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Structural analysis of AlF3CaF2YF3 glass by diffraction methods
] O U R N A L OF
Journal of Non-Crystalline Solids 140 (1992) 249-254 North-Holland
NON-CItYSTALLINE SOIIDS
Structural analysis of A1F3-CaF2-YF 3 glass by diffraction methods Y. A k a s a k a a T. N a n b a b H . I n o u e c, T. O s u k a d a n d I. Y a s u i a a Institute of Industrial Science, University of Tokyo, Roppongi 7-22-1, Minato-ku, Tokyo 106, Japan b Faculty of Engineering, Okayama University, Tsusimanaka 3-1-1, Okayama-shi, Okayama 700, Japan c Faculty of Engineering, University of Tokyo, Hongo 7-3-1, Bunkyo-ku, Tokyo 113, Japan dAsahi Glass Company, Hazawa-cho 1150, Kanagawa-ku, Yokohama-shi, Kanagawa 221, Japan
The structure of a ternary 40AIF 3 •20YF 3 -40CaF 2 glass (AYC glass) was examined by a combination of X-ray and neutron diffraction analysis and molecular dynamics simulations. The comparison with the structure of 40AIF3.20BaF2.40CaF 2 glass (ABC glass) [T. Nanba et al., Mater. Sci. Forum 32&33 (1988) 385] indicated that the network structures of both glasses were similar to each other, with AIF6 octahedra joining at their corners to form chains with branches. Although A1F7 polyhedra were not found in ABC glass, they existed in AYC glass, as one third of the A1F, octahedra, and these polyhedra were distorted due to the presence of y 3 + ions.
1. Introduction Infrared transmitting fluoride glasses are divided into four groups based on BeF 2, A1F3, Z r F 4 and H f F 4. Fluoride glasses containing BeF 2 vitrify easily but their deliquesence and toxicity have been a disadvantage [1]. Glasses based on A1F3, Z r F 4 and H f F 4 are less hygroscopic [2]. Glasses based on Z r F 4 or H f F 4 are stable and, in particular, in the c a s e of the former glasses, optical fibers of a few meters in length have been fabricated. A rapid quench is needed to form AIF 3based glasses, but it is reported that their glassforming region spread and fabrication become relatively easy when BaF 2, CaF 2 or YF 3 are added [3-5]. Thus, we tried to analyze the structure of A Y C glass with the use of X-ray and neutron Table 1 Atomic scattering factors for X-ray and scattering lengths for neutron Atom
Charge
X-ray
Neutron
F A1 Y Ba Ca
13+ 3+ 2+ 2+
10 10 36 54 18
0.5564 0.3449 0.775 0.525 0.490
diffraction. This glass vitrifies more easily than ABC glass which needs a rapid quench to form glass. Diffraction analysis is generally effective to analyze the mid-range order of glass structures. In X-ray diffraction studies, strong signals from heavy metals, in this case y 3 + , often hide information from lighter atoms, in this case network formers (A13--, F - ) , but in neutron diffraction all constituent atoms have comparable scattering lengths (table 1) and information about light atoms is also obtained.
2. Experimental The composition of the p r e p a r e d glass was 40A1F 3 • 20YF 3 • 40CaF 2. Mixtures of pure CaF2, YF 3 and A1F 3 were melted at 1000°C for 2 h under N 2 atmosphere. Melts were quenched rapidly by pressing between brass plates. X-ray diffraction was measured with Mo K s radiation monochromatized with balanced filters ( Z r - Y ) and a graphite m o n o c h r o m a t o r in a diffracted b e a m using a Rigaku Denki RU-200 diffractometer. Neutron diffraction m e a s u r e m e n t s were carried out by time of flight (TOF) with pulsed neutrons using the H I T facility in the National
0022-3093/92/$05.00 © 1992 - Elsevier Science Publishers B.V. All rights reserved
250
Y. Akasaka et al. / AIF3-CaF2-YF3 glass
Table 2 Parameters used in MD simulations
Table 3 Depth of pair potentials (Born-Mayer)
Repulsive constants, B (× 10-16 j) AI Y Ca F A1 Y Ca F
1.95 -
3.13 6.10 -
2.25 5.89 3.86 -
1.30 3.61 2.42 0.84
Number of atoms 24 12 24 156
AI-F r = 1.475 A Y-F r = 1.948 * Ca-F r = 1.950 ,&
zl~ij (J)
Single bond strength (J)
346.9
346.1 glass former 272.3 intermediate 181.7 modifier
297.1 192.5
Cell lengths = 13.63 A.
I n s t i t u t e o f H i g h - E n e r g y Physics. A f t e r d a t a correction, r a d i a l d i s t r i b u t i o n functions ( R D F ) w e r e calculated. The structure models were calculated by m o l e c u l a r d y n a m i c s ( M D ) s i m u l a t i o n s using Born-Mayer type pair potentials:
ff)ij
-
e2
ZiZ j
4~e0
rij
m o d e l s o b t a i n e d by M D s i m u l a t i o n s w e r e calcul a t e d using a p a i r - f u n c t i o n m e t h o d .
3. Results
3.1. Observed R D F
+ B exp(-riJp),
w h e r e e 2 / 4 w e 0 is a c o n s t a n t t e r m (230.717 × 10 -3o J m), rij is an i n t e r a t o m i c distance, Z is an ionic c h a r g e , B is a r e p u l s i v e c o n s t a n t , a n d p is an e m p i r i c a l c o n s t a n t (0.3 A). T h e values o f t h e B c o n s t a n t s a n d t h e sizes o f unit-cells a r e listed in t a b l e 2. A l t h o u g h t h e repulsive c o n s t a n t s w e r e d e t e r m i n e d e x p e r i m e n tally, t h e c o m p a r i s o n of d e p t h o f p o t e n t i a l s (table 3) with single b o n d s t r e n g t h [6] shows g o o d a g r e e ment. This suggests t h a t t h e p o t e n t i a l s u s e d h e r e s e e m to b e p r o p e r . T h e R D F s for t h e s t r u c t u r e
A c o m p a r i s o n of o b s e r v e d R D F s b e t w e e n 4 0 A 1 F 3 - 2 0 Y F 3 . 4 0 C a F 2 glass ( A Y C glass) a n d 4 0 A 1 F 3 - 2 0 B a F 2 . 4 0 C a F 2 glass ( A B C glass! is shown in fig. 1. T h e first p e a k n e a r 1.85 A is a t t r i b u t e d to t h e n e a r e s t n e i g h b o r A I - F a t o m i c pairs, a n d t h e s e c o n d p e a k a r o u n d 2 . 5 - 3 . 0 consists o f t h e n e a r e s t Y - F , C a - F a n d F - F pairs. W h i l e t h e first p e a k is h i d d e n by t h e s e c o n d large p e a k in X - r a y R D F s , the first s h a r p p e a k is clearly s e p a r a t e d f r o m t h e s e c o n d p e a k in n e u t r o n R D F s . I n X - r a y diffraction, R D F s show little similarity b e c a u s e t h e ion r a d i u s o f heavy m e t a l s
( barns2/A2)
( el 2//~ )
16 --
Neutron
.';'*"....... ..."''""
............s***'"*""
200(
,", ~
0
2
.../"
0 2 4 6 4 6 8 10 radius (A) radius (/k) Fig. 1. Observed RDFs of AYC and ABC glasses.
--
AYC
8
i0
251
Y. Akasaka et al. / A1F3-CaF2- YF3 glass OALOF
©¥
20-
@ca
12
'/"/
Y-F
/-
4
//'~-'- Ca-F
0// 2
3
4
Radius (A/ Fig. 4. Accumulated coordination numbers of each bonds.
Fig. 2. Structure model of AYC glass obtained by MS simulations.
and/or RDF.
Ca-F
a n d / o r Y - F bonds in the X-ray
3.2. Calculated R D F
(Ba 2+= 1.43 A, y 3 + = 0.96 A) are different. O n the contrary, in n e u t r o n diffraction, the peaks are similar, except for the first half of the second p e a k which is assigned to m e t a l - F bonds. Peakfitting analysis by a pair function m e t h o d indicates that one A1 a t o m is c o o r d i n a t e d by 6.42 F atoms in this glass. T h e positions and the areas of the second peaks are mainly affected by the number of F - F bonds in the n e u t r o n R D F and B a - F
( e12/]~)
T h e structure m o d e l obtained by M D simulation is shown in fig. 2. Calculated R D F s from this model (dotted line) and observed one (solid line) are shown in fig. 3. T h e y show g o o d a g r e e m e n t in p e a k positions. In this glass, A1F6 o c t a h e d r a are joined at their corners to form chains with b r a n c h e s as in A B C glass. Y F x and C a F x polyhedra tend to crowd each o t h e r with edge-sharing. Figure 4 shows the a c c u m u l a t e d coordination
( barns2]A2)
X-ray
16
2000
Neutron~/.~..,,,"'
/ 1 0
i 2
i
J
i
i
4 6 radius (~)
i
i 8
i
i
10
0
,v/
""~"'"""""""o~-s, "'"' \
......~- talc.
e-), 2
Fig. 3. Observed and calculated RDFs of AYC glass.
4 6 radius (A)
8
10
252
Y. Akasaka et al. / A l F 3 CaF2-YF3 glass
4. Discussion
numbers of A1, Y, Ca atoms. The slopes of the accumulated coordination numbers corresponding to the first coordination become steeper in the order A1 < Y < Ca. This order is opposite to the order of single bond strengths metal-F.
4.1. Peak comparison The distribution function of each bond was compared with that of ABC glass. They show
A1-F
(el'/~) X-ray
A1-F
( barns~/X~)
.,,,
Neutron
i....... ABC
100
_d. i
J
2
i
i
,
4
6
2
4
radius (,~)
Ca-F
( e1~/N)
6
radius (~.)
(ba~s~
Ca-F
2)
Neutron
X-ray 200
.." C ~ - ~ yc
,_./
i
4
0
6
0
4
radius (A)
F-F
( el2/N )
6
radius (A)
F-F
( barns2//~ 2)
Neutron
X-ray 400
/... i
2
4
6
radius (/~) Fig. 5. Distribution function of each bond.
2
t
4 radius (]~)
Y. Akasaka et al.
253
/ AIF3-CaF2-YF3 glass
good agreement, except for B a - X and Y - X bonds (fig. 5). This indicates that the structure of these two glasses are very similar. A close observation revealed that the first peak of the A1-F bond in A Y C glass shifts longer than that in ABC glass and the second p e a k starts at shorter range. These peak shifts are probably caused by the influence of y 3 + ions. Thus, we tried to investigate this influence. 4.2. Bonding angle in A l F 6 octahedra
The average of the F - A 1 - F bond angles of AIF 6 octahedra in A Y C and A B C models are 90.47 ° and 90.2 °, respectively, which are almost equal to the ideal bond angle 90 °. On the other hand, the standard deviation of the angles in the A Y C glass model is 13.23 °, which is much bigger than that for the A B C glass model (8.6 ° ). This shows that the octahedra in A Y C glass are distorted to a greater extent than those in ABC glass.
tt:Y
® :Ca
o :F
4.3. A I - F Distance
Fig. 6. Linkage structure of AIF6 octahedra and YFx polyhedra.
A 1 - F distances of A1F6 octahedra which do not connect with y3+ ions are the same as those in A Y C and A B C glasses, but A I - F distances where the F is connected with y 3 + are shifted 0.03 .& longer. We think these changes in the A I - F bond length will be due to the distortion probably caused by the presence of strong Y - F bonds.
consist of sheets or chains in contrast to crystal structures. The structure of Ca2A1F 7 crystal [7], the F / A 1 ratio of which is 7, was compared with A B C and A Y C glasses. Except that A1-F bonds in A Y C glass are a little longer, most of interatomic distances in glasses and crystals are much the same as each other. This fact can be understood since the difference in the mode of connec-
4.4. Network structure
Figure 6 shows the linkage structure between A1Fx chains and YF x polyhedra. YF x polyhedra link the A1Fx chains and isolated ones. This will be one of the explanations for the higher tendency for A Y C glass to vitrify. 4.5. Comparison with crystal structures
Crystal structures of fluoroaluminates can be generally classified by the F / A 1 ratio (table 4). The comparison of glass structures with crystal structures indicates that glass structures generally
Table 4 Classification of structure F/AI 3 4 4.7 5 6
Crystal framework sheet interrupted sheet chain isolated
6.5 isolated (Ca2AIF7)
Glass sheet and chain chain with branches chain with branches (ABC) chain with branches and isolated (AYC)
254
Y. Akasaka et al. / AIF3-CaF2-YF 3 glass
tion b e t w e e n A1Fx polyhedra in the glass and crystal structures does not affect the bind distance. Therefore, we conclude that the slightly longer distance b e t w e e n A1 and F in A Y C glass can be ascribed to the presence of Y atoms, which form strong Y - F bonds.
T h e authors wish to thank Professor M. Misawa in K E K and D r H. F u k u n a g a of N a g o y a University who have given m u c h help in the neutron diffraction m e a s u r e m e n t s and t r e a t m e n t of data and wish to thank the C o m p u t e r Center, Institute for Molecular Science, Okazaki National R e s e a r c h Institutes, for the use of the H I T A C M - 6 8 0 H and $ 8 2 0 / 8 0 computers.
5. Conclusion In the structure of A Y C glass, A1Fx polyhedra were joined at their corners to form chains with b r a n c h e s as in the structure of A B C glass. Alt h o u g h isolated A l F x were not f o u n d in A B C glass, they existed in A Y C glass, as only one eighth of AlFx polyhedra. T h e s e polyhedra in A Y C glass were distorted to a greater extent by the presence of y 3 + ions than those in A B C glass. T h e network structure consisting of Y F x polyhedra combining the A1Fx chains and isolated ones will be the key feature of the structure to explain the higher t e n d e n c y of A Y C glass to vitrify.
References [1] K.H. Sun, Glass Techol. 20 (1979) 36. [2] S. Shibata, T. Kanamori, S. Mitachi and T. Manabe, Mater. Res. Bull. 15 (1980) 129. [3] T. Kanamori, K. Oikawa, S. Shibata and T. Manabe, Jpn. J. Appl. Phys. 20 (1981) 326. [4] L Yasui, H. Hagihara and Y. Arai, Mater. Sci. Forum 32&33 (1988) 178. [5] H. Hefang, L. Fengying, G. Donghang and L. Min, Mater. Sci. Forum 5 (1985) 145. [6] D. Mueller, W. Gessner, H. Behrens and G. Scheler, Chem. Phys. Lett. 79 (1981) 59. [7] R. Domesle and R. Hoppe, Z. Kristallogr. 153 (1980) 317.
Journal of Non-Crystalline Solids 140 (1992) 249-254 North-Holland
NON-CItYSTALLINE SOIIDS
Structural analysis of A1F3-CaF2-YF 3 glass by diffraction methods Y. A k a s a k a a T. N a n b a b H . I n o u e c, T. O s u k a d a n d I. Y a s u i a a Institute of Industrial Science, University of Tokyo, Roppongi 7-22-1, Minato-ku, Tokyo 106, Japan b Faculty of Engineering, Okayama University, Tsusimanaka 3-1-1, Okayama-shi, Okayama 700, Japan c Faculty of Engineering, University of Tokyo, Hongo 7-3-1, Bunkyo-ku, Tokyo 113, Japan dAsahi Glass Company, Hazawa-cho 1150, Kanagawa-ku, Yokohama-shi, Kanagawa 221, Japan
The structure of a ternary 40AIF 3 •20YF 3 -40CaF 2 glass (AYC glass) was examined by a combination of X-ray and neutron diffraction analysis and molecular dynamics simulations. The comparison with the structure of 40AIF3.20BaF2.40CaF 2 glass (ABC glass) [T. Nanba et al., Mater. Sci. Forum 32&33 (1988) 385] indicated that the network structures of both glasses were similar to each other, with AIF6 octahedra joining at their corners to form chains with branches. Although A1F7 polyhedra were not found in ABC glass, they existed in AYC glass, as one third of the A1F, octahedra, and these polyhedra were distorted due to the presence of y 3 + ions.
1. Introduction Infrared transmitting fluoride glasses are divided into four groups based on BeF 2, A1F3, Z r F 4 and H f F 4. Fluoride glasses containing BeF 2 vitrify easily but their deliquesence and toxicity have been a disadvantage [1]. Glasses based on A1F3, Z r F 4 and H f F 4 are less hygroscopic [2]. Glasses based on Z r F 4 or H f F 4 are stable and, in particular, in the c a s e of the former glasses, optical fibers of a few meters in length have been fabricated. A rapid quench is needed to form AIF 3based glasses, but it is reported that their glassforming region spread and fabrication become relatively easy when BaF 2, CaF 2 or YF 3 are added [3-5]. Thus, we tried to analyze the structure of A Y C glass with the use of X-ray and neutron Table 1 Atomic scattering factors for X-ray and scattering lengths for neutron Atom
Charge
X-ray
Neutron
F A1 Y Ba Ca
13+ 3+ 2+ 2+
10 10 36 54 18
0.5564 0.3449 0.775 0.525 0.490
diffraction. This glass vitrifies more easily than ABC glass which needs a rapid quench to form glass. Diffraction analysis is generally effective to analyze the mid-range order of glass structures. In X-ray diffraction studies, strong signals from heavy metals, in this case y 3 + , often hide information from lighter atoms, in this case network formers (A13--, F - ) , but in neutron diffraction all constituent atoms have comparable scattering lengths (table 1) and information about light atoms is also obtained.
2. Experimental The composition of the p r e p a r e d glass was 40A1F 3 • 20YF 3 • 40CaF 2. Mixtures of pure CaF2, YF 3 and A1F 3 were melted at 1000°C for 2 h under N 2 atmosphere. Melts were quenched rapidly by pressing between brass plates. X-ray diffraction was measured with Mo K s radiation monochromatized with balanced filters ( Z r - Y ) and a graphite m o n o c h r o m a t o r in a diffracted b e a m using a Rigaku Denki RU-200 diffractometer. Neutron diffraction m e a s u r e m e n t s were carried out by time of flight (TOF) with pulsed neutrons using the H I T facility in the National
0022-3093/92/$05.00 © 1992 - Elsevier Science Publishers B.V. All rights reserved
250
Y. Akasaka et al. / AIF3-CaF2-YF3 glass
Table 2 Parameters used in MD simulations
Table 3 Depth of pair potentials (Born-Mayer)
Repulsive constants, B (× 10-16 j) AI Y Ca F A1 Y Ca F
1.95 -
3.13 6.10 -
2.25 5.89 3.86 -
1.30 3.61 2.42 0.84
Number of atoms 24 12 24 156
AI-F r = 1.475 A Y-F r = 1.948 * Ca-F r = 1.950 ,&
zl~ij (J)
Single bond strength (J)
346.9
346.1 glass former 272.3 intermediate 181.7 modifier
297.1 192.5
Cell lengths = 13.63 A.
I n s t i t u t e o f H i g h - E n e r g y Physics. A f t e r d a t a correction, r a d i a l d i s t r i b u t i o n functions ( R D F ) w e r e calculated. The structure models were calculated by m o l e c u l a r d y n a m i c s ( M D ) s i m u l a t i o n s using Born-Mayer type pair potentials:
ff)ij
-
e2
ZiZ j
4~e0
rij
m o d e l s o b t a i n e d by M D s i m u l a t i o n s w e r e calcul a t e d using a p a i r - f u n c t i o n m e t h o d .
3. Results
3.1. Observed R D F
+ B exp(-riJp),
w h e r e e 2 / 4 w e 0 is a c o n s t a n t t e r m (230.717 × 10 -3o J m), rij is an i n t e r a t o m i c distance, Z is an ionic c h a r g e , B is a r e p u l s i v e c o n s t a n t , a n d p is an e m p i r i c a l c o n s t a n t (0.3 A). T h e values o f t h e B c o n s t a n t s a n d t h e sizes o f unit-cells a r e listed in t a b l e 2. A l t h o u g h t h e repulsive c o n s t a n t s w e r e d e t e r m i n e d e x p e r i m e n tally, t h e c o m p a r i s o n of d e p t h o f p o t e n t i a l s (table 3) with single b o n d s t r e n g t h [6] shows g o o d a g r e e ment. This suggests t h a t t h e p o t e n t i a l s u s e d h e r e s e e m to b e p r o p e r . T h e R D F s for t h e s t r u c t u r e
A c o m p a r i s o n of o b s e r v e d R D F s b e t w e e n 4 0 A 1 F 3 - 2 0 Y F 3 . 4 0 C a F 2 glass ( A Y C glass) a n d 4 0 A 1 F 3 - 2 0 B a F 2 . 4 0 C a F 2 glass ( A B C glass! is shown in fig. 1. T h e first p e a k n e a r 1.85 A is a t t r i b u t e d to t h e n e a r e s t n e i g h b o r A I - F a t o m i c pairs, a n d t h e s e c o n d p e a k a r o u n d 2 . 5 - 3 . 0 consists o f t h e n e a r e s t Y - F , C a - F a n d F - F pairs. W h i l e t h e first p e a k is h i d d e n by t h e s e c o n d large p e a k in X - r a y R D F s , the first s h a r p p e a k is clearly s e p a r a t e d f r o m t h e s e c o n d p e a k in n e u t r o n R D F s . I n X - r a y diffraction, R D F s show little similarity b e c a u s e t h e ion r a d i u s o f heavy m e t a l s
( barns2/A2)
( el 2//~ )
16 --
Neutron
.';'*"....... ..."''""
............s***'"*""
200(
,", ~
0
2
.../"
0 2 4 6 4 6 8 10 radius (A) radius (/k) Fig. 1. Observed RDFs of AYC and ABC glasses.
--
AYC
8
i0
251
Y. Akasaka et al. / A1F3-CaF2- YF3 glass OALOF
©¥
20-
@ca
12
'/"/
Y-F
/-
4
//'~-'- Ca-F
0// 2
3
4
Radius (A/ Fig. 4. Accumulated coordination numbers of each bonds.
Fig. 2. Structure model of AYC glass obtained by MS simulations.
and/or RDF.
Ca-F
a n d / o r Y - F bonds in the X-ray
3.2. Calculated R D F
(Ba 2+= 1.43 A, y 3 + = 0.96 A) are different. O n the contrary, in n e u t r o n diffraction, the peaks are similar, except for the first half of the second p e a k which is assigned to m e t a l - F bonds. Peakfitting analysis by a pair function m e t h o d indicates that one A1 a t o m is c o o r d i n a t e d by 6.42 F atoms in this glass. T h e positions and the areas of the second peaks are mainly affected by the number of F - F bonds in the n e u t r o n R D F and B a - F
( e12/]~)
T h e structure m o d e l obtained by M D simulation is shown in fig. 2. Calculated R D F s from this model (dotted line) and observed one (solid line) are shown in fig. 3. T h e y show g o o d a g r e e m e n t in p e a k positions. In this glass, A1F6 o c t a h e d r a are joined at their corners to form chains with b r a n c h e s as in A B C glass. Y F x and C a F x polyhedra tend to crowd each o t h e r with edge-sharing. Figure 4 shows the a c c u m u l a t e d coordination
( barns2]A2)
X-ray
16
2000
Neutron~/.~..,,,"'
/ 1 0
i 2
i
J
i
i
4 6 radius (~)
i
i 8
i
i
10
0
,v/
""~"'"""""""o~-s, "'"' \
......~- talc.
e-), 2
Fig. 3. Observed and calculated RDFs of AYC glass.
4 6 radius (A)
8
10
252
Y. Akasaka et al. / A l F 3 CaF2-YF3 glass
4. Discussion
numbers of A1, Y, Ca atoms. The slopes of the accumulated coordination numbers corresponding to the first coordination become steeper in the order A1 < Y < Ca. This order is opposite to the order of single bond strengths metal-F.
4.1. Peak comparison The distribution function of each bond was compared with that of ABC glass. They show
A1-F
(el'/~) X-ray
A1-F
( barns~/X~)
.,,,
Neutron
i....... ABC
100
_d. i
J
2
i
i
,
4
6
2
4
radius (,~)
Ca-F
( e1~/N)
6
radius (~.)
(ba~s~
Ca-F
2)
Neutron
X-ray 200
.." C ~ - ~ yc
,_./
i
4
0
6
0
4
radius (A)
F-F
( el2/N )
6
radius (A)
F-F
( barns2//~ 2)
Neutron
X-ray 400
/... i
2
4
6
radius (/~) Fig. 5. Distribution function of each bond.
2
t
4 radius (]~)
Y. Akasaka et al.
253
/ AIF3-CaF2-YF3 glass
good agreement, except for B a - X and Y - X bonds (fig. 5). This indicates that the structure of these two glasses are very similar. A close observation revealed that the first peak of the A1-F bond in A Y C glass shifts longer than that in ABC glass and the second p e a k starts at shorter range. These peak shifts are probably caused by the influence of y 3 + ions. Thus, we tried to investigate this influence. 4.2. Bonding angle in A l F 6 octahedra
The average of the F - A 1 - F bond angles of AIF 6 octahedra in A Y C and A B C models are 90.47 ° and 90.2 °, respectively, which are almost equal to the ideal bond angle 90 °. On the other hand, the standard deviation of the angles in the A Y C glass model is 13.23 °, which is much bigger than that for the A B C glass model (8.6 ° ). This shows that the octahedra in A Y C glass are distorted to a greater extent than those in ABC glass.
tt:Y
® :Ca
o :F
4.3. A I - F Distance
Fig. 6. Linkage structure of AIF6 octahedra and YFx polyhedra.
A 1 - F distances of A1F6 octahedra which do not connect with y3+ ions are the same as those in A Y C and A B C glasses, but A I - F distances where the F is connected with y 3 + are shifted 0.03 .& longer. We think these changes in the A I - F bond length will be due to the distortion probably caused by the presence of strong Y - F bonds.
consist of sheets or chains in contrast to crystal structures. The structure of Ca2A1F 7 crystal [7], the F / A 1 ratio of which is 7, was compared with A B C and A Y C glasses. Except that A1-F bonds in A Y C glass are a little longer, most of interatomic distances in glasses and crystals are much the same as each other. This fact can be understood since the difference in the mode of connec-
4.4. Network structure
Figure 6 shows the linkage structure between A1Fx chains and YF x polyhedra. YF x polyhedra link the A1Fx chains and isolated ones. This will be one of the explanations for the higher tendency for A Y C glass to vitrify. 4.5. Comparison with crystal structures
Crystal structures of fluoroaluminates can be generally classified by the F / A 1 ratio (table 4). The comparison of glass structures with crystal structures indicates that glass structures generally
Table 4 Classification of structure F/AI 3 4 4.7 5 6
Crystal framework sheet interrupted sheet chain isolated
6.5 isolated (Ca2AIF7)
Glass sheet and chain chain with branches chain with branches (ABC) chain with branches and isolated (AYC)
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Y. Akasaka et al. / AIF3-CaF2-YF 3 glass
tion b e t w e e n A1Fx polyhedra in the glass and crystal structures does not affect the bind distance. Therefore, we conclude that the slightly longer distance b e t w e e n A1 and F in A Y C glass can be ascribed to the presence of Y atoms, which form strong Y - F bonds.
T h e authors wish to thank Professor M. Misawa in K E K and D r H. F u k u n a g a of N a g o y a University who have given m u c h help in the neutron diffraction m e a s u r e m e n t s and t r e a t m e n t of data and wish to thank the C o m p u t e r Center, Institute for Molecular Science, Okazaki National R e s e a r c h Institutes, for the use of the H I T A C M - 6 8 0 H and $ 8 2 0 / 8 0 computers.
5. Conclusion In the structure of A Y C glass, A1Fx polyhedra were joined at their corners to form chains with b r a n c h e s as in the structure of A B C glass. Alt h o u g h isolated A l F x were not f o u n d in A B C glass, they existed in A Y C glass, as only one eighth of AlFx polyhedra. T h e s e polyhedra in A Y C glass were distorted to a greater extent by the presence of y 3 + ions than those in A B C glass. T h e network structure consisting of Y F x polyhedra combining the A1Fx chains and isolated ones will be the key feature of the structure to explain the higher t e n d e n c y of A Y C glass to vitrify.
References [1] K.H. Sun, Glass Techol. 20 (1979) 36. [2] S. Shibata, T. Kanamori, S. Mitachi and T. Manabe, Mater. Res. Bull. 15 (1980) 129. [3] T. Kanamori, K. Oikawa, S. Shibata and T. Manabe, Jpn. J. Appl. Phys. 20 (1981) 326. [4] L Yasui, H. Hagihara and Y. Arai, Mater. Sci. Forum 32&33 (1988) 178. [5] H. Hefang, L. Fengying, G. Donghang and L. Min, Mater. Sci. Forum 5 (1985) 145. [6] D. Mueller, W. Gessner, H. Behrens and G. Scheler, Chem. Phys. Lett. 79 (1981) 59. [7] R. Domesle and R. Hoppe, Z. Kristallogr. 153 (1980) 317.