The Origin of Colour in Diamond

 

The Origin of Colour in Diamond

A perfect diamond, made up of a regular array of carbon atoms, and containing no impurities, would be completely colourless. In practice, no diamond is perfect and most diamonds are coloured. Brown and yellow are the most common colours encountered in natural diamonds, although other colours (red, orange, green, blue, violet) are found occasionally. These colours result from the presence of defects – either point defects which have dimensions comparable with the distance between carbon atoms in the diamond crystal or extended defects which may be large enough to see with an optical microscope. The scientific understanding of point defects is more advanced than that for extended defects, but, even so, is far from comprehensive. What is clear, however, is that nitrogen, which is a very common impurity in diamond, is involved in many of the point defects. Nitrogen can exist in several different forms in diamond, which we describe below. The properties of boron in diamond are also mentioned, as are details of one important extended defect. Vacancies can occur naturally in diamond, and can be introduced by radiation to produce ‘treated colours’. A few vacancy-nitrogen complexes are also described which have particular relevance to the colour of diamond.

 

Nitrogen

Nitrogen can be present in diamond in at least three different forms. Each of these causes absorption in the infrared part of the spectrum at frequen- cies less than 1332 cm 1 (wavelengths longer than 7.5 m). This is known as the ‘one-phonon region’. Diamonds that contain sufficient nitrogen to produce absorption are generically known as ‘type I’. If there is no easily detectable absorption in this region the diamond is classified as ‘type IIa’. It is believed that nitrogen is initially incorporated into diamonds on isolated substitutional sites. Very few natural diamonds are found with nitrogen in this form, but almost all manufactured diamond produced commercially by HPHT synthesis has most of the nitrogen in the isolated substitutional form. This sort of diamond is referred to as ‘type Ib’. The presence of the nitrogen causes optical absorption in the visible region, starting at approximately 500 nm and increasing

 

towards shorter wavelengths. Diamonds like this therefore have a distinctive yellow colour, which is sometimes described as a ‘canary yellow’. At higher nitrogen concentrations, or in larger diamonds, the colour is yellow/brown.

 

Nitrogen as a Donor

To understand some of the colour phenomena in diamond we need to use a concept familiar in the study of semiconductors. Single substitutional nitrogen in diamond acts as an electrical donor. A nitrogen atom may there- fore donate an electron to a certain defect if the nitrogen and the defect are sufficiently close together. This causes the defect to be in a negative charge state (because an electron has a negative charge), producing a different absorption than if the defect had no electrical charge and so was neutral.

 

Aggregated Nitrogen

The majority of natural diamonds have spent substantial periods of time (millions of years to perhaps 3000 million years) in the earth’s mantle at geological temperatures up to 1200 °C. At this temperature, given suffi- cient time, the single nitrogen atoms come together to form aggregates. The first aggregate to be produced is the ‘A aggregate’ which comprises two nitrogen atoms on adjacent lattice sites. The next stage of aggregation leads to the ‘B aggregate’ in which four nitrogen atoms symmetrically surround a vacancy. (A vacancy is a position in the diamond crystal which would normally contain a carbon atom, but from which the carbon atom has been removed.) The formation of the B aggregate therefore requires a carbon atom to be ejected into an interstitial position (i.e. in between positions normally occupied by carbon atoms in the diamond structure). In the majority of cases these self-interstitials also aggregate to form extended defects called ‘platelets’. The precise structure of these platelets is still not known, and they may also include some nitrogen. The platelets give rise to a distinctive absorption peak in the infrared spectrum, at approximately 1365 cm 1 (7.3 m), but do not cause any absorption in the visible region.

Diamonds containing aggregated nitrogen are known generically as ‘type Ia’. Those in which most of the nitrogen is present as A aggregates are classified as ‘type IaA’ and those that have the majority of the nitrogen present as B aggregates are classified as ‘type IaB’. Diamonds containing significant concentrations of both the A form and the B form of nitrogen are termed ‘type IaAB’ or ‘type IaA/B’. Neither the A aggregates nor the B aggregates produce any absorption in the visible region, and therefore do not affect the colour. However, when the B aggregate is formed there is

 

another minor product produced which is referred to as the ‘N3 centre’. This has three nitrogen atoms on a {111} plane, surrounding a common vacancy. Optical transitions at this centre give rise to an absorption band known as ‘N3’. This has a sharp line at 415.2 nm and a structured band to shorter wavelengths. There is, at the N3 centre, another transition which produces the broad ‘N2’ peak at 478 nm with weaker peaks at shorter wavelengths. (The nomenclature of the N2 and N3 absorption peaks stems from early observations in the 1950s when the naturally occurring absorp- tion lines in type Ia diamonds were listed in order of decreasing wave- length.) Because of the response of the eye, it is predominantly the N2 peak, and the peaks at slightly shorter wavelengths, that are responsible for the colour. Diamonds containing this absorption are known as ‘cape stones’. At low concentrations of the N3 centre the colour of the diamond is just perceptibly yellow when compared with a colourless master stone; at progressively higher concentrations the yellow colour becomes increas- ingly obvious, eventually tending to a fancy colour.

The N3 absorption is one of the most common in diamond, and its

presence underlies the system of colour grading. Colourless (D colour) diamonds command the highest price, and as the colour increases through the grades E, F, G, etc., a slightly tinted yellow colour is reached which is regarded as least desirable; then the selling price moves up again as the fancy yellow colour grades are approached.

If it is possible to take a diamond with a colour near the minimum point on this curve and increase the intensity of the colour by some technique, there is clearly a possibility of increasing the selling price. We shall see later two methods by which this may be achieved.

 

Boron

Natural diamonds in which substitutional boron is the major impurity are extremely rare. These are the type IIb diamonds, and the presence of boron results in optical absorption which starts at a wavelength of approximately 4 m in the infrared region and extends into the red region of the visible spectrum. In favourable cases this absorption gives the diamond an attrac- tive blue colour.

 

Extended Defects

We have already encountered platelets earlier in this chapter. Another type of extended defect in diamond is associated with plastic deformation; it appears that in certain stones an external shearing stress has caused some planes of carbon atoms to slip with respect to each other. Many of the brown and pink diamonds from the Argyle production are in this category.

 

If the crystals are examined under a microscope it is observed that the colour is not uniform, but is striated with the striations oriented in the direction of slip. These striations are referred to as ‘coloured graining’ in the gem trade. It is not known in detail why the plastic deformation pro- duces coloured diamonds, or why many are brown and some are pink. One possibility is that a point defect is also involved, and this is trapped at, or ‘decorates’ the dislocation. An alternative explanation has been pro- posed by Tom Anthony of General Electric (GE): he has suggested that the pink colour is associated with small displacements of the planes of carbon atoms and that larger displacements result in a brown colour.

Type IIa brown diamonds simply exhibit a featureless absorption that rises continuously from the red end to the blue end of the visible spec- trum. The brown type Ia diamonds from the Argyle production also have this increasing absorption towards short wavelengths. In addition they generally have some N3 absorption present, and a broad absorption band with a maximum near 560 nm. There may also be a weak absorption band known as ‘H3’ with a sharp line at 503 nm.

 

Vacancies

Vacancies can be produced in diamond by radiation. In summary an energetic particle, such as a high-energy electron or a fast neutron, displaces some of the carbon atoms, leaving a vacancy and placing the carbon atoms into interstitial positions. Absorption associated with the vacancy in its neutral charge state produces the GR absorption features. Vacancies are produced by all forms of sufficiently energetic radiation in all types of diamonds and the GR stands for General Radiation. The absorption spectrum shows a sharp line at 741 nm (the GR1 line) and a band to shorter wavelengths. This absorption gives the diamond a green or blue-green colour. Many uncut natural diamonds have a green ‘skin’ which has been produced by alpha particles, but this is only a few m deep. Diamonds with a naturally produced absorption due to vacancies throughout their bulk are extremely rare; the Dresden Green is one example.

Vacancies are also released when plastically deformed diamonds are

subjected to HPHT processing, and this process will be considered in more detail in a later section.

 

Complexes Involving Nitrogen and the Vacancy

When diamonds containing vacancies are heated in the laboratory to 800 °C the vacancies become mobile, and after an hour or so the GR1 absorption band disappears.

 

In type I diamonds the vacancies are captured by the nitrogen to form new colour centres. The dominant defect in type Ib diamonds is the nitrogen-vacancy centre in the negative charge state (N–V) and this produces an absorption system with a sharp line at 637 nm and a band to shorter wavelengths. This gives the diamond a pink colour or, with larger amounts of radiation, a pink/red colour. The reason why most of the (N–V) centres are in the negative charge state is that type Ib dia- monds have a high concentration of single-nitrogen atoms which act as donors. Some nitrogen-vacancy centres may be in the neutral charge state (N–V)0 and this defect produces a sharp line at 575 nm. The relative absorption intensity of the 575 nm line to the 637 nm line is higher in diamonds with a low concentration of single nitrogen because the N–V centres are, on average, further away from the nitrogen donors and so remain neutral.

In type IaA diamonds vacancies are trapped by the A aggregates, and a rearrangement of the atoms occurs to produce the structure N–V–N. Research described below has shown that when all the nitrogen is in the A-aggregate form this centre is in the neutral charge state (N–V–N)0, and produces absorption in the H3 system with a sharp line at 503 nm and a band to shorter wavelengths. This absorption produces a yellow, orange or reddish brown colour, depending on the amount of radiation to which the diamond was subjected. (The ‘H’ series of optical bands are those that are produced by radiation followed by Heat treatment.)

Following irradiation and annealing of type IaB diamond, the vacancies are trapped by the B form of nitrogen to produce H4 centres which give rise to an absorption band with a sharp line at 496 nm and a band to shorter wavelengths. Again, this results in a yellow, orange or reddish brown colour.

 

 

Other Colours

There are many other absorption bands in diamond for which there is, as yet, no adequate scientific explanation. These include blue and violet diamonds from the Argyle production, orange diamonds which owe their colour to a broad absorption band with a maximum at 480 nm, and ‘chameleon diamonds’ which change colour, depending on their environment and/or temperature. The blue and violet diamonds show evidence of a high hydrogen concentration which causes substantial absorption in the near-infrared region, but it has not yet been demon- strated whether the hydrogen is, in fact, responsible for these unusual colours.