Growth of Incipient Ferroelectric KTaO3 Single Crystals by a Modified

Jun 22, 2010 - Department of Ceramics and Glass Engineering, CICECO, University of Aveiro, 3810-193 Aveiro, Portugal. ‡ IFIMUP and IN- Institute of ...
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DOI: 10.1021/cg100036v

Growth of Incipient Ferroelectric KTaO3 Single Crystals by a Modified Self-Flux Solution Method

2010, Vol. 10 3397–3404

Sebastian Zlotnik,† Paula M. Vilarinho,*,† M. Elisabete V. Costa,† J. Agostinho Moreira,‡ and Abilio Almeida‡ †

Department of Ceramics and Glass Engineering, CICECO, University of Aveiro, 3810-193 Aveiro, Portugal, and ‡IFIMUP and IN- Institute of Nanoscience and Nanotechnology. Departamento de Fı´sica da Faculdade de Ci^ encias da Universidade do Porto. Rua do Campo Alegre, 687. 4169-007 Porto. Portugal. Received January 11, 2010; Revised Manuscript Received June 4, 2010

ABSTRACT: High-quality potassium tantalate (KTaO3, KT) single crystals are grown by a high-temperature self-flux solution modified method in which potassium carbonate (K2CO3) and boron oxide (B2O3) are utilized as a complex flux. Additions of small amounts of boron oxide, used because of its low melting temperature (450 °C) and tendency to decrease the weight losses, increased the metastable region, requiring lower temperature (e1300 °C) for the growth of relatively large KT crystals thereby suppressing the K volatilization tendency. By changing the flux composition and flux to solute proportion growth conditions are modified. The as-grown potassium tantalate crystals exhibit a dielectric permittivity of 6600 and dielectric losses of 0.004 at 13 K and 100 kHz. These results suggest a new promising approach for growing relatively large size and high quality single crystals within KT-based system.

*To whom correspondence should be addressed. E-mail: paula.vilarinho@ ua.pt.

toward a high value of several thousands.4,5 The absence of the establishment of a ferroelectric order keeps the losses at very low levels. At high temperatures, KT has the ideal cubic perovskite structure,6 and pure KT do not exhibit any dipolar relaxational effects, but in a manner similar to that of ST,7 new dipolar and relaxation features can be induced by diluted impurities, defects, pressure, and external electric fields.8-10 For some applications, single crystals are required, which is the case of KT crystals to be used as substrates for the fabrication of superconducting devices. Because of the very good lattice match between KT (a = 3.9877 A˚11) and the superconducting material, epitaxial films with optimized performance can be grown.12-14 In optical applications, single crystals are used to minimize scattering or absorption of energy. KTaO3-based crystals have been already tested in order to be adopted for nonlinear and electro-optics, or as solid immersion lens made for a near-field optical disk system.15-17 Although crystals of suitable size are required for most of the applications,18,19 the growth of high-quality single crystals (i.e., with a high homogeneity of chemical and physical properties) still remains challenging. Many various methods have been developed for crystal growth, but in recent years high-temperature solution (flux method) has been successfully used to grow high-quality pure single crystals for a wide range of electronic materials.19 In the growth of crystals from this technique, the oxide material is dissolved at high temperature in a suitable solvent (flux) and crystallization is allowed during the cooling, which makes the solution critically supersaturated. The main advantage of this method is that the crystals are grown below the melting temperature of the oxide material,20 making this process particular suitable for the growth of crystals with compositions that melt incongruently,21 possess volatile components22 or are highly refractory,23 as KT. The key point of this process is the appropriate selection of a multicomponent flux system. And some principles apply for the choice of the flux

r 2010 American Chemical Society

Published on Web 06/22/2010

Introduction ABO3 perovskite oxides constitute an important family of functional materials. Because of their wide variety of physical properties, including nonlinear dielectric behavior, piezoelectric, pyroelectric, ferroelectric, and relaxor response, perovskites are particularly relevant for electronic and microelectronic applications.1 Some of the most important perovskite materials are lead-based ones, such as lead titanate (PT), lead zirconate titanate (PZT), lead lanthanum zirconate titanate (PLZT), or lead magnesium niobate (PMN). The morphotropic phase boundary compositions of the solid solution between PbTiO3 and PbZrO3 present some of the highest piezoelectric coefficients, and as so, it is currently the most commonly used system for piezoelectric applications.2 However, because of environmental concerns lead-free materials have been considered as substitutes to the former ones. Barium titanate (BT), strontium titanate (ST), potassium niobate (KN), potassium sodium niobate (KNN), and potassium tantalate niobate (KTN) are some of the perovskite lead-free compositions, which are technologically important.2 Indeed K0.5Na0.5NbO3 (KNN) is at present one of the lead-free ferroand piezoelectric systems considered as the most plausible substitute of PZT because piezoelectric constants comparable to those of PZT at room temperature have been reported for this composition.3 Within the dielectrics, incipient ferroelectric perovskites are also relevant because of their very high dielectric permittivity and low losses. KaTO3 (KT) and SrTiO3 (ST) are some of the most important quantum paraelectrics in which the condensation of a low-lying transverse optical mode is prevented by atomic quantum fluctuations.1 As a result, the ferroelectric phase transition does not occur down to 0 K, although the dielectric constant increases with the decrease of temperature

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materials: flux should have a low melting point, low solubility in the growth crystal, the viscosity of the solution should be low, the solution cannot attack the growth container, and the residual melt should be easily separated from the crystals.20 The commonly used fluxes are basic oxides or fluorides, such as PbO, Bi2O3, K2O, WO3, MoO3, Na2O, B2O3, PbF2, and KF, selection of which depends, of course, on the composition of the crystal to be grown. The main disadvantages of this process are the impurities that may be trapped in the crystal (if the flux contains additional elements), the growth is relatively slow and the growth of very large crystals is generally difficult.20 Undoped and doped KT single crystals have been synthesized by different techniques including Czochralski method, growth from melt, top-seed solution method and flux one.24-27 In the latter case KT single crystals were obtained by self-flux solution process, using K2CO3 as the flux.7,26-28 The crystals were obtained by heating up the starting reagents (potassium carbonate and tantalum oxide) to 1450 °C and held at this temperature for 1-2 h. The average size of the asgrown crystals varied from tens of millimeters to almost one centimeter.27,28 However, the high synthesis temperature used increased markedly the difficulty in controlling KT stoichiometry. The aim of the present work is to grow KT single crystals at temperatures below the melting point of KT system, T = 1360 °C, to allow a better control of the potassium stoichiometry, which can be perturbed by its relative high volatilization. For that a modified flux method was used being the K2CO3 rich flux altered by adding a second component, in this case B2O3. The flux charge consists of potassium carbonate, which plays an important role in the solvent because is one of the starting precursors of KT and contributes to the stabilization of the potassium content in the composition, though does not decrease the saturation temperature. By enriching the flux with B2O3 the solute solubility will increase and the saturation temperature will decrease reducing the volatility of K.

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Figure 1. Schematic representation of the setup for the high temperature self-flux growth of KTaO3 single crystals.

Experimental Procedure High-purity chemical reagents of K2CO3 (Merck, g99%), Ta2O5 (Aldrich, 99%), and B2O3 (Merck, 95%) were used as raw powders. Polycrystalline KT powders were first synthesized by a conventional solid-state reaction. The starting reagents, potassium carbonate and tantalum oxide, in the stoichiometric ratio and with 10 wt % excess of K2CO3 to compensate potassium evaporation were mixed by ball-milling, in Teflon pots and in ethanol medium for 5 h using zirconia balls. Before weighting and mixing, K2CO3 was dried at 250 °C for 12 h. Then the mixture was calcined at 850 °C for 5 h to obtain the perovskite phase. A mixture of K2CO3 and B2O3 was selected as the flux (proportions between K2CO3 and B2O3 varied from 11:1 to 7.75:1). The precalcined KT powders were mixed with the flux mixture by ballmilling for 12 h. Then 6 g of KT/flux mixture (ratio from 7:3 to 6:4) was loaded into a Pt crucible and tightly sealed with a Pt lid. The Pt crucible was then inserted into a larger alumina crucible, and sealed with alumina powder, to minimize the evaporation of the flux at high temperatures. The schematic representation of the used setup for the growth of KT single crystals is presented in Figure 1. The alumina crucible was placed into a vertical furnace equipped with an automatic temperature controller. The mixture was then heated following the thermal profiles illustrated in Figure 2, that is, (1) heating from room temperature to 950 °C at a rate of 150 °C/h and dwelling at this temperature for 4 h (premelting step), (2) heating to a maximum temperature (between 1230 and 1300 °C) at 100 °C/h and dwelling there for 20 h, and (3) slow cooling from the maximum temperature to 1150 at 5 °C/h, from 1150 to 1000 °C at 10 °C/h, from 1000 to 800 °C at 20 °C/h and then down to room temperature at 150 °C/h.

Figure 2. Furnace thermal profiles used for the growth of KTaO3 single crystals (TP1, 2, 3, and 4 = thermal profile 1, 2, 3, and 4). The gradually accelerated slow cooling process was used to create the required supersaturation conditions for crystal nucleation and growth. Different thermal profiles were used in order to decrease the dwelling temperature. The longest thermal profiles lasted four days. The growth conditions and results are specified in Table 1, and eight distinct growth runs were performed. Run no. 1 was conducted at 1300 °C (20 h) without B2O3 addition to the flux charge, and the proportion of flux to KT was 6:4. The following four runs (runs 2 to 5) were conducted for the same flux/solute ratio, but 5 wt % of B2O3 was added, and the proportions of KT and complex flux components KT/K2CO3/B2O3 were 4:5.5:0.5, respectively. The dwelling temperature was decreased from 1300 (run 2) to 1230 °C (run 5). The last three runs (runs 6-8) were carried out using 8 wt % of B2O3 in the flux charge, and flux/KT ratio was 7:3 (runs 7 and 8). The plateau temperature for these last three runs was varied from 1270 °C (run 7) down to 1230 °C (runs 6 and 8). After growth procedure, the crystals were separated from the solidified flux. This process is slow and takes a few days. The crucible was placed in a beaker with water for several hours to dissolve the potassium carbonate. Then, the crystals were washed by sonication in an aqueous mixture of acids (HNO3 and HClO4) to dissolve any flux remaining. Finally, the crystals were repeatedly cleaned in water and dried with acetone. As-grown KT single crystals were translucent and dyed from light yellow to greenish-yellow.

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Table 1. Preparation Conditions and Maximum Crystal Size of As-Produced KT Crystals complex flux composition (wt %) maximum thermal temperature for crystal profile run no. (Figure 2) growth (°C) 1 2 3 4 5 6 7 8

TP1 TP1 TP2 TP3 TP4 TP4 TP2 TP4

1300 1300 1270 1250 1230 1230 1270 1230

K2CO3

B2O3

60 55 55 55 55 52 62 62

0 5 5 5 5 8 8 8

flux/solute maximum ratio crystal (wt %) size (mm) 60:40

70:30

2.5  1.5 3.8  3.0 3.2  2.2 3.0  2.0 2.7  1.8 2.8  2.0 3.3  2.3 3.0  2.3

The flux behavior of the different mixtures was assessed by a simple test, here designated by “button” test. Pellets with 1 cm of diameter of K2CO3 þ B2O3 þ KTaO3 powder mixtures were pressed and heat-treated on alumina substrates at 930 °C, for 5 min. The deformation of the pellet, indicative of the mixture flux behavior, was quantified by the angle formed between the pellet height and the substrate. The crystal-phase composition and crystallographic orientation of the KT single crystals were evaluated by X-ray powder diffraction (XRD, Rigaku, D/Max-B, Cu KR radiation) and by single crystal X-ray diffraction (XRD, Bruker APEX II) at room temperature in the 2θ range from 4° to 90° with a step length of 0.02°, and by transmission electron microscopy (TEM, Hitachi H9000). The single crystal XRD data were collected using graphite-monochromatized Mo KR radiation with the crystal positioned at 35 mm from the CCD. The surface of the single crystals was observed by using optical (Zeiss Jenaphot 2000 with digital camera Nikon E4500) and scanning electron microscopy (SEM, Hitachi S-4100). The elemental composition of the crystals was characterized by energy dispersive X-ray spectroscopy (EDS) attached to SEM (SEM/EDS, Hitachi SU-70) and inductive coupled plasma atomic emission spectrometry (ICPS, Jobin Yvon Activa-M). The unpolarized Raman spectra were recorded in the spectral range from 10 to 1000 cm-1, using a Spectra Physics argon laser operating at λ = 514.5 nm in a retro-scattering geometry using a microRaman setup. The scattered radiation was analyzed using a Jobin-Yvon T64000 spectrometer equipped with a CCD and a photon-counting detector. The spectral slit width was about 1.5 cm-1 wide. For electrical characterization, parallel faces of naturally shaped crystals were polished, and gold electrodes sputtered onto them. Then, the sample was mounted on a homemade sample holder, located inside a He-closed-cycle cryostat (Displex APDCryogenics CH-2), which provided a temperature rate of 0.8 K/min in the range 12-300 K. Dielectric constant (εr), and dielectric losses (tan δ) were measured by a Precision LCR Meter (HP 4284A) under a weak ac electric field of 0.5 V/mm in the frequency range from 1 kHz to 1 MHz.

Results and Discussion The typical crystal growth conditions and KT crystal size are summarized in Table 1 and illustrated in Figure 3 that indicates that small single crystals grew in the form of cubelike-shaped crystals at the bottom of the Pt crucible. They were translucent and light yellow in color. Some other crystals, bigger and with more complicated shapes, were also formed out of the Pt crucible, on the Pt lid and walls, indicating that the high temperature favored melt evaporation and the resulting vapor phase found on Pt surfaces adequate sites for KT heterogeneous nucleation and growth. Different dwelling temperatures, flux compositions and concentrations were tried. For low flux concentration (40 wt % of KT and 60 wt % of flux containing 5 wt % of boron) and high reaction temperature (1300 °C), the obtained crystals

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are relatively large, 3.8  3.0 mm. As temperature decreases down to 1230 °C, crystal size decreases too, until 2.7  1.8 mm. Concomitantly, flux compositions with different contents of boron oxide were tried for KT crystal growth, as well. Relatively small KT crystals, 2.5  1.5 mm, were obtained without boron-modified flux, just by using potassium carbonate (run 1, Table 1). In comparison with the growth under flux without B2O3 (0 wt %), the addition of boron oxide (5 and 8 wt %) to the K2CO3 flux improved the crystal growth and crystals up to 3.8  3.0 mm in size were obtained. Increasing the amount of B2O3 addition from 5 to 8 wt % led to the formation of even larger crystals. Similarly, large Pb[(Sc0.5Nb0.5)0.42]O3 or NdTa7O19 crystals were grown from a boron oxide based flux.29-31 The increase of flux concentration at the same reaction temperature (1230 °C) also benefited crystal growth as documented by runs 6 and 8 from which crystals of 3.0  2.3 mm were obtained for the highest flux concentration (70 wt % of flux and 30 wt % of KT). The observed results suggest a decrease of the viscosity of the flux mixture, that was indeed proved via the “button” test. Figure 4 represents the results obtained after annealing the flux mixture at 930 °C, for 5 min. The degree of wettability of the flux is increasing (the angle between the substrate and the flux mixture decreases) with the increase of the amount of B2O3 in the flux mixture, suggesting that the observed crystal growth improvements reflects the melt viscosity decrease induced by the variations of the flux composition or concentration; a lower viscosity at a given temperature promotes the solute mass transport within the melt solution thereby enhancing the grown crystal size. X-ray powder diffraction was performed on the milled crystals at room temperature, and Figure 5 depicts the diffraction patterns of KT crystals prepared under run 3 (Table 1). It is worth mentioning that all the analyzed crystals, obtained under distinct growth conditions, exhibit the same diffraction patterns. The diffraction peaks can be well indexed to the cubic perovskite structure of potassium tantalate, and no impurity phases were found for any of the prepared crystals, under the resolution limits of the used X-ray diffractometer. (100), (200), and (300) reflections are obviously preferred, even though the crystals had been milled before the XRD measurements were carried out. Those three peaks are in fact much stronger, compared to X-ray diffraction pattern of a polycrystalline KT powder (Figure 5), and revealed also in XRD spectra of KT single crystals. The computed unit cell parameter of the KT crystals is a = 3.9801 A˚ (very close to the values reported in the literature, such as a = 3.9877 A˚11), and the X-ray density is F =7.06 g/cm3. Figure 6 depicts the electron diffraction pattern that clearly confirms the simple cubic structure of these KT single crystals. Moreover, the diffraction spots were indexed, according to the crystal structure of KTaO3. The SEM microstructure of KT single crystals prepared under the run 3 (Table 1) is portrayed in Figure 3b, and represents the general microstructure of all the obtained KT crystals. The crystals, whose contour is predominantly irregular, have a layered structure suggesting a layered growth mechanism. Detailed surface micromorphological observation (Figure 3c), shows that the growth steps align along the [100] crystallographic direction, indicating a dominant layer growth mechanism for the (100) faces. Thus, the crystal growth mechanism is controlled by typical bidimensional nucleation phenomena on the particle surface.

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Figure 3. Selected KTaO3 crystals formed from self-flux method (a), SEM micrographs of KTaO3 crystals edge (b), and micro morphology of near-edge KTaO3 crystals (c).

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Figure 4. “Button” test results of three different KCO3 þ B2O3 þ KTaO3 mixtures heat-treated at 930 °C, for 5 min. The angle formed between the pellet height and the substrate is indicative of the mixture flux behavior.

Figure 5. X-ray powder diffraction patterns of polycrystalline KTaO3 powders and crushed KTaO3 crystals. X-ray single crystal pattern of KTaO3 single crystals.

The EDS spectrum of the crystals prepared under run no. 8 conditions (Table 1) reveals only K and Ta elements. Boron from the flux was not detected (Figure 7c). In addition, the EDS elemental mapping for K and Ta was carried out (Figure 7a and b), confirming that the distribution of the elements on the surface of KT crystal is very uniform in all the evaluated specimens, ascertaining the good chemical quality of the as-grown crystals. The results of EDS analysis at different regions of the crystals obtained from run 2 (Table 1), clearly indicate that the disparity of the constituent elements is very low (Table 2). The average atomic ratio of K/Ta/O (out from four distinct points) is about 17.41:15.27:67.32. In addition the nominal composition of KT single crystals prepared with 5 wt % (run 5, Table 1) and 8 wt % (run 8, Table 1) of boron oxide was evaluated by inductively coupled plasma spectroscopy (ICPS). The results tabulated in Table 3 ascertain

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Figure 6. Electron diffraction pattern of KTaO3 crystals.

that the composition of KT crystals is quite close to the nominal stoichiometric one. Only minor amounts of boron were found in the analyzed samples and the difference in B content between the samples is very small. The Raman spectra of three different KT single crystals, grown under distinct conditions (runs 1, 4, and 7), are presented in Figure 8. KT possess an ideal cubic perovskite structure with the space group Pm3m and exhibits no firstorder Raman spectra since all long-wavelength phonons are of odd parity. The spectrum consists entirely of second-order Raman, and the main structural features of the spectrum are caused by the combinations and associations between transverse acoustic modes (TA) and transverse low-frequency modes (TO). The obtained results are in good agreement with previously published ones.33,34 It is worth stressing that no firstorder Raman modes are clearly apparent, which ascertains for an undistorted centered cubic perovskite lattice. Moreover, the fact that Raman spectra of several crystals are very similar, reveals that though different growth conditions still allow obtaining pure and high structural quality KT single crystals. Platelets of KT crystals of various thickness (200-500 μm) were polished parallel to the (100) facets, and the dielectric properties were measured. Figure 9 depicts the temperature and frequency dependence of the dielectric constant and dielectric loss of KT single crystals emphasizing the typical quantum paraelectric behavior, that is, εr increases with the decrease of the temperature and apparently tends to saturate at very low temperatures. Dielectric constant at room temperature is about 400, and at low temperature (13 K at 100 kHz), it reaches the value of 6600, which is higher than the results previously reported in the literature.27,32,35,36 εr(T) does not apparently change with frequency in the range 1 kHz to 1 MHz. In Figure 9, the dielectric losses versus temperature at 100 kHz is presented, and its value is quite low, around 0.004 at 100 K. One small peak at ∼40 K is observed in the dielectric losses plot, which is according to literature,8 very strongly dependent on the sample quality, becoming smaller for increasingly pure samples. Indeed, it is well-known that losses of incipient

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Figure 7. (a) SEM/EDS elemental mapping of KTaO3 crystals: potassium distribution (left; red) and tantalum distribution (right; green). (b) Distribution of constituent elements of KTaO3 crystals and random positions of four points over the surface. (c) EDS spectra of KTaO3 crystals.

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Table 2. Composition Analysis by EDS at Four Random Positions over the Surface of KT Single Crystal elements (at %)

1 2 3 4 average

K

Ta

O

17.65 16.67 17.59 17.73 17.41

15.58 14.86 15.25 15.39 15.27

66.77 68.47 67.16 66.88 67.32

Table 3. Chemical Analysis of KT Crystals Performed by ICPS Analysis elements (wt %/at %)

KT_1 KT_2 stoichiometric composition

K

Ta

B

14.9/17.5 15.2/21.0 14.6/20.0

61.9/15.7 67.3/20.1 67.5/20.0

0.24 0.25

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fluxes were used to obtain KT crystals, with a layered habit at low temperatures. The addition of small amounts of B2O3 (e8 wt %) facilitates the powder dissolution, optimizes melt viscosity and increases the critical supersaturation of potassium, thus providing optimal conditions for KT crystals growth, even using appreciable lower temperatures (Tmax e 1300 °C) than those reported before for similar method (1450 °C). As a result, very close to nominal stoichiometric KT crystal were grown at low temperatures. Crystal size was seen to be dependent on the used temperature profile and on the amount of boron oxide used in the complex flux. XRD and microstructural studies performed in the as-grown crystals provide clear evidence for their chemical and structural high quality. Raman scattering measurements carried out in various single KT crystals confirm the composition and structure quality, as well. The absence of first-order Raman modes ascertains that all the studied crystals have a high undistorted cubic perovskite lattice. The high quality of the grown crystals is also supported by a dielectric permittivity of 6600 and dielectric losses of 0.004 at 13 K and 100 kHz. Acknowledgment. The authors are thankful to Prof. Ian Reaney for TEM characterizations. This work was funded by FCT under the project PTDC/CTM/64805/2006 and FEDER.

References

Figure 8. Raman spectra of three different KTaO3 crystals at room temperature.

Figure 9. Temperature and frequency dependence of the dielectric constant (εr) and dielectric losses (tan δ) of KTaO3 crystals (thickness ≈ 300 μm).

ferroelectrics are very sensitive to impurities, and in the present case, small amounts of B were still detected by ICPS, that together with the trace amounts of impurities carried by the precursors might account for the observed peak. Conclusions Potassium tantalate single crystals were successfully fabricated by a modified self-flux method, in which boron-modified

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