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NMR and XAS study on doped LaF3
SOUD STATE lONgS
Solid State Ionics 52 (1992) 327-331 North-Holland
NMR and XAS study on doped LaF3 M.A. Denecke, W. G u n B e r Institute for Physical Chemistry, Universityof Hamburg, BundesstraJ3e45, D-2000 Hamburg 13, Germany A.V. P r i v a l o v a n d I.V. M u r i n Department of Chemistry, Leningrad State University, Universitetskaya nab 7/9, 199164 St. Petersburg, Russia Received 20 October 199l; acceptedfor publication 25 October 1991
Temperature dependent NMR measurementsas well as XAS investigations on La~_~r~Fj_x (x= 0.0, 0.01, 0.05, 0.16) have been performed to study the fluorine diffusion in these solid electrolytesand to elucidate the effect that doping with heterovalent SrF2 has on LaF3. A three-stageprocessof diffusion was observed. Upon increasingtemperature, initial ionic exchangebeginsin the F, sublanice. At even higher temperatures, dependent upon dopant concentration, ionic exchange between the FI and F2.3 sublattices takes place. A further increase in temperature eventuallyleads to exchangeover all F- structural positions.
1. Introduction
For over 20 years the superionic conductor lanthanum trifluoride LaFa has been the subject of many studies. There are many materials based on this compound which are of practical importance as superionic conductors with very high ionic mobility. One of these is the solid solutions of SrF2 in LaFa. Many investigations of ionic mobility in LaF3 and other structural analogues have yielded contradictory results as to which of the fluorine sublattices in these compounds initially become mobile. In addition, the effect of doping with heterovalent SrF2 on these processes is of great importance. We have, therefore, performed nuclear magnetic resonance ( N M R ) and X-ray absorption spectroscopy (XAS) experiments on LaFa and solid solutions of SrF2 in LaF3 in order to clarify these questions. LaF3 possesses the same structure as the mineral tynosite, belonging to the P~cl space group [ 1-3 ]. In this structure (see fig. 1 ) the La-atoms form layers in the hexagonal ( a - b ) crystallographic plane. Each of the La cations is surrounded by nine fluorine nearest neighbors in a distorted tricapped trigonal prisma configuration. There are two additional F L Authorto whom correspondenceshould be addressed.
anions located somewhat further away, over the other two remaining faces of the prisma so that the coordination number of La in LaF3 may be considered to be 9 + 2 = I 1. Table 1 summarizes the atomic distances in the L a - F coordination polyhedron in LaF3. There exist three inequivalent fluorine sites in the ratio 12:4:2, designated F1, F2 and F3. In many experimental methods the F2 and F3 sites are nearly indistinguishable [4,5 ] so that the structure can be simplified and be considered to be composed of two fluorine sublattices Fm and F2,3. Reports found in the literature as to which of these sublattices initially becomes mobile are contradictory. Some authors conclude that the more mobile fluorine ions are those in the F1 sublattice [6-9 ], while other authors maintain the F2,a ions are faster [ 10-14]. It is obvious that the mechanism of fluorine diffusion in LaF3 is intricate and not yet resolved. In this paper we present 19F N M R studies on LaF3 and La~_~SrxF3_x as a function of temperature. N M R measurements were performed on single crystals in a magnetic field Bo= 7 T. The high spectral resolution, resulting from the more pronounced chemical shifts (CS) in a strong magnetic field, and detailed numerical calculations of the spectra allow us to separate and to identify the resonance components. From the temperature behavior of individual spec-
0167-2738/92/$ 05.00 © 1992 ElsevierSciencePublishers B.V. All rights reserved.
328
M.A. Denecke et al. / N M R and XAS study on doped LaF~
/ X, Fig, 1. Above: unit cell in LaF3; below: unit cell as viewed from above, along the c axis; small at right: corresponding views of the La first coordination shell. Table 1 La-F distances in the La first coordination sphere from ref. [ 3 ]. Ion
Number
Distance (A)
F2 F3 Fi Fl Fl Ft
2 1 2 2 2 2
2.4224 2.4467 2.4672 2.4925 2.6466 3.0084
tral components, the ionic m o t i o n o f the sublattices was determined. XAS m e a s u r e m e n t s were also perf o r m e d to elucidate the effect that doping has on LaF3.
2. Experimental The 19F N M R spectra were measured at temperatures ranging from 130 K to 490 K with a frequency o f 282 M H z using a Bruker CXP-300 spectrometer. U n d e c o u p l e d spectra were o b t a i n e d by F o u r i e r t r a n s f o r m a t i o n o f the F I D after a 90 ° pulse. The simulation technique described in ref. [ 3 ] was used to calculate spectra. XAS m e a s u r e m e n t s were card e d out at the E X A F S II Beamline o f the H a m b u r ger S~nchrotronstrahlungslabor ( H A S Y L A B ) using a Si (111 ) double crystal m o n o c h r o m a t o r . To optimize sample homogeneity, the fluoride powders were p r e p a r e d as pressed pellets with polyethylene as support material. D a t a analysis was done analogously to that reported in ref. [ 15 ].
M.A. Denecke et a L I NMR and XAS study on doped LaF~
329
3. Results and discussion
In fig. 2 the dipolar broadened spectrum of a LaF3 single crystal for Bollc axis at 173 K is shown along with the calculated spectra of all three sublattices. The dipolar spectrum consists of two components, differing in their CS, with a 2:1 intensity ratio. Upon comparing experimental with calculated data, one can assign the larger signal as being from the F~ sublattice fluorine atoms and the smaller one from the superposition of the F 2 and Fa components. The calculations also show that the doublet splitting of the F~ signal is caused by the strong dipole-dipole interaction between F~ ions and their nearest neighboring F 1 ions 2.42 A away in the c-direction. The temperature dependent behavior of the LaF3 was also investigated. The spectra for this compound, scaled to the same maximal amplitude and recorded at various temperatures, are shown in fig. 3. Above 290 K the F~ line loses its doublet structure, indicating that the onset of fluorine diffusion is in the F~ sublattice (correlation time~ l0 -4 s). Above 300 K the doublet structure has coalesced to a single peak. This peak then continues narrowing up to 410 K. In this temperature range the CS and the intensity ratio between the F~ and F~,3 signal exhibits no changes. This is also indicative that the F~ ions are the first to become mobile. Also, because the CS and width of the F2,3 peak remains broad, one may assume that these fluorine sites are not only structurally similar, but also similar dynamically, both remaining in relative slow motion.
//'/// ///~
/I
~. /" ,/ // /
~\
~,
/ /'
~ \
32O
\
" i/'
~
~00
/ /
x
/
~
~
, {I i
/+80 ~ 520
~
~
~.~/
Fig. 3. Dipolarbroadened spectra of LaFa at indicated temperatures in Kelvin.
X 0.16
O.OS
/-//~. ~ ~ ~'
~-
~"~
0.01
-.
,
0.0
200
300
400
S00
T/K/
~
7
3
}
Fig. 4. Schematic representation of the three stages of fluorine diffusion in Lal_~rxF3_x (x=O.O, 0.01, 0.05, 0.16) as elucidated from ngFNMR data (data points for the ion exchange in LaF3lie above 520 K).
¢
t
I0
i ~Hz
Fig. 2. (a) Dipolar broadened spectrum of a LaFs single crystal for Bol[caxis at 173 K; (b) calculated spectra of F~, 1=2,F~ and
F2,3.
At low temperatures similar behavior was also observed for the Lal_~SrxF3_x (x=0.01, 0.05, 0.16) crystals. The data for the samples studied is schematically represented in fig. 4. The lowest temperature (220 K) for the onset of the loss of the F~ doublet structure was recorded for Lao.95Sro.osF2.95.Upon increasing the temperature further changes in the NMR spectra of these doped crystals were observed.
330
M.A. Denecke et al. / N M R and XAS study on doped LaF~
2 "I0
/ S.45
5149'
'
'5:$3
.
.
.
.
5.$7 . .
5.61
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E [keY] Fig. 5. La Llll-edge XANES spectra of LaF3 (--) and Lao.9Sro.lF2.9 (...).
At a certain temperature, dependent upon the dopant concentration, ionic exchange begins between the F, and F2, 3 sublattiees. Evidence for this is the broadening of both signals (correlation time ~ 10 -4 s), a sharp increase in the F~/F2.3 intensity ratio and a decrease in the difference in the CS's. The minimum temperature recorded for this exchange was also found in Lao.95Sro.osF2.95, at 350 K. For LaF3, this temperature lies above 520 K. At even higher temperatures, eventually only one resonance line is observed. This reflects the fact that, at high temperatures, ionic diffusion occurs over all structural positions (correlation t i m e ~ 10 -5 s). The X-ray absorption near-edge structure (XANES) spectra of L a F 3 and Lao.9Sro.lF2.9 m e a s u r e d at the La Lln-edge are shown in fig. 5. It is evident that the introduction of one mole fraction SrF2 into L a F 3 leads to an increase in the white line intensity of the La Lm-edge. The LiH-edge white lines result from 2P3/2 to 5d transitions and this observed intensity increase is indicative of a more effective screening of the nuclear potential as seen by the 5d states [ 16]. Tentative results of extended X-ray absorption fine structure (EXAFS) measurements on CeF3:SrF2 solid solutions, also having tysonite structure, suggest that F - defects are preferentially formed in the F~ sublattice. Temperature-dependent EXAFS mea-
surements are now underway to test if these vacancies contribute to a greater mobility of this sublattice and verify our N M R results of the stages of ionic motion in these electrolytes.
4. Conclusion This investigation into the ionic diffusion process in four single crystals ofLa,_~SrxF3_x (x=0.0, 0.01, 0.05, 0.16) using N M R yields the following threestage process. Initially the ionic exchange begins in the F~ sublattice. A further increase in temperature leads to exchange between F~ and F2.3 sublattices. At even higher temperatures diffusion takes place over all F - positions. It is evident that for the samples studied x = 0 . 0 5 shows the lowest temperature for these stages.
Acknowledgement We wish to thank the Bundesminister f'tir Forschung und Technik for financial support. The project number is GU2HAM-O.
M.A. Denecke et al. / NMR and XAS study on doped LaFj
References [ 1] M. Mansmann, Z. Kristallogr. 122 (1965) 375. [2 ] A. Zalkin and D.H. Templeton, Acta Cryst. B41 ( 1985 ) 91. [3] B. Maximov and H. Schultz, Acta Cryst. B41 (1985) 88. [4] M. Goldman and L. Shen, Phys. Rev. 144 (1966) 321. [5] A.F. Privalov, H.-M. Vieth and I.V. Murin, J. Phys. Chem. Solids 50 (1989) 395. [6] IC Lee, Solid State Commun. 7 (1969) 367. [7] M. Goldman and L. Shen, Phys. Rev. 144 (1966) 321. [8] A.F. Privalov, H.-M. Vieth and I.V. Murin, in: Proc. XXIV Congress Ampere, Pozna6, Poland, eds. J. Stankowski, N.P. Pislewski and S. Idzak (1988) p. 79. [9] L. Shen, Phys. Rev. 172 (1968) 259.
331
[ 10] A.F. Aalders, A.F.M. Arts and H.W. de Wijin, Phys. Rev. B32 (1985) 5412. [ 11 ] A.G. Lundin, S.P. Gabuda and A.I. Lifshits, SOv.Phys. Solid State 9 (1967) 237. [ 12] F.C. Case and P.P. Mahendroo, J. Phys. Chem. Solids 42 (1981) 385. [13] A. Roos, F.C.M. van de Poi, R. Keim and J. Schoonman, Solid State Ionics 13 (1984) 191. [ 14] A. Roos, M. Buijs, ICE.D. Wapenaar and J. Schoonman, J. Phys. Chem. Solids 46 (1985) 655. [ 15 ] W. Niemann, W. Malzfeldt, P. Rabe and R. Haensel, Phys. Rev. B. 35 (1987) 1099. [ 16 ] M.A. Denecke and W. GunBer, ISSI Letters 2 ( 1991 ) 9.
Solid State Ionics 52 (1992) 327-331 North-Holland
NMR and XAS study on doped LaF3 M.A. Denecke, W. G u n B e r Institute for Physical Chemistry, Universityof Hamburg, BundesstraJ3e45, D-2000 Hamburg 13, Germany A.V. P r i v a l o v a n d I.V. M u r i n Department of Chemistry, Leningrad State University, Universitetskaya nab 7/9, 199164 St. Petersburg, Russia Received 20 October 199l; acceptedfor publication 25 October 1991
Temperature dependent NMR measurementsas well as XAS investigations on La~_~r~Fj_x (x= 0.0, 0.01, 0.05, 0.16) have been performed to study the fluorine diffusion in these solid electrolytesand to elucidate the effect that doping with heterovalent SrF2 has on LaF3. A three-stageprocessof diffusion was observed. Upon increasingtemperature, initial ionic exchangebeginsin the F, sublanice. At even higher temperatures, dependent upon dopant concentration, ionic exchange between the FI and F2.3 sublattices takes place. A further increase in temperature eventuallyleads to exchangeover all F- structural positions.
1. Introduction
For over 20 years the superionic conductor lanthanum trifluoride LaFa has been the subject of many studies. There are many materials based on this compound which are of practical importance as superionic conductors with very high ionic mobility. One of these is the solid solutions of SrF2 in LaFa. Many investigations of ionic mobility in LaF3 and other structural analogues have yielded contradictory results as to which of the fluorine sublattices in these compounds initially become mobile. In addition, the effect of doping with heterovalent SrF2 on these processes is of great importance. We have, therefore, performed nuclear magnetic resonance ( N M R ) and X-ray absorption spectroscopy (XAS) experiments on LaFa and solid solutions of SrF2 in LaF3 in order to clarify these questions. LaF3 possesses the same structure as the mineral tynosite, belonging to the P~cl space group [ 1-3 ]. In this structure (see fig. 1 ) the La-atoms form layers in the hexagonal ( a - b ) crystallographic plane. Each of the La cations is surrounded by nine fluorine nearest neighbors in a distorted tricapped trigonal prisma configuration. There are two additional F L Authorto whom correspondenceshould be addressed.
anions located somewhat further away, over the other two remaining faces of the prisma so that the coordination number of La in LaF3 may be considered to be 9 + 2 = I 1. Table 1 summarizes the atomic distances in the L a - F coordination polyhedron in LaF3. There exist three inequivalent fluorine sites in the ratio 12:4:2, designated F1, F2 and F3. In many experimental methods the F2 and F3 sites are nearly indistinguishable [4,5 ] so that the structure can be simplified and be considered to be composed of two fluorine sublattices Fm and F2,3. Reports found in the literature as to which of these sublattices initially becomes mobile are contradictory. Some authors conclude that the more mobile fluorine ions are those in the F1 sublattice [6-9 ], while other authors maintain the F2,a ions are faster [ 10-14]. It is obvious that the mechanism of fluorine diffusion in LaF3 is intricate and not yet resolved. In this paper we present 19F N M R studies on LaF3 and La~_~SrxF3_x as a function of temperature. N M R measurements were performed on single crystals in a magnetic field Bo= 7 T. The high spectral resolution, resulting from the more pronounced chemical shifts (CS) in a strong magnetic field, and detailed numerical calculations of the spectra allow us to separate and to identify the resonance components. From the temperature behavior of individual spec-
0167-2738/92/$ 05.00 © 1992 ElsevierSciencePublishers B.V. All rights reserved.
328
M.A. Denecke et al. / N M R and XAS study on doped LaF~
/ X, Fig, 1. Above: unit cell in LaF3; below: unit cell as viewed from above, along the c axis; small at right: corresponding views of the La first coordination shell. Table 1 La-F distances in the La first coordination sphere from ref. [ 3 ]. Ion
Number
Distance (A)
F2 F3 Fi Fl Fl Ft
2 1 2 2 2 2
2.4224 2.4467 2.4672 2.4925 2.6466 3.0084
tral components, the ionic m o t i o n o f the sublattices was determined. XAS m e a s u r e m e n t s were also perf o r m e d to elucidate the effect that doping has on LaF3.
2. Experimental The 19F N M R spectra were measured at temperatures ranging from 130 K to 490 K with a frequency o f 282 M H z using a Bruker CXP-300 spectrometer. U n d e c o u p l e d spectra were o b t a i n e d by F o u r i e r t r a n s f o r m a t i o n o f the F I D after a 90 ° pulse. The simulation technique described in ref. [ 3 ] was used to calculate spectra. XAS m e a s u r e m e n t s were card e d out at the E X A F S II Beamline o f the H a m b u r ger S~nchrotronstrahlungslabor ( H A S Y L A B ) using a Si (111 ) double crystal m o n o c h r o m a t o r . To optimize sample homogeneity, the fluoride powders were p r e p a r e d as pressed pellets with polyethylene as support material. D a t a analysis was done analogously to that reported in ref. [ 15 ].
M.A. Denecke et a L I NMR and XAS study on doped LaF~
329
3. Results and discussion
In fig. 2 the dipolar broadened spectrum of a LaF3 single crystal for Bollc axis at 173 K is shown along with the calculated spectra of all three sublattices. The dipolar spectrum consists of two components, differing in their CS, with a 2:1 intensity ratio. Upon comparing experimental with calculated data, one can assign the larger signal as being from the F~ sublattice fluorine atoms and the smaller one from the superposition of the F 2 and Fa components. The calculations also show that the doublet splitting of the F~ signal is caused by the strong dipole-dipole interaction between F~ ions and their nearest neighboring F 1 ions 2.42 A away in the c-direction. The temperature dependent behavior of the LaF3 was also investigated. The spectra for this compound, scaled to the same maximal amplitude and recorded at various temperatures, are shown in fig. 3. Above 290 K the F~ line loses its doublet structure, indicating that the onset of fluorine diffusion is in the F~ sublattice (correlation time~ l0 -4 s). Above 300 K the doublet structure has coalesced to a single peak. This peak then continues narrowing up to 410 K. In this temperature range the CS and the intensity ratio between the F~ and F~,3 signal exhibits no changes. This is also indicative that the F~ ions are the first to become mobile. Also, because the CS and width of the F2,3 peak remains broad, one may assume that these fluorine sites are not only structurally similar, but also similar dynamically, both remaining in relative slow motion.
//'/// ///~
/I
~. /" ,/ // /
~\
~,
/ /'
~ \
32O
\
" i/'
~
~00
/ /
x
/
~
~
, {I i
/+80 ~ 520
~
~
~.~/
Fig. 3. Dipolarbroadened spectra of LaFa at indicated temperatures in Kelvin.
X 0.16
O.OS
/-//~. ~ ~ ~'
~-
~"~
0.01
-.
,
0.0
200
300
400
S00
T/K/
~
7
3
}
Fig. 4. Schematic representation of the three stages of fluorine diffusion in Lal_~rxF3_x (x=O.O, 0.01, 0.05, 0.16) as elucidated from ngFNMR data (data points for the ion exchange in LaF3lie above 520 K).
¢
t
I0
i ~Hz
Fig. 2. (a) Dipolar broadened spectrum of a LaFs single crystal for Bol[caxis at 173 K; (b) calculated spectra of F~, 1=2,F~ and
F2,3.
At low temperatures similar behavior was also observed for the Lal_~SrxF3_x (x=0.01, 0.05, 0.16) crystals. The data for the samples studied is schematically represented in fig. 4. The lowest temperature (220 K) for the onset of the loss of the F~ doublet structure was recorded for Lao.95Sro.osF2.95.Upon increasing the temperature further changes in the NMR spectra of these doped crystals were observed.
330
M.A. Denecke et al. / N M R and XAS study on doped LaF~
2 "I0
/ S.45
5149'
'
'5:$3
.
.
.
.
5.$7 . .
5.61
.
E [keY] Fig. 5. La Llll-edge XANES spectra of LaF3 (--) and Lao.9Sro.lF2.9 (...).
At a certain temperature, dependent upon the dopant concentration, ionic exchange begins between the F, and F2, 3 sublattiees. Evidence for this is the broadening of both signals (correlation time ~ 10 -4 s), a sharp increase in the F~/F2.3 intensity ratio and a decrease in the difference in the CS's. The minimum temperature recorded for this exchange was also found in Lao.95Sro.osF2.95, at 350 K. For LaF3, this temperature lies above 520 K. At even higher temperatures, eventually only one resonance line is observed. This reflects the fact that, at high temperatures, ionic diffusion occurs over all structural positions (correlation t i m e ~ 10 -5 s). The X-ray absorption near-edge structure (XANES) spectra of L a F 3 and Lao.9Sro.lF2.9 m e a s u r e d at the La Lln-edge are shown in fig. 5. It is evident that the introduction of one mole fraction SrF2 into L a F 3 leads to an increase in the white line intensity of the La Lm-edge. The LiH-edge white lines result from 2P3/2 to 5d transitions and this observed intensity increase is indicative of a more effective screening of the nuclear potential as seen by the 5d states [ 16]. Tentative results of extended X-ray absorption fine structure (EXAFS) measurements on CeF3:SrF2 solid solutions, also having tysonite structure, suggest that F - defects are preferentially formed in the F~ sublattice. Temperature-dependent EXAFS mea-
surements are now underway to test if these vacancies contribute to a greater mobility of this sublattice and verify our N M R results of the stages of ionic motion in these electrolytes.
4. Conclusion This investigation into the ionic diffusion process in four single crystals ofLa,_~SrxF3_x (x=0.0, 0.01, 0.05, 0.16) using N M R yields the following threestage process. Initially the ionic exchange begins in the F~ sublattice. A further increase in temperature leads to exchange between F~ and F2.3 sublattices. At even higher temperatures diffusion takes place over all F - positions. It is evident that for the samples studied x = 0 . 0 5 shows the lowest temperature for these stages.
Acknowledgement We wish to thank the Bundesminister f'tir Forschung und Technik for financial support. The project number is GU2HAM-O.
M.A. Denecke et al. / NMR and XAS study on doped LaFj
References [ 1] M. Mansmann, Z. Kristallogr. 122 (1965) 375. [2 ] A. Zalkin and D.H. Templeton, Acta Cryst. B41 ( 1985 ) 91. [3] B. Maximov and H. Schultz, Acta Cryst. B41 (1985) 88. [4] M. Goldman and L. Shen, Phys. Rev. 144 (1966) 321. [5] A.F. Privalov, H.-M. Vieth and I.V. Murin, J. Phys. Chem. Solids 50 (1989) 395. [6] IC Lee, Solid State Commun. 7 (1969) 367. [7] M. Goldman and L. Shen, Phys. Rev. 144 (1966) 321. [8] A.F. Privalov, H.-M. Vieth and I.V. Murin, in: Proc. XXIV Congress Ampere, Pozna6, Poland, eds. J. Stankowski, N.P. Pislewski and S. Idzak (1988) p. 79. [9] L. Shen, Phys. Rev. 172 (1968) 259.
331
[ 10] A.F. Aalders, A.F.M. Arts and H.W. de Wijin, Phys. Rev. B32 (1985) 5412. [ 11 ] A.G. Lundin, S.P. Gabuda and A.I. Lifshits, SOv.Phys. Solid State 9 (1967) 237. [ 12] F.C. Case and P.P. Mahendroo, J. Phys. Chem. Solids 42 (1981) 385. [13] A. Roos, F.C.M. van de Poi, R. Keim and J. Schoonman, Solid State Ionics 13 (1984) 191. [ 14] A. Roos, M. Buijs, ICE.D. Wapenaar and J. Schoonman, J. Phys. Chem. Solids 46 (1985) 655. [ 15 ] W. Niemann, W. Malzfeldt, P. Rabe and R. Haensel, Phys. Rev. B. 35 (1987) 1099. [ 16 ] M.A. Denecke and W. GunBer, ISSI Letters 2 ( 1991 ) 9.