Nanorods of Potassium Tantalum Niobate Tetragonal Tungsten

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Nanorods of Potassium Tantalum Niobate Tetragonal Tungsten Bronze Phase Grown by Pulsed Laser Deposition Q. Simon,†,§ V. Dorcet,† P. Boullay,‡ V. Demange,*,† S. Députier,† V. Bouquet,† and M. Guilloux-Viry† †

Institut des Sciences Chimiques de Rennes, ISCR - UMR 6226 CNRS/Université de Rennes 1, 263, avenue du Général Leclerc, CS 74205, 35042 Rennes Cedex, France ‡ Laboratoire de Cristallographie et Sciences des Matériaux, CRISMAT − UMR 6508 CNRS/ENSICAEN, 6, boulevard du Maréchal Juin, 14050 Caen Cedex 4, France S Supporting Information *

ABSTRACT: K−Ta−Nb−O tetragonal tungsten bronze phase was grown on (11̅02) Al2O3 (R-plane sapphire) by pulsed laser deposition. The microstructure, structure, and chemical composition of the deposit were studied by scanning electron microscopy, X-ray diffraction, energy-dispersive X-ray spectrometry, and transmission electron microscopy. The crystal structure was solved by precession electron diffraction as being tetragonal tungsten bronze-type structure with space group P4/mbm, refined cell parameters a = 12.537 ± 0.003 Å, c = 3.975 ± 0.001 Å, and composition K5.06(Ta0.57Nb0.43)10.99O30. The tetragonal potassium tantalum niobate growth follows two modes with respect to the substrate surface: (i) as single vertical right parallelepiped-shaped nanorods (50 to 100 nm wide and up to 1 μm in length) along the [001] direction and (ii) as in-plane attached crystals along the ⟨310⟩ direction. These two growth modes are understood as being governed by the plane termination of the substrate. This new phase is of potential interest due to the physical (dielectric, catalytic, etc.) properties evidenced for tetragonal tungsten bronze phases in numerous systems. KEYWORDS: KTN, TTB, PLD, TEM, crystal structure, precession electron diffraction

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INTRODUCTION Compounds in the K−Ta−Nb−O (KTN) system have attracted considerable interest for many decades for various applications in microelectronics, electro-optics, and photocatalysis. Indeed, among the numerous phases which were reported, the KTaxNb1−xO3 perovskite is well-known to present strong electro-optic effect,1−3 large piezoelectric coefficients,4,5 and highly tunable dielectric properties for microwaves applications,6,7 as well as attractive photocatalytic activity for water splitting,8 with functional performances which are closely related to the Ta:Nb ratio. Besides, other oxides like tetragonal tungsten bronzes (TTB) are particularly promising for numerous physical properties, such as ferroelectricity,9−13 piezoelectricity,14,15 optic properties,16,17 and photocatalytic activities.18−21 Despite that TTB phases in K−Ta−O and K−Nb−O systems are reported in the literature,22−25 no work has been published until now on TTB phases in the K−Ta−Nb−O system, neither as bulk nor as supported nanostructure. The synthesis by pulsed laser deposition (PLD) of compounds in the K−Ta−Nb−O system is difficult owing to the high volatility of potassium at the required processing temperatures. For instance, KTaxNb1−xO3 perovskite thin films are commonly obtained on various substrates at deposition temperature around 700 °C26,27 by introducing an appropriate © XXXX American Chemical Society

potassium excess in the target. In such conditions, the high volatility of potassium represents a drawback to control the stoichiometry of the deposited thin films. By changing the synthesis parameters (as deposition temperature or potassium excess), several other phases can be obtained together with the perovskite phase, such as the pyrochlore compound K2Ta2O6 which is deleterious for the dielectric properties.28 The same difficulty is encountered when the formation of pure TTB phase is the aim. In this work, we report the synthesis of highly oriented nanorod arrays of pure TTB phase by PLD in the KTN system and the results of the structural characterization of this phase. First, the compositional, morphological, and structural features of the obtained nanostructures are introduced basing on scanning electron microscopy (SEM), X-ray diffraction (XRD), energy-dispersive X-ray spectrometry (EDXS), and transmission electron microscopy (TEM) analyses. Due to the low diffracting volume and the strong preferential growth of the compound, XRD characterization interpretation is of limited use, and TEM and precession electron diffraction (PED) studies were therefore performed for structural and microReceived: March 29, 2013 Revised: June 12, 2013

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dx.doi.org/10.1021/cm401018k | Chem. Mater. XXXX, XXX, XXX−XXX

Chemistry of Materials

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structural characterizations. PED29 is a powerful approach to solve crystalline structures of the nanometric object.30,31 Finally, the potential physical properties and growth mechanisms of the obtained phase are discussed according to data from literature on other TTB phases.

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EXPERIMENTAL SECTION

The PLD target was prepared by sintering a mixture of KTaO3, KNbO3, and KNO3 powders at 350 °C during 12 h. KNO3 was used to introduce a potassium excess to counterbalance the potassium depletion occurring during the deposition, due to the high volatility of potassium oxide K2O under high temperature.32,33 The KTaO3 (KNbO3) compound was prepared by solid state reaction from Ta2O5 (Nb2O5) and K2CO3·1.5H2O precursors at 1000 °C during 12 h. The KNO3, Ta2O5, and Nb2O5 oxides were supplied by Alfa Aesar and the K2CO3·1.5H2O hydrated potassium carbonate by Fluka. The target composition was KTa0.65Nb0.35O3 + 60 mol % KNO3, as commonly used when the perovskite phase is targeted.26 Prior to the deposition, the target was polished on dry 320 and 1200 grit SiC papers and cleaned by pulsed air. The substrates were 5 × 5 mm2 single crystal R-plane sapphire (11̅02)Al2O3, (JCPDS card No. 00-010-0173, space group (SG): R3̅c (No. 167), a = 4.758 Å, c = 12.991 Å) supplied by Crystal GmbH Company. They were ultrasonically cleaned in acetone for 5 min and then in isopropyl alcohol for 5 min. KTN deposits were grown by PLD using a KrF excimer laser (Tuilaser Excistar, pulse duration 20 ns, λ = 248 nm) operating at 2 Hz with an energy of 210 mJ (corresponding to a fluence of 2 J·cm−2). The target−substrate distance D was 70 mm. During the deposition (30 min), the substrate heater temperature (845 °C) and the oxygen pressure (0.3 mbar) were kept constant. SEM was performed with a field emission gun Jeol JSM 6310F instrument working at 7 kV. Chemical composition of the samples was characterized by SEM-EDXS by using a Jeol JSM 6400 instrument operating at 10 kV equipped with an Oxford Inca EDS system. The samples were coated with carbon prior to the analysis. XRD analysis of obtained nanostructures was carried out using a θ-2θ instrument (Brüker AXS D8 Advanced) working with a monochromatized Cu Kα1 radiation and equipped with a 1D detector (192 channels). Data were collected across a 2θ range of 5−80°, using a 0.02° step and acquisition time of 0.3 s/step. Samples for TEM were prepared by scratching the deposit from the substrate with a diamond tip and by collecting the so-obtained particles on an amorphous carbon copper grid (Agar). TEM was performed with a LaB6 Philips CM200 200 kV instrument and a LaB6 Jeol 2010 200 kV instrument equipped with an Oxford EDXS system (TEM-EDXS). PED was performed with LaB6 Jeol 2010 200 kV and a LaB6 FEI Tecnai G230 300 kV instruments equipped with a precession module (DigiStar from Nanomegas) at a precession semiangle α = 2°. PED data were acquired from several electron diffraction patterns (EDPs) using a GATAN Ultrascan CCD camera. The reflection intensities were extracted from PED patterns by using the EXTRAX ImageJ plugin,31 scaled and merged in the predetermined space group with the help of the PSM3 program.34 The structure was finally determined by direct methods using the SIR2008 program35 and refined with JANA2006 where electron scattering amplitudes are implemented.36

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Figure 1. SEM micrographs at two different magnifications of the KTN deposit showing (a) the formation of vertical nanorods and (b) of a horizontal continuous crystalline layer on the substrate.

1b), shows the presence of a crystalline layer on the substrate surface in addition to the vertical rods. This layer consists in contiguous elongated horizontal crystals, aligned along two perpendicular directions. Its thickness is about 100 nm. The average composition determined by SEM-EDXS, from three measurements on 10 × 7.5 μm2 areas, is K0.490±0.02Ta0.623±0.02Nb0.376±0.02Ox (K: (Ta,Nb) atomic ratio = 0.490) with a standard deviation of 0.017 for K and 0.013 for Ta and Nb (Table 1). Analyses performed on areas which are only constituted of the horizontal layer reveal that the latter has a similar composition. SEM-EDXS showed therefore that the composition of the deposit deviates from the target composition, especially regarding the potassium content (more than 50%) due to the high deposition temperature and a decrease of the Ta:Nb atomic ratio (from 1.86 for the target to 1.66 for the deposit). X-ray Diffraction. Figure 2 shows the θ/2θ XRD pattern of the sample plotted at two different scales. At full scale (Figure 2a), in addition to peaks due to the substrate, one can observe two diffraction peaks at 2θ = 22.348° (d = 3.9748 ± 0.001 Å) and 45.606° (d = 1.9875 ± 0.001 Å), corresponding to a same family of planes, and therefore to a single orientation. At first sight, these peaks could be assigned to the cubic KTaxNb1−xO3 perovskite phase with the [100] orientation (JCPDS card No. 01-070-2011) or to a TTB phase with the [001] or ⟨310⟩ orientation (JCPDS card No. 01-70-1088). A magnification of the pattern in the very low intensities area reveals that additional diffraction peaks are also present (Figure 2b): these ones do not correspond to a perovskite phase, some of them can be assigned to a TTB phase, but not all of them, and

RESULTS

Scanning Electron Microscopy and SEM-EDXS. SEM micrographs of the supported nanosystem are shown in Figure 1a, revealing the growth of isolated right parallelepiped-shaped nanorods (50 to 100 nm wide and up to 1 μm long) on the substrate, contrasting dramatically with the usual microstructure of both perovskite and pyrochlore KTN films grown on R-plane sapphire.26,27,37 The micrograph of one edge of the sample, performed at higher magnification (Figure B



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Table 1. Average Compositions and Standard Deviations for Each Element Measured by SEM- and TEM-EDXS, Recalculated Composition for Comparison Purpose with the Refinement Result and Values of Ta:Nb Ratio technique

average measured composition

standard deviation for K

standard deviation for Ta and Nb

recalculated composition

Ta:Nb ratio

SEM-EDXS TEM-EDXS refinement

K0.490±0.02Ta0.623±0.02Nb0.376±0.02Ox K0.495±0.02Ta0.57±0.02Nb0.43±0.02Ox

0.017 0.071

0.013 0.058

K5.39(Ta0.623Nb0.376)10.99Ox K5.44(Ta0.57Nb0.43)10.99Ox K5.06(Ta0.57Nb0.43)10.99O30

1.66 1.32 1.32

Figure 2. X-ray diffraction pattern in θ/2θ mode of the obtained sample on R-plane sapphire: (a) full scale view; (b) magnification of the low intensity area (log scale). Peaks of the film are indexed according to the tetragonal tungsten bronze structure (*: sapphire; #: unknown structure).

Figure 3. (a) Brightfield micrograph of a vertical nanorod; the arrow shows the growth direction. Inset: EDP of the rod along the [11̅0] zone axis of the TTB phase; the arrow shows the growth direction, the line shows the direction of linear diffuse scattering. (b) Darkfield micrograph of the same nanorod in 2-beam condition with g = 3̅3̅0 (reflection shown by an arrow in the EDP in inset). (c) Same with g = 002 (reflection shown by an arrow in the EDP in inset). (d) Brightfield micrograph of a part of the horizontal layer. Inset: EDP of the corresponding area: arrow indicates the orientation of the substrate, and square corresponds to the (a,b)* reciprocal cell of the tetragonal phase. (e) Darkfield micrograph of the same area (g = 004).

reversely, most of the peaks expected for a TTB phase are not observed. Therefore, it is not possible to index unambiguously the diffraction pattern. Transmisssion Electron Microscopy. Figure 3 shows brightfield and darkfield images of a single nanorod oriented along the zone axis shown in the inset (Figures 3a−c) and of a part of the horizontal layer (Figure 3d,e). The nanorod observed in Figures 3a−c is 575 nm long and 50 nm wide. The growth direction is indicated by an arrow in the image (Figure 3a) and in the EDP in the inset. Linear diffuse scattering (denoted by a white line) is observed in the EDP along the direction perpendicular to the growth direction. Actually, in all the recorded EDPs of this rod (not shown here), diffuse scattering is systematically observed along the direction which is perpendicular to the growth direction, indicating that there is some disorder along this last one. Darkfield micrographs were recorded in two-beam conditions with selected reflections g shown by an arrow in insets in Figure 3b,c. These images show a particular contrast effect at the edges of the rod, which is related to different diffraction conditions in this area, due to several possible phenomena as relaxation effects or presence of

twinned crystals. A nanorod presenting low diffuse scattering intensities was selected, and several EDPs were obtained after rotation of this rod along its elongation axis (i.e., growth direction) (horizontal axis in Figure 4a−f, dhkl ≈ 3.96 Å). The crystal unit cell was then determined by reconstruction of the 3D reciprocal space based on this series of 2D patterns as being a tetragonal cell with lattice parameter a ≈ 12.5 Å and c ≈ 3.96 Å. Figure 4g presents the reconstructed (a,b)* reciprocal plane of this cell. Within this cell, the conditions limiting reflections: h0l; h = 2n (Figures 4a−f) and 0kl; k = 2n (Figure 4h) are observed and lead to the three following possible space groups: P4̅b2 (SG: No. 117, acentric and nonpolar), P4bm (SG: No.100, acentric and polar), and P4/ mbm (SG: No. 127, centric). The comparison of the micrographs (Figure 3) to EDPs (Figure 4) allows also to identify the growth direction of the nanorod as being the [001] direction of the tetragonal cell. The second film fragment is constituted of several small crystals attached to a part of the Al2O3 substrate (Figure 3d−e). The crystals constituting the horizontal thin layer have a small size, as shown in the darkfield image in which few of these crystals C



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Figure 5. Microdiffraction EDPs obtained after rotation of a crystal belonging to the horizontal layer around the [100]* direction: (a) [001] zone axis (relative position: 0°); (b) [016] (−31.28°); (c) [013] (55.12°). Some of the reflections due to the double-diffraction phenomena are denoted by white lines; (d) reconstruction of reciprocal space built on previous patterns, corresponding to the [100] ZAP (open circles: reflections due to double-diffraction effect); horizontal axis: [010]*, vertical axis: [001]*.

in the Figure 3d presents the EDP of the area shown in the brightfield image: contributions of both substrate and tetragonal phase were simultaneously observed. This pattern shows that the (a,b)* tetragonal cell is rotated about 18.3° relative to the [11̅02]* direction of the sapphire. The average composition determined by TEM-EDXS from five crystals is about K0.495±0.02Ta0.570±0.02Nb0.430±0.02Ox with a standard deviation of 0.071 for K and 0.058 for Ta and Nb (Table 1). TEM-EDXS showed therefore that the composition of the nanorods deviates from the target composition with a loss of about 50% of potassium and a decrease of the Ta:Nb atomic ratio from 1.86 for the target to 1.32 for the nanorods, in agreement with the tendency observed by SEM-EDXS. Crystal Structure Determination. One series of PED patterns, having the [110]* direction in common, is shown in Figure 6. It is observed that the Zero Order Laue Zone (ZOLZ) widely overlaps with the First Order Laue Zone (FOLZ) (see white arrows in Figure 6b, for example). Diffraction intensities were extracted from the PED patterns with the EXTRAX program.31 The Line 2D method (see ref 31 for details) was used so that the intensity of each ZOLZ reflection was integrated along a line profile perpendicular to the [hh0]* direction. By this way, FOLZ reflection intensities cannot false the ZOLZ reflection intensities measurements. An hkl intensities file was created for each PED pattern. Scale factors were calculated by linear least-squares measurements using the common hh0 reflections, and a total of 333 independent reflections were obtained after merging the data in the 4/mmm Laue group. No Lorentz geometric corrections38 were used since it was shown that they generally do not benefit the structure resolution.39 The g values (g = 1/d) were also extracted with EXTRAX and were used to refine the cell parameters. Calibration used to measure the g values was made

Figure 4. EDPs obtained after rotation of a nanorod around the [001]* direction: (a) [010] zone axis (relative position: 0°); (b) [140] (15.17°); (c) [130] (19.57°); (d) [120] (28.23°); (e) [230] (36.23°); (f) [110] (47.89°); (g) reconstruction of reciprocal space built on previous patterns, corresponding to the [001] zone axis pattern (ZAP); horizontal axis: [100]*, vertical axis: [010]*; (h) [011̅] ZAP.

are in contrast (Figure 3e). Due to the small lateral size of crystallites, EDPs were recorded in microdiffraction mode. A series of patterns of a single crystal was obtained by rotation around the same row (horizontal axis in the Figure 5, dhkl ≈ 12.5 Å). The result of the reconstruction is shown in Figure 5d. The indexation of these EDPs leads to the same cell as the one previously determined, demonstrating that the nanorods and the thin layer have the same crystalline structure but that they were growing along a different direction on the substrate. On some of these patterns, (h00) and (0k0) reflections, usually extinct for h (k) odd, are present (denoted by a white line on the patterns) due to a double-diffraction effect. The pattern shown in Figure 5a confirms the tetragonal cell and corresponds to the calculated pattern in Figure 4g. The inset D



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Figure 6. PED patterns of a nanorod obtained at a precession semiangle of 2° along the (a) [1̅11], (b) [2̅23], (c) [1̅12], (d) [2̅25], (e) [1̅13], (f) [1̅14] zone axis. White arrows denote some of the FOLZ reflections observed on each pattern.

from XRD measurement by considering that d001 = c = 3.975 ± 0.001 Å. The refined a-parameter is then equal to a = 12.537 ± 0.003 Å. The structure was determined by direct methods using the SIR2008 program.35 The choice of the input composition was determined taking into account on one hand the experimental composition analysis and on the other hand the known phases which present structural similarities (lattice parameters and space group). Indeed, the K6Nb10.88O30 (JCPDS card No. 01-087-1856) 24 and K 6 Ta 10.8 O 30 (JCPDS card No. 01-70-1088)22 compounds have the TTB K0.57WO3-type structure (SG: P4/mbm (No. 127)).40 Lattice constants of the niobate are a = 12.582 Å, c = 3.992 Å, and that of the tantalate are a = 12.569 Å and c = 3.978 Å. Their respective K:Nb and K:Ta atomic ratios (0.551 and 0.555) are close to the value which has been experimentally found by EDX for the K: (Nb,Ta) atomic ratio on the sample (i.e., ≈ 0.5). SIR2008 does not take mixed site occupancy into account for solution, and an order between Ta and Nb is not probable considering they have the same radius and charge. The formulas K5Ta10O30 and K5Nb10O30 were then used for structure solution and gave almost the same results. Therefore, only those of the former are displayed. The raw results of the resolution of the structure are displayed in Figure 7 for the three possible space groups, i.e., P4/mbm, P4bm and P4̅b2. In the figure, the projections of the crystalline cells along the

Figure 7. Projections along the [001] direction of the structure found by SIR2008 with the space groups P4/mbm (a), P4bm (b) and P4̅b2 (c). Linked oxygen atoms are those that fit with a tetragonal tungsten bronze structure. Encircled ones are supposed to be Ta/Nb atoms. Oxygen atoms encircling the 0 0 0 and 0.5 0 0.5 positions are SIR2008 residual densities.

[001] direction are drawn. The three results present strong similarities. The main differences consist of the z position of the atoms and the R-values (27.56, 23.93, and 23.74%, respectively). These high values are due to the dynamical effects and are satisfactory for PED. If only the cationic positions are considered, the structure corresponds to a TTB structure as shown in Figure 8. In general, the TTB oxide phases have for chemical formula (A1)2(A2)4C4(B1)2(B2)8O30 E



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various valences, and the C site (often empty) by small cations.42 In Figure 7, potassium occupies only the A2 site. According to these results, the K:(Ta,Nb) atomic ratio is 0.4, which is not consistent with the EDXS results (i.e., 0.490). This is probably due to the fact that the oxygen position found by SIR2008 in the A1 site (0,0,0 coordinates for P4/mbm space group) is actually partially occupied by potassium, with an occupancy of about 0.5. Moreover, the site which is occupied by oxygen atoms, encircled in Figure 7, corresponds to the C site of the TTB structure, which should be in fact partially occupied by Ta/Nb atoms in order to respect the electroneutrality of the compound like in the K6Ta10.8O30 and K6Nb10.88O30 oxides. From these results, and similarly to these two phases, the space group of the new phase is considered as being P4/mbm although the R-value for this group is the highest found. The differences between the R-values of the three space groups may be due to wrong oxygen positions and to the smaller degree of freedom of each element to move in the P4/mbm space group (more symmetric structure). In the model found by the SIR2008 program, some oxygen positions are missing compared to a TTB structure. These positions were added to the P4/mbm model, and this last one was refined with the JANA2006 program where electron scattering amplitudes were implemented.36 The structure refinement against PED data using kinematic theory rarely improves the model due to remaining dynamical diffraction effects, but a reasonable resulting structure is a good validation of the model. The Ta:Nb atomic ratio was fixed for every Ta/Nb site to the average value obtained by TEM-EDXS measurement (i.e., 57/43, this value being close to the composition of the nanorod on which PED was performed), oxygen Uiso was fixed to 0.02 Å2 to stabilize the refinement. Potassium occupancy in the A1 site was refined together with the occupancy of the C site by Ta/Nb in order to fulfill the electroneutrality condition. All of the other parameters were allowed to vary. The refined structural data are given in Table 2, and the final refined structure is displayed in Figure 9. The final result is in good agreement with a TTB structure. The R-value is slightly higher after the refinement, due to the suppression of the spurious oxygen atoms given by SIR2008. In the refined structure, octahedral are much distorted. This distortion of the octahedra

Figure 8. Projection of the tetragonal tungsten bronze structure along the [001] direction showing the three cationic tunnels A1, A2, and C and the cationic octahedral sites B1 and B2. Generally, A1 and A2 sites can be occupied by alkali (K+, Na+, Cs+), alkaline earth (Ca2+, Sr2+, Ba2+), rare-earth (Ce4+, Dy3+, Eu2+, Gd3+, Ho3+, Nd3+, Pr3+, Sm3+, Tb3+), and some other (Tl+, Pb2+, Ag+, Cd2+, Ln3+, Bi3+, Th4+, U4+) large cations, whereas B1 and B2 sites can be occupied mainly by small and highly charged cations as Nb5+, Ta5+, or W6+ which can be partially substituted by transition metals cations of various valences (V2+, Fe2+, Co2+, Ni2+, Cu2+, Zn2+, V3+, Cr3+, Mn3+, Fe3+, Ti4+, Zr4+, Nb4+, V5+, W5+, Mo6+, etc.), and C sites (often empty) by small cations (Li+, Na+, Mg2+, Ni2+, Cu2+, etc.).

where A1, A2, B1, B2 and C are the crystallographic sites partially or fully occupied by cations located in squared (A1), pentagonal (A2), and triangular (C) tunnels formed by B1O6 and B2O6 distorted octahedra which are connected by their corners.41 The coordinances of the A1, A2, and C sites are 12, 15 and 9, respectively. Generally, the A1 and A2 sites can be occupied by alkali, alkaline earth, rare-earth, and some other large cations, whereas B1 and B2 sites can be occupied mainly by small and highly charged cations such as Nb5+, Ta5+, or W6+ which can be partially substituted by transition metal cations of Table 2. Results of the JANA2006 Refinement site

atom

X

Y

Z

Uiso (Å2)

A1 A2 B1 B2

K1 K2 Ta1/Nb1 Ta2/Nb2 O1 O2 O3 O4 O5 Ta3/Nb3

0 0.318(2) 0.5 0.2092(9) 0.056(4) 0.198(4) 0.5 0.2959(4) 0.507(4) 0.374(4)

0 0.818(2) 0 0.0724(8) 0.130(4) 0.096(4) 0 0.204(4) 0.823(4) 0.12(11)

0.5 0.5 0 0 0 0.5 0.5 0 0 0.5

0.01(2) 0.011(6) 0.017(4) 0.018(3) 0.02 0.02 0.02 0.02 0.02 0.11(6) 333 7.18% 32.03% 28.51% 28.51% 32.03%

C no. ind. ref. Rint Robs Rwobs Rall Rwall

F

occ. 0.53(5) 1 1 1 1 1 1 1 1 0.25(2)



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composition K0.490Ta0.623Nb0.376Ox, as determined by SEMEDXS). This loss of potassium is due to the high volatility of K2O which is very sensitive to the temperature.43 As a consequence, formation of a less-rich potassium compound than the perovskite phase occurs. Also, the Ta:Nb ratio of the deposit (1.66) measured by SEM-EDXS (i.e., on a large area) is slightly smaller than in the target (1.85). Local composition measurements on single nanorods by TEM-EDXS show that there is an important dispersion of results from a rod to another (Table 1). Nevertheless, the average value for K amount measured by both techniques is the same. Dispersion of K amount values can be related to an inhomogeneous volatility of this element between the rods. Dispersion of Ta and Nb amount values can correspond to an inhomogeneous distribution on these species in B sites from a crystal to another and also throughout a same crystal. Similarly, the difference between the Ta:Nb ratio of the deposit and that of the target may also be related to an inhomogeneous distribution of these two elements throughout the whole sample. In particular, the small parts of the sample which were analyzed by TEM were taken on the edge of the sample while SEM analyses were performed on its central part, these two areas being distant of 2.5 mm. PLD is known for its easy stoichiometric transfert (except for volatile elements). Nevertheless, diffusion of film elements in the substrate can slightly modify the desired composition: in a previous study, an important diffusion of Ta in the sapphire substrate was observed by secondary neutral mass spectrometry (SNMS) for samples prepared at 700 °C,37 while the diffusion of Nb is weaker. Therefore, the composition difference observed between the target and the deposit may be due to the diffusion of Ta into the substrate, which is reinforced in the present case by the higher deposition temperature. Average compositions have been recalculated by fixing the (Ta + Nb) amount at 10.99 for comparison with the refined composition (Table 1). It appears that the single analyzed rod is less K-rich than the average composition; this fact can be related to the existence of vacancies in A1 sites.42 K6(TaxNb1−x)10O30 Solid Solution and Expected Effect of the Substitution of Nb5+ by Ta5+ on Ferroelectric Properties. The present result is the first report of a TTB in the K−Ta−Nb−O system, either as bulk material or thin film. Since TTB phase is referenced in both K−Ta−O and K−Nb− O systems, the phase found in the present work indicates that a solid solution K6(TaxNb1−x)10.8−10.9O30 exists, with K6Ta10.8O30 and K6Nb10.88O30 as end members. The dispersion of Ta and Nb amounts according to the analyzed single rods confirms the existence of the solid solution. Several other solid solutions of TTB niobo-tantalates were reported, as Na 2 Sr 4 (Ta x Nb 1 − x ) 1 0 O 3 0 , 4 4 Na 2 Ba 4 (Ta x Nb 1 − x ) 1 0 O 3 0 , 4 4 K 2 Sr 4 (Ta x Nb 1 − x ) 1 0 O 3 0 , 4 5 K 2 Ba 4 (Ta x Nb 1 − x ) 1 0 O 3 0 , 4 5 , 4 6 K6Li4(TaxNb1−x)10O30,10 and Ba6Ti2(TaxNb1−x)8O30.47 For Ba6Ti2(TaxNb1−x)8O30 and K6Li4(TaxNb1−x)10O30 compounds, it was shown that replacement of Nb5+ ions by Ta5+ ions in the B sites of the structure leads to a reduction of the cell volume.10,47 For the K−(TaxNb1−x)−O system, no tendency can be observed since the reported values of lattice parameters of TTB niobates and tantalates are quite dispersed, with the a‑parameter lying between 12.537 and 12.582 Å and the c‑parameter between 3.973 and 3.992 Å.23−25,48 TTB phases are known to be ferroelectric materials. In particular, all the TTB potassium niobates and tantalates which were reported so far possess either the paraelectric space group

Figure 9. Projections of the KTN P4/mbm structure obtained after refinement with JANA2006 along the [001] direction (a) and along an unoriented direction showing the strong anisotropy of the cell (b).

framework is mainly due to the oxygen atoms that were added to the model, their positions being consequently less accurate. Furthermore, the Ta3/Nb3 (C site) Uiso value is one order higher than the Uiso values of the other cations. This high value can result from several reasons: (i) oxygen positions surrounding this site are not enough accurate; (ii) the site could be preferentially occupied by Nb5+ rather than Ta5+ which has a higher scattering amplitude; and (iii) the possible presence of oxygen vacancies could lead, to respect the electroneutrality, to a lower occupancy of the C site. In all cases, it is not possible to refine accurately these parameters from PED data since the diffracted intensities are still dynamical. Considering our model, occupancies give the formula K0.506(Ta0.57Nb0.43)1.099O3 (i.e., K5.06(Ta0.57Nb0.43)10.99O30 if compared to the usual formula for TTB phases), which is very close to the values obtained by TEM-EDXS. According to these results, 47% of the K-containing A1 sites are vacant, the C Ta/Nb-occupied positions are partially filled at 24.7%, and the B1 and B2 Ta/Nb sites are fully occupied.

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DISCUSSION Chemical Composition. Increasing the PLD temperature deposition from 700 °C (formation of the KTa0.65Nb0.35O3 perovskite phase)26,27 to 845 °C leads to a decrease of about 50% of the potassium content in the obtained compounds (of G



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Growth and Crystals Anisotropy. According to the results of the structural resolution, most of the diffraction peaks on XRD patterns (Figure 2) were labeled with Miller indices of the TTB phase, excepted two of them (see below). Diffraction intensities of the TTB phase do not correspond to those of a powder due to the strong preferred orientation of the vertical and horizontal rods. The strong peaks can correspond either to the {001} or {310} families of planes or both. Indeed, the lattice spacings d001 (= c = 3.975 Å) and d310 (= a/√10 = 3.964 Å) are almost equal. TEM study has revealed that the vertical nanorods are single crystals and that they were growing along the [001] direction. From literature results,56,57 we made the assumption that the horizontal layer was growing along the ⟨310⟩ orientation, and we checked this point by performing XRD phi-scan experiments. This is also consistent with the observed orientation of the TTB cell relatively to the substrate surface in the Figure 3d, since the angle between (100) and (310) planes of the TTB phase is equal to 18.43°, therefore close to the measured angle of 18.3° between the (100) TTB plane and the substrate surface (a more complete study on epitaxial growth of TTB phase on (100)STO and on R-plane sapphire will be the subject of another paper). Therefore, the XRD strong peaks were labeled as (001)/{310}, (002)/{620}, etc., since the TTB phase has grown according to two modes: vertically, with c perpendicular to the substrate ([001] growth direction), and horizontally, with c lying parallel to the substrate surface (⟨310⟩ growth direction). In other studies on TTB thin films, columnar growth of grains along the [001] direction was previously observed for Sr0.5Ba0.5Nb2O6 on (100)SrTiO3 (STO)56 and Ca0.28Ba0.72Nb2O6 on (100)MgO,58 in both cases prepared by PLD. The second type of growth (along the ⟨310⟩ directions) was also observed for the Sr0.5Ba0.5Nb2O6 phase (obtained by PLD)56 and for the Sr0.3Ba0.7Nb2O6 phase (obtained by sol−gel methods)59,60 on (100)STO substrates, with the formation of horizontal rods of 10−50 nm in width and 200 nm−1 μm in length. Infortuna et al.56 have shown that the TTB orientation is a function of substrate termination for the (Sr,Ba)Nb2O6 phase on (100)STO. Pretreatment of the substrate allows the formation of a unique termination (either SrO or TiO2) plane: SrO termination leads to formation of vertical rod-like grains with (001) orientation, whereas TiO2 termination leads to growth of horizontal rod-like crystals with ⟨310⟩ growth direction. On the contrary, Jia et al.57 have shown that the growth on (100)STO of the Ca0.28Ba0.72Nb2O6 TTB phase along the ⟨001⟩ direction is preferred on TiO2-plane termination to SrO termination. In the present work, the fact that the two kinds of orientation ([001] and ⟨310⟩) have been obtained simultaneously indicates that the surface of R-plane sapphire should present two different states as well (it has been shown that the R-plane can present several terminations, see ref 61 and references therein). Previous works on TTB [001]-oriented films showed formation of a dense coating,56,58,62 while in the present work, [001]-oriented single nanorods were observed, which is an unreported result so far. The lattice spacing c being considerably smaller than a, the structure presents a strong anisotropy (see Figure 9b). As a consequence, the TTB phases often show an anisotropic growth with the c axis as growth direction. This behavior is observed for materials obtained in the form of powder by polymerizable complex method,20 solidstate reaction,12,20,63,64 sol−gel method,65 or molten salt synthesis,48 the anisotropy of the powder particles being dependent on the preparation method20,48 or on the variation

(P4/mbm) or the ferroelectric one (P4bm) (excepted for the Ba0.27Sr0.75Nb2O5.78 compound which showed a transition via the P4̅b2 space group41 and for the SrTi2M8O30 (M = Ta, Nb) compounds which crystallize in the Pba2 space group)49 As far as we know, there is no information available about the Curie temperature (TC) of K6Ta10.8O30 and K6Nb10.88O30 oxides. Nevertheless, they were both reported to possess the paraelectric space group, i.e., P4/mbm (No. 127), at room temperature (RT). Therefore, it is assumed that the paraelectric−ferroelectric transition occurs at a temperature below the RT, supporting our assumption that the phase studied in the present work possesses the P4/mbm space group. This is also consistent with the work of Ravez and Simon46 who have shown that TC is far below RT for several niobates and tantalates (including potassium-based oxides as La2K4Nb10O30, Bi2K4Nb10O30, K2Sr4Ta10O30, and K2Ba4Ta10O30). It is not possible to give a general picture on the type of transition (either displacive or order/disorder type) for TTB phases, because this one depends actually on the composition of the phase.50 Indeed, it was shown that for several niobo− tantalate systems, the replacement of Nb5+ by Ta5+ transforms the macroscopic polarization into a local one,46 leading to the decrease of TC and to appearance of relaxor behavior. For some ferroelectric structures (as the Ba(Ti,Zr)O3 perovskite), the value of TC decreases as the average size of the cation located in the octahedral sites increases.51 In TTB phases, the substitution of Nb5+ for Ta5+ in the octahedral sites leads also to a large decrease in TC, but since the Nb5+ and Ta5+ cations have similar charge configurations and size, this is associated to an increase of covalency of M−O bonds (M = Ta, Nb) from niobates to tantalates, which may inhibit the ferroelectric distortion by increasing the bond rigidity and the stability of the paraelectric structure.42,52,53 As a consequence of the transformation of the macroscopic polarization to a local polarization with the substitution of Nb5+ by Ta5+, A12A24(TaxNb1−x)10O30 compounds, with A1 = Na+, K+ and A2 = Sr2+, Ba2+, present a transition from classical ferroelectric behavior to relaxor behavior for a certain value of x (for instance, K2Sr4(TaxNb1−x)10O30 phase becomes relaxor for x ∼ 0.15).54 Thus, if it is requested to work with a potassium niobo− tantalate having a TC close to the RT, substitution of K+ by Na+ on A1 site and K+ by Sr2+ or Ba2+ on A2 site is an interesting way to get this result, keeping in mind that above a low value of x, the TC value would decrease strongly and that the relaxor behavior would appear.46,55 Structure Resolution. The present work is another example of the powerful character of precession electron diffraction for the structure solution of nanomaterials, although it is not possible to determine if the distribution of cations in B sites is ordered or disordered. The determined composition indicates that the A1 site is not fully occupied by K+ ions and that the C site is partially occupied by highly charged Ta5+/Nb5+ ions. Nevertheless, in most TTB phases, the C site is either empty or occupied by small and low-charged ions. 42 For K 6 Nb 10.8−10.88 O 30 , 24,25 K 6 Ta 10.8 O 30 , 22 and K5.06(Ta0.57Nb0.43)10.99O30 (this work) compounds, the partial occupancy of C site by Nb5+ (0.20−0.22), Ta5+ (0.20), and Ta5+/Nb5+ (0.247) ions is necessary to achieve electroneutrality. These partial occupancies in A1 and C sites may have an effect on distortion of octahedra and on lattice parameters as well, and therefore further studies on single crystals having different compositions is needed to fully understand this point. H



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of synthesis conditions, such as composition12,48 or temperature.48,65 The powder grains have either a needle-, fiber-, plate-, or cuboid-like morphology with sizes up to a few micrometers in width and a few millimeters in length.12,20,48,63−65 For compounds prepared by the same method, amplitude of the crystal shape anisotropy depends on the cations ordering at the A1 and A2 sites.20 Some preliminary results on effect of variation of PLD deposition parameters on KTN TTB (not shown here) confirm that the rod morphologies can be modified by the process parameters. Also, the growth of these nanorods is a catalysis- and a template-free process, in opposition to numerous growth methods of nano-objects.66,67 Assumption that this fact may also be attributed to the substrate preparation could be made but needs further investigation. Finally, it is believed that the presence of the smaller peaks in the XRD pattern (Figure 2b) comes from fallen rods since these peaks are indexed with hk0 labels. The two peaks which do not correspond to the TTB phase are denoted by # in Figure 2 and located at 2θ = 10.3 and 10.437°. Efforts to identify origin of the unattributed peaks indicate the possible presence of a thin intermediate layer between the TTB film to the substrate. This point needs also further investigation.

Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors acknowledge the french CNRS/CEA METSA (Microscopie Electronique et Sonde Atomique) network for the financial support and facilities, the Region Bretagne and the CNRS for the financial support, the CMEBA for SEM analyses and EDXS analyses, and André Perrin for fruitful discussions. KTN, K−Ta−Nb−O; TTB, tetragonal tungsten bronze; PLD, pulsed laser deposition; SEM, scanning electron microscopy; XRD, X-ray diffraction; EDXS, energy-dispersive X-ray spectrometry; TEM, transmission electron microscopy; PED, precession electron diffraction; JCPDS, Joint Committee on Powder Diffraction Standards; SG, space group; EDP, electron diffraction pattern; ZAP, zone axis pattern; ZOLZ, Zero Order Laue Zone; FOLZ, First Order Laue Zone; SNMS, secondary neutral mass spectrometry; TC, Curie temperature; RT, room temperature

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CONCLUSIONS Tetragonal tungsten bronze structure was successfully synthesized in the K−Ta−Nb−O system, by pulsed laser deposition and identified by the precession electron diffraction technique, which has shown a partial occupancy of K and Ta/Nb in the A1 and C site of the TTB structure, respectively. The as-grown crystals have vertical and horizontal nanorods morphology, with [001] and ⟨310⟩ growth orientations, respectively. This phase is of potential interest due to the large number of physical properties (as ferroelectricity, large dielectric, piezoelectric and electro-optic coefficients, etc.), which were evidenced for other TTB phases in numerous systems. As a perspective to this work, beyond study of epitaxial relationships and influence of elaboration parameters, we plan to work on the substrate termination for tailoring the film orientation on different substrates to optimize its properties, to synthesize compounds with other compositions to reach Curie temperature closer to room temperature for applicative functions, and to target some particular properties as piezoelectricity or photocatalysis in relation with the particular morphologies of the crystals. Work is also in progress to obtain thin films on electrodes to perform electrical measurements.

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ASSOCIATED CONTENT

S Supporting Information *

An electron crystallographic file (CIF) is available. This material is available free of charge via the Internet at http:// pubs.acs.org.

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REFERENCES

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel.: (+33) 223236788. Fax: (+33)223236799. Present Address §

Q.S.: Institut de Chimie de la Matière Condensée de Bordeaux, ICMCB − UPR 9048 CNRS, 87, Avenue du Docteur Schweitzer, 33608 PESSAC Cedex, France. I



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Nanorods of Potassium Tantalum Niobate Tetragonal Tungsten

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