Yu. M. Borzdov

Diamond formation from mantle carbonate fluids

Analysis of inclusions has shown that natural diamond forms at pressures of 5-6 GPa and temperatures in the range 900-1,400 °C. In non-metallic systems,, diamond has been synthesized only at pressures greater than 7 GPa and temperatures of more than 1,600 °C. We find that diamond can crystallize in alkaline carbonate-fluid melts at pressures and temperatures that correspond to those of natural diamond formation.

Diamond and graphite crystallization from C–O–H fluids under high pressure and high temperature conditions

Abstract Crystallization of diamond was studied in the CO2–C, CO2–H2O–C, H2O–C, and CH4–H2–C systems at 5.7 GPa and 1200–1420°C. Thermodynamic calculations show generation of CO2, CO2–H2O, H2O and CH4–H2 fluids in experiments with graphite and silver oxalate (Ag2C2O4), oxalic acid dihydrate (H2C2O4·2H2O), water (H2O), and anthracene (C14H10), respectively. Diamond nucleation and growth has been found in the CO2–C, CO2–H2O–C, and H2O–C systems at 1300–1420°C. At a temperature as low as 1200°C for 136 h there was spontaneous crystallization of diamond in the CO2–H2O–C system. For the CH4–H2–C system, at 1300–1420°C no diamond synthesis has been established, only insignificant growth on seeds was observed. Diamond octahedra form from the C–O–H fluids at all temperature ranges under investigation. Diamond formation from the fluids at 5.7 GPa and 1200–1420°C was accompanied with the active recrystallization of metastable graphite.

HPHT synthesis of diamond with high nitrogen content from an Fe3N–C system

Abstract The capability of iron nitride, Fe3N for converting graphite to diamond was explored at P=7 GPa and T=1550–1850 °C in experiments with a duration of 20 h. It was established that depending on the synthesis temperature the iron nitride melt provides conditions for crystallisation of diamond and/or graphite, with the minimal temperature for spontaneous diamond nucleation being approximately 1700 °C. Based on the results obtained it was argued that the iron nitride acts as the solvent-catalyst for diamond formation. The crystallised diamonds were found to contain nitrogen in concentration up to approximately 3300 ppm, which depending on the synthesis temperature was present in either the A form or both A and C forms. Absorption peaks caused by hydrogen-related defects were observed in IR spectra of all diamonds examined. For the 3107 cm−1 line a tendency to increase in intensity with increasing the nitrogen content was found. The well-known blue band-A, N3, H3 and 2.156 eV systems as well as a band with zero-phonon energy at 1.787 eV were observed in cathodoluminescence.

High-pressure synthesis and characterization of diamond from a sulfur–carbon system

Abstract Diamond crystallization in the sulfur–graphite system has been studied at P=7 GPa and T=1750–1850°C in experiments with a duration up to 7 h. It has been found that diamond nucleation and crystallization occur both at the interface between the graphite and sulfur melt and directly within the carbon–saturated sulfur melt. Diamond crystals with maximum size up to 500 μm were synthesized. The crystals had cube–octahedral morphology with minor faces of trapezohedron. Goniometric measurements revealed that crystallographic indexes of the trapezohedron faces are {411} and {944}. Spectroscopic characterization of sulfur–synthesized diamonds by means of infrared absorption microscopy and cathodoluminescence has been made for the first time. It was found that crystals contain nitrogen impurity in the form of A aggregates with concentration up to approximately 700 at. ppm. An absorption band with a maximum at 1050 cm−1, whose origin is not clear, was observed in the IR spectra. The cathodoluminescence spectra of these diamonds were found to comprise of the well-known H3 and 575-nm systems as well as a broad emission band.

Effect of Crystal Structure on the Behavior of Diamond Electrodes Electrochemical Characteristics of Individual Crystal Faces

Effects of crystal structure on the electrochemistry of boron-doped high temperature-high pressure (HTHP) diamond single crystals grown from a Ni-Fe-C melt are studied. On the {111}, {100}, and {311} faces, the linear and nonlinear electrochemical impedance measurements were performed and the rate of electron transfer for Fe(CN) 3-/4- 6 was evaluated. Like polycrystalline chemical vapor deposited films the HTHP electrodes equivalent circuit includes a constant phase element. The uncompensated acceptor concentration in the semiconductor diamond was determined from Mott-Schottky plots and amplitude-demodulation measurements and was found to vary in the range of 10 18 to 10 21 cm -3 . The difference in the electrochemical behavior of individual crystal faces is primarily attributed to different boron concentrations in the growth sectors associated with the faces.

Crystal growth and characterization of HPHT diamond from a phosphorus-carbon system

Crystallization of diamond in the phosphorus-graphite system has been studied at 7 GPa and 1750 °C in a series of experiments with duration from 5 to 20 h. Spontaneous diamonds crystallized through both film growth (FG) and temperature gradient growth (TGG) processes as well as diamond layers grown on seed crystals were obtained. Morphology of the crystallized diamonds was studied by scanning electron microscopy and goniometry. For TGG diamonds faces of tetrahexahedron and trapezohedron indexed as {310} and {911}, respectively, which are the new growth forms of synthetic diamond, were established. By FTIR measurement it was shown that bluish coloration characteristic of crystallized diamonds arises from a continuum of absorption, which is similar to the phosphorus photo-ionization continuum seen in P-doped CVD diamond films.

High-pressure synthesis and characterization of diamond from an Mg–Si–C system

Diamond crystallization in the Mg–Si–C system has been studied at high-pressure high-temperature conditions of 7 GPa and 1500–1900 °C. The features of nucleation and growth of diamond from the carbon solution in the Mg–Si melt are established. The degree of the graphite-to-diamond transformation is found to depend significantly on the crystallization temperature. As opposed to the pure Mg–C system where the cubic morphology dominates, the octahedron with the antiskeletal structure of faces is the dominant form of growth in the Mg–Si–C system over the entire temperature range. The possibility of epitaxial growth of silicon carbide tetrahedral crystals on diamond upon their co-crystallization was noted. Synthesized diamonds are found to contain optically active silicon-vacancy (Si-V) centers and inactive substitutional silicon defects, giving rise to the 1.68 eV system in the photoluminescence spectra and an absorption peak at 1338 cm−1 in the infrared absorption spectra, respectively.

The peculiarities of the local thermal diffusivity of large synthetic diamonds

Abstract A series of large diamond single crystals was prepared using FeNi solvent-catalyst by the temperature gradient method. Local thermal diffusivity was measured by an original experimental technique and the results obtained were: 2.2 to 8.0 cm 2 /s for nitrogen-containing samples and ∼ 11 cm 2 /s for nitrogen-free samples (grown with Ti getter). To explain the observed peculiarities of the distribution of the local thermal diffusivity, visible-IR absorption spectra, photo- and cathodoluminescence in the visible range and X-ray projection topographs were studied. The body of the results obtained enabled us to suggest that, in synthetic diamonds of the type studied, the main contribution to the thermal resistance is made by phonon scattering on defects including Ni ions in their structure.

Photochromic effect in irradiated and annealed nearly IIa type synthetic diamond

We examined the effect of radiation damage and annealing on the optical properties of nitrogen-gettered nearly IIa type synthetic diamonds. It was found that the 2.156 eV centre, whose absorption is usually very weak, appears in these diamonds as one of the dominant absorption features. A new vacancy-related vibronic absorption system with zero-phonon line at 3.420 eV was observed. A pronounced photochromic effect was established for the 1.945, 2.085, 2.156, 3.420 and 4.325 eV absorption bands. Of all these bands only the 2.156 eV band remains in the absorption spectra after a proper photoexcitation. In contrast, for similarly treated type Ib diamonds we did not reveal any photoinduced changes in the 1.945 eV band absorption. Based on the results of optical bleaching and thermal recovery experiments, we conclude that the assignment of the 2.156 eV centre to the neutral charge state of the nitrogen-vacancy defect needs further verification.

Interaction of iron carbide and sulfur under P–T conditions of the lithospheric mantle

Experimental studies were performed in the Fe3C–S system at P = 6.3 GPa, T = 900–1600°C, and t = 18–20 h. The study aimed to characterize the conditions of iron carbide stability in a reduced lithospheric mantle and to reveal the possibility of the formation of elemental carbon by the interaction of iron carbide and sulfur. It was found that the reaction at T < 1200°C proceeds with the formation of a pyrrhotite–graphite assemblage by the following scheme: 2Fe3C + 3S2 → 6FeS + 2C0. The crystallization of graphite at T < 1200°C is accompanied by the generation of sulfide and metal–sulfide melts and via 2Fe3C + 3S2 → 6[Fe–S(melt) + Fe–S–C(melt)] + 2C(graphite)0 reaction. Resulting from the carbon-generating reactions, not only graphite crystallized in sulfide or metal–sulfide melts, but the growth of diamond also takes place. The obtained data allow one to consider cohenite as a potential source of carbon in the processes of diamond and graphite crystallization under the conditions of a reduced lithospheric mantle. The interaction of iron carbide and sulfur under which carbon extraction proceeds may be one of possible processes of the global carbon cycle.