Oriented Nucleation and Crystal Growth of Ba-Fresnoite (Ba2TiSi2O8

Article Cite This: Cryst. Growth Des. XXXX, XXX, XXX−XXX

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Oriented Nucleation and Crystal Growth of Ba-Fresnoite (Ba2TiSi2O8) in 2 BaO·TiO2·2 SiO2 Glasses with Additional SiO2 Wolfgang Wisniewski,* Franziska Döhler, and Christian Rüssel Otto-Schott-Institut, Jena University, Fraunhoferstr. 6, 07743 Jena, Germany ABSTRACT: Glasses of the mol composition 2 BaO·TiO2·2 SiO2 + x SiO2 (with x = 0.0, 0.1, 0.2, 0.5, 0.75, 1.5, 2.0, and 3.0 mol) were prepared, polished, and crystallized at 810 °C for 5 h or more. All glasses showed the sole crystallization of Ba-fresnoite (Ba2TiSi2O8). The crystal orientations at the immediate surfaces were analyzed by EBSD and showed that all glasses exhibit a nonrandom orientation distribution, i.e., oriented nucleation at the surface. While the glasses with x = 0.0, 0.1, and 0.2 show rather broad orientation preferences in the form of c-axes tilted from the surface by roughly 10−50°, the glasses with x = 1.5 or higher show discrete textures where specific crystal orientations are significantly preferred with orientation spreads of less than ±15°. While the glasses with x = 0 and 0.1 show polygon crystals at the surface and in the bulk, dendritic crystals are observed in the bulk of all other analyzed compositions. Phase separation solely occurred in the glass with x = 1.5 during the applied heat treatments.

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INTRODUCTION Ba-fresnoite (Ba2TiSi2O8) containing glass-ceramics have been of interest due to the intriguing properties of this phase, especially noting the pyroelectricity,1 piezoelectricity,2−5 and nonlinear optical properties6 which result from the polarity of the fresnoite crystal. Glass-ceramics containing Ba-fresnoite also have an extremely low dielectric dissipation, varying from 0.00003 to 0.0006 depending on the sample composition.7 Fresnoite is one of the few non-ferroelectrics reported in the literature which can easily be obtained as polycrystalline but polar materials. Melts in the fresnoite system also contribute to glass science as they show very high nucleation rates8,9 and crystal growth velocities.10,11 The work concerning glassceramics containing Ba-fresnoite, Sr-fresnoite (Sr2TiSi2O8), or Ge-fresnoite (Ba2TiGe2O8) as well as the properties of these fresnoites was recently reviewed.12 One contribution of the fresnoite system to the fundamental aspects of glass science was the observation of oriented nucleation during the surface crystallization of glasses which was first detected after the surface crystallization of Ba-fresnoite from a glass.13 This observation is in contrast to the basic assumption of randomly oriented nuclei in the classical nucleation theory for glasses. Oriented nucleation has been described to occur in glass-ceramics containing Ba-fresnoite,11,13 Sr-fresnoite,14,15 or Ge-fresnoite.16 It has also been detected in other systems during the surface crystallization of BaAl2B2O7,17,18 diopside,19 Sr1−x/2Al2−xSixO4 (x = 0.9, 0.7, and 0.5),20 walstromite,21 probably ε-Y2Si2O7, as well as yttrium stabilized ZrO222 and after the crystallization of mullite at the surface of a glass melt during cooling.23 The arguments for oriented nucleation have been presented in detail in ref 12. Oriented nucleation was first observed in a glass of the mol composition 2 BaO-TiO2-2.75 SiO2, i.e., stoichiometric Bafresnoite with 0.75 mol of SiO2 added to the melt.13 A nomenclature for glasses used for the crystallization of fresnoite has been presented12 and will be used throughout this article. It © XXXX American Chemical Society

is based on the stoichiometric fresnoite composition and is normalized to a constant BaO content. A glass of the stoichiometric composition is thus denoted as “BTS” while a glass of the composition 2 BaO-TiO2-2.75 SiO2 is denoted as “BT0.75S” to indicate the additional 0.75 mol of SiO2 in the glass.12 It has been shown that the oriented nucleation of Bafresnoite in the BT0.75S-glass depends on the annealing temperature, and multiple orientations have been shown to occur preferably within a single glass-ceramic.11 Hence, the question arises whether the orientation is also affected by the chemical composition of the melt. As the BaO-SiO2-TiO2system is the most frequently analyzed fresnoite system12 and extensive data is available, this system is well suited to analyze this aspect. Adding SiO2 enables a simple chemical modification as the resulting glass-ceramic microstructure can be approximated to be composed of Ba-fresnoite and residual SiO2-glass. The pseudobinary phase diagram of 2BaO-SiO2-2TiO2 and SiO2 shows a eutectic composition at ca. 2 BaO-TiO2-4.25 SiO2, i.e., for BT2.25S.12 Values for the glass-transition temperature (Tg) of the stoichiometric BTS-glass have been published from 700 to 730 °C.12 Although adding increasing amounts of SiO2 to the melt affects the crystallization temperature, the Tg remains relatively constant at ca. 750 ± 5 °C.12 Furthermore, various crystal growth mechanisms of fresnoites have been reported ranging from polygon growth in stoichiometric BTS-glass12,24 to dendritic growth in BT0.75S and a mechanism proposed to be viscose fingering in a glass of the composition ST0.75S where Sr-fresnoite crystallized from the surface.25 The growth of the hexagonal phase Sr0.75Al1.5Si0.5O4 in a glass of that same composition20 showed Received: February 28, 2018 Revised: March 22, 2018

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DOI: 10.1021/acs.cgd.8b00312 Cryst. Growth Des. XXXX, XXX, XXX−XXX

Crystal Growth & Design

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Table 1. Compositions, Glass Transition Temperatures (Tg), and Temperatures of the Crystallization Peak (Tp) of the Glasses Prepared and Crystallized for This Article According to ref 12 composition [mol] 2.0 2.0 2.0 2.0 2.0 2.0 2.0

BaO-1.0 BaO-1.0 BaO-1.0 BaO-1.0 BaO-1.0 BaO-1.0 BaO-1.0

BaO [mol %]

TiO2 [mol %]

SiO2 [mol %]

abbrev.

Tg [°C]

Tx [°C]

40 39.2 38.5 34.8 30.8 28.6 25

20 19.6 19.2 17.4 15.4 14.3 12.5

40 41.2 42.3 47.8 53.8 57.1 62.5

BTS BT0.1S BT0.2S BT0.75S BT1.5S BT2S BT3S

700−730 719 725−738 730−765 749 744−746 747

803−832 825 837−843 883 954 957−958 963

TiO2-2.0 SiO2 TiO2-2.1 SiO2 TiO2-2.2 SiO2 TiO2-2.75 SiO2 TiO2-3.5 SiO2 TiO2-4.0 SiO2 TiO2-5.0 SiO2

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RESULTS AND DISCUSSION The compositions of the glasses produced for subsequent analysis, along with the abbreviations for the corresponding glass-ceramics according to the nomenclature proposed in ref 12, are presented in Table 1. The respective glass-transition temperatures (Tg) and temperatures of the crystallization peak (Tp) were taken from the literature.12 The glasses were crystallized for 5 h at 810 °C, i.e., ∼60 K above the average Tg values of the compositions BT0.75S to BT3S. The CTEs of two bulk glasses were measured from 100 to 800 °C. The glass BT0.2S showed a CTE(100−800 °C) = (9.4 ± 0.1)*10−6 K−1 while the glass BT0.75S showed a slightly enhanced value of CTE(100−800 °C) = (9.5 ± 0.1)*10−6 K−1. The glass composition BT0.2S was chosen because glasses containing less SiO2 could not be produced with the thickness required for acceptable samples without crystallizing. BT0.75S is a glass intensely analyzed in the context of electrochemically induced nucleation5,12 and hence also of special interest. SEM micrographs showing the surface morphologies representative for the respectively stated glass-ceramics are presented in Figure 1. The individual grains are generally smaller than 1 μm in diameter at the surface. The glass BT0.75S shows a very strong surface topography in agreement with previous observations,11,13 making the individual grains difficult to discern. While the grains show a direct contact to their neighbors in the close-to-stoichiometric compositions and

remarkable similarities to the growth of Sr-fresnoite in the ST0.75S-glass. As it has been proposed that all of these growth mechanisms may be linked via a specific growth velocity25 and adding SiO2 to the stoichiometric BTS-glass increases the viscosity of the glass at a given temperature, the influence of the additional SiO2 on the crystal growth mechanism is of fundamental interest. The aim of this article is to systematically analyze if and how adding network formers to the chemical composition of the glass affects the oriented nucleation of Ba-fresnoite and the crystal growth into the bulk at a given crystallization temperature.

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EXPERIMENTAL SECTION

Glasses of the mol composition 2 BaO·TiO2·2 SiO2 + x SiO2 (with x = 0.0, 0.1, 0.2, 0.5, 0.75, 1.5, 2.0, and 3.0 mol) were prepared from BaCO3, TiO2, and SiO2 (quartz). The compounds were mixed and subsequently melted in a 250 mL Pt crucible at a temperature of 1550 °C using an inductive furnace. After keeping the melts at 1550 °C for 1 h, they were stirred for 1 h using a frequency of 60 min−1 and subsequently poured on a copper block. The melts were rapidly quenched using a copper stamp to avoid crystallization. The coefficients of thermal expansion (CTE) for the bulk glasses BT0.2S and BT0.75S were measured using samples of 6×6×20 mm3 in a dilatometer (NETZSCH 402PC) with a heating rate of 5 K/min. Pieces of the produced glasses were polished with abrasive slurries down to diamond paste of 0.75 μm grain size and then crystallized in an electric furnace at 810 °C for 5 h or more under a normal air atmosphere. In order to ensure reproducibility, at least two samples of each glass composition were independently polished, crystallized, and analyzed during the process. In order to perform scanning electron microscopy (SEM) studies, all samples were contacted with Ag paste and coated with a thin layer of carbon at about 10−3 Pa to avoid surface charging. Some samples were embedded in Araldite CY212 and cut perpendicular to the surface polished before crystallization in order to produce cross sections. These cross sections were manually polished with abrasive slurries down to a diamond paste of 0.75 μm grain size and a final finish of at least 30 min using colloidal silica (Logitech Syton Typ SF1 (pH = 13.3, Grain size 32 nm)) was applied. SEM analyses were performed using a Jeol JSM 7001F SEM equipped with an EDAX Trident analyzing system containing a Digiview 3 EBSD-camera. EBSD scans were performed using a voltage of 20 kV and a current of ca. 2.40 nA. The scans were captured and evaluated using the software TSL OIM Data Collection 5.31 and TSL OIM Analysis 6.2. Grain CI-standardization clean-ups were applied to scans used to present maps but not for texture evaluation. Unreliable data points were removed in all data sets used for orientation analysis by applying a Confidence Index (CI) filter of 0.1. Pole figures of textures are presented in multiples of a random distribution (MRD). X-ray diffraction (XRD) was performed using Cu Kα radiation in a Rigaku MiniFlex 300 diffractometer using a θ−2θ arrangement in the 2θ range from 10° to 60°.

Figure 1. SEM micrographs representative for the immediate surfaces of the respectively stated glass-ceramics in the BaO·TiO2·SiO2 system after crystallization at 810 °C for 5 h. B



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one to judge the intensity of the orientation preference observed in a scan. As the samples deform during crystallization and are hence not perfectly parallel to the SEM stage, slight shifts from these ideal values indicated the tilt of the sample surface with respect to the SEM stage. As none of the results presented in Figure 2 indicate a random orientation distribution, oriented nucleation occurred at the surfaces of all analyzed glasses. The indexing rates range from 35% to 61% which can be considered to be representative, taking into account the large area fraction attributed to grain boundaries or to residual glass and the topography of these surfaces. The glass-ceramics BTS, BT0.1S, and BT0.2S show only slight preferences for orientations where the c-axes are tilted from the surface normal by ca. 10−50°. Orientations with the c-axis tilted from the surface normal by 35−90° preferably occur at the immediate surface of the glass-ceramic containing 0.75 mol of additional SiO2. The glass-ceramics containing 1.5 mol of additional SiO2 or more show discrete textures; all of them show a strong preference of orientations where the c-axes are tilted from the surface normal by ∼60°. The primary preferences are c-axes tilts of 63 ± 20° for BT0.75S, 61 ± 15° for BT1.5S, 58 ± 8° for BT2S, and basically 56 ± 7° for BT3S, although here the primary peak is split into two subpeaks of 53 ± 3° and 60 ± 3°. This peak splitting was also observed in data acquired from a different BT2S sample and is also indicated in the presented Φ distribution of BT1.5S. The primary orientation preference in BT0.75S is in acceptable agreement with previous results obtained from this glass at 810 °C (58 ± 4°13 and 60 ± 15°11) considering that the samples were polished manually and different furnaces were used (i.e., different errors for the heat treatment temperatures are possible). Secondary orientation preferences occur for orientations where the c-axes are oriented perpendicular or parallel to the sample surface. Furthermore, apart from these discrete preferences, orientations where the c-axes are tilted further from the surface normal (60 < Φ < 120°) occur more often than those tilted less from the surface normal (0 < Φ < 60° and 120 < Φ < 180°) in the glass-ceramics BT0.75S (in agreement with ref 11), BT1.5S, and BT2S as is discernible by comparison to the dashed reference lines. While fresnoite glass-ceramics have been extensively analyzed,12 comparably few articles feature the microstructure resulting from the crystallization of a glass with the stoichiometric composition BTS.12,24,26 It has been stated that the BTS-glass only shows bulk nucleation,24,27 but, e.g., figures of the microstructure adjacent to the surface have not been presented so far. Figure 3 a presents an SEM micrograph of the crystal growth at the immediate edge to the surface of a crystallized BTS-glass. A thin layer less than 5 μm thick seems to contain slightly elongated crystals aligned with their long axes perpendicular to the surface. The IPF + IQ map of an EBSD scan performed at this edge is presented in Figure 3b but fails to clearly confirm this impression. A statistically significant orientation preference of crystals within this topmost layer of crystallization could not be measured via EBSD from cross sections of these glass-ceramics, which is understandable with respect to the comparably low degree of orientation preference indicated for this glass-ceramic in Figure 2. It should be noted that these elongated crystals were not observed elsewhere at the edge of the same cut plane, but it is possible that a single layer of crystals flaked off locally while polishing the edge. While the nonrandomly oriented nucleation indicated for stoichiometric BTS-glass in Figure 2 implies a preferred surface

BT0.75S, those with a larger amount of additional SiO2 show grains separated by residual glass. Some of the grains in the BT3S-glass appear brighter in the SEM micrograph. It should be noted that some of the samples crystallized from the stoichiometric glass showed even smaller grain sizes, making the acquisition of EBSD patterns problematic so that indexing rates of 10% or less were obtained during the subsequent EBSD analyses. However, the texture information obtained from these surfaces is qualitatively comparable to that obtained from the surface featured in Figure 1. It should be remembered that the surface crystallization is affected by the atmosphere, as, e.g., the glass BT0.75S showed much larger crystals of dendritic morphology when crystallized in situ in the vacuum of an SEM chamber.11 EBSD scans containing at least 100 000 data points were performed on these surfaces using a small step size of only 200 nm which was necessary because of the observed grain sizes. At least two such scans were initially performed at different locations of each sample and they always led to comparable texture results; hence, the reproduction samples were only scanned once. The texture information of EBSD scans performed on these surfaces is presented in Figure 2 via pole

Figure 2. 001-PFs and normalized histograms of the Euler Angle Φ obtained from EBSD scans performed on the immediate surfaces of the respectively stated glass-ceramics after crystallizing at 810 °C for 5 h. The percentage of data points reliably indexed during each scan is stated in %, and the horizontal dashed lines are included as guides for the eye. The red lines serve as references marking Φ = 4, 60, 90, 120, and 176°.

figures (PFs) and normalized histograms of the Euler Angle Φ. The effect of the individual Euler Angles φ1, Φ, and φ2 on the PF of Ba-fresnoite has been illustrated in ref 13: Φ describes the tilt of the crystallographic c-axes from the surface normal; i.e., Φ = 0° or Φ = 180° means the c-axes are perpendicular to the surface, while Φ = 90° means they are parallel to it. Normalizing these values to a range from 0% to 100% enables C



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Figure 3. (a) SEM micrograph obtained from the cross section of a crystallized BTS-glass at the edge to the initial surface. (b) IPF map of an EBSD scan performed at the edge to the immediate surface.

crystallization because the surface functions as a reference system for nucleation, Figure 3 shows that its effect is limited to the topmost layer of crystals which can, at best, grow to a slightly elongated shape before bulk nucleation occurs. Hence, a preferred nucleation at the surface is probable, but the small induction time, perhaps only a few seconds, between surface and bulk nucleation in the stoichiometric BTS-glass is basically impossible to detect using the methods generally applied for measuring the Avrami parameter on which the statements concerning the sole bulk nucleation in the BTS-glass are based.24,27 It should be noted that the observed microstructure leaves minimal space for a possible kinetic selection during growth into the bulk. SEM micrographs featuring the surface near regions in the other analyzed glasses BT0.1S to BT3S after 5 h at 810 °C are presented in Figure 4: the glass BT0.1S shows the same microstructure as the stoichiometric composition in Figure 3. By contrast, the cross sections of the crystallized glasses BT0.2S to BT0.75S clearly show layers of surface crystallization as well as nucleation in the bulk, and it is noteworthy that the crystals no longer show a polygon but instead the dendritic morphology. Bulk nucleation is not observed in the glasses containing 1.5 mol additional SiO2 or more after the provided heat treatment. The thickness of the crystallization layer originating at the polished surface is reduced from BT0.2S to BT1.5S and then slightly increases for the glasses BT2S and BT3S even though the glasses contain increasing amounts of SiO2. The thin layer of surface crystallization in BT1.5S is counterintuitive as an increasing amount of network former should increase the glass viscosity and hence lead to a smaller growth velocity and perhaps longer induction times, i.e., thinner layers of crystallization. Hence, such a sample was heat treated for another 12 h at 810 °C in order to grow a thicker layer of surface crystallization. A cross section of this sample was analyzed by SEM as well as EBSD. An overview of the barely discernible layer of surface crystallization is presented in Figure 5a, where it reaches a thickness of more than 30 μm, proving that the layer continued to grow for a time during the second heat treatment step. The more detailed micrographs of the edge to the initial surface (Figure 5b) and the growth front (Figure 5c) show that the microstructure is completely saturated by dark spheres, most likely droplets of SiO2 resulting from a

Figure 4. SEM micrographs illustrating the crystal growth into the bulk in the respective glasses thermally treated for 5 h at 810 °C. The edge to the initial surface is always at the top of each figure.

Figure 5. SEM micrographs obtained from a BT1.5S glass-ceramic after annealing for 5 h at 810 °C and then for another 12 h at 810 °C. (a) Overview of the layer of surface crystallization, (b) edge to the initial surface in greater detail, and (c) growth front in greater detail.

phase separation. Phase separation has been reported in various compositions in the BaO-TiO2-SiO2 system.24,28,29 It should be noted that the phase separation in Figure 5c clearly occurs in the crystallized layer as well as beyond the growth front in the residual glass. This is important because no droplets were observed beyond the crystallization front in the system D



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proposed to show phase separation in a related Sr-fresnoite glass-ceramic.30 Results of an EBSD scan performed on this area are presented in Figure 6: the combined inverse pole figure (IPF) +

Figure 7. SEM micrographs obtained from a cross section of a BT3Sglass heat treated for 30 h at 810 °C. (a) Surface crystallized layer and (b) the framed area in greater detail. The 001-PF of an EBSD scan performed over the crystallized layer is also presented.

microstructure in Figure 7 features Ba-fresnoite dendrites growing perpendicular to the surface, similar to the observations made for BT0.75G in a related glass-ceramic.16 Bulk nucleation was not detected in this BT3S glass-ceramics after the applied heat treatment. While the growth front interactions of the related systems STS14 and BTG16 have been analyzed by SEM and EBSD to see whether the local growth velocity is increased, reduced, or unaffected,12,14 this has not been done for Ba-fresnoite growing in glasses. Figure 8 presents an SEM micrograph obtained from Figure 6. EBSD results of the sample featured in Figure 5: (a) IPF + IQ map superimposed on the SEM micrograph, (b) IQ map of all data points, and (c) PF of a texture calculated from the reliably indexed data points.

image quality (IQ) map shown in Figure 6a illustrates orientation domains which appear to be homogeneous but contain deviations of up to 10° within a domain. The IQ map in Figure 6b shows a clear decrease of the pattern quality after the growth front beyond which EBSD patterns could not be obtained at all. Hence, either the area beyond the surface layer of crystallization is not crystallized or the crystal lattice is so irregular at a nm scale that electron diffraction does not occur at an intensity significant to the available EBSD camera. The 001-PF of a texture calculated from the data set featured in Figure 6a is presented in Figure 6c and shows a 001-texture related to that described in BT0.75S13 in the scanned area. A texture where the c-axes are tilted from the main growth direction comparable to that observed during the crystallization of Sr-fresnoite25 of Sr0.75Al1.5Si0.5O420 is not detected despite the less discrete orientation preference probably caused by the phase separation. A BT3S shard was annealed for 30 h at 810 °C so as to produce a thicker layer of surface crystallization as well as test this glass for phase separation similar to that observed in BT1.5S when annealed for such a long time. SEM micrographs of the resulting layer of surface crystallization are presented in Figure 7. Figure 7a shows the complete layer of surface crystallization where neither the crystallized layer nor the uncrystallized glass shows any indication of phase separation. This impression is confirmed by Figure 7b which features the growth front in detail. The superimposed 001-PF of an EBSD scan performed on the crystallized layer shows that the c-axes of the crystals are aligned but tilted from the current cut plane by ca. 20°. Hence, the plane of this cross section is probably not perfectly perpendicular to the initial surface but tilted by ca. 20°. Considering this tilt, it can be concluded that the

Figure 8. SEM micrograph featuring the collision of two growth fronts in a cross section through a BT0.75S glass-ceramic annealed for 5 h at 810 °C.

a cross section of the BT0.75S glass-ceramic showing an area where the growth fronts from the top and right side of the sample collided. In analogy to Ge-fresnoite,16 Ba-fresnoite shows no growth front interaction. Hence, the dendritic crystals growing in the BT0.75S-glass at 810 °C simply collide and prevent each other from further growth, similar to the observations made for Ge-fresnoite.16 Although texture analysis on compact samples by XRD in the Θ−2Θ setup can be problematic due to the large information depth and an orientation resolution limited to the occurring diffraction peaks as outlined in ref 12, it is frequently used to gain a first insight into textures of surface crystallized fresnoite glass-ceramics.12 Hence, XRD patterns acquired from the surfaces of compact samples of each glass-ceramic analyzed in this article are presented in Figure 9. The pattern of the uncrystallized BTS-glass is also presented to confirm that, despite its high nucleation rate, the produced glass showed no crystals detectable by XRD. The theoretical pattern of Bafresnoite (JCPDS file no. 01-70-1920) is presented for comparison. Despite the weak orientation preferences detected by EBSD at the immediate surfaces of the crystallized glasses BTS to E



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into the bulk. As, e.g., the glass BT0.75S has been shown to crystallize via dendritic crystals at the immediate surface when heated in the vacuum of an SEM chamber,11 a probable explanation is that the surfaces crystallized in all samples before the final heat treatment temperature was reached when heated in air. The growth mechanism change to dendritic growth occurs at some point as the temperature is increased to the final 810 °C. It seems likely that, due to the high nucleation rate and small crystallite size, the glasses BTS and BT1S were completely crystallized by the time they reached 810 °C and the crystals actually grew at a lower temperature. While the surface layers formed in the glasses BT0.2S and BT0.75S clearly show an induction period between the surface and bulk nucleation, bulk nucleation was not detected in the glasses BT1.5S, BT2S, or BT3S after the applied heat treatments at 810 °C. While the glass BT1.5S showed a clearly discernible phase separation when annealed for 17 h at 810 °C, phase separation was not detected in the glass BT3S after annealing for 30 h at 810 °C. The BT1.5S glass shows the thinnest layer of surface crystallization after 5 h at 810 °C, perhaps due to an already present phase separation with droplets too small to be detected in the SEM but still hinder crystal growth. All analyzed glasses show oriented nucleation. An increasing amount of additional SiO2 in the BaO·TiO2·SiO2 system seems to have a similar effect as increasing the annealing temperature for the BT0.75S-glass:11 both increases lead to increasingly discrete textures and a reduced intensity of the secondary orientation preferences with 60 < Φ < 120°. In the BT0.75S glass-ceramic, the Φ = 90° preference is very weak when crystallizing at 790 °C, strong when crystallizing at 830 or 850 °C, but is less pronounced when the glass is crystallized at 970 °C.11 Similarly, the Φ = 90° preference in Figure 2 is weak for BT0.75S, clearly discernible for BT1.5S and BT2S, but not observed for BT3S. Adding network formers to the glass reduces the nucleation rate, and the orientation preferences observed at the immediate surfaces are of a statistical nature, i.e., not absolute. It is plausible that the orientations statistically preferred during nucleation only show a slight energetic preference in comparison to other orientations. Thus, a reduction of the nucleation rate should increase the significance of the energetic difference which would be in agreement with the increasingly discrete textures observed for the glasses BT0.75S to BT3S. The peak splitting observed in the Euler Angle distributions of BT2S and BT3S could also be explained by this model as the applied surface polish may vary (manual polish) and an ideal surface could provide a further separation into two energetically preferred orientations of the subpeaks which combine into one if the surface preparation is less perfect. As the XRD patterns of the glass-ceramics BT1.5S to BT3S solely originate from the layers of surface crystallization, the enhanced peak intensities for the 221- and 201-peaks support the oriented nucleation detected by EBSD as these peaks can be formed by crystals with their c-axes tilted by 60° or 120° from the surface normal, both orientations indicated at the immediate surfaces by EBSD.

Figure 9. XRD patterns obtained from the uncrystallized BTS-glass as well as from the surfaces of the respective compact samples thermally treated for 5 h at 810 °C. The JCPDS file no. 01-70-1920 is provided as a reference.

BT0.2S, the XRD patterns acquired from them clearly show (00n)- and (221)-peaks of enhanced intensity. This is especially surprising for the glass-ceramics BTS and BT0.1S which at best show a single layer of polygon crystals in the Figures 3 and 4 that may have nucleated just before bulk nucleation occurred, allowing only minimal kinetic selection during growth into the bulk. It is noteworthy that the XRD patterns acquired from the same glass polished using CeO2 and crystallized at 770 or 790 °C did not show such clear peak intensity exaggerations;26 sadly, the 001-peak is not included in the presented 2Θ range in ref 26.The glass-ceramics BT0.75S to BT3S all show enhanced intensities for the (00n)-, (201)-, and (221)-peaks. The enhanced (221) peaks in the XRD patterns are in agreement with the EBSD results as a {221} plane can be parallel to the surface when the c-axis of the unit cell is tilted from the surface normal by ∼60°. Similarly, a {211} plane can be parallel to the surface when the c-axis is tilted by 120°; hence, both peak intensity exaggerations can result from the orientation preference where the c-axes are tilted by ∼60° from the surface normal. It should be noted that the information concerning the oriented nucleation at the immediate surface is usually lost in the XRD measurements due to the comparably thick 001 oriented layers beneath them, but is detected here because the analyzed crystal layers are very thin. In summary, the results presented above show that an increasing amount of SiO2 leads to increasingly separated grains at the surface and that the BT0.75S glass-ceramics show the strongest surface topography in the form of valleys surrounded by sharp ridges. BTS and BT0.1S show polygon growth in the bulk, and a possible induction period between surface and bulk nucleation is minimal. By contrast, the other analyzed glasses from BT0.2S to BT3S all show dendritic growth of Ba-fresnoite

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CONCLUSION All analyzed glasses show oriented nucleation of Ba-fresnoite at the surface. Adding network formers to the stoichiometric BTSglass seems to have a similar effect as increasing the heat treatment temperature in BT0.75S: both predominantly affect F



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the intensity of the texture detected at the surface. An increased network former content leads to a more discrete texture formation in the analyzed glasses while secondary orientation preferences are reduced in intensity. Stoichiometric BTS-glass shows surface and bulk nucleation, but the induction period between them is so small that it at best affects a single layer of crystals at the surface. The high nucleation rates in BTS-glass and BT0.1S lead to a complete crystallization of these glasses by polygon crystals before the samples reach a temperature where Ba-fresnoite grows via the dendritic mechanism observed in the bulk of all analyzed glasses containing a higher amount of SiO2. The glass BT0.2S shows a slightly higher CTE than the glass BT0.75S. The textures detected by EBSD at the immediate surface can be verified by XRD if the layer of surface crystallization is thin enough.

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AUTHOR INFORMATION

Corresponding Author

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

Wolfgang Wisniewski: 0000-0001-6390-4750 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 nr. RU 417/14-1.

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REFERENCES

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Oriented Nucleation and Crystal Growth of Ba-Fresnoite (Ba2TiSi2O8

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