Temporal Evolution of Diffusion Barriers Surrounding ZrTiO4 Nuclei in

Article pubs.acs.org/crystal

Temporal Evolution of Diffusion Barriers Surrounding ZrTiO4 Nuclei in Lithia Aluminosilicate Glass-Ceramics Thomas Höche* and Christian Patzig Fraunhofer Institut für Werkstoffmechanik IWM, Walter-Hülse-Strasse 1, D-06120 Halle, Germany

Thomas Gemming Leibniz-Institut für Festkörper- und Werkstoffforschung, Helmholtzstraße 20, D-01069 Dresden, Germany

Roman Wurth and Christian Rüssel Otto-Schott-Institut, Universität Jena, D-07743 Jena, Germany

Isak Avramov Institute of Physical Chemistry, Bulgarian Academy of Sciences, G. Bonchev Str. Block 11, 1113 Sofia, Bulgaria

ABSTRACT: Glasses are usually synthesized by quenching a melt rapidly enough to avoid crystallization. Nanocrystalline materials can subsequently be derived from glasses by controlled crystallization with applying a tailored heat treatment. Upon the latter, nucleation agents are widely used to adjust the desired nanostructures. Nano glass-ceramics often possess intriguing properties. For example, they can be ultratransparent; that is, they hardly scatter light or possess thermal expansion coefficients very close to zero. Such properties have a high potential for future applications in optical devices. In this paper, the role of zirconia and titania used as nucleation agents in a lithia aluminosilicate glass is studied on the nanoscale using cutting edge analytical and imaging techniques performed using the transmission electron microscope. Precipitation of ZrTiO4 nanocrystals [Bhattacharyya, S., et al. Nano Lett. 2009, 9, 2493] was found earlier to be accompanied by the formation of a circumjacent diffusion barrier consisting of alumina. In addition to this, here we study the temporal evolution of the alumina barrier and the size distributions of ZrTiO4 nanocrystals and lithia aluminosilicate high-quartz solid solution crystals promoted by the nucleation agent. In the light of these findings, the theory of self-limited growth is refined.

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INTRODUCTION Li2O−Al2O3−SiO2 (LAS) glass-ceramics, possessing thermal expansion coefficients close to zero in a wide temperature range,1 have gained considerable commercial importance. Because of this property, LAS glass-ceramics are widely used as cooktop panels, telescope mirrors, and high-temperature (furnace) windows.2−4 Zero thermal expansion (ZTE) is accompanied by a high thermalshock resistance (being important for cooktop panels and furnace windows) as well as by a minimization of shape changes with temperature (being crucial for telescope mirrors). © 2012 American Chemical Society

The conceptual idea behind such glass-ceramics is to precipitate very fine-grained phases with negative thermal expansion coefficients along at least one crystallographic axis, such as β-quartz solid solutions or keatite.5−7 Thermal annealing schedules can be chosen such that glass-ceramics become fully transparent by applying processing schemes that exclusively result in a high Received: December 6, 2011 Revised: January 11, 2012 Published: January 31, 2012 1556

dx.doi.org/10.1021/cg2016148 | Cryst. Growth Des. 2012, 12, 1556−1563

Crystal Growth & Design

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results obtained are discussed on the basis of a refined theoretical model.

volume fraction of nanosized crystallites of the functional phase, whose diameters are in the range of several tens of nanometers.8−10 Either nanosized functional crystals become directly precipitated10−12 or the formation of the functional grains is preceded by a spatial enrichment of a nucleation phase. The latter approach has been pursued for decades by tailoring sophisticated nanostructures utilizing nucleation agents such as titania, zirconia, or both.13−16 However, it took until very recently that the theory of selflimited growth in nonisochemical systems was set out17 and became experimentally confirmed.11,12,18 According to this theory, the composition of the residual glass in close proximity to the crystal changes upon crystallization if the composition of the nucleating phase is different from that of the original glass phase. If network forming oxides become enriched in the residual glass immediately surrounding the nucleated crystals, a diffusion barrier encircling the latter is formed. The latter barrier is effectively limiting the growth of nanocrystals. Such barriers were proved experimentally for various glassceramics by applying advanced techniques of nanoanalytics in aberration-corrected transmission electron microscopes.11,12,18 Beyond this, the action of nucleation agents, formerly assumed to become epitaxially overgrown,15 was put into a new perspective.19 The advantages of conventional transmission electron microscopy (TEM) imaging for the characterization of the nanostructure of glasses and glass-ceramics are widely accepted.20 One crucial limitation, however, is radiation damage, that proved to be particularly severe when studying lithia aluminosilicate (LAS) glasses and glass-ceramics on the nanometer scale at medium acceleration voltages [200 keV or alike]. The benefits of using reduced electron energies [e.g., 80 keV] have been demonstrated before.12,18,19,21 The key point is that with aberration-corrected electron optics, imaging as well as electron energy-loss nanoanalytics can be used with much improved spatial resolution (on the atomic scale) at reduced acceleration voltages. For LAS glass-ceramics, it was proposed that the compositional gradient formed at the periphery of the diffusion barrier around ZrTiO4 nanocrystals would facilitate nucleation of the secondary LAS phase.19 Moreover, nucleation agents, a blend of titania and zirconia, have been shown to undergo liquid−liquid phase separation forming tiny droplets of just a few nanometers in diameter.19 Upon annealing, these droplets, possessing an extremely narrow size distribution, crystallize without size coarsening.22 Ostwald ripening was shown to be effectively suppressed by an alumina-rich shell surrounding ZrTiO4 nano precipitates. This barrier is believed to hamper diffusion of Zr and Ti from the residual glass matrix toward the tiny crystal, thus effectively suppressing a further growth of the latter. Using X-ray absorption near-edge structure spectroscopy, the course of crystallization of ZrTiO4 from phase-separation droplets was monitored.22 In order to draw the full picture of nucleation and growth in zero-thermal-expansion LAS glass-ceramics, two major factors have not been determined yet: (i) the number density of nucleation-agent ZrTiO4 nanocrystals and the same figure for the functional phase LAS and (ii) the temporal evolution of the composition profile within the matrix surrounding ZrTiO4 nanocrystals upon annealing. These remaining questions are addressed in the present paper, where we again apply aberration-corrected TEM to image the nanostructure and use scanning TEM to perform nanoanalytics. Experimental

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EXPERIMENTAL SECTION

The glass studied was melted by Schott AG, Mainz, Germany (laboratory melt) with the composition 7.6Li2O·0.16Na2O·0.13K2O·1.85MgO· 0.33BaO·1.2ZnO·12.73Al2O3·72.58SiO2·2.11TiO2·0.9ZrO2·0.39As2O3 (all mol %). Commercial Robax glass-ceramics exhibit similar compositions but do not contain arsenic. Melting was accomplished in an electric furnace in quantities of 1 L using a platinum crucible in air. Glass samples of approximately 10 × 20 × 1 mm3 were heat treated at a temperature of 750 °C (heating rate: 10 K/min) for different periods of time (0 h, i.e., just ramping up to 750 °C with no hold time, 4 h, 8 h, and 120 h). After annealing, the furnace was switched off to allow for a slow cooling down to room temperature. Samples for TEM investigations were prepared from four differently annealed glass-ceramics by cutting slices, plane-parallel grinding, onesided dimpling to a residual thickness of about 10−15 μm, and ionbeam thinning using Ar+ ions. Utilizing double-sided ion-beam etching at small angles (5°) and low ion-beam energy (acceleration voltage: 2.5 kV; beam current: < 9 μA), substantial heating of the TEM foils and, hence, the introduction of artifacts was avoided. Prior to TEM investigation, the nonconducting samples were selectively coated with carbon to reduce charging effects under the electron beam.23 For bright-field imaging, a not aberration-corrected HITACHI H8100 TEM, operated at an acceleration voltage of 75 kV and equipped with a LaB6 filament, was used. For high-resolution transmission electron microscopy (HRTEM) and electron energy-loss spectroscopy (EELS) line scans, a FEI TITAN 80-300 TEM equipped with a Gatan Imaging filter (Tridiem) was used at an accelerating voltage of 80 kV. The Digital Micrograph software (Gatan Inc., Pleasanton, CA, USA) was employed for image processing and analysis. The evaluation of TEM micrographs in order to count the numbers of crystals per area was done using the software ImageJ (http:// rsbweb.nih.gov/ij/). TEM micrographs taken of samples tempered with different annealing times were first scanned and digitized and then processed with the aim to enhance the contrast of the crystals that appear dark in the micrographs due to their large mass-thicknesscontrast in comparison to the surrounding matrix. The image processing was done in the following way: first, a quadratic area to be analyzed was cut out of the original, unprocessed micrograph (150 nm × 150 or 200 nm × 200 nm for counting the ZrTiO4 crystals, and 1000 nm × 1000 nm for counting the LAS crystals). Then, a Gaussian filter was applied for smoothing the micrographs, followed by a bandpass filtering to segment out too large or too small structures to be taken into account as crystals. Then, the still existing background of the micrographs was eliminated using the “subtract background” tool of the software. A following binarization of the as-processed micrographs led to images as exemplarily shown in Figure 2 on the right side. From those processed images, it is easy to count the remaining island-like features (only features with an area larger than 4 nm2 were taken into account). Care was taken to analyze all micrographs with the same routine, to make sure that the inevitable errors implied with so many processing steps (not to mention the error coming from not knowing the sample thicknesses at the positions were the micrographs were taken) are of a systematic nature. Thus, it is possible to predict maybe not perfectly correct numbers of crystal densities but still the trends in the nucleation and growth process with growing holding times of tempering.

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RESULTS AND DISCUSSION While the base LAS glass turned out to be fully amorphous (cf. Figure 2 in ref 19), after ramping up the glass to 750 °C immediately followed by a cooling to room temperature, tiny, spherical areas of enhanced scattering were observed (cf. Figure 1a) that turned out to be significantly enriched in zirconium. Zirconium, due to its high atomic number (relative to the other majority elements in the base glass), is visible while the 1557



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those phase-separation droplets (PSDs) are fully crystallized after annealing at 750 °C for 2 h. After 8 h, besides crystallized ZrTiO4 nanocrystals, larger, crystalline LAS precipitates can be easily discerned (Figure 1b) and after 120 h, almost the entire glass matrix is transformed into crystalline LAS high-quartz solid-solution crystals (Figure 1c). As illustrated by Figure 2, and described in detail in the Experimental Section, the number density and size distribution of PSDs (the early stage) or ZrTiO4 nanocrystals (the final stage), respectively, was determined as a function of the annealing time at 750 °C. As shown in Figure 3a, despite the large error bars due to the unknown thickness of the electrontransparent section, the number density is increasing within the first 15 to 30 min, while the average area, that is, the size of the precipitates, may be assumed to enlarge until 1 h of hold time (see Figure 3b). Image-processing analyses of LAS functional crystals reveal that the latter can be first discerned after 2 h of annealing time, and their number density increases by a factor of 4 between 2 and 4 h. In parallel, the average projected area, reflecting crystal size, steadily shrinks; that is, after 4 h, LAS crystals possess approximately half the area of crystals observed after 2 h. In Figures 4−7, STEM images of PSDs (or, at later stages of heat treatment, ZrTiO4 nano crystals, respectively) are shown together with line scans of the molar Al/Si concentration ratio for 0 h, 4 h, 8 h, and 120 h. The latter line scans were recorded along trajectories indicated in the respective STEM images using EELS. The Al/Si ratio was determined for all annealing times (Figures 4−7) following a routine described in detail in ref 19. From Figure 4 (0 h sample), it can be concluded that besides zirconia and titania, alumina is found in the PSDs. Upon heat treatment, alumina is squeezed toward the rim of the PSD (cf. Figure 5), forming a rigid shell around the ZrTiO4 crystals formed from the PSD. All EELS data taken together, a dependency of the maximum Al/Si ratio near ZrTiO4 nanocrystals can be plotted as shown in Figure 8. After some steep increase of the ratio in the first few hours, the diffusion barrier formed around ZrTiO4 nucleation agent crystals is dissolved after very long times (120 h). By the time the crystallization of the functional LAS phase is broadly finished, the barrier is vanishing due to alumina diffusion which is noticeable after such long times. The disintegration of this diffusion barrier facilitates the shape change of ZrTiO4 nanocrystals from spherical into more elongated, as evident after a hold time of 120 h at 750 °C (cf. Figure 1c): being no longer confined by the rigid shell, the nanocrystals habit approximates its equilibrium shape which is not necessarily spherical. The first stage of the nucleation process is obviously the phase-separation process; it has solely thermodynamic reasons and is a prerequisite for the formation of ZrTiO4 nuclei. However, in contrast to earlier assumptions, phase-separation droplets contain not just zirconia and titania but also alumina, as Figure 4 clearly shows. After the first ZrTiO4 nuclei are formed in the phase-separation droplets upon annealing at 750 °C, a layer enriched in aluminum is formed around the crystals, which in Figures 5−7 is illustrated by the Al/Si ratio along the EELS line-scan trajectories. This leads to the slight reduction of the PSD size between 60 and 480 min annealing time shown in Figure 3b: the PSDs, not fully crystallized until an annealing time of approximately 2 h,23 squeeze out alumina upon crystallization of ZrTiO4, thus gradually reducing the size of the areas with enhanced electron scattering that can be observed

Figure 1. Bright-field TEM image of the samples annealed at 750 °C for (a) 0 h, (b) 8 h, and (c) 120 h.

distribution of other elements can only be determined by analytical methods like energy-dispersive X-ray spectrometry (EDXS) or electron energy-loss spectroscopy (EELS). As shown earlier,22 TEM imaging of such small objects (in the present case: phase-separation droplets), which are furthermore embedded into a glassy matrix, is not capable of discriminating between amorphous or partly crystalline arrangements. It is known, however, from X-ray absorption investigations22 that 1558



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Figure 2. Illustration of image-processing thresholding (0 h @ 750 °C) used for automated determination of particle number densities and projected area of nanocrystals.

Figure 3. Particle number density per sample area (a and c) and average projected area (b and d) of ZrTiO4 (open squares) and LAS crystals (open circles).

crystals.18 However, redistribution of elements within phaseseparation droplets has also been reported (for an order of magnitude larger precipitates).12,24 In the present case, the phase-separation droplets are enriched in octahedrally coordinated ions such as Zr4+, Ti4+, and Al3+, but not in tetrahedrally coordinated Si4+.25 If in the course of nucleation, tiny ZrTiO4 crystals are formed within the zriconia-, titania-, and aluminarich droplets, this leads  irrespective of the larger diffusion coefficients of Al with respect to Si  to the high aluminacontent shell around ZrTiO4 crystals.

with TEM. The repulsion of alumina from the PSDs as a consequence of ZrTiO4 formation can be visualized analytically by comparing the maximum ratio cAl/cSi in close proximity to the PSDs, which increases notably with increasing annealing times. At a first glance, the formation of an alumina-rich barrier is surprising since the diffusion coefficient of aluminum should be larger than that of silicon (SiO2 is the main component which builds up the network). And in fact, in other glass-ceramics, silica was already found to become enriched around growing 1559



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Figure 4. (Top) Dark-field STEM image (annual dark-field detector, camera length: 48 mm, GIF aperture entrance: 25 mm) of the sample annealed for 0 h with the location of line scan EELS analyses indicated across a crystal; (bottom) composition ratio of Al/Si along the line scan across the crystal shown in the top panel.

Figure 5. (Top) Dark-field STEM image (annual dark-field detector, camera length: 48 mm, GIF aperture entrance: 25 mm) of the sample annealed for 4 h with the location of line scan EELS analyses indicated across a crystal; (bottom) composition ratio of Al/Si along the line scan across the crystal shown in the top panel.

For short annealing times, the Al/Si ratio in these shells increases due to residual alumina pushed toward the rim of the Zr−Ti−Al−O PSDs upon ZrTiO4 crystallization within these droplets. The increase in the Al/Si ratio (Figure 8) slows down for longer annealing times because all zirconia and titania enriched PSDs have been transformed into crystalline ZrTiO4 after a few hours. In other words, if all ZrTiO4 has been formed, no further alumina enrichment occurs in the crystals' periphery. In the final stage, when ZrTiO4 crystals do not grow anymore, the Al/Si ratio in the layer decreases with heat-treatment time (blue line in Figure 8). This is because alumina is striving to form a uniform distribution in the residual glass outside the former PSDs. In this stage of microstructure development, LAS crystals are formed as well, taking up alumina. Close inspection of the data reveals that the maximum alumina enrichment in the barrier formed around former PSDs is detected after 8 h of annealing, while, according to XANES measurements, ZrTiO4 crystallization within the PSDs is

completed already after 2 h. There are two effects that might contribute to this discrepancy. First, the formation of a highly alumina-enriched layer might be thermodynamically favorable for the residual glass. This layer is obviously very thin (just a few nanometers) so that in analogy to chemical surface reconstruction discussed in the context of surface crystallization,26 also at internal interfaces upstream diffusion can possibly occur. In this respect, experiments bringing ZrTiO4 crystals in contact with glasses of appropriate composition (hence lacking the kinetic effects of precipitation and growth) will help evaluate this option. Upon proceeding LAS crystallization (consuming alumina), the composition of the residual glass will change and the barrier will get reduced. A second contribution might stem from a not perfectly symmetric repulsion of alumina out of the PSDs. As can be clearly seen for prolonged annealing times, ZrTiO4 nanocrystals are not necessarily spherical. Precipitation in a spherical PSD might well lead to 1560



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Figure 6. (Top) Dark-field STEM image (annual dark-field detector, camera length: 48 mm, GIF aperture entrance: 25 mm) of the sample annealed for 8 h with the location of line scan EELS analyses indicated across a crystal; (bottom) composition ratio of Al/Si along the line scan across the crystal shown in top panel.

Figure 7. (Top) Dark-field STEM image (annual dark-field detector, camera length: 48 mm, GIF aperture entrance: 25 mm) of the sample annealed for 120 h with the location of line scan EELS analyses indicated across a crystal; (bottom) composition ratio of Al/Si along the line scan across the crystal shown in top panel.

slightly faceted crystals and therefore alumina enrichment might vary with position. Since such details could not become clearly discriminated upon EELS line-scan acquisition, the 8 h heat treatment data in Figure 8 might just be related to hitting such an alumina “pocket”. This interpretation is supported by the linescan shown in Figure 6, where the trajectory is hitting two ZrTiO4 nanocrystals. For the left one, an Al/Si ratio of 2.9 was determined while the vicinity of the right nanocrystal possesses an Al/Si value of just 1.3. In this respect, the maximum Al/Si ratio encountered for at least six linescans for each temperature might be afflicted with quite some errors. The general tendency, however, is just following the solid lines shown in Figure 8. There are two possible reasons why crystalline lithia aluminosilicate is formed after annealing times of 2 h or more. First, there is a composition change in the glass matrix: Zr and Ti are no longer present here, since the enrichment in phaseseparation droplets followed by ZrTiO4 crystallization is completed. In parallel, an inhomogeneous alumina distribution evolves on the nanoscale due to the cosegregation in the droplets

Figure 8. Temporal evolution of the maximum value of the Al/Si ratio derived from Figures 4−7. Lines are meant as guides to the eye and represent ZrTiO4 precipitation related and internal strain related stopping of growth (red line, 0−8 h) as well as exponential decay for longer times. 1561



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Figure 9. Schematic representation of the microstructure evolution in LAS glass-ceramics illustrating the action of nucleation agents.

movement in the glass structure. After many hours of heat treatment at 750 °C, LAS occupies almost the entire volume of the glass-ceramics. Because of finite diffusion coefficients and LAS formation consuming alumina, alumina-rich shells are diminishing as alumina gets redistributed after very long times. ZrTiO4 crystals, although no longer restricted in their growth and shape, do not coarsen but tend to adopt an elongated habit. On the basis of our current knowledge, the question as to why LAS crystals do not undergo Ostwald ripening after long times of heat treatment (but rather even seem to get reduced in size, Figure 3d) remains open. But most likely also in this respect, internal stresses play a major part (negative CTE of LAS versus positive CTE of the residual glass).

followed by the formation of alumina-rich barriers surrounding the ZrTiO4 crystals. Consequently, each former droplet possesses a circumjacent alumina rich shell in which optimum conditions for LAS nucleation are likely to be met. The second possible reason for LAS crystallization in the vicinity of ZrTiO4 nanocrystals is stress: the temporal increase in the ZrTiO4 crystal-number density in the first stages of nucleation is accompanied by stresses formed around the growing crystals on the nanoscale.27,28 In summary, the following model (sketched in Figure 9) is suggested. From the plain glass, zirconia, titania, and alumina enriched liquid−liquid phase-separation droplets of some 4 nm diameter are precipitated upon just heating up to 750 °C. Within 2 h at 750 °C, these droplets fully crystallize and form ZrTiO4 nanocrystals. In parallel, alumina is pushed toward the periphery of the droplets and forms an inherent barrier. This barrier is hampering growth and coarsening of the ZrTiO4 nanocrystals. Within this compositional gradient, decaying with time, ideal conditions for nucleation of the functional phase LAS high-quartz solid solution are given. In addition, internal stress is likely to also contribute to LAS nucleation. Both of these contributions do relate to neither homogeneous nor heterogeneous nucleation of the LAS high-quartz solid-solution crystals, Li2Al2Si3O10. While, in the classical sense, homogeneous nucleation is characterized by the absence of any foreign boundaries as loci for nucleation and is due, exclusively, to local density fluctuations, heterogeneous nucleation is associated with abrupt boundaries between adjacent phases. Beyond the aforementioned, LAS nucleation is postulated here to take place within the decomposing compositional gradient formed around former phase-separation droplets. This novel kind of nucleation mechanism could be best described as dif f usive nucleation since the compositional equilibration of the alumina enrichment zone by diffusion is vital for the formation of nuclei. This concept also readily explains the time lack between ZrTiO4 crystallization (80% between completion already after 1 h)22 and onset of Li2Al2Si3O10 crystallization after only 2 h: not the barrier itself matters for nucleation but its decomposition by diffusion causing plenty of

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CONCLUSION From a series of electron energy-loss spectroscopy linescans performed in a cutting-edge analytical transmission electron microscope, the temporal evolution of a compositional gradient form around phase-separation droplets in a lithia aluminosilicate glass containing zirconia and titania as nucleation agents is studied. It is found that alumina gets expelled from former amorphous, titania, zirconia, and alumina containing phaseseparation droplets. Consequently, an alumina-enriched layer is formed around the ZrTiO4 nanocrystals crystallized from the droplets. This explains the diameter reduction observed for the zirconia−titania−alumina phase-separation droplets upon ZrTiO4 crystallization. By monitoring the temporal evolution of the spatial extent and maximum concentration of the alumina-enriched diffusion barriers it is found that peak alumina concentrations increase within the first few hours of heat treatment but drop down for longer heat-treatment times when growth of the Li2Al2Si3O10 solid-solution crystals is completed. On the basis of the experimental results, a novel nucleation mechanism, namely, “diffusive nucleation”, that is, nucleation in the decomposing alumina gradient around former phase-separation droplets, perhaps supported by internal strain, is proposed. 1562



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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS T.H. is indebted to Dres. George H. Beall and Somnath Bhattacharyya for fruitful discussions. Part of the work was performed at Leibniz Institute for Surface Modification and one series of EELS line scans was acquired at FEI Nanoport in Eindhoven by Dr. Anna Carlsson. Funding by the European Commission under contract Nr. NMP3-CT-2006-033200 is gratefully acknowledged.

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REFERENCES

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Temporal Evolution of Diffusion Barriers Surrounding ZrTiO4 Nuclei in

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