Article pubs.acs.org/crystal
Crystal Growth and Characterization of Alexandrite Denis A. Vinnik,† Dmitry A. Zherebtsov,† Sergey A. Archugov,† Markus Bischoff,‡ and Rainer Niewa*,‡ †
South Ural State University, 76 Lenin Avenue, 454080 Chelyabinsk, Russia Institute of Inorganic Chemistry, University of Stuttgart, Pfaffenwaldring 55, 70550 Stuttgart, Germany
‡
ABSTRACT: Large, high-quality alexandrite single crystals (Cr-doped Al2BeO4) with dimensions of up to 120 mm were grown from molten alexandrite-based fluxes in the temperature range of 1800− 1853 °C using the Stepanov and Kyropoulos methods. Etching experiments resulted in a maximum pits density of 105 cm−2. Powder X-ray diffraction was used to establish the dependence of unit cell dimensions on the Cr content. Optical absorption spectroscopy (UV−vis) revealed nonlinear absorption dependence on the Cr content.
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INTRODUCTION
Alexandrite, chromium-doped Al2BeO4, is the most valuable variety of chrysoberyl, which is the third hardest naturally occurring gemstone (after diamond and corundum). It is wellknown for the strong pleochroism effect with different colors along different crystallographic directions and for the so-called alexandrite effect, meaning changing color from bluish-green in daylight to red in artificial light. Chrysoberyl, an isotype of olivine, contains aluminum in two different crystallographic positions both with octahedral coordination by oxygen, but with the Al(II) site with distinctly larger distances than Al(I). It was established from X-ray diffraction performed on single crystals of natural alexandrite that Cr prefers to substitute the Al(II) site,1 probably due to the larger ionic radius of Cr3+ (r(Cr3+) = 0.615 Å, r(Al3+) = 0.535 Å in octahedral coordination).2 However, the actual Cr distribution over both sites is a function of the crystal growth technique and temperature history of the material and ranges from 1:2 to 1:3 for the Al(I) and Al(II) sites.3,4 The aforementioned alexandrite effect strongly depends on the distribution of Cr3+ over the two crystallographic positions.5 Alexandrite solid state lasers are based on the excellent activator properties of Cr3+ substituted for Al3+ in chrysoberyl. However, the working performance of the laser strongly depends on the Cr content, its distribution over both crystallographic sites, and the crystal quality.6 The main challenge in production of alexandrite laser materials is the growth of large crystals of several millimeters in size with homogeneous Cr distribution. Here we describe the growth of large, high-quality crystals up to 50 mm in diameter from solution in molten salts with a high growth rate and with excellent homogeneity of chromium distribution. © 2012 American Chemical Society
EXPERIMENTAL SECTION
Alexandrite single crystals were grown using a high-temperature solution method with two solvents (flux) based on molten alexandrite with addition of extra aluminum oxide (5−6 wt %) allowing the growth process to be performed in the temperature range of 1800− 1853 °C.8 From these solutions it is possible to grow alexandrite single crystals: (a) 8−12 mm in diameter with a length of 120 mm using the Stepanov (edge-defined film-fed growth) method and (b) 40−50 mm in diameter with a length of 30 mm using the Kyropoulos method. A detailed description of the construction of the crystal growth system and growth details was reported earlier.9−11 Chromium was added in form of Cr2O3 in substitution for Al2O3 in quantities of up to 1 wt % in excess to appropriate molar ratios for BeAl2−xCrxO4. In the majority of experiments 5 wt % of Al2O3 was added to the solvent to lower the melting point of the flux. Exact batch compositions are presented in Table 1. A typical feed weight of 50 g was loaded in a molybdenum (MCh type) crucible of 26 mm height and an outer diameter of 47 mm. Crystal growth was performed with a cooling rate of 0.5−4 K/h and a rotation rate of 1−5 min−1 under Ar atmosphere of 1.2 bar. Pulling speed was about 0.2−1 mm/h for crystals grown with the Stepanov
Table 1. Solvent Batch Composition for Growth of Alexandrite Crystals (BeO)·[(Al2O3)1−x·(Cr2O3)x]a x
wt % BeO
wt % Al2O3
wt % Cr2O3
wt % Cr
0.001 0.002 0.004 0.008
19.69 19.68 19.67 19.64
80.19 80.08 79.85 79.41
0.12 0.24 0.48 0.95
0.04 0.11 0.20 0.53
The Cr content according to WDX results obtained from the final crystals is given in the last column. a
Received: March 13, 2012 Revised: June 26, 2012 Published: July 9, 2012 3954
dx.doi.org/10.1021/cg300344g | Cryst. Growth Des. 2012, 12, 3954−3956
Crystal Growth & Design
Article
method. Growth parameters (charge composition, temperature gradient, and rotation rate) for the Kyropolous method were the same as for the Stepanov method, but the Kyropolous method was carried out without application of pulling. Growth times of 24−30 h resulted in alexandrite crystals of 35−40 g, corresponding to alexandrite yields of up to 75% of the feed weight. The Cr content in the resulting crystals was measured by wavelength dispersive X-ray spectroscopy (WDX) analysis using a Rigaku Supermini X-ray fluorescence spectrometer (Table 1). For powder X-ray diffraction pieces of alexandrite single crystals with chromium concentrations up to 0.53 wt % were powdered. Patterns were taken in transmission mode on a STADIP diffractometer (STOE) equipped with a solid state strip detector MYTHEN 1K (DECTRIS) using Mo Kα1 radiation in the 2θ range from 10° to 70°. Alexandrite crystals have been etched using molten PbO·B2O3 flux. The most accurate pictures of distribution of etch pits have been obtained within 30 s at 950 °C. Etch pits distributions were studied using optical microscopes as well as scanning electron microscopy (SEM) on a Jeol JSM-6460LV. For the recording of optical absorption spectroscopy (UV−vis) spectra a TIDAS (J & M) diode array spectrometer was used with a cell holder specifically adapted to the measurement of large crystals. Alexandrite crystals were cut to plan parallel plates with a thickness of about 2.5 mm and the surfaces were polished. The crystals were adjusted in the incident light beam so that minimal reflection resulted. The spectra thus obtained at ambient temperature were rather insensitive to small changes in the orientation of the crystals.
Figure 1. Alexandrite single crystals with different Cr concentrations grown by the Stepanov (two cylinders in the upper left) and Kyropoulos methods (upper right crystal 0.04 wt % Cr, down right 0.11 wt %, down left 0.20 wt %, upper left 0.53 wt % (two cylinders)).
Table 2. Unit Cell Parameters of Alexandrite Single Crystals with Different Cr Content
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RESULTS AND DISCUSSION It was reported that alexandrite possesses a high-temperature βmodification stable from 1853 °C to the melting point (1870 °C).7 This may be the reason for the observed difficulties in growing single crystals of the room temperature α-modification from alexandrite melts. In the present work high-temperature solution (flux) growth methods using molten alexandrite with addition of some aluminum oxide were used to reduce the melting point and allow the crystal growth at temperatures of 1800−1853 °C.8 Depending on the exact technique used, crystals of 120 mm in length and up to 12 mm in diameter (Stepanov edge-defined film-fed growth method) or 30 mm in length and up to 50 mm in diameter (Kyropoulos method) with different Cr contents were obtained. Figure 1 shows selected alexandrite single crystals with different Cr concentrations grown by the Stepanov and Kyropoulos methods. The homogeneity of the Cr distribution across the obtained crystals was checked by EDX measurements in the central and peripheric areas of cross-sectioned samples. The Cr concentration varies between different samples but stays the same within the error of the measurement. Because of the rotation of the crystal in the melt, alexandrite crystals grown using the Kyropoulos technique were found to have a slightly more uniform Cr distribution across the crystal than those from the Stepanov method. All studied samples presented powder X-ray diffraction patterns in agreement with pure alexandrite. No additional reflections were observed. Table 2 gathers the refined unit cell parameters. The increasing unit cell parameters are readily explained by the larger ionic radius of Cr3+ as compared to Al3+;2 thus powder X-ray diffraction can be used for fast Cr content determination. Figure 2 presents a typical etch pits distribution by optical microscopy, and Figure 3 presents the shape of typical etch pit by SEM for all obtained crystals. According to the obtained images, the maximum etch pits density does not exceed 105 cm−2. In cross section we found an etch pits density of 102−103
wt % Cr
a/Å
b/Å
c/Å
V/Å3
0.04 0.11 0.20 0.53
9.3882(7) 9.3909(4) 9.392(1) 9.3934(6)
5.4669(4) 5.4678(2) 5.4686(6) 5.4687(2)
4.4191(4) 4.4200(2) 4.4212(4) 4.4209(2)
226.81(2) 226.96(1) 227.07(3) 227.10(1)
Figure 2. Optical microscopy image of etch pits distribution on alexandrite single crystals (exemplary figure of one crystal grown by the Kyropolous method; images of all other crystals including those grown from Stepanov techniques appear basically identical).
cm−2 and 104−105 cm−2 nearby the seed, similar to data earlier presented in the literature.12 The etch pits distribution is rather 3955
dx.doi.org/10.1021/cg300344g | Cryst. Growth Des. 2012, 12, 3954−3956
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spectra even revealed that this line consists of two closely neighboring bands due to the two Al sites substituted by Cr.16 The transitions at wavelengths below 300 nm were regarded to charge transfer or to transitions to higher levels of the 3d3 configuration.17
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CONCLUSIONS Large, high-quality alexandrite crystals with different homogeneous Cr distributions were grown from high-temperature molten flux. The maximum etch pits density was determined to be 105 cm−2. Unit cell parameters reflect the Cr content. The quality of crystals grown with the Kyropolous and Stepanov methods is similar. However, crystals produced with the Stepanov technique generally have an economic advantage, since the loss of valuable single-crystalline raw material at the machining stage is much smaller than for crystals obtained from the Kyropolous method.
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AUTHOR INFORMATION
Corresponding Author
*Tel. 0711-685-64217. Fax 0711-685-64241. E-mail rainer.
[email protected].
Figure 3. SEM image of a typical etch pit on alexandrite single crystals (exemplary figure of one crystal grown by the Kyropolous method; images of all other crystals including those grown from Stepanov techniques appear basically identical).
Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We thank Dr. Brigitte Schwederski (University of Stuttgart) for collecting the optical spectroscopy data.
homogeneous for different crystallographic directions. The low defect concentration found in etching experiments is in good agreement with earlier investigations on heat conductivity on the presented alexandrite crystals revealing k = 29 W/(m·K) at 300 K and a temperature dependence typical for dielectrics.13 Figure 4 shows the absorption spectrum of polished alexandrite crystals with different Cr contents in the UV−vis
REFERENCES
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Figure 4. UV−vis spectra of polished alexandrite crystals with various Cr contents. The legend indicates the Cr content in wt %.
range. Two main absorption bands were observed in the visible range. The absorption at about 419 nm was assigned to the 4A2 → 4T1 transition and exhibits only little change in energy with Cr content (maximum varying from 414 to 422 nm) (e.g., refs 14−17). However, the second transition at about 580 nm, assigned to the 4A2 → 4T2 transition, increased in wavelength from λmax = 578 nm (0.04 wt % Cr) and 575 nm (0.11 wt % Cr) to 588 and 587 nm for the 0.20 and 0.53 wt % Cr samples, although the shift is small and the scatter considerable. Both transitions significantly gain intensity with increasing Cr content. A further small band at 679 nm was earlier attributed to the absorption of the R line. A detailed study of the emission 3956
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