Rare-earth metal catalysts for high-pressure synthesis of rare diamonds

The combination of the unique properties of diamond and the prospects for its high-technology applications urges the search for new solvents–catalysts for the synthesis of diamonds with rare and unusual properties. Here we report the synthesis of diamond from melts of 15 rare-earth metals (REM) at 7.8 GPa and 1800–2100 °C. The boundary conditions for diamond crystallization and the optimal parameters for single crystal diamond synthesis are determined. Depending on the REM catalyst, diamond crystallizes in the form of cube–octahedrons, octahedrons and specific crystals bound by tetragon–trioctahedron and trigon–trioctahedron faces. The synthesized diamonds are nitrogen-free and belong to the rare type II, indicating that the rare-earth metals act as both solvent–catalysts and nitrogen getters. It is found that the REM catalysts enable synthesis of diamond doped with group IV elements with formation of impurity–vacancy color centers, promising for the emerging quantum technologies. Our study demonstrates a new field of application of rare-earth metals.

Diamond formation in an electric field under deep Earth conditions

Most natural diamonds are formed in Earth's lithospheric mantle; however, the exact mechanisms behind their genesis remain debated. Given the occurrence of electrochemical processes in Earth's mantle and the high electrical conductivity of mantle melts and fluids, we have developed a model whereby localized electric fields play a central role in diamond formation. Here, we experimentally demonstrate a diamond crystallization mechanism that operates under lithospheric mantle pressure-temperature conditions (6.3 and 7.5 gigapascals; 1300° to 1600°C) through the action of an electric potential applied across carbonate or carbonate-silicate melts. In this process, the carbonate-rich melt acts as both the carbon source and the crystallization medium for diamond, which forms in assemblage with mantle minerals near the cathode. Our results clearly demonstrate that electric fields should be considered a key additional factor influencing diamond crystallization, mantle mineral-forming processes, carbon isotope fractionation, and the global carbon cycle.

Effect of Oxygen on Diamond Crystallization in Metal-Carbon Systems

In this article, we report the influence of oxygen concentration in the transition-metal solvent-catalyst on the crystallization processes, morphology, and defect-and-impurity content of diamond crystals. In a series of experiments, the concentration of oxygen (C _O) in the growth system was varied by adding Fe₂O₃ to the charge, and the other parameters and conditions of the growth were constant: Ni₇Fe₃ solvent-catalyst, P = 6.0 GPa, T = 1400 °C, and duration of 40 h. It is found that on increasing C _O in the growth system from 0 to 10 wt %, the crystallization of diamond proceeds through the following stages: single crystal → block crystal → spontaneous crystals → aggregate of block crystals and twin crystals. At C _O ≥ 5 wt %, diamond crystallizes jointly with wustite (FeO) and metastable graphite. The oxygen solubility in the iron-nickel melt is estimated at about 2 wt %. With increasing oxygen content in the system, the range of nitrogen concentrations in diamonds crystallized in one experiment significantly broadens with the maximum nitrogen concentrations being increased from 200-250 ppm in the experiment without O additives to 1100-1200 ppm in the experiment with 10 wt % O added. The established joint growth of diamond and wustite suggests possible crystallization of natural diamonds in the Fe-Ni-O-C system over a wide range of oxygen concentrations up to 10 wt %.

Effect of sulfur on diamond growth and morphology in metal–carbon systems

Sulfur additives inhibit diamond crystallization in the Fe–Ni–C system at 6 GPa and 1400 °C and affect the diamond crystal morphology and nitrogen impurity content.

High-pressure crystallization and properties of diamond from magnesium-based catalysts

HPHT diamond synthesis using catalysts based on magnesium demonstrates a number of intriguing characteristics. In this highlight, we review the major characteristics of the growth, morphology, internal structure, and defect and impurity content of diamonds crystallized using Mg-based catalysts.

Germanium: a new catalyst for diamond synthesis and a new optically active impurity in diamond

Diamond attracts considerable attention as a versatile and technologically useful material. For many demanding applications, such as recently emerged quantum optics and sensing, it is important to develop new routes for fabrication of diamond containing defects with specific optical, electronic and magnetic properties. Here we report on successful synthesis of diamond from a germanium-carbon system at conditions of 7 GPa and 1,500–1,800 °C. Both spontaneously nucleated diamond crystals and diamond growth layers on seeds were produced in experiments with reaction time up to 60 h. We found that diamonds synthesized in the Ge-C system contain a new optical centre with a ZPL system at 2.059 eV, which is assigned to germanium impurities. Photoluminescence from this centre is dominated by zero-phonon optical transitions even at room temperature. Our results have widened the family of non-metallic elemental catalysts for diamond synthesis and demonstrated the creation of germanium-related optical centres in diamond.

X-ray topography of diamond using forbidden reflections: which defects do we really see?

Natural and synthetic diamonds with various concentrations and types of point and extended defect were investigated using X-ray topography employing allowed (111, 004) and forbidden (222) reflections. On the topographs of the forbidden reflections, weak stress fields from lattice imperfections and extended defects are readily observed. Comparison of the topographs with IR maps of the distribution of point defects suggests that certain types of point defect may increase the structure factors of the forbidden reflections.

The role of mantle ultrapotassic fluids in diamond formation

Analysis of data on micro- and nano-inclusions in mantle-derived and metamorphic diamonds shows that, to a first approximation, diamond-forming medium can be considered as a specific ultrapotassic, carbonate/chloride/silicate/water fluid. In the present work, the processes and mechanisms of diamond crystallization were experimentally studied at 7.5 GPa, within the temperature range of 1,400-1,800 degrees C, with different compositions of melts and fluids in the KCl/K(2)CO(3)/H(2)O/C system. It has been established that, at constant pressure, temperature, and run duration, the mechanisms of diamond nucleation, degree of graphite-to-diamond transformation, and formation of metastable graphite are governed chiefly by the composition of the fluids and melts. The experimental data suggest that the evolution of the composition of deep-seated ultrapotassic fluids/melts is a crucial factor of diamond formation in mantle and ultrahigh-pressure metamorphic processes.