Scholarly article on topic ' Revealing the role of cationic displacement in potassium–sodium niobate lead-free piezoceramics by adding W 6+ ions '

Revealing the role of cationic displacement in potassium–sodium niobate lead-free piezoceramics by adding W 6+ ions Academic research paper on "Materials engineering"

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Academic research paper on topic " Revealing the role of cationic displacement in potassium–sodium niobate lead-free piezoceramics by adding W 6+ ions "

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Cite this: J. Mater. Chem. C, 2015, 3, 4168

Received 18th December 2014, Accepted 17th March 2015

DOI: 10.1039/c4tc02908a www.rsc.org/MaterialsC

Revealing the role of cationic displacement in potassium-sodium niobate lead-free piezoceramics by adding W6+ ionsf

L. Ramajo,$a M. Castro,a A. del Campo,b J. F. Fernandezb and F. Rubio-Marcos$*b

The effect of the structural modifications induced by the replacement of the B sites with W6+ ions in (K0.44Nao.52Lio.o4)[(Nbo.86Tao.ioSbo.o4)i-xW5X/6]O3 lead free ceramics is investigated. Here we show the coexistence between a tetragonal symmetry (T) and an orthorhombic symmetry (O) in the perovskite structure, which is tuned by varying the doping level. In addition, this polymorphic behaviour is accompanied by the appearance of a secondary phase, which can be detected through XRD and Raman results. A correlation between the presence of both structural features and the W6+ content has been evaluated, and therefore this lead free system reveals a transition from a normal ferroelectric to a 'relaxor-like' ferroelectric due to the cation disorder in the perovskite-structure in doped samples. A large diffusivity value (g) has been attained when the x value reached o.o5 owing to the involved O-T polymorphic phase, as well as due to the appearance of a secondary phase. The experimental proof makes clear that the role of the secondary phase is to capture the alkali ions of the (KNa)NbO3-based system, provoking a cation disorder in the perovskite-structure matrix. The significance of this work lies in evaluating whether such a material can benefit the understanding of (KNa)NbO3-based ceramics and the expansion of their application range.

1. Introduction

Lead titanate-zirconate ceramics (Pb(Ti,Zr)O3-PZT) are the most widely used piezoelectric ceramics, on account of their high piezoelectric properties, large-scale production capability and the tailoring of their properties through composition. However, due to the toxicity of lead, a wide range of regulations concerning environmental preservation are increasingly being introduced worldwide.1'2 PZT ceramics are still allowed because of the lack of an adequate alternative. Among the available lead-free ferroelectric ceramics, one promising candidate is the family of sodium potassium niobate (K,Na)NbO3 (KNN), on account of its good electromechanical properties.3-5 As for PZT, the sinterability of KNN-based materials can be improved by using different sintering aids and dopants.6-9

Numerous studies on lead-free piezoelectric ceramics such as (K,Na)NbO3, BaTiO3-based, Bi-layered, bismuth sodium titanate and

a Instituto de Investigaciones en Ciencia y Tecnología de Materiales (INTEMA), Av. Juan B Justo 4302 (B7608FDQ), Mar del Plata, Argentina b Electroceramic Department, Instituto de Cerámica y Vidrio, CSIC, Kelsen 5, 28049, Madrid, Spain. E-mail: frmarcos@icv.csic.es; Fax: +34 91 735 58 43; Tel: +34 91 735 58 40 f Electronic supplementary information (ESI) available. See DOI: 10.1039/ c4tc02908a

t L. Ramajo and F. Rubio-Marcos contributed equally.

tungsten bronze-type materials have been recently published.10-12 In this way, niobate (K,Na)NbO3 (KNN)-based ceramics have shown good piezoelectric and electric properties, high Curie temperature and environmental inequity. Dai et al.13 investigated the influence of the morphotropic phase boundary and electrical properties of K1_xNaxNbO3 ceramics with x = 0.48-0.54 prepared by a conventional solid-state reaction method. They found that a typical morphotropic phase boundary exists at x = 0.52-0.525, separating the monoclinic and orthorhombic phases. Moreover, the sample with the composition near x = 0.52 showed the maximum value of the piezoelectric constant (d33 = 160 pC N-1).13

Saito et al. reported exceptionally high piezoelectric properties in the system (K,Na)NbO3-LiTaO3-LiSbO3.14 This study was based on chemical modifications, in the vicinity of the MPB of (K,Na)NbO3 (KNN), by complex simultaneous substitutions in the A (Li) and B (Ta and Sb) site of the perovskite lattice. Besides these chemical modifications, they developed a novel processing route for producing textured polycrystals of the KNN-based compositions by additional engineering of the microstructural design. However, the compositional inhomogeneity, particularly the inhomogeneous distribution of Nb, Ta and Sb on the B-site of the perovskite lattice, is rather difficult to be avoided due to the phase segregation of end members over a wide temperature interval.15 In addition, apparent compositional segregation in KNN has been evidenced in ceramics annealed for a long time.16

Recently, we have reported in the (K,Na,Li)(Nb,Ta,Sb)O3 system (abbreviated as KNL-NTS) that the dielectric, piezoelectric and elastic material responses are fundamentally related to extrinsic effects.17 Nonetheless, the dielectric and mechanical losses at room temperature are similar to those of a soft PZT ceramic and too high to be used in power devices. Thus, these materials need to be modified by the use of suitable dopants, like for ''pure'' KNN. The effect of doping on various physical and chemical properties of this material is considered as a classic story in the field of piezoelectric materials, which are used to modify the crystal structure and the piezoelectricity of these systems. Many aliovalent compositional modifications to the KNN-based system have been studied either with higher valence substitutions (donors), or with lower valence ions (acceptors).

There are few reports available on WO3 incorporation in KNN-based ceramics. Zang et al.1s synthesized lead-free

(Ko.4SNao .52)(W2/3Bi1/3)xNb1-xO3 (KNN-WBi) piezoceramics

with x ranging from 0.004 to 0.010 by conventional ceramic processing. The sintered ceramics showed the perovskite structure without a detectable secondary phase containing W and Bi. With increasing x, the orthorhombic-tetragonal phase transition temperature (TO-T) decreased from 200 to 1S4 °C whereas, the tetragonal-cubic phase transition temperature (TC) decreased slightly. With the doping of (W2/3Bi1/3), the piezoelectric properties were greatly improved and the piezoelectric constant exhibited a maximum value of 136 pC N-1, at x = 0.00S. Rani et al.19 reported the effect of WO3 addition on electrical and optical properties of (K0.17Na0.S3)NbO3. They established that WO3 addition causes the increase in the volume of the unit cell of (K0.17Na0. S3)NbO3 ceramics and promotes densification. The value of the dielectric constant at room temperature and at Curie temperature was found to increase with increasing WO3 concentration and it was found to be maximum for x = 5 wt% WO3.

In the present work, W6+ is selected as the dopant for KNL-NTS ceramics. Thus, on the basis of ionic radii,20 the W6+ ion (rw6+: 0.60 A for a coordination number CN = 6) falls in the size range of the B-site position (rNb5+: 0.64 A, rTa5+: 0.64 A, rSb5+: 0.60 A CN = 6). Considering its valency, W6+ can act as a donor-dopant if introduced into the B-site. Such behaviours may cause several effects on the dielectric behaviour through interaction with the perovskite structure. Therefore, in order to introduce the W6+ ion into the B-site of the perovskite lattice, we selected

the B-site deficiency, with a global formula (K0.44Na0.52Li0.04)-

[(Nb0 S6Ta010Sb0 04)1_xW5x/6]O3, hereafter abbreviated as KNL-(NTS)1-xW5x/6. We synthesized the (K,Na)NbO3-LiTaO3-LiSbO3 system, replacing the B-sites with W6+ ions [KNL-(NTS)1-xW5x/6], which shows a polymorphic behaviour between a tetragonal symmetry and an orthorhombic symmetry. The substitution at the B (Nb+5, Ta+5 and Sb+5) sites of the perovskite lattice resulted in the presence of a new secondary phase, which modifies the phase transitions of the system. These structural changes are due to the partial retention of the alkali elements involved in (KNa)NbO3-based perovskite structure, which move to form the secondary phase. The main achievement is related to the tuning of the ferroelectric behaviour, which evolves from

a normal ferroelectric to a 'relaxor-like' ferroelectric because of the heterogeneous distribution of the W6+ ions.

2. Experimental details

Sample preparation

The (K0.44Na0.52Li0 .04)[(Nb0.86Ta0.10Sb0.04)1-xW5x/6]O3 composition

was prepared by the conventional ceramic processing route. Na2CO3, Li2CO3 (Panreac, 99.5%), K2CO3 (Merck, 99%), WO3, Nb2O5, Ta2O5, and Sb2O5 (Sigma-Aldrich, >99.5%, 99.9%, 99% and 99.995%, respectively) were used as starting raw materials. They were individually milled, in order to obtain an appropriate distribution of the particle size.

Powders with different WO3 concentrations (x = 0, 0.005, 0.010, 0.030 and 0.050), abbreviated as KNL-(NTS)1-xW5x/6, were weighed using an electronic balance and ball milled for

3 h in ethanol medium in a high energy laboratory ball-mill with zirconia balls. Afterwards, the resulting powders were dried and calcined at 700 °C for 2 h at 3 °C min-1. These calcined powders were attrition milled again and pressed at 200 MPa into disks of 10 mm in diameter and 0.7 mm thick. The pellets were finally sintered in air at 1125 °C for 2 h. Bulk densities of the samples were determined using the Archimedes method.

X-ray diffraction (XRD)

Crystalline phases were characterized by X-ray diffraction (XRD) (DS Advance, Bruker, Germany), using CuKa radiation, on powders obtained by milling the sintered ceramics. The patterns were recorded over the angular range 5-70° (20) with a step size of 0.0334° and a time per step of 100 seconds, using Cu Ka radiation (l = 0.154056 nm) with a working voltage and current of 40 kV and 100 mA, respectively. Peak positions were fitted assuming a Lorentz peak shape.

Atomic force microscopy and confocal Raman microscopy mapping

The experiments were performed using a confocal Raman microscope (CRM) coupled with atomic force microscopy (AFM, Witec alpha-300R). The sample was imaged in AFM AC Mode using ArrowFM cantilevers (Nanoworld, Germany) with a resonance frequency in the range of 70-90 kHz and a damping of r = 50%, recording both topography and phase images simultaneously. Raman spectra were obtained using a frequency-doubled NdYAG laser operating at 532 nm and a 100 x objective lens (NA = 0.9). The incident laser power was 20 mW. The optical diffraction resolution of the confocal microscope was limited to about b250 nm laterally and ~500 nm vertically. Raman spectral resolution of the system was down to 0.02 cm-1. The microscopy sample was mounted on a piezo-driven scan platform having

4 nm lateral and 0.5 mm vertical positional accuracy. The piezoelectric scanning table allows steps of 3 nanometres (0.3 nm in the vertical direction), giving a very high spatial resolution for both the AFM and the confocal Raman microscopy. The microscope base was also fitted with an active vibration isolation system, active 0.7-1000 Hz. Collected spectra

were analysed by using Witec Control Plus Software, and Raman mode positions were fitted assuming a Lorentz peak shape.

Electron microscopy

The microstructure was evaluated on polished and thermally etched samples (1000 °C for 5 min) using a field emission scanning electron microscope, FE-SEM (Hitachi S-4700). The composition of the ceramics was estimated using energy dispersive spectroscopy, EDS.

Electrical properties

For the electrical measurements, silver paste electrodes were coated on both sides of the sintered samples. After being fired at 700 °C for 20 minutes, these disks can be used for characterizing electrical performance. In order to test the piezoelectric constant, the samples were polarized under a direct current (dc) electric field of 40 kV cm-1 in a silicone oil bath at 25 °C for 30 min. The piezoelectric constant d33 was measured using a piezo d33 meter (YE2730A d33 METER, APC International, Ltd, USA). Dielectric properties were determined at different temperatures and frequencies using an impedance analyzer HP4294A in the frequency range 100 Hz to 1 MHz.

3. Results

3.1. Structural characterization of the I<NL-(NTS)1-xW5x/6 ceramics by XRD: the identification of the polymorphic behaviour

Fig. 1 presents the X-ray diffraction patterns of KNL-(NTS)1-xW5x/6 ceramics for different W6+ amounts and sintered at 1125 °C for 2 hours. The diffraction patterns correspond to a perovskite structure. Furthermore, doped-samples present the same main perovskite phase and a minor secondary phase, which are assigned to K6LiNb6O17 (PDF# 36-0533) or K6Nb10.88O30 (PDF# 87-1856), with a tetragonal tungsten-bronze phase structure (TTB).21'22 This phase is more relevant in high doped ceramics, indicating that the W6+ concentration affects the formation of secondary phases.

The insets of Fig. 1 display the splitting of the (200) pseudo-cubic peak into (200) and (002), which suggests non-cubic symmetry in these samples. The insets show in detail the XRD diffraction pattern in the 20 range 44.5° to 46.5° of the KNL-(NTS)1-xW5x/6 ceramic system. Moreover, as represented in the insets, the W6+ doping produces changes in the symmetry of the perovskite structure. All samples display this splitting associated with the coexistence of a tetragonal symmetry, T, and an orthorhombic symmetry, O, (see inset of Fig. 1). The coexistence of different polymorphs (tetragonal and orthorhombic phases) was previously reported in KNL-NTS bulk ceramics.23-26 It is well known that the tetragonal symmetry, T, of the perovskite phase can be deconvoluted in two Lorentzian peaks, (002)T, and (200)T. In addition, in these samples two peaks located at ~45.4 and ~45.6° (20) which are associated with the orthorhombic symmetry, O, are detected. At low W6+ contents (x < 0.01), the peaks associated with tetragonal symmetry are more relevant than in the ceramics with high W6+ contents (x > 0.03),

Fig. 1 Influence of the W concentration on the crystalline structure of the KNL-(NTS)1-xW5x/6 ceramic: the figure shows X-ray diffraction patterns of KNL-(NTS)1-xW5x/6 ceramics sintered at 1125 °C for 2 h. In the ceramics with high W6+ content (x > 0.03) different peaks that are associated with the occurrence of the secondary phase appear and are signaled with a circle symbol. The insets of each figure show in detail the XRD diffraction pattern in the 20 range of 44.5° to 46.5° of the KNL-(NTS)1-xW5x/6 ceramics. These patterns are fitted to the sum of four Lorentzian peaks, which are indexed as 2 tetragonal peaks (in blue colour) plus 2 orthorhombic peaks (in red colour) of the perovskite phase. (T: tetragonal symmetry and O: orthorhombic symmetry).

which implies a stabilization of tetragonal symmetry at low W6+ concentrations (see inset of Fig. 1). As a consequence, the most probable origin of this behaviour must be related to the solubility of W6+ ions in the perovskite structure, and therefore to the chemical homogeneity of the system. So, we suppose that the perovskite lattice cannot accommodate the nominal W6+ content since it corresponds to B-site excess. Thus, the corresponding excess should be directly compensated by the eviction of some Nb5+ ions, with the transformation of the perovskite structure towards an orthorhombic symmetry, and the apparition of the secondary phase.

3.2. Study of the secondary phase location by Raman imaging

To verify that the crystalline symmetry and the appearance of the secondary phase are influenced by the doping on the

KNN-based system, additional experiments were performed by confocal Raman microscopy (CRM). According to the nuclear site group analysis, Raman active modes of the tetragonal P4mm (C4v) crystal symmetry are associated with the BO6-perovskite octahedron.27'28 So, the vibrations of the BO6-octahedron consist of 1A1g (n1) + 1Eg (n2) + 2F1u (n3, n4) + F2g (n5) + F2u (n6). From these vibrations, 1A1g (n1) + 1Eg (n2) + 1F1u (n3) are stretching modes and the other ones are bending modes. In particular, A1g (n1) and F2g (n5) have been detected as being relatively strong scatterings in systems similar to the ones studied in this paper due to a near-perfect equilateral octahedral symmetry. The presence of the main vibrational modes of the NbO6- octahedron as shown in Fig. 2(a) is another evidence for the formation of the perovskite structure, which corresponds to the red regions marked in Fig. 2(b)-(f). Clear differences can be observed in the shape of the spectra as a function of the doping amount, particularly in the 500 to 700 cm-1 region. The details of the Raman modes of the perovskite are magnified in connecting insets in Fig. 2(a), which can be assigned to Eg (n2) and A1g (n1) Raman modes, respectively. The evolution of the A1g mode shows a continuous Raman red-shift of this mode when the x increases, see insets of Fig. 2(a). So, the inset in the top of Fig. 2(a) shows an enlargement of the Raman modes A1g (n1) and Eg (n2), exhibiting a Raman red-shift of 8.1 cm-1 for x = 0.05 with respect to the undoped ceramics. Thus, one can deduce that the incorporation of W6+ into the perovskite lattice slightly alters the observed vibrational frequencies, shifting the A1g (n1) mode to a lower wavenumber due to a decrease in the strength constant force, caused by the lengthening of the distance between B5+ type ions and their coordinated oxygens.

A careful examination of the Raman spectra, Fig. S1 (ESIf), reveals the presence of Raman modes corresponding to the KNN-based phase and the appearance of an additional Raman peak. This new Raman peak is attributed to the appearance of a characteristic Raman mode of the TI B secondary phase at ~690 cm-1,29 [blue regions marked in Fig. 2(c)-(f)], which emerges concomitantly with the increase in x (more information on this secondary phase is in the ESIf). Fig. 2(b)-(f) shows a sequential series of the Raman maps in which we can observe and quantify the KNN-based phase (marked in red) and TTB secondary phase distribution (marked in blue) into ceramics as a function of the W6+ content. As mentioned above, the average Raman spectrum of the secondary phase can be indexed on the basis of a phase mixture constituted by a majority of the KNN-based phase and a minority of the secondary phase, see ESI.f

As a result, it can be observed that the secondary phase amount increases with the W6+ concentration, Fig. 2(g), while changes in the Raman shift of Raman modes associated with the BO6-octahedron, Fig. 2(a), allow determining variations in polarization, which are associated with modifications of the constant force of the octahedron due to deformation or stress.30'31 The last phenomenon (polarization changes) must be associated with the evolution of polymorphic behaviour observed by XRD.

3.3. Determination of the secondary phase composition

The FE-SEM micrographs of the KNL-(NTS)1-xW5x/6 ceramics with x between 0.00 to 0.05 are shown in Fig. 3(a)-(e).

The microstructure of undoped materials consisted of homogeneously cubic-shaped grains with an average equivalent diameter of ~1 mm. These ceramics present a porous microstructure without secondary phases. The addition of a low amount of the W®+ produces an inhomogeneous grain growth and a reduction in the porosity level (see Table 1). A change in the average grain size and the grain morphology is observed with the W6+ content, as depicted in Fig. 3(d) and (e). In addition, for a higher W6+ content (x > 0.03), a secondary re-crystallization appears forming small plate-like grains, see Fig. 3(d) and (e). According to XRD results, secondary phases are observed and their amount increased with the doping level. Additionally, the appearance of the secondary amorphous phase is associated with the transitory liquid phase that assisted the sintering process. This liquid phase has also already been observed in undoped samples only for the short sintering time.32 In this work, the liquid phase is stabilized for the low doping content. The presence of W6+ ions enhances probably the amount of the transient liquid phase and thus the grain growth is promoted.

EDS analysis was carried out on all the specimens in order to identify the composition of the TTB phase. Fig. 3(f)-(h) illustrate the EDS spectrum corresponding to the matrix and TTB phase, respectively, and the composition analysis is also summarized in the included table. Na, K, Nb, Sb, W and Ta ions were found in the matrix and its composition is confirmed to be the very close to KNL-NTS, see the composition table in Fig. 3 derived from the EDS spectra. In the TTB phase, however, a low relation Na/K has been expected, see the composition table in Fig. 3. Through the EDS analysis, the composition of the secondary phase is close to the K6Nb10 SSO30-based phase with a tetragonal tungsten-bronze structure.

3.4. Influence of the structural and microstructural changes on the functional properties of the KNL-(NTS)1-xW5x/6 ceramics

Table 1 shows density, the real part (e') of dielectric permittivity, dielectric loss (tan d) and the piezoelectric constant of sintered samples. It can be observed that the ceramics with 0.005 < x < 0.01 show higher density values than pure KNL-NTS ceramics, although a diminution in e' and d33 values has been observed when the doping level increases. This behaviour can be attributed to the secondary phase formation, the density diminution, and the coexistence of different polymorphic phases. Moreover, the presence of the secondary phase correlates with the higher dielectric losses.

In order to further determine the influence of the phase stabilization on dielectric properties, the e'-T curves of each sample were measured in the temperature range of 25-450 °C, as shown in Fig. 4(a). Fig. 4(a) shows the temperature dependence of the dielectric permittivity e0 (at 50 kHz) of KNL-(NTS)1-xW5x/6 ceramics as a function of x. These curves present two transitions with temperature. The first one, near room temperature, is associated with the orthorhombic-tetragonal phase transition (TO-T), while the second one, at 295 ± 5 °C, corresponds to the ferroelectric-paraelectric phase transition (Curie temperature, TC). Considering the results of both XRD

Fig. 2 Detection of the secondary phase observed in the KNL-NTS ceramics doped with different W amounts: (a) average Raman spectra of ceramics. These Raman spectra are fitted to the sum of two Lorentzian peaks, ascribed to the Eg (n2) and A1g (nj corresponding to Raman modes of the KNL-NTS perovskite phase. Moreover, the inset in the top of Fig. (a) shows an enlargement of the Raman shift occurred between the ceramics with x = 0.00 and x = 0.05, exhibiting an increased Raman shift of 8.1 cm-1. (b) to (f) Raman image of ceramics with different W6+ amounts, exhibiting the secondary phase location, blue areas. The Raman image is derived by summing the total spectral pixel intensity from 150 cm-1 to 1000 cm-1. The secondary phases signalled in regions with blue colours, whereas the modified KNN perovskite phase corresponds to red regions. (g) Evolution of the secondary phase

concentration obtained from Raman spectroscopy as a function of W6 phase in the ESI.f

content. In addition, the reader can find more information about this secondary

patterns and e'-T curves [see Fig. 1 and 4(a) and (b)], we can deduce that all samples belong to orthorhombic (O) and tetragonal (T) phase coexistence.24-26 It is also known that

in (K,Na,Li)(Nb,Ta)O3 ceramics, the orthorhombic-tetragonal phase transition temperature, TO-T, decreases as a result of the Li+ addition, which stabilizes the tetragonal symmetry.33

Fig. 3 SEM images of KNL-(NTS)1-xW5x/5. (a) Undoped; (b) x = 0.005 (c) x = 0.01; and (d) x = 0.03, (e) x = 0.05; and energy dispersive spectra (EDS) at five different regions (1-5) of KNL-(NTS)1-xW5x/5 with x between 0.00 (f) to 0.05 (g-h). The table presents the composition on the points shown on a micrograph derived from EDS spectra. The table represents the atomic percentages of elements.

Previous studies have shown that doping with Mo6+, Ni2+, or Mn2+ increases TO-T in KNN.32'34'35 The observed shifts in the TO-T indicate that the W6+ incorporation into the perovskite structure promotes the stabilization of the orthorhombic symmetry at room temperature, as can be observed by X-ray diffraction, due to the secondary phase formation which partially retains the alkaline elements (Li, Na, K). Here we will also mainly focus on the variations of the TC values as a function of W6+, Fig. 4(b). As compared with the KNN-based ceramics

Table 1 Density (p), relative density (pr), dielectric properties (e', tan d), and piezoelectric constant (d33) of KNL-(NTS)1-xW5x/5 ceramics sintered at 1125 °C for 2 h. The p and d33 values are the average values of five specimens. Data are measured at 25 °C and 24 hours after poling. Theoretical density 4.59 g cm-321

KNL-(NTS)1-xW5x/6 (x) p (g cm-3) pr (%) e' tan(d) d33 (pC N-1)

0.000 0.005 0.010 0.030 0.050

4.41 ± 0.024 4.53 ± 0.043 4.50 ± 0.022 4.28 ± 0.028 4.26 ± 0.037

94.1 96.6 96.0 91.3 90.8

880 861 664 566 529

0.007 0.025 0.037 0.059 0.062

230 195 180 140 95

without W6+ we can notice that the ceramics with W6+ have a lower TC. This phenomenon is consistent with the previously reported results in KNN with the addition of another doping at the B-site.34-37

In addition, some differences in the TO-T and TC peak shapes were also observed, as shown in Fig. 4(c) and (d). The TO-T peak shape of the ceramics with 0.00 < x < 0.005 matches effectively with the T phase of KNN and the ceramics with 0.01 < x < 0.05 possess O and T mixed phases [see Fig. 4(c) and (d)]. Moreover, we can observe from Fig. 4(c) and (d) a more frequency dependence in the ceramics with large W6+content. Thus, the cause of this behaviour can be related to chemical inhomogeneities. These inhomogeneities are characteristics of polymorphism and generally appear as polar nanoregions (PNRs), which are typical to relaxor systems, or when phase diffusion is present. As mentioned above, the doping effects on the ceramics are also reflected in the TC transition. One can see that the ceramics with high W6+ contents (x > 0.03) have a larger frequency dependence and presents some characteristics of relaxor-like behaviour:38 (i) a broad maximum in the thermal dependence of the dielectric permittivity and (ii) a frequency

Fig. 4 (a) Real permittivity (e0) as a function of temperature of KNL-(NTS)1-xW5x/6 sintered ceramics (at 50 kHz). The part (b) shows the evolution of the TC and the phase transition temperatures from orthorhombic to tetragonal phases (TO-T) of the KNL-(NTS)1-xW5x/6 ceramics with different contents in W6+ (the sensitivity of the phase transition temperatures, TO-Tand TC, were estimated at ±5 °C). The parts (c and d) show the temperature dependence of

er for the undoped ceramic (c) and the ceramic with x colour) are shown, respectively.

0.05 (d) as a function of frequency, in which TO-T (marked in blue colour) and TC (marked in red

dependence of the maximum of the dielectric constant (the maximum is slightly shifted to higher temperatures when the frequency is increased). Thus, this ''relaxor-like'' behaviour is particularly clear for the sample with high W6+ contents (x > 0.03) (Fig. 4(d)) and seems to disappear for the undoped ceramics (Fig. 4(c)).

To verify such behaviour, the diffuseness of the phase transition can be determined from the modified Curie-Weiss law, 1/e' - 1/em = C-1(T - Tm)g (ref. 38) for which g = 2 corresponds to a relaxor behaviour while g = 1 corresponds to a classical ferroelectric-paraelectric phase transition.39 Fig. 5(a) shows the plots of ln(1/e' - 1/em) vs. ln(T - Tm) of the KNL-(NTS)1-xW5x/6 ceramics for different W6+ amounts and sintered at 1125 °C for 2 hours. All the samples exhibit a linear relationship. The g value was determined by least-squares fitting of the experimental data to this modified Curie-Weiss law, Fig. 5(b). For the ceramics with x = 0.00, the g value calculated is 1.27, suggesting a normal ferroelectric behaviour. As the W6+ content increases, g intensifies gradually, Fig. 5(b), reaching a value of 1.75 for the ceramics with x = 0.05, indicating that the ceramic has been evolved from a normal ferroelectric state toward a ''relaxor'' state. It is suggested that this behaviour is due to the cation disorder in the perovskite unit cell and the consequent micro domain formation.40 Such disorder and clusters are presented in PZT because solid solutions tend to have compositional fluctuations.41-43 Thus, the combination of the XRD, CMR, FE-SEM and e0-T curves allows us to infer that the large

variations in g values are a consequence of the appearance of the secondary phase, which provokes an increase in the local chemical heterogeneity of the system.

Following the above analysis, it is clear that the addition of W6+ has induced a spontaneous normal tetragonal to relaxor orthorhombic ferroelectric phase transformation. This kind of phase transition behaviour may be accompanied by the corresponding change in domain switching behaviour and domain morphology. Electric field-induced polarization hysteresis loops measured at room temperature for the KNL-(NTS) 1-xW5x/6 ceramics with the value of x lying between 0.00 to 0.05 are shown in Fig. 6. A saturated square hysteresis loop was observed for the sample with low W6+ concentration (x < 0.01), showing a relatively large spontaneous polarization (Ps) and remnant polarization (Pr). This is a typical characteristic of the phase that contains long-range interaction between dipoles and owns a micron-sized ferroelectric domain state,44,45 indicating that KNL-(NTS)1-xW5x/6 ceramics with low W6+ content belongs to a normal ferroelectric. With an increase in the W6+ content, the loops become much slimmer, especially for the x = 0.05 sample with an orthorhombic ''relaxor'' state. Compared to the Pr value, the saturated polarization (Pmax) for the sample with x = 0.05 is only slightly less than the value of samples with a low W6+ concentration (x < 0.01). The results indicate that the polarization of the orthorhombic phase can be aligned to the saturated state, but cannot be maintained when the applied electric field is removed. The relaxor behaviour of ferroelectrics

result, the sample finally exhibits a lower Pr. In addition, it is also believed that the absence of long-range dipoles restricts not only the polarization, but also tends to induce the formation of polar nanodomains.47

For ferroelectric materials, doping elements can act in various ways: (i) they can be incorporated into the structure, which is thus modified, i.e. intrinsic effects, (ii) they can modify the sintering and grain growth mechanisms, thus changing the microstructure (grain size, morphology) and the ferroelectric domain configuration and size, i.e. extrinsic effects and (iii) they can induce structural defects (like oxygen vacancies) thus modifying the pinning of ferroelectric domains. As a consequence, the role of the doping elements appears as quite complex and must be considered carefully. The ac conductivity analysis can allow us to identify the possible conduction mechanisms, and their variation with the frequency, beside it can specify us if its origin is intrinsic or extrinsic. Thus, from the imaginary part of permittivity e00 values at different frequencies (f), the ac conductivity (sac) data can be calculated according to the following equation:

: ë"-Ë0-W

Fig. 5 (a) Plots corresponding to the modified Curie-Weiss law for the KNL-(NTS)1_xW5x/6 sintered ceramics. The symbols denote experimental data, while the red dotted lines correspond to the least-squares fitting of the modified Curie-Weiss law. (b) Diffuseness, g, of the ceramics as a function of the W6+ content.

is generally considered to originate from the local orderdisorder of the crystal structure which causes the formation of PNRs and a local electric field, owing to the heterovalent occupation of the B-site on the perovskite structure with W6+ ions. The presence of the random field can hinder the longrange dipole alignment and thus suppress the ferroelectric interaction, resulting in a lower Ps value. Although the normal ferroelectric state with long-range dipoles in a relaxor state can be excited by an electric field (i.e., larger Pmax),46 yet it cannot be maintained after the external electric field is released. As a

where e0 = 8.85 x 10-12 F m-1 and o = 2pf. As a result, it can be observed that sac clearly depends on the W®+ concentration, Fig. 7. At high frequencies ( f > 105 Hz) the ac conductivity slightly increased with doping, which are associated with intrinsic effects. This fact clearly shows that the solubility of W6+ acting as donor dopant in the B-site position of the perovskite is very limited, in agreement with the structural results.

At low frequencies (f < 105 Hz), the doped ceramics exhibit a significant increase in ac conductivity of ~ 1 order of magnitude compared to the undoped sample, which is associated with extrinsic effects. As a logical consequence, the explanation of this evolution is that the solubility limit of the doping element is quickly reached and that excess W6+ ions together with some alkaline cations generate a liquid phase that enhances the formation of a secondary phase (extrinsic effect), as evidenced in Fig. 2 and 3. This secondary phase is preferably located at grain boundaries and/or at the triple point, and it seems to be responsible for the grain refinement as an indication of their liquid nature during the sintering step. Moreover, the secondary phase

Fig. 6 Ferroelectric behaviours of the KNL-(NTS)1-xW5x/5 ceramics: P-E loops of the ceramics as a function of the composition (x = 0.00; x = 0.005; x = 0.01; x = 0.03; and x = 0.05). The blue dotted line shows the evolution of the remnant polarization (Pr).

Fig. 7 Plots corresponding to the ac conductivity (sac) versus the frequency of KNL-(NTS)1-xW5x/6 sintered at room temperature.

possesses a low crystallinity and only large amounts result in the appearance of the crystallized phase. The evidence of the presence of the secondary phase is demonstrated here by confocal Raman microscopy (Fig. 2), and as second evidence the ac conductivity is valuable to determine this fact. To sum up, excess W6+ ions promote the appearance of such a secondary phase and the displacement of the alkaline equilibrium that it is also translated in small grains. The general approach to consider the XRD pattern as the proof of perovskite purity is here demonstrated to be not valid enough and resembles that the presence of the secondary amorphous phase is a main

parameter to determine the lower piezoelectric properties in (K,Na)NbO3-based lead-free piezoceramics. In addition the diversity of piezoelectric properties of this system could be explained in part by the fact that the amorphous secondary phase is not well understood. Thus, higher piezoelectric properties in (K,Na)NbO3-based lead-free piezoceramics will be obtained after careful control and reduction of the secondary phase.

3.5. Role of the secondary phase in cationic displacement of the alkali ions forming the perovskite structure

To verify that the secondary phase formation plays a relevant role in partial retention of the alkaline elements (Li, Na, K) forming the KNL-(NTS)1-xW5x/6 perovskite structure, additional experiments were performed by atomic force microscopy (AFM) and CRM. The ceramic with higher W®+ contents, x = 0.05, was chosen to set so that the secondary phase concentration was maximized. Fig. 8(a) depicts an optical microscopy image of the sample with x = 0.05 aligned perpendicularly to the AFM cantilevers. The area of 14 x 14 mm (Fig. 8(a)) delimits the range where topographic information was collected by AFM. Fig. 8(b.1) shows a detailed AFM topographic image of two grains corresponding to the secondary phase structure with a plate-like shape. The AFM scans along the white arrow of Fig. 8(b.1) are illustrated in Fig. 8(b.2). The grain associated with the secondary phase (i) has a grain size of ~4 mm and (ii) close to the grain boundary protrusions appear (height difference of ~400 nm).

CRM is here combined with AFM in the same experimental setup, thus giving direct correlations between topography and local structure. The selected area is the one previously studied by AFM. Raman spectra having the same Raman shift are classified by the colours and colour intensity corresponds to the Raman intensity as shown in Fig. 2. The colour combination results in (i) the Raman image of the surface (Fig. 8(c.1)) and (ii) the Raman depth scan image of the cross-section (Fig. 8(c.2)). From these results we can deduce that there is a

Fig. 8 (a) Optical micrograph of the polished surface and thermally etched KNL-(NTS)1-xW5x/6 sintered ceramics with x = 0.05. (b.1) AFM image of KNL-(NTS)1-xW5x/6 sintered ceramics with x = 0.05, showing the topography of the secondary phase inside the marked white box of (a). (b.2) AFM topography scans along the white arrow of (b.1). The white rectangle in the panel (a) shows the positions where the XY Raman image is obtained and corresponds with the area of previous AFM analysis. The Raman image shows the secondary phase distribution (blue regions) at the surface by a colour code (c.1) as well as in the depth scan (c.2). The white arrow in the panel (c.1) shows the position where the XZ Raman depth scan image is obtained.

clear correlation between the secondary phase region (marked in blue colour) and the protrusion evidenced by AFM. This phenomenon results from the partial retention of the alkali elements (Li, Na, K) induced by the secondary phase formation at high temperature during the sintering step, which promotes the displacement of the alkaline elements from the main KNL-NTS phase (red region in Fig. 8(c)1-2) to the new secondary phase (blue region in Fig. 8(c)1-2). As a consequence, we can conclude that the partial retention of the alkaline elements (Li, Na, K) in the secondary phase, plays an important role in the increase of TO-T and the simultaneous decrease of TC in the KNL-NTS phase, as shown in Fig. 4.

4. Conclusions

(K0.44Na0.52Li0.04)[(Nb0.86Ta0.10Sb0.04)i-xW5X/6]O3 lead-free piezo-ceramics have been prepared by solid state reaction, and the effects of the W6+ content on their phase structure, microstructure, and electrical properties were investigated in detail. The replacement in the B (Nb+5, Ta+5 and Sb+5) sites of the perovskite lattice resulted in the presence of a polymorphic behaviour between a T symmetry and an O symmetry in the perovskite structure, and in the formation of a new secondary phase. Moreover, the partial retention of the alkaline elements (Li, Na, K) in the secondary phase provokes the increase of TO-T and the simultaneous decrease of TC in KNL-NTS. Both structural features can be modified by the W®+ content, resulting in the tuning of the phase transitions of the system. Thus, it is worth noting that the ceramics with higher W®+ content develops a high diffusivity value (g), which can be explained as a consequence of an increase in the local chemical heterogeneity of the system. As a result, we believe that the significance of this work lies in the understanding of the structural effects caused by the dopant addition in (KNa)NbO3-based lead-free ceramics, and hence this knowledge can lead to the expansion of their application range.

Acknowledgements

The authors are grateful to CONICET, ANPCyT (Argentina) and MICINN project MAT2013-480089-C04-1-P for the financial support provided for this research. Dr F. Rubio-Marcos is also indebted to MINECO for a ''Juan de la Cierva'' contract (ref: JCI-2012-14521), which is cofinanced by FEDER funds.

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