Lead-Free Relaxor Ferroelectric Na0.47K0.47Li0.06Nb0.94Sb0.06O3

Article pubs.acs.org/crystal

Lead-Free Relaxor Ferroelectric Na0.47K0.47Li0.06Nb0.94Sb0.06O3 Crystals for Optoelectronic Applications Geeta Ray,† Nidhi Sinha,†,‡ Budhendra Singh,†,§ Igor Bdikin,§ and Binay Kumar*,† †

Crystal Lab, Department of Physics & Astrophysics and ‡Department of Electronics, SGTB Khalsa College, University of Delhi, Delhi-7, India § TEMA-NRD, Mechanical Engineering Department, Aveiro Institute of Nanotechnology University of Aveiro, 3810-193 Aveiro, Portugal ABSTRACT: In the present study, we found that Na0.47K0.47Li0.06Nb0.94Sb0.06O3 (NKLNS) crystals are good piezo-/pyro-/ferroelectric candidate for various electronic and optical applications. Its large single crystals (up to 9 × 8 × 4 mm3) were successfully grown by self-flux method. Relaxor type broad and diffused Curie phase transition with maximum dielectric constant around 355 °C was obtained. High values of dielectric constant, d33 coefficient (125 pC/N), and pyroelectric coefficient (∼821 μC m−2 °C−1) were measured. Fatigue-free ferroelectric properties via hysteresis loops and piezoresponse force microscopy were observed. The crystals were found to show good transparency in the UV−visible region and a wide direct-energy band gap of 3.8 eV, along with sharp UV, blue, and green emissions. The crystals exhibited reverse indentation size effect with Meyer’s index n = 2.19. Thus, NKLNS crystals are a promising lead-free alternative for dielectric, piezo/ferroelectric, and optical applications.

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piezoelectric and ferroelectric crystals has been created.8 Alkalibased titanates such as Na0.5Bi0.5TiO3−K0.5Bi0.5TiO3 (BNKT) and niobates such as (1 − x)Na0.5K0.5NbO3-xLiNbO3 (NKLN) are among the most promising lead-free candidates owing to their excellent piezoelectric and ferroelectric properties.9−12 Concerning lead-free perovskite systems, Na 0.5 K0.5 NbO3 (KNN) crystals show high Curie phase transition above 400 °C along with a high piezoelectric charge coefficient.13,14 LiNbO3 (LN) crystals have good optical and piezoelectric properties and are widely used in optoelectronic industries.15,16 LN-added KNN, i.e., 0.94[Na 0.5 K 0.5 NbO 3 ]−0.06LiNbO 3 (NKLN), is a good lead-free candidate with properties of normal ferroelectrics because there is no distortion created on the B-site ion.11,17,18 To make NKLN crystals relaxor ferroelectric type, its B site can be substituted with 6 mol % of Sb5+, which makes it to belong to the category of A(B′B″)O3-type relaxors. Lead-free crystals belonging to the class of perovskite niobates such as LiNbO3, KNbO3, and KNN show excellent optical properties that are useful in optical devices such as optical modulators, optical switching, optical communications, and high-energy laser generation.15,19,20 Therefore, it is expected that a ternary system of these crystals will also show good optical properties. NKLNS ((1 − x)[Na0.5K0.5NbO3]−xLiSbO3) is an Sb-substituted NKLN perovskite showing good piezoelectric and ferroelectric properties. NKLNS, when synthesized as a ceramic, shows a high

INTRODUCTION Perovskite materials have always attracted the interest of researchers for their technological applications in optical, piezo-/pyro-/ferroelectric, and dielectric devices. Lead titanate (PbTiO3)-based relaxor perovskite ferroelectric single crystals such as Pb(Zn1/3Nb2/3)O3−PbTiO3 (PZN-PT) and Pb(Mg1/3Nb2/3)O3−PbTiO3 (PMN-PT) have dominated the industry of ferroelectric memory devices, actuators, piezoelectric sensors, transducers, optical mixers, pyroelectric detectors, and electro-optics, owing to their unsurpassable electromechanical and optical properties.1−4 These lead-based relaxor ferroelectrics have a general chemical formula described as A(B′B″)O3−PbTiO3 (where A is Pb; B′ is Mg2+, Zn2+, Ni2+, Fe3+, Sc3+, In3+, etc.; and B″ is Nb5+, Ta5+, W6+, etc.).5 The relaxor ferroelectric crystals are very useful in tunable microwave devices, owing to their excellent dielectric constant and relatively low dielectric loss.6 Relaxor ferroelectric materials can be characterized by a broad dielectric dispersion around Curie temperature (Tc), as compared to normal ferroelectric materials that display a sharp and narrow peak at Tc. Relaxortype phase transition is also known as diffused phase transition that has been attributed to B-site disordering.7 A(B′B″)O3based ferroelectric crystals have B cation ordering, i.e., charge disorder and random local electric fields between B′ and B″, that results in a polar nano region even after Tc, making them useful for ferroelectric switching devices.7 PbTiO3-based relaxor ferroelectric single crystals contain a large amount of lead oxide/volatile heavy transition metal, i.e., Pb2+, that adversely affect the environment. Because of the toxicity of lead-based materials, a requirement of environment friendly lead-free © 2015 American Chemical Society

Received: December 31, 2014 Revised: February 10, 2015 Published: February 25, 2015 1852

DOI: 10.1021/cg501891r Cryst. Growth Des. 2015, 15, 1852−1860

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Crystal Growth & Design piezoelectric coefficient for 0.05 ≤ x ≤ 0.06 because of the existence of a PPT (polymorphic phase boundary) near room temperature.21−23 NKLNS ceramics show a high d33 coefficient (∼215 pC/N for x = 0.05); with a slight variation in its composition, i.e., for x = 0.06, d33 decreases to 161 pC/N as reported by Palei et al.21 G.-Z. Zang et al. have found NKLNS ceramic to show a high d33 (∼286 pC/N for x = 0.052).22 Single crystals of NKLNS are not yet reported; therefore in this work, single crystals of Na0.47K0.47Li0.06Nb0.94Sb0.06O3 (NKLNS) were grown by the self-flux method because this method has an advantage of growing crystals at a temperature far below their melting point, hence reducing volatilization of the material. High-temperature-grown crystals are always preferred in ferroelectric industries because of their thermal stability and better mechanical properties. The structural, dielectric, piezo-/ pyro-/ferroelectric, optical, and mechanical properties of NKLNS crystals were investigated in the present work.

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EXPERIMENTAL SECTION

Materials and Crystal Growth. Materials. For single crystal growth of Na0.47K0.47Li0.06Nb0.94Sb0.06O3 (NKLNS), high-purity starting materials K2CO3 (>99% purity), Li2CO3 (99.997% purity), Na2CO3 (>99% purity), Nb2O5 (99.99% purity), and Sb2O5 (99.995% purity) from Sigma-Aldrich were taken according to the stoichiometry of NKLNS. Crystal Growth. The oxides were mixed thoroughly and ground to a fine powder. The powder was calcined at 850 °C in an alumina crucible for 10 h to ensure the desired phase formation of NKLNS. The following reaction took place during calcination:

Figure 1. (a) Top view of platinum crucible after crystal growth. (b) NKLNS crystals in the crucible after partial washing. (c) NKLNS crystals extracted from the flux.

0.47Na 2CO3 + 0.47K 2CO3 + 0.06Li 2CO3 + 0.94Nb2 O5 + 0.06Sb2 O5 → 2Na 0.47K 0.47Li 0.06Nb0.94 Sb0.06O3 + CO2 (g)↑

m−1) was used to apply the voltage to the surface and to measure the mechanical response of the sample. For this, a commercial atomic force microscope (AFM, Ntegra Prima, NT-MDT) equipped with an external lock-in amplifier (SR830, Stanford Research) and a function generator (FG120, Yokogawa)) were used to apply both ac and dc voltages to the surface of the crystals for poling and piezoresponse image acquisition.24,25 Both topography and piezoelectric images (512 × 512 points) with a scan rate of 0.1 Hz were carried out by the PFM method under an applied ac voltage of amplitude = 5 V and frequency = 50 kHz. All signals were acquired in the form Acos Φ, where A is the amplitude of the piezoelectric vibrations and Φ is the phase shift between the driving and detected signals. In this way, domains with opposite polarities exhibit different contrast. Dark areas (hereafter referred to as negative domains) correspond to domains with polarization oriented toward the substrate (Φ = 180°), and bright regions (positive domains) to areas with polarization terminated at the free surface of the film (Φ = 0°). UV−vis spectrum of the crystals was recorded using a PerkinElmer UV Win Lab spectrophotometer. For UV−vis analysis, the unpoled crystals of NKLNS were ground to a fine powder and dissolved in distilled water to prepare a clear solution. Photoluminescence of the crystals was obtained on a Varian Cary Eclipse fluorescence spectrophotometer at room temperature with an excitation wavelength of 240 nm. A Vickers microhardness tester (MVH-1 by Metatech), equipped with a pyramidal diamond indenter and a computerized camera, was used to test their mechanical properties.

To the calcined powder of NKLNS was then added 25 wt % of selfflux mixture of K2CO3, Na2CO3, and Li2CO3. The flux-added powder was then put into a Pt crucible of wall thickness ∼1.0 mm, base ∼1.5 mm, and volume ∼100 cc. The Pt crucible was sealed in an alumina crucible by high-temperature cement. The powder was heated inside the furnace with the same temperature profile as reported in the case of NKLN crystals.17 The program consisted of fast heating rate of 100 °C/h up to 900 °C, followed by a constant temperature ∼900 °C for 4 h to form a stable product. Then a slow heating rate of 50 °C/h was set from 900 to 1200 °C to melt the mixture. The mixture was kept at 1200 °C for 4 h of soaking time to form homogeneous liquid solution. The solution was cooled down to 1000 °C at a very slow rate of 3 °C/ h for proper growth of NKLNS crystals. The crystals were kept at 1000 °C for 4 h to minimize cracks. The cooling rate was then increased to 5 °C/h up to 900 °C followed by 900 °C for 4 h. Further cooling to 500 °C was done at a rate of 10 °C/h, followed by natural cooling. After the completion of the furnace program, the Pt crucible was flooded with distilled water in order to obtain NKLNS crystals as shown in Figure 1a−c. The crystals were taken out of the crucible and washed by distilled water several times to remove the flux completely. The largest NKLNS crystal obtained was 9 × 8 × 4 mm3. Measurements and Characterizations. A D8 Discover diffractometer was used for structure analysis of NKLNS crystals with Cu Kα radiation of wavelength 1.5405 Å (at 20 °C). For the functional group confirmation, a Raman spectrophotometer in the range 100−1000 cm−1 was used with excitation radiation at 514 nm with an Ar-ion laser at room temperature. The dielectric constant and loss were measured as a function of temperature (30−500 °C) and frequency (0.5 kHz−2 MHz) by an Agilent E4980A LCR meter. The crystals were poled using a dc poling unit while immersed in silicon oil. A Piezotest piezometer system (PM300) was used for piezoelectric charge coefficient (d33) measurements. The Sawyer−Tower circuit of an automatic P−E loop tracer was used for the ferroelectric measurements. In piezoresponse force microscopy (PFM) study, a conductive Si cantilever (Nanosensors, with a nominal force constant of 15 N

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RESULTS AND DISCUSSION Powder X-ray Diffraction. Figure 2 shows the powder Xray diffraction pattern of NKLNS crystals measured at room temperature. They exhibited reflections of a pure perovskite structure, meaning that Sb5+ has successfully substituted the B site of the NKLN crystal lattice. To accurately study the crystal structure of NKLNS, the splitting of (002), (020), and (200) 1853


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Crystal Growth & Design

Figure 2. X-ray powder diffraction of NKLNS crystals; inset shows the peak splitting around 45°, confirming orthorhombic structure.

peaks around 45° is shown in the inset of Figure 2. NKLNS crystals have a mixed structure of KNN (KNa0.5Nb0.5O3) and LS (LiSbO3), in which KNN has an orthorhombic structure with (002)O and (020)O peaks at room temperature. Now, peaks of (002)T and (200)T for tetragonal crystal structure emerge when LS is added to the crystal lattice of KNN. When these peaks combine together for the crystal structure of NKLNS, three prominent peaks of (002)T, (020)O, and (200)T can be observed.21 This implies that for a pure orthorhombic crystal structure the intensity of (020)O should be greatest. Because in the present case the (020) peak has much higher intensity than the other two peaks, this suggests that the structure of NKNS crystals are orthorhombic. The lattice parameters of the crystals were calculated to be a = 3.939 Å, b = 3.956 Å, and c = 3.938 Å with space group Amm2. Raman Spectroscopy. To further analyze the structure of NKLNS crystals, Raman spectroscopy was carried out. The obtained Raman spectrum of NKLNS crystals is shown in Figure 3. The spectrum exhibited three obvious asymmetric broad bands around 250, 613, and 865 cm−1, confirming the formation of KNN-based pure perovskite structure.26 The three broad bands were found to be shifted to lower frequencies as compared to the case of the NKLN crystals.17 This is due to the distortions of the NbO6 octahedral cluster that is caused by the partial substitution of Nb5+ ion by Sb5+ ion on the B site of NKLN. The Sb−O bond is shorter and stronger as compared to the Nb−O bond that results in the lower-frequency shifting of the bands.27 The Raman peaks in the lower frequency region (below 200 cm−1) can be assigned to the translational modes of Na+/K+/Li+ and rotations of BO6 (B = Nb5+/Sb5+) octahedra. The vibrational modes of BO6 octahedra, which mainly consist of 1A1g(ν1) + 1Eg(ν2) + 2F1u(ν3,ν4) + F2g(ν5) + F2u(ν6), appear in the region of 200−900 cm−1. Among these vibrational modes, F1u(ν4), F2g(ν5), and F2u(ν6) are the bending modes, whereas the rest are the stretching modes. The main band located at 250 cm−1 can be attributed to F2g(ν5); because it is

Figure 3. Raman spectrum of NKLNS crystals.

relatively stronger than the rest, it indicates nearly perfect equilateral octahedral symmetry. Hence, the Sb cation has completely diffused in the NKLN lattice to form a pure perovskite structure of NKLNS crystals. Dielectric Property. The variation of dielectric constant with temperature in a wide temperature range (30−500 °C) at various frequencies is shown in Figure 4a. The dielectric constant was found to be 833 at room temperature and increased with temperature. There are few bumps in the curves of the dielectric constant and loss tangent (Figure 4a) as a result of varying electrical conductivity with frequency and temperature. This anomaly in dielectric study is a result of site exchange among A- and B-site cations that is one of the mechanisms of electrical conduction.28 The crystals of NKLNS consist of three competing A-site cations, i.e., Li+, Na+, and K+, and two competing B-site cations, i.e., Sb5+ and Nb5+, that give rise to anomalies in ac conductivity, hence also in tan δ and ε′. A very sharp phase transition peak caused by orthorhombic to tetragonal phase transition was observed at 205 °C. 1854


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Figure 4. (a) Variation of dielectric constant with temperature at various frequencies for NKLNS crystals. Loss tangent variation with temperature at various frequencies is shown in the inset. (b) The plot of log ((1/ε) − (1/εm)) vs log (T − Tm) for the calculation of diffuseness parameter.

Figure 5. (a) P−E hysteresis loops of NKLNS crystals traced at temperature range (40−90 °C). (b) Variation of spontaneous polarization with temperature for the calculation of pyroelectric coefficient. (c) P−E loops traced at various switching cycles. (d) Variation of remnant polarization, maximum polarization and coercive field with switching cycles.

transition has been attributed to B-site disordering. The B-site disordering has been caused by the substitution of Nb5+ by Sb5+. Sb5+ has a smaller radius than Nb5+ that may give rise to a larger off-centering of the cation and induce distortion among the equivalent lattice sites. This chemical/ionic substitution that results in the disordered structure is caused by the difference of electrostatic and elastic energies of the structure.7 This phenomenon is also associated with the formation of the polar nanoregion (PNR). In relaxor ferroelectrics, a correlated bound state between the dipoles and the PNR exists until the

Furthermore, the dielectric curve showed another peak around 355 °C for Curie phase transition that was found to shift toward higher temperature with increasing frequency, i.e., from 355 °C at 500 Hz and to 370 °C at 2 MHz. This type of phase transition in the dielectric study is known to be a diffused phase transition. In the simple ABO3-type structures of normal ferroelectrics such as NKLN, an isovalent substitution at the B site can lead to a relaxor-type diffused phase transition.6 The relaxor behavior of NKLNS crystals gives direct evidence of Sb5+ entering the NKLN lattice because the diffused phase 1855


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Figure 6. (a) Topography image and (b) OOP and (c) IP polarization PFM images for NKLNS crystal.

thermal energy (kBT) becomes greater than the electrostatic energy associated with the charges, leading to the formation of the local dipoles even after Tc.7 This phenomenon is very useful in ferroelectric switching devices where the change of the states is required to happen gradually. The variation of tan δ with temperature is shown in the inset of Figure 4a. The value of tan δ was found to be less than 0.9 up to 370 °C, which indicates the applicability of NKLNS crystals in high-temperature chargestorage devices up to 370 °C. The value of tan δ showed a sharp increase at 435 °C, which may be due to the formation of cubic phase. Thus, Sb5+ substitution on the B site of NKLN has resulted in lower dielectric loss than for NKLN crystals17 because of relaxor behavior. The degree of diffuseness for NKLNS crystals was calculated using the modified Curie− Weiss law represented as28 (1/ε) − (1/εm) = ((T − Tm)γ )/C

5b. For long-term operation of the ferroelectric devices, consistency of polarization of the ferroelectric material is required over a number of switching cycles, which can be examined by performing a fatigue test. Electric fatigue of ferroelectric crystals is the major obstacle for their potential application in devices based on the reversal of spontaneous polarization, e.g., high strain actuators and memory devices.31 Fatigue testing was carried out on the NKLNS crystals over a wide range of cycles (up to 10 000 cycles), as shown in Figure 5c,d. NKLNS crystals showed negligible change in their ferroelectric property up to 10 000 cycles because the loops are overlapping up to 10 000 cycles in Figure 5c. Only a small change in the remnant polarization, from 3.66 to 3.36 μC/cm2, was observed after 10 000 cycles. The coercive field remained almost completely consistent during the cyclic loading. Hence, the NKLNS crystals can be used for long-duration ferroelectric applications. Piezoelectric Property. Piezoelectric materials are the most crucial component of ultrasonic transducers and actuators. The piezoelectric charge coefficient (d33) of the unpoled NKLNS crystals was determined to be 80 pC/N in the ⟨001⟩ directions. When the crystals were poled with a dc poling field of 1.4 kV/mm, the value of the d33 coefficient was found to be 125 pC/N. The unpoled crystals of NKLNS grown at high temperatures under the influence of various flux conditions normally exhibit multidomains that are slightly misaligned, showing a piezoelectric coefficient less than the value shown by the poled sample because the poling process improves the alignment of the domains and reorients them to improve polarization. The value of the d33 coefficient of NKLNS crystals is higher than that of NKLN crystals17 because of the partial substitution of the Nb−O bond by Sb−O that caused distortions of the NbO6 octahedron cluster, i.e., a larger offcentering of the cation, giving rise to stronger polarization. Hence, partial substitution of the Sb5+ cation can result in better piezoelectric properties of the crystals. Piezoresponse Force Microscopy. PFM was used for high-resolution domain studies of NKLNS single crystals. An AFM topography image of the well-polished NKLNS single crystal along the (101) plane is shown in Figure 6a. Following the topography acquisition, the AFM was switched to the PFM regime, in which the conducting tip was scanned in contact mode with the application of an ac voltage (Vac) between the tip and the bottom electrode to obtain both out-of-plane (OOP) and in-plane (IP) polarization components. Here, the OOP polarization component represents polarization perpen-

(1)

where εm is the maximum value of dielectric constant and Tm is the temperature corresponding to εm. C is a constant, and γ represents the degree of diffuseness. γ was found to vary with the frequency (1.36, 1.34, 1.27, 1.01, 1.18, and 1.29 at 1, 5, 10, and 100 kHz and 1 and 2 MHz, respectively; Figure 4b), which confirms the relaxor-type ferroelectric behavior of NKLNS crystals. Therefore, NKLNS crystals are useful for hightemperature tunable microwave devices. Ferroelectric and Pyroelectric Property. The ferroelectric property of NKLNS crystals was analyzed by tracing the well-developed polarization−electric-field (P−E) hysteresis loops at various temperatures, as shown in Figure 5a. The remnant polarization (Pr) and coercive field (Ec) at room temperature were found to be 3.66 μC/cm2 and 15.45 kV/cm, respectively. This value of Pr is lower than that for pure NKLN crystals,17 which is due to the relaxor behavior of NKLNS crystals because the hysteresis loop of a normal ferroelectric retains a larger polarization at zero field and relaxor ferroelectrics have randomly distributed nanodomains leading to lower Pr.29 The coercive field, Ec, of NKLNS crystals is higher than that for NKLN crystals,17 which indicates its better stability for the poled state. The values of Pr and Ec were found to increase with increasing temperature because of domain relaxation. The pyroelectric coefficient of NKLNS crystals was measured by differentiating the spontaneous polarization with temperature, using the equation represented as30

p = ΔPs/ΔT

(2)

The pyroelectric coefficient was found to be 821 μC m−2 °C−1 for the temperature range 40−90 °C, as plotted in Figure 1856


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Figure 7. NKLNS crystal images: (a) topography, (b) OOP polarization PFM of a smaller area, and (c) central part of self-correlation.

Figure 8. (a) Piezoresponse for 50 × 50 μm2 images for as-grown polished surface, (b) switching response after poling +75 V and −75 V, and (c) relaxation after 100 min of poling for NKLNS crystal.

Figure 9. (a) UV−vis absorption and transmittance spectra of NKLNS crystals. (b) Plot of (αhν)2 vs photon energy for the band-gap energy calculation.

for a very small area (3 × 3 μm2) with RMS roughness of 16 nm are shown in Figure 7a and b. In the OOP contrast image, the bright areas correspond to the polarization directed toward the bottom electrode, whereas dark areas show domains in the opposite direction. Irregular domain patterns with typical sizes of 50−100 nm have been observed on the 010-oriented surfaces of unpoled samples of NKNLS. The existence of nanodomains in NKNLS crystals is tentatively attributed to the relaxor nature of NKNLS where small polar clusters may form under zero-field-cooling conditions. The self-correlation function of the PFM image

dicular to the surface (d33), and IP shows the polarization parallel to the sample surface (d15). The topography image (Figure 6a) and PFM images for OOP and IP (Figure 6b,c, respectively) give evidence of the piezoelectric nature of the contrast as no correlation between the topography was evident in OOP and IPP signals.32 However, a very sharp contrast for OOP polarization and comparatively low contrast for IP polarization were observed suggesting a large d33 component of polarization as compared to d15 and hence further analysis was carried out on OOP contrasts only. High resolution topography and OOP contrast 1857


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Crystal Growth & Design was used to obtain the average domain size as shown in Figure 7c. The average highest and lowest values of the domain size were calculated as ∼660 and 50−100 nm, respectively. To establish the mechanism controlling polarization dynamics, tipinduced switching experiments with subsequent depth analysis were carried out. In ferroelectrics, tip-induced switching results in formation of localized domain with well-saturated polarization, i.e., switching proceeds through formation of a welldefined area within which polarization switchs from state −Ps to Ps. The resulting domain can be metastable and will relax through radial shrinking, albeit maintaining the same phase state in the core. The contrasts from positive and negative poling voltages are different. Our poling results reveal relaxation existence of the bias-induced domain states with relaxation time dependence from composition when compared to NKLN crystals.17 Domain patterns on the surface of the crystal before poling on which two rectangular portions were chosen for applying poling field of +75 and −75 V is shown in Figure 8a. The switching of polarization domains as a result of poling field is shown in Figure 8b. A contrast change in the PFM images was observed after 100 min (as shown in Figure 8c when compared to Figure 8b) and is due to possible relaxation phenomena of the domains. It reveals the relaxation phenomenon for the bias-induced domain states existing at a local level with a relaxation time of 20 min. Presence of local relaxation signifies the presence of the nanoscale domains for the stability of the domain states in the crystals. UV−Vis Analysis. Perovskite ferroelectric materials can also be used in optical and photovoltaic devices because of their good optical properties and large energy band gaps.33 The band gap energy of NKLNS crystals was determined using the absorption and transmittance spectra of the crystals as shown in Figure 9a,b. NKLNS crystals exhibited a wide transparency window with high transmittance for the UV−visible−NIR wavelength region of 240−1100 nm. The absorption coefficient, α, was calculated from the absorption spectra using the relation expressed as

α = 2.303A /t

Figure 10. Photoluminescence spectra of NKLNS crystals.

visible light region of 300−600 nm. The emission consisted of very sharp blue and green peaks. Sharp UV emission at 360 nm was observed near the band gap, which makes it useful in UVlaser generation applications. Sharp blue emission peaks at 422 and 457 nm along with sharp green emissions at the wavelengths 497, 527, and 543 nm were also observed. In the oxide materials, the defect energy levels caused by the oxygen vacancies play an important role for the phenomenon of blue and green emissions.35−39 In addition, the existence of such large energy band gap also promotes formation of an intermediate level that results in emissions in the visible light region.40 Therefore, crystals with a large band gap are capable of emitting UV−visible light. Thus, NKLNS crystals are a new photoluminescent material. Microindentation Hardness. A hardness test is frequently used to study the mechanical properties of crystals by producing indentations on a small area of the crystal surfaces using a hard indenter. Microindentation hardness testing is a nondestructive technique used to calculate the resistance of a solid to any local plastic/permanent deformation.41 Loads of 5−200 g were applied by a diamond pyramid indenter on the flat surface of the crystal for a dwell time of 30 s. The applied loads of 5 and 10 g could not produce recognizable indents. Nearly square microindentations were observed as a result of applied loads of 20−200 g on the surface of the crystals, as shown in Figure 11a−f (black circle). The size of the indents on NKLNS crystals increased with the applied loads. The diagonals (d) of the indents were measured using Quantimet software and were used for the calculation of Vickers hardness (Hv) parameters using the relation expressed as41

(3)

where A is the absorption and t is the thickness of the sample. The absorption coefficient was used to calculate the direct energy band gap of NKLNS crystals with the help of the equation expressed as34 α = ((hν − Eg )1/2 )/hv

(4)

where Eg is the direct energy band gap of the crystals and hv is the incident photon energy. The direct energy band gap of the crystals was found to be 3.8 eV, as shown in Figure 9b. The band gap energy of NKLNS crystals is lower than that for NKLN crystals,35 which indicates incorporation of Sb5+ ion on the B site. The A−O and B−O bonds of an ABO3-type perovskite give rise to such a high energy band.33 For the case of NKLNS crystals, NbO6 and SbO6 bonds resulted in a high energy band gap because of the large difference of electronegativity of their metals and oxygen ion. The results suggest NKLNS to be a good optical candidate. Photoluminescence. In the photoluminescence (PL) process, a material emits spontaneous light under optical excitation. NKLNS crystals were excited using a wavelength of 240 nm for PL emission, which is shown in Figure 10. The crystals were found to exhibit PL emissions in the UV and

H v = 1.854P /d 2

(5)

where P is the applied load in kilograms and d is in millimeters. A graph showing the variation of Hv with respect to the applied loads is shown in Figure 12a. Hv was found to increase with the increasing loads, suggesting that these crystals exhibit a reverse indentation size effect (ISE). The relation between the indentation size and the load is given by Meyer’s law, represented as42

P = kd n

(6)

where k is a material constant and n is the Meyer’s index. The value of n was calculated using the slope of a graph between log P and log d, shown in Figure 12b. Meyer’s index, n, was found to be 2.19, which confirms the reverse ISE behavior of the 1858


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Figure 11. Micro indentations on the surface of NKLNS crystals as a result of applied loads of (a) 20, (b) 30, (c) 40, (d) 50, (e) 100, and (f) 200 g.

Figure 12. (a) Plot of Vickers hardness (Hv) with respect to the applied loads (P) (b) Graph between log P and log d for the calculation of Meyer’s index (n).

crystals. For n > 2, the material exhibits a reverse ISE behavior. When the values of n lies between 1 and 1.6, the material is a hard material. If n > 1.6, then the material is a soft one. Thus, NKLNS crystals can be classified as soft crystals.

ferroelectric fatigue up to 10 000 switching cycles in the hysteresis loop. PFM images also revealed good piezoelectric behavior of the crystals. The crystals were found to show good transparency in the UV−visible−NIR region and a wide direct energy band gap of 3.8 eV. The crystals exhibited sharp UV, blue, and green emissions at 360, 422, 457, 497, and 527 nm, which established good photoluminescent behavior in NKLNS crystals. Meyer’s index n was calculated to be 2.19, which categorized the crystals as soft materials and confirmed the reverse ISE. Thus, NKLNS crystals are a good candidate for lead-free piezoelectric and ferreolectric applications. In addition, the crystals have good optical and mechanical properties, also suggesting their potential applicability in the optoelectronic industries.

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CONCLUSIONS Single crystals of Na0.47K0.47Li0.06Nb0.94Sb0.06O3 (NKLNS) with a maximum size up to 9 × 8 × 4 mm3 were successfully grown by self-flux method. Powder X-ray diffraction and Raman analysis confirmed the orthorhombic structure and phase formation of NKLNS crystals at room temperature. The crystals exhibited broad and diffused Curie phase transition. The maximum dielectric constant around 355 °C was found to be shifted toward higher temperature with increasing frequency, indicating the relaxor behavior of the crystals. The value of tan δ was found to be less than 0.9 up to 350 °C, showing the applicability of the crystals in charge-storage devices. The crystals showed high piezoelectric charge coefficient (125 pC/ N), coercive field (15.45 kV/cm), and remnant polarization (3.66 μC/cm2) at room temperature. A high value of the pyroelectric coefficient (∼821 μC m−2 °C−1 for the temperature range 40−90 °C) also indicated its potential application in pyroelectric devices. The crystals did not show any

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Fax: +91-011-27667061. Phone: +91-9818168001. Notes

The authors declare no competing financial interest. 1859


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ACKNOWLEDGMENTS We are thankful for the financial support received from the DST (Sanction No SR/S2/CMP-0068/2010) and DU R&D Grant (Sanction No. RC/2014/6820). Geeta Ray is thankful to the Council of Scientific and Industrial Research (CSIR), India for providing the Senior Research Fellowship (SRF). Budhendra Singh is thankful to Fundaçaõ para a Ciência e a Tecnologia for SFRH/BPD/76184/2011 grants (Funded by NSRF−POPH).

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Lead-Free Relaxor Ferroelectric Na0.47K0.47Li0.06Nb0.94Sb0.06O3

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