Single crystal growth of 67%BiFeO3-33%BaTiO3 solution by the floating zone method

Accepted Manuscript Single Crystal growth of 67%BiFeO3-33%BaTiO3 solution by the floating zone method Y. Rong, H. Zheng, M.J. Krogstad, J.F. Mitchell, D. Phelan PII: DOI: Reference:

S0022-0248(17)30525-0 http://dx.doi.org/10.1016/j.jcrysgro.2017.08.032 CRYS 24283

To appear in:

Journal of Crystal Growth

Received Date: Revised Date: Accepted Date:

7 April 2017 21 August 2017 29 August 2017

Please cite this article as: Y. Rong, H. Zheng, M.J. Krogstad, J.F. Mitchell, D. Phelan, Single Crystal growth of 67%BiFeO3-33%BaTiO3 solution by the floating zone method, Journal of Crystal Growth (2017), doi: http:// dx.doi.org/10.1016/j.jcrysgro.2017.08.032

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Single Crystal growth of 67%BiFeO3-33%BaTiO3 solution by the floating zone method Y. Ronga, H. Zhenga, M. J. Krogstada,b, J. F. Mitchella, and D. Phelana,* a

Materials Science Division, Argonne National Laboratory, Lemont, IL 60439, USA b

Department of Physics, Northern Illinois University, DeKalb, IL 60115, USA *Corresponding Author, email address: [email protected]

Abstract: The growth conditions and the resultant grain morphologies and phase purities from floatingzone growth of 67%BiFeO3-33%BaTiO3 (BF-33BT) single crystals are reported. We find two formidable challenges for the growth. First, a low-melting point constituent leads to a pre-melt zone in the feed-rod that adversely affects growth stability. Second, constitutional super-cooling (CSC), which was found to lead to dendritic and columnar features in the grain morphology, necessitates slow traveling rates during growth.

Both challenges were addressed by

modifications to the floating-zone furnace that steepened the temperature gradient at the meltsolid interfaces. Slow growth was also required to counter the effects of CSC. Single crystals with typical dimensions of hundreds of microns have been obtained which possess high quality and are suitable for detailed structural studies.

Keywords: A2. Floating zone technique; B2. Ferroelectric materials; B1. Titanium Compounds; A1. High resolution X-ray diffraction

I. Introduction Relaxor ferroelectrics are an important class of materials typified by broad and rounded maxima in their temperature dependent dielectric susceptibilities that shift with frequency[1]. This contrasts with normal ferroelectrics that have sharp, cusp-like peaks at their Curie temperature that have little-to-no shift with frequency. Pb-based perovskite oxides, such as PbMg1/3Nb2/3O3, PbZn1/3Nb2/3O3, PbSc1/2Nb1/2O3, and their respective solutions with PbTiO3 possess relaxor-behavior, and a great deal of research has recently been devoted to employing these systems as platforms for unraveling the underlying microscopic physics of relaxors[2–5]. In particular, attention has been focused on probing the temperature-dependent inelastic and elastic neutron and x-ray scattering cross-sections in an effort to connect the lattice dynamics and local short-range order to the dielectric response. Strongly anisotropic diffuse scattering patterns have been observed in all Pb-based relaxor ferroelectrics[3,4,6,7], and a number of authors have attributed the scattering to polar nanoregions, which can be thought of as nano-sized regions of polar order embedded in a non-ferroelectric matrix[8–11].

Nevertheless, a definitive

interpretation of the diffuse scattering is still lacking, as a number of alternative viewpoints on this issue have been expressed[12–14].

Moreover, lattice dynamical studies (and their

interpretations) remain a controversial issue[15–19]. To further the understanding of relaxor physics, it is useful to study the lattice dynamics, long-range structure, and local structural modulations of a wider array of materials than the Pbbased systems. Detailed studies of lattice dynamics (temperature dependent phonon dispersion) and anisotropic diffuse scattering necessitate the use of single crystals.

The (1-x)BiFeO3-

xBaTiO3 (BF-xBT) system is promising to compare and contrast with Pb-based systems because it possesses relaxor properties near the phase boundary where the long-range symmetry changes

from rhombohedral to cubic. This boundary occurs at x=0.33 (BF-33BT)[20,21], and classic relaxor dielectric behavior has been reported for BF-33BT[3].

Specifically, a strongly

frequency-dependent peak was reported in the permittivity of at Tm ~ 580 K at a measuring frequency of 200 Hz[22]. Furthermore, single crystal neutron diffraction measurements revealed temperature-dependent diffuse scattering in the cubic (112) zone[22].

This temperature-

dependent diffuse scattering motivated our interest in this system, as there is a potential for further, more detailed measurements that could be performed as a function of temperature and electric field with x-rays and neutrons. The BF-33BT crystal reported by Soda et al. was grown by the floating-zone method[22]; however, little information about the growth process and the quality of the grown crystals was reported. Such information is useful for advancing the field of relaxor ferroelectrics and guiding the growth of analogous systems in future, and this is what we focus on here. The rest of the paper is organized as follows. In Section II, we discuss the technical details of our growth experiments as well as our diffraction experiments. In Section III, we discuss results of the crystal growth, focusing on factors leading to stability (III.A.), resulting grain morphologies (III. B.), obtained phase purities (III.C.), and single crystal quality (III.D.).

In Section IV we

conclude by summarizing the results and discussing the prospective for future single crystal growth experiments.

II. Experimental Details Polycrystalline BF-33BT samples were synthesized via solid state reaction with stoichiometric Bi2O3 (melting point 817 °C) BaCO3 (decomposition temperature 811 °C), Fe2O3 (melting point 1565 °C) and TiO2 (melting point 1843 °C) powders. Excess quantities of 0.5 mol% or 2.0 mol% Bi2O3 were added to compensate for the evaporative loss of Bi2O3 during crystal growth. The mixture was ground and then sintered in air at 800 °C for 10 hours and 1050 °C for 3 hours, with intermediate grinding. The resulting single phase powder was pressed into a 100 mm long, 8mm diameter rod and sintered in air at 1050 °C for 4 hours. Single crystal growth was performed using the floating-zone method in an NEC image furnace (KHC model #SC-K-15HD). To further improve the density, sintered rods were pre-melted by rapidly passing a melt zone through the feed rod (40 mm/hour and 15 mm/hr for the seed and feed rods, respectively) in 10% O2 atmosphere. The gas flow-rate during growth was 0.2 L/min, and the gauge pressure varied but was less than 1 bar; further details of the crystal growth are described in Section III. High-resolution synchrotron powder X-ray diffraction was performed at the 11-BM-B beamline at the Advanced Photon Source (APS) at Argonne National Laboratory. An x-ray wavelength of 0.41423 Å was employed. Lattice constants were calculated by indexing eight selected peaks with DICVOL06 [23], the positions determined from their maxima.

This

procedure was employed rather than a LeBail or Rietveld refinement due to asymmetric peak profiles as described later in the text. Single crystal measurements were performed at the Cornell High Energy Synchrotron Source (CHESS) on beamline A2 using an x-ray energy of 56.7 keV. Measurements were made in transmission with a Pilatus 6M Si detector, with measurements taken as the crystal was rotated by 360° in 0.1° degree intervals.

III. Single crystal growth In this section, we discuss the conditions that improved the stability of the floating-zone growth (III.A) and how the growth conditions affected the grain morphology in the growth rods (III.B). We then discuss the effects of the growth parameters on the phase purity and composition achieved from the growth rods (III.C). Finally, we characterize the quality of obtained crystals via synchrotron x-ray diffraction (III. D).

Figure1: (a) Schematic and photograph demonstrating the pre-melt and floating-zones observed during growth. We note that a similar diagram can be found in Ref.[24] where growth of La2-xSrxCuO4 single crystals is described. (b) Modification of the floating-zone IR furnace to increase the temperature gradient. (c) Photograph of a sliver obtained from growth performed at a speed of 2 mm/hr. The entire sliver was not a single crystal, but the lower right corner (marked in red) was a single crystal as verified by single crystal x-ray diffraction.

III.A. Zone Stability First, we discuss the factors that lead to stability of the floating-zone during growth, which we qualitatively assessed by the frequency at which changes in lamp power were required to maintain the zone or the frequency at which the zone was lost. Three different atmospheres were tested during growth, all with the flow-rate controlled at 0.2 L/min: (i) pure oxygen; (ii) 10% oxygen / 90% argon; (iii) 1% oxygen / 99% argon. We found that the zone could be wellcontrolled only using the 10%/90% mixture. Initially, we attempted to grow in an unmodified image furnace. However, it was found that the growth stability was difficult to maintain due to formation of a large pre-melt zone (see Fig. 1a). This resulted in frequent loss of the floating-zone. The pre-melt zone arose either due to incongruent melting or due to capillary action resulting from porosity of the feed rod. This pre-melting has similarly been observed and reported in floating-zone growth of La2-xSrxCuO4 (LSCO) single crystals, where it is described as a “ditch”[24]. For LSCO, improvements in growth were reported by modifications that increased the temperature gradient of the floatingzone[24], and we have employed a similar solution. In order to improve the stability, we increased the temperature gradient extending from the floating-zone over the pre-melt region, thereby reducing the size and effect of the pre-melt zone. As shown schematically in Fig. 1b, Au foil was wrapped around the cylindrical quartz tube that surrounded the crystal growth providing a controlled atmosphere. The Au foil provided a larger temperature gradient by providing a sharper axial gradient of IR incident upon the feed rod.

The sharper axial gradient is a

consequence of the decreased vertical divergence in the incident IR, and a consequence is an increase in the lamp power required to melt the floating zone. We settled upon leaving a total opening of 15 mm, though the upper and lower foils were not equidistant from the center of the

floating-zone. Rather, the upper gold foil (feed-rod side) was placed at ~ 10 mm above the zonecenter compared to the lower gold foil (seed side) ~ 5 mm below the zone-center (the positions of each foil were varied over a range of values, and specific foil positions can be found in Table I for selected growths). The bottom foil more strongly affects the temperature gradient of the crystalline interface and thus significantly impacts constitutional supercooling (CSC), which was a challenge that was encountered and is discussed below. The top foil more strongly affects the pre-melt zone. In practice, optimizing the trade-off between angular divergence and lamp-power requires testing different positions of the Au foil so that melting can still be achieved with the lamp power available while both CSC and the pre-melt zone can be controlled. For our setup, this caused the lamp power to increase by approximately a factor of two compared to growing without foil. We note that Au foil was used because the temperature was hot enough to cause oxidation or even melting of other metal foils (Al, Cu) that were tried.

With this simple

modification, we were able to maintain stable growth up to 30 hours, leading to boules of length 31 mm. It is not possible to measure the temperature profile during the actual growth in our setup. However, it can be estimated by embedding a thermocouple in a black-body and measuring the temperature at the thermocouple as a function of vertical position. We have therefore carried out such a measurement by embedding a thermocouple in a rod of La 1.36Sr1.64Mn2O7.

The

measurement was performed with and without the gold mask, below the melting point of La1.36Sr1.64Mn2O7 (estimated to exceed 1800 °C) with foil positions of ~ 12 mm above the zone center and ~ 3 mm below the zone center. The absolute temperature of the thermocouple is not the same as during the actual growth of BF-33BT due to differences in applied lamp power as well as light absorption; however, the obtained profile can be considered as a reasonable estimate

of the profile during growth which is not anticipated to depend as strongly on absolute temperature as it does on foil positions. Fig. 2 shows the resulting profiles, where it is clear that the zone is vertically sharper and that the gradient on both the seed and feed sides is considerably steeper with the foil in place.

Figure 2: Temperature profile as a function of vertical position measured with a thermocouple embedded in a black body with and without foil masking. The “0” position corresponds approximately to the focal point of the mirrors. The shaded areas denote the approximate positions of the foils. The unshaded position is not masked by foil.

Figure 3: Cross-sectional images along the growth axis taken from samples grown at (a) 4 mm/hr, (b) 2 mm/hr, (c) 1 mm/hr, and (d) 0.5 mm/hr. The grains evolve from dendritic to columnar upon decreasing growth rate.

No.

Foil location a, c [mm]

Rate [mm/h]

Excess Bi2O3 [mol %]

Remarks

1

+10, -4.5

4

0.5

Dendritic crystals

2

+11.5, -5.0

2

0.5

Columnar crystals

3

+10.5, -4.5

1

0.5

Big crystals outside, columnar in the center

4

+10, -4.5

0.5

2.0

No obvious columnar feature

Table I: Representative growth conditions and resulting microstructure. “a” refers to the approximate distance from the upper gold foil to the center of the floating-zone, while “c” refers to the approximate distance from the lower gold foil to the center of the floating-zone. The 2% extra Bi2O3 was only added for the slowest growth with 0.5 mm/h because of the more significant Bi2O3 evaporation.

III. B. Grain Morphology We now discuss the effects of the growth rate upon the size and shape of single-crystal grains. Despite extensive efforts, large single crystals extending across the entire cross-section of the melted rod were never achieved. Rather, small crystals were found (an example of a sliver containing a single crystal is shown in Fig. 1c). To study the microstructure of the boule and to understand the factors affecting the growth, the rods (post-growth) were cut along the growth direction, polished, and then etched with 5 vol% HCl solution. This procedure, performed upon rods grown at different rates, made manifest the grain boundaries. At a growth rate of 4 mm/h, shown in Fig. 3a, the grain boundaries appear as dendtritic, suggestive of CSC[25]. Consistent with this observation, as the growth rate was decreased, the grains evolved into columnar (Fig. 2(b)) and finally larger grains (Fig. 3c,d). For the 1 mm/hr growth, there were some large grains (1.5 mm  0.5 mm) located near the perimeter of the growth rod, whereas the inner part of the rod still possessed columnar features. This may result from the presence of a larger temperature gradient on the surface due to the flowing gas, suggesting that larger crystals might be grown if CSC were further reduced. For the 0.5 mm/hr growth, the columnar feature in the center has been significantly suppressed, and crystals with size 1.0  0.5 mm could be observed at the edge. We note that sintered BF-BT possesses grain sizes on the order of 10 m [26], so the enlargement in the grain size during the floating zone growth is approximately two orders of magnitude along a given direction. Table I describes the growth conditions and the observed grains resulting from representative growths.

III. C. Phase Purity Secondary phases were present in the boules after the floating-zone growth.

We

specifically note that the following discussion refers to the phase purity of the boule after growth and not to small single crystals obtained from the rod. To study the effect of growth parameters on the resultant phase purity, high resolution synchrotron x-ray powder diffraction measurements were performed on samples obtained from crushed growth rods. As shown in Fig. 4, powders obtained from two growth conditions were measured – one with a rate of 4 mm/h and 2 mol% excess Bi2O3 added and the other at 2 mm/h with 0.5 mol% excess Bi2O3. These measurements led to several important observations. First, by slowing the growth rate, the phase purity could be dramatically improved, which is evidenced by the far cleaner pattern obtained for the sample grown at 2 mm/h compared to that grown at 4 mm/h. Indeed, non-perovskite impurity peaks are substantial for the 4 mm/h growth, as are more pronounced shoulders on the high angle of each Bragg reflection. The fact that these shoulders are systematically present for each Bragg reflection is a strong indication that they do not arise from a lattice distortion of the main phase, but are rather a secondary phase that closely matches the symmetry of the main phase but with a smaller lattice constant. It is well-known that a rhombohedral phase is present for x<33 in BFxBT (a structural phase diagram can be found in [21]), but this can be ruled out as the source of the shoulders because a rhombohedral phase will not cause a splitting of h00 Bragg reflections, whereas the shoulders are present on these reflections. We speculate that the dramatic CSC present at faster growth rates leads to a higher concentration of impurities. Second, the main phase obtained from both samples possessed cubic symmetry and had very similar roomtemperature lattice constants (3.9984(1) for 4mm/h and 3.9988(1) Å for 2 mm/h). This indicates that the stoichiometry of the main phase obtained after melting is essentially identical for both

growth rates. Moreover, the fact that cubic symmetry was observed indicates that the obtained stoichiometry is not significantly BiFeO3 rich because rhombohedral symmetry is expected for BiFeO3 compositions and BF-33BT lies extremely close to the rhombohedral-cubic phase boundary. The starting room-temperature lattice parameter for the rods (before melting) was slightly less at 3.9954(1) Å. Given that the lattice parameter decreases by ~ 0.001 Å for an increase of x of 0.01 in the cubic phase[21], this would suggest that the obtained composition is within 0.03 in x of the target composition (x=0.33). Regarding the excess Bi2O3 added to compensate for volatilization, we consider it optimal to use the least amount of excess to obtain a stable growth because too much excess may enhance the ‘pre-melt’ zone due to the low melting point of Bi2O3 compared to the target composition. We were able to achieve relatively stable growth conditions with 0.5% excess at 2 mm/h and 1 mm/h so this relatively modest excess appears sufficient. However, once the rate was slowed to 0.5 mm/h, the loss of Bi2 O3 became more severe, and growths with 0.5% excess Bi2O3 were unsuccessful as a stable zone could not be achieved. When the amount of excess Bi2O3 was increased to 2%, the stability improved. Beyond these qualitative findings, we have not systematically studied the effect of excess Bi2O3 as we consider growth speed and temperature gradient to be the primary factors in the success or failure of the growth.

Figure 4: High resolution synchrotron x-ray diffraction measured on crushed parts of a rod grown at 4 mm/hour and with 2% excess Bi 2O3 and 2 mm/hour with 0.5% excess Bi2O3. Reflections marked by an * denote those originating from impurities.

III. D. Single Crystal Quality Single crystal synchrotron x-ray diffraction was performed on a small single crystal (dimensions of  100 m) removed from a grown rod after mechanical shattering. The high incident energy (56.7 keV) ensured that the bulk volume of the crystal was sampled in the measurement. No splitting of Bragg reflections was observed in either radial or transverse directions, again consistent with a cubic, un-twinned crystal. Moreover, a very tight mosaic was observed (<0.2° at full-width-half-maximum), evidencing the high crystalline quality of the

specimen. The Bragg reflections were indexed as a single cubic crystal using a custom peaksearch algorithm, and the room-temperature lattice parameter was estimated as 3.993(2) Å. This estimate is in close agreement with the starting material 3.9954(1) Å suggesting that the crystal stoichiometry is very close to BF-33BT.

Figure 4: (hk0) scattering plane measured on a BF-33BT single crystal.

IV. Conclusions We have studied the factors related to single crystal growth in the BF-BT system, specifically focusing on BF-33BT. The system tends to form a pre-melt zone on the feed rod

that leads to instability during growth. Indeed, nearly the identical problem occurs in BiFeO3. In that case, it was shown that a solution to the problem is to employ a laser diode furnace[27], which gives a substantially larger thermal gradient due to the narrower laser divergence as compared to a traditional floating zone furnace. In the absence of such a laser-diode furnace, our solution has been to increase the gradient at the zone by limiting the angular divergence of the IR radiation in the furnace by employing a gold foil mask. Similar modifications have proven useful for growth of high-Tc cuprates[24] and may prove useful for a broader range of materials including ferroelectrics. CSC tends to lead to dendritic and columnar morphology of single crystal grains; we have shown that decreasing the growth rate leads to improved size and morphology of single crystal grains, which has allowed us to obtain single crystals of narrow mosaic and composition faithful to that targeted. Crystals with dimensions of approximately 1× 0.5 × 0.5 mm have been grown. Single crystals suitable for synchrotron x-ray diffraction studies have been obtained, and we hope that this work will serve as a guide for growth of BF-BT crystals as well as those of similar systems. For seeded floating zone growth, a seed is needed with a length of a few millimeters on an edge. By slowing down the growth rate further and increasing the temperature gradient, this size may very well be attainable.

Acknowledgements: All work was supported by the U.S. Department of Energy, Office of Science, Basic Energy Sciences, Materials Science and Engineering Division.

Y.R. thanks the China Scholarship

Council for partial sponsorship of his appointment at Argonne National Lab. The authors thank Saul Lapidus at the APS for assistance on beam-line 11-BM-B, Jacob Ruff at CHESS for assistance on beamline A2, and Yang Ren at the APS for assistance in screening crystals on beam-line 11-ID-C.

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Highlights for: “Single Crystal growth of 67%BiFeO3-33%BaTiO3 solution by the floating zone method”

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Floating-zone crystal growth of BiFeO3-BaTiO3 solution is described. Constitutional supercooling and pre-zone melting present challenges. Slow growth speeds and large temperature gradient are key. Small crystals of high quality were obtained.