Brief introduction of the first author: Chen, the first and second members of the Artificial Products Professional Committee of China Insurance Association, the third senior consultant, and a researcher at Southwest Institute of Technical Physics.

I. Introduction

Emerald, commonly known as emerald, is the most precious member of beryl family and one of the most precious gems in the world. Since ancient times, emeralds, together with diamonds, rubies, sapphires and emeralds, have been called the five most precious gems in the world. Due to the scarcity of natural output, the price of high-quality emeralds can be comparable to that of high-quality diamonds, which is expensive. Emerald's beautiful and unique green color is fascinating and intoxicating, which is incomparable to any other green gemstone, so it has the reputation of "the king of green gemstones".

Because of this, people try their best to study the growth technology of artificial emerald. There are two series of synthetic emeralds and imitation emeralds. The difference between the two is that:

Synthetic emeralds-synthetic emeralds with the same chemical composition, physical and chemical properties, the same crystal structure and even similar inclusion morphology as natural emeralds are grown by artificial methods. The main growth methods are cosolvent method and hydrothermal method.

Imitation emerald-it is grown artificially, and it is different from natural emerald in chemical composition and crystal structure, but it is very close to synthetic emerald in color and other appearance characteristics, even very close to natural emerald. The main growth method is Czochralski method.

All kinds of imitations of natural gems are sold in the international jewelry market, including imitations of natural emeralds. However, the color of natural gemstones, especially natural emeralds, is difficult to simulate or imitate.

In order to change a crystal whose crystal structure and chemical composition are completely different from that of emerald into a crystal whose appearance characteristics are very similar to those of natural emerald, we must first understand the reasons why natural emerald appears emerald, and then try our best to meet the conditions of emerald, so that the emerald effect can appear.

This paper aims to evaluate the simulation effect of green YAG artificial gem on natural emeralds and its growth method.

Second, the coloring mechanism of emerald

Emerald is a member of beryl mineral family, belonging to hexagonal system, usually in the form of long hexagonal column. Pure beryl without impurities is colorless, and it may have color when it contains impurity elements. For example, beryl is known to be pink when it contains cesium, which is called cesium beryl; It is blue when it contains iron and is called aquamarine; It is green when it contains chromium, which is called emerald. Therefore, emerald green is mainly due to the coloring of beryl containing chromium ions.

The main chemical composition of emerald is Be3Al2Si6O 18, and it also contains trace elements such as chromium, vanadium, iron and nickel. The green color of emerald is mainly caused by the crystal field splitting effect of transition metal chromium ion (Cr3+), and the existence of other transition metal trace elements will affect its green hue, which can change it from yellow-green to blue-green. Among them, the content of dark green emerald chromium oxide can reach 0.5% ~ 0.6%, and the content of light green emerald chromium oxide is 0. 15% ~ 0.2%.

The crystallographic characteristics of emerald are: hexagonal system, hexagonal column. Transparent-translucent; Glass luster; Mohs hardness is 7.5 ~ 8; Low toughness, only 5.5, brittle; The density is 2.67 ~ 2.78 g/cm3 (the density is related to the content of chromium and other elements in the crystal, and the darker the color, the greater the density, and the density of dark green emerald can reach 2.78 g/cm3). Emerald is a uniaxial negative crystal with a refractive index of 1.564 ~ 1.602. Ultraviolet fluorescence: generally no fluorescence, a few are red; Under the Charles filter, most emeralds are pink to red, a few emeralds containing iron are green, and Colombian emeralds are dark red. The color of emerald is mainly determined by the content of chromium. However, the transparency depends on the content of iron oxide. The less iron oxide, the more transparent. When the iron content exceeds 0.6%, the green color will become dark.

The chemical molecular formula of beryl is Be3Al2Si6O 18, and its structure is mainly composed of rings (Si6O 18). One Si4 ++ and four O2- form a silicon-oxygen tetrahedron and two silicon-oxygen tetrahedrons with vertex angles of * * *, as shown in figure 1. Annular (Si6O 18) layered deposits form pores, which are bonded by aluminum and beryllium atoms, as shown in Figure 2.

Figure 1 Tetrahedral skeleton of circular silica in beryl

(according to/book/books/print/packcolor/link/5-4-2 html)

Fig. 8 Several "metamerism" color schemes of orange.

(According to Nassau, 199 1)

It should be pointed out that the artificial gem color modulated by this method usually has the same color as the imitation natural gemstone only under the illumination conditions specified in the experiment. There will be some differences between the two colors when the lighting conditions change. This is because their absorption or transmission spectra are not exactly the same.

2. Spectral matching color scheme

The absorption spectrum and fluorescence spectrum of gem crystal are the main factors that determine the color of gem. If the absorption spectrum and fluorescence spectrum of imitation gemstones are completely consistent with natural gemstones, then naturally, this imitation gemstone will have very similar or even identical color characteristics with natural gemstones. Therefore, it is the most ideal method to directly fit the (absorption and fluorescence) spectra of natural gemstones to be developed from spectral fitting, so as to get the desired gemstone color. The advantage of this method is that under different lighting conditions (including fluorescence and color filter), imitation gems and imitation natural gems will have the same or similar colors. But it will be much more difficult to develop imitation gems in this way.

Fourthly, the choice of jadeite materials.

1. Selection of Emerald Matrix Crystals

Based on the coloring mechanism of natural emeralds, we systematically screened matrix crystals with the possibility of imitating emeralds. Firstly, we consider a host crystal that may have "green" and "chromophore" similar to emerald: it has an oxygen octahedral ligand that can contain Cr ions, and the crystal field strength is similar to that of emerald (medium size). These crystals are closest to emeralds in color mechanism, as listed in table 1 In addition, other factors should be considered, such as hardness of gemstone materials, production cost and maturity of growth process. We finally chose the best matrix crystal material YAG to imitate emerald.

Table 1 Cr3 ++ ion in the coordination field of oxygen octahedron with medium crystal field strength and comparison of related parameters.

2. Crystal structure of YAG and Cr3+:YAG crystals.

YAG, that is, yttrium aluminum garnet net crystal with the molecular formula of Y3Al5O 12, belongs to cubic crystal system and space group Ia3d. According to the different lattice positions of ions in the crystal, we can regard the crystal structure of yttrium aluminum garnet as a link network of oxygen tetrahedron, oxygen octahedron and oxygen dodecahedron ligands (Figure 9). Y3+ ions occupy the central position of dodecahedron [c], and Al3 ++ occupies the central positions of octahedron [a] and tetrahedron [d].

The Al-O octahedron in YAG crystal structure is very similar to that in beryl crystal (the Al-O distance is about 1.92A). When chromium ion is doped into YAG crystal, it can enter the oxygen octahedral site in trivalent (Cr3+) state to replace the cation Al3+ and form the Cr-O octahedral chromophore. In addition, it also has a variety of other crystal positions (tetrahedral position and dodecahedral position) for other colored ions to enter, thus facilitating further color adjustment. The specific ion occupation and valence state are related to doping concentration and growth conditions, and the situation is complicated, so further experiments can be carried out through experiments.

Comparing Cr:YAG and Cr:Be3Al2Si6O 18 (Emerald), we can see that they are very similar in color ions and crystal field environment. This similarity is that YAG crystal can be used as the material basis of emerald matrix. At first, we planted Cr:YAG crystals as gems to understand the similarities and differences between Cr:YAG crystals and emeralds as gems. It is found that Cr:YAG crystal is bright green under transmitted light. However, this artificial gem green has obvious yellow tone, and the surface of the gem presents bright red fluorescence. Therefore, it is obviously different from the emerald green of natural emeralds and needs to be further improved.

Table 2 Comparison of Crystal Field and Spectral Characteristics of Cr3+ in Yttrium Aluminum Garnet and Emerald

Fig. 9 schematic diagram of crystal structure of yttrium aluminum garnet

(According to Lu Xueshan 1972)

3. Selection of "Emerald" Doping Ion Combination

The finally selected main crystal YAG is the most commonly used laser main crystal material grown by melt Czochralski method. Its melting point is about 1970℃, Mohs hardness is 8.5, density is 4.55 g/cm3, refractive index is 1.83, dispersion is 0.028, and there is no refraction. When Cr ~ (3+) ions are doped, the crystal field symmetry and strength of Cr ~ (3+) ions in YAG crystal structure are quite close to that of emerald, and the Mohs hardness is also similar. However, there is still a certain gap between the color of Cr:YAG crystal and emerald.

First, we compare the absorption spectra of Cr:YAG and Cr:Be3Al2Si6O 18, that is, emeralds, as shown in Table 3 and Figure 10. We found that they are similar in the main characteristics of the spectrum (compare Figure 5 and Figure 10):

1) absorption spectrum type: broad absorption band spectrum;

2) Number of main absorption bands: two;

3) The peak positions of the main absorption bands are basically the same;

4) Fluorescent line: one, and the position is close (700nm and 730nm, both red).

Table 3 Spectral comparison between Cr-Y-Al garnet gem and emerald

However, their spectra are obviously different:

1) In the absorption spectrum of emerald, the peak intensity of 600nm absorption band is basically the same as that of 430nm absorption band, and the ratio is close to 1 (slightly changed according to the origin of emerald); Moreover, we can see a trend that emeralds with a slightly stronger absorption band at 600nm tend to be bluer. In the absorption spectrum of Cr:YAG crystal, the peak intensity of 600nm absorption band is obviously lower than that of 430nm absorption band, so Cr:YAG crystal is yellow-green.

Fig./absorption spectrum of kloc-0/0cr: YAG crystal (mass fraction of Cr2O3 is 0.3%).

2) The red fluorescence in Cr:YAG crystal is relatively strong, which makes the crystal appear obviously red under the reflected light, and the red light is also strong under Charles filter, so it is obviously different from the dark red of natural emerald under Charles filter. This fluorescence is the R-ray fluorescence emission of Cr ions, so the R-ray fluorescence emission of Cr:YAG crystal is too strong compared with natural emerald.

According to the above analysis, in order to get the color of emerald, it is necessary to change the relative intensity of absorption band and fluorescence intensity in Cr:YAG crystal spectrum. But first, the improvement experiment should be carried out without changing the basic characteristics of Cr:YAG crystal band spectrum.

The main chromogenic ions in 1) gemstones can be roughly divided into two categories: transition metal ion colorants and rare earth ion colorants. The absorption spectrum of the former in gem crystal is mainly broadband spectrum, while the absorption spectrum of the latter is linear spectrum. The former is consistent with the fact that all known colored ions in emeralds are transition metal ions. Therefore, we first decided to use only a variety of dopant combinations of transition metal ions, mainly Cr3 ++ ions, and change the relative intensity ratio of the two absorption peaks by adjusting their relative doping amounts. Its advantage is that it will not change the basic type of absorption spectrum.

2) The next work will focus on the characteristics of absorption spectra formed by different transition metal ions in YAG and their effects on the fluorescence emission intensity of R-rays, and select appropriate combinations through experiments to make the absorption spectra and fluorescence spectra of (Cr, Re):YAG close to natural emeralds (Re is a transition metal other than Cr).

Specifically, we found that doping some transition metal ions will help to make the relative intensities of the two main absorption bands gradually close, and make the green color of the transmitted light more pure; Further doping other transition metal ions will help to weaken the intensity of red fluorescence. The effect is shown in figure 1 1 (compare figure 5) and the physical photos.

Verb (abbreviation of verb) A growth method of emerald-like YAG gem

Emerald YAG gem was grown by induction heating Czochralski method, and the schematic diagram of the growth device is shown in Figure 12.

Table 4 Analysis and comparison of emerald YAG gemstones and emerald spectra

Fig. 1 1 absorption spectrum of emerald YAG gem

Table 5 chromaticity coordinates of emerald YAG gemstones

Fig. 12 schematic diagram of emerald YAG gem growth device

1-gem melt; 2- Gem seed crystal; Three gem crystals were grown; 4 zirconia insulation cover; 5 iridium cover; 6 iridium crucible; 7 zirconia thermal insulation sand; 8— Induction coil

The growth process of emerald YAG gem is shown in figure 13.

The growth process flow is described as follows:

1) According to our patent, the raw materials for growing emerald YAG gems are alumina, yttrium oxide, chromium oxide, iron oxide, vanadium oxide, titanium oxide and so on. The purity of raw materials is above 99.99%. The melt composition is prepared according to Y3Al(5-x-y)CrxreyO 12, wherein Re is one or more of other transition metal elements such as Fe, Co, Ti or V, x = 0.02-0.10, y = 0.001~ 0.

2) After the prepared raw materials are accurately weighed, they need to be evenly mixed and compacted into blocks, and pre-sintered in a muffle furnace at 1300℃ for 24h. And then put into a crucible for heating, melting and pulling the single crystal, wherein the crucible used is an iridium crucible with the size of φ 80 mm× 80 mm ..

3) Growth parameters of emerald YAG crystal grown by Czochralski method: crystal rotation speed; 10 ~ 30r/min； Casting speed: 2 ~ 5 mm/hour; Seed crystal orientation [111]; The growth atmosphere in the furnace is high purity nitrogen.

4) The grown emerald YAG crystal and the processed gem ring surface are shown in figure 16 and figure 17.

Fig. 13 growth process of emerald YAG gem

Figure 14 crystal pulling furnace for growing emeralds.

Fig. 15 furnace growth device for growing emeralds.

Ending of intransitive verbs

Jade imitation began to be developed, tested and improved in 1989, and the formula and technology were constantly improved. 1On September 25th, 1996, the emerald-imitating growth technology passed the expert appraisal in Beijing, and on August 27th, 1997 "emerald-imitating" was patented in China with patent number ZL 95 165438+.

Figure 16 "Original Ecology" Imitation Emerald YAG Gem Crystal

Figure 17 Emerald Faceted Gems after Processing

Jade imitations have been sold in small quantities through various channels and are welcomed by users at home and abroad.

This new emerald imitation has been appraised by Beijing Gaode Jewelry Appraisal Institute and Gemological Appraisal Institute of China Geo University (Beijing), and the conclusions are as follows: "Its luminosity is isotropic, with no dichroism, the density is 4.55g/cm3, the refractive index is 1.833, the Mohs hardness is 8.5, the appearance color is bright green, and the glass luster under the color filter is dark red.

To sum up, the emerald imitation we developed has the following characteristics:

1) With YAG as the matrix and only Cr3 ++ as the colorant, the color, hardness and color of the product under the color filter are very close to natural emeralds, and the appearance characteristics of its best formula are close to the world-famous Colombian emeralds.

2) Not only the appearance is similar, but also the absorption spectrum and fluorescence of natural emerald (chrome) are similar.

3) This emerald imitation may or may not contain defects like veil.

At present, the production cost of this emerald-like YAG gem is still relatively high by Czochralski method, and further research and development of a new process that can reduce the production cost will help the market promotion of this high-quality emerald-like YAG gem.

Reference materials and materials

Chen,, etc. Two kinds of green gem materials simulating natural emeralds. The 68th Annual Meeting of ISSC, May 5-7, Vancouver, British Columbia, Canada.

Chen, Huang Jinrong. 1997. "Jade imitation". China patent ZL95 1 15498+0.

Lu xueshan, ed. 1972. Development of laser-based yttrium aluminum garnet (1 version). Beijing: Science Press.

Nassau Railway Company. Li Shijie and Zhang Zhisan. 199 1. the physics and chemistry of color (version 1). Beijing: Science Press.

/book/books/print/pack color/link/5-4-2 . html。

liguowu，http://www . crystal star . org/Photo/show pho-to . ASP？ PhotoID=78(2005-08-3 1)。