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In-situ Observation of Phase Separation During Growth of CsLiLaBr:Ce Crystals Using Energy-Resolved Neutron Imaging 2

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Anton S. Tremsin, Didier Perrodin, Adrian S. Losko, Sven C. Vogel, Takenao Shinohara, Kenichi Oikawa, Jeff H Peterson, Chang Zhang, Jeffrey J. Derby, Alexander M. Zlokapa, Gregory A Bizarri, and Edith D. Bourret Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.7b01048 • Publication Date (Web): 16 Oct 2017 Downloaded from http://pubs.acs.org on October 19, 2017

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Crystal Growth & Design

In-situ Observation of Phase Separation During Growth of Cs2LiLaBr6:Ce Crystals Using EnergyResolved Neutron Imaging Anton S. Tremsin1, Didier Perrodin2, Adrian S. Losko3, Sven C. Vogel3, Takenao Shinohara4, Kenichi Oikawa4, Jeff H. Peterson5, Chang Zhang5, Jeffrey J. Derby5, Alexander M. Zlokapa1, Gregory A. Bizarri2, Edith D. Bourret2. 1

Space Sciences Laboratory, University of California at Berkeley, Berkeley, CA 94720, USA

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Lawrence Berkeley National Laboratory, Berkeley, CA 94720, USA

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MST-8, Los Alamos National Laboratory, Los Alamos, NM 87545, USA

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Japan Atomic Energy Agency, Naka-gun Ibaraki 319-1195, Japan

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Department of Chemical Engineering and Materials Science, University of Minnesota,

Minneapolis, MN 55455, USA KEYWORDS Crystal growth, Phase separation, In-situ diagnostics, Neutron imaging, Elpasolite, Halide

ABSTRACT In-situ imaging and characterization of Cs2LiLaBr6:Ce crystal growth is performed utilizing energy-resolved neutron imaging. The unique capability of neutrons to penetrate the furnace and

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to provide direct information on the materials within the furnace is used to visualize the growth dynamics, the location and shape of liquid/solid interface and to map the elemental composition. Non-trivial dynamics of phase separation within the liquid and solid phases were observed and investigated. Quantitative projected 2D maps of Li concentrations were obtained with sub-mm spatial resolution delineating Li-rich and Li-depleted areas. Concurrent variations in Cs and Br concentrations were identified. Good transparency was obtained in part of the ingot where the liquid phase separation has reached steady state, suggesting that non-stoichiometric materials may be optimal for the original charge. The results demonstrate that energy-resolved neutron imaging and its associated modalities can provide unique information for the optimization of crystal growth conditions, in particular having the potential to accelerate scale-up from laboratory to commercial production by improving the yield and quality of single crystal materials.

1. INTRODUCTION Cs2LiLaBr6:Ce (or CLLB:Ce), a new scintillator of the elpasolite family, was described for its dual gamma-neutron detection capability (1) and has now found its way into a commercial product. The hygroscopic crystals are grown by the Bridgman-Stockbarger technique and crystals up to 2"inch in diameter have been produced (2). However, as with many other elpasolites, the growth of the crystals is difficult and has been particularly hindered by issues of phase separation (3). The growth of CLLB crystals was found to produce nearly stoichiometric and optically transparent material only in the middle section of the grown crystal, while the top and the bottom sections typically contain CsBr and LiBr residual phases, as revealed by X-ray diffraction (3). That substantially reduces the yield of CLLB crystals.


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Discovery and characterization of novel single crystal materials is usually performed with small samples on a millimeter scale (3), (4). If successful, that exploratory stage needs to be followed by the development of crystal growth procedures for much larger sample dimensions required for their use in specific applications. The difficulties of large, single crystal growth in many cases introduce very long delays between the material discovery and commercial production. Among the main difficulties are the limited diagnostics available during crystal growth, resulting in the need to assess the quality of resulting material after the entire growth sequence is finished by indirect, post-growth destructive observations. As a result, in many cases a lengthy, trial and error approach is implemented by arbitrarily varying growth conditions followed by post-growth examination, which makes process development slow and inefficient. Methods that allow for in-situ observation of crystal growth under different conditions, providing quantitative information on parameters such as location and shape of the liquid/solid interface, distribution of dopant atoms, elemental composition of the crystal and melt, temperature profiles, uniformity of crystal lattice and its orientation, and stresses in the material, would be quite valuable for efficient optimization of growth conditions. Such information would substantially accelerate the development of new materials and ultimately increase the yield and decrease the production cost. In many cases, where conventional probes such as X-ray or electron beams are utilized, in-situ diagnostics are impeded by the opacity of the growth equipment or the material itself. Indeed, Xray absorbing materials, such as gamma scintillators, cannot be probed by X-ray techniques as photons cannot penetrate these materials, and growth parameters can at best be assessed in situ by the measurements at the sample periphery. It has been demonstrated previously that neutrons are


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unique probes for such materials, as they can penetrate both growth equipment (including many metals and ceramics) and the crystal materials themselves (5), (6). In this paper we utilize energy resolved neutron imaging for the in-situ diagnostics of growth conditions of Cs2LiLaBr6:Ce gamma scintillator crystals, grown by a Vertical Gradient Freeze (sub-type of Bridgman) technique (7). We demonstrate in-situ observation of phase separation and other growth related phenomena during growth. The location and shape of the liquid/solid interface, the elemental composition and granularity of crystals are visualized with sub-mm resolution within the 12 mm diameter samples. Phase separation was clearly observed appearing in the melt, where Cs-rich/Li-deficient phase is formed just above the liquid/solid interface with a typical time constant of several hours. Supporting information contains the movies depicting the dynamics of several processes observed in-situ during Cs2LiLaBr6:Ce melting from the original charge (movie S1), crystal growth (movies S2-S4) and the scan through the energies representing the spatially-resolved diffraction contrast by the solid phase of the sample (movies S5,S6).

2. EXPERIMENTAL METHODS The results presented in this paper were obtained in three separate experiments at two pulsed spallation neutron beamline facilities: the Materials and Life Sciences Facility at the Japan Proton Accelerator Research Complex (J-PARC, Raden beamline for the Experiment 1 (8), Noboru beamline, Experiment 3 (9)) and Experiment 2 was conducted at the Lujan Neutron Scattering Center at Los Alamos National Laboratory (LANSCE, FP5 beamline (10)). A small clam-shell furnace was installed in front of a fast neutron counting detector capable of simultaneous detection of transmission spectra in each 55 µm pixel of 512x512 quad Timepix readout (11), (12), Figure


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1. The energy or wavelength of each detected neutron was calculated from the time it took to reach the detector active area, measured relative to the time of neutron production by pulsed neutron sources, operating at 25 Hz and 20 Hz frequency at J-PARC and LANSCE, respectively. Our furnace was designed with the aim of minimizing the distance between the sample and the detector to prevent the image blurring by finite beam divergence, which in our experiments was ~0.2 degrees. All measured spectra were corrected for background signal and normalized by the spectrum obtained with no sample placed in the furnace installed in the neutron beam, allowing removal of the beam own spectral and spatial distributions and detector non-uniformities. No chopper systems were used in our experiments, and the entire neutron spectrum produced in a spallation process was illuminating the sample. A 5 cm thick bismuth filter (J-PARC) and 5 cm Lead filter (LANL) were used to reduce the unwanted gamma flux from reaching the detector while only slightly attenuating the neutron spectrum without introducing spectral features (i.e. neither material has any neutron absorption resonances in the energy region utilized here).

Energy-resolved neutron imaging enables quantitative studies of elemental composition (through neutron resonance absorption in the epithermal range of energies) and crystallographic properties of crystalline materials through neutron coherent scattering in thermal range of energies (13), (14). Simultaneous measurement of neutron transmission spectra in each pixel allows these quantifications to be performed in each 55 µm pixel of our data set, provided there are sufficient neutron statistics for such analysis, as described in detail in reference (6) and illustrated in Figure 1. Conventionally, neutron diffraction experiments utilize neutrons which are diffracted by the sample away from the direct beam. Many crystallographic and microstructural properties can be


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measured in such experiments, from crystal structure (15), (16), (17), to magnetic structure (18), to texture (14), (19), (20), to strain (21), (22), (23), (24) and other parameters. However, in the present experiments we measured the neutrons which were left in the transmitted beam, allowing spatially resolved study in one measurement, without the need to scan the neutron beam across the sample, with the penalty of averaging through the sample thickness. For crystalline materials, neutrons are diffracted at wavelengths which fulfill the Bragg equation hkl=2dhklsin(). In case of a single crystal or large grained materials, this results in the reduced intensity of the transmitted neutron beam at a specific set of wavelengths for which Bragg's law is fulfilled. At the same time there is no coherent neutron scattering by the liquid phase. As a result, at certain wavelengths hkl there is a change in the value of transmission (which also depends on sample orientation relative to the beam) between the liquid and solid phases of the sample, allowing imaging location and shape of the liquid/solid interface. The uniformity of single crystal and even shape and location of single crystal grains have been already visualized by neutrons in the transmission mode (25), (26). Here we extend this method to confirm the location of the interface between the solid and liquid phases visualized by the change in the elemental composition as seen in Figures 3-5.

Sample Preparation. Cs2LiLaBr6:0.5mole%Ce (CLLB) samples were pre-synthesized within a silica glass ampule (~12 mm diameter) sealed in vacuum. For the 20g of CLLB sample the following quantities of raw materials were used (all 5N purity): CsBr beads from Aldrich 9.427g, LiBr anhydrous beads (Aldrich) 1.943g, LaBr3 anhydrous beads (Aldrich) 8.460g, and CeBr3 powder 0.170g. The raw materials were dried overnight at 110 oC under 10-7 bar vacuum prior to sealing the ampule. Synthesis of the material was performed by heating the prepared mixture of raw materials up to 800°C in 4h, followed by a dwell time of 2 hours at 800°C, then a ramp down


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to 540°C in 2 hours, dwelling at that temperature for 8 hours, and finally cooling down to room temperature over an 8 hour period.

In-situ Crystal Growth. The ampule containing the pre-synthesized polycrystalline charge was placed in a vertically-oriented, single zone clam-shell furnace, which produced the maximum temperature at the mid-point of the furnace, with temperature gradually decreasing towards its bottom with a much slower decrease towards the top (6). The furnace wall was located at a distance of ~5 mm from the neutron detector aligned with the neutron beam (Figure 1), such that sample was positioned ~7 cm from the detector’s active area. The furnace steel shell had cutouts of 3x3 cm2 within the beam path. The neutron transmission of the furnace was greater than 80% allowing for efficient interrogation of the sample during crystal growth. A simple Vertical Gradient Freeze method, as reported in (7), was used to grow CLLB crystals. The temperature was actively controlled by a feedback loop, with a control thermocouple placed in the close proximity to the ampule wall in the middle of the furnace. While the sample remained stationary within the furnace, the location of the solid/liquid interface could be moved within the length of the ampule by varying the temperature set point of the control thermocouple within ~50 degree. At the beginning of each imaging experiment the temperature was ramped up to ~575 °C (the melting point of Cs2LiLaBr6 is at 493 oC (3)) over a period of ~1 hour, while the location of the liquid/solid interface was visualized by ~30 s acquisition images, The location of the interface was visible, provided by the difference in neutron attenuation between the solid and liquid phases (due to the change in the elemental composition and density of the melt) and further confirmed by imaging where contrast was provided by neutron coherent scattering, Section 3.2.


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After an initial ramp up to ~450 °C, the temperature of the furnace was slowly raised to the point where the liquid phase was in the desired position within the active area of the detector, with melt maintained above that point and solid phase formed below as determined by the temperature profile of the furnace described in detail in reference (6). The steady state condition followed by the growth procedure was started at that point (later referred as time T0) with slow temperature ramp down for a typical solidification rate of ~2 mm/hr. With this simple procedure, the crystal is not properly self-seeded and the growth proceeds from a polycrystalline charge. The timing of procedures for Experiments 1-3 is described in Table 1.

Table 1. Summary of experimental procedures during in-situ crystal growth Procedure Duration Initial T, oC Final T, oC Ramp up ~2 hr. 25 450 Melting ~2 hr. 450 555 Steady state ~11.5 hr. 555 555 Growth ~ 7 hr. 555 530 Quenching N/A. 530 25 ----------------------------------------------------------------------------------------------N2 Ramp up ~2 hr. 25 520 Melting ~4.5 hr. 520 565 Steady state ~10 hr. 565 565 Growth ~3.5 hr. 565 520 Quenching N/A 520 25 ----------------------------------------------------------------------------------------------N3 Ramp up ~2 hr. 25 500 Melting ~1 hr. 500 560 Steady state 0 hr. Growth ~3 hr. 560 530 Quenching N/A 530 25

Experiment N1

A steady temperature was held at the beginning of the process for several hours in two experiments (Experiments 1 and 2), as described in the next section, in order to observe the


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dynamics of liquid phase separation that occurred independent of crystal growth. The energyresolved neutron transmission images were acquired every 20-45 minutes. At the end of the growth sequence, the samples were cooled to room temperatures at rates much higher than for proper single crystal growth in order to save valuable time at the beamline facility. For the same reason, no special procedures were implemented to promote proper nucleation at the initiation of growth in either of these experiments. Rather, the main aim of these experiments was to investigate the dynamics of phase separation within the liquid, which was not affected by these compromises in growth procedures.

3. RESULTS AND DISCUSSION 3.1 Liquid phase separation. It has been shown previously that neutron transmission imaging can be used for the in-situ visualization of liquid/solid interfaces and for studies of elemental composition and segregation within the sample materials during crystal growth (5), (6). A more detailed study of Cs2LiLaBr6:Ce (CLLB) crystal growth was performed in our present experiments, where phase separation within the liquid phase was observed in-situ. A typical sequence of full spectrum neutron transmission images obtained in these in-situ experiments is shown in Figure 2 (Experiment 1), with the corresponding times measured from the start time of the process T0. As mentioned earlier, the bottom of the charge was not re-melted and remained in its pre-synthesized polycrystalline condition. The volume above the dashed lines in Figure 2 was melted (see section 3.2). The first ~11 hours the sample was held at a constant temperature of ~555 °C during which the dynamics of phase separation within the liquid phase was studied. First of all, it was noticed that a band with higher neutron transmission at thermal neutron energies was formed at the very bottom of the melt, just above the liquid/solid interface. The


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contrast in the images shown in Figure 2 is relatively low due to small changes in elemental composition. A better way to visualize the changes in the material composition in the grown volume is to look at the ratio of two consecutively acquired images, as shown in Figure 3. There only the areas where the composition of the CLLB sample changed within the ~2.2 hour time period deviate from the gray value. The whiter sections of images in Figure 3 correspond to an increase in transmission with time, while darker areas show sections of the sample where transmission has decreased. The contrast in all images of Figures 2 and 3 is almost exclusively provided by the variation of Li concentration, since Li atoms have largest attenuation cross section for thermal and cold neutrons, exceeding 1000 barns, (27). Therefore, the lighter areas of images in Figure 3 correspond to the sample volume where concentration of Li was reduced within the last ~2.2 hour time period relative to the time shown in the legend of Figures 2 and 3. It should be noted here that the location of the solid/liquid interface remained practically stationary during the first ~11.2 hours of our experiment, when the temperature was held steady at ~555 °C. On the opposite, the darker areas in Figure 3 correspond to the region of the sample where concentration of Li atoms has increased during the interval between two measurements. Therefore the Li concentration in the grown solid volume (at 15.8 and 18 hours, Figure 3) is larger than in the liquid phase above the interface. In addition to obvious liquid/solid interface, a second interface (marked as “Interface II” in Figure 3) was clearly observed within the liquid phase (as seen in Figure 2), which moved downward slowly within the first ~11 hours while the furnace temperature was unchanged, forming top dark areas in Figure 3, until it merged with the growing Li-deficient band of higher transparency. That top interface (within the liquid phase) represents the transition from Cs-rich/Lideficient melt to the liquid with nearly stoichiometric composition, at least for Li and Cs atoms. It


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is interesting to note that crystal growth within hours ~11 to 18 did not affect the elemental composition of the Li-deficient lighter band (Figure 2). The most interesting observation in that experiment was the fact that the crystal grown immediately above the Li-deficient band was transparent as indicated by the dashed rectangle of Figure 2 (photo of crystal on the right was taken after the experiment), while it appeared to be milky at the location of the lighter band as well as in the ingot top areas where the melt was quenched relatively rapidly. This is in agreement with previously reported growth CLLB crystals (3). As suggested in that report, one explanation of the observed phase separation can be partitioning of some fraction of CLLB melt into CsBr and LiBr inclusions at certain temperature range above melting point, where CsBr settles down driven by gravity due to large difference in atomic weight between CsBr and LiBr. Above that temperature range the melt remained in a uniform state as observed in our experiments. Another interesting phenomenon observed in our imaging experiments was the fact that there was some redistribution of elements occurring within the solid phase of the sample, as highlighted by the dashed ovals in Figure 3. At the center of the sample a Li-deficient cluster was formed within the solid phase (in the volume which was not melted in our experiment and which remained in the original charge form) and it moved downward with time, as better seen in the movies in the supplemental material associated with this article. The in-situ imaging Experiment 1 was repeated twice at two different neutron sources in order to verify the reproducibility of the observed phenomenon of phase separation, as shown in Figures 4 and 5. Phase separation was also observed during melting of the original charge, seen in Figure 4. The bottom white band in these images indicates the location of solid-liquid interface, with decreased Li-concentration in the melt just above the solid; the top dark band corresponds to the interface between two liquid phases, where concentration of Li increases at the upper liquid phase


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as temperature of the furnace is increased. The dynamics of two liquid phases similar to those in the first experiment was observed here at the steady state conditions where a Li-deficient/Cs-rich band was formed between the white and dark bands. No liquid phase remixing was observed during following crystal growth, just as in the previous experiment. In the experiment depicted in Figure 5, there was no steady state temperature conditions and the phase separation within the liquid phase did not reach a saturation point. A higher transmission region continued to exist with nearly constant thickness within the sample volume where sample was grown steadily at a rate of ~2 mm/hr immediately after the charge was melted. The darker band in these images corresponds to the area where CLLB crystal was grown during the time between two measurements (with more Li incorporated into solid phase compared to the liquid phase above it), while the upper lighter band indicates the location of the interface between two liquid phases, where Li has lower concentration towards the end of the time interval compared to the starting time. Thus the interface between the solid and liquid phases moved up with nearly the same speed as the interface between two liquid phases.

3.2.

Location of solid/liquid interface through transmission diffraction. The exact location

and shape of the interface between liquid and solid phases was confirmed by the analysis of neutron diffraction by the CLLB sample. Figure 6.a shows narrow-energy neutron transmission image, acquired in-situ during the equilibration phase. Bragg scattering within the crystalline area of the sample was sufficient to change the transmission contrast at this particular wavelength. The contrast in this narrow-energy image is not due to the variation of sample elemental composition, as shown in Figures 2-5, but rather due to variation of neutron coherent scattering which is determined by the crystal structure. Only the solid phase had coherent neutron scattering removing


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neutrons from the transmitted beam. The location of the interface becomes more apparent in the movies which can be seen in the supplemental material associated with this article. The narrow energy transmission images allowed us to explicitly reconstruct the location of the solid/liquid interface. Since there was no proper nucleation performed in our in-situ crystal growth it is expected to have multiple grains within the solid volume of CLLB material, as seen in Figure 6.b acquired at the end of growth. The large clear area (supposedly in a single crystal form) seen in Figure 2 was grown above the area where phase separation reached steady condition. From these observations, we note that this technique can be used to reveal in-situ the formation of unwanted grain nucleation if they were to occur in properly seeded experiments, another useful diagnostic tool for crystal growth.

3.3.

Quantification of elemental composition through neutron resonance absorption. In

addition to crystallographic properties, revealed by neutron transmission in thermal and cold energy regions, neutron resonance absorption (appearing for most materials in epithermal range of energies, typically above 10 eV and for some materials only in MeV ranges) can be used to evaluate the elemental composition (13), (28), (29), (30), providing neutron transmission spectra can be measured at these energies by the experimental setup. Since our experiments were conducted at pulsed neutron sources where transmission was measured in a very wide range of energies (~0.001 – 1000 eV) the concentration of certain elements within our crystal samples can be reconstructed from the presence of sharp neutron attenuation (referred to as resonances) at a specific set of neutron energies characteristic to a particular element, or more specifically its isotope. In case of CLLB material, Cs, Br and Li provide sufficient signal for elemental mapping. Cs and Br have relatively large resonance cross sections and Li has the largest absorption of


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thermal and cold neutrons (27). The uniqueness of this technique is in its ability to study elemental distribution within both liquid and solid phases of the materials as crystals are being grown. However, the neutron attenuation by surrounding materials (e.g. furnace, sample container, etc.) need to be separated from the raw measured transmission spectra. That is usually achieved by measurements of neutron transmission with no sample installed in the beam, as demonstrated in Figure 7. The raw neutron spectra of an empty furnace marked in Figure 7.a as a direct neutron beam shows spectral features related to the neutron beam itself (e.g. > 800 eV resonances of Bismuth filter used in measurements, thermal peak of neutron beam at ~15 meV, Bragg dips formed by large grains of Bismuth filter and others). With the sample placed in the furnace within the incoming neutron beam, the transmission spectrum shows multiple resonances as seen in Figure 7. The largest uncertainty of our elemental mapping was determined by the limited accuracy in the determination of the background spectrum, which needs to be taken into account for the reconstruction of the sample transmission, shown in Figure 7.b. The complication of background characterization comes from the fact that it consists of both neutron and gamma components, which were not properly characterized in present experiments. Some of those gamma photons are produced at the source and reach the detector through a direct path, others are produced by neutron interaction with shielding, furnace, grown material and other equipment. Those secondary gammas arrive to the detector at various times relative to the time of spallation. One of the background calibration techniques uses a set of filters made from various materials with an appropriate set of resonances, placed in the beam far from the detector active area. A set of opaque resonances (energies at which such filters completely absorb neutron flux) can be used to measure the background signal only at those resonance energies. Providing those opaque resonances can sparsely span the entire spectrum used in the data analysis an accurate reconstruction of measured


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spectra can be performed. Multiple filter materials need to be measured in order to cover the wide spectrum used in our study. A more detailed beam characterization will be used in the future experiments for a more precise reconstruction of the background spectra for the accurate elemental quantification. However, analysis of Cs and Li composition can still be performed very accurately for present experiments because those elements have high cross sections at low energies (250000 transmission spectra in a wide range of neutron energies (0.004 – 6.3 Å). Figure 2. Experiment 1. Full spectrum neutron transmission images obtained at different times of crystal growth sequence and a photograph of the grown crystal. In this experiment, the solid CLLB charge sealed within silica glass was melted to the dashed line of 2 hr image (top left), followed by ~11 hours during which the sample was held at constant temperature of 555 degrees oC, followed by crystal growth with a rate of ~2 mm/hr to the top of the dashed rectangle. The dashed rectangle indicates the volume of the sample where a crystal of good quality (seen as transparent material in photo of the crystal) was grown. The lighter band in the middle of the transmission images corresponds to the volume where phase separation within the melted material was observed during steady-state condition. The yellow dashed horizontal lines indicate the location of liquid/solid interface. The white dash-dotted lines indicate the location of the second interface observed within the melt, where Li concentration changes from low (below that line) to high (above that line) value. Figure 3. Same data as in Figure 2, except shown as the ratio to the previously acquired image. That is each image of Figure 2 is divided by the image acquired ~2.2 hours earlier. This normalization procedure enhances the contrast in the areas which have composition changed over that ~2.2 hour period. The lighter areas correspond to the region of the CLLB material where Li concentration is reduced within latter period, while darker areas indicate relative increase of Li


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concentration compared to the previously taken image. The dashed ovals indicate the region where some movement of Cs-rich/Li-deficient cluster was observed within the solid phase. Figure 4. Experiment 2. Relative change in neutron transmission during melting, steady temperature and growth of CLLB crystal. Phase separation within the liquid phase was clearly observed during melting and steady-temperature stages, while during crystal growth no further increase in phase separation was observed, indicating that phase separation process has a limited time for a given charge volume. The subsequent crystal growth did not alter the elemental composition of Cs-rich/Li-deficient material formed during steady-state conditions. Figure 5. Experiment 3. Full spectrum neutron transmission images of CLLB charge during the third crystal growth experiment, during which a nearly constant speed of ~2 mm/hr was immediately applied, with no steady-state pre-conditioning performed before the growth. Ratio to the previous image is shown, indicating the relative change in sample transmission over the latter period. The two bands persisted through the entire growth process, as phase separation and crystal growth speeds were comparable to each other. The interface between the solid and liquid phases was at the location of dark band, while the white band indicates the location of interface between two liquid phases. Note upward concave shape of both interfaces. Figure 6. Narrow-energy (~0.034 Å/~0.16 meV wide, centered near ~3 Å/~14 meV) neutron transmission images of CLLB charge taken at the beginning (a) and at the end of the growth sequence (b) during Experiment 1. The ratio between consecutive narrow-energy neutron transmission images is shown in this figure, enhancing the contrast formed by neutron coherent scattering within the solid phase. No coherent neutron scattering is present in the liquid phase which appears as gray area with no sharp changes in neutron attenuation. This difference in neutron


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attenuation allows explicit imaging of the interface location between the solid and the liquid phases during crystal growth. The two arrows indicate the location of solid/liquid interface. The pixels with no CLLB material are masked out as no information on the crystallinity of CLLB is presented there. Figure 7. Neutron transmission spectra within CLLB sample. (a) Raw spectra measured with and without sample placed in the neutron beam. (b) Neutron transmission spectrum of CLLB reconstructed from data shown in (a). Low transmission below ~1 eV neutron energies is mainly due to the high absorption by Li atoms, while sharp resonance features of Cs, Br and La nuclei are seen at epithermal energies above 1 eV. Figure 8. Theoretical transmission spectra calculated for different elemental composition of 10 mm thick Cs2LiLaBr6:Ce material. The assumed elemental concentration for all three graphs is defined in Table 2. (a) Transmission values calculated as a function of neutron energy. (b) The ratio of transmission curves shown in Figure 8.a. Higher Cs concentration in composition 3 leads to a dip for all three curves at Cs resonances (e.g. at ~6, ~22, ~48 eV), higher Br concentration in composition 3 leads to dips at Br resonances (~35, ~53 ~100 eV), while higher concentration of La in compositions 1 and 4 leads to a peak observed at La resonance of ~72 eV. (c) Detailed view of the relative transmission near Cs resonance of ~6 eV, demonstrating the potential for Cs quantification. Figure 9. Calibration curve used for the reconstruction of Li composition within the CLLB sample. An integrated transmission between 2.66 meV and 91.1 meV is calculated for a given Li concentration within a 10 mm thick sample. Variation from stoichiometric concentration in Li is compensated by corresponding change in concentration of Cs and Br (e.g. 5% increase in Li


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concentration corresponds to 2.5 % decrease in both Cs and Br concentration). That curve is used for the spatially-resolved reconstruction of Li composition shown in Figure 10. Figure 10. Concentration of Li measured within CLLB charge in-situ at the beginning and the end of experiment shown in Figure 2. The horizontal dashed line indicates the location of solid/liquid interface. The vertical rectangle and two arrows indicate the location of cross sections through the measured concentration maps, shown in Figure 11. The color bar indicates the atom % concentration of Li within the sample. A very clear band of substantial Li deficiency is seen in (b), which was formed by phase separation during steady-temperature part of the growth process. The numbers indicate the areas where transmission spectra were taken for the data shown in Figure 13. The pixels with no CLLB material are masked out during data analysis. Figure 11. Cross sections through the maps of Li concentration shown in Figure 10. (a) Vertical cross sections across two images of Figure 10. A sharp change in Li concentration is clearly seen at the interface between solid and liquid phases in the vertical cross section shown in (a) at ~3.5 mm distance. (b) Radial cross sections through the Li concentration images at “Radial 1” (solid lines) and “Radial 2” (dotted lines) locations defined in Figure 10 by two arrows. Decrease of Li concentration is observed in the radial direction in both solid and liquid phases. Figure 12. Concentration of Li measured in-situ within CLLB charge for the growth experiment shown in Figure 2. The top row images correspond to steady temperature conditions, the bottom row acquired during crystal growth. The time since the beginning of the process is indicated in each image. The pixels with no CLLB material are masked out during data analysis. The location of solid/liquid interface is shown by dashed lines. Each image corresponds to ~40 min acquisition. The legend indicates the atom % concentration of Li measured in each pixel (integrated through


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the thickness of the sample). The variation of sample thickness and the transmission of ampule are taken into account during the analysis. Figure 13. (a) Measured (in area 1 of Figure 10) and calculated thermal neutron transmission for different Li concentrations. (b), (c) Relative ratio of measured neutron transmissions for the regions indicated by numbers in Figure 10. (b) Thermal and cold neutron energies, where Li attenuation dominates above all other elements; (c) Epithermal neutron energies where resonances of Cs, Br and La can be used to compare their concentrations within the sample material.


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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


For Table of Contents Use Only In-situ Observation of Phase Separation During Growth of Cs2LiLaBr6:Ce Using Energy-Resolved Neutron Imaging A.S. Tremsin, D. Perrodin, A.S. Losko, S.C. Vogel, T. Shinohara, K. Oikawa, J.H. Peterson, C. Zhang, J.J. Derby, A.M. Zlokapa, G.A. Bizarri, E.D. Bourret.

Synopsis This study demonstrates the use of a non-destructive technique, namely energy-resolved neutron imaging, for in-situ diagnostic of crystal growth. This technique allows real-time visualization of the melt-growth process, enabling direct measurements of various growth parameters crucial for process optimization. The dynamics of phase separation within the melt is studied in-situ during growth of Cs2LiLaBr6:Ce crystals.


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Figure 1


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Figure 2

Liquid Solid

Liquid Solid

Solid Liquid

Solid Liquid

Interface II

Solid Liquid

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60



33


11.8 mm

Interface II

Liquid

Solid

Liquid

Figure 3

Solid

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 4 Melting

Steady state

12 mm

Growth


35


Figure 5

Solid Liquid

Interface II

Solid Liquid

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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Figure 6 11.8 mm 3.8 hrs.

Liquid Solid

Liquid Solid (a)

(b)

20 hrs.


37


Figure 7

Neutron intensity (arb. units)

4 Direct neutron beam Transmitted through the sample Background

3

(a)

2

1

0 0.001

0.1

10

1000

Neutron energy (eV)

1 0.8

Transmission

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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(b) 0.6 0.4 0.2 0 0.001

0.1

10

1000

Neutron energy (eV)


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Figure 8 1

(a)

1 2 3 0.6

4

0.4 Transmission

Transmission

0.8

0.2 0 0.001

0.01

1 0.8 0.6 0.4 0.2 0

1 2 3 4 20

Neutron energy (eV)

0.1

1

200

10

100

1000

Neutron energy (eV)

2

Ratio of transmissions

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(b) 1.5

1

0.5

0 0.001

3/1. 3/2. 3/4. 0.01

0.1

1

10

100

1000

Neutron energy (eV)


39


1


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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(c)

0.5

3/1. 3/2. 3/4. 0 5

5.5

6

6.5

7

Neutron energy (eV)


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Figure 9

30

Li concentration (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


y = 4.6559E+01x2 ‐ 1.0792E+02x + 4.4404E+01

25 20

10 mm thickness

15 10 5 0 0.2

0.3

0.4

0.5

0.6

Avg. transm. btw. 2.66 and 91.1 meV


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Figure 10 Radial 2

4

Liquid Solid

3 Radial 1 Liquid

2

Solid

1

~2 hrs.

~18 hrs.

(a)

(b)


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Li concentration (atom %)

Figure 11

12

(a)

10 8 6

Vertical, 18 hrs.

4

Vertical, 2 hrs.

2 0 0

5

10

15

20

Distance along the ampule (mm)

Li concentration (atom %)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


12

(b)

10 8 6

Radial 1, 18 hrs. Radial 1, 2 hrs.

4

Radial 2, 18 hrs. Radial 2, 2 hrs.

2 0 0

2

4

6

8

10

12

Distance along the ampule (mm)


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Figure 12


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Figure 13

1

(a)

Transmission

0.8 0.6 Experiment N1 Theory Li 0% Theory Li 5% Theory Li 10% Theory Li 15%

0.4 0.2


0 0.001 2.2 2 1.8 1.6 1.4 1.2 1 0.8 0.6 0.4 0.2 0.001

0.01 0.1 Neutron energy (eV)

1

Region 4 / Region 2 Region 3 / Region 1 Region 2 / Region 1 Region 3 / Region 4

(b) 0.01

0.1

1

Neutron energy (eV)


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


2.2 2 1.8 1.6 1.4 1.2 1 0.8 0.6 0.4 0.2

Region 4 / Region 2 Region 3 / Region 1 Region 2 / Region 1 Region 3 / Region 4

(c)

Cs‐133 4

Cs‐133

Cs‐133 Br‐79 La‐139 40


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In-Situ Observation of Phase Separation During ... - ACS Publications

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