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Controlled crystallization of emerald from the fluxed melt
Journal of Crystal Growth 198/199 (1999) 716—722
Controlled crystallization of emerald from the fluxed melt S.N. Barilo , G.L. Bychkov *, L.A. Kurnevich , N.I. Leonuk, V.P. Mikhailov , S.V. Shiryaev , V.T. Koyava, T.V. Smirnova Institute of Physics of Solids and Semiconductors, The National Academy of Sciences, Minsk 220072, Belarus Moscow State University, Department of Geology, Moscow 119899, Russia **Diagem Crystals++ Research Laboratory, Minsk 220050, Belarus
Abstract The problem of controlled crystallization of emerald single crystals from a fluxed melt, its colour characteristics and optic parameters are discussed. Properties of the as-grown single crystals are very much like those of natural gems. Emeralds weighting as much as 150 ct grown on oriented seeds in dynamical regime feature small dichroism, uniform distribution of colour in the volume to offer good jewelry characteristics. The (1 0 1 0) and (1 1 2 0) cuts of previously grown crystals is established to be the optimal seed. The technique has the advantage of maintaining the optimal concentration ratio of the solute near the crystallization front through adequate stirring by a platinum crystal holder is rotated at a rate of 30 rounds per minute, and seed positioning. To examine emerald crystals quality we have performed a laser experiment and threshold measurements. Lasing was achieved at absorbed pump energy threshold of less than 0.6 mJ. 1999 Elsevier Science B.V. All rights reserved. PACS: 81.10.!h; 78.20.!e; 74.76.!w; 64.75.#g; 78.45.#h Keywords: Emerald; Single crystals; Flux melt growth; Solubility; Segregation; Stimulated emission
1. Introduction Emerald, one of the most attractive precious gemstones of green colour, is the beryl—beryllium aluminum silicate family with the formula
* Corresponding author. Tel.: #375 172 841162; fax: #375 172 840888; e-mail: [email protected].
Ba Al Si O : Cr and classified with the first grade precious gemstones. Apart from jewelry, emerald can be used in low-noise microwave amplifiers as well as serve as the most effective lasing medium in the range 450—600 nm [1,2]. The emerald crystals refer to the dihexagonal dipyramid class of symmetry of hexagonal syngony (D P6/mcc) and demonstrate well developed (1 0 1 0) prism, and the (0 0 0 1) pinacoid faces, for a while the (1 1 2 1), (1 0 1 0) dipyramid and the (1 1 2 0)
0022-0248/99/$ — see front matter 1999 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 0 2 4 8 ( 9 8 ) 0 1 1 3 4 - 8
S.N. Barilo et al. / Journal of Crystal Growth 198/199 (1999) 716–722
prism faces are more rare in occurrence [3]. Since the mining of the natural emerald stones has been reduced the last years there is a growing demand for man-made emeralds. Owing to incongruent melting of beryl [4] one has to take recourse to such growth methods that allow the process to be run at temperatures below ¹ "1420°C. These
are the crystallization from the flux melt, the hydrothermal method, and gas-transport reaction. Thus, the US Company ‘‘Linde Air Products’’ has employed the flux melt technique for growing of emeralds using various fluxes and temperature conditions (methods of temperature gradient and slow cooling). V O as well as lithium tungstate or mo lybdate were utilized as a flux, however, preference was given to lithium molybdate Li O—nMn O , here n"2.25—3.25 [5]. Ushio et al. [6] were growing emerald single crystals from V O flux in the temperature range from 1010°C to 1050°C. The temperature gradient between the seed and the nutrient was made in the range 10—35°C and the crystal exhibited a developed growth morphology with dipyramid faces. Emerald single crystals grown by Miyata et al. from the lithium molybdate based solvent [7] also showed well-developed faces. In the Former Soviet Union emerald crystals have been first manufactured by Bukin et al. [8]. The emeralds were crystallized using alkaline salts of tungsten, vanadium and molybdenium acids (in particular Li O—3MoO ) as well as a mixture of Mo, V, Pb and B oxides. The crystals were grown either by slow cooling of the flux melt or at temperature gradient. The best results have been realized in the PbO—V O flux. Emerald single crystals have been produced commercially for several decades by two companies “Carol F. Chatham” and “Pierre Gilson”. Both the manufactures have not revealed the know-how of the technologies used, though, the literature data evidence serious crystal defects, non-uniform distribution of colour in volume of the crystals, and worse optical properties as compared to the best natural analogues [9]. In this paper we report the results of the studies on the controlled growth of emerald single crystals from high temperature flux as well as optimization of their colour characteristics, optical properties, and tolerance for cutting.
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2. Experimental results and discussion 2.1. Choosing of solvent and growth temperature range Based on the quality check of beryllium solubility, the phase composition analysis of the end products in alkaline salts of tungsten, vanadium and molybdenum acids, mixture of the Mo, V, Pb and B oxides as a function of temperature, a number of solvents suitable for growing of emerald crystals have been selected. Despite partial emerald dissociation into phenacite and quartz in the V O based solvents we have achieved success with the use of the PbO—V O flux proposed earlier by Linares [10]. Studies on equilibrium of V O with oxygen against temperature and oxygen pressure over the melt and determination of the equilibrium constant for the dissociation process were made in Ref. [11]. It has been known that the adding of lead oxide to vanadium pentaoxide affect essentially the melt stability as temperature increases. At the PbO concentration over 40 mol%, the compositions are chemically resistant. To estimate the starting mixture composition and optimal temperature range for emerald crystallization a series of runs has been carried out. We determined initially its solubility in the PbO—V O flux. To this end, a block of natural beryl was dissolved in the flux mixture at constant temperature till complete saturation of the melt occurred. As shown by the solubility curves (Fig. 1) taken at 1000°C, the flux melt becomes 90% saturated
Fig. 1. Solubility curves for natural beryl in the PbO—V O flux depending on: (a) temperature; (b) dissolving duration.
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within 50 h, while complete saturation is attained for 4—5 days. The solubility curve for beryl in the PbO—V O flux appears as a parabola in Fig. 1b. The saturation concentration varies up to 7.5% in the temperature range from 1250°C to 900°C and crystallization of emerald under slow cooling conditions can therefore be applied. It is known that the growth of high-quality crystals at sufficiently high rates is feasible only from solutions with small viscosity. Therefore, it is desirable to carry on crystallization at temperatures no less than 900°C, with the solvent compositions containing from 30 to 70 wt% of PbO. Besides, the primary crystallization range of chrysoberyl in the system phase diagram corresponds to lower temperatures. In its turn, the upper limit of the temperature range of growth from the given solvent was 1250°C, above which the primary crystallization range of phenacite and tridymite is located. 2.2. Single crystal growth The tentative experiments on growing of emerald from the above mentioned fluxes were conducted by the technique of spontaneous crystallization with slow cooling of the flux melt and steadily growing rate of overcooling from 0.5°C to 4°C a day. Both the static mode of growing and the mode, wherein the rotation of crucible was employed in the growth process, were utilized. To avoid the rising of the nucleated crystals to the flux melt surface a platinum-perforated screen had been placed into the platinum crucible. The size of the crystals yielded in 2—3 months experiments depended upon of the crucible volume and the overcooling rate of the melt did not exceed 15—20 mm of the edge. When the volume of a platinum crucible increasing average dimensions of the emerald crystals are rising. For a while the overcooling rate increasing leads to smaller the crystals sizes. The as-obtained crystals demonstrated a large number of defects visible to the naked eye in the form of flux inclusions, cracks, aggregates of small crystals of the accompanying phases. The best results were obtained during spontaneous crystallization at a minimal cooling rate of 0.5°C a day. In this event, the morphology of crystals changed and apart from the most abundant (0 0 0 1) and (1 0 1 0) faces there
appeared well-developed (1 0 1 0) prism faces of the second order. To grow high-quality emerald single crystals with uniform distribution of colour in the volume we have used the method of growing on oriented seeds in the dynamic mode. This method allowed us to adjust the colour of gem growing on test seed crystals while searching for the temperatures of saturation and spontaneous crystallization on trial seeds. The advantage of the method of controlled growth on seeds is also a possibility of maintaining the optimal concentration ratios of the melt near the crystallization front due to the expense of effective stirring. The cuts of single crystals, grown by the method of spontaneous crystallization, parallel to the natural prism (1 0 1 0) and (1 1 2 0) faces and pinacoid (0 0 0 1) faces and later on the corresponding faces of the crystals, grown on seeds, served as seeds. The seeds were arranged parallel each other in the planes oriented perpendicular to the flux melt surface and fixed to specially shaped platinum holders. Such configuration of the seeded growth experiment provides continuous washing of the growing crystal face and stirring of the melt at optimal rotation rate of 30 rs/min. The total area of the seed plates varied from 600 mm to 2000 mm depending on the crucible volume and the flux melt mass. The main experimental parameters of seeded growth in crucibles of various volumes are summarized in Table 1. We tried to optimize and unify as much as possible the emerald growth process. For this purpose the comparison experiments have been performed with thermal apparatus operating on single cycle with various orientation of the seed crystals. The experimental data obtained are listed in Table 2. The highest quality optical crystals have been prepared with the cooling rate of the flux melt varying in the range 0.5—2°C a day. In this situation the quasi-equilibrium crystal growth at a practically constant growth rate of &0.1 mm a day in the [1 0 1 0] and [1 1 2 0] crystal directions was ensured. It is established that two crystal cuts parallel to the prism (1 0 1 0) and (1 1 2 0) faces (indicated in Table 2 the m and a faces, respectively) correspond to the optimal seed crystal orientation. As the experimental analysis shows the mass of high optical quality single crystals on the average is 6 times as large as the yield with the crystal cut
S.N. Barilo et al. / Journal of Crystal Growth 198/199 (1999) 716–722
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Table 1 The selected growth experiments of emerald data depending on crucible dimension Run no.
Flux melt super-cooling range (°C)
D¹ (°C)
Growth process duration (days)
Flux melt super-cooling rate (°C/day)
Total mass of crystals (g)
Crystal quality
1 2 3
Crucible 1170—1049 1152—1132 1118—881
121 20 237
Diameter 70 mm 19 18 20
5—8 1—1.6 4—16
Height 70 mm 23.82 6.54 22.18
Poor High Poor
4 5 6
Crucible 1040—1008 1081—1018 1066—1005
32 63 61
Diameter 100 mm 27 40 50
1—1.6 1.2—2 1—1.6
Height 100 mm 6.35 21.18 22.73
Accep. High Accep.
7 8
Crucible 1146—1093 1100—1006
53 94
Diameter 120 mm 26 117
2—2.4 0.25—1.2
Height 120 mm 41.63 61.2
Poor High
9 10 11 12
Crucible 1207—733 1150—979 1130—1030 1030—962
474 171 100 68
Diameter 130 mm 152 121 140 96
1—5 0.25—4 0.25—1 0.125—1
Height 230 mm 462.53 284.98 122.7 49.05
Poor Accep. High High
Table 2 Growth experiments of emerald data in the single cycle of 4 furnaces equipped by the same crucible (80 mm in height and 80 mm in diameter) depending on the seed orientation Growth parameter
Mass of solute (g) Flux melt saturation temperature (°C) Total range of overcooling (°C) Flux melt overcooling rate interval (°C/days) Duration of growth (days) Number of seeds Seed orientation Seed rotation rate (rs/min) Total mass of crystals (g) Optical quality
Installation number I
II
III
IV
90 1104
85 1083
90 1114
90 1092
95
100
100
110
1.6
1—2.4
0.5—1.6
1—2.4
53 10 c, m 14 16.5 Accept.
66 10 c 32 16.3 Poor
94 12 a 32 16.0 Perfect
78 8 a, m 32 15.0 High
parallel to the most rapidly growing pinacoid (0 0 0 1) face (c face in Table 2) employed as a seed. The optimal experiment cycle lasted, in the average, for 3—4 months. The best single crystals grown in
the experiments were almost perfect in terms of the optical parameters weighing as much as 150 carats (Fig. 2). These crystals revealed high tolerance for cutting off and subsequent facetting. The test experiments on cutting of seeded grown crystals showed that it was possible to obtain cut gems weighing as much as 7 carats, that corresponding to the highest quality of crystals. 2.3. Optical spectra and quality of emerald crystals The absorption spectra of the samples indicate the high quality of emerald crystals. As can be seen from Fig. 3 the broad absorption bands with peaks at 595 nm (ENC6), 642 nm (E#C6) and 475 nm (ENC6), 430 nm (E#C6) correspond to transition from the 4A2 ground state to the 4T2 and 4T1 excited states of the Cr> ion in octahedral coordination splitted due to symmetry distortion. Two sharp peaks at about 680 nm (R-lines) are attributed to the spin-forbidden transition to the 2E state. The samples exhibit high absorption coefficient at 600 and 640 (21 and 30 cm\) and small parasitic absorption and scattering losses in the
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Fig. 2. Emeralds grown from flux melt: (a) as-grown crystals (40.3 and 45.2 carats); (b) faceted stones, emerald-cut, 3.67 and 3.3 carat.
cing between mirrors was about 170 mm. The pump beam was focused into the sample by 40 cm focal length lens. Lasing was achieved at absorbed pump energy threshold of less then 0.6 mJ that corresponds to energy fluence of 1.9 J/cm. This value is considerably smaller than the value reported for both hydrothermally and flux grown emerald crystals [12,13]. 2.4. Controlled color variation of emerald single crystals
Fig. 3. Absorption spectra of the emerald crystal in polarized radiation.
gain region. The emission lifetime was measured to be 45$5 ls. To examine crystal quality we have performed also the laser experiment and threshold measurements. The sample was 1.5 mm thick plate with 4 mm by 8 mm — rectangular cross section. Laser experiments were performed using a Nd : YAG laser pumped Ba(NO3)2 Raman laser. The second stoks output at 600 nm was used for pumping of emerald crystal. A near hemispherical laser cavity was formed by concave mirror having a radius of curvature of 20 cm with 99.9% reflectivity in the range 700—780 nm and by flat output coupler with transmission less then 1%. The spa-
The colour of emerald is known to depend on the presence of chromium that substitutes isomorphously aluminum in the crystal structure. High quality natural emeralds also contain iron and vanadium. We have followed the influence of impurity concentration of transition metal ions on the scale of colours, tone and optical characteristics of emerald. It is common knowledge that the method of spontaneous crystallization does not allow one to adequately vary the colour of the growing emerald crystal. Moreover, the nonuniform distribution of chromium dopes at the stage of nucleation and, ensuing development of the crystal lead to variation in the intensity and tone of colour. The method of single crystals growth on oriented seeds in dynamic mode employed by us, in its turn, is devoid of the above mentioned disadvantages. The purpose of the search for optimal colour characteristics for the grown emerald single crystals was to
S.N. Barilo et al. / Journal of Crystal Growth 198/199 (1999) 716–722
obtain reference single crystals whose characteristics of colour, tone, refractive index, birefringence were very close to those of the finest first-grade gem stones from the “Muzo” and “Chivor” (Columbia) as well as from the “Malyshevo” (Ural mountains) mines. The sequence of process enabling us to achieve this objective is as follows: (1) the experimental determination of the coefficient of distribution of ions of the main chromium components of the flux melt Cr> and V>&0.1 and 0.01, respectively; (2) the growing of a test crystal; (3) the comparison of the Cr and V concentration in the test crystal with the optimal one (0.4 and 0.04 wt%, respectively — Colombian emerald; 0.3—0.01 wt% — the Urals emerald); (4) the refining of the concentration and vanadium oxides in the flux melt; (5) the repeat comparison of the chemical composition of the sample with natural analogues; (6) the comparison of optical characteristics of the as-grown emeralds with natural ones; (7) the refining of chromium concentration in crystals with a tendency towards increasing up to the specific gravity value 2.69—2.70 g/cm near to maximum theoretically possible for emerald single crystals grown by the flux method; (8) the refining of the flux melt composition through introduction of an additional amount of iron oxide to achieve yellowish green color typical of the Urals emeralds; (9) the comparison of the Ce concentration with the optimal one — 0.3 wt% characteristic of finest samples from the Urals mountains; (10) the repeat refining of the melt by adding of small amounts of Ce and Mo
721
with subsequent comparison with natural sample; (11) the crystals of bluish green colour in the initial state (concentration Cr'0.4 wt%; V'0.04 wt% are refined by introducing small amounts of Ni with a tendency towards more light color. The use of analogous technique made it possible to obtain emerald samples grown with the colour varying in the range of bright green color, as well as to get reference colours of emerald used for visual sorting of single crystals. Table 3 gives the representative characteristics of the crystals grown in the reported experiments for comparison with those of single crystals grown by the hydrothermal (“Linde” Co) [5], flux melt (“Chatham”) growth techniques and natural gemstones.
3. Conclusions Analysis of the experimental data obtained allow us to draw the following conclusions: (1) to obtain a maximum actual yield of gem quality single crystals it is essential that the growth on oriented seeds in dynamic mode should be performed; (2) at the same duration of the process equal to the fraction of actual use of raw material when the crucible of 100 mm in weight and 100 mm in diameter is employed (the mixture weighing 2.6 kg) the actual yield of crystals is 4.5 times as great as that when the crucible of 70 mm in height and 70 mm in diameter (the mixture weighing 1 kg) is utilized; (3) The optimal orientation of seeds while growing emeralds is the one with the cuts parallel to the
Table 3 Optical properties and specific gravity data for seeded grown crystals in comparison with hydrothermal grown emerald and natural gems Emerald
Hydrothermal (“Linde” Co.) [5] Flux grown (“Chatham”) Natural (“Muzo” mine) Natural (“Chivor” mine) Bright green test specim. (this work) Dark green test specim. (this work) Bluish-light green (this work)
Impurity concentration (wt%)
Refractive index
Specific gravity (g/cm)
Cr>
V>
n
n
0.3 0.6 0.4 1.1 0.43 1.0 0.24
0.008 0.002 0.04 0.02 0.04 0.07 0.1
1.571 1.562 1.577 1.584 1.570 1.574 1.560
1.566 1.559 1.571 1.578 1.562 1.566 1.554
2.67 2.65 2.69 2.71 2.68 2.68 2.65
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prism (1 0 1 0) and (1 1 2 0) faces as the mass of gem single crystals with this orientation of seeds is 6 times greater than the yield obtained with the use of seeds parallel to the most fast growing (0 0 0 1) crystal face. The absorption spectra of the samples have been measured as well as a considerably smaller, in comparison with the previous data, pump energy threshold to be used for lasing achievement confirmed the high quality of the emerald crystals.
Acknowledgements The Committee of Science and Technology of Belarus Republic is greatly acknowledged for the financial support of this work in the applied scientific program “Diamonds” framework by grant C3203/1.03. The investigations were supported in part by Belarus—USA joint venture “Diagem”.
References [1] F.E. Goodwin, J. Appl. Phys. 32 (1961) 1624. [2] S.A. Payne, L.L. Chase, L.K. Smith et al., J. Appl. Phys. 66 (1989) 1051. [3] I. Kostov, Minerology, Mir, Moscow, 1971 (in Russian). [4] R.P. Miller, R.A. Mercer, Mineral. Mag. 35 (N27) (1965) 250. [5] E.M. Flanigen, D.W. Breck, N.R. Mumbach, A.M. Taylor, Amer. Mineral. 32 (1967) 744. [6] M. Ushio, Y. Symiyoshi, Nippon Kagaku Kaishi 9 (1972) 1648. [7] T. Miyata, Y. Kavabata, I. Takeda, H. Kojima, J. Crystal Growth 99 (1990) 869. [8] G.V. Bukin, A.A. Godovikov, V.A. Klyakhin, V.S. Sobolev, Crystal Growth, XIII, Nauka, Moscow, 1980, p. 215 (in Russian). [9] R. Diehl, Z. Dtsh. Gemmol. Ges 26 (1977) 61. [10] R.C. Linares, Amer. Mineralogist 52 (1967) 1554. [11] C. Blaauw, J. Phys. Chem.: Solid State Phys. 8 (1975) 456. [12] J. Buchert, A. Katz, R.R. Alfano, IEEE J. Quant. Electr. 19 (1983) 281. [13] V.S. Gulev, A.P. Eliseev, V.P. Solntsev et al., Quantum Electron. 14 (1987) 1990 (in Russian).
Controlled crystallization of emerald from the fluxed melt S.N. Barilo , G.L. Bychkov *, L.A. Kurnevich , N.I. Leonuk, V.P. Mikhailov , S.V. Shiryaev , V.T. Koyava, T.V. Smirnova Institute of Physics of Solids and Semiconductors, The National Academy of Sciences, Minsk 220072, Belarus Moscow State University, Department of Geology, Moscow 119899, Russia **Diagem Crystals++ Research Laboratory, Minsk 220050, Belarus
Abstract The problem of controlled crystallization of emerald single crystals from a fluxed melt, its colour characteristics and optic parameters are discussed. Properties of the as-grown single crystals are very much like those of natural gems. Emeralds weighting as much as 150 ct grown on oriented seeds in dynamical regime feature small dichroism, uniform distribution of colour in the volume to offer good jewelry characteristics. The (1 0 1 0) and (1 1 2 0) cuts of previously grown crystals is established to be the optimal seed. The technique has the advantage of maintaining the optimal concentration ratio of the solute near the crystallization front through adequate stirring by a platinum crystal holder is rotated at a rate of 30 rounds per minute, and seed positioning. To examine emerald crystals quality we have performed a laser experiment and threshold measurements. Lasing was achieved at absorbed pump energy threshold of less than 0.6 mJ. 1999 Elsevier Science B.V. All rights reserved. PACS: 81.10.!h; 78.20.!e; 74.76.!w; 64.75.#g; 78.45.#h Keywords: Emerald; Single crystals; Flux melt growth; Solubility; Segregation; Stimulated emission
1. Introduction Emerald, one of the most attractive precious gemstones of green colour, is the beryl—beryllium aluminum silicate family with the formula
* Corresponding author. Tel.: #375 172 841162; fax: #375 172 840888; e-mail: [email protected].
Ba Al Si O : Cr and classified with the first grade precious gemstones. Apart from jewelry, emerald can be used in low-noise microwave amplifiers as well as serve as the most effective lasing medium in the range 450—600 nm [1,2]. The emerald crystals refer to the dihexagonal dipyramid class of symmetry of hexagonal syngony (D P6/mcc) and demonstrate well developed (1 0 1 0) prism, and the (0 0 0 1) pinacoid faces, for a while the (1 1 2 1), (1 0 1 0) dipyramid and the (1 1 2 0)
0022-0248/99/$ — see front matter 1999 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 0 2 4 8 ( 9 8 ) 0 1 1 3 4 - 8
S.N. Barilo et al. / Journal of Crystal Growth 198/199 (1999) 716–722
prism faces are more rare in occurrence [3]. Since the mining of the natural emerald stones has been reduced the last years there is a growing demand for man-made emeralds. Owing to incongruent melting of beryl [4] one has to take recourse to such growth methods that allow the process to be run at temperatures below ¹ "1420°C. These
are the crystallization from the flux melt, the hydrothermal method, and gas-transport reaction. Thus, the US Company ‘‘Linde Air Products’’ has employed the flux melt technique for growing of emeralds using various fluxes and temperature conditions (methods of temperature gradient and slow cooling). V O as well as lithium tungstate or mo lybdate were utilized as a flux, however, preference was given to lithium molybdate Li O—nMn O , here n"2.25—3.25 [5]. Ushio et al. [6] were growing emerald single crystals from V O flux in the temperature range from 1010°C to 1050°C. The temperature gradient between the seed and the nutrient was made in the range 10—35°C and the crystal exhibited a developed growth morphology with dipyramid faces. Emerald single crystals grown by Miyata et al. from the lithium molybdate based solvent [7] also showed well-developed faces. In the Former Soviet Union emerald crystals have been first manufactured by Bukin et al. [8]. The emeralds were crystallized using alkaline salts of tungsten, vanadium and molybdenium acids (in particular Li O—3MoO ) as well as a mixture of Mo, V, Pb and B oxides. The crystals were grown either by slow cooling of the flux melt or at temperature gradient. The best results have been realized in the PbO—V O flux. Emerald single crystals have been produced commercially for several decades by two companies “Carol F. Chatham” and “Pierre Gilson”. Both the manufactures have not revealed the know-how of the technologies used, though, the literature data evidence serious crystal defects, non-uniform distribution of colour in volume of the crystals, and worse optical properties as compared to the best natural analogues [9]. In this paper we report the results of the studies on the controlled growth of emerald single crystals from high temperature flux as well as optimization of their colour characteristics, optical properties, and tolerance for cutting.
717
2. Experimental results and discussion 2.1. Choosing of solvent and growth temperature range Based on the quality check of beryllium solubility, the phase composition analysis of the end products in alkaline salts of tungsten, vanadium and molybdenum acids, mixture of the Mo, V, Pb and B oxides as a function of temperature, a number of solvents suitable for growing of emerald crystals have been selected. Despite partial emerald dissociation into phenacite and quartz in the V O based solvents we have achieved success with the use of the PbO—V O flux proposed earlier by Linares [10]. Studies on equilibrium of V O with oxygen against temperature and oxygen pressure over the melt and determination of the equilibrium constant for the dissociation process were made in Ref. [11]. It has been known that the adding of lead oxide to vanadium pentaoxide affect essentially the melt stability as temperature increases. At the PbO concentration over 40 mol%, the compositions are chemically resistant. To estimate the starting mixture composition and optimal temperature range for emerald crystallization a series of runs has been carried out. We determined initially its solubility in the PbO—V O flux. To this end, a block of natural beryl was dissolved in the flux mixture at constant temperature till complete saturation of the melt occurred. As shown by the solubility curves (Fig. 1) taken at 1000°C, the flux melt becomes 90% saturated
Fig. 1. Solubility curves for natural beryl in the PbO—V O flux depending on: (a) temperature; (b) dissolving duration.
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within 50 h, while complete saturation is attained for 4—5 days. The solubility curve for beryl in the PbO—V O flux appears as a parabola in Fig. 1b. The saturation concentration varies up to 7.5% in the temperature range from 1250°C to 900°C and crystallization of emerald under slow cooling conditions can therefore be applied. It is known that the growth of high-quality crystals at sufficiently high rates is feasible only from solutions with small viscosity. Therefore, it is desirable to carry on crystallization at temperatures no less than 900°C, with the solvent compositions containing from 30 to 70 wt% of PbO. Besides, the primary crystallization range of chrysoberyl in the system phase diagram corresponds to lower temperatures. In its turn, the upper limit of the temperature range of growth from the given solvent was 1250°C, above which the primary crystallization range of phenacite and tridymite is located. 2.2. Single crystal growth The tentative experiments on growing of emerald from the above mentioned fluxes were conducted by the technique of spontaneous crystallization with slow cooling of the flux melt and steadily growing rate of overcooling from 0.5°C to 4°C a day. Both the static mode of growing and the mode, wherein the rotation of crucible was employed in the growth process, were utilized. To avoid the rising of the nucleated crystals to the flux melt surface a platinum-perforated screen had been placed into the platinum crucible. The size of the crystals yielded in 2—3 months experiments depended upon of the crucible volume and the overcooling rate of the melt did not exceed 15—20 mm of the edge. When the volume of a platinum crucible increasing average dimensions of the emerald crystals are rising. For a while the overcooling rate increasing leads to smaller the crystals sizes. The as-obtained crystals demonstrated a large number of defects visible to the naked eye in the form of flux inclusions, cracks, aggregates of small crystals of the accompanying phases. The best results were obtained during spontaneous crystallization at a minimal cooling rate of 0.5°C a day. In this event, the morphology of crystals changed and apart from the most abundant (0 0 0 1) and (1 0 1 0) faces there
appeared well-developed (1 0 1 0) prism faces of the second order. To grow high-quality emerald single crystals with uniform distribution of colour in the volume we have used the method of growing on oriented seeds in the dynamic mode. This method allowed us to adjust the colour of gem growing on test seed crystals while searching for the temperatures of saturation and spontaneous crystallization on trial seeds. The advantage of the method of controlled growth on seeds is also a possibility of maintaining the optimal concentration ratios of the melt near the crystallization front due to the expense of effective stirring. The cuts of single crystals, grown by the method of spontaneous crystallization, parallel to the natural prism (1 0 1 0) and (1 1 2 0) faces and pinacoid (0 0 0 1) faces and later on the corresponding faces of the crystals, grown on seeds, served as seeds. The seeds were arranged parallel each other in the planes oriented perpendicular to the flux melt surface and fixed to specially shaped platinum holders. Such configuration of the seeded growth experiment provides continuous washing of the growing crystal face and stirring of the melt at optimal rotation rate of 30 rs/min. The total area of the seed plates varied from 600 mm to 2000 mm depending on the crucible volume and the flux melt mass. The main experimental parameters of seeded growth in crucibles of various volumes are summarized in Table 1. We tried to optimize and unify as much as possible the emerald growth process. For this purpose the comparison experiments have been performed with thermal apparatus operating on single cycle with various orientation of the seed crystals. The experimental data obtained are listed in Table 2. The highest quality optical crystals have been prepared with the cooling rate of the flux melt varying in the range 0.5—2°C a day. In this situation the quasi-equilibrium crystal growth at a practically constant growth rate of &0.1 mm a day in the [1 0 1 0] and [1 1 2 0] crystal directions was ensured. It is established that two crystal cuts parallel to the prism (1 0 1 0) and (1 1 2 0) faces (indicated in Table 2 the m and a faces, respectively) correspond to the optimal seed crystal orientation. As the experimental analysis shows the mass of high optical quality single crystals on the average is 6 times as large as the yield with the crystal cut
S.N. Barilo et al. / Journal of Crystal Growth 198/199 (1999) 716–722
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Table 1 The selected growth experiments of emerald data depending on crucible dimension Run no.
Flux melt super-cooling range (°C)
D¹ (°C)
Growth process duration (days)
Flux melt super-cooling rate (°C/day)
Total mass of crystals (g)
Crystal quality
1 2 3
Crucible 1170—1049 1152—1132 1118—881
121 20 237
Diameter 70 mm 19 18 20
5—8 1—1.6 4—16
Height 70 mm 23.82 6.54 22.18
Poor High Poor
4 5 6
Crucible 1040—1008 1081—1018 1066—1005
32 63 61
Diameter 100 mm 27 40 50
1—1.6 1.2—2 1—1.6
Height 100 mm 6.35 21.18 22.73
Accep. High Accep.
7 8
Crucible 1146—1093 1100—1006
53 94
Diameter 120 mm 26 117
2—2.4 0.25—1.2
Height 120 mm 41.63 61.2
Poor High
9 10 11 12
Crucible 1207—733 1150—979 1130—1030 1030—962
474 171 100 68
Diameter 130 mm 152 121 140 96
1—5 0.25—4 0.25—1 0.125—1
Height 230 mm 462.53 284.98 122.7 49.05
Poor Accep. High High
Table 2 Growth experiments of emerald data in the single cycle of 4 furnaces equipped by the same crucible (80 mm in height and 80 mm in diameter) depending on the seed orientation Growth parameter
Mass of solute (g) Flux melt saturation temperature (°C) Total range of overcooling (°C) Flux melt overcooling rate interval (°C/days) Duration of growth (days) Number of seeds Seed orientation Seed rotation rate (rs/min) Total mass of crystals (g) Optical quality
Installation number I
II
III
IV
90 1104
85 1083
90 1114
90 1092
95
100
100
110
1.6
1—2.4
0.5—1.6
1—2.4
53 10 c, m 14 16.5 Accept.
66 10 c 32 16.3 Poor
94 12 a 32 16.0 Perfect
78 8 a, m 32 15.0 High
parallel to the most rapidly growing pinacoid (0 0 0 1) face (c face in Table 2) employed as a seed. The optimal experiment cycle lasted, in the average, for 3—4 months. The best single crystals grown in
the experiments were almost perfect in terms of the optical parameters weighing as much as 150 carats (Fig. 2). These crystals revealed high tolerance for cutting off and subsequent facetting. The test experiments on cutting of seeded grown crystals showed that it was possible to obtain cut gems weighing as much as 7 carats, that corresponding to the highest quality of crystals. 2.3. Optical spectra and quality of emerald crystals The absorption spectra of the samples indicate the high quality of emerald crystals. As can be seen from Fig. 3 the broad absorption bands with peaks at 595 nm (ENC6), 642 nm (E#C6) and 475 nm (ENC6), 430 nm (E#C6) correspond to transition from the 4A2 ground state to the 4T2 and 4T1 excited states of the Cr> ion in octahedral coordination splitted due to symmetry distortion. Two sharp peaks at about 680 nm (R-lines) are attributed to the spin-forbidden transition to the 2E state. The samples exhibit high absorption coefficient at 600 and 640 (21 and 30 cm\) and small parasitic absorption and scattering losses in the
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Fig. 2. Emeralds grown from flux melt: (a) as-grown crystals (40.3 and 45.2 carats); (b) faceted stones, emerald-cut, 3.67 and 3.3 carat.
cing between mirrors was about 170 mm. The pump beam was focused into the sample by 40 cm focal length lens. Lasing was achieved at absorbed pump energy threshold of less then 0.6 mJ that corresponds to energy fluence of 1.9 J/cm. This value is considerably smaller than the value reported for both hydrothermally and flux grown emerald crystals [12,13]. 2.4. Controlled color variation of emerald single crystals
Fig. 3. Absorption spectra of the emerald crystal in polarized radiation.
gain region. The emission lifetime was measured to be 45$5 ls. To examine crystal quality we have performed also the laser experiment and threshold measurements. The sample was 1.5 mm thick plate with 4 mm by 8 mm — rectangular cross section. Laser experiments were performed using a Nd : YAG laser pumped Ba(NO3)2 Raman laser. The second stoks output at 600 nm was used for pumping of emerald crystal. A near hemispherical laser cavity was formed by concave mirror having a radius of curvature of 20 cm with 99.9% reflectivity in the range 700—780 nm and by flat output coupler with transmission less then 1%. The spa-
The colour of emerald is known to depend on the presence of chromium that substitutes isomorphously aluminum in the crystal structure. High quality natural emeralds also contain iron and vanadium. We have followed the influence of impurity concentration of transition metal ions on the scale of colours, tone and optical characteristics of emerald. It is common knowledge that the method of spontaneous crystallization does not allow one to adequately vary the colour of the growing emerald crystal. Moreover, the nonuniform distribution of chromium dopes at the stage of nucleation and, ensuing development of the crystal lead to variation in the intensity and tone of colour. The method of single crystals growth on oriented seeds in dynamic mode employed by us, in its turn, is devoid of the above mentioned disadvantages. The purpose of the search for optimal colour characteristics for the grown emerald single crystals was to
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obtain reference single crystals whose characteristics of colour, tone, refractive index, birefringence were very close to those of the finest first-grade gem stones from the “Muzo” and “Chivor” (Columbia) as well as from the “Malyshevo” (Ural mountains) mines. The sequence of process enabling us to achieve this objective is as follows: (1) the experimental determination of the coefficient of distribution of ions of the main chromium components of the flux melt Cr> and V>&0.1 and 0.01, respectively; (2) the growing of a test crystal; (3) the comparison of the Cr and V concentration in the test crystal with the optimal one (0.4 and 0.04 wt%, respectively — Colombian emerald; 0.3—0.01 wt% — the Urals emerald); (4) the refining of the concentration and vanadium oxides in the flux melt; (5) the repeat comparison of the chemical composition of the sample with natural analogues; (6) the comparison of optical characteristics of the as-grown emeralds with natural ones; (7) the refining of chromium concentration in crystals with a tendency towards increasing up to the specific gravity value 2.69—2.70 g/cm near to maximum theoretically possible for emerald single crystals grown by the flux method; (8) the refining of the flux melt composition through introduction of an additional amount of iron oxide to achieve yellowish green color typical of the Urals emeralds; (9) the comparison of the Ce concentration with the optimal one — 0.3 wt% characteristic of finest samples from the Urals mountains; (10) the repeat refining of the melt by adding of small amounts of Ce and Mo
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with subsequent comparison with natural sample; (11) the crystals of bluish green colour in the initial state (concentration Cr'0.4 wt%; V'0.04 wt% are refined by introducing small amounts of Ni with a tendency towards more light color. The use of analogous technique made it possible to obtain emerald samples grown with the colour varying in the range of bright green color, as well as to get reference colours of emerald used for visual sorting of single crystals. Table 3 gives the representative characteristics of the crystals grown in the reported experiments for comparison with those of single crystals grown by the hydrothermal (“Linde” Co) [5], flux melt (“Chatham”) growth techniques and natural gemstones.
3. Conclusions Analysis of the experimental data obtained allow us to draw the following conclusions: (1) to obtain a maximum actual yield of gem quality single crystals it is essential that the growth on oriented seeds in dynamic mode should be performed; (2) at the same duration of the process equal to the fraction of actual use of raw material when the crucible of 100 mm in weight and 100 mm in diameter is employed (the mixture weighing 2.6 kg) the actual yield of crystals is 4.5 times as great as that when the crucible of 70 mm in height and 70 mm in diameter (the mixture weighing 1 kg) is utilized; (3) The optimal orientation of seeds while growing emeralds is the one with the cuts parallel to the
Table 3 Optical properties and specific gravity data for seeded grown crystals in comparison with hydrothermal grown emerald and natural gems Emerald
Hydrothermal (“Linde” Co.) [5] Flux grown (“Chatham”) Natural (“Muzo” mine) Natural (“Chivor” mine) Bright green test specim. (this work) Dark green test specim. (this work) Bluish-light green (this work)
Impurity concentration (wt%)
Refractive index
Specific gravity (g/cm)
Cr>
V>
n
n
0.3 0.6 0.4 1.1 0.43 1.0 0.24
0.008 0.002 0.04 0.02 0.04 0.07 0.1
1.571 1.562 1.577 1.584 1.570 1.574 1.560
1.566 1.559 1.571 1.578 1.562 1.566 1.554
2.67 2.65 2.69 2.71 2.68 2.68 2.65
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prism (1 0 1 0) and (1 1 2 0) faces as the mass of gem single crystals with this orientation of seeds is 6 times greater than the yield obtained with the use of seeds parallel to the most fast growing (0 0 0 1) crystal face. The absorption spectra of the samples have been measured as well as a considerably smaller, in comparison with the previous data, pump energy threshold to be used for lasing achievement confirmed the high quality of the emerald crystals.
Acknowledgements The Committee of Science and Technology of Belarus Republic is greatly acknowledged for the financial support of this work in the applied scientific program “Diamonds” framework by grant C3203/1.03. The investigations were supported in part by Belarus—USA joint venture “Diagem”.
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