- Email: [email protected]
Awaxs study of amorphous WO3 and WO3 · nH2O
__ __ l!iB
&
Nuclear Instruments
and Methods in Physics Research B 97 (1995) 180-183
NOMB
Beam Interactions with Materials & Atoms
ELSEVIER
Awaxs study of amorphous WO, and WO, - nH,O B. Bouchet-Fabre h Laboratoire
a3*, S. Laruelle b, M. Figlarz b
a L.U.R.E. Unil,ersit~ paris-sud, Bat 2090, 91405 Orsay Cedex, France de R&activit6 et de Chimie des Solides, URA-CNRS 1211, UnicaersitC de Picardie Jules Verne, 33 rue Saint-Leu, 80039 Amiens, France
Abstract Anomalous Wide Angle X-Ray Scattering (AWAXS) has been applied to glassy bulk WO,. In this system the partial distribution of pairs W-W and W-O may be reliably extracted. Glasses have been prepared by non-reactive grinding from both the metastable pyrochlore type structure and from the hydrates WO, nH,O. A weighted differential analysis has been developed, leading to the knowledge of the oxygen surrounding and to the extraction of the partial W-W first correlations. The influence of water content on the octahedra’s network in the amorphous states is analyzed and the medium range order are considered in the framework of hexagonal, pyrochlore and ReO, type structures.
1. Introduction Amorphous WO, thin films have been extensively studied because of their electrochromic properties used in display devices. Depending on the water content, the local order is dominated by the ratio of short double bonds W=O to single bonds W-O and W--OH, [3]. This induces a broad distribution of first neighbours around 1.9 A. The local structure of pyrochlore H0,65W,,63505,23r which is CFC, may be seen as layers made up of distorded WO, octahedra sharing their corners and arranged in 6membered rings; those layers are linked by octahedra along the [ll l] direction [4]. In contrast, hydrates WO, nH,O are described like MOO,. nH,O, consisting of octahedra stacked in sheets, sharing four corners in the equatorial plane. The layers are displaced by a/2 in WO, . lH,O so that the octahedra lie over holes, or stacked alternately with a layer of water molecules for WO, .2H,O. The sheets are held by the network of hydrogen bonds. Nevertheless in the amorphous state, if the local range order has been precisely studied [2,5], the data on the medium range order remain doubtfull. The latter are based on the analysis of the total Radial Distribution Function (RDF) [6,7]. Due ‘,” the strong peak of the W-W correlation close to 3.7 A, the deconvolution of a RDF is quite impossible. Moreover, the differences between hexagonal or ReO, type packing requires a fine analysis of W-W
* Corresponding author, fax +33 1 64464148, 64468099, e-mail: [email protected].
tel.
+33
1
correlations over 5 A, in other words a differential method separating at least those correlations from the others. Thus we present here the AWAXS study of amorphous samples. Bulk samples have been prepared by non-reactive grinding of both the metastable pyrochlore-like lacunar phase the stable phases WO, . lH,O and WO, Ha.,sW,.,,,Os.,,~ 2H z0. The evolution of the structures during amorphization has been followed by usual X-Ray diffraction [8]. The desordered samples will be called a-WO, pyro, a-WO, lH,O and a-WO, .2H,O.
2. X-ray measurements
and differential analysis
Measurements have been performed at LURE-DC1 on the wiggler DW31B beam line equipped with a 2-circles goniometer supporting a solid multidetector [9]. The diffracted X-Ray intensity at an energy E by a glass, for one average atom can be written as: I(k,
E) = (f?> = (f’>
(f2> = xAji(k,
+ (f>?*[S(k, + xAlj(k,
E)/xi
E) - l] E)[S,,(k,
E) - I.],
and (f>2 = xAij(k,
with
E).
f,(k, E) =f,,&k) +f;‘(E) + if;‘(E) is the complex atomic scattering factor of the atomic species j. After correction for absorption, the reduction of the recorded data to I(k, E) has been done by integration, after removing the Compton contribution [lO,ll] and the fluorescence Ly (using the theoretical ratio R = Ly/La). The quality of the normali-
0168-583X/95/$09.50 0 1995 Elsevier Science B.V. All rights reserved SSDI 0168-583X(94)00391-2
181
B. Bouchet-Fabre et al. /Nucl. Instr. and Meth. in Phys. Rex B 97 (1995) 180-183
sation has been improved zero of g(K): g(K)=lKk’(s(k)-l)dk= II The weighted fined as:
through the oscillations
around
lim2m’(p,(r)-p,,). r-n
differential
-A;,(k,
intensity
E,)l(k,
by atom has been de-
Ez).
Then the related differential structure factors Aijs(k, E,, E,) do not include ij correlations. AWAXS study has been achieved through spectra recorded near the W-L,, edge at 11.530 eV (f’ = -16, f” = 9.4) and 13563 eV (f’ = -3.5, f” = 10.35). We have extracted two differential structure factors excluding W-O correlations denoted S, and W-W correlations denoted S,. For a-W03-pyro, the contrast between the two measurements leads to a contribution of -0.16 for an O-O pair and 4.8 for a W-W pair in the radial distribution function R,(r) corresponding to 5,. S, is a picture of the oxygen surrounding. The contribution of a O-W pair to the radial distribution function R,(r), is about 10 times that of an O-O pair in any sample. The error on O-O correlations remains great due to its low weight in I(k, E). The local structures have been analysed through a Gaussian fit of the coordinations and using p. = 0.06 at./ia.
3. Results Fig. 1 shows the total structure factors TSF for the three samples at 13563 eV. The great similarity between the curves is surprising if we consider that the original crystalline phases were completely different. The oscilla-
0
2
Fig. 2. Differential structure tions are removed for a-WO, a-WO,.2H,O.
4
(A)
k
6
8
10
factors S,(k) where W-O corrclapyrochlore type, a-WO,. IH,O and
tions over 4 A-’ going very far at high K are almost periodic. The a priori differences seem to be in the $ze of the domains remaining0 after grinding, about 1.5 A for a-WO, 1HzO and 10 A for the others. The low K range is characterized by a very strong double prepic in any case, much stronger in hydrated samples than in a-WO,-pyro. which means more disorder in the medium range for the latter. The extracted partial S,(k) and S,(k) functions are shown in Figs. 2 and 3. The S,(k) curves, which are mainly related to W-W correlations, are similar for aWO,-pyro and a-WO, lH,O except for the intensity, more slowly decreasing in the latter. The period of the oscillations at high K is shorter in a-WO, 2H,O, even if the damping of the oscillations remains similar to that in a-WO,-pyro. S,(k) functions show very poor contrast, playing a role of small modulation of S,(k) in the TSFs. They give a strong negative contribution to the second
5
z
-5 0
2
4 k
Fig. 1. Total structure factors S(k) extracted from measurements performed at 13563 eV in a-WO, pyrochlore type, a-WO, lH,O and a-W0,‘2Hz0.
(A,
6
8
1 10
Fig. 3. Differential structure factors S,(k) where W-W correlations are removed for a-WO, pyrochlore type, a-WO,. lH,O and a-WO,‘2H,O.
IV. GLASSES
182
B. Bouchet-Fabre et al. /Nucl. Instr. and Meth. in Phys. Res. B 97 (19951 180-183
1
2
4
6
R (A)
10
’
12
0
1
2
3
4 R
/
(A) 5
7
6
8
Fig. 4. Differential radial distribution function of pairs R,(r) where W-O correlations are removed for a-WO, pyrochlore type, a-WO,.lH,O and a-WO,.2H,O.
Fig. 5. Differential radial distribution function of pairs R,(r) around oxygen atoms for a-WO, pyrochlore type, a-WO, lH,O and a-WO,.2H,O.
maximum of the prepeak, indicating the strong chemical order in those glasses. R,(r) functions are shown in Fig. 4. In both a-WO,pyro and a-WO, lH,CJ, each tungsten is surrounded by about 4 others at 3.72 A with a narrow distribution (a= 0.16). The correlations around 7.3 A and 8.2 A are well seen in a-WO, . lH,O, due to the big size of the remaining domains. They correspond to the 3rd distances between the octahedra in a plane ReO,-type. The correlations perpendicular to the planes are completely smoothed in aWO, . lH,O.OThe distribution of the distances in the vicinity of 5-6 A is much spread in a-WO,-pyro than in a-WO, . lH,O: the medium range in a-WO,-pyro may a priori be seen as a mixing of the two kinds of nearly flat packing: ReO, type and hexagonal. In contrast, the first shell W-W in a-WO, 2H,O consist of two coordinations: one at 3.76 A related to the octahedra sharing their corners and an other near 3.26 A, which may be related to octahedra sharing their edges. The total coordination number 3.2 remains low. The medium range is as disordered as in a-WO,-pyro. Fig. 5 shows R,(r) functions describing the surrounding of oxygen. The O-W first coordination shell is at least composed by two distances (Table ll, compatible with the existence of double W=O and single
W--OH, bonds previously described. Each tungsten is surrounded by 5:2 oxygen, at an average disiance decreasing from 1.96 A in a-WO,-pyro to 1.93 A in a-WO, 2HzO. In the second set of correlations, we also find 2 kinds of coordinations, a narrow and a broad at about 2.07 times the first W=O and single W--O correlations. In hydrates glasses, the broad distribution of oxygen leads rapidly to a complete loss of order in R,(r), in contrast with a-WO,-pyro.
4. Discussion The W-W skeleton of the a-WO,-pyro does not seem sensitive to water content and may be compared with that of a-WO, . lH,O whereas 0 atoms are not distributed in the same way. It seems that WO, . lH,O keeps small 2D curved coherent domains through amorphization when the others show more defects and correlations in all directions. In a-WO, . 2H20, W-W correlations are completely changed through grinding, indicating a deviation of the local structure from the crystalline one. In contrast the surrounding of oxygen shows the same features in the medium range for both hydrates.
Table 1 Neighbourhood
of oxygen
atoms in bulk amorphous Ex-WO,
Ex-pyrochlore d [.&I
oxides extracted
from a Gaussian
fit of R, and R, (for 0-O)
Ex-WO,
lH,O d [A]
(T
N
tungsten
N
v
.2HaO d [z&l
N
IJ
W W 0
1.82 2.2 2.87
1 0.6 7
0.06 0.10 0.10
W W 0
1.75 2.0 2.78
0.3 1.0 7.0
0.06 0.13 0.08
W W 0
1.8 1.96 2.74
_ 0.2 0.9 6
0.10 0.10 0.10
W W 0
3.77 4.40 4.4
1.8 < 6.8
0.08 0.40 _
W W 0
3.75 4.27 4.4
1.3 < 4.0 - 12
0.06 0.20 0.08
W W 0
3.70 4.38 4.4
1.9 < 3.0 -6
0.15 0.20 0.13
B. Bouchet-Fabre et al. /Nucl. Instr. and Meth. in Phys. Res B 97 (1995) 180-183
5. Conclusion We have shown that it is possible to extract the partial distribution of W-W pairs in amorphous tungsten oxides from AWAXS data recorded around the W L,-edge. A picture of oxygen surrounding has also been extracted. This
not conventional
differential
analysis
has allowed
us
more precisely the local and medium range order in amorphous tungstates in relationship to water content and the precursory crystalline phases. to compare
References [l] M. Figlarz, Prog. Solid St. Chem. 19 (1989) 1. [2] A. Kuzmin and J. Purans. J. phys.: Condens. Matter (1993) 2333.
183
[3] T. Nanba, Y. Nishiyama and 1. Yasui. J. Mater. Res. 6 (19911 1324. [4] A. Coucou, A. Driouiche, M. Figlarz, M. Touboul and G. Chevrier, J. Solid St. Chem. 99 (1992) 283. 151 E. Burattini, A. Kuzmin and J. Purans, Jpn J. Appl. Phys. 32 (1993) 655. [6] T. Nanba and I. Yasui, Diffusion and Defect Data 53 54 (1987) 105. [7] T. Nanba and I. Yasui, J. Solid St. Chem. 83 (1989) 304. [81 S. Laruelle and M. Figlarz, J. Solid St. Chem. (to be published). [9] R. Andouard, C. Barbier, D. Dagneaux. M. De Santis. D. Raoux and G. Nicoli, Proc. European Int. Conf. on Progress in X-Ray Synchrotron Radiation Research, Rome, eds. A. Balerma, E. Bernieri and S. Mobilio (1990) p. 351. [lo] R.G. Palinkas, Acta Crystallogr. A 29 (1973) 10. [Ill F. Hajdu, Acta Crystallogr. A 27 (1971) 73.
IV. GLASSES
&
Nuclear Instruments
and Methods in Physics Research B 97 (1995) 180-183
NOMB
Beam Interactions with Materials & Atoms
ELSEVIER
Awaxs study of amorphous WO, and WO, - nH,O B. Bouchet-Fabre h Laboratoire
a3*, S. Laruelle b, M. Figlarz b
a L.U.R.E. Unil,ersit~ paris-sud, Bat 2090, 91405 Orsay Cedex, France de R&activit6 et de Chimie des Solides, URA-CNRS 1211, UnicaersitC de Picardie Jules Verne, 33 rue Saint-Leu, 80039 Amiens, France
Abstract Anomalous Wide Angle X-Ray Scattering (AWAXS) has been applied to glassy bulk WO,. In this system the partial distribution of pairs W-W and W-O may be reliably extracted. Glasses have been prepared by non-reactive grinding from both the metastable pyrochlore type structure and from the hydrates WO, nH,O. A weighted differential analysis has been developed, leading to the knowledge of the oxygen surrounding and to the extraction of the partial W-W first correlations. The influence of water content on the octahedra’s network in the amorphous states is analyzed and the medium range order are considered in the framework of hexagonal, pyrochlore and ReO, type structures.
1. Introduction Amorphous WO, thin films have been extensively studied because of their electrochromic properties used in display devices. Depending on the water content, the local order is dominated by the ratio of short double bonds W=O to single bonds W-O and W--OH, [3]. This induces a broad distribution of first neighbours around 1.9 A. The local structure of pyrochlore H0,65W,,63505,23r which is CFC, may be seen as layers made up of distorded WO, octahedra sharing their corners and arranged in 6membered rings; those layers are linked by octahedra along the [ll l] direction [4]. In contrast, hydrates WO, nH,O are described like MOO,. nH,O, consisting of octahedra stacked in sheets, sharing four corners in the equatorial plane. The layers are displaced by a/2 in WO, . lH,O so that the octahedra lie over holes, or stacked alternately with a layer of water molecules for WO, .2H,O. The sheets are held by the network of hydrogen bonds. Nevertheless in the amorphous state, if the local range order has been precisely studied [2,5], the data on the medium range order remain doubtfull. The latter are based on the analysis of the total Radial Distribution Function (RDF) [6,7]. Due ‘,” the strong peak of the W-W correlation close to 3.7 A, the deconvolution of a RDF is quite impossible. Moreover, the differences between hexagonal or ReO, type packing requires a fine analysis of W-W
* Corresponding author, fax +33 1 64464148, 64468099, e-mail: [email protected].
tel.
+33
1
correlations over 5 A, in other words a differential method separating at least those correlations from the others. Thus we present here the AWAXS study of amorphous samples. Bulk samples have been prepared by non-reactive grinding of both the metastable pyrochlore-like lacunar phase the stable phases WO, . lH,O and WO, Ha.,sW,.,,,Os.,,~ 2H z0. The evolution of the structures during amorphization has been followed by usual X-Ray diffraction [8]. The desordered samples will be called a-WO, pyro, a-WO, lH,O and a-WO, .2H,O.
2. X-ray measurements
and differential analysis
Measurements have been performed at LURE-DC1 on the wiggler DW31B beam line equipped with a 2-circles goniometer supporting a solid multidetector [9]. The diffracted X-Ray intensity at an energy E by a glass, for one average atom can be written as: I(k,
E) = (f?> = (f’>
(f2> = xAji(k,
+ (f>?*[S(k, + xAlj(k,
E)/xi
E) - l] E)[S,,(k,
E) - I.],
and (f>2 = xAij(k,
with
E).
f,(k, E) =f,,&k) +f;‘(E) + if;‘(E) is the complex atomic scattering factor of the atomic species j. After correction for absorption, the reduction of the recorded data to I(k, E) has been done by integration, after removing the Compton contribution [lO,ll] and the fluorescence Ly (using the theoretical ratio R = Ly/La). The quality of the normali-
0168-583X/95/$09.50 0 1995 Elsevier Science B.V. All rights reserved SSDI 0168-583X(94)00391-2
181
B. Bouchet-Fabre et al. /Nucl. Instr. and Meth. in Phys. Rex B 97 (1995) 180-183
sation has been improved zero of g(K): g(K)=lKk’(s(k)-l)dk= II The weighted fined as:
through the oscillations
around
lim2m’(p,(r)-p,,). r-n
differential
-A;,(k,
intensity
E,)l(k,
by atom has been de-
Ez).
Then the related differential structure factors Aijs(k, E,, E,) do not include ij correlations. AWAXS study has been achieved through spectra recorded near the W-L,, edge at 11.530 eV (f’ = -16, f” = 9.4) and 13563 eV (f’ = -3.5, f” = 10.35). We have extracted two differential structure factors excluding W-O correlations denoted S, and W-W correlations denoted S,. For a-W03-pyro, the contrast between the two measurements leads to a contribution of -0.16 for an O-O pair and 4.8 for a W-W pair in the radial distribution function R,(r) corresponding to 5,. S, is a picture of the oxygen surrounding. The contribution of a O-W pair to the radial distribution function R,(r), is about 10 times that of an O-O pair in any sample. The error on O-O correlations remains great due to its low weight in I(k, E). The local structures have been analysed through a Gaussian fit of the coordinations and using p. = 0.06 at./ia.
3. Results Fig. 1 shows the total structure factors TSF for the three samples at 13563 eV. The great similarity between the curves is surprising if we consider that the original crystalline phases were completely different. The oscilla-
0
2
Fig. 2. Differential structure tions are removed for a-WO, a-WO,.2H,O.
4
(A)
k
6
8
10
factors S,(k) where W-O corrclapyrochlore type, a-WO,. IH,O and
tions over 4 A-’ going very far at high K are almost periodic. The a priori differences seem to be in the $ze of the domains remaining0 after grinding, about 1.5 A for a-WO, 1HzO and 10 A for the others. The low K range is characterized by a very strong double prepic in any case, much stronger in hydrated samples than in a-WO,-pyro. which means more disorder in the medium range for the latter. The extracted partial S,(k) and S,(k) functions are shown in Figs. 2 and 3. The S,(k) curves, which are mainly related to W-W correlations, are similar for aWO,-pyro and a-WO, lH,O except for the intensity, more slowly decreasing in the latter. The period of the oscillations at high K is shorter in a-WO, 2H,O, even if the damping of the oscillations remains similar to that in a-WO,-pyro. S,(k) functions show very poor contrast, playing a role of small modulation of S,(k) in the TSFs. They give a strong negative contribution to the second
5
z
-5 0
2
4 k
Fig. 1. Total structure factors S(k) extracted from measurements performed at 13563 eV in a-WO, pyrochlore type, a-WO, lH,O and a-W0,‘2Hz0.
(A,
6
8
1 10
Fig. 3. Differential structure factors S,(k) where W-W correlations are removed for a-WO, pyrochlore type, a-WO,. lH,O and a-WO,‘2H,O.
IV. GLASSES
182
B. Bouchet-Fabre et al. /Nucl. Instr. and Meth. in Phys. Res. B 97 (19951 180-183
1
2
4
6
R (A)
10
’
12
0
1
2
3
4 R
/
(A) 5
7
6
8
Fig. 4. Differential radial distribution function of pairs R,(r) where W-O correlations are removed for a-WO, pyrochlore type, a-WO,.lH,O and a-WO,.2H,O.
Fig. 5. Differential radial distribution function of pairs R,(r) around oxygen atoms for a-WO, pyrochlore type, a-WO, lH,O and a-WO,.2H,O.
maximum of the prepeak, indicating the strong chemical order in those glasses. R,(r) functions are shown in Fig. 4. In both a-WO,pyro and a-WO, lH,CJ, each tungsten is surrounded by about 4 others at 3.72 A with a narrow distribution (a= 0.16). The correlations around 7.3 A and 8.2 A are well seen in a-WO, . lH,O, due to the big size of the remaining domains. They correspond to the 3rd distances between the octahedra in a plane ReO,-type. The correlations perpendicular to the planes are completely smoothed in aWO, . lH,O.OThe distribution of the distances in the vicinity of 5-6 A is much spread in a-WO,-pyro than in a-WO, . lH,O: the medium range in a-WO,-pyro may a priori be seen as a mixing of the two kinds of nearly flat packing: ReO, type and hexagonal. In contrast, the first shell W-W in a-WO, 2H,O consist of two coordinations: one at 3.76 A related to the octahedra sharing their corners and an other near 3.26 A, which may be related to octahedra sharing their edges. The total coordination number 3.2 remains low. The medium range is as disordered as in a-WO,-pyro. Fig. 5 shows R,(r) functions describing the surrounding of oxygen. The O-W first coordination shell is at least composed by two distances (Table ll, compatible with the existence of double W=O and single
W--OH, bonds previously described. Each tungsten is surrounded by 5:2 oxygen, at an average disiance decreasing from 1.96 A in a-WO,-pyro to 1.93 A in a-WO, 2HzO. In the second set of correlations, we also find 2 kinds of coordinations, a narrow and a broad at about 2.07 times the first W=O and single W--O correlations. In hydrates glasses, the broad distribution of oxygen leads rapidly to a complete loss of order in R,(r), in contrast with a-WO,-pyro.
4. Discussion The W-W skeleton of the a-WO,-pyro does not seem sensitive to water content and may be compared with that of a-WO, . lH,O whereas 0 atoms are not distributed in the same way. It seems that WO, . lH,O keeps small 2D curved coherent domains through amorphization when the others show more defects and correlations in all directions. In a-WO, . 2H20, W-W correlations are completely changed through grinding, indicating a deviation of the local structure from the crystalline one. In contrast the surrounding of oxygen shows the same features in the medium range for both hydrates.
Table 1 Neighbourhood
of oxygen
atoms in bulk amorphous Ex-WO,
Ex-pyrochlore d [.&I
oxides extracted
from a Gaussian
fit of R, and R, (for 0-O)
Ex-WO,
lH,O d [A]
(T
N
tungsten
N
v
.2HaO d [z&l
N
IJ
W W 0
1.82 2.2 2.87
1 0.6 7
0.06 0.10 0.10
W W 0
1.75 2.0 2.78
0.3 1.0 7.0
0.06 0.13 0.08
W W 0
1.8 1.96 2.74
_ 0.2 0.9 6
0.10 0.10 0.10
W W 0
3.77 4.40 4.4
1.8 < 6.8
0.08 0.40 _
W W 0
3.75 4.27 4.4
1.3 < 4.0 - 12
0.06 0.20 0.08
W W 0
3.70 4.38 4.4
1.9 < 3.0 -6
0.15 0.20 0.13
B. Bouchet-Fabre et al. /Nucl. Instr. and Meth. in Phys. Res B 97 (1995) 180-183
5. Conclusion We have shown that it is possible to extract the partial distribution of W-W pairs in amorphous tungsten oxides from AWAXS data recorded around the W L,-edge. A picture of oxygen surrounding has also been extracted. This
not conventional
differential
analysis
has allowed
us
more precisely the local and medium range order in amorphous tungstates in relationship to water content and the precursory crystalline phases. to compare
References [l] M. Figlarz, Prog. Solid St. Chem. 19 (1989) 1. [2] A. Kuzmin and J. Purans. J. phys.: Condens. Matter (1993) 2333.
183
[3] T. Nanba, Y. Nishiyama and 1. Yasui. J. Mater. Res. 6 (19911 1324. [4] A. Coucou, A. Driouiche, M. Figlarz, M. Touboul and G. Chevrier, J. Solid St. Chem. 99 (1992) 283. 151 E. Burattini, A. Kuzmin and J. Purans, Jpn J. Appl. Phys. 32 (1993) 655. [6] T. Nanba and I. Yasui, Diffusion and Defect Data 53 54 (1987) 105. [7] T. Nanba and I. Yasui, J. Solid St. Chem. 83 (1989) 304. [81 S. Laruelle and M. Figlarz, J. Solid St. Chem. (to be published). [9] R. Andouard, C. Barbier, D. Dagneaux. M. De Santis. D. Raoux and G. Nicoli, Proc. European Int. Conf. on Progress in X-Ray Synchrotron Radiation Research, Rome, eds. A. Balerma, E. Bernieri and S. Mobilio (1990) p. 351. [lo] R.G. Palinkas, Acta Crystallogr. A 29 (1973) 10. [Ill F. Hajdu, Acta Crystallogr. A 27 (1971) 73.
IV. GLASSES