High-pressure High-temperature Synthetic Diamond Research Articles

Spectroscopic analysis of microdiamonds in ophiolitic chromitite and peridotite

Microdiamonds ∼200 μm in size, occurring in ophiolitic chromitites and peridotites, have been reported in recent years. Owing to their unusual geological formation, there are several debates about their origin. We studied 30 microdiamonds from 3 sources: (1) chromitite ore in Luobusa, Tibet; (2) peridotite in Luobusa, Tibet; and (3) chromitite ore in Ray-Iz, polar Ural Mountains, Russia. They are translucent, yellow to greenish-yellow diamonds with a cubo-octahedral polycrystalline or single crystal with partial cubo-octahedral form. Infrared (IR) spectra revealed that these diamonds are type Ib (i.e., diamonds containing neutrally charged single substitutional nitrogen atoms, N s 0 , known as the C center) with unknown broad bands observed in the one-phonon region. They contain fluid inclusions, such as water, carbonates, silicates, hydrocarbons, and solid CO 2 . We also identified additional microinclusions, such as chromite, magnetite, feldspar (albite), moissanite, hematite, and magnesiochromite, using a Raman microscope. Photoluminescence (PL) spectra measured at liquid nitrogen temperature suggest that these diamonds contain nitrogen-vacancy, nickel, and H2 center defects. We compare them with high-pressure–high-temperature (HPHT) synthetic industrial diamond grits. Although there are similarities between microdiamonds and HPHT synthetic diamonds, major differences in the IR, Raman, and PL spectra confirm that these microdiamonds are of natural origin. Spectral characteristics suggest that their geological formation is different but unique compared to that of natural gem-quality diamonds. Although these microdiamonds are not commercially important, they are geologically important in that they provide an understanding of a new diamond genesis.

Microwave Plasma CVD Grown Single Crystal Diamonds - A Review

Diamond offers a range of unique properties, including wide band of optical transmission, highest thermal conductivity, stiffness, wear resistance and superior electronic properties. Such high-end properties are not found in any other material, so theoretically it can be used in many technological applications. But the shortcoming has been the synthesis of the diamond material in the laboratory for any meaningful use. Although microwave plasma chemical vapour deposited (MPCVD) has been in practice since 1980s for the diamond growth but it is in the past 7-8 years that its potential has been realised by the industry due to capability of MPCVD to deposit diamond, pure and fast, for commercial uses. There are many CVD techniques for growing diamond but among them MPCVD can only make single crystal diamond (SCD) effectively. SCD grown by MPCVD is also superior to other forms of diamond produced in the laboratory. For example, SCD is necessary for the best electronic properties - often outperforming the polycrystalline diamond (PCD), the high pressure high temperature (HPHT) diamond and the natural diamond. In many applications the short lateral dimensions of the lab-grown diamond available is a substantial limitation. Polycrystalline CVD diamond layers grown by hot filament CVD solved this problem of large area growth, but the presence of grain boundaries are not appropriate for many uses. On the other hand, there is still limitation in the area over which SCDs are grown by MPCVD, only upto 10-15 mm lateral sizes could have been achieved so far, while there are recipes which rapidly grow several mm thick bulk SCDs. This lateral size limitation of SCDs is primarily because of the small seed substrate dimension. Although natural and HPHT diamonds may not be suitable for the intended application, still they are routinely used as substrates on which SCD is deposited. But the problem lies in the availability of large area natural SCD seeds which are extremely rare and expensive. Moreover, large diamond substrate plates suitable for CVD diamond growth have not been demonstrated by HPHT because of the associated high economic risk in their fabrication and use. Other than lateral dimension, purity of SCD is also very important for technological use. Natural diamond is often strained and defective, and this causes twins and other problems in the CVD overgrowth or fracture during synthesis. In addition, dislocations which are prevalent in the natural diamond substrate are replicated in the CVD layer, also degrading its electronic properties. HPHT synthetic diamond is also limited in size, and generally is of poorer quality in the larger stones, with inclusions being a major problem. There will be much research interest in the next 10 years for the MPCVD growth of SCD. Purer and bigger SCDs will be tried to grow with faster and reproducible MPCVD recipes. Here the MPCVD growth of SCD is being reviewed keeping in mind its huge technological significance in the next decades or so. Discoveries of the commercial productions of silicon, steel, cement different materials have built modern societies but higher scales will be achieved with the advent of lab-grown diamond.

Fluid CH4 and H2 trapped around metallic inclusions in HPHT synthetic diamond

Diamonds grown by the high-pressure, high-temperature (HPHT) method commonly contain solid inclusions from the metallic solvent-catalyst they are grown in. Such inclusions give rise to their familiar magnetism. Upon recent analysis of 4 commercial HPHT synthetic diamonds from different sources, several of these solid inclusions were found to have a thin, fluid film or jacket of methane (CH4) and hydrogen (H2) trapped at the interface between the solid inclusion and surrounding diamond. Raman spectroscopy reveals peaks near 2918cm−1 and 4156cm−1 that correspond to the main vibrations of CH4 and H2, respectively. In some cases the peak position is shifted down by a few wavenumbers due to elevated pressure within the inclusions.The CH4 and H2 probably formed as the inclusion cooled, when dissolved hydrogen atoms were no longer soluble in the solidifying inclusion phases and were forced to exsolve as H2, as well as CH4 by combining with available carbon. Inclusions can provide an indirect means of studying the effective growth environment. These findings show that hydrogen is commonly present at the site of diamond growth in HPHT synthesis, even if it is not intentionally added among the starting materials. Hydrogen is known to influence diamond growth, based on doping experiments, suggesting that there could be unrealized benefits to gaining better control over hydrogen during routine HPHT diamond synthesis.

Nitrogen isotope systematics and origins of mixed-habit diamonds

Nitrogen isotope values from mantle diamonds are a commonly used tracer in the quest to track volatiles within the Earth’s mantle through deep time. Interpretations of this isotope data are valid so long as stable isotope fractionation processes in the mantle are understood. The fractionation of nitrogen isotopes between {111} and {100} growth sectors is well documented for high-pressure high-temperature (HPHT) synthetic diamonds, but there is little data on whether it also occurs in natural mixed-habit diamonds. We present 91 in-situ nitrogen isotope (δ15N) measurements, along with carbon isotope (δ13C) values and nitrogen abundances [N], obtained from three mixed-habit diamonds by secondary ion mass spectrometry (SIMS). While the well-documented enrichment of nitrogen concentrations in octahedral sectors compared to contemporaneous cuboid sectors is observed, a similarly clear disparity is not obvious in the δ15N data. Whereas HPHT synthetic diamonds exhibit 15N enrichment in the {100} sectors by ∼+30‰, the mixed-habit diamonds studied here show enrichment of the octahedral sectors in 15N by only 0.4–1‰. This major difference between HPHT synthetic and natural mixed-habit diamonds is proposed to be the result of different physical properties of the growth interfaces. The smooth interfaces of the octahedral sectors are the same in both types of crystal, but the outermost atoms on the smooth cube interfaces of an HPHT synthetic diamond behave differently to those on the rough cuboid interfaces of the natural mixed-habit diamonds, resulting in different δ15N values. Both the δ13C (average of ∼−8.7‰) and δ15N (average of ∼0‰) data show only minor offsets from the typical mantle values (δ13C=−5±3‰, δ15N=−5±4‰). This may indicate diamond formation from a mantle derived fluid/melt containing a minor subducted component (lowering δ13C values and elevating δ15N) or relate to moderate degrees of isotopic fractionation of a pure mantle fluid/melt by prior diamond precipitation. The homogeneous nature of both the carbon and nitrogen isotopic compositions of all three diamonds, however, documents continuous and unlimited supply of diamond forming fluid/melt, with a constant composition. Such homogenous isotopic compositions exclude fluid mixing or isotopic fractionation close to the site of diamond formation and preclude distinguishing between these two processes based on diamond analyses alone.

Nickel–nitrogen complexes in synthetic diamond: the 1.660 eV luminescence system

In this paper, we report the effects of applied uniaxial stress and temperature dependence of the 1.660eV zero phonon line (ZPL) observed in high-pressure high-temperature synthetic diamonds, grown using an Ni solvent catalyst and annealed at 1600°C. It is shown that stress dependence of the 1.660eV line can be understood in terms of an electric dipole transition between A and B states at a centre of monoclinic I symmetry, with the electric dipole lying in the plane normal to the main symmetry axis at an angle of ∼75° to the 〈1−10〉 crystal axis.The shape of the vibronic sideband is explained using a simple electron–lattice coupling model. The temperature effects on the ZPL are discussed in the framework of linear and quadratic electron–phonon coupling.

Things we still don’t know about optical centres in diamond

Abstract In this article the optical spectra associated with some of the major defects in diamond are considered. In particular, we deal with isolated substitutional boron, isolated substitutional nitrogen and the nitrogen aggregates, the self-interstitial and defect complexes involving the self-interstitial with nitrogen or boron. Although some aspects of the spectra are understood, many puzzles and inconsistencies remain. The absorption bands responsible for the colours of natural pink and amber diamonds are discussed and studies of transition metal complexes in high-pressure high-temperature synthetic diamond are briefly mentioned.