CRYSTAL GROWTH & DESIGN 2009 VOL. 9, NO. 2 671–675
Articles X-ray Topography of Hydrothermally Grown Emerald Single Crystals Using Laboratory and Synchrotron Sources R.-U. Barz*,† and T. B. Bekker‡ Crystallography Section, Faculty of Earth Sciences, Ludwig-Maximilians-UniVersita¨t Mu¨nchen, Theresienstrasse 41, 80333 Mu¨nchen, Germany, and Institute of Mineralogy and Petrography, Siberian Branch of Russian Academy of Science, Prosp. Acad. Koptyug 3, NoVosibirsk, 630090, Russia ReceiVed June 8, 2007; ReVised Manuscript ReceiVed May 9, 2008
ABSTRACT: Hydrothermally grown emerald crystals have been investigated by X-ray topography methods using both laboratory and Synchrotron sources. Because of the disintegration of the given nonsingular seed face (556) into smaller pyramidal faces, growth sectors have been formed. These are accompanied by growth bands consisting of the isochrones of the dopants that occur because of the fluctuating hydrodynamic conditions during the growth process. Both kinds of defects have been topographically detected and are discussed referring to their appearance on the imaged topographs.
1. Introduction Because of the sporadic character of the convective mixing in hydrothermal solutions, crystals grown under such conditions commonly show growth zonalities, which indicate the varying growth conditions. Therefore, the analysis of such so-called striations or growth bands in a grown crystal gives an idea of the chronology of the respective crystallization process, which is especially useful when a method is applied where it is rather difficult to observe the crystal growth directly. In hydrothermal processes, this is valid due to the occurring relatively high pressures making it necessary to use thick-walled sealed autoclaves. On the other hand, these growth zonalities represent defect structures in a crystal and should be minimized as much as possible if one wishes to grow perfect single crystals. In emerald, which is chemically beryl Al2Be3Si6O18 with Al3+ being substituted up to certain concentrations by other trivalent elements (e.g., Cr, Fe, V), the growth bands occur due to local alterations of the dopant concentrations caused by the change of the effective microscopic growth velocity. This results in varying optical properties, and therefore, the growth bands can be resolved by optical microscopy or even with the naked eye. Furthermore, the mutual substitution of ions having slightly different sizes leads to corresponding changes of the unit cell parameters, and consequently to some strain within the structure. Thus, the resulting stress fields may be detected by X-ray topography methods. * To whom correspondence should be addressed. Present address: SiCrystal AG, Gu¨nter-Scharowsky-Str. 1, D-91056 Erlangen. E-mail: ralph-uwe.barz@ sicrystal.de. † Ludwig-Maximilians-Universita¨t Mu¨nchen. ‡ Siberian Branch of Russian Academy of Science.
The study reported here has been carried out on emerald single crystals grown under hydrothermal conditions on seed plates, which have been cut parallel to the (556) plane. This seed orientation has been chosen because relatively high growth rates can be achieved normal to this face. However, (556) does not occur as a growth face in beryl crystallization, and consequently, the growth on such seeds is realized by a disintegration of the originally flat plane into small pyramidal faces being terraces between (556) and other major faces.1 These microfaces are indicated by the growth bands within the crystals appearing as zigzag lines in their projections in optical micrographs. Thus, growth sectors occur, which are determined by the shift of different individual microfaces. At the boundaries between such sectors, the defect density is usually increased, which can be observed either directly or via their strain field. Whereas the characterization of natural beryl crystals by X-ray topography has already been the subject of several papers,2-5 no such publications have been found on synthetic beryl crystals. It is the aim of this paper to report on the characterization of growth defects in synthetic hydrothermally obtained beryl by topography using both a laboratory as well as a Synchrotron source.
2. Experimental Conditions Sample Preparation. The crystals under investigation have been grown by recrystallization of natural beryl charges from the Izumrudny Kopi gem mine (Ural mountains) by the standard autoclave method6 as well as under the action of rotating heat fields, which have been applied to regularize the convective flow and, thus, finally to decrease the temperature fluctuations within the solution.7 The source charge and the seed plates were placed in an autoclave with an outer diameter
10.1021/cg070521z CCC: $40.75 2009 American Chemical Society Published on Web 12/12/2008
672 Crystal Growth & Design, Vol. 9, No. 2, 2009
Barz and Bekker
of 80 mm, an inner diameter of 30 mm, and a height of 290 mm. Its filling was chosen to result in a pressure of 1.2-1.5 kbar for pure water at the growth temperature.8 The sealed autoclave was then heated to an average temperature of about 600 °C. The temperature difference between the dissolution and the crystallization zone was between 35 and 50 K with the higher temperature being applied to the lower part of the autoclave (620-640 °C) and the lowest temperature occurring outside of the growth zone (580-590 °C). Use was made of seed plates, which usually have been cut parallel to the nonsingular (556) plane. The duration of the growth runs was about 30 days. Prior to the topography experiments, the samples have been appropriately oriented and cut using a diamond wire saw before the surfaces were mechanically polished. Plates having been cut parallel to (001), (11j0), (110), and (113j), which is nearly perpendicular to the given seed surface (556), have been investigated. Lang Topography. Laboratory X-ray topography experiments have been carried out by the Lang method in transmission mode (Laue case). Use was made of Mo KR1 radiation, which was maintained by filtering (155 µm Zr) and narrowing the horizontal width of the beam below a maximum value smax according to
smax ) l · tan(θ2 - θ1)
(1)
with l being the distance between the beam source and the slits and θ1 and θ2 being the Bragg angles of the applied reflections belonging to Mo KR1 and Mo KR2 radiation, respectively. Then the samples were adjusted to meet the Bragg conditions of such net planes, which are inclined to the surface by an angle close to 90°. The topographs were imaged on Agfa Structurix D3 films. White-Beam Topography. The white-radiation topographs were taken using the topography equipment at the Fluo-Topo beamline of the synchrotron light source ANKA, Karlsruhe. The experimental station is situated about 30 m from the tangent point of the ring. The 2.5 GeV second generation storage ring delivers radiation with a characteristic wavelength of 2 Å.9 Taking into account the absorption by the 700 mm air path of the primary beam between the Be-window and the sample, the maximum intensity is found at 1.1 Å while more than 10% of this intensity can be expected between 0.35 and 2.3 Å. Imaging was done both in transmission (Laue case) as well as in back-reflection mode (Bragg case) on Agfa Structurix D3 films with typical sizes of 180 × 130 mm2. The distance between sample and film was chosen to 110 mm and the beam size was 4 × 4 mm2, which offers a reasonable geometrical resolution with a sufficiently large lighted part of the sample. Under these conditions, overlaps of the topographs have been widely avoided.
3. Results and Discussion Indexing of the Laue Patterns. Whereas the respective reflection used for imaging with Lang-topography is inherently known from the adjustment of the sample, the characteristics of the white-beam method lead to the simultaneous occurrence of several topographs, forming patterns known from the common Laue method. Indexing of these patterns have been done with the help of the simulation software LAUEPT,10 while taking into account the characteristics of the beam line. The results have been checked by geometrical considerations based on the angles between the surface under investigation and the reflecting net plane. Because of the characteristics of the beamline, the intensity of the primary beam contributing to the reflection can be estimated from the spectrum given in ref 9. Together with the structure factor (Table 1), a first approximation of the reflected intensity resulting in an individual topograph can be obtained. Considering the point group of beryl 6/mmm, the symmetry of the Laue pattern can be deduced for the different observation directions. Consequently, the pattern imaged with the primary beam being parallel (001), (11j0), and (113j) show symmetry 6mm, 2mm and m, respectively, as it is shown for two examples in Figure 1. Tilting of the Seed. In Figure 2, the Lang topograph of a crystal can be seen, where only the diffraction conditions for
Table 1. List of Strong Reflections in the Laue Topographs of Emeralda hkl
λ [Å]
Fc
2.1519 1.7110 1.4567 1.4162 1.3684 1.2776
1.008 0.752 0.921 0.436 0.815 0.710
29.9 47.0 68.6 25.9 50.2 105.2
78.718 0.9014 sample surface (11j0)
1.768
55.1
1.988 1.140 1.029 0.797 0.611 0.420 0.807 0.794 0.935 0.614 0.602 0.601 0.727
54.5 22.2 86.8 50.1 25.7 49.1 59.7 70.6 47.5 82.2 74.2 55.3 69.3
2θB [°]
d [Å]
sample surface (001) transmission mode 311 411 332 511 512 602 back-reflection mode 1 1 10 transmission mode 102 210 211 212 104 320 313 2 1j6 1 3j 6 520 521 415 3 1j 7 transmission mode 4 3j 0 1j 4 1 5 2j 0 062 9 5j 1j back-reflection mode 3 3 6j 0 5 8j 5 2 7j 0 6 1j0j
13.539 10.728 18.478 8.863 17.321 16.140
14.450 10.893 10.344 9.091 7.953 6.587 12.993 15.858 20.018 13.898 13.763 13.747 17.562
3.9827 3.0163 2.8660 2.5217 2.2083 1.8308 1.7942 1.4537 1.3659 1.2779 1.2657 1.2643 1.2040 sample surface (113j) 7.670 11.283 19.341 13.436 19.655
2.2134 2.1519 1.8296 1.2776 1.0155
0.591 0.830 1.213 0.594 0.683
30.9 29.9 49.1 105.2 45.7
78.685 72.877 74.984 73.268
1.0846 0.9325 0.9158 0.7562
2.127 1.782 1.769 1.448
70.2 45.6 63.2 90.1
a h k l: reflecting net plane, 2θB: diffraction angle, d: distance of the reflecting net planes, λ: wave length of the reflected beam, Fc: calculated structure factor.
the grown parts on both sides of the original seed plate have been met. When analyzing the original seed crystal, however, the maximum reflected intensity has been found by turning the sample about 0.1°. As shown by rotating the sample around the direction of the primary beam by 180°, this results from a tilting of the grown crystal against the original seed crystal, which is caused by the use of an undoped, that is, colorless beryl crystal for the seeding. Since the experimental conditions restrict the horizontal divergence of the primary beam to below 0.035°, the range of this tilting angle can be estimated to be 0.065° e ∆ e 0.135°. Further, the difference of the reflection angle between the seed and the grown crystal due to the different lattice parameters can be calculated to be smaller than 0.01° (with aseed ) 9.209 Å, cseed ) 9.197 Å, acrystal ) 9.232 Å, ccrystal ) 9.198 Å - data taken from powder diffraction measurements). Thus, the misorientation of the growing crystal relative to the seed has to be considered as the cause of the observed difference of the diffraction conditions. Imaging the same crystal with white-beam topography, the different orientations between the seed and the grown crystal yields a shift of the respective images in the topographs (Figure 2). The tilt angle ∆ can be found from this shift by
(
a · ∆x 1 ∆ ) a tan 2 2 (a - x2) + x · ∆x
)
(2)
where a is distance between sample and film, x is distance
Hydrothermally Grown Emerald Single Crystals
Crystal Growth & Design, Vol. 9, No. 2, 2009 673
Figure 3. White beam topographs of emeralds normal to the (113j) face (a) Bragg case and (b) Laue case. The position of the individual topographs has been shifted against the original Laue pattern. In the topograph due to reflection 527j a single dislocation line can be seen, which has its origin in the seed and runs into the grown crystal.
Figure 1. Pattern of reflections obtained by white-beam topography of emerald (a) normal to (001) (Laue case) and (b) normal to (113j) (Bragg case), including the respective guide for indexing.
Figure 2. Lang-topograph due to reflection 112 and white-beam topograph of a beryl sample being tilted to the original seed crystal.
between primary beam and image of the grown crystal on the film, and ∆x is gap between the images of the edge between the seed and the grown crystal) when a reflecting net plane is considered that belongs to zone [110]. With the values taken from a back-reflection topograph due to reflection 336j (a ) 100 mm, x ) 45 mm, and ∆x ) 0.3 mm) a tilt angle of 0.1° can be obtained, which is the same as it has been observed while using Lang topography. Growth Sector Boundaries and Growth Bands. As mentioned earlier, the growth uses seed plates with their major faces being parallel to (556), which does not occur as a singular face. Instead, the shift of the growth interface is maintained by the disintegration of the given face into small growth faces. These
are mainly pyramidal faces belonging to forms {212} and {312} that are slightly inclined against the originally given face, thus, being vicinales to the given seed plane.1 Because of the growth of the pyramidal faces, growth sector boundaries are observed, which are caused by the locally increased density of inclusions and other defects at the edges formed by the respective neighboring faces. The increased defect densities at these boundaries are the origins of deformation of the adjacent crystal regions, which can be observed by X-ray topography. The strain fields of these defects are resolved when using both kinds of sources, that is, by the Lang- as well as by the white-beam method (Figures 2-4). The number of the growth sector boundaries has been found to decrease during the growth process, which is equivalent to an increasing size of each growth sector. This can be explained taking into account the commonly accepted growth mechanism from solutions, where after the formation of a twodimensional seed on an atomically flat surface, the growth toward the edge of the face happens rather rapidly. Considering the existence of two growth faces at the growth sector boundary, the enlargement of one face can be understood by the partial overgrowth of the adjacent face, which may be repeated after forming the next surface seed. Looking at the individual growth sectors in more detail, a second defect type can be found. Because of the varying hydrodynamic conditions in the solution, the microscopic growth rate changes continuously, thus, leading to an alteration of the effective distribution coefficient, and consequently, of the dopant concentration within the crystal. These local composition variations, which are further referred to as striations, can also be observed by X-ray topography due to the accompanying strain fields within the crystal structure. However, these defects cannot be resolved in the Lang topographs taken under the given conditions, which is assumed to be a consequence of the apparatus resolution of the laboratory equipment on hand. Furthermore, the restriction to the transmission mode with the laboratory equipment leads to a superposition of all defect strain fields that occur in the transmitted bulk of the crystal. This influence on the geometrical resolution of the growth bands is confirmed by comparing white-beam topographs taken in the transmission and the back-reflection mode, respectively. Thus, the striations can be resolved in all samples imaged in back-
674 Crystal Growth & Design, Vol. 9, No. 2, 2009
Barz and Bekker
Figure 4. White beam topographs of emeralds normal to the (001) face (a) Bragg case and (b) Laue case. The position of the individual topographs has been shifted against the original Laue pattern.
reflection mode, which is because only a relatively thin part near the surface contributes sufficiently to the image. In the transmission mode, however, it turned out that even with the small divergence of the beam provided by the Synchrotron source the detection of the growth bands is affected. This can be seen in Figure 3 where striations are resolved in the Bragg case and are not visible in the Laue case. In one of the crystals, very strong variations of the growth temperatures have been applied in order to induce rather distinguished striations. Therefore, the growth bands can be observed in topographs taken under both Laue and Bragg conditions (Figure 4). The growth bands are resolved in all symmetrically equivalent reflections taken in the Bragg case. While comparing transmission topographs of symmetrically equivalent reflections, however, the growth bands as well as the growth sector boundaries cannot be resolved in all topographs. This is due to the different components of the strain fields induced by the defect structures and their relations to the diffraction vector. In an ideally flat growth band the distortion is normal to the growth front. That is, the growth band is invisible when
b g ·b n )0
(3)
where b g is the diffraction vector, and b n is the vector parallel to the growth direction.11 However, this ideal condition is not met in our topographs while taking into account the orientation of the growth pyramid surfaces and the reflections found on the Laue pattern. Nevertheless, the origins of the imaged contrasts are the alterations in the lattice geometry due to the respective dopant content, and therefore, the resulting strain fields can be assumed to have their maximum components in growth direction and only smaller components parallel to the growing faces. Consequently, topographs imaged by reflections of net planes nearly parallel to the growth interface verticals show rather strong contrasts. This is equivalent to the scalar product of the respective normalized vectors being close to 1 (e.g., reflections 1j51, 141, 411, and 51j1 in Figure 4). The components of the strain fields perpendicular to the diffraction vectors of other reflections are not that strong since they are not parallel to the growth direction. Thus, the growth bands yield only weak contrast in the referring topographs. Because of the rather massive occurrence of the described two-dimensional defects other real structure defects can only be observed occasionally within the grown crystals. As it can
be seen in the back-reflection topograph due to the 527j reflection in Figure 3, the white beam topograph can resolve single dislocations, if their strain field does not show much influence from other disturbances of the crystal real structure.
4. Conclusions The study of hydrothermally grown crystals by topographic methods using X-radiation from both a laboratory (Lang topography) and a Synchrotron source (white-beam topography) has shown the common defects of the crystals. Mainly, twodimensional defects referred as growth bands and growth sector boundaries have been observed. Whereas growth sector boundaries are resolved both in transmission as well as in the backreflection mode, the growth bands could only be observed by white-beam topography. The contrast of the imaged strain fields of the growth bands has been found to depend on the reflection conditions. By considering the correspondence between the defect structure and the applied growth conditions, the obtained results are now to be used for the evaluation and optimization of the hydrothermal growth process. Acknowledgment. The authors thank Mrs. R. Enders for sample polishing as well as A. Danilewsky and R. Simon for assistance with white-beam topography. Financial support by ANKA (beam time) and INTAS (Grant No. 03-55-1212) is greatly acknowledged.
References (1) Bekker, T. B.; Barz, R.-U. Cryst. Growth Des. 2007, 7, 1898. (2) Graziani, G.; Scandale, E.; Zarka, A. J. Appl. Crystallogr. 1981, 14, 241. (3) Herres, N.; Lang, A. R. J. Appl. Crystallogr. 1983, 16, 47. (4) Yoshimura, J.; Koishi, Y.; Suzuki, C. K. J. Cryst. Growth 1985, 73, 275. (5) Jiang, S. S.; Li, Q.; Xu, X. Y. J. Cryst. Growth 1992, 116, 93. (6) Wilke, K.-Th.; Bohm, J. Kristallzu¨chtung; Verlag Harry Deutsch: Thun Frankfurt/Main, 1988; pp 978, 1054. (7) Thomas, V. G.; Demin, S. P.; Foursenko, D. A.; Bekker, T. B. J. Cryst. Growth 1999, 206, 203. (8) Rivkin, S. L.; Alexandrov, A. A. Thermodynamic Properties of Water and Water Vapor; Energiya, Moscow, 1980, p 423 (in Russian).
Hydrothermally Grown Emerald Single Crystals (9) Simon, R.; Danilewsky, A. N. Nucl. Instrum. Methods Phys. Res., Sect. B 2003, 199, 550. (10) Huang, X.-R. Lauept: White-Beam X-ray Diffraction Patterning; Copyleft: Stony Brook, NY, 2003.
Crystal Growth & Design, Vol. 9, No. 2, 2009 675 (11) Tanner, B. K. Science of the Solid State; Pergamon Press: Oxford, 1976; Vol. 10, p 95.
CG070521Z
