Transport properties of LaF3 fast ionic conductor studied by field gradient NMR and impedance spectroscopy

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Abstract

The self-diffusion coefficient and the ionic conductivity in LaF3 single crystals are measured over a temperature range from 300 to 1400 K. Three thermally activated processes are found in this range. After heating, both the diffusion coefficient and the conductivity of the as-grown samples undergo strong irreversible changes at T<1100K. An activation energy of Ea=1.2eV and a Haven ratio of about 0.1 are determined in the intrinsic region. The mechanism of the ion transport and the effect of thermal treatment are discussed.

Introduction

The transport properties of superionic conductors can only be understood in terms of their dynamics on a microscopic level. There are several relevant observables to be looked at, e.g. the ionic conductivity, the self-diffusion coefficient, the mobility and jump rates. Different experimental methods used in this context are sensitive to different characteristic processes. Combining results from several methods therefore not only improves the quality of the analysis, but also yields additional information on the microscopic nature of the ion dynamics.

The transport properties of LaF3 in a wide temperature range have been investigated during long time mainly using impedance spectroscopy [1], [2], [3], [4], [5]. When analyzing the literature data, several discrepancies concerning charge carriers, anisotropy of diffusion and activation energies occur.

Crystalline LaF3 has a trigonal, so called tysonite structure (space group P3̄c1) where the fluorine ions are located on three distinct sublattices, called F1, F2 and F3, in the ratio of 12:4:2 per unit cell [6]. The immobile lanthanum cations are located on layers perpendicular to the c-axis. The F1 anions, which form layers between the La planes, are highly mobile [7], [8]. It has been assumed that the ionic migration in LaF3 is caused by fluorine vacancies of Schottky type [9]. However, also some evidence for Frenkel defects at temperatures above 1150 K, i.e. about 600 K below the melting point has been reported [10]. Further, from the tysonite structure one expects a significant anisotropy of the ionic conductivity in LaF3 along (σ) and perpendicular (σ) to the c-axis. However, literature data is controversial in this respect. Some authors [2], [3] observed a very minor anisotropy of the ionic conductivity only at low temperatures (300–415 K). On the other hand, an anisotropic ionic conductivity has been found above room temperature up to 650 K [1] where σ values exceed those in the perpendicular direction, σ, by a factor of 1.4–2, depending on temperature. Moreover, an even higher anisotropy σ/σ, ranging from 3 to 6, has been reported recently [4].

Furthermore, discrepancies are present in the reported activation energies. Usually three temperature ranges with different activation energies are distinguished. Disagreement in published data occurs in both, Ea-values and their ranges of validity. In Ref. [3] the temperature ranges given as below 415 K, in between 415 and 715 K and above 715 K are assigned to activation energies of around 0.45, 0.26 and 0.84 eV, respectively. Chadwick et al. [1] observed similar temperature ranges with activation energies of 0.38 eV below 400 K, 0.28 eV in the intermediate range of 400–555 K and ∼0.8 eV above 555 K. Sorokin et al. [5] reported approximately the same activation energies for the low and intermediate temperature ranges, however the high activation energy regime was not found up to 800 K. According to Hoff et al. [4] the activation energy is around 0.3 eV in the whole temperature range from 300 to 750 K. In Ref. [3] it has been concluded that the conductivity mechanism in the high temperature range is intrinsic and ionic migration is caused by thermally generated defects. However, conductivity in this range shows a strong dependence on the heterovalent impurity concentration (see Fig. 2 in Ref. [3]), which contradicts to the assumption of the intrinsic regime.

Trying to clarify these discrepancies and to get more detailed information about transport properties in LaF3 we carried out measurements of both diffusion coefficient and ionic conductivity in a wide temperature range.

Section snippets

Experimental

Self-diffusion coefficients are measured using 19F ultrahigh static field gradient (SFG) NMR. The idea of this method is similar to that of the tracer diffusion experiment, but instead of a radioactive marker the ions are tagged by their NMR frequency. Roughly speaking, in a magnetic field gradient the ionic diffusion will lead to an additional decrease of the NMR echo signal with increasing time. The method has several advantages: it is nondestructive, it does not influence the dynamic

Results

In presenting our experimental results as obtained from both diffusion and conductivity measurements we proceed by consecutively pointing out three important phenomena.

Discussion

The temperature dependence of the ionic conductivity in as-grown LaF3 can be described by three regimes with different activation energies. According to Refs. [1] these regimes are denoted as association (range I, low temperatures), extrinsic (range II, intermediate temperatures) and intrinsic (range III, high temperatures) regime. A similar behavior is found in many superionic conductors [15], [16]. The values of the activation energy in ranges I and II are in good agreement with the published

Acknowledgements

The authors thank Prof. H. Hahn (Institute of Material Sciences, TU Darmstadt) for providing us measuring time at his impedance spectroscopy equipment. The work is supported by the German Science Foundation (DFG) under grant no. FU 308/4.

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1

On leave from Institute of Solid State Physics of the Russian Academy of Sciences, Chernogolovka, Moscow District, 142432, Russia.

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