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Physical vapour deposition of rare-earth fluorides
] O U R N A L OF
Journal of Non-Crystalline Solids 161 (1993) 23-26 North-Holland
~l~J~i|.|.~
~.~I~
Physical vapour deposition of rare-earth fluorides O. P e r r o t , B. B o u l a r d a n d C. J a c o b o n i Laboratoire des Fluorures, URA CNRS 449, Facultd des Sciences, Universitd du Maine, 72017 Le Mans c~dex, France
A systematic study of evaporation for single or binary mixtures of rare earth (RE) fluorides (LaF3, PrF3, NdF 3, GdF 3, ErF 3 and YbF 3) has been performed. Single RE fluorides follow the physical evaporation laws, while for binary RE fluoride mixtures the role of crystal chemistry on the evaporation behaviour has been shown.
1. Introduction Vapour phase deposition has attracted great attention as a technique for producing highly pure glasses. The possibility of obtaining transition metal fluoride glasses ( P b F 2 - Z n F 2 - G a F 3 = P Z G glass) by this method has been demonstrated earlier [1-3]. More recently, rare earth (RE) doped fluoride glasses have received increasing interest due to their ability to generate a large number of wavelengths in the visible and IR and to amplify optical signals in fibres. The challenge is thus to introduce R E ions in the films; since their vapour pressure is much lower than P Z G starting components (at least eight orders of magnitude) they must be evaporated at a higher temperature and so in separated crucibles to achieve convenient doping level (i.e., 0.1-1%). The aim of this work is to measure the evaporation rates of the R E fluorides. Binary mixtures are also investigated since energy transfers are expected in co-doped systems ( Y b - E r , P r - E r , for example). The results are correlated to crystallographic and thermodynamic data.
2. Experimental conditions The evaporations are conducted in a 33 cm diameter inox vessel, connected to a vacuum sysCorrespondence to: Dr B. Boulard, Laboratoire des Fluorures, URA CNRS 449, Facult6 des Sciences, Universit6 du Maine, 72017, Le Mans c6dex, France. Telefax +33 43 83 35 06.
tern with a liquid nitrogen trap, allowing pressure around 10 - 4 mbar. The rare-earth fluorides are heated by RF coil (1000-1500°C) in a graphite crucible. The target holder, which is allowed to rotate and translate vertically above the crucible, can be heated up to 250°C to ensure a good adherence of the deposited film. For this preliminary study the target is a polished silica slice (25 mm diameter).
3. Results
3.1. Single RE fluoride evaporation The evaporation rates, or, of R E fluorides are measured at different temperatures in the range 1000-1300°C. A constant mass (200 mg) is heated in the graphite crucible for 20 min, and the crucible is weighed before and after evaporation (fig. 1). The sublimation is accompanied by a partial reduction of LnF 3 to LnF 2 caused by the graphite crucible. Reduction occurs at high temperature and is much more important for solids than for liquids because of the phenomenon of calefaction: liquids tend to form a ball which is insulated from the crucible by a thin layer of gas. No reduced species are detected on the X-ray spectra of the deposit. The divalent fluorides do not evaporate due to their lower vapour pressure compared to the trivalent one.
0022-3093/93/$06.00 © 1993 - Elsevier Science Publishers B.V. All rights reserved
24
O. Perrot et al. I
180
"~
I
Irj
'
/ Physical vapour deposition of rare-earth fluorides i
3.2. E v a p o r a t i o n o f binary fluorides m i x t u r e s x R F 3 - ( 1 -- x ) R ' F 3
Sublimation • vaporation'T :/" o YFs ,, ~/,'
160
•
E 140
:://;
L~ a
,'; :~ ;,' ,it
V PrF 3 • NdFa
,2o
[]
"~
/
Odr
~"Ybr3~i/" eJ' . Zrrs
100
",~
The drastic conditions of quenching for films (from a vapour) as compared to bulk samples (from a liquid) may enhance different site distributions for R E in the glassy matrix. In order to analyze the optical properties and site distributions of active R E as a function of doping rate, a global constant proportion of R E is required. This can be achieved by a substitution between active (ErF3, YbF3, PrF 3 and NdF 3) and inactive R E fluorides (LaF3, YF 3 or GdF3). Binary mixtures of active R E for which energy transfer processes are expected, are also studied. Since the evaporation rates between the R E fluorides are different in most cases, non-congruent evaporation process is predicted. For this reason, a systematic study of binary systems at different compositions is necessary.
.
.J
8o ~6o 0 >
40
t y,y • • ~ . ,y,~o' :~=
zo 0
~li~==r'~'j T
1000
j-ll
I
11 O0
,
1200
I
1300
Temperature (oC) Fig. 1. Evaporated mass versus temperature for RE fluorides (starting charge: 200 mg; evaporation time: 20 min).
100
t
!
t ~
100
t
/
E 75
- LoFs/E~//!
_E 75 - •
/"
25 0
0
25
50
i 75
t .,/']
100
'
°
25
50
' A
/ ~ ']
1180°C
/
/'1
E75
Q ,4=
0
c 50
t
YFJFrFs
~o -
_c 50 ~ 25
1O0
0
0
25
50
75
1O0
0
0
1 O0
lOO I
,
75
I O0
~Erf'~i In the mixing
ZErF3 In the mixing
:grrf" 3 In the mixing
,
lOO
,
,
I 25
I 50
' /]
75 Q
_~ 50 J~
25
2"--,' 25
50
75
~gYbF3 in the mixing
1 O0
~ 25
~'~ 25 0
0
25
50
75
ZYbF3 In the mixing
1O0
0
/ 0
I 75
II I O0
ZYbF 3 In the mixing
Fig. 2. Comparison between starting charge and observed composition (in mol%) for binary mixtures of RE fluorides. The dot-dashed line represents the theoretical curve deduced from results of fig. 1.
25
O. Perrot et aL / Physical vapour deposition o f rare-earth fluorides
Table 1 Data [5] and results for evaporation of RE fluorides (ionic radii are given for eightfold coordination [6]
YF 3 LaF 3 PrF 3 NdF 3 GdF 3 ErF 3 YbF 3
r i (~k)
S.G.
Tm (°C)
AHvap (kcal/mole)
AHmelt (kcal/mole)
Evaporation type
Bexp ( x 104)
BKent ( X 104)
1.16 1.29 1.25 1.24 1.20 1.15 1.13
Pnma P63/mcm P63/mcm P63/mcm Pnma Pnma Pnma
1152 1490 1395 1374 1231 1140 1157
60 62 62 62 60 60 60
13 8 8 8 8 8 8
vaporization sublimation a) sublimation a) sublimation a) sublimation b) vaporization vaporization
1.57 2.09 1.80 1.57 1.37 1.07 1.02
2.19 2.02 1.87 1.91 -
a) Reduction. b) Melting at 1190°C.
To get homogeneous results the binary mixtures are first heated at 1200°C during 20 h in platinum tubes sealed under inert atmosphere. The formation of a total solid solution is confirmed by X-ray powder diffraction between fluorides having the same crystalline structure (i.e., hexagonal or orthorhombic), while fluorides with a different structure only form partial solid solution. R E fluorides are deposited on polished silica substrates distant of 4 cm from the crucible for 5 min (during this time the composition of the charge hardly varies). The composition of the crystallized deposit is analyzed by I C P - A E S (inductively coupled plasma-atomic emission spectrometry) after acidic attack; it is supposed to be the same as the vapour and to be constant for a short deposition time. X-ray micro-analysis performed with SEM (scanning electronic microscopy) confirms an homogeneous composition of the deposit on the whole surface of the substrate Figure 2 gathers the relative compositions of the starting charge and deposit. The experimental curves are compared to the one deduced from the data obtained for single fluoride using the formula % R F 3 in the film a
OrRF3
aOrRF 3 + bO'RF3 '
where a is % RF 3 in the mixture and b is % R ' F 3 in the mixture.
4. Discussion
4.1. Single RE fluoride evaporation We have noticed two kinds of behaviour during the evaporation: (i) sublimation for LaF 3, PrF3, and NdF3, (ii) vaporization for ErF3, YbF 3 and YF 3. On the other hand, G d F 3 sublimates up to 1180°C, then melts and evaporates. The sublimation process induces a greater evaporation rate than for vaporization except for YF3; due to its similarity in ionic size, coordination number and valence state, this element is assimilated to rare earth;
Log (o-T1,/2) I
3.50 3.o
Y~3
I
"
l
~
•
-\\\
\\
NdF3 X~'~ \ \ \
•
2.s
O'
ErF 3 YbF 3
-
,
I
6.5
~,
X~&W
,X,
I
,
7.0
I
,
7.5
1/T (K-1"1 0'6) Fig. 3. Linear dependence of evaporation rate for RE fluorides.
26
O. Perrot et al. / Physical vapour deposition of rare-earth fluorides
however its higher evaporation rate compared to ErF 3 and YbF 3 is consistent with a lower molar weight. The theory of evaporation states that the evaporation rate, tr, is dependent of the temperature, T, following the law: log(~T 1/2) = C - B / T ; the slope B is the same as in the vapour pressure law: log p = A - B / T . Experimental curves of log(o-T 1/2) versus 1 / T show a linear variation (fig. 3) for each fluoride; B values are listed in table 1 and compared to those given by Kent [4] which correspond only to sublimation. The GdF 3 curve also exhibits linear variation; also it melts within the temperature range chosen for the measurements. This is not surprising since the enthalpies of vaporization and sublimation are close (60 and 68 kcal mol-1, respectively).
rate of the less volatile fluoride is increased in the case of solid solutions YF3-ErF 3 and YF 3YbF 3. For the ErF3-YbF 3 binary system, congruent evaporation is achieved.
5. Conclusion
The experimental conditions for the evaporation of the RE, fluorides have been determined. The doping rate of PZG films could be monitored either by changing the temperature o r / a n d the relative position of the crucibles. This work was granted by Groupement Scientifique 'Fibres actives et passives' with CNRS and Alcatel for financial support.
4.2. Evaporation of binary fluorides mixtures As shown in fig 2, strong deviation from theoretical compositions are observed for YFa-ErF 3, NdF3-ErF3, YF3-YbF 3. These results are related to miscibility between RE fluorides in the solid state. When the ionic radii of the RE are strongly different or, in other words, when the solid solution domain is narrow, the two curves are superimposed: the two fluorides are evaporated independently (this is the case for LaF3-ErF 3 and PrF3-YbF3). On the contrary, the evaporation
References [1] B. Boulard and C. Jacoboni, Mater. Res. Bull 25 (1990) 671. [2] C. Jacoboni and B. Boulard, patent, PCT/FR90/000083. [3] B. Boulard and C. Jacoboni, in: Proc. SPIE, Int. Soc. Opt. Eng. 1513 (Glasses Optoelectron. 2) (1991) p. 204. [4] R.A. Kent, in: Proc. Conf. on Nuclear Applied Non Fissionable Ceramics (1966) p. 249. [5] T. Moeller, in: Comprehensive Inorganic Chemistry (Pergamon, Oxford, 1973) table 34, p. 87. [6] Y.Q. Jia, J. Solid State Chem. 95 (1991) 184.
Journal of Non-Crystalline Solids 161 (1993) 23-26 North-Holland
~l~J~i|.|.~
~.~I~
Physical vapour deposition of rare-earth fluorides O. P e r r o t , B. B o u l a r d a n d C. J a c o b o n i Laboratoire des Fluorures, URA CNRS 449, Facultd des Sciences, Universitd du Maine, 72017 Le Mans c~dex, France
A systematic study of evaporation for single or binary mixtures of rare earth (RE) fluorides (LaF3, PrF3, NdF 3, GdF 3, ErF 3 and YbF 3) has been performed. Single RE fluorides follow the physical evaporation laws, while for binary RE fluoride mixtures the role of crystal chemistry on the evaporation behaviour has been shown.
1. Introduction Vapour phase deposition has attracted great attention as a technique for producing highly pure glasses. The possibility of obtaining transition metal fluoride glasses ( P b F 2 - Z n F 2 - G a F 3 = P Z G glass) by this method has been demonstrated earlier [1-3]. More recently, rare earth (RE) doped fluoride glasses have received increasing interest due to their ability to generate a large number of wavelengths in the visible and IR and to amplify optical signals in fibres. The challenge is thus to introduce R E ions in the films; since their vapour pressure is much lower than P Z G starting components (at least eight orders of magnitude) they must be evaporated at a higher temperature and so in separated crucibles to achieve convenient doping level (i.e., 0.1-1%). The aim of this work is to measure the evaporation rates of the R E fluorides. Binary mixtures are also investigated since energy transfers are expected in co-doped systems ( Y b - E r , P r - E r , for example). The results are correlated to crystallographic and thermodynamic data.
2. Experimental conditions The evaporations are conducted in a 33 cm diameter inox vessel, connected to a vacuum sysCorrespondence to: Dr B. Boulard, Laboratoire des Fluorures, URA CNRS 449, Facult6 des Sciences, Universit6 du Maine, 72017, Le Mans c6dex, France. Telefax +33 43 83 35 06.
tern with a liquid nitrogen trap, allowing pressure around 10 - 4 mbar. The rare-earth fluorides are heated by RF coil (1000-1500°C) in a graphite crucible. The target holder, which is allowed to rotate and translate vertically above the crucible, can be heated up to 250°C to ensure a good adherence of the deposited film. For this preliminary study the target is a polished silica slice (25 mm diameter).
3. Results
3.1. Single RE fluoride evaporation The evaporation rates, or, of R E fluorides are measured at different temperatures in the range 1000-1300°C. A constant mass (200 mg) is heated in the graphite crucible for 20 min, and the crucible is weighed before and after evaporation (fig. 1). The sublimation is accompanied by a partial reduction of LnF 3 to LnF 2 caused by the graphite crucible. Reduction occurs at high temperature and is much more important for solids than for liquids because of the phenomenon of calefaction: liquids tend to form a ball which is insulated from the crucible by a thin layer of gas. No reduced species are detected on the X-ray spectra of the deposit. The divalent fluorides do not evaporate due to their lower vapour pressure compared to the trivalent one.
0022-3093/93/$06.00 © 1993 - Elsevier Science Publishers B.V. All rights reserved
24
O. Perrot et al. I
180
"~
I
Irj
'
/ Physical vapour deposition of rare-earth fluorides i
3.2. E v a p o r a t i o n o f binary fluorides m i x t u r e s x R F 3 - ( 1 -- x ) R ' F 3
Sublimation • vaporation'T :/" o YFs ,, ~/,'
160
•
E 140
:://;
L~ a
,'; :~ ;,' ,it
V PrF 3 • NdFa
,2o
[]
"~
/
Odr
~"Ybr3~i/" eJ' . Zrrs
100
",~
The drastic conditions of quenching for films (from a vapour) as compared to bulk samples (from a liquid) may enhance different site distributions for R E in the glassy matrix. In order to analyze the optical properties and site distributions of active R E as a function of doping rate, a global constant proportion of R E is required. This can be achieved by a substitution between active (ErF3, YbF3, PrF 3 and NdF 3) and inactive R E fluorides (LaF3, YF 3 or GdF3). Binary mixtures of active R E for which energy transfer processes are expected, are also studied. Since the evaporation rates between the R E fluorides are different in most cases, non-congruent evaporation process is predicted. For this reason, a systematic study of binary systems at different compositions is necessary.
.
.J
8o ~6o 0 >
40
t y,y • • ~ . ,y,~o' :~=
zo 0
~li~==r'~'j T
1000
j-ll
I
11 O0
,
1200
I
1300
Temperature (oC) Fig. 1. Evaporated mass versus temperature for RE fluorides (starting charge: 200 mg; evaporation time: 20 min).
100
t
!
t ~
100
t
/
E 75
- LoFs/E~//!
_E 75 - •
/"
25 0
0
25
50
i 75
t .,/']
100
'
°
25
50
' A
/ ~ ']
1180°C
/
/'1
E75
Q ,4=
0
c 50
t
YFJFrFs
~o -
_c 50 ~ 25
1O0
0
0
25
50
75
1O0
0
0
1 O0
lOO I
,
75
I O0
~Erf'~i In the mixing
ZErF3 In the mixing
:grrf" 3 In the mixing
,
lOO
,
,
I 25
I 50
' /]
75 Q
_~ 50 J~
25
2"--,' 25
50
75
~gYbF3 in the mixing
1 O0
~ 25
~'~ 25 0
0
25
50
75
ZYbF3 In the mixing
1O0
0
/ 0
I 75
II I O0
ZYbF 3 In the mixing
Fig. 2. Comparison between starting charge and observed composition (in mol%) for binary mixtures of RE fluorides. The dot-dashed line represents the theoretical curve deduced from results of fig. 1.
25
O. Perrot et aL / Physical vapour deposition o f rare-earth fluorides
Table 1 Data [5] and results for evaporation of RE fluorides (ionic radii are given for eightfold coordination [6]
YF 3 LaF 3 PrF 3 NdF 3 GdF 3 ErF 3 YbF 3
r i (~k)
S.G.
Tm (°C)
AHvap (kcal/mole)
AHmelt (kcal/mole)
Evaporation type
Bexp ( x 104)
BKent ( X 104)
1.16 1.29 1.25 1.24 1.20 1.15 1.13
Pnma P63/mcm P63/mcm P63/mcm Pnma Pnma Pnma
1152 1490 1395 1374 1231 1140 1157
60 62 62 62 60 60 60
13 8 8 8 8 8 8
vaporization sublimation a) sublimation a) sublimation a) sublimation b) vaporization vaporization
1.57 2.09 1.80 1.57 1.37 1.07 1.02
2.19 2.02 1.87 1.91 -
a) Reduction. b) Melting at 1190°C.
To get homogeneous results the binary mixtures are first heated at 1200°C during 20 h in platinum tubes sealed under inert atmosphere. The formation of a total solid solution is confirmed by X-ray powder diffraction between fluorides having the same crystalline structure (i.e., hexagonal or orthorhombic), while fluorides with a different structure only form partial solid solution. R E fluorides are deposited on polished silica substrates distant of 4 cm from the crucible for 5 min (during this time the composition of the charge hardly varies). The composition of the crystallized deposit is analyzed by I C P - A E S (inductively coupled plasma-atomic emission spectrometry) after acidic attack; it is supposed to be the same as the vapour and to be constant for a short deposition time. X-ray micro-analysis performed with SEM (scanning electronic microscopy) confirms an homogeneous composition of the deposit on the whole surface of the substrate Figure 2 gathers the relative compositions of the starting charge and deposit. The experimental curves are compared to the one deduced from the data obtained for single fluoride using the formula % R F 3 in the film a
OrRF3
aOrRF 3 + bO'RF3 '
where a is % RF 3 in the mixture and b is % R ' F 3 in the mixture.
4. Discussion
4.1. Single RE fluoride evaporation We have noticed two kinds of behaviour during the evaporation: (i) sublimation for LaF 3, PrF3, and NdF3, (ii) vaporization for ErF3, YbF 3 and YF 3. On the other hand, G d F 3 sublimates up to 1180°C, then melts and evaporates. The sublimation process induces a greater evaporation rate than for vaporization except for YF3; due to its similarity in ionic size, coordination number and valence state, this element is assimilated to rare earth;
Log (o-T1,/2) I
3.50 3.o
Y~3
I
"
l
~
•
-\\\
\\
NdF3 X~'~ \ \ \
•
2.s
O'
ErF 3 YbF 3
-
,
I
6.5
~,
X~&W
,X,
I
,
7.0
I
,
7.5
1/T (K-1"1 0'6) Fig. 3. Linear dependence of evaporation rate for RE fluorides.
26
O. Perrot et al. / Physical vapour deposition of rare-earth fluorides
however its higher evaporation rate compared to ErF 3 and YbF 3 is consistent with a lower molar weight. The theory of evaporation states that the evaporation rate, tr, is dependent of the temperature, T, following the law: log(~T 1/2) = C - B / T ; the slope B is the same as in the vapour pressure law: log p = A - B / T . Experimental curves of log(o-T 1/2) versus 1 / T show a linear variation (fig. 3) for each fluoride; B values are listed in table 1 and compared to those given by Kent [4] which correspond only to sublimation. The GdF 3 curve also exhibits linear variation; also it melts within the temperature range chosen for the measurements. This is not surprising since the enthalpies of vaporization and sublimation are close (60 and 68 kcal mol-1, respectively).
rate of the less volatile fluoride is increased in the case of solid solutions YF3-ErF 3 and YF 3YbF 3. For the ErF3-YbF 3 binary system, congruent evaporation is achieved.
5. Conclusion
The experimental conditions for the evaporation of the RE, fluorides have been determined. The doping rate of PZG films could be monitored either by changing the temperature o r / a n d the relative position of the crucibles. This work was granted by Groupement Scientifique 'Fibres actives et passives' with CNRS and Alcatel for financial support.
4.2. Evaporation of binary fluorides mixtures As shown in fig 2, strong deviation from theoretical compositions are observed for YFa-ErF 3, NdF3-ErF3, YF3-YbF 3. These results are related to miscibility between RE fluorides in the solid state. When the ionic radii of the RE are strongly different or, in other words, when the solid solution domain is narrow, the two curves are superimposed: the two fluorides are evaporated independently (this is the case for LaF3-ErF 3 and PrF3-YbF3). On the contrary, the evaporation
References [1] B. Boulard and C. Jacoboni, Mater. Res. Bull 25 (1990) 671. [2] C. Jacoboni and B. Boulard, patent, PCT/FR90/000083. [3] B. Boulard and C. Jacoboni, in: Proc. SPIE, Int. Soc. Opt. Eng. 1513 (Glasses Optoelectron. 2) (1991) p. 204. [4] R.A. Kent, in: Proc. Conf. on Nuclear Applied Non Fissionable Ceramics (1966) p. 249. [5] T. Moeller, in: Comprehensive Inorganic Chemistry (Pergamon, Oxford, 1973) table 34, p. 87. [6] Y.Q. Jia, J. Solid State Chem. 95 (1991) 184.