Effect of Structural Modification by MnO2 Addition on the

Article pubs.acs.org/crystal

Effect of Structural Modification by MnO2 Addition on the Electrical Properties of Lead Free Flux Grown (Na0.5Bi0.5)TiO3−(K0.5Bi0.5)TiO3 Single Crystals Sonia Bhandari and Binay Kumar* Crystal Lab, Department of Physics and Astrophysics, University of Delhi, Delhi 110007, India ABSTRACT: In this study, single crystals of 0.25 mol % Mnmodified lead-free (Na0.5Bi0.5)TiO3−(K0.5Bi0.5)TiO3 grown by selfflux method are investigated. The crystal shows coexistence of tetragonal and cubic phases in the perovskite structure. EDAX analysis shows a uniform Mn doping for the BNKMT single crystals. The depolarization temperature (Td) and the temperature of dielectric constant maximum (Tm) are found to be 175 and 310 °C respectively. The complex impedance plot exhibited single impedance semicircle identified over the frequency range of 200 Hz to 2 MHz. The ac conductivity results indicate activation energies in the range of 0.035− 0.040 eV at low temperatures and 0.1−0.5 eV at high temperatures. The piezoelectric charge coefficient d33 reaches a maximum of 202 pC/N. Nonswitchable ferroelectric hysteresis showing a strong anomaly along the polarization axis with temperature are observed. Fatigue-free behavior up to 10,000 electrical cycles is observed, making the crystals favorable for high precision sensors, capacitors, and actuator applications.

1. INTRODUCTION Scientific research communities across the world have been looking for novel lead-free materials that can be used as excellent piezoelectric and ferroelectric materials. Piezoelectric materials are well suited to generate high frequency movement and are often employed as ultrasonic transducers.1,2 These materials act as a sensor and an actuator, thus find applications in accelerometers, displacement actuators, and force generators. PZT for many years has been the most widely used material in piezoelectric and ferroelectric applications.3 But increased cognizance for environmental safety has commenced considerable efforts to reduce the amount of hazardous substances in consumer products. So far, lead-free systems with perovskite structure such as (Na0.5Bi0.5)TiO3 (NBT) and (K,Na)NbO3 (KNN) are found to be potential nontoxic candidates to replace omnipresent PZT in such applications. These materials appear to be most promising in the lead-free class because of their high piezoelectric properties, large electromechanical coupling coefficient, and high Curie temperature.4 In the class of lead-free materials, relaxors have attracted continued interest because of their broad maximum in temperature dependent dielectric permittivity and high piezoelectric coefficients. With their unusual properties, relaxors are being used as a material of choice for high-end industrial applications converting between mechanical and electrical forms of energy. NBT, being a relaxor ferroelectric, exhibits maximum relative permittivity εm ∼ 2800−3200 at phase transition maxima of ∼320 °C (Tm) and possesses a distorted © 2015 American Chemical Society

perovskite structure (R3c) showing extensive cation-displacement and octahedral tilt disorder.5 The temperature evolution of NBT structure is still a matter of discussion with studies showing changes from rhombohedral to tetragonal to cubic phases in a broad temperature range between 250 and 550 °C.6 The structural complexity in NBT is closely consorted to cationic disorder in its A-site, which can be enhanced synthesizing its solid solution with other perovskites. Solid solutions of NBT−(K0.5Bi0.5)TiO3 (BNKT) and NBT−BaTiO3 (NBBT) have been reported by many researchers with compositions near and far from morphotropic phase boundary (MPB).7,8 Both BaTiO3 (BT) and (K0.5Bi0.5)TiO3 (KBT) are well-known lead-free piezoelectric materials with tetragonal symmetry, but BT has a low Curie temperature (Tc = 120 °C) as compared to KBT (Tc = 380 °C). However, these binary systems still suffer from the problems of depolarization temperature, Td, which limits their wide working temperature range for capacitor and piezoelectric applications, and hence it is required to increase the Td value. K.-S. Moon et al. reported high quality NBT−5BT single crystals with Td = 145 °C, attributing it to a phase transition from rhombohedral ferroelectric to tetragonal antiferroelectric.9 R. Sun et al. studied the pyroelectric properties of Mn-doped 94.6NBT− 5.4BT single crystals showing Td of 120 °C.10 Because of high Received: November 13, 2014 Revised: December 31, 2014 Published: January 5, 2015 867

DOI: 10.1021/cg5016669 Cryst. Growth Des. 2015, 15, 867−874

Article

Crystal Growth & Design

Figure 1. (a) Temperature profile for crystal growth and top view of crucibles arrangement. (b) As grown BNKMT crystals in Pt crucible. (c) Flux grown crystal dimensions after separating from the flux and (inset) polished BNKMT crystals.

Tc value of KBT, a binary system of NBT−KBT was chosen in this manuscript as it is expected to increase the Td value for the grown single crystals. The MPB composition for (1−x)NBT−x(KBT) is reported to lie in the range 0.16 < x < 0.20 by Sasaki et al.11 respectively. The piezoelectric and ferroelectric response in these binary systems depends on the nominal starting composition, the origin of which has not been resolved. The role of MPB compositions exhibiting coexistence of ferroelectric phases in the equilibrium state for enhanced piezoelectric properties is reported many times, but several others obtained it for pseudocubic compositions.12 Recently, most investigations have concentrated on the titanate perovskite modifications with cations of matching ionic radii selected as substitute for A- and B-sites.13−15 Appropriate cation modifications are likely to influence the dielectric, piezoelectric, and ferroelectric properties of lead-free BNKT system. The effect of Mn doping on the dielectric and ferroelectric properties of perovskite materials has attracted considerable attention in the field of research. As per literature, MnO2 converts into Mn2O3 in the temperature range from 600 to 950 °C in air showing a change of valence state.16 Mn is believed to be incorporated to the B-site of the lattice as the ionic radius of Mn4+ (0.53 Å) is close to that of Ti4+ (0.61 Å).17 But because of the high temperature reaction process, the different oxidation states of Mn ions, i.e., Mn2+, Mn3+, and Mn4+, are expected to show A-site incorporation as well and are reported by many researchers.18,19 Y. Wang et al. showed suppression of dielectric and piezoelectric responses along with hardening effects in P−E hysteresis loops making Mn-modified 0.24PIN−0.47PMN− 0.29PT single crystals more suitable for high-power electromechanical devices.20 H. Liu et al. showed enhancement of dielectric, piezoelectric, and ferroelectric properties of NBT:Mn single crystal demonstrating that Mn doping is an effective tool to improve electrical performance in these crystals.21 As a result, Mn substitution depicts an unusual and interesting effect on both structural and electrical properties and helps in a detailed understanding of the defect chemistry of such materials. In this manuscript, Mn doping was chosen for studying the aliovalent substitution in the BNKT lattice. Because of its similar cationic radius as the Ti-site of the host lattice, it is

expected to show some peculiarities in terms of structural and electrical properties, which mainly originate from the displacement of B−O6 octahedra in the A-BO6 type perovskite structure. Considering its multiple valence states (Mn2+, Mn3+, Mn4+), it can act either as a donor-dopant if introduced at Na+ or K+ ionic sites or as an acceptor-dopant if introduced at the Bi3+ or Ti4+ ionic site of the perovskite lattice. The objective of this work is to study the effect of Mn-modified BNKT single crystals (hereafter BNKMT crystals) and to correlate the results of structural studies with dielectric, piezoelectric, and ferroelectric properties of such crystals.

2. EXPERIMENTAL SECTION 2.1. Crystal Growth Process. The polycrystalline material of Bi0.5(Na0.65K0.35)0.5Mn0.0025Ti0.9975O3 (BNKMT) was synthesized first according to stoichiometric ratio using high purity Bi2O3, Na2CO3, K2CO3, MnO2, and TiO2 (>99.99% from Sigma Aldrich) as starting materials, by conventional solid state reaction technique. The summary reaction is as follows:

Bi 2O3 + 0.65Na 2CO3 + 0.35K 2CO3 + 0.01MnO2 + 3.99TiO2 → 4Bi 0.5(Na 0.65K 0.35)0.5 Mn 0.0025Ti 0.9975O3 + CO2

(1)

Then BNKMT polycrystalline material was ground and mixed with 25 wt % excess of Bi2O3, Na2CO3, and K2CO3 as self-flux in equal ratios. The use of alkali based fluxes for high temperature crystal growth has been known for some time. As crystal growth at high temperature leads to a significant loss of volatile alkali materials and Bi, making use of self-flux also acts as a source in the final product. The role of flux in crystal growth is generally to enhance the rates of diffusion in solid state reactions and promote crystallization at lower temperature.22 The charge and flux mixture was placed in a Pt crucible covered with a lid, which was then placed in an Al2O3 crucible and sealed using high temperature industrial grade adhesive cement in order to avoid the volatilization of A-site oxides at high temperature and possible damage to the furnace. The sealed crucibles were placed in a computer controlled resistance furnace under air atmosphere, for crystal growth. In order to improve crystal size and quality, a step cooling method was followed with intermediate soaking at 1000 and 900 °C for 3 h. The temperature profile as followed for BNKMT crystal growth is shown in Figure 1a. The BNKMT single crystals were obtained from different parts of the crucible with dimension 5−15 mm, but a majority of them were at the center of the crucible as show in Figure 1b. The obtained single crystals were rinsed in warm and diluted nitric acid to 868


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Figure 2. (a) X-ray powder diffraction patterns of the pure BNKT and Mn-modified BNKT single crystals. (b) Topas refined pattern for BNKMT single crystal. (c) EDAX spectrum of BNKMT single crystal. separate them from residual flux. The crystals hence obtained were blackish brown in color and crack free as shown in Figure 1c. The crystals hence obtained were cut using a diamond saw along their natural faces and polished as shown in Figure 1c inset for the purpose of characterization. 2.2. Characterization Process. The structural modification produced by Mn addition on the perovskite structure of BNKT was determined after pulverizing the single crystals (model: Rigaku Ultima IV) with Cu Kα radiation (λ = 1.5405 Å). To analyze the chemical components in the crystals hence obtained energy dispersive X-ray analysis (EDAX) was performed using a JEOL JSM 6610LV scanning electron microscope. For electrical characterization, parallel faces of the polished crystals were coated with silver paste to act as a parallel plate capacitor. The variations of the dielectric constant and loss in the frequency range 200 Hz to 2 MHz were studied from room temperature to 500 °C at 3 °C/min using an E4980A LCR meter. Crystals were poled by applying a dc field of 4 kV/mm for 20 min at 40 °C, and the variation in the piezoelectric charge coefficient with field was measured using a piezometer (PM 300, Piezotest, U.K.). The hysteresis loops were traced by using a computer-controlled P−E loop tracer.

are given in Table 1. Examination of the XRD pattern showed the structure to be pseudo-cubic. The fundamental structure Table 1. Lattice Parameters for BNKMT Single Crystal crystal structure

space group

a (Å)

c (Å)

vol (Å3)

tetragonal cubic

P4mm Pm3m

3.9014 3.9010

3.9072 3.9010

59.4720 59.3678

refinement parameters for the shown profile fit came out as profile factor Rp = 10.8; weighted profile factor Rwp = 13.6; expected weighted profile factor Re = 13.2, giving goodness of fit = Rwp/Re = 1.03. Further, to verify the compositional consistency and Mn incorporation, EDAX analysis was performed and is shown in Figure 2c. However, it is difficult to determine concentration of each element quantitatively for these nonstoichiometric crystals with small amount of Mn modifier. The results obtained show a uniform Mn doping for the BNKMT single crystals. Moreover, on the basis of EDAX analysis with an average 1.82 atom % K content, the composition for the as grown crystal was found to be near MPB region. The composition is different from presynthesized polycrystalline material and is produced inevitably during the high temperature crystal growth process due to volatility of Asite components. The structural changes taking place in a near MPB composition with Mn substitution imply its strong coupling with oxygen atoms. Because of similar ionic radii of Mn ions and Ti4+, a difference in B−O6 octahedral vibrations is induced that suppresses the tetragonal nature of octahedral tilting and makes the structure pseudo-cubic. 3.2. Dielectric Studies. The dielectric response of the electrode BNKMT single crystal over the heating run from room temperature to 500 °C is shown in Figure 3a. The first hump observed at 175 °C, with clearly distinguishable broad

3. RESULTS AND DISCUSSION 3.1. Phase Structure Analysis. The powder X-ray diffraction patterns of the pure and BNKMT single crystals in the 2θ range from 10 to 80° are shown in Figure 2a. The diffraction profile exhibits a pure perovskite structure without the formation of any secondary phase which confirms the incorporation of Mn ions into the BNKT lattice. In the XRD pattern no distinguishable splitting of peaks was observed at 2θ of 40° that depicts characteristic rhombohedral symmetry of the crystal structure and at 46−47° depicting characteristic tetragonal symmetry.23,24 For the determination of cell parameters and phases present, Le Bail profile fit was carried out using Topas software and is shown in Figure 2b. The obtained lattice parameters and corresponding crystal system 869


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Figure 3. (a) Evolution of dielectric constant for BNKMT single crystal with frequency over temperature range 30−500 °C. (b) Temperature dependence of tan δ for the BNKMT crystal. (c) Dielectric permittivity data fitted with modified Curie−Weiss law for BNKMT single crystal. (d) Dielectric permittivity data fitted with Lorentz type quadratic fitting at 50 kHz.

peak at lower frequency, corresponds to the depolarization temperature Td, as it is usually associated with a temperature dependent flexion in dielectric constant and tan δ values. The second hump corresponds to the permittivity maximum (εm) at the maximum temperature (Tm) of 310 °C. The single crystals, in their unpoled state, showed strong frequency dependence below Td, and subsequently became less dispersive on further heating beyond 175 °C. The frequency dispersion behavior, as observed around Td, is a result of thermal evolution because of breaking of long-range polar correlation in the BNKT lattice due to Mn substitution. A similar kind of behavior is observed in many NBT based structures modified with BaTiO3 or MnO2 which showed lower depolarization temperatures.25,10 Moreover, the diffuseness of the complex perovskite relaxor is sensitive to the degree of compositional fluctuation at the B-site ion, as a result of which frequency dispersion around permittivity maximum becomes less apparent. Figure 3b shows the dielectric loss study for the BNKMT single crystal. The phase transition in Td is masked at frequencies below 100 kHz, as shown in the Figure 3b inset, but the anomaly is evident in the plot of dielectric loss at higher frequencies. The loss value being less than 0.10 up to temperatures far higher than dielectric peak maximum (Tm = 310 °C) establishes low loss nature of the as grown BNKMT single crystal. Moreover the dielectric loss appears to increase with frequency increase whereas the relative effect of defects on losses decreases with increasing frequency.26 When the electrical response of a material lags behind the applied field, it results in dielectric losses. In this case, at higher frequencies

the real part of permittivity decreases slowly with imaginary part showing increase in a linear fashion. As a result, loss tangent values increase with increase in frequency. Also, in BNKMT single crystal, as Bi evaporates during the high temperature crystal growth process, some A-site vacancies are generated, which is associated with the occurrence of oxygen vacancy. In that case, some Mn ions (Mn3+) occupy the Bi3+ site for charge compensation, but because they have different ionic radii from Bi3+ (1.17 Å), this enhances the site disorder, which results in pinning of domain walls and reduction in dielectric losses as experimentally evident. Together, BNKMT crystal with high dielectric permittivity, high depolarization temperature, and low tan δ values proves to be a temperature stable low loss dielectric material for applications in the microwave frequency region as waveguides and capacitors. The degree of diffuseness is shown in Figure 3c in the frequency range 500 Hz to 2 MHz. It is normally measured in terms of diffusion factor γ, using modified Curie−Weiss law given as27 (T − Tm)γ 1 1 = + ε εm 2εmδ 2

(2)

where “γ” value ranges from 1 to 2 and “δ” is the diffuseness parameter. The diffusion factor as determined from the slope of the ln(1/ε − 1/εm) vs ln(T − Tm) curve is shown in the Figure 3c inset. The variation of γ with frequency is found to increase in the range 500 Hz to 100 kHz and then decreases as frequency increases to 2 MHz. Such γ values are representative of a high degree of disorder in A-site ions induced as a result of 870


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Figure 4. (a) Nyquist plot for BNKMT crystal. (b) Variation of relaxation time τ with temperature for BNKMT single crystal.

Figure 5. (a) Variation of ac conductivity for BNKMT crystal with temperature and frequency. (b) Activation energies calculated for low and high temperature region.

bulk resistance (Rb) of the material. With increasing temperature, the slope of the semicircular arc decreases and the lines were observed to bend toward the Z′ axis, indicating an increase in conductivity of BNKMT crystal. The crystal shows negative temperature coefficient of resistance (NTCR) behavior, a typical semiconducting property where bulk resistance of the material decreases with increase in temperature. NTCR behavior is found in materials that are highly disordered due to the presence of cation−anion vacancies thus creating free electrons to act as mobile charged species. The free electrons were generated and annihilated from ions in the form of

octahedral distortion and responsible for its DPT behavior. Asite cationic disorder gives rise to a random local field, developing a long-range polar order. This results in the formation of polar nano regions (PNR) each with a local Curie temperature (Tc). With increase in temperature thermal fluctuations of this local field develop a large distribution of local Curie temperature resulting in the broadening of the dielectric peak and giving a DPT behavior. Lorentz-type behavior is universally observed in ferroelectrics with or without diffuse phase transition. It is given as a quadratic formula describing the dependence of the permittivity on temperature at T > Tm in a DPT and is given as28 εA (T − TA )2 =1+ ε 2δ 2

Mn 4 + + e− ↔ Mn 3 + (3)

Ti4 + + e− ↔ Ti 3 +

where εA, TA, and δ are the fitting parameters. The deviation from Curie−Weiss law in DPT materials is a result of shortrange correlations between polar regions in the temperature range from Tm to TB (Burns temperature). Figure 3d shows the temperature dependence of 1/ε for the grown crystal. An excellent fit is obtained in the temperature range 350−380 °C. The fitting parameters obtained are εA = 5885, TA = 265 °C, and δ = 132.49 °C respectively confirming diffused dielectric response in the case of the BNKMT crystal. 3.3. Impedance Spectroscopy. The temperature dependent spectra (Nyquist plot) of BNKMT single crystal are shown in Figure 4a. The impedance spectra, when characterized by the appearance of a single semicircular arc, give an estimate of the

These trapped electrons of Mn4+ and Ti4+ ions or oxygen vacancies forming the color centers are easily activated to the conduction process by thermal energies resulting in NTCR behavior. Also, the observed peak maximum of the plots decreases and shifts toward higher frequency with increase in temperature. The obtained Nyquist plots were used for studying the nature of variation of relaxation time τ with temperature for BNKMT single crystal. The frequency corresponding to the peak maximum (ωmax) from the Nyquist plot gives the most probable relaxation time from the condition ωmaxτ = 1. The curve hence obtained was found to follow the Arrhenius equation: τ = τo exp(−Ea/kBT) where τo is the exponential factor, Ea is the activation energy, kB is the 871


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Figure 6. (a) P−E response of the as grown BNKMT single crystal with temperature. (b) Variation of remnant polarization and coercive field as a function of temperature.

Boltzmann constant, and T is the temperature in kelvins. The activation energy was evaluated from the slope of the log τ vs 1000/T curve and is shown in Figure 4b. The observed τ value in BNKMT crystal decreases with increasing temperature, which is a typical behavior of a semiconductor. The value of Ea obtained is 0.63 eV for BNKMT crystal, showing a close resemblance of the dielectric relaxation process to the presence of oxygen vacancies that are known to be most mobile ionic defects in such perovskites.7 3.4. Conductivity Analysis. The temperature dependence of ac conductivity measured in the frequency range 500 Hz to 2 MHz and temperature range 30−500 °C is shown in Figure 5a. At high temperatures (above Td), the temperature dependence of σ(ω) becomes strong, showing variation that can be attributed to the relaxation process associated with domain wall motion and dipolar behavior, while at T < Td, σ(ω) is proportional to temperature. The activation energies calculated from the slope of the log σ(ω) vs 1000/T plot are shown in Figure 5b. The results show that, with increasing frequency, activation energy Ea increases slightly in the temperature range T < Td while it shows a decreasing trend in the high temperature region. The temperature dependence of conductivity can be described by using the Arrhenius equation: σ(ω) = σo exp(−Ea/kBT),29 where the symbols have their usual meanings. The motion of domain wall becomes complex as a result of structural modification induced by Mn ions because of its existence as a multivalent substituent. This causes a difference in conductivity mechanism with temperature as carrier transport mechanism is associated with the hopping between localized states via oxidation or reduction processes. The Ti ions show easy transition from Ti4+ to Ti3+ described by the equation

temperature. But reduction of oxygen vacancies will be more pronounced in the case of Mn2+ valence facilitating the movement of ferroelectric domains. Here, oxygen vacancies pin the domain walls, a behavior also observed in ferroelectric studies, evidently supporting that a majority percentage of Mn ions are in the Mn3+ oxidation state. 3.5. Piezoelectric and Ferroelectric Studies. Crystal symmetry plays a decisive role in piezoelectric effect, which is defined as the ability of materials to develop an electric charge that is directly proportional to an applied mechanical stress. It is measured in terms of piezoelectric charge coefficient d33. One possible way to measure d33 is to apply a known force (static or dynamic) and measure the corresponding charge developed on the opposite surfaces. For BNKMT crystal, piezoelectric performance was measured at room temperature and with electric field optimization at tapping frequency and force of 110 Hz and 0.25 N, respectively. The maximum value obtained for the piezoelectric charge coefficient d33 is 202 pC/N. The value hence obtained is higher than that of many other lead-free Mndoped single crystals reported, with d33 of 120 pC/N for NBT crystal32 and 161 pC/N for NKN crystal,33 respectively. The present results indicate that BNKMT single crystals are promising candidates for piezoelectric device applications. Figure 6a represents the temperature evolution of nonswitchable ferroelectric P−E loops in the case of BNKMT crystal. The P−E curve for BNKMT crystal indicates that the ferroelectric behavior has been greatly reduced in the sample. Hysteretic polarization−electric field relation is required in memory applications that are based on polarization switching, but in high precision sensors, capacitors, and actuator applications, hysteresis is undesired. To reduce the hysteresis in piezoelectric devices, keeping a high piezoelectric response is a challenging task as most ferroelectric materials with high piezoelectric properties have the strongest electromechanical hysteresis.34 The best known method for hysteresis control in ferroelectric material is by chemically modifying the material primarily with two types of dopants, i.e., the donor type (soft type) and the acceptor type (hard type). The former is caused by the substitution of higher valence ions for A- or B-site in a perovskite, while lower valence ion substitution for the A- or Bsite atoms corresponds to “hard” doped characteristics. In our case, the shape of the loop is slim, showing a small remnant polarization (Pr) of 4.01 μC/cm2 and coercive field (Ec) of 10.35 kV/cm at room temperature and is significantly different

2Ti4 + + O2 − ↔ 2Ti 3 + + (1/2)O2 ( ↑ ) + Vo••

These oxygen vacancies, together with Mn ions, form defect dipoles leading to the generation of electrons (space charge) that can hop between different Ti ions, i.e., Ti4+ and Ti3+. With Mn ions as acceptor dopant (Mn2+ or Mn3+), the reduction of Ti4+ to Ti3+ is prevented by neutralizing the donor action of the oxygen vacancies. The activation energy values corresponding to oxygen vacancies are on the order of 0.2−0.4 eV.30,31 The Ea values in the range 0.035−0.040 eV suggested a low oxygen vacancy migration at low and intermediate temperature in Mnmodified crystal and confirmed their contribution at high 872


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Figure 7. P−E response for fatigue in the range 500−10000 cycles at room temperature.

from a square-shaped P−E hysteresis loop for typical ferroelectric material. The most interesting feature evidenced in our P−E data is the existence of an anomalous vertical broadening of loops along the polarization axis. In most cases, the origin of hysteresis in single crystals is the result of field induced displacement of domain walls.35,36 Because of the presence of different oxidation states of Mn ions, defect dipoles are created that do not respond fast enough to the movement of domains, resulting in pinning of domain walls and leading to hard ferroelectric effects. With rise in temperature, defect dipole migration rate increases, enhancing the domain wall width to expand and making the pinning force weaker. As a result, such a hysteresis evolution is obtained in BNKMT crystal. 3.6. Fatigue Analysis. For device application, the degradation of polarization properties will result in losing capability for information storage that directly restricts device reliability for repeated applications. Hence it is important to understand the intrinsic fatigue behavior of BNKMT single crystals under electrical switching cycles. Figure 7 represents polarization fatigue in the switching range from 500 to 10,000 cycles. The BNKMT single crystal clearly shows almost no polarization fatigue after 104 electrical cycles. The variation in the ferroelectric behavior is very small for repeated cycles with remnant polarization and coercive field confined in the range 4.1−4.4 μC/cm2 and 11.1−11.5 kV/cm, as shown in the inset of Figure 7. A fatigue-free behavior is desired in normal product lifetime for its device fabrication.37,38 The observed cycle endurance ensures its use as a high frequency capacitor material for transducers. Table 2 represents a comparison of the dielectric, piezoelectric, and ferroelectric properties of the grown BNKMT single crystals with other reported BNKT single crystals.

Table 2. Comparison of Dielectric, Piezoelectric, and Ferroelectric Properties of Lead-Free NBT−KBT Single Crystals single crystal NBT− 8KBT39 NBT− 12KBT40 NBT− KBT23 Mn: NBT− KBTa a

εRT (10 kHz)

εm (10 kHz)

Td (10 kHz) (°C)

Tm (10 kHz) (°C)

Pr (μC/ cm2)

Ec (kV/ cm)

d33 (pC/ N)

∼805

∼4318

150

316

8.1

50

175

877

∼4700

300

7

22

208

316

9.23

25.38

216

310

4.01

10.35

202

981 1192

5462

175

This work.

4. CONCLUSIONS In summary, high quality lead-free Mn-modified BNKT single crystals of dimension 5−15 mm across were grown by the selfflux method. The XRD pattern indicates their structure to be pseudo-cubic with uniform Mn doping. EDAX analysis depicts that BNKMT single crystal owns a near MPB composition. The dielectric constant shows strong frequency dependence below Td that was observed to be 175 °C, making them a material of choice for wide working temperature range in capacitor applications. The crystal shows low losses up to 375 °C, which is much higher than its depolarization temperature, making them useful for high temperature microwave device applications. Impedance studies show activation of trapped electrons of Mn4+ and Ti4+ ions or oxygen vacancies resulting in NTCR behavior. Conductivity analysis helps in understanding the complex domain wall motion. The piezoelectric charge coefficient d33 was found as high as 202 pC/N. Nonswitchable 873


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(21) Liu, H.; Ge, W.; Jiang, X.; Zhao, X.; Luo, H. Mater. Lett. 2008, 62, 2721−2724. (22) Shivakumara, C.; Hegde, M. S.; Srinivasa, T.; Vasanthacharya, N. Y.; Subbanna, G. N.; Lalla, N. P. J. Mater. Chem. 2001, 11, 2572−2579. (23) Bhandari, S.; Sinha, N.; Ray, G.; Kumar, B. CrystEngComm 2014, 16, 4459−4466. (24) Cheng, S.-Y.; Shieh, J.; Lu, H.-Y.; Shen, C.-Y.; Tang, Y.-C.; Ho, N.-J. J. Eur. Ceram. Soc. 2013, 33, 2141−2153. (25) Ge, W.; Maurya, D.; Li, J.; Priya, S.; Viehland, D. Appl. Phys. Lett. 2013, 102, 222905. (26) Sebastian, M. Dielectric Materials for Wireless Communication; Elsevier: 2008; pp 11−82. (27) Kumar, K.; Singh, B. K.; Gupta, M. K.; Sinha, N.; Kumar, B. Ceram. Int. 2011, 37, 2997−3004. (28) Singh, B.; Bdikin, I.; Kaushal, A.; Kumar, B. CrystEngComm 2014, 16, 9135−9142. (29) Ray, G.; Sinha, N.; Kumar, B. Mater. Chem. Phys. 2013, 142, 619−625. (30) Rani, R.; Sharma, S.; Rai, R.; Kholkin, A. L. J. Appl. Phys. 2011, 110, 104102. (31) Moreno, B.; Urones-Garrote, E.; Chinarro, E.; Fuentes, L.; Moran, E. Chem. Mater. 2011, 23, 1779−1784. (32) Ge, W.; Li, J.; Viehland, D.; Luo, H. J. Am. Ceram. Soc. 2010, 93, 1372−1377. (33) Inagaki, Y.; Kakimoto, K. Appl. Phys. Express 2008, 1, 061602. (34) Mayergoyz, I.; Bertotti, G. The Science of Hysteresis; Elsevier: 2005; Vol. 3, pp 337−465. (35) Aravind, V. R.; Morozovska, A. N.; Bhattacharyya, S.; Lee, D.; Jesse, S.; Grinberg, I.; Li, Y. L.; Choudhury, S.; Wu, P.; Seal, K.; Rappe, A. M.; Svechnikov, S. V.; Eliseev, E. A.; Phillpot, S. R.; Chen, L. Q.; Gopalan, V.; Kalinin, S. V. Phys. Rev. B 2010, 82, 024111. (36) Lin, D.; Li, Z.; Li, F.; Zhang, S. CrystEngComm 2013, 15, 6292− 6296. (37) Bhandari, S.; Sinha, N.; Ray, G.; Kumar, B. Scr. Mater. 2014, 89, 61−64. (38) Ray, G.; Sinha, N.; Bhandari, S.; Singh, B.; Bdikin, I.; Kumar, B. CrystEngComm 2014, 16, 7004−7012. (39) Sun, R.; Zhao, X.; Zhang, Q.; Fang, B.; Zhang, H.; Li, X.; Lin, D.; Wang, S.; Luo, H. J. Appl. Phys. 2011, 109, 124113. (40) Zhang, H.; Chen, C.; Zhao, X.; Deng, H.; Ren, B.; Li, X.; Luo, H.; Li, S. Solid State Commun. 2015, 201, 125−129.

slim P−E loops were observed that became broader with increase in temperature. The observed hard doping effects confirm Mn substitution in BNKT binary perovskite that most preferably stabilizes in its Mn3+ oxidation state. With high piezoresponse, reduced hysteresis, and fatigue-free behavior, BNKMT crystals are demonstrated to be a potential piezoelectric material for high power transducer applications as they will help in suppressing heat generation during the operation of the device. This complete electrical characterization for leadfree BNKMT crystal can be of great help in facilitating simulation design of electromechanical devices and replacing lead based materials therein.

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AUTHOR INFORMATION

Corresponding Author

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

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We are thankful for the financial support received in DST project (Sanction No. SR/S2/CMP-0068/2010). S.B. is thankful to DST for providing the Senior Research Fellowship (SRF).

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REFERENCES

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