-structured network, with growth possibly proceeding by insertion of active carbon species (e.g., C2\mathrm{C_2}) into C–H bonds of hydrogen-terminated seed surfaces, or whether another mechanism operates; (2) establish whether the diamond crystals arise by genuinely new nucleation in the fluid or by incorporation or partial reorganization of the seed material; and (3) quantify the yield of diamond-structured material and its dependence on reagent ratio, NaOH concentration, pressure, temperature, reaction time, heating rate, and substrate/nucleation-agent role. These three deliverables jointly constitute one objective — a chemically demonstrated account of this metastable diamond-forming reaction. A submitted solution may be a documented, evidence-grounded analysis rather than new experimentation: it should establish (1) and (2) from the existing chemical and spectroscopic literature, specifying the measurements that would resolve what remains open, and address (3) across the seven listed factors to the depth the available evidence permits. As an interpretive extension conditional on (1)–(3), evaluate whether natural colloidal pneumatolytic–hydrothermal environments, with their actual reagent compositions, fluxes and timescales, could produce macroscopic diamond; this extension is separable and its absence does not by itself defeat a solution to the core objective.

References

  1. S. Korablov, K. Yokosawa, D. Korablov, K. Tohji, N. Yamasaki, 'Hydrothermal formation of diamond from chlorinated organic compounds', Materials Letters 60 (25-26), 3041-3044 (2006). DOI: 10.1016/j.matlet.2005.07.092, https://doi.org/10.1016/j.matlet.2005.07.092
  2. S. Korablov, K. Yokosawa, T. Sasaki, D. Korablov, A. Kawasaki, K. Ioku, E. H. Ishida, N. Yamasaki, 'Synthesis of diamond from a chlorinated organic substance under hydrothermal conditions', Journal of Materials Science 42, 7939-7949 (2007). DOI: 10.1007/s10853-006-1487-4, https://doi.org/10.1007/s10853-006-1487-4
  3. Y. Gogotsi, S. Welz, D. A. Ersoy, M. J. McNallan, 'Conversion of silicon carbide to crystalline diamond-structured carbon at ambient pressure', Nature 411, 283-287 (2001). DOI: 10.1038/35077031, https://doi.org/10.1038/35077031
  4. M. Alkahtani, P. Hemmer, 'Charge stability of nitrogen-vacancy color centers in organic nanodiamonds', Optical Materials Express 10 (5), 1224 (2020). DOI: 10.1364/ome.392503, https://doi.org/10.1364/ome.392503
  5. Y. A. Alzahrani, M. Alkahtani, 'Growth of sub-10 nm fluorescent nanodiamonds', Optical Materials Express 13 (8), 2192 (2023). DOI: 10.1364/ome.495033, https://doi.org/10.1364/ome.495033
  6. M. Alkahtani, Y. A. Alzahrani, A. Aljuwayr, A. Hazrathosseini, A. M. Alessa, A. Alromaeh, A. N. Alhazaa, F. Jelezko, M. Scully, P. R. Hemmer, 'Hydrothermal Growth of Nanodiamonds for Quantum Applications', Advanced Optical Materials 14 (5), 2503553 (2026). DOI: 10.1002/adom.202503553, https://doi.org/10.1002/adom.202503553
  7. G. Demazeau, 'Solvothermal reactions: an original route for the synthesis of novel materials', Journal of Materials Science 43 (7), 2104-2114 (2008). DOI: 10.1007/s10853-007-2024-9, https://doi.org/10.1007/s10853-007-2024-9
  8. S. Korablov, K. Yokosawa, K. Tohji, N. Yamasaki, 'Decomposition of Chlorinated Organic Compounds to Diamond Structured Carbon at Moderate Hydrothermal Conditions', Transactions of the Materials Research Society of Japan 29 (5), 2371-2374 (2004), https://www.mrs-j.org/pub/tmrsj/vol29_no5/vol29_no5_2371.pdf