V.A. Gusev

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.

Effect of nitrogen impurities on the Raman line width in diamonds

The dependence of the Raman line width in diamonds on the nitrogen impurity is experimentally studied. A linear relation between the nitrogen content and the width is found. It is demonstrated that the slope of the linear fit depends on the type of nitrogen defect. A relation between the Raman experiment and results from lattice parameter experiments is found and discussed.

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.

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.

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.

Nickel-Related Optical Centers in Hpht Treated Synthetic Diamonds: Photoluminescence Study

The results of a photoluminescence (PL) study of synthetic diamonds annealed over the temperature range 1500-2500°C are presented. For nickel- and nitrogen-containing diamonds a number of new, previously undocumented PL systems with zero-phonon lines (ZPLs) at 598.5, 622.6, 638.9, 752.1 and 877.2 nm has been found. Photoluminescence excitation (PLE) spectra for PL systems with ZPLs at 727.1 and 746.6 nm have been measured for the first time. Preliminary results on annealing behavior of the PL centers have been obtained. Of all the PL systems observed only that ones with ZPLs at 752.1 and 793.0 nm together with the S2 and S3 optical centers are present in diamonds annealed at temperatures higher than 2200°C.

Photoluminescence excitation study of cobalt-related optical centers in high-pressure high-temperature diamond

Abstract The results of a photoluminescence excitation (PLE) study of cobalt–nitrogen optical centers in high-pressure high-temperature (HPHT) treated synthetic diamonds are presented. A single vibronic band with zero-phonon line (ZPL) at 3.085 eV was found in the PLE for the 2.367 eV system and two bands with ZPLs at 3.004 and 3.673 eV for the 2.277 eV system. From the continuum excitation threshold the ground state was tentatively located at 4.3 and 4.45 eV below the conduction band for the 2.277 and 2.367 eV systems, respectively. For the minor photoluminescence bands with ZPLs at 2.207 and 2.135 eV general features of the PLE spectra were established.