On the structure and dynamics of heterovalently doped β-PbF2

SOLID STATE

Solid State Ionics 72 (1994) 7-11 North-Holland

IOIIICS On the structure and dynamics of heterovalently doped [3-PbF2 J. ten Eicken, W. Gunsser, M. Karus, A. M e y e r Institut Jfir Physikalische Chemie, Universitiit Hamburg, Bundesstr. 45, 20146 Hamburg, Germany

and I.V. M u r i n Department of Chemistry, University of St. Petersburg, Universitetskii pr. 2, Petrodvoretz, 198904 St. Petersburg, Russia

Heterovalent isomorphous replacement is one of the main methods to increase the ionic conductivity of solid electrolytes. Understanding the nature of these impurity defects and their relaxation can help in establishing the connection between the electrophysical properties and crystallochemical peculiarities of doped superionic conductors. EPR spectra of Gd 3+ in PbF2 show a well resolved fine structure as well as spin forbidden transitions with AMs> 1. The angular dependence in both cases indicates that Gd 3+ occupies a purely cubic symmetric site. This stands in contrast to earlier results and is indicative of the importance of the history of a sample, especially its thermal and pressure treatment, as shown by X-ray analysis and conductivity data. A method of investigating dipole defects and polarizing phenomena is proposed, whereby RE-Fi dipole defects are fixed and their relaxation is studied by means of EPR-spectroscopy.

1. Introduction

The growing interest in superionic materials has lead to a rising interest in investigating fluorite-type materials. Although heterovalent isomorphous replacement is one of the main methods to increase the ionic conductivity of solid electrolytes many questions remain to be answered especially for systems such as Pb] _xRExF2+x . PbF2 itself is well known and has the best conductivity within the fluorite group. Understanding the nature of these impurity defects and their relaxation can help in establishing the connection between the electrophysical properties and crystallochemical peculiarities of doped superionic conductors. The local charge compensation of PbF2 doped with rare-earth trifluorides takes place via nearest neighbor ( N N ) or next nearest neighbor ( N N N ) dipole complexes, whereby the cation site symmetry becomes tetragonal or trigonal, respectively, o r via distant compensation, which leads to a cubic symmetry for the cation site. The ratio of these charge compensating mechanisms is determined by the radii of both the host and the rare earth

cation. With this paper we wish to continue our studies [ 1-3 ] on the defect structures in doped [3-PbF2 systems, extending this to their dynamics at temperatures up to 900 K.

2. Experimental Polycrystalline samples of Pb~ _xRExF2+x (RE = Sc, Gd) solid solutions were prepared by solid state synthesis in HF gas atmosphere starting from the well ground mixtures of fluorides. Syntheses were carried out at 930-1020 K for 7.5 h. Single crystals were grown by the Bridgman-Stockbarger technique starting from high purity fluorides and their structure verified by X-ray powder diffraction, Laue images and X-ray spectroscopy. Conductivities of pressed pellets were measured using a standard ac bridge technique. The temperature range studied was 300-770 K. We used an externally heated hydrostatic high pressure vessel which allows us to measure electro-conductivity at pressures up to 2.5 × 10 s Pa and temperatures between 220 K and 770 K. EPR

0167-2738/94/$ 07.00 © 1994 Elsevier Science B.V. All rights reserved.

8

3. ten Eicken et al. /Heterovalently dopedfl-PbF2

experiments were carried out using a Bruker ER 420 spectrometer and an ERS 300 spectrometer, both working at frequencies 9.1 to 9.5 GHz. Temperatures as low as 4 K were reached by means of a continuous-flow cryostat and high vacuum equipment. For temperatures between 300 K and 900 K samples were kept under inert gas and heated by a pre-heated H2/N2 (10:90) gas mixture.

leads to a partial transition from the [3- to the a-modification of PbF2 as shown in fig. 1 for the indicated reflexes relative to the signal due to the sample mount. These multiphase systems exhibit higher conductivities than the samples having pure [3-form, but they lack thermal stability. This is depicted in fig. 2 where the first temperature cycle of measuring the conductivity of the 10 mol% ScF3-doped sample at a frequency of 40 kHz is shown, whereas fig. 3 shows the conductivity dependence on the applied pressure during the measurements. Until now, it has not been possible to verify to what extent the conductivity of these systems can be increased by means of the combination of temperature and mechanical pressure. High pressure technique allows us to receive direct information about elastic volume relaxation associated with the formation and motion of lattice defects. As shown in table 1 A VF is much less than the molar volume ( - 3 2 cm3/mol). This is an additional confirmation that [3-PbF2 has Frenkel disordering. We can also define separately the activation volume of migration of vacancies and interstitial ions. The comparison of these volumes shows that the vacancy mechanism of ionic conductivity is preferable. We can also estimate the activation volumes of

3. Results Heterovalent doping improves the ionic conductivity of [3-PbF2 significantly. The best results can be achieved at a concentration of 7.5 mol% ScF3. The concentration-dependent conductivity exhibits a maximum. This is caused by the aggregation (clustering) of the dipole defects due to the heterovalence such that higher concentrations would reduce the number and mobility of the free fluoride ions [ 1,2]. Using ceramics one has to be very careful with the preparation because during grinding, pressing or temperature treatment it is very easy to change the content of the cubic [3- and the orthorhombic ~t-phase of PbF2. Especially the pressure applied to press the pills in preparation for conductivity measurements

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Fig. 1. Pressure dependence of the intensities of the predominant reflexes of the ct- and 13-modificationin relation to the external peak due to the sample mount.

J. ten Eicken et a L I Heterovalently doped fl-PbF2

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tool%) on the applied pressure during the measurements at 292 K, 418 K and 450 K; the pressure range is restricted by the change from 13-to a-modification. quasichemical complexes dissociation (heterovalent impurity-local compensator) e.g. [Na ÷-VF], [Gda+-F~ ]. Table 1 gives some results for the best F--conducting materials, based on 13-PbF2, especially for PbSnF4 with the lowest value of activation volume. Our experimental results allowed us to test the validity of proposed theoretical models and at first time Zener model (strain energy model) in

Griineisen approximation. The measured and calculated values of A V are in good agreement. Various EPR investigations on heterovalently doped alkaline earth fluorides have shown that tetragonal and trigonal dipoles as well as the non-local charge compensation (Gd 3+ at sites of cubic symmetry) can be distinguished by their characteristic splittings and angular dependencies. In dependence to the thermal treatment and the preparation technique it is possible to realize a situation where Gd 3+ ions are only located at sites of cubic symmetry. In contrast to previous results [ 1 ] our EPR-investigations did not detect any existing dipoles having axial symmetry within the new crystals. The spectra of the Gd3+-ions are dominated by a well resolved fine structure (fig. 4, right). The rotational diagrams speak for the pure cubic symmetry of the Gd3+-sites and the spectra are to be described by the parameters shown in table 2. At low temperatures and in an external magnetic field < 220 mT we observe lines which are by the factor of 100 smaller than the seven fine-structure lines of the Gd3+-ion. Due to their angular dependence (fig. 5), we can ascribe them to spin-forbidden transitions (AMs> 1 ) of the Gd 3+ at sites with cubic symmetry. In our TSD-experiments we were able to identify

10

J. ten Eicken et al, /Heterovalently doped fl-PbF2

Table 1 Summery of the activation volumes of the lattice defects in PbF2. Compound

Process

T (K)

Ah (eV)

AV (cm3/mol)

A V~lc (cm3/mol)

A V/A V=~c (cm3/mol)

[]-PbF2

Frenkel defect formation fluorine vacancy migration fluorine interst. migration dissociation [Na+-Vi~] dissociation [Gd3+-FT] defect migration defect migration

300-350

1.04

4.6

4.9

0.9

340-500

0.22

1.4

1. l

1.3

325-435

0.33

2.4

1.7

1.4

273-340

0.24

2.0

1.2

1.7

273-325

0.34

2.1

1.7

1.2

400 285-335

0.13 0.4

0.53 0.7

-

-

defect migration

300-450

0.34

1.7

-

Frenkel defect formation fluorine vacancy migration fluorine interst. migration

300-500

-

6.9

-

300-500

-

2.8

-

300-500

-

3.0

-

[~-PbF2-NaF (0.1%) ~-PbF2-GdF3 (0.1%) 13-PbSnF4 ~PbF2-CdF2 (34%) ~PbFz-UF4 (15%) a-PbF2

Table 2 EPR-parameters of the PbF2 samples doped with 0.1, 0.5 and 1.0 mol% GdF3 in comparison to data from literature [6-8]; no amount of GdF3 in the samples was given.

Af

(mol%) 0.1 0.5 1.0

110.0

220.0

330.0

-

g _+0.002 1.992 1.991 1.996

c (10-25J) +0.02 3.15 3.04 3.18

d (10-25 J) -+0.08 0.01 0.00 0.01

B [rn~

Refs. Fig. 4. EPR spectra (X-band) of the PbF2, sample doped with 0.1 mol% GdF3 at 77 K and Bll < 100 >; left: spin forbidden transitions (AM,>~2) magnified by a factor of 100; right: fine structure spectra of Gd 3+.

dipole complexes with trigonal and tetragonal symm e t r i e s [ 1,9 ]. D i s c r e p a n c i e s b e t w e e n t h e T S D a n d E P R r e s u l t s are e x p l a i n e d b y d i f f e r e n c e s i n p r e p a ration, especially in the thermal treatment of the s a m p l e s [ 1 0 - 1 2 ]. T h e s e l e a d u s t o t h e i d e a o f i m plementing a new method for investigating dipole c o m p l e x e s . T h e s a m p l e is p o l a r i z e d a n d c o o l e d in t h e a p p l i e d e l e c t r i c a l field, s u c h as t h e y are i n t h e T S D -

[6] [7] [8 ]

g -+0.001 1.992 1.990 1.990

c (10-25 J) _+0.02

d ( 1 0 25j)

3.18 -3.18 3.14

0.0 -0.0008+__0.0001 0.016 + 0.004

e x p e r i m e n t , w i t h i n a special s y s t e m a d j u s t a b l e to t h e E P R cavity. U p o n i n c r e a s i n g t h e t e m p e r a t u r e o n e can observe the relaxation of the dipoles by means of EPR spectroscopy through changes in the position a n d w i d t h o f t h e signals. W e did, in fact, o b s e r v e s u c h c h a n g e s i n o u r i n i t i a l m e a s u r e m e n t s (fig. 6 ) . I m -

J. ten Eicken et al. /Heterovalently doped fl-PbF2

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Fig. 5. Measured (dots) and calculated (lines) angular dependence of the spin-forbidden transitions with AMs>/2 of Gd 3+.

350

400

450

5~

550

6~

650

700

750

800 T [K]

Fig. 7. Position of the fine structure lines with increasing temperature (crystal doped with 0.1 mol% GdF 3). o n l y be caused b y a c o u p l i n g m e c h a n i s m with G d 3+ . High t e m p e r a t u r e E P R spectra from r u b y a n d clay m a t e r i a l s [ 15 ] fail to show such changes in lineshape, i n d i c a t i n g the observed changes are characteristic for these solid electrolytes.

Acknowledgement

Fig. 6. EPR spectra of PbF2:GdFa (0.5 mol%) before being polarised (top) and after polarisation (bottom), both at 45 K. p r o v e m e n t s o f this setup are n o w u n d e r way to further investigate such effects. W i t h increasing t e m p e r a t u r e (T--.800 K ) , changes in the crystalline lattice are directly o b s e r v a b l e in the spectra t h r o u g h the decrease in line splitting a n d the increase in the differential linewidth. T h i s leads to a single line s p e c t r u m at high t e m p e r a t u r e s (fig. 7). However, the t e m p e r a t u r e range available for investigating the solid electrolytes is l i m i t e d by a t e m p e r ature m a x i m u m a b o v e which the m i c r o w a v e cavity c a n n o t be a d j u s t e d [ 3 ]. T h e higher the c o n d u c t i v i t y o f the sample, the lower the l i m i t i n g t e m p e r a t u r e . At high t e m p e r a t u r e s , we also observe a n o t h e r interesting effect: the l i n e s h a p e o f the r e m a i n i n g high temp e r a t u r e spectra is n o t L o r e n t z i a n . Q u a l i t a t i v e l y it is D y s o n i a n - l i k e [ 3 , 1 3 - 1 5 ]. O n e m a y d r a w s o m e h y p o t h e t i c a l similarities between the m o b i l e fluoride ions in PbF2 a n d cond u c t i n g electrons i n metals. H o w e v e r , while D y s o n lines result f r o m m o b i l e electrons as p a r a m a g n e t i c species, i n o u r case the m o b i l e fluoride ions are diamagnetic a n d a n y effect these have on the spectra can

We gratefully appreciate the s u p p o r t given to o n e o f us (J.t.E.) by the Alfried K r u p p v o n B o h l e n u n d H a l b a c h Stiftung.

References [ 1] J. ten Eicken, W. GunBer, S.V. Chernov and I.V. Murin, Solid State Ionics 53-56 (1992) 843. [2 ] J. ten Eicken, W. GunBer, S.V. Chernov and I.V. Murin, Ber. Bunsenges. Physik. Chem. 96 (1992) 1723. [ 3 ] J. ten Eicken, W. GunBer and I.V. Murin, in: Defects in InsulatingMaterials, eds. O. Kanert and J.M. Spaeth (World Scientific, Singapore, 1993). [4] G.A. Samara, J. Phys. Chem. Solids 40 (1989) 509. [ 5 ] I.V. Murin and O.V.Glumov, Fisica Tverd. Tela 23 ( 1981 ) 2155. [6] F.Z. Gil'fanov, M.M. Zaripov, L.D. Livanova, A.L. Stolov and V.G. Stepanov, Sov. Phys. Solid State 10 (1968 ) 260. [7] T. Rewaj, Proc. Congr. Ampere 20th, 1978, Magn. Reson. Relat. Phenom. (1979) 342. [8] J. Kuriata and T. Rewaj, Acta Phys. Pol. A 75 (1989) 427. [ 9 ] I.V. Murin and W. Gunger, Solid State lonics 53-56 ( 1992 ) 843. [ 10 ] F.K. Fong, J. Chem. Phys. 64 (1976 ) 4243. [ 11 ] W. Low and E. Secemski, J. Chem. Phys. 64 ( 1976 ) 4240. [ 12 ] Y. Ito, K. Kato, S. Yoshikado and T. Ohachi, Solid State Ionics 15 (1985) 253. [ 13 ] G. Feher and A.F. Kip, Phys. Rev. 98 ( 1955 ) 337. [14] F.J. Dyson, Phys. Rev. 98 (1955) 349. [ 15 ] J. ten Eicken, Dissertation (Universit~it Hamburg, 1993 ).