Synthetic Diamonds - Final Report |
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Distinguishing between Natural and Synthetic Diamonds. researched by James Portsmouth in July 1995 There are currently two methods of synthesizing diamonds. High pressure - high temperature (HPHT) methods subject graphite to conditions similar to those under which natural diamonds are formed in the Earth’s mantle. Most large, gem quality synthetic diamonds are grown using variants of this method. The most successful involves growing the diamonds in a ‘flux’ of molten metal alloy. There are also vapour deposition methods which operate at lower temperatures and pressures, and are usually used to produce thin films for industrial applications. There are 4 known types of natural diamond (Ia, Ib, IIa, IIb), classed according to the presence of nitrogen in the crystal, and certain other properties. Synthetic diamonds can also be categorized according to this scheme. However there can still be a wide variation in some properties between diamonds of the same type. Almost all synthetics are of type Ib, having an even distribution of nitrogen atoms substituted for carbon atoms in the lattice (up to about 500ppm). It is believed that in the earlier stages of their history, all natural diamonds were type Ib Most natural diamonds (~99.9%) are of type Ia, with a large amount of nitrogen concentrated in various aggregates in the crystal. The initially type Ib diamonds are considered to have changed to type Ia after many years in a HPHT environment, in which the nitrogen diffused and coalesced into aggregates. Types IIa and IIb are very rare in nature but can be synthesized for industrial purposes. Natural diamonds consisting of several different types in one stone are sometimes seen. Comparison of various properties Natural diamonds can be colourless, yellow or brown. Synthetic diamonds of all these colours have been made, as well as greenish-yellow, blue and colourless varieties. At the moment most synthetic diamonds are yellow or brownish-yellow. Red synthetics can be made (probably by heat treatment), but are rare. Crystals tend to grow on certain sets of energetically favoured planes. In uncut diamonds the symmetry of the planes is obvious from the shape of the crystal. In cut diamonds the growth planes can still be identified using optical techniques (which involve cutting and polishing the diamond). Natural diamond crystals most often grow on octahedral {111} faces. Cubic {100} faces are rarely seen. In synthetic diamonds cubic faces are often seen, although in cut diamonds the planes are difficult to identify because of the abundance of defects. In all natural diamonds there exist impurities of more than 50 elements. The most abundant are nitrogen, hydrogen, boron and oxygen. It has been shown that nitrogen and boron can exist as substitutes for carbon in the crystal lattice, but most impurities exist as small inclusions. Boron has been shown to be responsible for the semiconducting behavior of type IIb diamond. Synthetic diamonds do not have as great a diversity of impurities, although mineral impurities do occur from the walls of the synthesis vessel. Nitrogen and hydrogen are present in significant amounts. There are frequently nickel and iron inclusions from the metallic flux in which the crystal grew. These inclusions have a metallic lustre and can be large. They have a tabular, elongate or needle-like shape and are oriented parallel to the cube, octahedral and dodecahedral faces as a result of the growth process. In addition, most synthetics contain ‘clouds’ of tiny, white pinpoints (rare in naturals). Thermal and Electrical properties Type I diamonds have a thermal conductivity about twice that of copper, and that of type IIa diamond is about 5 times greater. Type IIb diamond has semiconducting properties. There are no clear differences in thermal or electrical conductivity between natural and synthetic diamonds at room temperature. It seems possible that synthetic diamonds can be produced with thermal and electrical properties tailored to some extent by ion implantation. In type Ia diamonds (most naturals) nitrogen is present mainly in the form of aggregates which cause a point defect in the crystal. There are two types of such aggregate, designated A and B. In type Ib diamonds (most synthetics) almost all the nitrogen is in substitutional sites. Platelets, seen commonly in type Ia diamonds, are small (~1µm) aggregations of nitrogen which collect in planar formations on the cubic faces of the lattice. Voidites, seen often along with platelets, are tiny (~10nm) octahedral structures of relatively low density, probably consisting of solid nitrogen. The A,B defects and the platelets show characteristic peaks in the IR absorption spectrum of the crystal. X-ray diffraction and fluorescence are also used to study these defects. In many synthetics, cathodo-luminescence shows up internal ‘growth sectors’ with different nitrogen concentrations as distinctly zoned areas. Small pyramidal depressions, called trigons, are usually seen (with magnification) in the octahedral surfaces of natural diamonds. They are only very rarely seen in synthetics, but raised pyramids (called triangular pyramids) are commonly seen on the octahedral faces. The rare cube faces of natural diamonds are rough and pitted. The relatively common cube faces on synthetics are flatter but have an irregular, granular appearance. On the octahedral faces of synthetics, and less commonly on the cube faces, slightly raised markings with a dendritic, tree-like structure are frequently seen. These are thought to be an imprint of the metal catalyst used during the synthesis, and are never seen in naturals. These features are removed to some extent by cutting and polishing, but can still be observed in small rough areas. Refractive index, birefringence and dispersion cannot distinguish between naturals and synthetics. Bands of different hues and intensities of colour can often be seen in synthetics (‘colour zoning’), although this is harder to see in faceted than in rough stones. Colour zoning can occur in natural diamonds but is rare. Under a hand lens or optical microscope, planar defects and large metallic inclusions are also often seen in synthetics. With a polarizing microscope synthetic diamonds often show a cross shaped strain pattern (although generally natural diamonds show more strain patterns than synthetics). This may be hard to see in faceted stones. . In faceted stones patterns of intersecting graining lines, often arranged in an ‘hourglass’ shape, can frequently be seen. During exposure to UV light, natural and synthetic diamonds sometimes glow a characteristic colour (and sometimes ‘phosphoresce’ for a while after exposure). In long wavelength (‘longwave’, ~350nm) UV some natural diamonds glow blue, green, orange, yellow or (rarely) red. The same colours are seen in shortwave (~250nm) UV but with variable intensity. Synthetic diamonds are usually inert in longwave UV (although some Russian made synthetics show weak longwave fluorescence). In shortwave UV some glow yellow or yellow-green. The fluorescence is frequently uneven, with different coloured or inert regions. In addition, colours are usually strongly zoned during fluorescence Diamond synthesized by HPHT methods usually have a dispersion of nickel and/or iron inclusions. The gems therefore have some permanent (ferro-) magnetism, and can be picked up with a magnet. Natural diamonds have some permanent magnetism, although not as a great as that in synthetics. It seems that much of the magnetism of natural stones is caused by metallic inclusions on the surface, which can be washed off with concentrated acid. After this treatment the level of magnetism in natural stones is at least one order of magnitude lower than that in the synthetics. (Treating synthetics in the same way does not change their magnetism significantly). Colourless or near-colourless synthetics may have lower levels of magnetism than other synthetics, most likely lower than that of natural diamonds but not as low as that of acid-washed naturals. Attraction to a hand magnet may not be sensitive enough to distinguish between these stones and naturals. Suspending the stone in a liquid increases the sensitivity of the test. However, both of these tests are not reliable measures of the amount of permanent magnetism in a stone because they also respond to any paramagnetic substances in the stone such as mineral inclusions and nitrogen aggregates. Devices which respond only to ferromagnetism exist, but are complicated (e.g. SQUID magnetometer). Such a device would be required for a definitive test.
Description of experimental techniques When white light is shone on a diamond, certain frequencies in the IR region of the spectrum excite impurities or defects in the lattice causing vibration of inter-atomic bonds. The frequency spectrum of the transmitted beam therefore shows troughs (peaks) at these absorbed frequencies. Characteristic peaks are seen corresponding to substitutional nitrogen, A/B defects, platelets and other features (usually nitrogen related). Consequently, each diamond type shows a characteristic and fairly distinctive spectrum. In addition, since some type Ia character is always seen in naturals and much less in synthetics, type Ib naturals can be distinguished from type Ib synthetics This technique therefore provides a very useful if not definitive test for synthetics. In the optical region, natural diamonds usually show sharp absorption bands whereas synthetic diamonds do not. This may be a useful additional test. Some naturals show characteristic peaks in the optical band (e.g., the ‘Cape’ series). When electrons impinge on a diamond crystal (in a vacuum), atoms are excited to higher energy states (by promotion of electrons). When they fall back to the ground state, monochromatic (one wavelength) radiation is emitted. The spectrum of emissions is different for each type of diamond. The technique requires low temperatures (e.g., 70K). This phenomenon is similar to cathodo-luminescence, with high energy photons rather than electrons exciting the crystal. Standard UV lamps are used to illuminate the crystal at room temperature. Optical microscopy is used to study surface features, defects such as grain boundaries, inclusions, and internal strain patterns (in polarized light). Phase contrast microscopy is an optical technique used to produce good quality images of diamond surfaces and interiors. Electron microscopy is used to provide high resolution images of nitrogen and lattice defects, and surface features. Electron (and X-ray) diffraction patterns can provide information about the crystal structure and impurities. Most synthetic diamonds can be moved with a powerful magnet. Natural diamonds are not so responsive. A test slightly more sensitive to differences in magnetism involves floating the diamond in a liquid, either by choosing a liquid of similar density or supporting the diamond with a float. A definitive test is provided by the SQUID (Superconducting Quantum Interference Device) magnetometer, which operates at very low temperatures (~4K) and can measure the magnetic field produced by tiny amounts of ferromagnetic material. The main physical features of synthetic diamond have been compared and contrasted with those of natural diamond. Several possible tests were proposed and the experimental techniques required briefly described. No simple test is available which can distinguish all synthetics from naturals. A reliable identification can be achieved by combining the results of several of the following relatively simple tests:
Definitive tests involve the use of the following specialized laboratory equipment:
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