Article pubs.acs.org/crystal
Origin of Crazing in Deuterated KDP Crystals Julien Zaccaro,*,†,‡ Jérôme Debray,†,‡ Sabine Douillet,†,‡ and Alain Ibanez†,‡ †
CNRS, Institut NEEL, 25 av. des Martyrs, BP 166, F-38042 Grenoble Cedex 09, France Université Grenoble Alpes, Institut NEEL, F-38042 Grenoble, France
‡
ABSTRACT: The H/D isotopic exchange between DKDP crystals [K(H(1‑x)Dx)2PO4] and their environment causes a surface damage known as crazing. By studying crystals, grown under perfectly stationary conditions, with a combination of analysis techniques (surface microtopography, micro-Raman spectroscopy, X-ray diffraction topography, trace element analysis) we were able to determine the localization of the isotopic exchange, quantify it, and connect it with a preferential incorporation of impurity. This enables us to propose a direct correlation between the growth mechanisms governing the impurity incorporation, the increase of proton diffusion in the structure in that part of the crystal hence on the localization, and the magnitude of crazing upon aging of the DKDP crystals.
1. INTRODUCTION KDP (KH2PO4) crystals have good electro-optic and nonlinear optical (NLO) properties. These properties, associated with the ability to grow rapidly bulk crystals in aqueous solution, led during the past decades to their wide involvement in commercial laser sources as well as large inertial laser fusion facilities.1,2 Moreover, the partial substitution of hydrogen by deuterium in DKDP (K(H(1‑x)Dx)2PO4) crystals leads to improved electro-optical and optical properties when compared with the pure KDP.3 However, these improvements come at a price. To begin with, high crystal quality is harder to achieve for DKDP than for KDP.4,5 Indeed, typical polythermal methods are ill adapted to achieve a perfect control of crystal growth for intermediate compositions of K(H(1‑x)Dx)2PO4 solid solutions6−8 inducing deuterium content D/(H + D) inhomogeneities and associated strains in the crystals. To overcome this drawback, methods have been proposed9−11 that allow the rapid growth of high quality DKDP crystals with homogeneous D/(H+D) distribution.12 On the other hand, for NLO devices, in addition to be slightly hygroscopic, DKDP optics exhibit a specific surface degradation referred to as crazing. This surface damage is considered to be the consequence of hydrogen− deuterium exchanges between the DKDP slab surface and its environment. The resulting exchange layer, with reduced D/(H +D),13,14 induces cell parameter evolutions at the crystal surface, associated tensile strains, and surface fractures. Surprisingly, this environmental degradation appears to be unsystematic, with some optical devices being more affected by this isotopic exchange than others. Explanations for this could be differences in surface preparation (lapping, polishing), material origin (the optic is cut from different part of the boule), or growth conditions13−15 (impurity content, variations in temperature-supersaturation in the polythermal growth method...). Sorting out all the combined possibilities makes the determination of which parameters are relevant to the © XXXX American Chemical Society
occurrence of crazing all the more challenging. That is why, to the best of our knowledge, only the later three referenced studies address this issue, and the parameters and mechanisms governing the occurrence of crazing remain unclear. In ref 15 it is noted that crazing concerns more the pyramid sectors (even stopping at the pyramid−prism boundary), and crystals grown at low temperature where the preferential Fe incorporation in the prism and exclusion from the pyramid are increased. This anticorrelation between the Fe impurity content and crazing occurrence led to the hypothesis that “crazing is more prevalent in higher purity crystals with more perfect lattice structure”. In that context, the observation of crazing in as-grown DKDP crystals elaborated under stationary conditions represents a unique opportunity to test that hypothesis. Indeed, the growth conditions are both perfectly stationary and reproducible, making comparisons straightforward, be it between different parts of a crystal (growth sectors) or between different crystals (different growth temperatures). Also, the observation on asgrown crystals allows eliminating any involvement of postgrowth processes (cutting, lapping, or polishing) and emphasizes the role of the growth conditions previously suggested to be a key parameter. In this study, we characterized the crazing of as-grown crystal faces by optical profilometer (microtopography), micro-Raman spectroscopy, ICP-AES and ICP-MS analyses, and X-ray topography in order to specify the origin of this phenomenon. All these methods combined allowed us to propose a mechanism favoring the formation of significant exchange layers producing crazing damages. Received: October 7, 2014 Revised: November 6, 2014
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dx.doi.org/10.1021/cg501491x | Cryst. Growth Des. XXXX, XXX, XXX−XXX
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Figure 1. K(D0.72H0.28)2PO4 crystal (view slightly tilted from the (110) mirror plan, c vertical) grown on a point seed at 20 °C with Rc = 7.8 mm/day and aged for about 30 months. Cracks of crazing are clearly visible on two adjacent pyramidal faces. Crazing is confined to triangular portions of the pyramidal faces that are oriented in opposite directions on two adjacent faces. No crazing is visible on the prismatic faces. The insert represents growth hillocks and corresponding vicinal sectors and their relative orientation in agreement with the I4̅2d space group. exchange layer at the crystal surface, which becomes strongly depleted in deuterium. In order to connect this layer formation with growth mechanisms it is important to determine if its occurrence is correlated with the growth face microtopography. The growth of the DKDP crystals occurred by step flow with dislocations serving as step sources. This results in the formation of growth hillocks with trihedral pyramid shape on all the pyramidal {101} faces and “rounded parallelogram” shaped ones on the prismatic {100} faces (see insert, Figure 1). The position of the hillocks and their different vicinal sectors (vicinal facets) were determined using a Bruker Contour GT optical profiler in vertical scanning interferometry mode with 5× or 50× objectives with a spatial sampling interval of 3.64 and 0.9 μm, respectively, and a vertical resolution of about 1 nm. 2.2.2. Micro-Raman Spectroscopy. To determine the depth of the exchange layer and the magnitude of the deuterium depletion we relied on micro-Raman spectroscopy.13 The deuterium content was derived, in the same way as presented in ref 12, from the measured position of the asymmetrical P(OD)2 stretching vibration (ν1, located around 880 cm−1). Raman scattering spectra were obtained between 700 and 1200 cm−1 at room temperature by a micro-Raman spectrograph (Jobin-Yvon T-64000). The excitation source used was a cw-Ar ion laser (514.5 nm) with a power of 1 mW. Illumination and collection of the backscattered light were made through a confocal microscope (aperture of 0.120 mm, 50× objective). The spectrograph was in the direct single spectrometer mode using a grating of 1800 gr/ mm. This setup results in a spectral resolution of 0.86 cm−1 with a confocal volume of 4 × 4 × 10 μm3 (10 μm in the axial direction). The lowering of the confocal volume allowed determination of the depth profile of the deuterium content in different parts of DKDP samples to correlate the H−D exchange with other structural features (growth sectors, crystal defects, impurities...). 2.2.3. X-ray Diffraction Topography. To characterize the crystal quality, locate the sector boundaries in the volume (growth and vicinal sectors), and identify possible defects, we used X-ray diffraction
2. EXPERIMENTAL SECTION The system used to grow the DKDP crystals has been presented in detail elsewhere,11 and the mechanism of their growth was identified.12 So, regarding the crystal elaboration, we will only summarize here the most important points that are relevant for the matter at hand. 2.1. Crystal Growth Conditions. The DKDP crystals were grown in a high purity (electronic grade) fused silica reactor by a modified Walker−Kohman method.11 The growth solutions and nutrients were prepared from high purity starting materials: type 1 ultrapure water (18.2 MΩ cm), heavy water, and KDP salt (Merck Optipur grade). The method we have developed allows rapid crystal growth under stationary conditions (temperature and supersaturation). Several sets of stationary conditions were tested. The growth temperature, Tgrowth, was fixed in the 17−26 °C range associated with a relative supersaturation, σ, in the range 10−35%. We evidenced that, under these experimental conditions, crystal growth always occurs in our reactor by step flow (ex-situ AFM images showed no sign of 2D nucleation) with screw dislocations acting as step sources and that growth kinetics are mass transport limited. As a result, the supersaturation available at the crystal surface differs significantly from that applied to the bulk solution. Hence, instead of referring to the applied supersaturations we will use in this article the growth rate along the c axis, Rc, as a more transposable indication of the supersaturation. With the applied supersaturations being stationary, the achieved growth rates were perfectly constant throughout the growth runs and ranged from Rc = 3 to 10 mm/day.11 Under these various sets of conditions, DKDP crystals up to 2 in. wide have been grown with several D/(H + D) ratios, between 0.61 and 0.84. They have all been found highly homogeneous in deuterium (no growth bands or segregation), in and between growth sectors, and free from extended defects aside from a couple of dislocations.12 2.2. Characterizations. 2.2.1. Crazing Localization Observation. As indicated above, crazing is associated with the formation of an B
dx.doi.org/10.1021/cg501491x | Cryst. Growth Des. XXXX, XXX, XXX−XXX
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Figure 2. 5× magnified surface microtopography of a growth hillock on a {101} face close to the edge showing the three vicinal sectors with different slopes: steep (bottom), shallow (upper left), and intermediate (upper right). The three inserts show in false colors 50× magnified surface profiles in each vicinal sector. Crazing (visible with fractures running along the ⟨010⟩ and ⟨001⟩ crystallographic directions) occurs only in the shallow sector.
rate, and deuterium content. The first important fact is that, upon aging at ambient conditions, crazing was systematically observed on the DKDP crystals independently of the growth temperature (in the range Tgrowth = 17−26 °C), the supersaturation and related growth rate (Rc = 3−10 mm/ day), or the deuterium content of the crystal: D/(H + D) = 0.61−0.84. Furthermore, crazing was always confined to triangular portions of the pyramidal {101} natural faces while no sign of surface degradation was observed on the prismatic ones {100} (see Figure 1). Moreover, on a given DKDP crystal, the orientations of the triangular crazed portions on the different pyramidal faces are not random but obey the I42̅ d symmetry of the crystal: they are mirrored through {110} planes. That is, two adjacent pyramidal faces have crazed portions orientated in opposite directions (see Figure 1). This indicates that crazing is neither a global process nor randomly appearing on the crystal surface but is governed by a phenomenon directly related to the atomic structure. In fact, the shape and orientation of the crazed portions is strongly evocative of the vicinal facets formed on the pyramidal faces. In crystals of the KDP family, the growth hillocks generated by screw dislocations on the pyramidal faces assume the shape of trihedral pyramids. At the molecular level, each one of the 3 triangular shaped facets of these hillocks corresponds to vicinal facets with {101} terraces but with different step orientations and advancement speeds. As a result, they present three distinct slopes that allow identifying the corresponding vicinal sectors: steep, intermediate, and shallow, labeled St, Int, and Sh, respectively, in the inset of Figure 1. To confirm the correlation between crazing and the vicinal sectors, the microtopography of pyramidal {101} growth faces was determined. Figure 2 presents an observed microtopography under 5× magnification and shows a characteristic growth hillock. The different vicinal sectors are easily identified from the different spacing of the interference fringes: the
topography in Lang geometry. The samples were {100} or {101} crystal slabs, 450 μm thick. Traverse topographs were recorded with the symmetrical (020) reflection using the Mo Kβ radiation from a sealed tube (F020 = 112.98, μt = 1.77). To reduce the sample surface contribution to the images and thus favor contrasts arising from possible structural defects present in the bulk of samples, the slabs were polished to high optical quality by a mechanochemical process. Topography images were recorded on high resolution KODAK INDUSTREX M100 films. 2.2.4. ICP-AES, ICP-MS. While X-ray diffraction topography allows imaging almost any kind of extended defects, it is not well-suited for point defect characterization (vacancies or impurities). To determine if a specific impurity is correlated with the formation of the exchange layer, the content of trace elements in different zones of a crystal was investigated using inductively coupled plasma atomic emission spectroscopy (ICP-AES, Thermo Fischer ICap 6300) and inductively coupled plasma mass spectrometry (ICP-MS, Agilent 7500CX). These techniques offer high sensitivity for trace elements, from fractions to several ppm depending on the elements and the detection technique. However, both methods require dissolving the solid to be analyzed and thus can only give content information averaged over whole samples. That is why samples were extracted from each type of vicinal and growth sectors in a single DKDP crystal (deuterium content 0.72, Tgrowth = 20 °C, Rc = 7.8 mm/day). Thus, slabs, 2 mm thick, were cut along {100} and {101} faces. The {101} slab, previously observed with the optical profiler, exhibited only one vicinal hillock, which was then separated into each individual vicinal sector. To minimize the surface pollution caused by the crystal slicing and handling, the specimens’ surfaces were thoroughly dissolved under a stream of type 1 ultrapure water (18.2 MΩ cm) and immediately dried by nitrogen gas. These samples were dissolved in ultrapure HNO3 for analysis. For the quantification of the impurities present, internal standardization was used, In for ICP-MS and Sc for ICP-AES. The resulting errors on the contents are typically around 5% of the reported values.
3. RESULTS AND DISCUSSION 3.1. Crazing Occurrence and Localization. Many crystal growth experiments have been conducted, each at a specific set point regarding growth temperature, supersaturation/growth C
dx.doi.org/10.1021/cg501491x | Cryst. Growth Des. XXXX, XXX, XXX−XXX
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Figure 3. Depth profiles of the isotopic composition determined by microRaman spectroscopy in different vicinal sectors and growth sectors of the DKDP crystal presented in Figure 1. The data points were fitted to Fick’s second law of diffusion; their size represents the error bars.
Figure 4. Traverse topograph of a (100) slab of DKDP (D/(H + D) = 0.72) taken with the symmetrical (020) reflection (diffraction vector g horizontal, c axis vertical) using the Mo Kβ radiation from a sealed tube (F020 = 112.98, μt = 1.77). The inset shows the D/(H + D) depth profile determined by micro-Raman perpendicularly to the slab [along (010)] at the two locations marked by colored squares (red in the shallow and blue in the steep vicinal sector).
shallow sector is at the upper left, the steep one is at the bottom, and the intermediate is on the upper right. Upon further magnification (50×, inserts) one can clearly see fractures running along ⟨001⟩ and ⟨010⟩ associated with crazing in the shallow vicinal sector. No such fractures are observed in the other two sectors, with the perturbation of the
fringe system in the intermediate sector only denoting a local deviation from flatness. Hence, as anticipated, crazing occurrence strongly depends on vicinal sectoriality. On all crystals, the fractures of crazing were found to be restricted to the shallow vicinal sectors that they cover completely. D
dx.doi.org/10.1021/cg501491x | Cryst. Growth Des. XXXX, XXX, XXX−XXX
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3.2. Deuterium Loss. To measure the extent of the D−H exchange layer associated with crazing and how sensitive it is to the vicinal and growth sectors, we determined depth profiles of deuterium content using micro-Raman spectroscopy (Figure 3) in different parts of DKDP crystals. Figure 3 presents the D/(H + D) profiles determined for the three vicinal sectors of {101} and for the two major sectors of {100} (referred to as top and bottom vicinal, even though they are symmetry equivalent), for the crystal shown in Figure 1. The first striking features are the magnitude of the deuterium loss (down to 12% at the surface from 72% in the bulk) and the depth of around 800 μm for the exchange layer observed in the shallow vicinal sector (green Figure 3). Both the loss and the depth in this sector are several times larger than those measured in the other vicinal sectors of the same face or in the other natural face (prismatic). While this difference is probably exacerbated in Figure 3 by the fractures of crazing that favors the exchange with the environment in the depth of the sector, this trend has been systematically observed in all the analyzed specimens, even those that hadn’t developed cracks (see insert Figure 4, for example). Far from the surface, the deuterium content in all sectors reaches the same asymptotical value (72% in Figure 3) corresponding the as-grown D/(H + D) ratio. This confirms the results presented in ref 12: the isotopic content of the DKDP crystals was found to be highly homogeneous. Contrary to what is suggested in page 35 of ref 5, we did not observe any preferential incorporation of H or D in one particular vicinal sector during the growth. Hence, the particularly low deuterium content found in the shallow sector is in no way due to an initial deficit in deuterium formed during the growth. Rather, a feature of the shallow sector enhances the deuterium loss in this specific area, forming thus a much more marked exchange layer ultimately leading to crazing. The remark in ref 5 (p. 35) can then be viewed as the observation of a postgrowth deuterium loss rather than a deuterium segregation during growth. Lastly, one can clearly see that the exchange layer is found to be all the more pronounced (in deuterium loss and depth) from the steep sector (purple Figure 3), through the intermediate (blue Figure 3), to the shallow one (green Figure 3). To quantify this trend we fitted Fick’s second law of diffusion to the experimental deuterium content depth profiles. In samples where the depth profiles were not altered by cracks, the exchange layer depths were in proportions of about 3.6:1.4:1 for the shallow, intermediate, and steep vicinal sectors, respectively. While there is a correlation in the pyramidal face between the slope of each sector and the magnitude of the exchange layer formed, the slope and more generally the sector microtopography are hardly the cause of that differentiation. Indeed, these three vicinal sectors consist of {101} terraces and only differ from one another by the step density (defining the vicinal slope) and the orientation of the steps risers. For the step orientation to significantly affect the deuterium loss would require both a substantial diffusion through the step risers and an anisotropy in the H/D diffusion. Yet, according to the literature, no major anisotropy of proton diffusion exists in pure KDP.16,17 On the other hand, if the step density played a significant role in the formation of the exchange layer, then one would expect the sector with more steps, the steeper one, to show the most important exchange layer as the total surface over which proton exchange may occur increases. Yet, we observe precisely the opposite.
Hence, the sectors’ microtopography itself does not explain the peculiar exchange layer of the shallow sector leading to crazing. The two are indeed correlated but through another phenomenon that is to be found in the bulk of the material 3.3. Correlation of Crazing and Vicinal Sectoriality. 3.3.1. X-ray Diffraction Topography. The formation of an exchange layer, large enough to induce crazing, requires the diffusion of protons deep into the structure. In order to determine if the shallow sector can be associated with specific extended defects in the bulk of DKDP crystals, we recorded Xray diffraction topographs of {100} slabs in traverse geometry. Before sample slicing, the disposition of the vicinal sectors was determined on the crystal dome with the optical profiler, and the cut was made along the (100) plane so as to include a shallow sector. A topogram obtained for the symmetric reflection (020) is presented Figure 4. It shows no extended defects in any sector and very few dislocations. Also, no misorientations between sectors, or growth bands, striations, or strains associated with deuterium content fluctuations are visible. Hence the crazing localization cannot be explained by the presence of extended defects in a particular vicinal sector. The most visible contrast is a geometrical shape in the pyramidal sector. It corresponds to a vicinal sector with enhanced contrast and strains at the sector boundary (VSB). On the basis of the microtopography of the pyramidal growth face, where this subsector emerges (upper right side of the specimen), it corresponds to a shallow vicinal sector while the rest of the pyramidal sector is associated with the steep one. Such high contrasts for the shallow subsector and the shallow/ steep sector boundary have already been observed in topograms of pure KDP18 and ADP (NH4H2PO4)19 and were attributed to specific impurities content. Such zonal inhomogeneities lead to sudden changes in stress at the vicinal sector boundaries that are relaxed by deformation of the lattice. This creates a strong contrast in weak X-ray absorption conditions, the shallow/steep boundary being the most pronounced. For KDP, J. J. De Yoreo and co-worker further noted in ref 18 that, with increasing growth rates, the compositional zoning decreased between prismatic and pyramidal growth sectors but increased between vicinal sectors in the pyramidal zone. That is also what we observe here with a faint contrast between the steep and prismatic sectors and a strong one between shallow and steep subsectors. With the two compounds involved in those previous studies (KDP and ADP) being deuterium free, the contrasts can solely be attributed to selective incorporations of impurities in the vicinal sectors of {101} faces. In this DKDP study, such impurity segregation is most probably at play too, but the change in the lattice parameters of the crystal can also be due in part to the peculiar deuterium loss in the shallow vicinal sector. To estimate that loss, D/(H + D) profiles in the depth of the (100) slab were measured in the shallow and steep subsectors (positions marked in red and blue, respectively, Figure 4) free from any extended defect. The deuterium loss (see insert Figure 4) through the cut surface of the slab is 4.3 times bigger in the shallow subsector than in the steep subsector, which is rather similar to the 3.6 ratio of D/(H + D) between the shallow and steep vicinal sectors observed for natural faces (see section 3.2). This further demonstrates that crazing in the shallow vicinal sector is not due to the microtopography of the surface or proton/deuteron diffusion anisotropy since the same behavior is observed with the same E
dx.doi.org/10.1021/cg501491x | Cryst. Growth Des. XXXX, XXX, XXX−XXX
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Table 1. Impurity Content of Different Growth and Vicinal Sectors of the DKDP Crystal Presented in Figure 1 in ppm vicinal sectors shallow intermediate steep
B
Na
Si
