A new, lead free, family of perovskites with a diffuse phase transition: NaNbO3-based solid solutions

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Abstract

(1−x)NaNbO3–(x)ABO3 perovskite solid solutions belonging to group II according to the Krainik classification [Izv. Akad. Nauk SSSR, Ser. Phys. 28 (1964) 643] exhibit a dramatic diffusion of the dielectric permittivity ε′ maximum and relaxor-type behavior when the second component concentration exceeds a threshold value x0. The concentration phase transition to this relaxor-like phase is abrupt (of the first order kind) that is seen from the step in the dependence of the ε′(T) maximum temperature, Tm, on x. Some relaxor-like properties appear even at x<x0 in the course of cooling while disappear during the course of heating. Due to this fact and because of coupling of the antiferroelectric (AFE) and ferroelectric (FE) order parameters a giant (up to 100 K) temperature hysteresis of ε′(T) arises at AFE–AFE first order phase transition. The Tm values of all the known NaNbO3–ABO3 relaxor-type compositions are well below the room temperature and the dielectric permittivity maximal values, εm, are much lower than in the case of Pb-containing relaxors. However both Tm and εm values can be increased substantially by Li or K-doping leading to the formation of NaNbO3–ABO3–LiNbO3 (KNbO3) solid solutions.

Introduction

For nearly 50 years the so-called ‘diffuse phase transitions’ (DPT) remain in the focus of the physics of ferroelectrics (FE) [1]. Both FE [1] and antiferroelectrics (AFE) [2] with DPT are characterized by a diffuse frequency-dependent permittivity, ε′, maximum. Due to the strong frequency dependence of both the maximal value, εm, and the temperature Tm of the ε′(T) peak, FE with DPT are often referred to as relaxors [3], [4], [5], [6]. The phase transition diffusion is associated with a nanoscale compositional inhomogeneity leading to the distribution of local phase transitions over a broad temperature range. Modern theoretical descriptions of the properties of relaxors usually take into account dipolar glass formation [5] and/or a random-field frustrated FE state [4].

The relaxor behavior in perovskites was studied predominantly in Pb-containing ternary compounds (PMN, PZN, PST, PSN) and solid solutions (PLZT, PMN-PT) [6]. During the past years some lead-free BaTiO3-derived solid solution compositions attracted much attention as environmentally benign relaxor materials [7], [8], [9]. However, up to now the obtained values of the dielectric permittivity maximum temperature, Tm, in the BaTiO3-based relaxors were too low for potential applications [7]. The aim of the present paper is to study the dielectric properties of some NaNbO3-based solid solutions forming a new family of lead-free materials with DPT.

It should be mentioned that though the diffused ε′(T) maxima and relaxor-type behavior were reported in some NaNbO3–BaTiO3 solid solution ceramics [9], [10], [11], the relaxor-like compositions belong to the BaTiO3-rich side of this system and are therefore out of scope of the present work. Hereafter we will refer to NaNbO3–ABO3 solid solutions as binary solid solution systems, as it is common in FE materials engineering, though we realize that in fact they are much more complicated objects.

On heating from liquid helium temperature, NaNbO3 exhibits a series of six phase transitions from the low temperature FE phase N (R3C) to the high temperature paraelectric (PE) cubic phase U (Pm3m) through different AFE and PE phases of other symmetries [12], [13]. An ε′(T) maximum is observed at 350–370 °C. It was explained as originating from the phase transition between two AFE phases: P (Pbma) and R (Pmnm) (using Megaw notations [12]). The P⇔R phase transition is of the first order and a thermal hysteresis ΔTh of the ε′(T) curve is observed [13], [14], [15], [16], [17], [18], [19], [20], [21] (Fig. 1).

Besides the ε′(T) maximum, a comparatively small ε′(T) anomaly is often observed at temperatures about 150 °C both in single crystals and ceramics of NaNbO3[14], [15], [16], [17] (inset in Fig. 1). Precise dielectric and X-ray studies have shown that this anomaly is likely to be related to the second order phase transition between two orthorhombic AFE phases [21]. This conclusion is in accordance with the results of Raman scattering studies [22].

Though NaNbO3 is believed to be an AFE above the room temperature, the FE phase can be rather easily induced by a small amount of appropriate dopants (e.g. Li or K) or by application of a moderate electric field [1], [20]. In the latter case the FE state, being induced once, remains stable after switching off the field up to approximately 280 °C. There are some evidences that intrinsic defects like oxygen and sodium vacancies can be responsible for the existence of nanosize regions of the FE phase, observed by TEM in NaNbO3 crystals at room temperature and above [23], [24].

Similar to solid solutions of other perovskite AFE, the NaNbO3–ABO3 binary solid solution systems can be divided into two groups [1], [25]. In the solid solutions of group I the high temperature FE phase appears at small (a few mol%) content x of the second component ABO3, the Tm(x) dependence is rather smooth and the ε′(T) maxima are sharp. These solid solutions, especially (Na,Li)NbO3 and (Na,K)NbO3 systems, are of great interest for high-frequency piezoelectric applications and have been extensively studied for the last three decades [1].

In the solid solutions of group II the AFE phase remains stable up to comparatively high x values. In contrast to the solid solutions of other perovskite AFE, the Tm(x) dependence in the NaNbO3-based solid solutions belonging to group II remains smooth only up to a threshold x=x0 value (x0 varies from 5 to 20 mol% in the majority of the solid solutions up to 50 mol% in the case of the NaNbO3–NaTaO3 system [26], [27]) while at x>x0 the phase, usually referred to as FE or ferrielectric becomes stable, which is accompanied by an abrupt drop in the Tm values and dramatic diffusion of the ε′(T) maximum (Fig. 2, Fig. 3). For the compositions with x>x0 hysteresis loops were observed below Tm while the compositions with x<x0 exhibit a linear PE relationship [27], [28]. Relatively low values of the spontaneous polarization PS (2.3 μC/cm2 for 0.875 NaNbO3–0.125CaTiO3 ceramics at −170 °C [28]) were a reason for attributing the materials with x>x0 to ferrielectrics. While the Tm values of the compositions with x<x0 do not depend on frequency, the compositions with x>x0 were reported to exhibit a frequency dispersion of ε′ and an increase of Tm with frequency [25]. Moreover, the low-temperature X-ray studies of the NaNb0.3Ta0.7O3 ceramics did not reveal any structural changes on cooling below Tm[27]. Thus, it seems that the compositions of the NaNbO3-based solid solutions belonging to group II exhibit a relaxor-like behavior at x>x0. Besides, the lack of systematic data on the properties of such materials prevents one from definite conclusions.

Later, we will consider experimental data concerning the NaNbO3–ABO3 solid solutions belonging to group II, their dielectric properties depending on the concentration of the second and third components as well as on the temperature and frequency.

Section snippets

Experimental

NaNbO3–Gd1/3NbO3 solid solution crystals were grown by the flux method using the Na2CO3–Nb2O5–B2O3 system with additions of Gd2O3. The crystals obtained had a platelet (up to 2 mm on edge and 0.2–0.3 mm thick) and isometric (edge dimensions 0.5–2 mm) form with the sides parallel to the {100} planes of the perovskite prototype lattice. The composition of the mixed crystals was evaluated by comparing their structural parameters and the phase transition temperatures with the experimental values

Dielectric properties and Tmx phase diagrams of some NaNbO3-based solid solutions

Fig. 3(a) shows the typical evolution of the ε′(T) dependencies with the composition for the solid solution of group II. The similarity of the character of this evolution for the ceramic samples and single crystals shows that the peculiar properties of the group II solid solutions are not due to the immiscibility effect typical of many solid solution ceramics.

Though the ε′(T) maxima of the group II NaNbO3-based solid solution compositions with x>x0 are smeared and a frequency dispersion of ε

Discussion

We believe that the essential experimental observation is the dramatic increase in the jump in thermal hysteresis, ΔTh(x) as xx0 from below, followed by its abrupt disappearance at x=x0. We discuss these data in terms of a phenomenological Landau-type expansion in three scalar order parameters: a FE order parameter, P; and two AFE order parameters, Q1 and Q2. We believe this is the simplest model that accounts for the observed behavior of ΔTh(x), but acknowledge that it ignores the full

Summary

Both the ceramics and single crystals of binary NaNbO3–ABO3 perovskite solid solutions belonging to group II (according to Krainik [25] classification), exhibit a dramatic diffusion of the dielectric permittivity ε′ maximum when the second component ABO3 concentration exceeds a threshold value x0 corresponding to the abrupt decrease of the maximum temperature Tm. In spite of the general proximity of the temperature and frequency behavior of ε′(T) at the diffuse phase transition in the

Acknowledgements

This work was partially supported by Russian Foundation for Basic Research (Grants #99-02-17575, 01-03-33119 and 01-02-16029). S.A.P. appreciates discussions with M.D. Glinchuk and B. Burton.

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