Ferroelectric Phase Transition of Lead Free (1x )(Na_{0.5}K_{0.5})NbO_{3} xLiNbO_{3} Ceramics
 Author: Park JongHo, Park HuiJin, Choi ByungChun
 Organization: Park JongHo; Park HuiJin; Choi ByungChun
 Publish: Transactions on Electrical and Electronic Materials Volume 13, Issue6, p297~300, 25 Dec 2012

ABSTRACT
Leadfree (1
x )(Na_{0.5}K_{0.5})NbO_{3}xLiNbO_{3}, i.e., NKNLNx (x =0.0, 0.1, 0.2, 0.3, 0.4 mol) was prepared using the conventional solid state reaction method. The effects of LN mixing on the ferroelectric properties of NKNLNx ceramics were studied using a dielectric constant andPE (Polarizationelectric field) measurements. Ferroelectricity was observed in the composition for x approximately varying between 0.0 and 0.4. Minimum remanent polarization 2P _{r}=5 C/cm^{2} was achieved in the composition forx = 0.2. The ferroelectric phase transition temperature T_{C} increased with increasing LN content. The ferroelectric phase transition of NKNLNx (x ≥ 0.1) is a secondorder phase transition, and that of NKNLNx (x ≤ 0.2) is a firstorder phase transition. These results indicate that the ferroelectric phase transition temperature of NKNLNx change from that of secondorder to weak firstorder phase transition according to the LN content.

KEYWORD
Leadfree , Oxides , Impedance spectroscopy , Dielectric , Phase transition

1. INTRODUCTION
In the field of piezoelectric ceramics, sodium potassium niobate ceramics with leadfree piezoelectric material have been investigated as alternative material for PZTbased ceramics [113]. Leadfree ferroelectric materials with perovskite structure have a general formula of ABO_{3}. In this structure, cations based on their valence states and coordination numbers occupy the A or Bsites. Na_{1y}K_{y}NbO_{3}, NKN is a material with perovskite structure, and it exhibits high piezoelectric properties because its structure permits spontaneous polarization to rotate along three orientations. Sodium potassium niobate (NKN) is a solid solution of potassium niobate (KN) a ferroelectric and sodium niobate (NN), with an Na/K ratio of ~50/50. The piezoelectric applications of Na_{0.5}K_{0.5}NbO_{3} (NKN), ceramics produced by hotpressing, are better than those produced by sintering in air atmosphere. Hotpressed NKN ceramics have been reported to have a high phase transition temperature (T_{c} ~ 420℃), good piezoelectric properties (d_{33} ~ 160 pC/N), and a high planar coupling coefficient (κp~45%) [14].
However, NKN ceramics are difficult to obtain using the conventional sintering method because their phase stability is limited to 1,140℃ and they are exposed to moisture. Therefore, attempts have been made to improve the sinterability and piezoelectric properties of KNN through the addition and/or substitution of several cationic elements in the A or Bsites [1013]
It is known that (1
x )(Na_{0.5}K_{0.5})NbO_{3}x LiNbO_{3}, NKN LNx ceramics are good, leadfree piezoelectric and ferroelectric ceramics. A morphotropic phase boundary between the orthorhombic phase and the tetragonal phase of NKNLNx was present whenx was approximately 0.05 ~ 0.07 mol of LN [8]. Guoet. al. , observed that the Curie temperatures (T_{C}) of NKN LNx ceramics were in the range of 452 ~ 510℃, according to their LN content, which is at least 100℃ higher than that of Pb(Zr, Ti)O_{3}. For (Na_{0.5}K_{0.5})NbO_{3}, Tc values were observed at 420℃ and 200℃, which correspond to the cubicorthorhombic and orthorhombictetragonal phase transitions, respectively. Two phase transitions were present atx = 0.04, 0.06 mol, similar to the case for NKN, except that the phase transition temperatures were shifted [9]. Many research efforts thus far have been based on the conditions for which a small amount of LN was added to the NKN composition.In this study, (1
x )(Na_{0.5}K_{0.5})NbO_{3}x LiNbO_{3}, i.e., NKNLNx (x = 0.0, 0.1, 0.2, 0.3, 0.4mol), was synthesized using the conventional solid state method. The purpose of this study is to investigate the phase transition and electrical properties of (Na_{0.5}K_{0.5})NbO_{3} in terms of its LiNbO_{3} content.2. EXPERIMENTS
Leadfree (1
x )(Na_{0.5}K_{0.5})NbO_{3}x LiNbO_{3}, i.e., NKNLNx (x = 0.0, 0.1, 0.2, 0.3, 0.4 mol), was prepared by mixing the oxides, K_{2}CO_{3} (99% purity), Na_{2}CO_{3} (99% purity), LiNbO_{3} (99% purity) and Nb_{2}O_{5} (99% purity) in a molar ratio used in the conventional solid state reaction method. Before being weighed, the K_{2}CO_{3} and Na_{2}CO_{3} powders were first dried in an oven at 200℃ for 10 h to minimize the effect of moisture. These powders were then milled with ZrO_{2} balls for 20 h using ethyl alcohol as a medium and dried. The dried powders were calcined at 850℃ for 2 h. After calcination, the powders were ballmilled again for 20 h and, dried, after which PVA(4 wt%) was added as a binder. They were then pressed into disks with diameter of under 13 mm. After burning off the PVA, the pellets were sintered at 1,070℃ for 2 h. The crystal structures were determined by Xray power diffraction analysis using CuКα radiation (Philips X’ Pert  MPD system). The remnant polarizationP _{r} and coercive fieldE _{c} were determined from the PE (Polarization  Electric field) hysteresis loops, as measured by a Radiant Precision Workstation. To examine their dielectric properties, the ceramics were polished and painted with silver paste on both surfaces, and fired at 800℃ for 30 min. The real and imaginary dielectric constants were measured using an SI1260 impedance analyzer at temperature ranging from room temperature to ~ 600℃ with heating and cooling rates of 0.2℃/min in the frequency range of 1 Hz to 1 MHz.3. RESULTS AND DISCUSSION
Figure 1 shows the XRD patterns of the (1
x )(Na_{0.5}K_{0.5})NbO_{3}x LiNbO_{3}, i.e., NKNLNx (x = 0.0, 0.1, 0.2, 0.3, 0.4 mol) ceramics. Studies have reported that a phase of K_{3}Li_{2}Nb_{5}O_{15} (KLN) with a tetragonal tungsten bronze structure starts to appear at x ≥ 0.08 [9]. In this study, it appeared at x ≤ 0.2 but for x ≥ 0.3, the KLN phase and LiNbO_{3} phase coexisted. This implies that the structures of the NKNLNx ceramics were transformed, again increasing their LiNbO_{3} content.PE hysteresis loops of (1
x )(Na_{0.5}K_{0.5})NbO_{3}x LiNbO_{3}, i.e., NKNLNx (x = 0.0, 0.1, 0.2, 0.3, 0.4 mol) ceramics measured at room temperature under a driven electric field are plotted in Figs. 2(a) (f). Generally, the presence of PE hysteresis loops is considered to be evidence that a material is ferroelectric.The capacitor is characterized by PE hysteresis curves. However, the shapes of the PE loops changed slightly with increasing LN contents. As shown in Fig. 2(f), the value of 2
P _{r} decreases with an increasing LN content below a certain critical level. 2P _{r} has a minimum value of 5 C/cm^{2} nearx = 0.2, and it first increases and then decreases after reaching this value. The coercive field 2E _{c} increases for an increase in the amount of LN in the range betweenx = 0.0 andx = 0.1 mol., and a further increase in the amount of LN abovex = 0.2 mol causes an increase in 2E _{c}.The tendency of varying 2
P _{r} is similar to that of 2E _{c} when the range ofx is approximately abovex = 0.2 mol.Du
et al. [8] reported the dielectric properties of NKNLNx ceramics for the case that the amount of LN is belowx = 0.2 mol; when the amount LN isx = 0.06 mol,E _{c} achieves its minimum value of 13.4 kV/cm andP _{r} reaches its minimum value of 20 C/ cm^{2}. They proposed that NKNLN0.06 ceramics are a promising candidate for leadfree hightemperature piezoelectric ceramics.Figures 3(a) and (e) show the real (
ε ') dielectric constant at 1 MHz as a function of temperature for of (1x )(Na_{0.5}K_{0.5})NbO_{3}x LiNbO_{3}, i.e., NKNLNx (x =0.0, 0.1, 0.2, 0.3, 0.4 mol) ceramics. In the case of NKNLN0.0 ceramics, the values ofε ' increase with decreasing temperature. At T_{C} (the temperature at whichε ' is maximized) = 409℃,ε ' beings to decrease, forming a large λtype peak in the dielectric constantvs . temperature curve upon heating and cooling.As the temperature decreases, if we assume that the phase
transition temperature is the midpoint of the steepest curve of
ε ', then the lower transition occurs at T_{OT,C} (low temperature phase transition point) = 176℃ upon cooling and at T_{OT,H}=195℃ upon heating with a thermal hysteresis of 19℃ This result is similar to that reported by Guoet al. [9].In the case of NKNLN0.1, a low temperature anomaly was not observed at T_{OT} upon heating or cooling.
At high temperatures, the complex dielectric response of NKNLN0.1 was found to be similar to that of NKNLN0.0. The sharp peaks around T_{C} for the NKNLN0.0 and NKNLN0.1 samples show a secondorder phase transition without thermal hysteresis.
In the case of NKNLN
x (x ≥ 0.2), the ferroelectric phase transition temperature T_{C} shifted to a higher value with an increase in the LN content, whereas the dielectric peak broadened. The temperature anomaly of the real dielectric constant appeared at T_{C} in all the samples upon heating and cooling with a small thermal hysteresis, which corresponded to at weak firstorder phase transition. A lowtemperature dielectric anomaly was not observed upon heating and cooling. In NKNLNx samples with 0 ≤x ≤ 0.07, Guo et al. [9] reported that the phase transition of NKNLN0.0 was observed at 420℃ and 200℃, which corresponds to the cubicorthorhombic (at T_{C}) and orthorhombictetragonal (at T_{OT}) phase transitions. Also, LiNbO_{3} has lithium niobate structure, which can be described as a heavily distorted perovskite or an ordered phase derived from the corundum structure with space group R_{3C} (C_{3V} ^{6} ). So, it is evident that two effects on the structure of NKN ceramics have been observed in NKNLiNbO_{3} ceramics. At lower LiNbO_{3} concentrations, Li mainly replaces Na and K in theA sites of ABO_{3} perovskite structure (i.e. form a solid solution), leading to a linear shift of the Curie point (T _{C}) to higher temperature [9]. However, the structure of solid solution transforms from orthorhombic to tetragonal symmetry due to the large distortion caused by Li^{+} [9].The phase transition temperatures also shifted increasing the
LN content. T_{C} shifted to a higher value, and T_{OT}, to a lower value [11]. Thus, we expect that a lowtemperature phase transition of this sample should appear at room temperature because these phase transition temperatures decrease with an increase in LN contents.
The values of T_{OT}, T_{C}, and ΔT obtained for all the samples are presented in Table 1. Here, ΔT indicates the degree of the firstand secondorder phase transition of NKNLN
x . These results indicate that the phase transition of NKNLNx ceramics occurs when T_{C} changes from a secondorder to weak firstorder phase transition with increasing LN contents. Our results also show the possibility that the concentration ofx = 0.2 may be the critical concentration for a first to secondorderferroelectric phase transition.4. CONCLUSIONS
In conclusion, (1
x )(Na_{0.5}K_{0.5})NbO_{3}x LiNbO_{3}, i.e., NKNLNx (x =0.0, 0.1, 0.2, 0.3, 0.4mol) ceramics, were synthesized using the solid state reaction method. The effects of LN mixing on the ferroelectric properties of these two ceramics were studied through dielectric and PE measurements. The value ofP _{r} increased with increasing Nb content. (1x )(Na_{0.5}K_{0.5})NbO_{3}x LiNbO_{3} ceramics exhibited a minimum remanent polarization of 2P _{r}=5 μC/cm^{2} at an LN content ofx ~ 0.2. These results indicate that LN doping can change the ferroelectric properties of NKNLNx ceramics. The phase transition temperature, T_{C}, increased with increasing LN contents. The ferroelectric phase transition of NKNLNx (x ≤ 0.1), is a secondorder transition without thermal hysteresis, and NKNLNx (x ≥ 0.2) is a weak firstorder transition with small thermal hysteresis. Thus, our results demonstrate the possibility that the concentration ofx ~ 0.2 may be the critical concentration for a firsttosecondorderferroelectric phase transition.

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[Fig. 1.] Xray diffraction patterns of the (1x)(Na0.5K0.5)NbO3xLiNbO3, NKNLNx ceramics.

[Fig. 2.] Ferroelelctric hysteresis loops of the (1x)(Na0.5K0.5)NbO3 xLiNbO3, NKNLNx ceramics for (a) x =0.0, x =0.1, (c) x =0.2, (d) x =0.3, and (e) x =0.4 mol, (f) remanent polarization and coercive field of NKNLNx ceramics as a function of the LN contents x.

[Fig. 3.] The temperature dependence of the real dielectric constant ε' in(1x)(Na0.5K0.5)NbO3xLiNbO3, NKNLNx ceramics at 1 MHz on heating (symbol) and cooling (solid line), (a) x=0.0, (b) x =0.1, (c) x =0.2, (d) x =0.3, and (e) x =0.4 mol.

[Table 1.] Phase transition temperature (TOT, TC) of NKNLNx ceramics on heating and cooling. unit: ℃