Surface Crystallization of Fresnoite from a Glass ... - ACS Publications

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Surface Crystallization of Fresnoite from a Glass Studied by Hot Stage Scanning Electron Microscopy and Electron Backscatter Diffraction Wolfgang Wisniewski,* Christian Bocker, Maher Kouli, Markus Nagel, and Christian Rüssel Otto-Schott-Institut, Jena University, Fraunhoferstrasse 6, 07743 Jena, Germany W Web-Enhanced Feature *

ABSTRACT: Polished pieces of a glass with the composition 2BaO·TiO2·2.75SiO2 (fresnoite + 0.75SiO2) were annealed at 790−970 °C and subsequently analyzed using X-ray diffraction (XRD) and scanning electron microscopy (SEM) including electron backscatter diffraction (EBSD). Surface morphologies and textures of samples annealed in a normal furnace are presented. While the texture of oriented nucleation occurs over the entire temperature range, the development of previously undetected secondary textures is described and explained. Insitu high temperature video sequences visualize the growth and a two-step nucleation of fresnoite. The velocity of crystal growth along the surface in the vacuum is measured and compared to the mean velocity of three-dimensional crystal growth in an ambient atmosphere. For example, growth velocities along the surface reach 3.6 μm/min in the vacuum at 830 °C compared to 0.16 μm/min during growth into the bulk.

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The term “oriented nucleation” was first used to explain the texture formation during the secondary recrystallization of metals after rolling in an historic experiment that was recently re-evaluated.17 While there still seems to be some controversy whether this effect is caused by oriented nucleation or selective growth,17−19 there seems to be a consensus that the driving force for this effect is the reduction of the grain boundary energy.18 The oriented nucleation observed in glasses is unlikely to be caused by exactly the same effects as there are no grain boundaries in the amorphous matrix where nucleation occurs. Instead, the orientation of a nucleus at the immediate glass surface is most probably governed by the minimization of the surface energy of the crystal versus the surrounding atmosphere. Additionally, the oriented nucleation discussed in metals leads to the formation of a macroscopic texture, while the textures of oriented nucleation detected in glass ceramics have never been stable in the cases analyzed so far but always been changed to other orientations by growth selection.6−8,10,12,13 Of course, energy minimization plays a role in the process as every nonstatistical effect needs to have a driving force, but with respect to the oriented surface nucleation in glasses it remains unknown whether this is a thermodynamic or a kinetic effect. In situ observations of glass crystallization are possible in a scanning electron microscope (SEM). A hot stage inside the

INTRODUCTION The crystallization of glasses under controlled conditions leads to a class of materials denoted as glass ceramics. This offers the possibility of combining glass properties, such as mechanical and thermal stability or the castability into nearly any form, with advantageous physical crystal properties.1 In certain compositional ranges, glasses do not crystallize in the bulk but exhibit sole2−9 or preferred10−13 nucleation at the surface or some nanometers below it. While the same phase is usually formed at the surface and in the bulk, the recent surface crystallization of YAG from a glass melt was followed by the independent bulk nucleation of a star-shaped phase which subsequently triggered the growth of three yttrium silicates.11 Surface crystallization is especially observed when large density changes during crystallization lead to notable stresses in the material which may even result in the formation of pores.11,14,15 Stress relaxation is easier at or near the surface, enhancing nucleation. Higher surface nucleation rates may also occur if the interfacial surface energy crystal/atmosphere is smaller than the interfacial energy crystal/glass. However, as some systems exhibit nucleation some nanometers below the surface,2−4,12,13,16 the interfacial or surface energies cannot be the only decisive effect. While, for example, cordierite, YAG, and cristobalite fulfill the classic nucleation theory by showing statistically oriented nuclei at the surface,3−5,11 although a stress-induced texture may be described for the surface crystallization of cristobalite in pure SiO2-glass,5 oriented nucleation has also been described in a number of systems including fresnoite.6−8,10,12,13 © 2013 American Chemical Society

Received: May 25, 2013 Revised: June 27, 2013 Published: July 2, 2013 3794

dx.doi.org/10.1021/cg4008087 | Cryst. Growth Des. 2013, 13, 3794−3800

Crystal Growth & Design

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RESULTS AND DISCUSSION Figure 1 presents an XRD pattern obtained from the casted glass which does not show any peaks and is hence X-ray

microscope chamber enables heating of the glass and monitoring of the formation of crystals at the surface.20−22 Ba2TiSi2O8-fresnoite (BTS) possesses the space group P4bm. Ti4+ forms pyramids which all show the same orientations within the unit cell. This leads to a macroscopically polar structure and to glass ceramics that show interesting piezoelectric, pyroelectric, and surface acoustic wave properties.23,24 In contrast to ferroelectric materials, fresnoite has a notably smaller relative dielectric constant. It is thus a favorable material for high frequency applications. Highly oriented fresnoite materials may also be obtained by electrochemically induced crystallization.25−27 This article presents ex situ and in situ scanning electron microscopy (SEM) experiments concerning the fundamental crystallization of glasses by the example of BTS formation from a glass with the chemical composition 2BaO·TiO2·2.75SiO2. Crystal orientations are determined using electron backscatter diffraction (EBSD). Stoichiometric BTS glass shows one of the largest nucleation rates reported for glass-ceramic systems28 and high growth velocities, which is why the additional SiO2 is necessary for controlled crystallization and to avoid spontaneous crystallization during glass quenching. This also enables visualization of crystal growth in the SEM hot stage experiment in an appropriate time scale.

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EXPERIMENTAL SECTION

A glass of the composition 2BaO·TiO2·2.75SiO2 (BTS + 0.75 SiO2) was prepared from BaCO3, TiO2, and SiO2 (quartz). The compounds were mixed and subsequently melted in a 250 mL platinum crucible at a temperature of 1550 °C using an electric furnace with MoSi2 heating elements. After the melt was kept at 1550 °C for 1 h, it was stirred for 1 h and subsequently poured on a copper block. The melt was rapidly quenched using another copper plate to avoid crystallization. In a series of experiments, the glass samples were annealed at temperatures from 790 to 970 °C for up to 20 h using an electric furnace. These samples were characterized by X-ray diffraction (XRD, Siemens D5000) using Cu Kα radiation. The XRD patterns were recorded directly from the surface of annealed samples or their cross sections without powdering. The samples where further characterized using a Jeol JSM-7001F FEG-SEM equipped with a TSL Digiview 3 EBSD camera. EBSD scans were captured and evaluated using the programs TSL OIM Data Collection 5.31 and TSL OIM Analysis 5.31. All EBSD measurements were performed using an accelerating voltage of 20 kV. To achieve a conductive surface, the samples were mounted using Ag-paste and coated with a thin layer of carbon at about 10−3 Pa. All EBSD measurements presented here were performed on the annealed surfaces without any further treatment apart from the carbon coating. The SEM was equipped with a GATAN heating stage H1004 for a series of in situ experiments during which a shield-shutter mechanism protects the objective pole-piece and the electron detector at high temperatures. The samples were placed in a crucible (molybdenum alloy) positioned within a ceramic furnace (corundum) and electrically heated by tantalum wires. This setup enables temperatures of up to 1250 °C. Glass samples 5 mm in diameter and 0.5 mm thick were used for the in situ measurements. Both sides were polished including a final finish using colloidal silica. The samples were heated using a rate of 10 K/min and could be studied at temperatures above 600 °C without charging effects due to the electric conductivity of the glass at these temperatures. The SEM was set to an acceleration voltage of 20 kV and a probe current of 0.89 nA for the in situ measurements. An Everhart−Thornley detector with a collector voltage of +300 V was used for imaging. The scanning speed was set to approximately 1 min for recording one micrograph.

Figure 1. XRD patterns obtained from the casted glass as well as from the surfaces of samples thermally annealed at temperatures in the range from 790 to 850 °C and their respective cross sections.

amorphous. Patterns obtained from cross sections and the solid surfaces of samples annealed for 20 h at the stated temperatures are also presented along with the theoretical pattern of statistically oriented BTS (JCPDS 70-1920). While the peak intensities of the cross sections all indicate the presence of untextured BTS, the patterns obtained from the solid surfaces all feature exaggerated 00n-peaks, indicating a preferred orientation with the c-axes perpendicular to the surface. This is in agreement with the results of ref 10. As outlined in ref 10, the 001 texture detected by XRD is only observed after about 7 μm of crystal growth into the bulk. The topmost 101-oriented layer of crystals caused by oriented nucleation at the surface10 fails to be detected with this XRD setup due to its comparably large information depth. Pieces of the polished glass were annealed at temperatures from 790 to 970 °C and analyzed by EBSD to see if oriented nucleation occurs and whether it is affected by the annealing temperature. The highest temperature of 970 °C was supplied to allow a comparison to the oriented nucleation observed in the corresponding Sr2TiSi2O8 (STS) system.7,8 Figure 2 shows SEM micrographs of the surfaces of samples annealed for 10 h at the stated temperatures as well as 001-pole figures (PFs) of textures calculated from performed EBSD scans. Each texture is based on at least 25 000 data points of reliably indexed Kikuchi patterns. Histograms of the Euler angle Φ are also presented. This angle describes the tilt of the c-axes of the tetragonal unit cell in the given setup: Φ = 0, 180° means the c-axis is perpendicular to the surface while Φ = 90° indicates a c-axis parallel to the surface. At Φ = 60° and 120°, the [101]-direction of the unit cell is perpendicular to the surface. 3795



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the topmost layer of crystals would be subjected to tensile stresses which could be relaxed by elongated crystals with their [101]-direction perpendicular to the surface shifting by 30° into a position where the c-axis is parallel to the surface as outlined in Figure 3. Local compressive stresses could lead to a few

Figure 3. Orientational preferences indicated by the texture analysis of Figure 2, possibly caused by tensile and compressive stresses during crystal growth.

crystals shifting into positions where their c-axis is perpendicular to the surface. The fact that these multiple textures predominantly appear in a certain temperature range but not at temperatures above and below this range may be due to the viscosity of the glass matrix. Below a certain temperature, the viscosity may be too high to allow crystal reorganization, while above a certain temperature the viscosity is low enough to allow stress relaxation without crystal reorganization, or the high growth velocities may simply lead to a fully crystallized sample before relaxation occurs. The cracks in the surface probably result from tensile stresses at the surface during cooling, and their appearance likely depends on the sample geometry. As in situ EBSD measurements cannot be performed in the given experimental setup, the analysis of the three orientation preferences possibly forming at three different times of the annealing processes must be postponed to future research. In situ experiments on surfaces polished using a final finish of colloidal silica were performed to visualize the actual crystal growth process during annealing. This polishing procedure was applied to reduce the nucleation rate as the latter is known to be affected by the polishing procedure,20 and chemomechanical polishing processes such as colloidal silica minimize residual stresses and scratches in the sample surface.31 The temperature/time schedule of a sample heated to a maximum of 790 °C during an in situ experiment is presented in Figure 4. The heating curve for the sample annealed at 830 °C is comparable. The dashed line represents the actual temperature of the sample which was measured by a platinum thermocouple positioned directly under the crucible. The crystallization behavior of the sample can be observed in an in situ video in AVI format published with this article (Movie S1). Image

Figure 2. SEM micrographs of samples annealed at the stated temperatures as well as the 001-pole figures of textures calculated from EBSD scans performed with a step size of 400 nm. The relative occurrence of the Euler angle Φ in the respective EBSD scans is visualized by the presented diagram.

The SEM micrographs show that crystal sizes are slightly smaller in the sample annealed at 790 °C. Annealing at 970 °C led to the formation of small cracks visualizing that tensions occurred in these samples. The density of BTS (4.43 g/cm3)29 is 7.5% larger than that of the glass (4.120 g/cm3)30 meaning crystallization is accompanied by a volume contraction. As crystallization is first initiated at the surface, the temperaturedependent viscosity of the residual glassy phase in the bulk may play a significant role in relaxing stresses formed during the initial crystallization. Hence, stresses in these samples may not only result from a mismatch in the coefficients of thermal expansion between the phases during cooling. The 001-PFs of the samples annealed for 790 and 810 °C show the clear ring previously reported for a crystallization temperature of 810 °C,10 and the corresponding Φ-distribution confirms that orientations with their c-axes tilted by about 30° from the surface normal (Φ = 60 and 120°) preferably occur. The Euler angle histograms presented in Figure 2 show that orientations with the c-axes more parallel to the surface (70° < Φ < 110°) occur more frequently than those with the c-axes more perpendicular to the surface. This effect is not clearly detected in the PFs. After annealing at higher temperatures, the texture notably changes as orientations with the c-axes parallel and perpendicular to the surface also become preferred. While the peak around Φ = 90° is well pronounced, the preference of Φ = 0, 180° is much lower, see arrows. This evolution of a double texture after annealing at higher temperatures may, for example, be explained by stress relaxation during initial growth. With increasing temperature, the viscosity of the glass matrix is lowered and crystals could rotate and shift more easily. Because of the anisotropic growth of BTS with higher growth rates along the c-axis, the resulting elongated crystals may relax stresses by shifting their position. As there is a volume contraction during initial crystallization,

Figure 4. The heating curve for the in situ crystallization at 790 °C. 3796



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recording was started immediately after reaching the desired temperature. During the observation time, the region of interest moved out of the frame due to a surprisingly strong drift of the electron beam before and during the formation of the first visible crystals. As far as the authors know, this effect is observed for the first time and up to now was not reported for other studied systems. The beam stabilizes during crystallization, slowly decreasing the amount of drift. One possible explanation for this initial drift could be the piezoelectricity of fresnoite. Since fresnoite is polar but not ferroelectric, it does not show a Curie temperature and hence does not lose its piezoelectricity with increasing temperature. Stresses formed during the early stage of crystallization could lead to charge transfers in the crystal lattice and hence an electrical field at the sample surface. The constant drift of the electron beam indicates a static field. As the crystals grow, this electrical field disappears, and the electron beam is stabilized. In order to remain at the same position on the sample surface, the micrographs were aligned after acquisition. The different micrographs where recorded by first focusing the beam using a reduced area and higher magnification. Then the micrograph with the magnification of 2000× was acquired followed by the micrograph with the magnification of 500×. Selected micrographs are shown in Figure 5. The first crystals

Figure 6. Feret diameter and minimum Feret diameter of the three crystals highlighted in Figure 5.

Figure 7. In situ SEM micrographs of a sample annealed at 830 °C after (a) 5 min, (b) 8 min, (c) 12 min, and (d) 30 min.

morphologically different from the other observed crystals. This particle was intentionally included in the region of interest as it greatly eases the focusing procedure and also visualizes the differences between the heterogeneous nucleation around the particle and the homogeneous nucleation of the other crystals. While the structures around the particle are comparatively diverse and imply a number of dendrites growing away from the particle, the other crystals are relatively compact, regularly shaped, and probably single crystals. Figure 7d shows a number of areas similar to that in frame 2 without a foreign particle in their centers. After 30 min, an almost complete crystallization was observed at the surface. It must be noted that a clearly bimodal size distribution of crystals is seen, again indicating two stages of nucleation. An effect of the electron beam on the sample surface, as, for example, the darkened area in Figure 5, was not observed. The sizes of the crystals 1 and 2 in Figure 7a were measured as a function of the annealing time and are presented in Figure 8. The initial growth rates are constant at 2.6 and 3.6 μm/min, respectively. After approximately 10 min, the growth velocities decrease rapidly until growth is stopped.

Figure 5. In situ SEM micrographs of a sample annealed at 790 °C after (a) 33 min, (b) 62 min, (c) 94 min, and (d) 158 min.

marked as 1−3 in Figure 5a are observed after annealing for 33 min. They continue to grow as irregular stars; see Figure 5b. The selected crystals do not increase in size after 94 min, but small crystals continue to nucleate around them. The experiment was stopped after 158 min because no further changes of the microstructure were detected. The dark area in the center of Figure 5c,d corresponds to the area where the higher magnification micrographs were recorded. While there seems to be no further nucleation between 33 and 70 min, tiny crystals with a much smaller growth velocity than the primary crystals appear after 72 min. The sizes of the crystals 1−3 in Figure 5 were measured as a function of annealing time by manually fitting an ellipsoid using the open source software ImageJ.32 Extrapolating the initial linear growth indicated in Figure 6 to the x-axis indicates all three crystals nucleated after 26−30 min. Repeating this experiment at 830 °C enables detection of the first crystals after about 5 min as shown in Figure 7 and in the in situ video in AVI format (Movie S2). There is a single bright particle (probably dust) in the upper left corner (frame 2) of the micrographs a−c around which crystal growth is

Figure 8. Feret diameter and minimum Feret diameter of the two crystals highlighted in Figure 7. 3797



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by the sample preparation (i.e., stresses and other surface defects introduced during mechanical treatment), and it is impossible to produce perfectly comparable surfaces. The enhanced nucleation around the particle in Figure 7 is not surprising as heterogeneous particles have for example been used to enhance nucleation in STS, which led to a faster orientation selection during growth and higher piezoelectric constants of the material.8 The number of crystals in a certain surface area increases with increasing annealing time, confirming the simultaneous nucleation and growth of crystals28 and making the number of nuclei dependent on the annealing time. The maximum nucleation temperature of the stoichiometric melt has been determined to be 740 °C.28 The grain size increases with the annealing temperature from 790 to 850 °C in Figure 2, which indicates decreasing nucleation rates with rising temperatures; that is, the supplied annealing temperatures should be above the temperature of the maximum nucleation rate. However, the sample annealed at 970 °C, which was polished separately from the other samples, shows smaller crystals, again highlighting the importance of comparable sample preparation for studies of nucleation rates which are, however, not the main topic of this article. The in situ experiments visualize that the morphology of heterogeneously nucleated crystals may be completely different from that of homogeneously nucleated crystals. They also prove that fresnoite nucleation may clearly still occur at temperatures as high as 830 °C in this system in a vacuum. An effect of the atmosphere during crystallization on crystal morphologies as well as on nucleation rates cannot be excluded, matching previous results where nucleation at pore surfaces was very different from that during initial YAG surface crystallization.11 Upon heating the samples to the desired temperatures, the first crystals were observed after about 26 to 30 min at 790 °C or only about 2 min at 830 °C. Some nucleation may also occur during heating despite the high heating rate (10 K/min). One explanation for the observed delay might be a nucleation time lag which has especially been described for isochemical systems in many publications.33−36 The nucleation time lag is for instance given by the Zeldovich equation (see ref 36) and is assumed to be caused by the structural rearrangement of the melt prior to nucleation. In the literature, the reason for this effect is still under discussion. The induction period is smaller at higher temperatures in the experimental and theoretical reports. An unbalanced temperature distribution is unlikely to cause the delay as it has been calculated that a sample of the geometry used here roughly needs 10 ms to increase its temperature by 10 K.22 It is noteworthy that the crystal growth velocities observed during the in situ experiments are more than an order of magnitude higher than those previously described using optical microscopy.37 While the latter were measured on a large number of crystals after annealing and represent the threedimensional (3D) growth of the crystals by using z-stacks, the in situ growth velocities detected here solely describe growth along the surface and do not include the nucleation time lag discussed above. Additionally, SEM analysis is much more suited to detect a thin layer of crystallization near the surface than optical microscopy. Hence, both methods have their advantages as SEM enables detection of specific growth velocities along the surface, while the results of ref 37 describe the mean 3D growth of a large number of crystals. Ostwald ripening was not observed within the studied time scale.

Extrapolating the slope of the constant growth velocity indicates primary nucleation occurred after about 2 min. In order to see if the bimodal size distribution observed in the in situ experiments is a result of the polishing procedure or an effect of the environment, that is, annealing the sample in the vacuum, samples polished using colloidal silica were annealed under ambient conditions. This should enable the comparison to those which produced the surfaces presented in Figure 2. Figure 9 presents the results obtained from a sample

Figure 9. SEM micrograph and texture of the sample polished using colloidal silica before annealing at 830 °C for 10 h.

after annealing at 830 °C for 10 h. The SEM micrograph enables discernment of drastically varying crystal sizes and hence nucleation rates; see, for example, white arrows. The traces of scratches, which leave a mechanically deformed zone in the polished surface, may clearly be discerned by the elevated nucleation rates in their vicinity. The superimposed IPF map of a performed EBSD scan shows that the prevailing orientations attributed to green and pink colors appear around these scratches as well as in areas with large and small crystals. The presented 001-PF of a texture calculated from the scan and matching the scale of Figure 2 shows the same double texture. Hence, it may be concluded that the formed textures are independent from the nucleation rate and the applied polishing procedures. The clearly bimodal size distribution is not observed. Hence, the secondary crystallization is only observed when the samples are annealed in the vacuum of the SEM. It is known that the atmosphere may affect the surface crystallization due to different surface energies, for example, by adsorption of foreign atoms (H2O) whose concentration is much smaller in the chamber of the SEM. However, this effect remains unclear. The results presented in this article confirm that the oriented nucleation of BTS with the [101]-direction perpendicular to the surface is observed over a large range of temperatures, that is, is probably temperature independent. However, the initial texture of nucleation may be changed by subsequent processes such as stress relaxation. The possibility of local stress induced texture modification has recently been shown to play a role in the surface crystallization of pure SiO2 glass.6 There seem to be some discrepancies between the results obtained from surfaces annealed under ambient conditions and in the vacuum: the clearly bimodal size distribution discernible in Figures 5 and 7 is not observed in Figures 2 and 9. Although the crystals annealed in the vacuum reach larger sizes indicating a lower nucleation rate, this may also be achieved by annealing samples polished using colloidal silica in a standard atmosphere; that is, this is not an effect of the vacuum but of the polishing procedure as already observed in the case of cordierite.20 The nucleation rate itself is known to be affected 3798



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a narrow crystal size distribution homogeneously dispersed in the glassy matrix.39−42 Sample modification during data acquisition is a general problem for in situ measurements, not only in electron microscopy. The darkening of the micrographs of Figure 5 and hence the decreased yield of secondary electrons might be an effect of enhanced diffusion, that is, locally changed chemical composition, and/or temperature triggered by the electron beam.42 In analogy, experimental evidence for the effect of Xrays on the nucleation of gold particles from a glass matrix has been given.35 It is proposed that nucleation sites can be created during the in situ measurements of small angle X-ray scattering (SAXS), introducing a way to control the number and size of these nanoparticles by irradiation and the annealing procedure. By contrast, the electron beam introduces negative charges into the sample in addition to energy, that is, heat. As the smaller crystals in the dark area of Figure 5 are definitely formed later than in the rest of the sample, it is clear that the electron beam notably affects crystal nucleation. In this case, it seems likely that the electron beam may lead to a slightly higher temperature, which may further delay the onset of the secondary nucleation due to a decreased thermodynamic driving force.

The secondary nucleation observed in Movie S2 and in Figure 7d is probably due to the low initial nucleation rate and the changed chemical composition of the glass matrix between the crystals. The investigated glass is not stoichiometric but contains excess SiO2 to decrease the crystallization tendency and to facilitate glass production. As the primary BTS crystals begin to grow during heating, the percentage of SiO2 in the residual glass matrix increases. The pseudo binary phase diagram Ba2TiSi2O8−SiO2 shows an eutectic point attributed to the composition Ba2TiSi2O8·2.5 SiO2 and a temperature of 1245 °C.30 Hence, the liquidus temperature of the melt decreases during the crystallization of fresnoite because the residual glass is continuously enriched in SiO2 until the eutectic point is reached. Since the undercooling of the melt decreases during crystallization until the eutectic composition is reached, the thermodynamic driving force for nucleation also decreases. It should be noted that an increasing SiO2 concentration is accompanied by a marginal increase in the glass transition temperature, which increases by only 22 °C from Ba2TiSi2O8 to Ba2TiSi2O8·2 SiO2.30 Assuming the surface energy between fresnoite and the residual glass melt does not change, the nucleation rate should hence decrease during the crystallization process. That means, nucleation occurs after reaching the appropriate nucleation temperature, but the nucleation rate decreases rapidly if a high enough volume concentration of fresnoite is reached. After the residual SiO2 enriched melt reaches the composition of the eutectic point, the liquidus temperature increases again and hence also the thermodynamic driving force. Since the glass transition temperature simultaneously increases, the nucleation rate should also increase. This is the reason for the second generation of crystals. In the sample thermally annealed under ambient conditions, the nucleation rate is notably higher which is why the secondary nucleation observed during annealing in the SEM under vacuum is not observed. The eutectic composition cannot completely crystallize because the temperature is too low for cristobalite formation. If both the glass transition temperature and the liquidus temperature of the SiO2 enriched glass melt are higher than before, the annealing temperature is closer to the new glass transition temperature and hence to the maximum in the dependency nucleation rate versus temperature. This explains why initial nucleation is followed by a low nucleation rate until the secondary nucleation is initiated after about 80 min at 790 °C and 9 min at 830 °C; see supporting Movies S1 and S2. This second generation of fresnoite crystals grows in a glass matrix of higher viscosity (elevated SiO2 content) which reduces the growth velocity and limits their size. It should be noted that the effects of a melt depleted of the crystal forming components on the crystallization process were recently described in refs 33, 34, and 38 for simple multicomponent systems. Apart from decreasing the oversaturation of the melt, the change in the chemical composition leads to a change in the viscosity. If network forming components are crystallized, that is, removed from the melt, the crystal growth velocity will decrease during crystallization. If, however, network modifying components such as alkaline earth fluorides or alkali fluorides crystallize from the melt, the viscosity increases, leading to a decrease of the crystal growth velocity.39,40 Hence, diffusion toward the crystals is hindered more than it would be simply due to the reduced oversaturation. The resulting diffusion barrier led to nanocrystals of

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CONCLUSION The oriented nucleation of BTS with the [101]-direction perpendicular to the surface was confirmed over a wide temperature range. Multiple textures appear at higher temperatures, probably due to stress relaxation processes. In situ experiments show that crystal growth along the surface in the vacuum is significantly faster than the average 3D growth of the crystals at ambient atmosphere. A secondary nucleation is observed if the initial nucleation rate is low enough and explained by compositional changes of the described nonstoichiometric melt. Nucleation and crystal morphologies are apparently affected by the atmosphere during annealing. The simultaneous occurrence of crystal nucleation and growth is confirmed.

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ASSOCIATED CONTENT

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Movies S1 and S2 in AVI format are available in the online version of this paper.

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AUTHOR INFORMATION

Corresponding Author

*Tel: (0049) 03641 948515. Fax: (0049) 03641 948502. Email: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by Deutsche Forschungsgemeinschaft (DFG) in Bonn Bad Godesberg (Germany) via Project No. RU 417/14-1.

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REFERENCES

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