Dielectric Spectroscopy of the (K0.44Na0.52Li0.04)0.97La0.01Nb0.9Ta0.1O3 ceramic obtained by Sol-Gel

K0.44Na0.52Li0.04)0.97La0.01Nb0.9Ta0.1O3 lead free ceramics were synthesized by solgel method. Their morphology, crystal structure and spectroscopic dielectric properties were analyzed by the appropriate techniques. The co-existence of an orthorhombic and a tetragonal ferroelectric phase at room temperature is confirmed by Rietveld analysis. La and Li cations are effective in shifting down the ferroelectric transition temperatures. The tetragonal to cubic transition, (TT-C), takes place at 371 °C while the orthorhombic to tetragonal, (TO-T), occurs at 110 °C, and the maximum relative permittivity (ɛmax) at TT-C is 3000, which is among the highest for KNN-based ceramics obtained by different methods. Conductivity behaves according to Jonscher’s law in the frequency and temperature range of this study. The activation energy values corresponding to the different conduction mechanisms were obtained. The results were compared with those for the same composition obtained by combining the reactive template grain growth (RTGG) technique and the solid-state method. 4


Prof. Chan-Wong Lai Wa Helen
Lead-free piezoelectric ceramics that could substitute the traditional Pb(Zr,Ti)O 3 (PZT) and the health hazard inherent to its synthesis, use and disposal are of great interest in the materials science community. One of the most promising substitutes is (K,Na)NbO 3 (KNN) [1], whose properties have been improved by the addition of dopants such as Li and Ta to reduce the orthorhombic to tetragonal (T O-T ) [2][3], and La to reduce the tetragonal to cubic (T T-C ) ferroelectric phase transition temperatures [4][5]. One of the first and more widely used methods to prepare KNN-based compounds is the solid-state reaction (SSR) procedure, but long processing times and high calcination and sintering temperatures are needed to produce high quality and dense ceramics. A different approach was taken by combining SSR with the reactive template grain growth (RTGG) method, producing thus not only dense but also textured ceramics, particularly of the incorporated, and finally Na 2 CO 3 , K 2 CO 3 , Li 2 CO 3 , and oxalic acid were also added and mixed; afterward the beaker was uncovered letting the gel formation to begin. This process was conducted with continuous stirring at 900 rpm and a temperature of 80 °C.
The obtained gel was heated at 250-300 °C to remove it from the beaker leaving a powder that was ground and then annealed at 800 °C. The powder was pressed to form 6 mm diameter, 0.5 mm thick pellets that were sintered at 1200 °C for 2h.
The crystal structure of the samples was determined by X-ray diffraction (XRD) using a PAN analytical XPert´PRO diffractometer, with a CuKα (λ 1 = 1.5406 Å) radiation, from 20° 2θ 90°, using a step-scanning mode with a step size of 0.05° and 100 s of step counting time. Rietveld refinement was performed to the x-ray diffraction data until a χ 2 < 2 was reached, using a Thompson-Cox-Hasting pseudo-Voigt function profile (TCHZ) for all the peaks. For morphology, a JEOL JSM 5300 Scanning Electron Microscope (SEM) was used on fractured samples.
After silver paint electrodes were thermally diffused on the samples' flat surfaces, relative permittivity, and dielectric loss, as functions of temperature were evaluated in a custom-made stage using an HP 3238 LCR. The samples were heated at a rate of 1 °C/min while applying a voltage of 1 V; measurements were taken in the 100 Hz to 1 MHz range in a 30-500 ±1 °C interval.

Crystal structure
As can be seen in Fig. 1, the peak at 2θ = 46° of the X-ray diffraction (XRD) pattern is a double hkl reflection with the smaller reflection at the left side indicating the coexistence of the tetragonal structure with P4mm and orthorhombic structure Amm2 symmetry in the KNNLiTaLa0.01 ceramic. We have refined the secondary phase in the XRD pattern as bronze tungsten with tetragonal structure (space group No.127, P4/mbm) [11]. From these results (see Table 1   gives rise to a relatively high density, 4.2 g/cm 3 , as compared to the theoretical density value of 4.51 g/cm 3 [12], that favors the good dielectric-piezoelectric properties discussed below. Nevertheless, the density is lower than that obtained by the RTGG preparation method [6,9]. confirms the incorporation of La into the structure [7]. The maximum permittivity of ɛ max= 3000 is obtained at T T-C = 371 °C, the tetragonal-to-cubic transition temperature. The maximum permittivity obtained is larger than those of other KNN compounds obtained by the conventional ceramic method [7] and comparable to that of any other similar ceramics obtained by the sol-gel method [10]. The downshift in temperature for the tetragonal-to-cubic transition is even higher for the ceramics of the same composition obtained by the RTGG method (T T-C = 214 °C) [9] highlighting the effect of density and texture associated to such preparation method. The orthorhombic-totetragonal transition, on the other hand, occurred at T O-T = 110 °C. Both transitions take place at lower temperatures as compared with those corresponding to undoped KNN [13]. The shift of the orthorhombic-to-tetragonal and tetragonal-to-cubic toward lower temperatures is with respect to the corresponding ones in undoped KNN and La 0.05 , Li 0.05 doped KNN obtained via conventional solid-state processing, is notable and is assumed to be related to the Li 0.04 and La 0.01 doping and the sol-gel processing of the samples [10].The permittivity ( ) analysis shows no dispersion with frequency at T T-C (see Fig. 3a)), meaning a normal-type transition. In contrast, the dispersion at T O-T indicates the existence of polymorphic phases, generated by La doping [9]. Table 2 shows the comparison of the transition temperatures and max with those of similar compounds. The obtained compound presents similar values of the temperature for the permittivity maxima for the tetragonal-to-cubic polymorphic transition than similar compounds obtained by sol-gel but somewhat lower than those in other reports obtained by the same method, due to the La doping.

Conductivity
Electric conductivity and dielectric properties are related in a ferroelectric system. The conductivity is affected by impurities, defects, changes in the polarization, and free charge transport. One way of analyzing the conductivity behavior is by following Jonscher´s formalism [14]. Here, the electric conductivity as a function of frequency (ω) is given by Jonscher´s universal law: Where σ dc , ω H , are the dc conductivity (long range), hopping frequency respectively and n and A are fitting exponent and coefficient. All terms are temperature dependent.
The first term in Eq. (1) refers to the Universal Dielectric Response (UDR) and the second to the Near Constant Loss (NCL). [15] As is well known, the temperature dependence of the dc conductivity and the hopping frequency expressed by an Arrhenius law: Where U dc and U H are the activation energies of the dc conductivity and the hopping conduction mechanism, respectively, σ 0 and ω 0 are pre-exponential factors and = 8.617×10 -5 eV K -1 is Boltzmann´s constant.
T  4 shows the frequency dependence of ac conductivity. It can be seen that it follows Jonscher´s law in the 100Hz-1MHz frequency and the 250-500 ºC temperature intervals.
The obtained conductivity values are significantly lower concerning those of the samples obtained by the RTGG method [9]. This fact, together with the XRD results presented in Fig. 1, shows the effectiveness of the homogenization of the KNNLiTaLa0.01 compound by Sol-Gel method as compared with that obtained via RTGG method [9]. Fig. 5a) shows the behavior of the hopping frequency obtained using Jonscher´s law with temperature. It can be seen that it obeys an Arrhenius law in intervals below and above the critical temperature T T-C . Similarly, Fig. 5b) shows the behavior of the dc conductivity obtained from Jonscher´s law, and it also obeys an Arrhenius law below and above the critical temperature T T-C . The change in the value of the activation energy around the critical temperature T T-C = 361 ºC in both hopping and dc conductivity processes has been demonstrated before by many researchers [6,[15][16].
In Fig. 5a), the change in the hopping activation energy value is registered while passing the tetragonal-to-cubic phase transition from 0.54 eV to 1.12 eV. This is attributed to a change in charge carrier type in the hopping mechanism as the transition of the sample from ferroelectric to paraelectric takes place. These activation energy values are associated to single and double ionized oxygen vacancies [17][18]. The activation energy jump of the hopping mechanism from the ferroelectric to the paraelectric behavior suggests that, in the ferroelectric state, the electric dipole ordering with the external field direction. The associated transport short distance hooping mechanism is by single ionized oxygen transport. In the paraelectric state, dipoles disappear and thermal energy (k B T) enhances the probability of the existence of doubleionized oxygen ions ( = ′′ + 2 ′ ) that would be responsible now for the hopping conduction process. Thermal energy would reduce the mobility of these ions [6]. Fig. 5b) shows the change in activation energy at the tetragonal-to-cubic phase transition from 0.86 to 1.00 eV for the dc conductivity mechanism. This is an indication that for the long-range conduction mechanism, double ionized oxygen atoms are the responsible ions according to = ′′ + 2 ′ [6]. and the energy jump signals the transition from ferroelectric to paraelectric state. The enhancement of the activation energy corresponding to the dc conductivity mechanism in the ferroelectric-paraelectric phase transition suggests that in the ferroelectric state, the dipole moments align with the external field direction while the long-range transport of charge carriers (doubleionized oxygen) behave according to the calculated dc activation energy. It is interesting to see that, in the paraelectric state, double-ionized oxygen is responsible for both dc conductivity and hopping conductivity. The obtained values for the activation energies for both mechanisms are similar to those reported by other authors for the KNN systems [15][16][17][18]. The values of the exponent in Jonscher´s law are shown in Fig. 6

Conclusions
The fabrication of the KNNLiTaLa0.01 compound seems to be not only dependent on the preparation conditions but also on the preparation method. In this work, we show that the sol-gel method renders a compound with values of the maximum dielectric permittivity larger than those of samples grown by the traditional solid-state technique but lower than those obtained by the RTGG method for similar compositions. For our samples, grown by sol-gel, we find that La doping induces shifts of the orthorhombicto-tetragonal and tetragonal-to-cubic transition temperatures toward lower temperatures.
The conductivity of the compound obeys Jonscher´s Universal Law in the studied temperature and frequency intervals showing lower values than the corresponding compound grown by the RTGG method, suggesting better composition homogeneity because of lower free ion mobile species. The studied compound shows a sharp change in the dc conductivity and hopping activation energies at the tetragonal-to-cubic phase transition. The calculated values of the activation energies for dc conductivity and hopping mechanisms allowed us to conclude that the corresponding charge carriers responsible for both processes were single and double ionized oxygen ions.