TAIRUS HYDROTHERMAL SYNTHETIC SAPPHIRES DOPED WITH NICKEL AND CHROMIUM By Victor G. Thomas, Rudolf I. Mashkovtsev, Sergey Z. Smirnov, and Vadim S. Maltsev Researchers of the Tairus joint venture have he history of hydrothermally grown synthetic developed the technology for the hydrother- corundum began nearly 40 years ago, when mal growth of gem-quality crystals of syn- Laudise and Ballman (1958) produced the first col- thetic sapphire. They have achieved a broad Torless synthetic sapphire. Their technique, when modified, spectrum of attractive colors by varying the also can be used to grow rubies (Kuznetsov and Shternberg, concentrations of Ni2+, Ni3+, and Cr3+. 1967). Until recently, however, attempts to grow blue sap- Comprehensive testing of 18 faceted synthet- ic sapphires and 10 synthetic crystals, repre- phire by the hydrothermal method had failed. This problem senting the range of colors, revealed a typical is significant because flux-grown and flame-fusion synthetic set of internal features that are specific to sapphire crystals typically have uneven color distribution. In hydrothermally grown crystals (microscopy), blue flame-fusion material, an intense blue hue is usually as well as the presence of OH– and various concentrated in thin outer zones of boules, while most of carbon-oxygen groups in the sapphire struc- the bodycolor is much lighter. In blue flux-grown synthetic ture (infrared spectroscopy). The reactions of sapphires, color inhomogeneity is apparent as a sequence of the test samples to ultraviolet radiation and blue and colorless angular growth zones that conform to the visible light were also found to be useful in crystal shape. Such color inhomogeneity makes cutting dif- distinguishing the Tairus synthetic sapphires ficult and greatly decreases the yield of faceted material. from their natural counterparts and other To address this market gap, the authors investigated the synthetics. use of hydrothermal growth techniques to produce blue crystals that would be free from the defects of synthetic sap- phire grown by other methods. This problem was approached in two ways: (1) by doping hydrothermal syn- thetic sapphire with iron (Fe2+) and titanium (Ti4+) ions (as proposed by, for example, Burns, 1981); and (2) by using other doping impurities that could reproduce the blue color of sapphire. Research into the development of a commercial ABOUT THE AUTHORS technique by the first approach is still ongoing. However, Dr. Thomas is a senior researcher, and Mr. the second approach has led to success: The use of nickel Maltsev is a technological researcher, at Tairus (a 2+ joint venture between the Siberian Branch of the (Ni ) as a dopant has resulted in crystals of “sky” blue syn- Russian Academy of Sciences and Pinky Trading thetic sapphire, from which thousands of faceted stones as Co. Ltd. of Bangkok, Thailand). Dr. Mashkovtsev large as 4 ct have been produced by the Tairus joint venture is a senior researcher, and Dr. Smirnov is a research scientist, at the Siberian Gemological and marketed internationally since 1995. Center, Novosibirsk The next step in the development of this technique was Gems & Gemology, Vol. 33, No. 3, pp. 188–202. to produce sapphires of different colors by varying the con- 2+ 3+ 3+ © 1997 Gemological Institute of America centrations of Ni , Ni , and chromium (Cr ). At this time, the Tairus research group has created the technology 188 Tairus Hydrothermal Synthetic Sapphires GEMS & GEMOLOGY Fall 1997 Figure 1. Tairus experts have produced hydro- thermal synthetic sap- phires in a broad range of colors, as illustrated here. These Tairus syn- thetic sapphires range from 1.00 to 4.74 ct. Photo © GIA and Tino Hammid. to grow commercial quantities of sapphires in blue prietary). Seed plates oriented parallel to the hexago- and a wide range of other colors (see, e.g., figure 1). nal prism {1010}– are used to grow cuttable crystals. This article presents the results of our research into For research purposes, we also grew two crystals on the growth of hydrothermal synthetic sapphires and a spherical seed to investigate the ideal arrangement their characterization, including the chemistry, of the crystal faces. All of the seeds in the samples gemological properties, and distinctive internal fea- used for this study were cut from Czochralski- tures of these new synthetics. grown colorless synthetic sapphire. The dopant (NiO and, for certain colors, Cr2O3) is introduced GROWTH TECHNIQUES via unsealed capsules placed at the bottom of the The autoclave arrangement used to grow Tairus ampoule. The concentrations of Ni and Cr in the hydrothermal synthetic sapphire is illustrated in fig- growing crystal are controlled by the diameter of ure 2. The growth process is carried out in a her- the opening in these capsules. metically sealed “floating” gold ampoule with a An oxygen-containing buffer plays a significant volume of almost 175 ml inside the autoclave, a role in the sapphire growth process. It is a powdered temperature of 600°C, and a pressure of about 2000 reagent that is placed at the bottom of each bar. Growth takes place when the initial synthetic ampoule. This substance governs the valence state colorless sapphire feed is transferred and crystal- of Ni in the solution, so that sapphires with differ- lized onto seeds, via a vertical temperature gradient ent Ni2+:Ni3+ ratios can be grown. Various metals of 60° to 70°C, through a carbonate solution with a and their oxides (none of which produces any color complex composition (the details of which are pro- in the synthetic sapphire) have been used for this Tairus Hydrothermal Synthetic Sapphires GEMS & GEMOLOGY Fall 1997 189 Figure 2. Hydrothermal synthetic sapphires are grown by Tairus scientists in an autoclave that mea- sures 60 cm high and 8 cm in diameter. The compo- nents are as follows: (1) lid; (2) push nut; (3) auto- clave body; (4) seal ring; (5) gold ampoule; (6) syn- thetic sapphire seeds; (7) diaphragm; (8) crushed syn- thetic colorless corundum charge; (9) capsules with Ni/Cr oxides; (10) powdered oxygen-containing buffer. Crystal growth proceeds via dissolution of the corundum charge (8) in a hydrothermal solution in the lower (i.e., hotter) part of the autoclave, followed by convective transfer of the saturated solution to the upper (i.e., cooler) part of the autoclave, where the synthetic sapphire crystallizes on the seeds (6). The autoclave is heated from the bottom and the side in a special furnace. both sides) plates were used for spectroscopy, inclu- sion study, and microprobe analysis. Standard gemological testing was carried out at the Siberian Gemological Center, Novosibirsk. The face-up colors of the 18 faceted synthetic sapphires were determined with a standard daylight-equiva- lent light source (the fluorescent lamp of a GIA GEM Instruments Gemolite microscope) and an ordinary incandescent lamp. For each faceted sam- ple, the color was compared to standards from the buffer (e.g., coupled Cu-Cu2O, Cu2O-CuO, etc. [see GIA GemSet and described in terms of hue, tone, table 1]). Synthetic sapphire crystals as large as 7 × and saturation. Specific gravity was determined 2.5 × 2 cm have been grown in a month, with a hydrostatically. Refractive indices were measured growth rate of almost 0.15 mm per day. with a GIA GEM Duplex II refractometer and a fil- tered, near-monochromatic, sodium-equivalent MATERIALS AND METHODS light source. To examine internal features and Two crystals of each color (10 pairs total) were polarization behavior, we used a GIA GEM grown under uniform conditions as described above. Illuminator polariscope and gemological micro- One crystal in each pair was used to produce faceted scope with various types of illumination. gems (from one to five faceted stones, each weigh- Detailed observations of inclusions, color zon- ing approximately 1 ct). The other was cut into ing, and growth sectors were made on crystal plates plates or used whole for crystallographic measure- of all 10 color varieties with a Zeiss Axiolab polariz- ments. Detailed crystallographic measurements ing microscope. Some samples were also immersed were also made on the two greenish blue crystals in methylene iodide for microscopic examination. (each about 0.8 cm across) that were grown on Appearance in visible light was observed with the spherical seeds. (These latter crystals were grown incandescent illumination provided by a GIA GEM both to illustrate the ideal arrangement of crystal Instruments spectroscope base, and UV lumines- faces for corundum produced under the experimen- cence was observed with the aid of a GIA GEM tal conditions described above, and because it was Instruments ultraviolet cabinet. Samples with weak easier to obtain accurate goniometric measure- luminescence were examined in a completely dark- ments on them.). We selected 18 faceted samples ened room. Dichroism was determined with a cal- (round and oval mixed cuts, weighing about 1 ct cite dichroscope and a GIA GEM Instruments each) that ranged from very light pink through FiberLite 150 fiber-optic illuminator. orange, yellow, and green to blue (see, e.g., figure 3) The arrangement of crystal faces was studied by for gemological testing. More than 20 polished (on means of a Zeiss ZRG-3 two-circle goniometer. The 190 Tairus Hydrothermal Synthetic Sapphires GEMS & GEMOLOGY Fall 1997 TABLE 1. Colors and partial chemistry of the Tairus synthetic sapphires.a Concentration (wt.%) b Sample Color Cr2O3 NiO FeO MnO TiO2 Oxygen buffer c 1 Greenish blue bdl 0.23 tr tr tr Cu-Cu2O 2 Greenish blue 0.15 0.24 tr tr tr Cu-Cu2O 3 Greenish blue 0.26 0.34 tr tr tr Cu-Cu2O 4 Light reddish purple 0.07 0.04 tr tr tr Cu-Cu2O 5 Pink 0.07 bdl tr tr tr Cu-Cu2O 6 “Padparadscha” 0.08 bdl tr tr tr PbO-Pb3O4 d 7 Yellow tr tr tr tr tr PbO-Pb3O4 e 8 Light greenish yellow bdl tr tr tr tr Cu-Cu2O 9 Green bdl 0.35 tr tr tr Cu2O-CuO 10 Blue-green bdl 0.23 tr tr tr not controlled 11 Colorless bdl bdl tr tr tr not controlled a Analyses by electron microprobe.