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Short-range order and F− ion diffusion inside the Pb1 − xAlxF2 + x solid solution Part I: 19F-NMR investigation
__ ;,__ Ff!!i
CZ$!3
SOLID STATE
EISEVIER
Short-range
IONICS
Solid State lonics 100 (1997) 233-240
order and F- ion diffusion inside the Pb I _,A1,F2,, solid solution Part 1: ‘“F-NMR investigation
Mohamed El Omari”, Jean Senegasb, Jean-Maurice Reaub** ‘Dqx~r~ment of Chemistry. Faculty of Scirnce, Moulry I.wuil Uniwmity, Meknes. Morocco bbrstitut de Chimie de la Matitre
Condenke
de Bordeaux
CNRS, Chateau
Brivazac,
F-33600
Pc.s,wc. France
Received 24 March 1997; accepted 24 April 1997
Abstract (O%c~O.12) solid solution and of the ordered Pb,AI,F?, phase is A “F-NMR investigation of’ the Pb,_,Al,l:,+,~ undertaken as a function of temperature. Several F - ion groups become progressively mobile at the NMR time scale at increasing temperature, the first, (F,),,, and (F,,),,,, are located in the surroundings of PbZ ’ cations, the second ones, (I;,,),, in that of AI’- cations. Four types of fluoride ions are distinguished in the 19F-NMR spectra of rigid lattice at and (PJ,,,, low temperature (T= 175 K) and the distribution of fluoride ions among the four sites is dctcrmincd by deconvolution of the spectra rcgistercd. The sites occupied by the (F,,),,, anions are identified with the normal anionic sites of the fluorite network, the others with different interstitial sites. Kepmrds:
F
ionic conduction: “F-NMK investigation; Fluorite-type
1. Introduction Various clustering processes wet-c proposed within the anion-excess M:‘,Mi2 “F,,,, (U = 1, 2) solid
solutions with fluorite-type str&turc. They dcpcnd on the nature of the cationic (M, M’) couples present. A clustering process model was set up relating, in a continuous way, the composition dependence of electrical propcrtics and the progrcssivc extension of clustering when x increases. It is based on results issued on the one hand from
“Corresponding author: Tel: + 33 5 5684 6332; 5 5684 6634: c-mail: [email protected] 0167-2738/97/$17.00 Q 1997 Elsevicr PII SO167-2738(97)00340-8
fax:
..33
Science B.V. All rights reserved
snucture
conductivity measurements and on the other hand from neutron diffraction and/or “F-NMR investigations [I ,2]. Three clustering processes have been in this way characterized: l
Larger and larger extension of column clusters involving single-tiles of vacancies (n + 1:2n: 1:O in or two-files of vacancies (2n + Pb,-Pi,F,+,) 2:3n:2:0 in Pb, rlnrF2__x and 2n+2:4n:2:0 in as x increases. Such clusters, Pb, rzrrF9+2r) labclled n,:n,:n,:n,, are electrically neutral and based on the association of n, vacancies in the normal anionic positions (I /4, l/4, l/4) of the fluorite-type network, n2 F’ (112, U, u: 0.355~5 0.40) n3 F” (u,, u,, u,: u, -0.41) and n4 F”’ (u,,
234
M. El Chari
c/ al. I Solid Sk&
interstitial fluoride ions, u 2’ u2: 0.28~u,~O.33) close to one or several substitutional cations. Progressive transformation of 4:4:3:0 clusters into 8: 12: I:0 cubooctahedral clusters in Ba, ,Bi,F, + ~ and Ca I-IM’IF2+x where M’ is Y or a small size rare earth (Er:..., Lu). Progressive transformation of clusters 3:0:3:2 (in where M’ is La,..., Gd and in Ca, .M:F,+, U), 3:2:3:0 (in Ba,_,M:‘F, ,_2X with M”=Th, Sr,..,Bi,F,.,>), 4:0:3:3 (in Ca,_,yM,rF,+,, and into Sr, _,M:‘F, , 2.r), 8:0:3:7 (in Pb,_,MtFZ_21) condensed 2:0:6:0 clusters. The 3:0:3:2, 3:2:3:0, 4:0:3:3 and 8:0:3:7 clusters derive from the same 1:0:3:0 basic cluster [2,3]. Inside all solid solutions studied till now, the substitutional cation is of a size large enough in order to be located in eight- or nine-coordinated sites. But what happens when the substitutional cation is of small size? To answer that question, we are interested in the fluorite-type Pb, _IAI,F,+ ‘: solid solution, which offers a trivalent cation of very small size. solid solution is long-range The Pb, AAl,F,+, disordered and has only a small existence range (O~x~O.12), due to the large size difference between the host and guest cations. An ordered Buoride, of Pb,AI,F2, composition, is located close to in the PbF,-AIF, system [4-61; it is PbI-,Al,F*+, characterized by a reversible phase transition at T, = 36.5-C IO K 171. A comparative study of electrical properties of Pb,_,Al,F,,, and Pb,Al,F?, has shown that the performance of the low- and hightemperature forms of Pb,AI,F,, are respectively close to those relative to (X=0.03) and (O.O6
Ionics
100 (1997)
233-240
2. Experiments The lead fluoride PbF, used (Ccrac 99.99%) was previously dried under vacuum at 473 K for 10 h. Aluminium fluoride AIF,, was prepared, as usual [IO], from Al,O, and (NH,)HF*, under dry nitrogen, at 1073 K for 5 h. AIF, was then treated under vacuum at 473 K, for 10 h. The (x=0.03; 0.06; 0.09; 0.12) Pb, _,AI,F,+, and Pb,AI,F,, samples were synthesized in the same conditions as those used for elect&al measurements 171. The mixtures of fluorides PbF2 and AIF, are ground in a dry box and inserted in a gold tube. After degassing under vacuum at 473 K during 2 h to eliminate the risk of hydrolysis at higher temperature, the gold tubes are filled up with dry argon and sealed. They arc then heated at the reaction temperature and cooled down to ambient temperatures. The thermal treatments of Pb, _,AI,rF, i_r; (0~~~0.12) and Pb,Al,F,, were 15 h at 873 K and quenching, 15 h at 753 K and natural cooling, respectively. were performed on a “F-NMR experiments Bruker MSL-200 spectrometer (H, =4.7 T) equipped with a standard variable temperature unit in the temperature range 125 to 430 K. For each temperature measurement, the powder samples are kept for 1 h at that tcmpcraturc, in order to reach a good thermal stability. Storage after 5 h of a large number of acquisitions allowed high resolution signals to be registered. The ‘Oncpulsc’ acquisition program was used in the following operating conditions gathered in Table 1. This program was selected because it delivers a spectral irradiation large enough to cover the different spectra1 widths of each type of F anions. As a matter of fact, different diffusion coefficients characterize the F ions in Pb, rAI,F1 rl and Pb,Al,F,,. Signals obtained were processed by Fourier transformation, using the ‘WINNMR ID’ program. ‘I’ablr:I Expcrimcntal Pb, ~Al,Fz.,
conditions of and Pb,IAlzt;2,
Spectrometer frequency Pulse width Dead rime delay Rccyclc delay time Spectral width Filter width
the
“F-NMR
investigarion
188.283 MHz 0.7 ps 6 KS IO s I MI-IL 2 MHz
of
M. El Omari rt ~1. I Solid Sture lonics 100 (1997) 233-240
235
Simulations of the 19F-NMR lines were performed using the WINFIT program [ 111. This program allows the adjustment of the peak position, peak height and line width ratio of Gaussian and Lorentzian functions and the relative percentage of their areas. When a single Gaussian does not fit exactly with the registered spectrum, an appropriate mixing of Gaussian and Lorentzian functions is used for the simulation. It was a case in particular for the spectra relative to the motional narrowing temperature range.
be simulated by the help of Gaussian functions. All F- ions arc consequently fixed in the NMR time scale in this temperature domain. Above T=200 K, a new peak called p,, located between the p, and p? peaks appears and grows with rising temperature. Simulated by a Lorentzian function, it represents the fluoride ions which are mobile at the NMR time scale above that tempcraturc. When tcmpcraturc incrcascs, the p2 peak, then the peak p, coalesce in turn with the p,, peak. This means that the number of mobile fluoride ions increases at the expense of the fluoride ions of p2 type, then of p,
3. r9F-NMR
type. Above a temperature T, , the p2 and p, peaks are completely vanished and the p,, peak is only observed on the “F-NMR spectrum. All the fluoride ions are consequently mobile at the NMR time scale above that temperature. T, is equal to 400 K for x=0.03 and 0.06, it is a little higher for x=0.09 and 0. I2 (T, =430 K). The rclativc contributions of the different peaks observed above T=200 K are determined by de-
results
The ‘“F-NMR spectrum at various temperatures is given in Fig. la, b, c, d for some Pb, _XAl,F,_, compositions corresponding to x = 0.03, 0.06, 0.09 and 0.12. At low temperatures (T < 175 K) two peaks called p, and p2 are detected at frequencies equal to = -20 and =5 kHz, respectively. The origin of the frequency scale corresponds to the nominal irradiation frequency (188.283 MHz). Both these peaks can
Ix-o.oB
(a)
40
20
Fi g. I. Thermal
variation
of the “F-NMR
-40
00
(KHz) x=0.09 and (d) x=0.12.
w1
RI
-60
(KH;;o spectrum
for rhe Pb,
1AlJ=z. compositions x
corresponding
to (a) x=0.03,
(b) x =0.06.
(c)
236
M. El Omari et al. I Solid Stcrte Ionics 100 (1997) 233-240
1
I_
7ie---
20
00
-00
-2s
Fig. 2. Deconvolution of the ‘%NMR spectrum Pb, w% MF, 06 (-: deconvoluted spectrum).
at 250 K for
convolution of the whole spectrum. Fig. 2 gives, as an example, the dcconvolution of the “F-NMR spectrum at 250 K for the Pb0.,,4A10,0hF2,0h composition. The fluoride ion rates considered as proportional to the areas of the different peaks are deduced from deconvolutions relative to each temperature. The temperature dependence of the rate (J;,) of fluoride ions mobile at the NMR time scale, determined in this way between 200 and 430 K is given in Fig. 3. At low temperatures (T(270 K), the temperature of appearance of the p,,, peak is all the lower and j;, is all the larger as x is higher; above T=270 K, x” increases all the quicker as x is smaller and it results that the maximum off,, equal to 1, is reached at T, =400 K for the Pb, _XAl,F,, I com-
positions relative to x=0.03 and 0.06 and only al T, =430 K for those relative to x=0.09 and 0.12. On the other hand. whatever x, a chemical shifting a(~,,,) of the p,, peak towards low frequencies can be observed on the “F-NMR spectrum when tcmperature increases (Fig. 4a). Considering the temperature range (2505Ts370 K), IS(p,,,)I, which is all the larger as x is smaller, appears tightly correlated to the f, increase in this temperature range. On the contrary, the maxima values of IS(l>,,,)I, I[S(y,)J,,,,I, obtained for each composition at the temperature T,, are all the larger as x is higher (Fig. 4b). The “F-NMR spectrum at various temperatures between 175 and 430 K is given in Fig. 5 for the fluoride Pb,,Al?F,,. The comparison of this spectrum with those of Ph, _,AI,F,+, compositions shows the existence of common characteristics: For the low temperatures
l
(200ST(K)S250),
the
4PWW-N -0.03
.
0.00
yzi:E
j:
.
(a)
100
’
7!00
-.
z.00
300
360
*.
*.
100
T(K) 400
I:-
Od. od
04, 0.2.
(b)
X
0
I 100
VK) zoo
2m
300
Jo0
400
400
Fig. 3. Temperature dependence of/,. rate of Iluoride ions mobile at the NMR time scale for the Pb , rAIzF,_x compositions studied.
I-
OdO
0.1
0.10
0.l
Fig. 4. (a) I‘hermal variation of the chemical shifting $p,,) for the Ph, .AI,F2+, compositions studied (b) variation of ![S(JJ,)],,~,~ as a function ofx for Ph,_,Al,F>,, and Pb.,hl,F,,.
M. El Oman’ et al. I Solid State Ionics 100 (1997) 233-240
115
210 250 200 345 400 420
(KHz) Fig. 5. Thermal Pb,Al,F,,.
variation
of
the
‘9F-NMR
spectrum
for
presence of the p, and p2 peaks located respectively at = -23 and =5 kHz. The appearance of the p, peak at rising temperature and the coalescing of the peaks pz with p,, then p, with p,. At the highest experimental temperature (T=430 K), the p, peak represents 86% of the total peak; yet, it remains, at that temperature, a residual contribution of the p, peak. The temperature T, could not consequently be determined for it could be estimated some thirty Pb,Al,F,,; degrees higher than 430 K. The value of IS( at T=430 K, considered as I[& p,)] max/ for is reported in the variation of Pb,Al,F,,, I[S(p,)],,,l vs. x (Fig. 4b). The point representative of Pb,Al,F,, is located in the lengthening of the curve corresponding to Pb,_,Al,F,+,. This result is in favor of the existence of a short-range order in Pb 1_XAl,F2+X deriving probably from the long range order in Pb,Al,F,,. The 19F-NMR spectrum of Pb&l,F,, at very low temperatures offers also some particular features. As a matter of fact, the spectrum observed at T= 175 K for Pb,A1,F2, is even more asymmetrical than the spectra relative to the Pb 1_,A1,FZcx compositions and the peak p, observed at T=250 K must be considered as the merge of two elementary peaks p, 1 and P,~ at T= 175 K (Fig. 5). On the other hand, on can remark that the diffusion properties of the F- ions at the NMR time scale
237
in Pb,Al,F,, appear independent of structural properties. As a matter of fact, Pb&l,F,, is characterized by a phase transition at T= 365 t 10 K and no peculiar difference is detected between the 19F-NMR spectra collected at T=345 and 400 K (Fig. 5). The comparison of the 19F-NMR spectra of different Pb, _,Al,F1+, compositions studied and Pb&l,F,, corresponding to a same temperature has allowed to identify different types of fluoride ions in these materials. The experimental temperatures selected are 175 K, which is in fact a rigid lattice temperature and 345 K, a significative temperature for which there is an important coalescing of the peaks p2 and p, but only a very small merging between the peaks p, and pm. Fig. 6a and b give the different spectra observed at T= 175 and 345 K. It results from the comparison of the “F-NMR spectra relative to T= 175 K that the P,~ and p12 peaks must be taken into account not only for Pb,Al,F,, but also for the Pbr_,Al,F,+, compositions; the p12 intensity increases when x increases, i.e. when the substitutional cation rate is growing. Examination of the spectra corresponding to T = 345
x = 0.182 0.12 0.09 0.66 0.03 T=345K
x = 0.182 0.12 0.09 0.06 0.03
(KHz) Fig. 6. Composition dependence of the ‘9F-NMR spectrum for Pb,-AF,+x and Pb,Al,F,, (x =0.182) at (a) T= 175 K and (b) T=345 K.
238
M. El Omari ct al. / Solid Stare Ionics 100 (1997) 233-240
K shows that the p1 peak is all the larger as x increases and its inlensity is maximum for Pb,Al,F*,. These comments induce us to assign the p,, and p,2 peaks to fluoride ions located in the closest surroundings of Al” cations and consequently the p2 peak to fluoride ions bound to Pb*cations . . The deconvolution of the “F-NMR spectra observed at T= 175 K, temperature of rigid lattice, has allowed the determination of the relative compositions of the different peaks and to deduce the percentage of fluoride ions rcprcsented by these peaks. The best simulation conditions possible have been sought from the Pb,A12F2, cp ectrum which offers the largest asymmetry. The simulation of this spectrum by the help of three Gaussian functions has not been enough; whatever the parameters selected, it was always a residue, small but not insignificant. The fitting has been obtained by splitting the pz peak into two peaks, called p2, and pz2, represented by Gaussian functions. It results that the dcconvolution of the whole spectrum of Pb,AI,F>, at 175 K consists of four gaussian functions (Fig. 7a). The ‘OF-NMR spectra at 175 K of the different Pb, XAI,F,_I compositions was dcconvolutcd in the same conditions as Pb,Al?F,, and consisted also of four Gaussian functions (Fig. 7b, c). The simulation parameters arc reported in Table 2. The deconvolution of spectra in four peaks shows the existence of four types of fluoride ions in these materials. The fuoridc ions located close to Al3 ’ cations and reprcscnted by the p, , and p,* peaks will be noted (F,,),, and (F,;,)*, respectively. In the same way, the fluoride ions located close to Pb” cations and simulated by the p2, and p2* peaks will be called Table 3 gives the (Fi )I’,> and (Fn)r,, respectively. percentages of fluoride ions represented by the p, peaks and their numbers per unit cell, ni =p,(2+x), for the Pb, rAI,F2_X compositions and the Pb9A12F2, (X= 0.182) ordered phase. The variation of numbers n, as a function of x is given in Fig. 8. n(F,),,, decreases quasi-linearly with increasing x and the extrapolation of this line at x=0 results in a n(F,,),, value very close to 2. Such a result allows the identification of (F”)r,,, to fluoride ions located in the normal sites (I /4, I /4, I /4) of the fluorite lattice. n(F,),,, n(F,,),, and n(F,,),,, increase regularly with increasing x and can be
(b)
x=0.02
50
25
5a
25
00
-25
-50
-25
-50
I
Fig.
7. Deconvolution
Pb,AI,F2,,
of
W Pb,, .,,A4 J,
I”F-NMR
spectrum
at 175 K for (a)
oi and (~1 Pb, ,,A~,, 2,
oq.
identified with three distinct sublattices of interstitial fluoride ions. The increases of n(Frd)*, and n(F,,,),, with x arc not linear; that of n(F,),, is, as n(F,),,,, quasi-linear with a slope close to 3 and the extrapola-
Table 2 Simulation Pb, ..4l,pz,, 0.182)
Line shape
Position-O.5 (kHz)
FWHH”‘-0.5 (kHz)
0.03
p2,
(Gaussian) (Gaussian) (Gaussian) (Gaussian) (Gaussian) (Gaussian) (Gaussian) (Gaussian) (Gaussian) (Gaussian) pI , (Gaussian) p,? (Gaussian) pz, (Gaussian) pz2 (Gaussian) p,, (Gaussian) plz (Gaussian) pr, (Gaussian) p2> (Gaussian) p, I (Gaussian) plz (Gaussian)
3 -5 -- 15.2 -23.6 3.8
6.8 12 25 10.2 15.3 12.9 23.4 11.5 I R.5 12 27.1 13.5 20.9 13.8 26.4 14.2 27.3 15.8 27.9 18
0.06
0.09
0.12
0. I82
n
spectra at 175 K for parameters of the “INMK (x = 0.03, 0.06, 0.09, 0.12) and Pb,AI,F,, (x=
x
pz2 p,, P,~ p2, pr2 p,, p12 pz, p2*
5.4 -21.6 - 30.4 4 -. 4.7 -21.5 - 33.3 5 -5.1 -21.8 -. 34.3 6.7 -6.7 - 20.8 - 34.6
2
t
0.0 .
“‘“\__ --._ -_ -i
o.*.
4.*/_____-----+
::
x *
0 0
0.01
0.00
0.12
0.10
0.04
0.w
0.12
0.10
I
n
1-
0
’ FWHH:
239
I(X) (19Y7) 233-240
M. I:1 Omori et (11. I Solid Stute Ionics
full width at half-height. Fig. 8. Composition
dependence
of ~z(P,)~,,, n(F,),,,
n(Pr,),,
and
n(P,,),,.
tion at x=0 results in a n(Fi)r,, value close to zero. The variation of n(Fi),,,, with x, of (y=3x) type, would be in agreement with the following substitution model:
fluoride ion into an interstitial site to ensure the electrical neutrality and the crossing of two fluoride ions from normal sites into interstitial sites, creating in that way two vacancies (Cl) in normal sites. On the other hand, whatever n,, the points representative of Pb,A12F2, are located in the lengthening of the line (or curve) relative to Pb,_,AI,F,_,, result confirming once more that the short range order in
Pb’ ’ + 2F,, + A13.:- + 3F- + 20
i.e. the replacement of one Pb2.’ ion by one A13-’ ion would involve the introduction of a supplementary
Table 3 Percentages Pb,AI,F,, x
of fluoride ions rcpresentcd
0.06 0.09 0.12 0.182
5.5
x.5 12 17 24.5
.I and
“1
P, PZl
0.03
by the p, peaks at T= 175 K and numbers of fluoride ions n, per unit cell for Pb,_JI,F,
PI2
PI,
PI2
(Pa),,
0=x,),.,
(F,, ),t,
(Fr,),,,
87 76 68.5 59 41.5
5.5 9 10.5 13.5 18
2 6.5 9 10.5 16
0.11-‘0.03 0.18-tO.03 0.25 20.03 0.36+0.03 0.54-co.03
I .77 -1-0.0s
0. I I zo.03 0.19zo.03 0.22zo.03 0.2g+o.o3 0.39 ro.03
o.w-‘-0.03 0.13 20.03 0.19-‘0.03 0.23+0.03 0.35-to.03
1.56-‘0.05 1.43~0.05 1.25:x0.05 0.91 TO.05
240
M.
El Omari
et al.
I Solid
Pb, .,AI,F,+, and are strongly
the long range order in Pb,AI,F2, correlated.
4. Conclusions The “F-NMR investigation of different compositions of the Pb , _XAI,F,. ~ solid solution and of the ordered Pb,Al,F,, phase shows that the short-range order in Pb, _,AI,F,+ ~ derives from the long-range order in Pb,Al,F,,.The dcconvolution, for each fluoride studied, of the “F-NMR spectrum of rigid lattice (T= 175 K) into four Gaussian functions induces the existence of four fluoride sublattices. Both of them, p,, and P,~, represent the (F,,,),,, and (F,,),,, anions located in the surroundings of Al’+ cations, both the others, p2, and pzr have been attributed respectively lo intcrstitial (F,),,,, and normal (Fn),, anions located in the surroundings of Pb2 ‘. cations. The variation of numbers of fluoride ions belonging to each fluoride sublattice has been determined as a function of X: decreases, n(F,),,, n(F,,),, and n(F,,),, n(F,),,, increase when x increases. The variation of n(Fi),+ with x is of (p=3x) type, in agreement with the substitution model: Pb”
+ 2F,T +A13’
The “F-NMR
+ 3F, + 20
(0: vacancy)
study in a wide temperature
range
State
( 125-430 K) has allowed the identification of the nature of the fluoride sublatticc mobile at increasing temperature. The fluoride ions, the most mobile at the NMR time scale are first the (F,),,, and then and (F,,),,, become, in turn, (P;,)p,,; the (F,,),, mobile at a higher temperature.
References Bull. Elcctrochcm. I I (19%) 34. 121J.-M. Reau, Xu Yong Jun. J. SenCgas, P. Hagenmuller, Solid State Ionicx 78 ( 1995) 315. 1x1E.F. Hairetdinov, N.F. Uvarov, .M. Wahbi, J.-M. Reau, Xu Yong Jun. P. Hagenmuller, Solid State lonics 86-88 (1996) 113. J. Ravcz, D. Dumora. C.R. Acad. Sci. 269C ( 1969) 33 I. A.V. Joshi, C.C. Liang, J. Electrochern. Sot. 124 (1977) 1253. J61 L.V. Moulton, R.S. Feigelson, J. Mater. Rcs. 6 (1991) 2188. 171 M. El Omari, J.-M. RCau, J. Ravez, Phase l‘ransitions (1997) in press. S. Kacim, J.C. Champarnaud-Mesjard, B. Frit, Rev. Chim. Min. 19 (1982) 99. J.P. Laval, C. Depierrefixe, B. Frit, G. Roult. J. Solid State Chem. 54 ( 1984) 260. S. Sarrautc, J. Ravez, Phase Transitions 57 (1996) 225. WJNFJ?‘ program, Bruker Spectrospin S.A., lineshape litting, version 950425. order No. SPP505.
[II J.-M. Ritau, I? Hagenmuller,
CZ$!3
SOLID STATE
EISEVIER
Short-range
IONICS
Solid State lonics 100 (1997) 233-240
order and F- ion diffusion inside the Pb I _,A1,F2,, solid solution Part 1: ‘“F-NMR investigation
Mohamed El Omari”, Jean Senegasb, Jean-Maurice Reaub** ‘Dqx~r~ment of Chemistry. Faculty of Scirnce, Moulry I.wuil Uniwmity, Meknes. Morocco bbrstitut de Chimie de la Matitre
Condenke
de Bordeaux
CNRS, Chateau
Brivazac,
F-33600
Pc.s,wc. France
Received 24 March 1997; accepted 24 April 1997
Abstract (O%c~O.12) solid solution and of the ordered Pb,AI,F?, phase is A “F-NMR investigation of’ the Pb,_,Al,l:,+,~ undertaken as a function of temperature. Several F - ion groups become progressively mobile at the NMR time scale at increasing temperature, the first, (F,),,, and (F,,),,,, are located in the surroundings of PbZ ’ cations, the second ones, (I;,,),, in that of AI’- cations. Four types of fluoride ions are distinguished in the 19F-NMR spectra of rigid lattice at and (PJ,,,, low temperature (T= 175 K) and the distribution of fluoride ions among the four sites is dctcrmincd by deconvolution of the spectra rcgistercd. The sites occupied by the (F,,),,, anions are identified with the normal anionic sites of the fluorite network, the others with different interstitial sites. Kepmrds:
F
ionic conduction: “F-NMK investigation; Fluorite-type
1. Introduction Various clustering processes wet-c proposed within the anion-excess M:‘,Mi2 “F,,,, (U = 1, 2) solid
solutions with fluorite-type str&turc. They dcpcnd on the nature of the cationic (M, M’) couples present. A clustering process model was set up relating, in a continuous way, the composition dependence of electrical propcrtics and the progrcssivc extension of clustering when x increases. It is based on results issued on the one hand from
“Corresponding author: Tel: + 33 5 5684 6332; 5 5684 6634: c-mail: [email protected] 0167-2738/97/$17.00 Q 1997 Elsevicr PII SO167-2738(97)00340-8
fax:
..33
Science B.V. All rights reserved
snucture
conductivity measurements and on the other hand from neutron diffraction and/or “F-NMR investigations [I ,2]. Three clustering processes have been in this way characterized: l
Larger and larger extension of column clusters involving single-tiles of vacancies (n + 1:2n: 1:O in or two-files of vacancies (2n + Pb,-Pi,F,+,) 2:3n:2:0 in Pb, rlnrF2__x and 2n+2:4n:2:0 in as x increases. Such clusters, Pb, rzrrF9+2r) labclled n,:n,:n,:n,, are electrically neutral and based on the association of n, vacancies in the normal anionic positions (I /4, l/4, l/4) of the fluorite-type network, n2 F’ (112, U, u: 0.355~5 0.40) n3 F” (u,, u,, u,: u, -0.41) and n4 F”’ (u,,
234
M. El Chari
c/ al. I Solid Sk&
interstitial fluoride ions, u 2’ u2: 0.28~u,~O.33) close to one or several substitutional cations. Progressive transformation of 4:4:3:0 clusters into 8: 12: I:0 cubooctahedral clusters in Ba, ,Bi,F, + ~ and Ca I-IM’IF2+x where M’ is Y or a small size rare earth (Er:..., Lu). Progressive transformation of clusters 3:0:3:2 (in where M’ is La,..., Gd and in Ca, .M:F,+, U), 3:2:3:0 (in Ba,_,M:‘F, ,_2X with M”=Th, Sr,..,Bi,F,.,>), 4:0:3:3 (in Ca,_,yM,rF,+,, and into Sr, _,M:‘F, , 2.r), 8:0:3:7 (in Pb,_,MtFZ_21) condensed 2:0:6:0 clusters. The 3:0:3:2, 3:2:3:0, 4:0:3:3 and 8:0:3:7 clusters derive from the same 1:0:3:0 basic cluster [2,3]. Inside all solid solutions studied till now, the substitutional cation is of a size large enough in order to be located in eight- or nine-coordinated sites. But what happens when the substitutional cation is of small size? To answer that question, we are interested in the fluorite-type Pb, _IAI,F,+ ‘: solid solution, which offers a trivalent cation of very small size. solid solution is long-range The Pb, AAl,F,+, disordered and has only a small existence range (O~x~O.12), due to the large size difference between the host and guest cations. An ordered Buoride, of Pb,AI,F2, composition, is located close to in the PbF,-AIF, system [4-61; it is PbI-,Al,F*+, characterized by a reversible phase transition at T, = 36.5-C IO K 171. A comparative study of electrical properties of Pb,_,Al,F,,, and Pb,Al,F?, has shown that the performance of the low- and hightemperature forms of Pb,AI,F,, are respectively close to those relative to (X=0.03) and (O.O6
Ionics
100 (1997)
233-240
2. Experiments The lead fluoride PbF, used (Ccrac 99.99%) was previously dried under vacuum at 473 K for 10 h. Aluminium fluoride AIF,, was prepared, as usual [IO], from Al,O, and (NH,)HF*, under dry nitrogen, at 1073 K for 5 h. AIF, was then treated under vacuum at 473 K, for 10 h. The (x=0.03; 0.06; 0.09; 0.12) Pb, _,AI,F,+, and Pb,AI,F,, samples were synthesized in the same conditions as those used for elect&al measurements 171. The mixtures of fluorides PbF2 and AIF, are ground in a dry box and inserted in a gold tube. After degassing under vacuum at 473 K during 2 h to eliminate the risk of hydrolysis at higher temperature, the gold tubes are filled up with dry argon and sealed. They arc then heated at the reaction temperature and cooled down to ambient temperatures. The thermal treatments of Pb, _,AI,rF, i_r; (0~~~0.12) and Pb,Al,F,, were 15 h at 873 K and quenching, 15 h at 753 K and natural cooling, respectively. were performed on a “F-NMR experiments Bruker MSL-200 spectrometer (H, =4.7 T) equipped with a standard variable temperature unit in the temperature range 125 to 430 K. For each temperature measurement, the powder samples are kept for 1 h at that tcmpcraturc, in order to reach a good thermal stability. Storage after 5 h of a large number of acquisitions allowed high resolution signals to be registered. The ‘Oncpulsc’ acquisition program was used in the following operating conditions gathered in Table 1. This program was selected because it delivers a spectral irradiation large enough to cover the different spectra1 widths of each type of F anions. As a matter of fact, different diffusion coefficients characterize the F ions in Pb, rAI,F1 rl and Pb,Al,F,,. Signals obtained were processed by Fourier transformation, using the ‘WINNMR ID’ program. ‘I’ablr:I Expcrimcntal Pb, ~Al,Fz.,
conditions of and Pb,IAlzt;2,
Spectrometer frequency Pulse width Dead rime delay Rccyclc delay time Spectral width Filter width
the
“F-NMR
investigarion
188.283 MHz 0.7 ps 6 KS IO s I MI-IL 2 MHz
of
M. El Omari rt ~1. I Solid Sture lonics 100 (1997) 233-240
235
Simulations of the 19F-NMR lines were performed using the WINFIT program [ 111. This program allows the adjustment of the peak position, peak height and line width ratio of Gaussian and Lorentzian functions and the relative percentage of their areas. When a single Gaussian does not fit exactly with the registered spectrum, an appropriate mixing of Gaussian and Lorentzian functions is used for the simulation. It was a case in particular for the spectra relative to the motional narrowing temperature range.
be simulated by the help of Gaussian functions. All F- ions arc consequently fixed in the NMR time scale in this temperature domain. Above T=200 K, a new peak called p,, located between the p, and p? peaks appears and grows with rising temperature. Simulated by a Lorentzian function, it represents the fluoride ions which are mobile at the NMR time scale above that tempcraturc. When tcmpcraturc incrcascs, the p2 peak, then the peak p, coalesce in turn with the p,, peak. This means that the number of mobile fluoride ions increases at the expense of the fluoride ions of p2 type, then of p,
3. r9F-NMR
type. Above a temperature T, , the p2 and p, peaks are completely vanished and the p,, peak is only observed on the “F-NMR spectrum. All the fluoride ions are consequently mobile at the NMR time scale above that temperature. T, is equal to 400 K for x=0.03 and 0.06, it is a little higher for x=0.09 and 0. I2 (T, =430 K). The rclativc contributions of the different peaks observed above T=200 K are determined by de-
results
The ‘“F-NMR spectrum at various temperatures is given in Fig. la, b, c, d for some Pb, _XAl,F,_, compositions corresponding to x = 0.03, 0.06, 0.09 and 0.12. At low temperatures (T < 175 K) two peaks called p, and p2 are detected at frequencies equal to = -20 and =5 kHz, respectively. The origin of the frequency scale corresponds to the nominal irradiation frequency (188.283 MHz). Both these peaks can
Ix-o.oB
(a)
40
20
Fi g. I. Thermal
variation
of the “F-NMR
-40
00
(KHz) x=0.09 and (d) x=0.12.
w1
RI
-60
(KH;;o spectrum
for rhe Pb,
1AlJ=z. compositions x
corresponding
to (a) x=0.03,
(b) x =0.06.
(c)
236
M. El Omari et al. I Solid Stcrte Ionics 100 (1997) 233-240
1
I_
7ie---
20
00
-00
-2s
Fig. 2. Deconvolution of the ‘%NMR spectrum Pb, w% MF, 06 (-: deconvoluted spectrum).
at 250 K for
convolution of the whole spectrum. Fig. 2 gives, as an example, the dcconvolution of the “F-NMR spectrum at 250 K for the Pb0.,,4A10,0hF2,0h composition. The fluoride ion rates considered as proportional to the areas of the different peaks are deduced from deconvolutions relative to each temperature. The temperature dependence of the rate (J;,) of fluoride ions mobile at the NMR time scale, determined in this way between 200 and 430 K is given in Fig. 3. At low temperatures (T(270 K), the temperature of appearance of the p,,, peak is all the lower and j;, is all the larger as x is higher; above T=270 K, x” increases all the quicker as x is smaller and it results that the maximum off,, equal to 1, is reached at T, =400 K for the Pb, _XAl,F,, I com-
positions relative to x=0.03 and 0.06 and only al T, =430 K for those relative to x=0.09 and 0.12. On the other hand. whatever x, a chemical shifting a(~,,,) of the p,, peak towards low frequencies can be observed on the “F-NMR spectrum when tcmperature increases (Fig. 4a). Considering the temperature range (2505Ts370 K), IS(p,,,)I, which is all the larger as x is smaller, appears tightly correlated to the f, increase in this temperature range. On the contrary, the maxima values of IS(l>,,,)I, I[S(y,)J,,,,I, obtained for each composition at the temperature T,, are all the larger as x is higher (Fig. 4b). The “F-NMR spectrum at various temperatures between 175 and 430 K is given in Fig. 5 for the fluoride Pb,,Al?F,,. The comparison of this spectrum with those of Ph, _,AI,F,+, compositions shows the existence of common characteristics: For the low temperatures
l
(200ST(K)S250),
the
4PWW-N -0.03
.
0.00
yzi:E
j:
.
(a)
100
’
7!00
-.
z.00
300
360
*.
*.
100
T(K) 400
I:-
Od. od
04, 0.2.
(b)
X
0
I 100
VK) zoo
2m
300
Jo0
400
400
Fig. 3. Temperature dependence of/,. rate of Iluoride ions mobile at the NMR time scale for the Pb , rAIzF,_x compositions studied.
I-
OdO
0.1
0.10
0.l
Fig. 4. (a) I‘hermal variation of the chemical shifting $p,,) for the Ph, .AI,F2+, compositions studied (b) variation of ![S(JJ,)],,~,~ as a function ofx for Ph,_,Al,F>,, and Pb.,hl,F,,.
M. El Oman’ et al. I Solid State Ionics 100 (1997) 233-240
115
210 250 200 345 400 420
(KHz) Fig. 5. Thermal Pb,Al,F,,.
variation
of
the
‘9F-NMR
spectrum
for
presence of the p, and p2 peaks located respectively at = -23 and =5 kHz. The appearance of the p, peak at rising temperature and the coalescing of the peaks pz with p,, then p, with p,. At the highest experimental temperature (T=430 K), the p, peak represents 86% of the total peak; yet, it remains, at that temperature, a residual contribution of the p, peak. The temperature T, could not consequently be determined for it could be estimated some thirty Pb,Al,F,,; degrees higher than 430 K. The value of IS( at T=430 K, considered as I[& p,)] max/ for is reported in the variation of Pb,Al,F,,, I[S(p,)],,,l vs. x (Fig. 4b). The point representative of Pb,Al,F,, is located in the lengthening of the curve corresponding to Pb,_,Al,F,+,. This result is in favor of the existence of a short-range order in Pb 1_XAl,F2+X deriving probably from the long range order in Pb,Al,F,,. The 19F-NMR spectrum of Pb&l,F,, at very low temperatures offers also some particular features. As a matter of fact, the spectrum observed at T= 175 K for Pb,A1,F2, is even more asymmetrical than the spectra relative to the Pb 1_,A1,FZcx compositions and the peak p, observed at T=250 K must be considered as the merge of two elementary peaks p, 1 and P,~ at T= 175 K (Fig. 5). On the other hand, on can remark that the diffusion properties of the F- ions at the NMR time scale
237
in Pb,Al,F,, appear independent of structural properties. As a matter of fact, Pb&l,F,, is characterized by a phase transition at T= 365 t 10 K and no peculiar difference is detected between the 19F-NMR spectra collected at T=345 and 400 K (Fig. 5). The comparison of the 19F-NMR spectra of different Pb, _,Al,F1+, compositions studied and Pb&l,F,, corresponding to a same temperature has allowed to identify different types of fluoride ions in these materials. The experimental temperatures selected are 175 K, which is in fact a rigid lattice temperature and 345 K, a significative temperature for which there is an important coalescing of the peaks p2 and p, but only a very small merging between the peaks p, and pm. Fig. 6a and b give the different spectra observed at T= 175 and 345 K. It results from the comparison of the “F-NMR spectra relative to T= 175 K that the P,~ and p12 peaks must be taken into account not only for Pb,Al,F,, but also for the Pbr_,Al,F,+, compositions; the p12 intensity increases when x increases, i.e. when the substitutional cation rate is growing. Examination of the spectra corresponding to T = 345
x = 0.182 0.12 0.09 0.66 0.03 T=345K
x = 0.182 0.12 0.09 0.06 0.03
(KHz) Fig. 6. Composition dependence of the ‘9F-NMR spectrum for Pb,-AF,+x and Pb,Al,F,, (x =0.182) at (a) T= 175 K and (b) T=345 K.
238
M. El Omari ct al. / Solid Stare Ionics 100 (1997) 233-240
K shows that the p1 peak is all the larger as x increases and its inlensity is maximum for Pb,Al,F*,. These comments induce us to assign the p,, and p,2 peaks to fluoride ions located in the closest surroundings of Al” cations and consequently the p2 peak to fluoride ions bound to Pb*cations . . The deconvolution of the “F-NMR spectra observed at T= 175 K, temperature of rigid lattice, has allowed the determination of the relative compositions of the different peaks and to deduce the percentage of fluoride ions rcprcsented by these peaks. The best simulation conditions possible have been sought from the Pb,A12F2, cp ectrum which offers the largest asymmetry. The simulation of this spectrum by the help of three Gaussian functions has not been enough; whatever the parameters selected, it was always a residue, small but not insignificant. The fitting has been obtained by splitting the pz peak into two peaks, called p2, and pz2, represented by Gaussian functions. It results that the dcconvolution of the whole spectrum of Pb,AI,F>, at 175 K consists of four gaussian functions (Fig. 7a). The ‘OF-NMR spectra at 175 K of the different Pb, XAI,F,_I compositions was dcconvolutcd in the same conditions as Pb,Al?F,, and consisted also of four Gaussian functions (Fig. 7b, c). The simulation parameters arc reported in Table 2. The deconvolution of spectra in four peaks shows the existence of four types of fluoride ions in these materials. The fuoridc ions located close to Al3 ’ cations and reprcscnted by the p, , and p,* peaks will be noted (F,,),, and (F,;,)*, respectively. In the same way, the fluoride ions located close to Pb” cations and simulated by the p2, and p2* peaks will be called Table 3 gives the (Fi )I’,> and (Fn)r,, respectively. percentages of fluoride ions represented by the p, peaks and their numbers per unit cell, ni =p,(2+x), for the Pb, rAI,F2_X compositions and the Pb9A12F2, (X= 0.182) ordered phase. The variation of numbers n, as a function of x is given in Fig. 8. n(F,),,, decreases quasi-linearly with increasing x and the extrapolation of this line at x=0 results in a n(F,,),, value very close to 2. Such a result allows the identification of (F”)r,,, to fluoride ions located in the normal sites (I /4, I /4, I /4) of the fluorite lattice. n(F,),,, n(F,,),, and n(F,,),,, increase regularly with increasing x and can be
(b)
x=0.02
50
25
5a
25
00
-25
-50
-25
-50
I
Fig.
7. Deconvolution
Pb,AI,F2,,
of
W Pb,, .,,A4 J,
I”F-NMR
spectrum
at 175 K for (a)
oi and (~1 Pb, ,,A~,, 2,
oq.
identified with three distinct sublattices of interstitial fluoride ions. The increases of n(Frd)*, and n(F,,,),, with x arc not linear; that of n(F,),, is, as n(F,),,,, quasi-linear with a slope close to 3 and the extrapola-
Table 2 Simulation Pb, ..4l,pz,, 0.182)
Line shape
Position-O.5 (kHz)
FWHH”‘-0.5 (kHz)
0.03
p2,
(Gaussian) (Gaussian) (Gaussian) (Gaussian) (Gaussian) (Gaussian) (Gaussian) (Gaussian) (Gaussian) (Gaussian) pI , (Gaussian) p,? (Gaussian) pz, (Gaussian) pz2 (Gaussian) p,, (Gaussian) plz (Gaussian) pr, (Gaussian) p2> (Gaussian) p, I (Gaussian) plz (Gaussian)
3 -5 -- 15.2 -23.6 3.8
6.8 12 25 10.2 15.3 12.9 23.4 11.5 I R.5 12 27.1 13.5 20.9 13.8 26.4 14.2 27.3 15.8 27.9 18
0.06
0.09
0.12
0. I82
n
spectra at 175 K for parameters of the “INMK (x = 0.03, 0.06, 0.09, 0.12) and Pb,AI,F,, (x=
x
pz2 p,, P,~ p2, pr2 p,, p12 pz, p2*
5.4 -21.6 - 30.4 4 -. 4.7 -21.5 - 33.3 5 -5.1 -21.8 -. 34.3 6.7 -6.7 - 20.8 - 34.6
2
t
0.0 .
“‘“\__ --._ -_ -i
o.*.
4.*/_____-----+
::
x *
0 0
0.01
0.00
0.12
0.10
0.04
0.w
0.12
0.10
I
n
1-
0
’ FWHH:
239
I(X) (19Y7) 233-240
M. I:1 Omori et (11. I Solid Stute Ionics
full width at half-height. Fig. 8. Composition
dependence
of ~z(P,)~,,, n(F,),,,
n(Pr,),,
and
n(P,,),,.
tion at x=0 results in a n(Fi)r,, value close to zero. The variation of n(Fi),,,, with x, of (y=3x) type, would be in agreement with the following substitution model:
fluoride ion into an interstitial site to ensure the electrical neutrality and the crossing of two fluoride ions from normal sites into interstitial sites, creating in that way two vacancies (Cl) in normal sites. On the other hand, whatever n,, the points representative of Pb,A12F2, are located in the lengthening of the line (or curve) relative to Pb,_,AI,F,_,, result confirming once more that the short range order in
Pb’ ’ + 2F,, + A13.:- + 3F- + 20
i.e. the replacement of one Pb2.’ ion by one A13-’ ion would involve the introduction of a supplementary
Table 3 Percentages Pb,AI,F,, x
of fluoride ions rcpresentcd
0.06 0.09 0.12 0.182
5.5
x.5 12 17 24.5
.I and
“1
P, PZl
0.03
by the p, peaks at T= 175 K and numbers of fluoride ions n, per unit cell for Pb,_JI,F,
PI2
PI,
PI2
(Pa),,
0=x,),.,
(F,, ),t,
(Fr,),,,
87 76 68.5 59 41.5
5.5 9 10.5 13.5 18
2 6.5 9 10.5 16
0.11-‘0.03 0.18-tO.03 0.25 20.03 0.36+0.03 0.54-co.03
I .77 -1-0.0s
0. I I zo.03 0.19zo.03 0.22zo.03 0.2g+o.o3 0.39 ro.03
o.w-‘-0.03 0.13 20.03 0.19-‘0.03 0.23+0.03 0.35-to.03
1.56-‘0.05 1.43~0.05 1.25:x0.05 0.91 TO.05
240
M.
El Omari
et al.
I Solid
Pb, .,AI,F,+, and are strongly
the long range order in Pb,AI,F2, correlated.
4. Conclusions The “F-NMR investigation of different compositions of the Pb , _XAI,F,. ~ solid solution and of the ordered Pb,Al,F,, phase shows that the short-range order in Pb, _,AI,F,+ ~ derives from the long-range order in Pb,Al,F,,.The dcconvolution, for each fluoride studied, of the “F-NMR spectrum of rigid lattice (T= 175 K) into four Gaussian functions induces the existence of four fluoride sublattices. Both of them, p,, and P,~, represent the (F,,,),,, and (F,,),,, anions located in the surroundings of Al’+ cations, both the others, p2, and pzr have been attributed respectively lo intcrstitial (F,),,,, and normal (Fn),, anions located in the surroundings of Pb2 ‘. cations. The variation of numbers of fluoride ions belonging to each fluoride sublattice has been determined as a function of X: decreases, n(F,),,, n(F,,),, and n(F,,),, n(F,),,, increase when x increases. The variation of n(Fi),+ with x is of (p=3x) type, in agreement with the substitution model: Pb”
+ 2F,T +A13’
The “F-NMR
+ 3F, + 20
(0: vacancy)
study in a wide temperature
range
State
( 125-430 K) has allowed the identification of the nature of the fluoride sublatticc mobile at increasing temperature. The fluoride ions, the most mobile at the NMR time scale are first the (F,),,, and then and (F,,),,, become, in turn, (P;,)p,,; the (F,,),, mobile at a higher temperature.
References Bull. Elcctrochcm. I I (19%) 34. 121J.-M. Reau, Xu Yong Jun. J. SenCgas, P. Hagenmuller, Solid State Ionicx 78 ( 1995) 315. 1x1E.F. Hairetdinov, N.F. Uvarov, .M. Wahbi, J.-M. Reau, Xu Yong Jun. P. Hagenmuller, Solid State lonics 86-88 (1996) 113. J. Ravcz, D. Dumora. C.R. Acad. Sci. 269C ( 1969) 33 I. A.V. Joshi, C.C. Liang, J. Electrochern. Sot. 124 (1977) 1253. J61 L.V. Moulton, R.S. Feigelson, J. Mater. Rcs. 6 (1991) 2188. 171 M. El Omari, J.-M. RCau, J. Ravez, Phase l‘ransitions (1997) in press. S. Kacim, J.C. Champarnaud-Mesjard, B. Frit, Rev. Chim. Min. 19 (1982) 99. J.P. Laval, C. Depierrefixe, B. Frit, G. Roult. J. Solid State Chem. 54 ( 1984) 260. S. Sarrautc, J. Ravez, Phase Transitions 57 (1996) 225. WJNFJ?‘ program, Bruker Spectrospin S.A., lineshape litting, version 950425. order No. SPP505.
[II J.-M. Ritau, I? Hagenmuller,