Ionic conductivity of the perovskites NaMgF3, KMgF3, KMgK3 and KZnF3 at high temperatures

Solid State Ionics 17 (1985) 143-145 North-Holland, Amsterdam

IONIC CONDUCTIVITY OF THE PEROVSKITES NaMgF3, KMgF 3 AND KZnF 3 AT HIGH TEMPERATURES N.H. ANDERSEN, J.K. KJEMS Physics Department, Ris¢~National Laboratory, DK-4000 Roskilde, Denmark and

W. HAYES Clarendon Laboratory, Parks Road, Oxford, OX] 3PU, UK

Received 1 May 1985 Revised manuscript received 24 May 1985

We have carried out a study of the ionic conductivity of NablgFa, KMgF3 and KZnF a up to temperatures close to the melting point. Our results, in contrast to previous reports in the literature, show no abnormal ionic conductivity at high temperatures. Care in interpretation of remits is required because of surface electronic conduction.

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The unusually high ionic conductivity of fast ionic conductors is associated with extensive lattice disorder. This may be due to nonstoichiometry, as in the case of sodium/3-alumina, or may be thermally induced, as in the case of CaF 2 [ 1 ]. In the latter material, conversion to the fast ionic state is accompanied by a diffuse order-disorder transition which gives rise to a specific heat anomaly at 1157°C, below the melting temperature T m = 1423°C; the disorder occurs in the fluorine sublattice. O'Keeffe and Bovin [2] reported the onset of fast ionic behaviour in NaMgF 3 at 900°C, the temperature at which the crystal changes from the tetragonal to the high-temperature cubic structure; they observed a rapid increase in conductivity, or, from 10 -2 S cm-1 to 1 S cm - 1 , between 900°C and T m = 1030°C, and found very little change in a on melting. Poirier et al. [3] reported a similar transition to superionic behaviour in KZnF3, below T m = 870°C. Possible relevance of these experiments to the lower mantle of the earth was pointed out [2,3]. Cheeseman and Angell [4] carried out a molecular dynamics study of NaMgF 3 but found no evidence

for the diffusion of any of the constituent ions at temperatures of up to 774°C, and higher. This negative result seems consistent with the close packing of ions in the perovskite structure. Chadwick et al. [5] studied both ionic conductivity and NMR relaxation up to 430 C m NaMgF 3, KMgF 3 and KCaF 3. They found no evidence of ionic motions in NaMgF 3 or KMgF 3 but observed rapid diffusion of F - ions in KCaF3, through a vacancy mechanism arising from oxygen doping (see also Bouznik et al. [6] for similar results for CsPbF3). We have carried out ac ionic conductivity measure. ments on single crystals of NaMgF 3, KMgF 3 and KZnF 3 up to temperatures close to T m , We find no evidence for fast ionic behaviour in these crystals. We have confirmed this result in the case of NaMgF 3 by a neutron scattering experiment, which showed no anomalous diffuse scattering indicative of ionic disorder in the high temperature cubic phase.

2. Experimental

Conductivity studies were performed by use of a standard two-electrode ac-measuring technique with

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N.H. Andersen et al./Ionic conductivity of the Perovskites

a Solartron FRA 1174 frequency response analyzer equipped with a preamplifier. A typical frequency range was 0.1 Hz to 1 MHz. Sample voltages were about 50 mV. Single-crystal samples were cut from ingots by a diamond saw running in kerosene and were polished by diamond paste. Typical dimensions were 10 mm X 10 mm X 1 mm. Platinum electrodes with an approximate thickness of 1000 A were dc-sputtered on to the surface of the samples and were connected to the external platinum electrodes by a spring load. The measurements were performed in a flow of nitrogen (less than 5 ppm H 2 0 and 3 ppm 02) or argon purified by a Ti-getter system (less than 0.1 ppm H20 and O2). The furnace used has a bifilar heating filament and is operable up to 1000°C. Temperature measurements were performed with chromel/alumel thermocouples. Conductivities were determined from the complex plane admittance spectra at the foot on the real axis ~, of the high-frequency geometrical capacitance line. No attempts were made to deduce the conductivity from fits to equivalent circuits. This procedure can give rise to errors of ~10%, not significant for present purposes. In the low frequency range, 0.1 Hz was often insufficient to close the depressed semicircle (see fig: 2). Since the electronic contribution to the conductivity shows up in the dc limit of the frequency response some of the measurements were performed at frequencies down to 10 -2 Hz.

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Fig. 1. Arrhenius plot of conductivity data from two different crystaLsof KZnF3 (o) 1. sample, (o) 2. sample. The conductivity is essentially pure electronic. a sample to 400°C in argon, it had a dc resistance of 80 k~2; polishing the edges of the sample removed the dc conductivity. All samples showing electronic conduc tivity had a white or grey (perhaps oxide) surface layer. The conductivity of NaMgF3, measured in a nitrogen atmosphere, was more complex. Usually, a reasonably well behaved depressed semicircle, as shown in fig. 2, was observed, especially at high temperatures. A finite dc conductivity, most likely of electronic

3. Experimental r e s u l t s In general, the oonductivity measurements were not reproducible. For alI,three materials, evidence for electronic conductivities were observed, most significantly in KZnF 3 where all the admittance spectra showed only the frequency variation resulting from the geometrical capacitance; ~f the blocking electrodes. A simple check by use of an ohmmeter showed clear evidence for electronic conduction without charging effects on the geometric and double layer capacitances. Results for two different crystals of KZnF 3 in an argon atmosphere are shown in fig. 1. The apparent discontinuity in o at 470°C for sample 1 was not found in sample 2. There is strong evidence for electronic conduction in a semiconducting surface layer because, after heating

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tivity measurements

N.H. Andersen et al./Ionic conductivity o f the Perovskites

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Conductivity studies were also performed on KMgF 3 in an argon atmosphere not purified by the Ti-getter. The data were, again, not very reproducible and showed unusual kinks in the Arrhenius plots. As before, dc conductivity developed during heating in air and also in argon gas. The upper limit of total conductivity at 1000°C was approximately 3 X 10 -4 S cm -1 .

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Fig. 3. Arrhenius plot of conductivity data from N a M g F 3 . (o) 1. ran, (v) 2. run. The conductivity is mixed ionic and electronic. nature, is found. However, the major contribution to the conductivity at high temperatures comes from the ionic part. Also the dc conductivity of the NaMgF 3 sample was directly susceptible to air because heating it to 600°C in the atmosphere increases the dc conductivity from about 10 -6 S cm -1 to 10 -4 S cm - 1 . The results of two runs on the same sample are shown in fig. 3. Between the two sets of measurements the sample was polished and new electrodes were sputtered on before the measurements. Apparently the surface for run no. 2 was cleaner than for run no. 1. It should be recalled that the data in fig. 3 represent the total conductivity. In principle, the electronic and ionic conductivity might be sepaJated. However, this would require very time consuming measurements at frequencies down to 1 0 - 3 - 1 0 -4 Hz. Since the electronic conductivity is a surface property of the material and the ionic conductivity is too low to be of further interest no attempts were made to separate th6m. Further it was observed that the platinum wires, and especially the chromel/alumel thermocouple wires, were attacked by the atmosphere (probably HF), generated during heating of the sample.

We have studied the ionic conductivity of single crystals of NaMgF3, KMgF 3 and KZnF 3 at temperatures up to 1000°C and we find no evidence for superionic behaviour of these close-packed materials. We have also carried out a study of the lattice dynamics of NaMgF 3 at temperatures up to T m using neutron scattering techniques. Again, we find no anomalous behaviour other than softening of the R point phonon for T ~ 900°C; this softening is characteristic of the change from tetragonal to cubic symmetry in perovskites [7] and is not due to disorder (see Hutchings et al. [8] for a description of neutron studies of disorder in fast ionic fluorites). It seems that surface electronic conductivity can give rise to confusion in the study of ion transport in perovskites and that single-frequency measuring techniques and powder samples should be avoided.

References

[1] W. Hayes, Cont. Phys. 19 (1978) 469. [2] M. O'Keeffe and J.O. Bovin, Science 206 (1979) 599. [ 3 ] J.P. Poirier, J. Peyrounneau, J.Y. Gesland and G. Brebec, Phys. Earth Planet. Inter. 32 (1983) 273. [4] P.A. Cheeseman and C.A. AngeH, Solid State Ionics 5 (1981) 597. [5 ] V. Chadwick, J.H. Strange, G.A. Ranieri and M. Terenzi, Solid State Ionics 9/10 (1983) 555. [6] M. Bouznik, Yu.N. Moskovich and V.N. Voronov, Chem. Phys. Letters 37 (1976) 464. [7] W. Cochran and A. Zia, Phys. Status Solidi 25 (1968) 273. [8] M.T. Hutchings, K. Clausen, M.H. Dickens, W. Hayes,

J.K. Kjems, P.G. Sclmabel and C. Smith, $. Phys. C 17 (1984) 3903.