Color by color center

Color center, referred to as color center, refers to crystal structure defects that can selectively absorb visible light. They are all point defects in crystals. Color centers are mainly produced by radiation damage, impurity aggregation, and both radiation damage and impurity aggregation. The color center is bound in the crystal lattice, and the electrons inside vibrate under the excitation of photon energy. This electronic vibration causes the surrounding carbon atoms to vibrate, causing the vibrational energy to spread throughout the diamond. The diamond color center is the center of this vibrating electron type, which is called the vibrational electronic center (Vibrational-Electronic Center, abbreviated as Vibronic Center). Vibrations in the vibronic center propagate through the diamond crystal in the form of longitudinal waves. In solid-state physics, this longitudinal vibration wave propagating in the crystal is called a phonon, which has the characteristics of energy and frequency. The spectral absorption of diamond color centers can be calculated from a theoretical simplified model of phonons. In general, the phonon absorption spectrum obtained by theoretical calculation is very similar to the actual measured spectrum. Conversely, the measured absorption spectrum can also be used to infer the phonon properties of the color center.

The phonon spectrum is characterized by a sharp zero phonon absorption line and a high-energy broad absorption band with multiple absorption peaks. The absorption peaks of the high-energy broad absorption band are the first, second, third... phonon peaks (or phonon lines). Figure 2-6 shows the phonon absorption curve of a typical N3 color center. The actual measured color center absorption spectrum is more complex than the theoretical phonon characteristic spectrum, and the absorption peak of a high-energy wide absorption band is difficult to identify. In the color origin, gemological research and identification of diamonds, the zero phonon line is used to represent the entire phonon absorption band, mainly because the zero phonon line is more obvious in the diamond absorption spectrum and other absorption peaks are difficult to identify.

The intensity and width of the zero phonon line are related to the temperature. The higher the temperature, the lower the intensity and the wider the width. The reason is that the higher the temperature, the stronger the thermal vibrations of the diamond lattice, making the zero phonon lines weaker and wider. When studying the color origin of diamonds, in order to obtain an absorption spectrum with clear resolution of zero phonon lines, diamonds are generally measured at liquid nitrogen temperature (77K). When studying the effect of color centers on diamond color, the diamond's visible transmission spectrum or visible reflection spectrum should be measured at room temperature in order to obtain true color measurement data.

Figure 2-6 Typical phonon absorption spectrum of the N3 color center

The peak of the N3 color center is located at 415nm, and its absorption band mainly extends to short-wave ultraviolet wavelengths, but also to long-wavelength The band extends to a wavelength of about 420nm

When the electrons in the color center are excited to the excited state, they will automatically return to the ground state and radiate visible light, which is a fluorescence phenomenon. The color center fluorescence radiation spectrum obtained by theoretical calculation and the corresponding phonon absorption spectrum are mirror images centered on the zero phonon wavelength. The color center fluorescence spectrum obtained by actual measurement is in good agreement with the theoretically calculated fluorescence spectrum. The property that the color center fluorescence spectrum and absorption spectrum are mirror images of the zero phonon line provides another way for qualitative research on diamond spectra: measuring the fluorescence spectrum of diamonds to study the color center. The fluorescence spectrum can be obtained by irradiating the diamond with a short-wave laser. At low temperatures, the laser fluorescence spectrum of the diamond color center is stronger, and the zero-phonon line absorption peak is also relatively clear. In addition, laser fluorescence spectra are easy to measure and are increasingly used.

The electrons in some color centers are very strongly bound, and the relative vibration of the electrons is very weak, making the intensity of the zero phonon line too weak to be measured, and the entire zero phonon absorption spectrum becomes one without The absorption band of the absorption peak. Although this color center cannot be determined directly by measuring the zero phonon line, it can be verified indirectly by exciting ultraviolet fluorescence.

For the sake of simplicity, when discussing the color center of diamonds below, the wavelength of the zero phonon line of the color center is used to represent the entire color center, in order to correspond to the convention of calibrating the color center in spectroscopic research. In the spectroscopic study of diamonds, we only need to know the zero phonon line of the color center to draw corresponding conclusions about the color center and diamonds. When researching and calculating diamond color, the measurement of diamond’s transmission spectrum and reflection spectrum should be carried out at room temperature. The zero phonon line and other absorption peaks of the obtained spectrum are not obvious, and the zero phonon line has no influence on the color. The influence of wide absorption band is generally much smaller than the influence of wide absorption band on color.

The following is a brief introduction to several typical and common color centers.

1.N3 color center

The discrete nitrogen atoms in type Ib diamond crystals will gradually polymerize under high temperature and high pressure conditions to form an aggregate of 2 or more nitrogen atoms, making Type Ib diamonds become Type Ia diamonds. In the polymerization process of nitrogen, it is most conducive to generate polymers containing 3 nitrogen atoms, followed by polymers containing 2 and 4 nitrogen atoms, and the possibility of generating polymers of other nitrogen atoms is less. The aggregates of these nitrogen atoms will absorb light to varying degrees. The aggregates of 2 and 4 nitrogen atoms will absorb in the infrared wavelength range. The aggregate of 3 nitrogen atoms will absorb blue visible light and make the diamond appear yellow. It is called the N3 color center and is one of the most important color centers in diamonds. one. The N3 color center consists of three nitrogen atoms bonded to one carbon atom. The zero phonon peak of the N 3 color center is located at 415 nm, and its absorption band mainly extends to short-wave ultraviolet wavelengths, and also extends to the long-wave band to a wavelength of about 420 nm, as shown in Figure 2-6.

Generally, the N 3 color center is always accompanied by an N 2 absorption peak with a peak at 478nm.

The intensity of the N2 absorption peak is related to the intensity of the N3 color center. The stronger the N3 color center, the stronger the N2 absorption peak. Compared with the N3 color center, the absorption intensity of the N2 absorption peak in the visible light range is weaker. The N2 absorption peak is not a zero phonon line, so the N2 absorption peak does not represent a color center. The reason is that the N2 absorption peak cannot produce corresponding fluorescence radiation. Since the shorter the wavelength of visible light, the lower the color vision of the human eye, the N 2 absorption peak at 478 nm is more effective in producing color than the N3 color center zero phonon line at 415 nm.

Figure 2-7 "Cape" absorption spectrum composed of N3 color center and N2 absorption peak

The N3 color center is produced by a polymer composed of 3 nitrogens, and the N2 absorption peak It is always accompanied by the N3 color center, but it neither belongs to the N3 color center nor is it produced by other phonon color centers. Its specific cause is unknown

The N3 color center and the N2 absorption peak form the famous "Cape" "Absorption spectrum, as shown in Figure 2-7. The "Cape" spectrum was first discovered in yellow diamonds produced near Cape City, South Africa, hence the name. The N3 color center and the N2 absorption peak form a visible light absorption band. Generally, a strong absorption peak line at 415nm is observed under a spectrometer, so the N3 absorption peak is also called the "Keeper" line. All type Ia diamonds have "Cape" lines, so most natural diamonds have "Cape" lines with different absorption strengths.

When type Ia and type Ib diamonds have the same nitrogen content, type Ia diamonds have a much lower yellow saturation than type Ib diamonds. This phenomenon shows that when the discrete nitrogen atoms in type Ib diamond form the aggregated nitrogen color center in type Ia diamond under high temperature and pressure, the absorption of visible light by the nitrogen atoms weakens. The discrete nitrogen atoms in type Ib diamond produce a broad absorption band in the short-wavelength range of visible light. The N3 color center produced by the polymerized nitrogen in type Ia diamond mainly produces a very narrow 415nm absorption peak in the visible wavelength range. Its broad absorption band is located at the short-wave end of visible light and the ultraviolet range, which has little visual impact on color. Another part of the nitrogen atoms forms aggregates of 2 and 4 nitrogen atoms, which have no absorption in the visible wavelength range. Due to the above reasons, the discrete nitrogen atoms in type Ib diamonds absorb short-wave light much more strongly than the same content of nitrogen atom aggregates in type Ia diamonds, and the yellow saturation produced by the diamond is correspondingly much higher. When synthetic diamonds are produced, nitrogen atoms exist in discrete forms and belong to type Ib. When synthetic diamonds containing discrete nitrogen are treated with high temperature and pressure, some of the discrete nitrogen atoms will form aggregates, producing polymerized nitrogen color centers, and their yellow color will become lighter, thereby improving the color.

2.GR1 Color Center

GR is the abbreviation of General Radiation in English. As the name suggests, the GR color center is the color generated by the holes created by radiation in the diamond crystal. center. When radioactive substances such as uranium, thallium and cobalt irradiate diamonds, they can knock carbon atoms out of the crystal lattice, creating a hole. This hole in the diamond lattice is called the GRl color center. The crystal hole in the GR1 color center is a point crystal defect of diamond, and there is no electron filling in the hole. The GRl color center of diamond is a permanent radiation damaged color center, and it is almost impossible to restore the original crystal structure by heating or other methods. The GRl color center produces a broad absorption band and a series of absorption peaks in the visible and infrared wavelength ranges, forming a typical phonon absorption spectrum. The wavelength of the zero-acoustic f absorption peak of the GR1 color center is 740.9nm, and its corresponding excitation energy is 1.673eV. It forms a broad absorption band in the wavelength range of 412-430nm and is accompanied by the absorption peak of GR2-8. The absorption spectrum of a typical GRl color center including the zero phonon absorption peak and absorption band is shown in Figure 2-8. Generally speaking, the GR2-8 absorption peak located in the broad absorption band is not obvious, and its contribution to the color of diamonds can be ignored.

From the GR1 absorption spectrum in Figure 2-8, it can be seen that the absorption rate of the absorption band generated by the GR1 color center gradually decreases from long wavelength to short wavelength until 430nm, and then increases slightly. The GRl color center spectrum itself produces a blue color in diamonds. When the GR2-8 absorption peak is strong, the color of the diamond is greenish blue. The blue color of irradiated type IIa is due to the GRl color center. In addition, the GRl color center can also make type Ia and Ib colorless diamonds with very low nitrogen content appear blue, that is, the color of some blue diamonds is actually produced by the GRl color center. If type IIb blue diamonds are irradiated, their blue saturation may increase.

Figure 2-8 The absorption band and absorption peak of the GR1 color center in the visible spectrum range

The wavelength of the zero phonon line is 740.9nm, and its corresponding excitation energy is 1.673eV: The absorption band of the GRl color center extends to the long-wavelength range of visible light, and the accompanying GR2-8 absorption peak produces weak absorption in the short-wavelength range of visible light

Yellow type Ia diamonds have N3 color centers and contain more Nitrogen atoms, their blue-violet light will be partially absorbed by nitrogen atoms. Radiation-treated type Ia diamonds have both GRl and N3 color centers. The GR1 color center absorbs long-wave visible light, and the N3 color center absorbs short-wave visible light, causing the hue of the radiation-treated type Ia diamond to change to green. The vast majority of natural green diamonds are type Ia. After natural radiation, their green color is produced by the GRl color center and the N3 color center.

Many literatures simply attribute the green color of diamonds only to the GRl color center. In fact, the contribution of the N3 color center to the green color of diamonds is essential.

Figure 2-9 Natural green diamond (Tino Hammid/Courtesy of Aurora Gem Collection)

Northern Lights Colored Diamond Collection No. 86, 0.63ct

According to GRl Color centers and N3 color centers differ in intensity, and the hue of a diamond's color varies from blue to yellow. When the diamond has only GR1 color center, the color is blue; when the absorption of the diamond's GR1 color center for long-wave visible light is greater than the absorption of the N3 color center for short-wave visible light, the color is green-blue; when the absorption of the GRl color center is approximately equal to N3 When the absorption of the diamond's color center is green, the color is green; when the absorption of the diamond's GR1 color center is less than the absorption of the N3 color center, the color is yellow-green; when the diamond only has the N3 color center, the color is yellow. Figure 2-9 is a green diamond from the Northern Lights Colored Diamond Collection.

Under the bombardment of high-energy particles, diamonds are more likely to produce more GR1 color centers, making the corresponding broad absorption band very strong. The GR2-8 absorption peak is also very obvious. The GR1 color center of diamond can be produced by any kind of high-energy radiation, including natural alpha and gamma rays, high-energy electron beams, high-energy neutron beams and fast neutrons from atomic reactors. Under normal circumstances, the energy of the high-energy radiation source that generates the GR1 color center should be greater than 1M eV. The energy level of the electron accelerator used in diamond color-changing experiments is generally above 2M eV, allowing electrons to penetrate more thicknesses.

The GR1 color center formed by the holes in the diamond crystal is the key to forming the H color center and N—V color center introduced below. The H color center and the N-V color center are both formed by combining holes with different forms of nitrogen elements.

3.H color center

H color center is a crystal defect produced by a nitrogen-containing type Ia diamond that is irradiated and then heat treated. After the diamond is irradiated, a hole without carbon atoms is created in the crystal, which is the GR1 color center. During further heat treatment, the GR1 color center may combine with the polymer of nitrogen atoms to form a new color center. When a hole is combined with an A polymer composed of two nitrogen atoms, an H3 (N—V—N) color center is formed. When a hole is combined with a B polymer composed of four nitrogen atoms, an H4 color center is formed.

The H 3 color center produces a broad absorption band with a zero phonon wavelength of 503.2nm. Since the broad absorption band of the H 3 color center is approximately between 400 and 500 nm in the long band of visible wavelengths, the H 3 color center itself will cause the diamond to produce a yellow hue, which is commonly seen in the absorption spectrum of treated fancy yellow diamonds.

Figure 2-10 Absorption spectra of H3 and H4 color centers

The zero phonon line of the H4 color center almost overlaps with the first absorption peak of the H3 color center

< The p>H4 color center produces a broad absorption band with a zero phonon line at 496 nm. Relative to the H3 color center, the H4 color center is generally weaker. The zero phonon line of the H4 color center is superimposed on the first absorption peak of the H3 color center, almost coinciding with it. Figure 2-10 shows the absorption spectra of H3 and H4 color centers.

The superposition of H3, H4 and N3 color centers can make diamonds appear highly saturated yellow, even orange-yellow and yellow-orange. The color of type Ia yellow natural diamonds is produced by the N3 color center and contains large amounts of A and B aggregates. After type Ia yellow natural diamonds are irradiated by high-energy particle accelerators, many holes will be generated in the diamond crystal. After the radiation-treated diamond is heat-treated, the holes generated by the radiation combine with the A polymer and the B polymer to produce strong H3 and H4 color centers. There is no significant change in the N3 color core after treatment, and the absorption intensity of short-wave visible light remains basically unchanged. The H3 and H4 color centers produced after processing absorb visible light shorter than 505nm. If the absorption of the N 3 color center is much greater than the absorption of the H 3 and H4 color centers, the diamond will still appear yellow, but the saturation will be higher than when there is only the N3 color center. If the absorption of the N3 color center is close to the absorption of the H3 and H4 color centers, the diamond may appear orange-yellow or even yellow-orange. If the absorption of the N3 color center is less than the absorption of the H3 and H4 color centers, the diamond will appear yellow-orange or even orange. Type Ia yellow diamonds may become fancy yellow or orange diamonds after radiation and heat treatment. If the H3, H4 and N3 color centers are all very strong, strong non-selective absorption may occur in the entire visible light range, making the diamond less bright and appearing in a low-brightness brown-orange or brown color.

The H2 color center is also composed of holes and nitrogen atom A aggregates, with negative charge (N—V—N), resulting in a wider absorption band, with the absorption peak center located at 986.1nm. . When the H 2 color center is strong, the absorption band extends to the long wavelength range of the visible spectrum. Since the H2 color center always co-exists with the N3, H3 and H4 color centers, the absorption of H2 in the visible light range is much smaller than the absorption of the other three color centers, so the H2 color center contributes very much to the color of the diamond or to the color change. Limited, the influence of H2 color center is usually not considered when discussing the cause of diamond color.

Because the H color center is formed by the combination of holes and nitrogen atom aggregates, diamonds with H color centers must belong to type Ia. In addition, H is the first letter of English Heat. Therefore, it can be seen literally that the H color center must have undergone some kind of high-energy radiation and heat treatment.

The heat treatment temperature range to produce H 3 and H 4 color centers is approximately 500 to 1800°C, and the commonly used temperature is 800°C.

Type Ia diamonds may also produce a 3H color center with a wavelength of 503.6nm and a 595nm color center after radiation and heat treatment. Although these two color centers play an auxiliary role in the identification of colored diamonds, they are relatively rare, and their spectral absorption is very low. Their contribution to the color of diamonds can generally be ignored. The zero phonon wavelength of the 3H color center is very close to the zero phonon wavelength of the H 3 color center. Sometimes they are confused.

Figure 2-11 shows a color-changed orange diamond collected by the author. This orange diamond has extremely strong H3, H4, and N3 color centers, which is why it appears orange.

Figure 2-11 Color-modified orange natural diamond (Photography by Liu Yan/Collection by Liu Yan)

Orange color is produced by the simultaneous production of H3, H4 and N3 color centers

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4.N—V color center

N—V is the abbreviation of English nitrogen (Nitrogen) and hole (Va-cancy). As the name implies. N—V color center is composed of nitrogen and A color center composed of holes. N-V color centers are crystal defects produced by radiation and heat treatment in type Ib diamonds containing discrete nitrogen. As mentioned earlier, irradiation of a diamond produces a GRl color center. When heat treated again, the GRl color center can combine with a single nitrogen atom to form a new color center. When a hole combines with a nitrogen atom, an N—V color center is formed. The absorption peak of the neutral (N-V) 0 color center is 575nm. The absorption peak of the negatively charged (N-V)- color center is 637nm. Figure 2-12 shows a typical N-V color center absorption spectrum.

Figure 2-12 Absorption spectrum of N-V color center

The zero phonon line of (N-V)0 is 575nm,

The zero phonon line of (N-V)- The line is located at 637nm

The N-V color center is often accompanied by the H color center and coexists in the absorption spectrum. In the process of generating the N-color V center, the H color center is inevitably generated

The importance of the N-V color center is that it produces a broad absorption band with main peak wavelengths of 574.8nm and 637.0nm in the visible light range. The N-V color center is located in the middle of visible light, which will cause the diamond to produce a reddish-purple color, such as pink or purple. In the diamond absorption spectrum obtained by general measurement, the absorption peak of the N—V color center is always accompanied by other absorption peaks. Spectra in which the N—V color center absorption peak exists alone are relatively rare. Only type IIb diamonds produce N-V color centers without accompanying H color centers. The reason is that type IIb diamonds contain almost no nitrogen. The lattice spacing of individual discrete nitrogen atoms is very large, making it impossible to aggregate together and produce H. Lust.

H3 and H4 absorption peaks are often accompanied by N-V absorption peaks in the absorption spectrum. This phenomenon shows that during the heat treatment process to generate N-V color centers, discrete nitrogen atoms may aggregate into A aggregates and B aggregates then combine with holes to generate H color centers. It is also possible that the N—V color centers combine with discrete nitrogen atoms under the action of heat to directly generate H color centers. No matter how H color centers are generated in the process of generating N-V color centers, the generation of H color centers will reduce the relative number of N-V color centers to varying degrees.

There are very few natural diamonds with N-V color centers. Even if N-V color centers exist, they are generally very weak, and their contribution to the color of colored diamonds is limited. Type IIa diamonds contain very little nitrogen, and all nitrogen exists in the form of single atoms. Individual type IIa diamonds will also produce N-V color centers after radiation and high temperature treatment. This kind of type IIa diamond with N-V color center all shows a very low saturation purple-red pink color. The reason is that the nitrogen content is extremely low. It is impossible to produce more N—V color centers. The color of a very small number of natural light pink diamonds is produced by N-V color centers. These natural light pink diamonds belong to type IIa and are reported to be produced in India.

Over the years, with the in-depth understanding of the causes of diamond color, the continuous improvement of equipment for synthetic diamonds, and the gradual improvement of synthesis and color processing technology, diamonds of many colors can be obtained by synthesis and processing. , especially important is the synthesis of red diamonds. During the artificial synthesis of diamonds and their radiation and heat treatment processes, N-V color centers are relatively easy to produce, and strong N-V color centers can be obtained. The wavelength of the N-V color center is in the middle of the visible light range, and its absorption efficiency of visible light is high. It plays a vital role in producing synthetic diamonds of certain colors, such as the color of some synthetic red diamonds that have been color-modified. It is produced by the N-V color center and free nitrogen. Figure 2-31 shows a color-modified type Ib red synthetic diamond collected by the author. The color is produced by the simultaneous production of free nitrogen atoms and N-V color centers. Free nitrogen atoms mainly absorb short-wave visible light, the N-V color center absorbs medium-wave visible light, and the remaining long-wave visible light appears red. If the absorption intensity of the N-V color center is greater than the absorption intensity of free nitrogen atoms, the relative intensity of short-wave visible light will be greater than the relative intensity of medium-wave visible light. In this case, the diamond will appear purplish red (Purplish Red). Under standard daylight light, the actual color of this synthetic red diamond is purplish red, not pure red.

According to the author's measurement research and visual observation, the actual hue of the few red synthetic diamonds on the market is purplish red, and its hue is similar to the hue of the red diamond in Figure 2-13.

Figure 2-13 Color-modified type Ib red synthetic diamond

(Photography by Liu Yan/Collection of Liu Yan)

The color is mainly caused by discrete nitrogen sources Synthetic red diamonds produced at the same time with f and N-V color centers are darker in color, that is, lower in brightness. The main reason is that the discrete nitrogen content is very high and the short-wavelength absorption of visible light is strong. ; Moreover, after strong radiation treatment and heat treatment, a strong N-V color center is produced, which produces strong absorption in the visible light band, so that the red synthetic diamond has strong absorption in the medium and short wavelength visible light, and produces strong absorption in the entire visible light band. The non-selective absorption results in a lower brightness red color. As shown in Figure 2-13, the table color distribution of this red synthetic diamond is a red flashing area against a dark reddish-brown background. The difficulties in synthesizing and treating red diamonds include: the nitrogen content of the diamond must be high, the radiation treatment must be strong, and the time, temperature, and pressure of the heat treatment must be just right. This is why synthetic red diamonds are also rare. It is not so much the purposeful synthesis of red diamonds as it is an accidental encounter during the synthesis and processing process. If the nitrogen content of the diamond is low and radiation and heat treatment are not suitable, the color of this synthetic diamond may be yellow-orange, orange, brown-orange, purplish-red-brown, brown or reddish-brown instead of red.

Under the same nitrogen content, the absorption of visible light by free nitrogen and N-V color centers is much greater than the absorption of visible light by N3, H3 and H4 color centers, and the absorption peak of N-V color centers Located in the medium wave range of visible light, therefore, color-modified synthetic diamonds are more likely to produce more saturated red and orange tones.

5. Other color centers

There are many vibrating electron centers in diamonds. Most of the vibrating electron centers absorb visible light too weakly and have no effect on the color of the diamond. In addition to the color centers introduced above, the color centers that affect the color of diamonds include the negatively charged hole color center (ND1), the 477nm absorption band and the 595nm color center.

After the diamond is bombarded by high-energy electrons, the high-energy electrons will be injected into the shallow layer of the diamond crystal, which will not only displace the carbon atoms to generate GR1 color centers, but may also remain in the lattice of the diamond crystal. These electrons left in the diamond form the negative hole color center ND1 (Negatively Charged Vacancy). The peak wavelength of the ND1 color center is 393nm, its intensity is weak, and it has little effect on the color of the diamond. A strong ND1 color center may cause type IIa diamonds to appear pale yellow.

The 477nm absorption band is a vibrating electron color center, and the zero phonon line disappears because the electron binding is very strong. According to the distribution of the absorption spectrum curve, it can be inferred that the position of the zero phonon line of the 477nm absorption band should be approximately around 520nm, and the color of the mirror-symmetrically distributed fluorescence radiation is orange-red. Diamonds with an absorption band of 477nm generally belong to type Ib, have low nitrogen content and are amber in color. Because it belongs to type Ib without A aggregates, diamonds with a 477nm absorption band have stronger fluorescence radiation, and their fluorescence colors are generally yellow to orange. This indicates that there may be an electronic vibration center with a shorter wavelength, and its fluorescence color is the superposition of these two fluorescence radiations.

Type Ia diamonds will produce a 595nm color center after radiation and heat treatment, accompanied by a weaker absorption peak at 425nm. Compared with the coexisting N 3, H 3 and H 4 color centers, the 595nm color center has very weak absorption of visible light and has almost no impact on the color of the diamond. Generally, the presence of the 595nm color center is evidence that the diamond has been treated. When the heat treatment temperature is higher than 1000°C, the 595nm color center will disappear. Therefore, a type Ia diamond without a 595nm color center does not prove that it has not been treated.