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Experimental study of βPbF2 thin films
Ekctrochimica
Am,
1977, Vol. 22. pp. 693-696.
PergamonPress. Printed in GreatBritain
EXPERIMENTAL J. D.
STUDY PISTRE,
Universitk
J.
OF j?PbFz THIN
FILMS
SALARDENNE and P. SMUTEK I, 33405 Talence, France
de Bordeaux
(Received 20 September 1976) Abstract-The high ionic conduction observed in pPbF2 single crystals and sintered samples has led us to study thin films of this compound. The films have been obtained using two methods: thermal evaporation in vacuum and cathode sputtering. The electric study of films involves an original method of admittance vs frequency measurement. The activation energy of conductivity is given for the strnetures in vacuum and when exposing them to the action of oxygen. The results of crystallographic structure studies (by X-rays), the search for impurities and the stoichiometry (by c( partichs retrodiffusion) are given for the two above mentioned methods. The observed variation of fluorine concentration all along the film thickness could be explained if one accepts a conduction by both anions and anion vacancies. The high anionic conductivity observed (about 10~7fi~‘_cm~’ at 323 K) in /IPbF2 is the basis of quite a lot of work either on single crystals[I, 2,3] or pressed pellets and thin films[3] or ‘sintered samples[4]. The activation energy obtained for conductivity, depends on the different samples used (0.6-0.7eV for single crystals, 0.47-0.51 eV in both
with a “Keithley 260” nanovolt-source. The current is measured and recorded by a “Keithley 410A” picoammeter associated with a recorder. With the nc method we obtain directly the real and imaginary parts of the complex admittance for each frequency with a synchronous detector. Figure 1 illustrates this method schematically. Method accuracy is & I%, for a current higher than 50 nA, as determined by measuring known resistors and capacitors. For lower values of the current, the uncertainty increases; according to the noise, this uncertainty can reach 10% for I = 5 nA.
sintered samples and pressed pellets, 0.55 eV in thin films with a coplanar configuration). Studies undertaken since those on samples including NaF or KF[S,h], have shown the considerable influence of these monovalent cation fluorides mixed in PbF2. This investigation for which only the first results are presented in this paper, is designed to show the eventual correlations between the electrical features
Nuclear analysis The study of c( particles retroditision[7,8] usually permits a very accurate quantitative analysis of the deposits and in the same way gives information on the possible concentration gradient of the different components, all along the film thickness. However, in the case of BPbF,, because of the wide lead spectrum, the identification of very small quantities of impurities in the deposit, is quite difficult. For the same reason it has not been possible to use the aX reaction method:Thus, this analysis gives information on the stoichiometry (ratio [F]/[Pb]) all along the film thickness.
observed and the characteristics (geometry, composition) of PPbF, thin films, and so permit a better comprehension of the conduction mechanisms. EXPERIMENTAL
Layered
TECHNIQUE
configurations
The films are deposited by thermal evaporation at pressures below 2.10-6 Torr. They are evaporated from MO boats and deposited on glass or mica at substrate temperatures of 373 K. The electrodes are Bi or Au, the rate of deposition is lO@s. Coplanar structure The electrodes are made by photogravure technique of a Ni film layered on a glass substrate, foflowed by the electrolytic deposition of the electrode metal. The fluoride is deposited on the electrodes either by thermal evaporation in vacuum (the same as for the layered configuration) or by cathode sputtering. Experiments have been made until now with AulPbF,IAu and AulPbF,/Cu structures. Electrical measurements With the dc method, the structures are polarized * Achieved with the help of DGRST, 75 7 0502. t Achieved at the Laboratoire de Chimie Bordeaux 1 University.
contract du
No.
EXPERlMENTAL
RESULTS
CrystuEIographic structure of the deposits The X-ray analyses? show that the cathode sputtered films were amorphous, whereas the thermal evaporated films were perfectly crystallized in the cubic form, with the (111) axis perpendicular to the substrate, whatever the substrate used or the lead fluoride evaporated ie orthorhombic (a) or cubic (p). Kennedy ef a![31 obtained the same results but with a growth in the direction of the (200) axis. We observed this orientation once, but we have not been able to point out what changes in experiment were responsible for this effect. In the case of a glass substrate we found parasitic X spectrum lines which could come from a chemical attack of the substrate.
Solide of
693
J. D. PISTKE,J.
694 PAR model
SALAKDENNE
AND
P. ShluTtK
efiect of reducible gases When we introduce some oxygen into the chamber whilst the structure is being polarized, the cell current increases almost instantaneously as a function of the partial pressure of oxygen. The results are reported Fig. 3. Nevertheless, when the struclure is polarized or when the reducible gas is maintained for a too long time, the cell sensibility decreases in a non reversible way (Fig. 4).
124
DISCUSSION
Fig.
1. Alternative
current
admittance
measurements
technique.
analysis
RetrodSgisiort
The results are reported in Table 1. The concentration of fluorite with regard to lead, measured at different film deepness, has been reported versus the film thickness (Figs 2a and 2b) for deposits of 450 and 2200 A. The value of the ratio has been found changing all along the film thickness. However the integral ratio was about 2 (Table 1). The fluorite concentration was higher near the external surface than in the proximity of the substrate. This resuh could be explained if we accept an attraction of the fluoride anions by the positive charge created by Auger effect at the film surface. This observed distribution is in agreement with the results of Lilly et aI[9] obtained on LaF,, concerning the formation of a positive space charge at the cathode when an electric field is applied to the structure. Conduction
measurements
We have reported in Table 2 the conductivity and the corresponding activation energies measured with an alternative current for the different structures, with an applied voltage of 100 mV. This conductivity has been obtained by extrapolating to o = 0, the circular part of the impedance diagram plot[Q]. We have also reported in this Table a “conductivity” measured by dc method.
We first consider the spread of the results due to the geometrical disposition of the electrodes and the kind of deposition. The cathode sputtered films and the thermal evaporated films do not contain the same quantity of impurities, as shown by the nuclear analysis results. Works done by Barriere and Dantoll 1, 121 on thermally evaporated AlF, and MgFz have shown that the obtained deposits contained much more Ca and K than the initial powder. This has been explained by a distillation effect in the boat. It seems possible in the case of OPbF, thin films that this variation in composition could involve the differences observed between the samples prepared by the two methods. Indeed measurements made by Schoonman et al[lO] on pressed pellets of PbF, doped MnF, have indicated the activation energies of 0.63 eV (by nmr) and 0.68 eV (by conduction). On the other hand, measurements made by Liang and Joshi[S] show that the activation energy of the conducting species in /IPbF, have changed from (0.62 + 0.024)eV for the pure product, to (0.18 + 0.012)eV for the product containing 0.1 mole % of KF. Moreover, the cathode sputtering films present a large deficit in fluorine anions, which seems electrically compensated by the presence of oxygen which has been found equably distributed in the film thickness. These films being amorphous, it could be considered that we have a PbF2, PbO mixture. So, the conduction mechanisms of these films might he very different from that of thermal evaporated films. On the other hand, the differences observed in conductivity, between the coplanar structure and the layered configuration, can be explained knowing the influence of the electrode-electrolyte interfacial effects, as it has been shown by Kennedy et al[3]. Indeed the ratio between the interfacial surface and the film thickness
Table 1. Nuclear analyses: tetrodiffusion Rutherford Kind
of target
Thickness 203
Sputtered PbF‘, un thick Carbonc
(A)
Ratio
[F]/[PbI
1.73 f O.Ob
0.46
500
Sputtered PbF, on thick Cnrbone
2003
Thermal evaporated on thin Carbone
PhF,
350
Thermal evaporated on thin Carbone
Pbb,
4w
Tlxrmal evaporated 0” thin Carbone
PbF,
2200
1.51 f rl.03
Undosable 1.86 f
0.12
0:29
f 9 + 0.5 5 40.5 5 4
Present hut not identifiable with cqtainty
II.045
Ovcrlappins
OV&tippiIlg of the pits Too weak
of the pits
to he idcnt18ed
Tooweakto be I.935 f
Undosable Cl,, and Cl,, K. Ca, Zn Mu. Ag, Cd, Sn.
Si 1 9 s:,
Sputtered PbF, on thick Carbone
Impunties %
Dasable
identified
wtth certainty with certainty
Experimental study of pPbF2 thin films
695
. (0) .
. w
.
.
. . l’ -. l* .
. l
l
2-
l
l.
.
. .
L5
.
l.
l
A L
.
.o $0” rrp 15 tn
$ s ZJ l
-
. . .
-* . . .-
I loo
0
I 200
I 300
Thickness,
1 400
a
Fig. 2(a). Behaviour of the ratio [F]/[Pb] vs thickness for a sample of 450 A. 4 .
(b)
. .
. l
353. 5
.
.
.
l
* _. ’
& LB* s
*
n.
. . *.*,_*
. 0. -
.
-
.
l
*.
.
. ..
.
. ‘.=.
.*.
: l
.
. *
.
. ..*
* . .
I
.._.
. .
.
l.
*. . l
.:
. ..*f
.
.*
.
l
.
.
l
l*
.
.
.
l .
.
.
.
. .* .*
.
. *
g
. .
*.
l. .
. . _
I-
: * .f .
8 ‘t 5
. .
Substrate I 1000
I 500
O
I IEoo
8,
Thickness,
Fig.
2(b). Behaviour
of the ratio [a/[PbJ
is very different, depending whether we have thick samples, thin coplanar structures or layered structures. Finally, for the thermal evaporated films, well crystallized in the cubic phase, the results obtained with the nuclear analysis seem to prove that the mobile Table 2. Conduction
measurements: Direct
Conductivities (n-‘.cm-‘) Sputtered electrodes
2 *
10-g
3
10-R
Y
I2QOO
us thickness for a sample of 2200 A.
species are both anions and anions vacancies. Indeed, at the anode side, the concentration ratio [F]/[Pb] is higher than 2. This is only possible if we have an anion migration due to the electric field; the fluorine anions necessarily occupying the interstitial sites (4 per lattice). On the other hand, at the cathode, the
conductivities
and activation
energies for
PbF,
Alternating
Current
Activation Encrgi’es W) 2 slopcs~ 0.36 and 0.56
Conductlvitics (n-‘,cm-‘) 25x
10 4
3 8 x
10-d
thin
films
Current Activation Pnargies @VI 046
evaporated electrodes AU-AU Layered
3 x lo-”
2 Slopes: 0.16 and 0.33
2.5 x 10-u
Unmeasurable (because of the shape of the admittance diagram)
J. D.
696
J. SALARDENNE
PISTRE,
AND
P. SMUTEK
Finally, we can note that the occupation of the anion vacancies by oxygen, observed by Schoonman and Macke in the case of PbBr, single crystals, could explain the aging effects shown by a decreasing of [T as the oxygen diffused into the films. CONCLUSION
P
I
0
1
II
2
3
Pressure
11
4
1
5
6
of oxygen
11
.
7
11
E
9
torr
IO
Fig. 3. Sensibility of a PbFz sample to a reducible gas. Electrodes were Bismuth; applied voltage 1ClOmV; temperature 353 K. o First experiment: anode on substrate side; l Second experiment: cathode on substrate side; V The same experiment after 5 h.
4
A more complete characterization of the interfacial electrochemical phenomena would give us a better understanding of the conduction mechanisms observed in these films and will be published in due
,Q h 3 \ : +‘x
;5 f f 5 0
2
We have studied the M-PPbF2-M film’s conductivity obtained by cathode sputtering and thermal evaporation, according to different configurations. The spread of the results, according to the different samples studied, can be simply explained if we take into account the conditions of preparation and the interfacial effects. We have been able to show with the. a particle retrodiffusion, that the mobile species were both anions and anion vacancies, in the case of well crystallized (cubic lattice) lead fluoride. This result is nevertheless not in agreement with some other authors[5, 6, lo], who consider the anions vacancies to be the only mobile species.
*.
\
course.
**I t~;~--_________________
REFERENCES I. C. Tubandt,
+-
1
l
2. 3.
I 0
IO
I 40
I 30
I 20
Time,
h
Fig. 4. Aging effect in a sample of PbF,. Electrodes were Bismuth ; voltage 50 mV applied only during measurement; temperature 333 K. + First experiment: aging under vacuum; 0 Second experiment: aging under 1 Torr of oxygen. (The correspondance between the two initial values of the current is casual.)
4. 5. 6. 7. 8. 9.
be more important pulling down the stability of the structure, as it has been observed. Such a concentration of fluorine ion vacancies might be responsible of the narrow positive space charge evidenced by Lilly et al. concentration
of fluorine ion vacancies than 4 per lattice without
can
IO. 11. 12. 13.
Z. anorg. Chem 115. 105 (1921). J. Schoonman. G. J. Dirksen and G. Blasse, J. solid state Gem. 7. 245 (1973). J. H. Kennedy, R. Miles and J. Hunter, J. electrochem. sot. 120, 1441 (1973). J. C. Gianduzzo, J. PistrC et J. Salardenne, Electra. SC. Tech. 2. 55 (1975). C. C. Liang and A. V. Joshi, J. electrochem. Sot. 122. 466 (1975). J. H. Kennedy and R. C. Miles. J. elechochem. Sot. 1233, 47 (1976). 8. Saboya, These de Doctorat de sp&cialiti, Bordeaux (1973). B. Saboya, J. F. Chemin, J. Roturier, A. Barr&e, Y. Danto et J. Salardenne, J. appl. Phys. 8. 1008 (1975). A. C. Lilly, Jr. B. C. La Roy, C. 0. Tiller and B. Whiting, J. e/e&-o&em Sot. 120. 1673 (1973). J. Schoonman, L. B. Eberl, C. H. Hsieh, and R. A. Huggins, J. appl. Phys. 46, 2873 (1975). A. Barri&e, Thise, Bordeaux (1976). Y. Danto, Th&se, Bordeaux (1976). J, Schoonman and A. J. H. Macke, J. solid state Chem. 5. 105 (1972).
Am,
1977, Vol. 22. pp. 693-696.
PergamonPress. Printed in GreatBritain
EXPERIMENTAL J. D.
STUDY PISTRE,
Universitk
J.
OF j?PbFz THIN
FILMS
SALARDENNE and P. SMUTEK I, 33405 Talence, France
de Bordeaux
(Received 20 September 1976) Abstract-The high ionic conduction observed in pPbF2 single crystals and sintered samples has led us to study thin films of this compound. The films have been obtained using two methods: thermal evaporation in vacuum and cathode sputtering. The electric study of films involves an original method of admittance vs frequency measurement. The activation energy of conductivity is given for the strnetures in vacuum and when exposing them to the action of oxygen. The results of crystallographic structure studies (by X-rays), the search for impurities and the stoichiometry (by c( partichs retrodiffusion) are given for the two above mentioned methods. The observed variation of fluorine concentration all along the film thickness could be explained if one accepts a conduction by both anions and anion vacancies. The high anionic conductivity observed (about 10~7fi~‘_cm~’ at 323 K) in /IPbF2 is the basis of quite a lot of work either on single crystals[I, 2,3] or pressed pellets and thin films[3] or ‘sintered samples[4]. The activation energy obtained for conductivity, depends on the different samples used (0.6-0.7eV for single crystals, 0.47-0.51 eV in both
with a “Keithley 260” nanovolt-source. The current is measured and recorded by a “Keithley 410A” picoammeter associated with a recorder. With the nc method we obtain directly the real and imaginary parts of the complex admittance for each frequency with a synchronous detector. Figure 1 illustrates this method schematically. Method accuracy is & I%, for a current higher than 50 nA, as determined by measuring known resistors and capacitors. For lower values of the current, the uncertainty increases; according to the noise, this uncertainty can reach 10% for I = 5 nA.
sintered samples and pressed pellets, 0.55 eV in thin films with a coplanar configuration). Studies undertaken since those on samples including NaF or KF[S,h], have shown the considerable influence of these monovalent cation fluorides mixed in PbF2. This investigation for which only the first results are presented in this paper, is designed to show the eventual correlations between the electrical features
Nuclear analysis The study of c( particles retroditision[7,8] usually permits a very accurate quantitative analysis of the deposits and in the same way gives information on the possible concentration gradient of the different components, all along the film thickness. However, in the case of BPbF,, because of the wide lead spectrum, the identification of very small quantities of impurities in the deposit, is quite difficult. For the same reason it has not been possible to use the aX reaction method:Thus, this analysis gives information on the stoichiometry (ratio [F]/[Pb]) all along the film thickness.
observed and the characteristics (geometry, composition) of PPbF, thin films, and so permit a better comprehension of the conduction mechanisms. EXPERIMENTAL
Layered
TECHNIQUE
configurations
The films are deposited by thermal evaporation at pressures below 2.10-6 Torr. They are evaporated from MO boats and deposited on glass or mica at substrate temperatures of 373 K. The electrodes are Bi or Au, the rate of deposition is lO@s. Coplanar structure The electrodes are made by photogravure technique of a Ni film layered on a glass substrate, foflowed by the electrolytic deposition of the electrode metal. The fluoride is deposited on the electrodes either by thermal evaporation in vacuum (the same as for the layered configuration) or by cathode sputtering. Experiments have been made until now with AulPbF,IAu and AulPbF,/Cu structures. Electrical measurements With the dc method, the structures are polarized * Achieved with the help of DGRST, 75 7 0502. t Achieved at the Laboratoire de Chimie Bordeaux 1 University.
contract du
No.
EXPERlMENTAL
RESULTS
CrystuEIographic structure of the deposits The X-ray analyses? show that the cathode sputtered films were amorphous, whereas the thermal evaporated films were perfectly crystallized in the cubic form, with the (111) axis perpendicular to the substrate, whatever the substrate used or the lead fluoride evaporated ie orthorhombic (a) or cubic (p). Kennedy ef a![31 obtained the same results but with a growth in the direction of the (200) axis. We observed this orientation once, but we have not been able to point out what changes in experiment were responsible for this effect. In the case of a glass substrate we found parasitic X spectrum lines which could come from a chemical attack of the substrate.
Solide of
693
J. D. PISTKE,J.
694 PAR model
SALAKDENNE
AND
P. ShluTtK
efiect of reducible gases When we introduce some oxygen into the chamber whilst the structure is being polarized, the cell current increases almost instantaneously as a function of the partial pressure of oxygen. The results are reported Fig. 3. Nevertheless, when the struclure is polarized or when the reducible gas is maintained for a too long time, the cell sensibility decreases in a non reversible way (Fig. 4).
124
DISCUSSION
Fig.
1. Alternative
current
admittance
measurements
technique.
analysis
RetrodSgisiort
The results are reported in Table 1. The concentration of fluorite with regard to lead, measured at different film deepness, has been reported versus the film thickness (Figs 2a and 2b) for deposits of 450 and 2200 A. The value of the ratio has been found changing all along the film thickness. However the integral ratio was about 2 (Table 1). The fluorite concentration was higher near the external surface than in the proximity of the substrate. This resuh could be explained if we accept an attraction of the fluoride anions by the positive charge created by Auger effect at the film surface. This observed distribution is in agreement with the results of Lilly et aI[9] obtained on LaF,, concerning the formation of a positive space charge at the cathode when an electric field is applied to the structure. Conduction
measurements
We have reported in Table 2 the conductivity and the corresponding activation energies measured with an alternative current for the different structures, with an applied voltage of 100 mV. This conductivity has been obtained by extrapolating to o = 0, the circular part of the impedance diagram plot[Q]. We have also reported in this Table a “conductivity” measured by dc method.
We first consider the spread of the results due to the geometrical disposition of the electrodes and the kind of deposition. The cathode sputtered films and the thermal evaporated films do not contain the same quantity of impurities, as shown by the nuclear analysis results. Works done by Barriere and Dantoll 1, 121 on thermally evaporated AlF, and MgFz have shown that the obtained deposits contained much more Ca and K than the initial powder. This has been explained by a distillation effect in the boat. It seems possible in the case of OPbF, thin films that this variation in composition could involve the differences observed between the samples prepared by the two methods. Indeed measurements made by Schoonman et al[lO] on pressed pellets of PbF, doped MnF, have indicated the activation energies of 0.63 eV (by nmr) and 0.68 eV (by conduction). On the other hand, measurements made by Liang and Joshi[S] show that the activation energy of the conducting species in /IPbF, have changed from (0.62 + 0.024)eV for the pure product, to (0.18 + 0.012)eV for the product containing 0.1 mole % of KF. Moreover, the cathode sputtering films present a large deficit in fluorine anions, which seems electrically compensated by the presence of oxygen which has been found equably distributed in the film thickness. These films being amorphous, it could be considered that we have a PbF2, PbO mixture. So, the conduction mechanisms of these films might he very different from that of thermal evaporated films. On the other hand, the differences observed in conductivity, between the coplanar structure and the layered configuration, can be explained knowing the influence of the electrode-electrolyte interfacial effects, as it has been shown by Kennedy et al[3]. Indeed the ratio between the interfacial surface and the film thickness
Table 1. Nuclear analyses: tetrodiffusion Rutherford Kind
of target
Thickness 203
Sputtered PbF‘, un thick Carbonc
(A)
Ratio
[F]/[PbI
1.73 f O.Ob
0.46
500
Sputtered PbF, on thick Cnrbone
2003
Thermal evaporated on thin Carbone
PhF,
350
Thermal evaporated on thin Carbone
Pbb,
4w
Tlxrmal evaporated 0” thin Carbone
PbF,
2200
1.51 f rl.03
Undosable 1.86 f
0.12
0:29
f 9 + 0.5 5 40.5 5 4
Present hut not identifiable with cqtainty
II.045
Ovcrlappins
OV&tippiIlg of the pits Too weak
of the pits
to he idcnt18ed
Tooweakto be I.935 f
Undosable Cl,, and Cl,, K. Ca, Zn Mu. Ag, Cd, Sn.
Si 1 9 s:,
Sputtered PbF, on thick Carbone
Impunties %
Dasable
identified
wtth certainty with certainty
Experimental study of pPbF2 thin films
695
. (0) .
. w
.
.
. . l’ -. l* .
. l
l
2-
l
l.
.
. .
L5
.
l.
l
A L
.
.o $0” rrp 15 tn
$ s ZJ l
-
. . .
-* . . .-
I loo
0
I 200
I 300
Thickness,
1 400
a
Fig. 2(a). Behaviour of the ratio [F]/[Pb] vs thickness for a sample of 450 A. 4 .
(b)
. .
. l
353. 5
.
.
.
l
* _. ’
& LB* s
*
n.
. . *.*,_*
. 0. -
.
-
.
l
*.
.
. ..
.
. ‘.=.
.*.
: l
.
. *
.
. ..*
* . .
I
.._.
. .
.
l.
*. . l
.:
. ..*f
.
.*
.
l
.
.
l
l*
.
.
.
l .
.
.
.
. .* .*
.
. *
g
. .
*.
l. .
. . _
I-
: * .f .
8 ‘t 5
. .
Substrate I 1000
I 500
O
I IEoo
8,
Thickness,
Fig.
2(b). Behaviour
of the ratio [a/[PbJ
is very different, depending whether we have thick samples, thin coplanar structures or layered structures. Finally, for the thermal evaporated films, well crystallized in the cubic phase, the results obtained with the nuclear analysis seem to prove that the mobile Table 2. Conduction
measurements: Direct
Conductivities (n-‘.cm-‘) Sputtered electrodes
2 *
10-g
3
10-R
Y
I2QOO
us thickness for a sample of 2200 A.
species are both anions and anions vacancies. Indeed, at the anode side, the concentration ratio [F]/[Pb] is higher than 2. This is only possible if we have an anion migration due to the electric field; the fluorine anions necessarily occupying the interstitial sites (4 per lattice). On the other hand, at the cathode, the
conductivities
and activation
energies for
PbF,
Alternating
Current
Activation Encrgi’es W) 2 slopcs~ 0.36 and 0.56
Conductlvitics (n-‘,cm-‘) 25x
10 4
3 8 x
10-d
thin
films
Current Activation Pnargies @VI 046
evaporated electrodes AU-AU Layered
3 x lo-”
2 Slopes: 0.16 and 0.33
2.5 x 10-u
Unmeasurable (because of the shape of the admittance diagram)
J. D.
696
J. SALARDENNE
PISTRE,
AND
P. SMUTEK
Finally, we can note that the occupation of the anion vacancies by oxygen, observed by Schoonman and Macke in the case of PbBr, single crystals, could explain the aging effects shown by a decreasing of [T as the oxygen diffused into the films. CONCLUSION
P
I
0
1
II
2
3
Pressure
11
4
1
5
6
of oxygen
11
.
7
11
E
9
torr
IO
Fig. 3. Sensibility of a PbFz sample to a reducible gas. Electrodes were Bismuth; applied voltage 1ClOmV; temperature 353 K. o First experiment: anode on substrate side; l Second experiment: cathode on substrate side; V The same experiment after 5 h.
4
A more complete characterization of the interfacial electrochemical phenomena would give us a better understanding of the conduction mechanisms observed in these films and will be published in due
,Q h 3 \ : +‘x
;5 f f 5 0
2
We have studied the M-PPbF2-M film’s conductivity obtained by cathode sputtering and thermal evaporation, according to different configurations. The spread of the results, according to the different samples studied, can be simply explained if we take into account the conditions of preparation and the interfacial effects. We have been able to show with the. a particle retrodiffusion, that the mobile species were both anions and anion vacancies, in the case of well crystallized (cubic lattice) lead fluoride. This result is nevertheless not in agreement with some other authors[5, 6, lo], who consider the anions vacancies to be the only mobile species.
*.
\
course.
**I t~;~--_________________
REFERENCES I. C. Tubandt,
+-
1
l
2. 3.
I 0
IO
I 40
I 30
I 20
Time,
h
Fig. 4. Aging effect in a sample of PbF,. Electrodes were Bismuth ; voltage 50 mV applied only during measurement; temperature 333 K. + First experiment: aging under vacuum; 0 Second experiment: aging under 1 Torr of oxygen. (The correspondance between the two initial values of the current is casual.)
4. 5. 6. 7. 8. 9.
be more important pulling down the stability of the structure, as it has been observed. Such a concentration of fluorine ion vacancies might be responsible of the narrow positive space charge evidenced by Lilly et al. concentration
of fluorine ion vacancies than 4 per lattice without
can
IO. 11. 12. 13.
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