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Part 2.125Te NMR study of M2O–TeO2 (M=Li, Na, K, Rb and Cs) glasses
Journal of Non-Crystalline Solids 243 (1999) 13±25
Part 2.125 Te NMR study of M2O±TeO2 (M Li, Na, K, Rb and Cs) glasses Shinichi Sakida b
a,* ,
Satoshi Hayakawa b, Toshinobu Yoko
a
a Institute for Chemical Research, Kyoto University, Uji, Kyoto-fu 611, Japan Faculty of Engineering, Okayama University, Okayama-shi, Okayama 700, Japan
Received 6 January 1998; received in revised form 18 September 1998
Abstract The structure of TeO2 and M2 O±TeO2 (M Li, Na, K, Rb and Cs) glasses has been investigated by means of 125 Te static and magic-angle spinning (MAS) NMR spectroscopies. On the basis of the relationship between the local structures around Te atoms and the NMR parameters obtained by 125 Te NMR spectra of various tellurite crystals, the local structures around Te atoms in TeO2 and M2 O±TeO2 glasses were examined. The spectral simulation of static NMR spectra enabled us to discriminate between TeO3 trigonal pyramid (tp) and TeO4 trigonal bipyramid (tbp) and to estimate the fractions of TeO3 tp (N3 ) and TeO4 tbp (N4 ). It was concluded that in TeO2 glass TeO4 tbps are connected by sharing their corners as in a-TeO2 crystal. N3 increased and N4 decreased with increasing M2 O content. On the basis of a previous structural model, the fractions of TeO4 tbp without a non-bridging oxygen (NBO) atom, TeO4 tbp with an NBO atom, and TeO3 tp with two NBO atoms were calculated. A new model which presents the structural change for M2 O±TeO2 glasses has been proposed. Ó 1999 Elsevier Science B.V. All rights reserved.
1. Introduction Tellurite glasses are considered as excellent candidate materials for optical switching devices [1] and laser hosts [2] because of their desirable physical properties, such as high refractive index, excellent infrared transmittance and high dielectric constant, good chemical durability and low temperature melting [3±7]. Structures of pure TeO2 glass and M2 O±TeO2 (M Li, Na, K, Rb and Cs) glasses have been investigated by means of X-ray diraction [8±12], neutron diraction [13,14], infrared spectroscopy
* Corresponding author. Tel.: +81-774 383 131; fax: +81-774 335 212; e-mail: [email protected].
[11,15±17], Raman spectroscopy [17±22], magicangle spinning (MAS) NMR [19,23], EXAFS [12,24] and M ossbauer spectroscopy [25,26]. The following are the recent ®ndings: (1) pure TeO2 glass consists of TeO4 trigonal bipyramids (tbp0 s) in which one equatorial site of the Te sp3 d hybrid orbitals is occupied by a lone pair of electrons and the other two equatorial and axial sites are occupied by oxygen atoms, and (2) addition of M2 O to pure TeO2 glass changes the Te coordination polyhedron from TeO4 tbp to TeO3 trigonal pyramid (tp) in which one of the Te sp3 hybrid orbitals is occupied by a lone pair of electrons. However, the symmetry around Te atoms with a lone pair of electrons is so low that a detailed analysis of the glass structure is very dicult. It can therefore be said that any decisive
0022-3093/99/$ ± see front matter Ó 1999 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 3 0 9 3 ( 9 8 ) 0 0 8 1 2 - 6
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S. Sakida et al. / Journal of Non-Crystalline Solids 243 (1999) 13±25
conclusion on the glass structures has not been obtained yet. NMR spectroscopy is a powerful tool that can reveal the detailed local structure around a nucleus, since it can acquire direct information about the detailed local structure around a given nucleus of interest. Although a 125 Te MAS NMR study of tellurite glasses was performed by Yoko et al. [19,23], spectra with low S/N ratio were only obtained of the Te nuclei with I )1/2 due to very long relaxation times (about 20 s). Accordingly, only qualitative analysis could be performed. In the present work, aiming at shorter measuring time and improvement of S/N ratio, 125 Te static and MAS NMR spectra are measured for M2 O±TeO2 (M Li, Na, K, Rb and Cs) glasses containing a trace amount of Fe2 O3 . On the basis of the more reliable ratio of TeO3 tp and TeO4 tbp in M2 O±TeO2 glasses obtained from the present NMR measurements, the structure of M2 O±TeO2 glasses is discussed quantitatively. A new model of vitri®cation and structural change for M2 O±TeO2 glasses is proposed. 2. Experimental 2.1. Preparation of glasses In order to attain both shorter measuring times and an improvement in S/N ratio by shortening the relaxation times of Te nuclei with I 1/2, tellurite glasses were doped with a trace of Fe2 O3 . Tests were conducted to ®nd the Fe2 O3 content giving the highest S/N ratio by using 30Li2 O á 70TeO2 á pFe2 O3 (p 0, 0.05, 0.1, 0.2, 0.3, 0.4 and 0.5) glasses. TeO2 , xLi2 O á (100 ) x)TeO2 (x 10, 20, 30 and 33.3), xNa2 O á (100 ) x)TeO2 (x 10, 20, 30 and 33.3), xK2 O á (100 ) x)TeO2 (x 10, 20 and 25), xRb2 O á (100 ) x)TeO2 (x 10, 20 and 25) and xCs2 O á (100 ) x)TeO2 (x 10 and 15) glasses containing a trace of Fe2 O3 were prepared. Reagent-grade b-TeO2 , Cs2 CO3 , Fe2 O3 , Li2 CO3 , Na2 CO3 , K2 CO3 and Rb2 CO3 were used as starting materials. 1±3 g batches of well-mixed reagents were melted in a Pt-5%Au crucible at 700±800°C for 10 min in air. The melt was poured onto a stainless plate and immediately pressed by
another stainless plate. For pure TeO2 glass, a very small amount of batch (50±100 mg) was melted in a Pt-5%Au crucible at 830°C for 10 min in air. The melt was quenched by immersing the bottom of the crucible into a freezing mixture consisting of NaCl and ice water kept at about )10°C. In most cases, the quenched melts were identi®ed to be glassy by visual inspection. X-ray diraction was used in some cases to con®rm visual observation. 2.2. NMR measurements 125 Te static and MAS NMR spectra of powdered tellurite glasses were measured at 126.32 MHz (9.4 T) on a JEOL JNM-GSX 400 MAS FT± NMR spectrometer. A single pulse sequence was used which is characterized by a pulse length of 2.5 ls and 1.0 ls, an accumulation of 1000 and 4325 scans, pulse delays of 2.5 s and 20 s, and a dead time of 0.5 and 4.0 ls for 30Li2 O á 70TeO2 á pFe2 O3 glasses with and without a trace of Fe2 O3 , respectively. A pulse length of 2.5 ls, an accumulation of 8000±20000 scans, a pulse delay of 2.5 s and a dead time of 1.5 ls were employed for M2 O± TeO2 glasses. A cylindrical zirconia rotor containing a glass sample was spun at a speed of about 5±6 kHz in the 125 Te MAS NMR measurements. Telluric acid Te(OH)6 was used as a secondary standard whose chemical shift was d 692.2 ppm with reference to (CH3 )2 Te [27].
2.3.
125
Te static NMR spectra
The principal components of the chemical shift tensors, d1 , d2 and d3 were estimated by ®tting the calculated theoretical spectra to the experimental static NMR spectra as described in a previous paper [28]. The isotropic chemical shift diso static , the chemical shift anisotropy Dd and the asymmetry parameter g were also determined by using the following de®nitions [29,30]: diso static
d 1 d2 d3 ; 3
1
Dd d3 ÿ
d1 d2 ; 2
2
S. Sakida et al. / Journal of Non-Crystalline Solids 243 (1999) 13±25
g
d 2 ÿ d1 : d3 ÿ diso static
15
3
The principal components of the chemical shift tensors can be determined based on the following relation. d3 ÿ diso static P d1 ÿ diso static P d2 ÿ diso static : 4 3. Results The 125 Te MAS NMR spectra of 30Li2 O á 70TeO2 á pFe2 O3 (p 0, 0.05, 0.1, 0.2, 0.3, 0.4 and 0.5) glasses are shown in Fig. 1. It is clear that the isotropic chemical shift does not change with the addition of a small amount of Fe2 O3 . This result was also demonstrated experimentally for several tellurite crystals in a previous paper [28]. Since the peaks of all glasses appeared at 2000 50 ppm, the structural change from the addition of a small amount of Fe2 O3 was considered to be negligible. This result enabled us to measure 125 Te NMR spectra at a pulse delay of 2.5 s, which was remarkably shortened compared to 20 s without Fe2 O3 . The S/N ratio of spectra containing 0.1±0.3 mol% Fe2 O3 was consequently higher than that without Fe2 O3 . Since the S/N ratio of spectra containing 0.3 mol% Fe2 O3 was the highest, 125 Te NMR spectra were measured for TeO2 and M2 O± TeO2 (M Li, Na, K, Rb and Cs) glasses containing 0.3 mol% Fe2 O3 . Fig. 2 shows the 125 Te MAS NMR spectra of Li2 O±TeO2 glasses. Whereas MAS NMR spectra of tellurite crystals (Fig. 5 in Ref. [28]) were composed of an isotropic chemical shift and a number of spinning side bands (SSB), all the spectra of glasses consisted of one broad peak without splitting in the range of 500±2800 ppm, which was obviously due to variations in site symmetry. Accordingly, isotropic chemical shifts could not be obtained from MAS NMR spectra of tellurite glasses. The intensity of the peaks in the range of 500±1800 ppm decreased with increasing Li2 O content, suggesting that the local structure around Te atoms in glasses changes with the Li2 O content. Fig. 3 shows the 125 Te static NMR spectra of Li2 O±TeO2 glasses together with those of a-TeO2
Fig. 1. 125 Te MAS NMR spectra of 30Li2 O á 70TeO2 á pFe2 O3 (p 0, 0.05, 0.1, 0.2, 0.3, 0.4 and 0.5) glasses. x 0: 1.0 ls pulse length, 20 s pulse delay, 4325 scans. Others: 2.5 ls pulse length, 2.5 s pulse delay, 1000 scans.
and Li2 TeO3 crystals. All the spectra of glasses consisted of one broad peak without splitting in the range of 500 to 2800 ppm and the line pro®les were very similar to those of the MAS spectra shown in Fig. 2. The line pro®les of the 125 Te static and MAS NMR spectra of M2 O±TeO2 (M Na, K, Rb and Cs) glasses which are not shown here were basically similar to those of Li2 O±TeO2 glasses. The similarity of the line pro®les between 125 Te MAS and static spectra have been observed in the
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S. Sakida et al. / Journal of Non-Crystalline Solids 243 (1999) 13±25
Fig. 2. 125 Te MAS NMR spectra of xLi2 O á (100 ) x)TeO2 (x 0, 10, 20, 30 and 33.3) glasses.
tellurite glasses as shown in Figs. 2 and 3. The reason is discussed below. It is generally known that the intervals between neighboring peaks depend on sample spinning speed, and the intensity and the number of SSBs decreases with increasing sample spinning speed. In the case of tellurite glasses, a speed of more than 290 kHz is required to eliminate most of the SSB's. In other words, the chemical shift anisotropy of tellurite glasses is too large to be eliminated at a speed of about 5±6 kHz employed in this work, and therefore the SSB
Fig. 3. 125 Te static NMR spectra of xLi2 O á (100 ) x)TeO2 (x 0, 10, 20, 30 and 33.3) glasses and a-TeO2 and Li2 TeO3 crystals.
peaks should have hardly decreased in intensity or number. 4. Discussion 4.1. 125 Te MAS and static NMR spectra and local structure of tellurite glasses In a previous paper [28], the local structure around Te atoms has been classi®ed into ®ve
S. Sakida et al. / Journal of Non-Crystalline Solids 243 (1999) 13±25
17
Fig. 4. Classi®cation of structural units of (1) TeO3 type which consists of (i) isolated TeO3 type and (ii) terminal TeO3 type, (2) TeO31 type, and (3) TeO4 type which consists of (i) a-TeO2 type and (ii) b-TeO2 type.
groups based on the structural units present in tellurite crystals as illustrated in Fig. 4. These are (1) TeO3 type which consists of an isolated TeO3 type and a terminal TeO3 type, (2) TeO31 type and (3) TeO4 type which consists of an a-TeO2 type and a b-TeO2 type. An isolated TeO3 type means that the TeO3 tp structural unit is present as a free anion; the terminal TeO3 type means that the TeO3 tp has one corner which is connected with a neighboring TeOn polyhedron; the TeO31 type means that the TeO3 tp has an additional O atom at a remote distance from 0.22 to 0.25 nm. Furthermore, the a-TeO2 type contains TeO4 tbp structural units which are connected by sharing their corners, and the b-TeO2 type contains Te2 O6 units consisting of two edge-shared TeO4 tbps. The MAS spectra of TeO2 and M2 O±TeO2 glasses are too broad to obtain an isotropic chemical shift. Therefore, further quantitative analysis based on the MAS spectra is impossible. To solve this problem, we turned to the 125 Te static NMR technique for this work. Fig. 5 plots the relationship between the asymmetry parameter g and the absolute value of the chemical shift anisotropy |Dd| for the structural units as found in various tellurite crystals as obtained in a previous paper [28]. A region determined by both g and |Dd|, which corresponds to each structural unit
Fig. 5. Plots of the g vs. the |Dd| for tellurite glasses. Pro®le 1 is the peak on the basis of TeO2 glass. Pro®le 2 was determined from M2 O±TeO2 (M Li, Na, K, Rb and Cs) glasses.
18
S. Sakida et al. / Journal of Non-Crystalline Solids 243 (1999) 13±25
shown in Fig. 4, is depicted in this g-|Dd| diagram. This diagram is useful for the examination of the local structure of tellurite glasses. Once the principal components of the chemical shift tensor, d1 , d2 and d3 , are estimated from the static spectrum of TeO2 glass (x 0), the isotropic chemical shift diso static , Dd and g are calculated from Eqs. (1)± (3). The result is listed in Table 1. The Dd and g of TeO2 glass are )1285 ppm and 0.74, respectively, and are plotted in the g-|Dd| diagram (Fig. 5). Since the point determined from both Dd and g of TeO2 glass is located just in the region of a-TeO2 type, it can be said that TeO2 glass is composed of TeO4 tbps of a-TeO2 type, that is, the TeO4 tbps are connected with each other by sharing corners. As clearly seen from Fig. 3, the line pro®le of 125 Te static NMR spectra of Li2 O±TeO2 glasses varies with increasing alkali oxide content and is getting close to that of the Li2 TeO3 crystal containing only a TeO3 tp as a structural unit. This suggests that the formation of TeO3 tp takes place on addition of Li2 O to TeO2 glass and the content increases with increasing Li2 O concentration. It seems reasonable to assume that M2 O±TeO2 glasses are mainly composed of TeO4 tbp and TeO3 tp as structural units, their fractions depending on composition. To con®rm this assumption, the experimental 125 Te static NMR spectra of alkali tellurite glasses are deconvoluted by using two spectral components: Pro®le 1 is due to TeO4 tbp and Pro®le 2 due to TeO3 tp. An example of line pro®le deconvolution performed for Li2 O±TeO2 glasses is shown in Fig. 6. Here the principal components of the chemical shift tensor, d1 , d2 and d3 , for TeO2 glass are used as the initial values for Pro®le 1. It was assumed for convenience that the shape of Pro®le 1 was the same as that for the TeO2 glass and that of Pro®le 2 was independent of glass composition. That is, the parameters |Dd| and g of Pro®les 1 and 2 of the present alkali tellurite glasses were assumed to be unchanged although their positions were not restricted. The values obtained by the spectral deconvolution for M2 O±TeO2 glasses are summarized in Table 1. The NMR parameters, g and Dd, of Pro®le 2 obtained for respective glass systems are shown in Fig. 5. It is interesting to note that all
Fig. 6. Simulated and experimental 125 Te static NMR spectra of xLi2 O á (100 ) x)TeO2 (x 0, 10, 20, 30 and 33.3) glasses.
the values fall in the area which is assigned to the structural unit TeO3 tp of terminal TeO3 type. This result supports the previous assumption that alkali tellurite glasses mainly consist of TeO4 tbp and TeO3 tp. Fig. 7 plots the fractions of TeO4 tbp (N4 ) and TeO3 tp (N3 ) as a function of M2 O content in the M2 O±TeO2 glasses. N3 increases and N4 decreases almost linearly with increasing M2 O content. Since the changes of N3 and N4 for each M2 O±TeO2
S. Sakida et al. / Journal of Non-Crystalline Solids 243 (1999) 13±25
19
Table 1 Chemical shift parameters used in the simulated spectra, chemical shift anisotropy and asymmetry parameter Glass x/mol%
Pro®le 1 (TeO4 ) d1
d2
Pro®le 2 (TeO3 ) d3
diso
Dd
g
d1
d2
d3
diso
Dd
g
Area/%
620
1477
)1285
0.74 100
)
)
)
)
)
)
0
720 520 610 690
1577 1377 1467 1547
)1285 )1285 )1285 )1285
0.74 0.74 0.74 0.74
79 58 35 28
2290 2290 2190 2210
2070 2070 1970 1990
1270 1270 1170 1190
1877 1877 1777 1797
)910 )910 )910 )910
0.36 0.36 0.36 0.36
21 42 65 72
640 610 620 750
1497 1467 1477 1607
)1285 )1285 )1285 )1285
0.74 0.74 0.74 0.74
78 57 35 27
2310 2470 2270 2250
2100 2260 2060 2040
1430 1590 1390 1370
1947 2107 1907 1887
)775 )775 )775 )775
0.41 0.41 0.41 0.41
22 43 65 73
xK2 O á (100)x)TeO2 glasses x 10 2350 1720 x 20 2190 1560 x 25 2240 1610
750 590 640
1607 1447 1497
)1285 )1285 )1285
0.74 80 0.74 58 0.74 47
2380 2340 2370
2150 2110 2140
1460 1420 1450
1997 1957 1987
)815 )815 )815
0.42 0.42 0.42
20 42 53
xRb2 O á (100)x)TeO2 x 10 2340 x 20 2300 x 25 2150
glasses 1710 1670 1520
740 700 550
1597 1557 1407
)1285 )1285 )1285
0.74 80 0.74 59 0.74 48
2210 2370 2310
2030 2190 2130
1400 1560 1500
1880 2040 1980
)730 )730 )730
0.37 0.37 0.37
20 41 52
xCs2 O á (100)x)TeO2 glasses x 10 2310 1680 x 15 2320 1690
710 720
1567 1577
)1285 )1285
0.74 78 0.74 69
2460 2390
2240 2170
1600 1530
2100 2030
)765 )765
0.45 0.45
22 31
TeO2 glass 2220 1590 xLi2 O á (100)x)TeO2 glasses x 10 2320 1690 x 20 2120 1490 x 30 2210 1580 x 33.3 2290 1660 xNa2 O á (100)x)TeO2 x 10 2240 x 20 2210 x 30 2220 x 33.3 2350
glasses 1610 1580 1590 1720
Area/%
All the values except g and area have a unit of ppm. The errors of d1 , d2 and d3 are 10 ppm. The errors of area are 2 %.
4.2. Structural model for tellurite glasses
glass system are almost the same, the structures of M2 O±TeO2 glasses are similar to one another. The broken curves in Fig. 7 denote the calculated amounts of TeO4 tbp remaining and TeO3 tp produced when the added M2 O is completely used to transform TeO4 into TeO3 . The calculated values of N3 and N4 can be given by Eqs. (5) and (6), respectively.
A model presenting a structural change on addition of a network modifying oxide to TeO2 glass is proposed by Himei et al. [22]. When the transformation of TeO4 tbp to TeO3 tp occurs, the following equilibrium reactions may be established in the glass melts:
N3 % 200x= 100 ÿ x;
5
TeO4=2 $ O2=2 Te@O;
7
N4 % 100 ÿ N3 %;
6
O3=2 Te±Oÿ $ O1=2 Te @OAOÿ
8
where x is the M2 O content (mol%). As seen in Fig. 7, the discrepancy between the experimental and theoretical values at the same composition increases with the increase of M2 O content, indicating that all the alkali oxides added are not used to form TeO3 tp.
where TeO4=2 and O2=2 Te@O units are represented as TeO2 , and O3=2 Te±Oÿ . The O1=2 Te(@O)±Oÿ unit makes up the basic structural units present in typical tellurite crystals, represented as 1/2[Te2 O5 ]2ÿ . Then the vitri®cation reaction can be described as
20
S. Sakida et al. / Journal of Non-Crystalline Solids 243 (1999) 13±25
MO±TeO2 (M Ba and Zn) glasses from the intensity ratio of the corresponding Raman peaks and discussed the glass structure on the basis of the model studied above. However, since their method does not give unique values of b and c, the molar ratios of the structural units in tellurite glasses cannot be determined. According to a molecular orbital calculation on clusters modeling TeO2 glass by Uchino et al. [31], it is pointed out that if the O2=2 Te O unit exists, a peak would appear near 900 cmÿ1 in the Raman spectrum. Such a peak, however, does not appear near 900 cmÿ1 in the experimental Raman spectra of tellurite glasses. Therefore, it can safely be said that the O2=2 Te O unit does not exist, that is, c 0. Eq. (9) is rewritten using c 0 and a 1 ) b: yO2ÿ TeO4=2 ! 2yf 1 ÿ bO3=2 TeOÿ bO1=2 Te @OAOÿ g 1 ÿ 2yTeO4=2 :
11
The ratio [TeO3 ]/[TeO4 ] is described as follows Fig. 7. Plots of the fraction of N4 (top) and N3 (bottom) vs. M2 O content in the M2 O±TeO2 (M Li, Na, K, Rb and Cs) glasses. The broken curves denote the theoretical curves (see text).
yO2ÿ TeO4=2 ! 2yfaO3=2 TeAOÿ bO1=2 Te @OAOÿ g 9 cyO2=2 Te@O 1 ÿ 2y ÿ cyTeO4=2 ; where a; b and c are the fractions and a + b 1. Eq. (9) holds only when 0 6 y 6 0.5. When y > 0.5, Eq. (9) cannot be used since (1 ) 2y ) cy) < 0. [O2=2 Te(±O)2 ]2ÿ (TeO4 tbp with two non-bridging oxygens (NBO)) and isolated (TeO3 tp) are considered to be hardly TeO2ÿ 3 formed when 0 6 y 6 0.5. The molar ratio of TeO3 /TeO4 is thus represented by Eq. (10) as a function of b, c and y TeO3 2b cy : TeO4 1 ÿ 2b cy
10
Himei et al. [22] estimated the molar ratio of TeO3 / TeO4 for R2 O±TeO2 (R Li, Na and K) and
TeO3 2yb : TeO4 1 ÿ 2yb
12
The y value is known from the glass composition. Since the ratio [TeO3 ]/[TeO4 ] is directly obtained in this NMR work, Eq. (12) gives the b values. The fractions of TeO4 tbp without a non-bridging oxygen (NBO) [TeO4=2 (N04 )], TeO4 tbp with an NBO [O3=2 Te±Oÿ (Nÿ 4 )] and TeO3 tp with two NBO's [O1=2 Te(@O)±Oÿ (Nÿ 3 )] can be calculated from the ÿ ÿ b values. The values of N04 , Nÿ 4 , N3 , N4 / ÿ + N ) and the average coordination number (Nÿ 3 4 of Te atoms Nave in M2 O±TeO2 glasses are summarized in Table 2. As clearly seen from this table, all these values are not sensitive to alkali oxide and show similar composition dependences. In Table 3 the average coordination numbers Nave of Te atoms in xLi2 O á (100 ) x)TeO2 glasses obtained by NMR (this work) are compared with those obtained by EXAFS [12,24] and RDF [12]. Although the Nave values have a tendency to decrease with increasing Li2 O content, the composition change of Nave is not systematic except in the present result. Therefore, it can be said that the Nave derived from the present NMR study appears
S. Sakida et al. / Journal of Non-Crystalline Solids 243 (1999) 13±25
21
Table 2 ÿ ÿ ÿ ÿ The percentage of the fractions of several tellurite structural units; N04 , Nÿ 4 and N3 , the ratio N4 /(N3 + N4 ), and the average coordination number Nave of Te atoms Glass x/mol%
N04 /%
Nÿ 4 /%
Nÿ 3 /%
ÿ ÿ Nÿ 4 /(N3 + N4 )
TeO2 glass xLi2 O á (100)x)TeO2 glasses x 10 x 20 x 30 x 33.3
100.0
0.0
0.0
Nave
)
4.0
77.8 50.0 14.3 0.0
1.2 8.0 20.7 28.0
21.0 42.0 65.0 72.0
0.06 0.16 0.24 0.28
3.8 3.6 3.4 3.3
xNa2 O á (100)x)TeO2 glasses x 10 x 20 x 30 x 33.3
77.8 50.0 14.3 0.0
0.2 7.0 20.7 27.0
22.0 43.0 65.0 73.0
0.01 0.14 0.24 0.27
3.8 3.6 3.4 3.3
xK2 O á (100)x)TeO2 glasses x 10 x 20 x 25
77.8 50.0 33.3
2.2 8.0 13.7
20.0 42.0 53.0
0.10 0.16 0.21
3.8 3.6 3.5
xRb2 O á (100)x)TeO2 glasses x 10 x 20 x 25
77.8 50.0 33.3
2.2 9.0 14.7
20.0 41.0 52.0
0.10 0.18 0.22
3.8 3.6 3.5
xCs2 O á (100)x)TeO2 glasses x 10 x 15
77.8 64.7
0.2 4.3
22.0 31.0
0.01 0.12
3.8 3.7
Table 3 The average coordination number Nave of Te atoms in xLi2 O á (100)x)TeO2 glasses Glass x/mol%
Nave NMR [This work]
TeO2 glass xLi2 O á (100)x) TeO2 glasses x5 x 10 x 15 x 20 x 25 x 30 a
4.0
a
) 3.8 ) 3.6 ) 3.3
EXAFS [24]
EXAFS [12]
RDF [12]
4.0
)
)
3.8 3.4 3.3 3.2 3.1 3.1
) ) 3.7 4.0 3.8 3.8
) ) 3.7 3.8 3.6 3.4
The value for TeO2 glass is assumed to be four.
more reliable than those obtained by EXAFS and RDF. ÿ Variations of N04 , Nÿ 4 and N3 with the Li2 O content in the Li2 O±TeO2 glass system are shown in Fig. 8 as an example. N04 decreases, while Nÿ 4 and Nÿ 3 increase with increasing Li2 O content. It is
interesting to note that the initial addition of M2 O to TeO2 glass results in the formation not of Nÿ 4 ÿ but of Nÿ 3 , and the N4 increases rapidly above 20 mol% M2 O. This behavior can be explained as follows. Owing to the resonance between the Te±Oÿ and
22
S. Sakida et al. / Journal of Non-Crystalline Solids 243 (1999) 13±25
Fig. 8. Plots of the fractions of several tellurite structural units; ÿ N04 , Nÿ 4 and N3 vs. composition in the Li2 O±TeO2 glasses.
Te@O bonds, the O1=2 Te(@O)±Oÿ unit seems more stable than the O3=2 Te±Oÿ unit and therefore the former unit is the main product at a low M2 O content. As the content of the O1=2 Te(@O)±Oÿ unit increases with increasing M2 O content, a repulsion between the lone pairs of electrons in the
O1=2 Te(@O)±Oÿ units increases markedly, and the relative stability of the O3=2 Te±Oÿ units may increase. The O3=2 Te±Oÿ unit has two variations with one NBO at an axial or an equatorial position since they are formed with equal probability on addition of M2 O to TeO2 as shown in Fig. 9. Recently, Uchino et al. [32,33] have reported that the trio of three-centered orbitals (bonding, nonbonding and antibonding orbitals) of which the electronic con®guration is shown in Fig. 10 is formed in the Oax ±Te±ax O bond of TeO4 tbp unit. When the Te±ax O bond has an NBO, excess electrons in the non-bonding orbital should have an increased opportunity to move into the antibonding orbital just above it, making the Oax ±Te±ax O bond unstable. By contrast, since the Te±eq O bond consists of r bondings formed by sp2 hybrid orbitals from Te5s;5p and O2p atomic orbitals, the formation of an equatorial NBO merely raises the energy level. Therefore, it can be said that the O3=2 Te±Oÿ unit with one NBO is present most probably as the O3=2 Te±eq Oÿ unit and the O3=2 Te±ax Oÿ units are easily transformed into the O1=2 Te(@O)±Oÿ (TeO3 tp) units. 4.3. Mechanism for the structural change of tellurite glasses Although a mechanism for the structural change of the tellurite glasses is proposed by
Fig. 9. A model showing a modi®cation of TeO2 glass network by M2 O.
S. Sakida et al. / Journal of Non-Crystalline Solids 243 (1999) 13±25
Fig. 10. The electronic con®guration of a Te atom in TeO4 tbp after Uchino et al. [32,33].
Yoko et al. [16] and Sekiya et al. [20], the structural unit of O1=2 Te(@O)±Oÿ which has been experimentally con®rmed to be present in this work has not been taken into account. We propose a new mechanism for the structural change which includes the structural unit of O1=2 Te(@O)± Oÿ . As illustrated in Fig. 11, when two MO1=2 are added to TeO2 glass containing deformed spirals formed by sharing the corners of TeO4 tbps as in a-TeO2 (step I), they break the Te±eq Oax )Te linkage to form two O3=2 Te±Oÿ units having Te±eq Oÿ and Te±ax Oÿ bonds. That is, O3=2 Te±eq Oÿ and O3=2 Te±ax Oÿ units are formed (step II). The mechanism of formation of the O1=2 Te(@O)±Oÿ unit from the O3=2 Te±eq Oÿ and O3=2 Te±ax Oÿ units is as follows: 1. Since the increase of the electron-donating ability in Te(1)±eq Oÿ (1) bond in O3=2 Te(1)±eq Oÿ and the formation of a Te(3)±ax Oÿ (2) bond in O3=2 Te(3)±ax Oÿ both make the charge around Te atoms excessively negative, a Te(1)±ax O1=2 (6) bond in O3=2 Te(1)±eq Oÿ and a Te(3)±ax O1=2 (4) bond in O3=2 Te(3)±ax Oÿ are weakened, resulting in the elongation of Te±ax O1=2 bonds and Te(3)- - -ax O1=2 (4) (Te(1)- - -ax O1=2 (6) bonds). 2. Simultaneously with (1), the Te(4)±ax O1=2 (5) bond in Te(4)O4=2 and the Te(2)±ax O1=2 (3) bond in Te(2)O4=2 are elongated, forming Te(4)- - -ax O1=2 (5) and Te(2)- - -ax O1=2 (3) bonds, respectively (step III). 3. The Te(1)- - -ax O1=2 (6) and the Te(4)- - -ax O1=2 (5) bonds formed at step III are cleaved by the transfers of electron, forming a O1=2 Te(1)(@O)±Oÿ (step IV). Similarly, the
23
Te(3)- - -ax O1=2 (4) and the Te(2)- - -ax O1=2 (3) bonds are cleaved by the transfers of electron, forming a O1=2 Te(3)(@O)±Oÿ (step IV). Then, although deformed spirals are formed, the deformation can be relaxed by the increase in the degree of freedom of a glass network. Thus, Te±eq Oax ±Te linkages are broken by the addition of a network modifying oxide and the constrains of the network are reduced in tellurite glasses. Therefore, tellurite glasses are easily vitri®ed by the addition of a network modifying oxide. However, the excessive addition of a network modifying oxide reverses the process and makes vitri®cation at a high M2 O composition dicult. 5. Conclusion The 125 Te static and MAS NMR spectra have been measured for TeO2 and M2 O±TeO2 (M Li, Na, K, Rb and Cs) glasses containing a trace of Fe2 O3 . The following conclusions were obtained. 1. The addition of Fe2 O3 allows the measurement of 125 Te NMR spectra at a pulse delay of 2.5 s, which was remarkably shortened compared to 20 s without Fe2 O3 . The S/N ratio of spectra containing 0.3 mol% Fe2 O3 was the highest. 2. TeO2 glass has a structure in which the TeO4 tbps are connected by sharing corners as in aTeO2 . 3. The fraction of TeO3 tp (N3 ) increases and that of TeO4 tbp (N4 ) decreases almost linearly with increasing M2 O content. The structures of M2 O±TeO2 glasses with the same composition are very similar to one another. 4. The fraction of TeO4 tbp without a non-bridging oxygen (NBO) [TeO4=2 (N04 )] decreases, and the fractions of TeO4 tbp with an NBO atom [O3=2 Te±Oÿ (Nÿ 4 )] and TeO3 tp with two NBO atoms [O1=2 Te(@O)±Oÿ (Nÿ 3 )] increase with increasing M2 O content. 5. The ratio of O3=2 Te±Oÿ (TeO4 tbp) to O1=2 Te(@O)±Oÿ (TeO3 tp) increases with increasing M2 O content. This is attributed to the repulsion between the lone pairs of electrons of excessive O1=2 Te(@O)±Oÿ tp groups. 6. When M2 O is added to the tellurite glass, TeO4=2 (TeO4 tbp) changes to O3=2 Te±Oÿ
24
S. Sakida et al. / Journal of Non-Crystalline Solids 243 (1999) 13±25
Fig. 11. Mechanism for the structural change induced by the addition of MO1=2 component to the network composed of TeO4 tbp's.
S. Sakida et al. / Journal of Non-Crystalline Solids 243 (1999) 13±25
(TeO4 tbp) ®rst, and then O3=2 Te±Oÿ (TeO4 tbp) to O1=2 Te(@O)±Oÿ (TeO3 tp) depending on the composition. Acknowledgements The authors thank Professor F. Horii, Mrs K. Omine and Dr H. Kaji of Kyoto University for their helpful advice and assistance in the NMR measurements. They also thank Dr T. Uchino of Kyoto University for Fig. 10 obtained on the basis of the ab initio molecular orbital calculation on clusters modeling TeO2 glass. One of the authors (T.Y.) also acknowledges a grant by the Asahi Glass Foundation. References [1] H. Nasu, O. Matsusita, K. Kamiya, H. Kobayashi, K. Kubodera, J. Non-Cryst. Solids 124 (1990) 275. [2] S. Tanabe, K. Hirao, N. Soga, J. Non-Cryst. Solids 122 (1990) 79. [3] J.E. Stanworth, Nature 169 (1952) 581. [4] J.E. Stanworth, J. Soc. Glass Tech. 36 (1952) 217T. [5] H. Burger, W. Vogel, V. Kozhukharov, Infrared Phys. 25 (1985) 395. [6] A.K. Yakhkind, J. Am. Ceram. Soc. 49 (1966) 670. [7] M.J. Redman, J.H. Chen, J. Am. Ceram. Soc. 50 (1967) 523. [8] G.W. Brady, J. Chem. Phys. 24 (1956) 477. [9] G.W. Brady, J. Chem. Phys. 27 (1957) 300. [10] T. Yoko, H. Takeuchi, K. Kamiya, K. Tanaka, The 28th Symposium on Glass Abstracts, Tokyo, (1987) 67. [11] Y. Dimitriev, V. Dimitrov, E. Gatev, E. Kashchieva, H. Petkov, J. Non-Cryst. Solids 95&96 (1987) 937.
25
[12] Y. Shimizugawa, T. Maeseto, S. Suehara, S. Inoue, A. Nukui, J. Mater. Res. 10 (1995) 405. [13] S. Neov, V. Kozhukharov, I. Gerasimova, K. Krezhov, B. Sidzhimov, J. Phys. C: Solid State Phys. 12 (1979) 2475. [14] M. Ueno, M. Misawa, K. Suzuki, Res. Rep. Lab. Nucl. Sci. Tohoku Univ. 14 (1981) 33. [15] N. Mochida, K. Takahashi, K. Nakata, S. Shibusawa, Yogyo-Kyokai-Shi 86 (1978) 316. [16] T. Yoko, K. Kamiya, K. Tanaka, H. Yamada, S. Sakka, J. Ceram. Soc. Jpn. 97 (1989) 289. [17] J. Heo, D. Lam, G.H. Sigel, Jr., E.A. Mendoza, D.A. Hensley, J. Am. Ceram. Soc. 75 (1992) 277. [18] T. Sekiya, N. Mochida, A. Ohtsuka, M. Tonokawa, J. Ceram. Soc. Jpn. 97 (1989) 1435. [19] T. Yoko, F. Miyaji, M. Fujita, S. Sakka, Trans. Ind. Ceram. Soc. 50 (1992) 158. [20] T. Sekiya, N. Mochida, A. Ohtsuka, M. Tonokawa, J. Non-Cryst. Solids 144 (1992) 128. [21] M. Tatsumisago, T. Minami, Y. Kowada, H. Adachi, Phys. Chem. Glasses 35 (1994) 89. [22] Y. Himei, A. Osaka, T. Nanba, Y. Miura, J. Non-Cryst. Solids 177 (1994) 164. [23] T. Yoko, M. Fujita, F. Miyaji, S. Sakka, Chem. Expr. 5 (1990) 549. [24] H. Yamamoto, H. Nasu, J. Matsuoka, K. Kamiya, J. NonCryst. Solids 170 (1994) 87. [25] T. Nishida, S. Saruwatari, Y. Takashima, Bull. Chem. Soc. Jpn. 61 (1988) 4093. [26] T. Nishida, M. Yamada, H. Ide, Y. Takashima, J. Mater. Sci. 25 (1990) 3546. [27] M.J. Collins, J.A. Ripmeester, J.F. Sawyer, J. Am. Chem. Soc. 109 (1987) 4113. [28] S. Sakida, S. Hayakawa, T. Yoko, this issue, p. 1. [29] K.A. Smith, R.J. Kirkpatrick, E. Old®eld, D.M. Henderson, Am. Mineral. 68 (1983) 1206. [30] S. Hayashi, K. Hayamizu, Bull. Chem. Soc. Jpn. 63 (1990) 961. [31] T. Uchino, T. Yoko, J. Non-Cryst. Solids 204 (1996) 243. [32] T. Uchino, T. Yoko, New Glass 10 (1995) 41 (in Japanese). [33] T. Uchino, S.H. Kim, T. Yoko, T. Fukunaga, J. Ceram. Soc. Jpn. 105 (1997) 201.
Part 2.125 Te NMR study of M2O±TeO2 (M Li, Na, K, Rb and Cs) glasses Shinichi Sakida b
a,* ,
Satoshi Hayakawa b, Toshinobu Yoko
a
a Institute for Chemical Research, Kyoto University, Uji, Kyoto-fu 611, Japan Faculty of Engineering, Okayama University, Okayama-shi, Okayama 700, Japan
Received 6 January 1998; received in revised form 18 September 1998
Abstract The structure of TeO2 and M2 O±TeO2 (M Li, Na, K, Rb and Cs) glasses has been investigated by means of 125 Te static and magic-angle spinning (MAS) NMR spectroscopies. On the basis of the relationship between the local structures around Te atoms and the NMR parameters obtained by 125 Te NMR spectra of various tellurite crystals, the local structures around Te atoms in TeO2 and M2 O±TeO2 glasses were examined. The spectral simulation of static NMR spectra enabled us to discriminate between TeO3 trigonal pyramid (tp) and TeO4 trigonal bipyramid (tbp) and to estimate the fractions of TeO3 tp (N3 ) and TeO4 tbp (N4 ). It was concluded that in TeO2 glass TeO4 tbps are connected by sharing their corners as in a-TeO2 crystal. N3 increased and N4 decreased with increasing M2 O content. On the basis of a previous structural model, the fractions of TeO4 tbp without a non-bridging oxygen (NBO) atom, TeO4 tbp with an NBO atom, and TeO3 tp with two NBO atoms were calculated. A new model which presents the structural change for M2 O±TeO2 glasses has been proposed. Ó 1999 Elsevier Science B.V. All rights reserved.
1. Introduction Tellurite glasses are considered as excellent candidate materials for optical switching devices [1] and laser hosts [2] because of their desirable physical properties, such as high refractive index, excellent infrared transmittance and high dielectric constant, good chemical durability and low temperature melting [3±7]. Structures of pure TeO2 glass and M2 O±TeO2 (M Li, Na, K, Rb and Cs) glasses have been investigated by means of X-ray diraction [8±12], neutron diraction [13,14], infrared spectroscopy
* Corresponding author. Tel.: +81-774 383 131; fax: +81-774 335 212; e-mail: [email protected].
[11,15±17], Raman spectroscopy [17±22], magicangle spinning (MAS) NMR [19,23], EXAFS [12,24] and M ossbauer spectroscopy [25,26]. The following are the recent ®ndings: (1) pure TeO2 glass consists of TeO4 trigonal bipyramids (tbp0 s) in which one equatorial site of the Te sp3 d hybrid orbitals is occupied by a lone pair of electrons and the other two equatorial and axial sites are occupied by oxygen atoms, and (2) addition of M2 O to pure TeO2 glass changes the Te coordination polyhedron from TeO4 tbp to TeO3 trigonal pyramid (tp) in which one of the Te sp3 hybrid orbitals is occupied by a lone pair of electrons. However, the symmetry around Te atoms with a lone pair of electrons is so low that a detailed analysis of the glass structure is very dicult. It can therefore be said that any decisive
0022-3093/99/$ ± see front matter Ó 1999 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 3 0 9 3 ( 9 8 ) 0 0 8 1 2 - 6
14
S. Sakida et al. / Journal of Non-Crystalline Solids 243 (1999) 13±25
conclusion on the glass structures has not been obtained yet. NMR spectroscopy is a powerful tool that can reveal the detailed local structure around a nucleus, since it can acquire direct information about the detailed local structure around a given nucleus of interest. Although a 125 Te MAS NMR study of tellurite glasses was performed by Yoko et al. [19,23], spectra with low S/N ratio were only obtained of the Te nuclei with I )1/2 due to very long relaxation times (about 20 s). Accordingly, only qualitative analysis could be performed. In the present work, aiming at shorter measuring time and improvement of S/N ratio, 125 Te static and MAS NMR spectra are measured for M2 O±TeO2 (M Li, Na, K, Rb and Cs) glasses containing a trace amount of Fe2 O3 . On the basis of the more reliable ratio of TeO3 tp and TeO4 tbp in M2 O±TeO2 glasses obtained from the present NMR measurements, the structure of M2 O±TeO2 glasses is discussed quantitatively. A new model of vitri®cation and structural change for M2 O±TeO2 glasses is proposed. 2. Experimental 2.1. Preparation of glasses In order to attain both shorter measuring times and an improvement in S/N ratio by shortening the relaxation times of Te nuclei with I 1/2, tellurite glasses were doped with a trace of Fe2 O3 . Tests were conducted to ®nd the Fe2 O3 content giving the highest S/N ratio by using 30Li2 O á 70TeO2 á pFe2 O3 (p 0, 0.05, 0.1, 0.2, 0.3, 0.4 and 0.5) glasses. TeO2 , xLi2 O á (100 ) x)TeO2 (x 10, 20, 30 and 33.3), xNa2 O á (100 ) x)TeO2 (x 10, 20, 30 and 33.3), xK2 O á (100 ) x)TeO2 (x 10, 20 and 25), xRb2 O á (100 ) x)TeO2 (x 10, 20 and 25) and xCs2 O á (100 ) x)TeO2 (x 10 and 15) glasses containing a trace of Fe2 O3 were prepared. Reagent-grade b-TeO2 , Cs2 CO3 , Fe2 O3 , Li2 CO3 , Na2 CO3 , K2 CO3 and Rb2 CO3 were used as starting materials. 1±3 g batches of well-mixed reagents were melted in a Pt-5%Au crucible at 700±800°C for 10 min in air. The melt was poured onto a stainless plate and immediately pressed by
another stainless plate. For pure TeO2 glass, a very small amount of batch (50±100 mg) was melted in a Pt-5%Au crucible at 830°C for 10 min in air. The melt was quenched by immersing the bottom of the crucible into a freezing mixture consisting of NaCl and ice water kept at about )10°C. In most cases, the quenched melts were identi®ed to be glassy by visual inspection. X-ray diraction was used in some cases to con®rm visual observation. 2.2. NMR measurements 125 Te static and MAS NMR spectra of powdered tellurite glasses were measured at 126.32 MHz (9.4 T) on a JEOL JNM-GSX 400 MAS FT± NMR spectrometer. A single pulse sequence was used which is characterized by a pulse length of 2.5 ls and 1.0 ls, an accumulation of 1000 and 4325 scans, pulse delays of 2.5 s and 20 s, and a dead time of 0.5 and 4.0 ls for 30Li2 O á 70TeO2 á pFe2 O3 glasses with and without a trace of Fe2 O3 , respectively. A pulse length of 2.5 ls, an accumulation of 8000±20000 scans, a pulse delay of 2.5 s and a dead time of 1.5 ls were employed for M2 O± TeO2 glasses. A cylindrical zirconia rotor containing a glass sample was spun at a speed of about 5±6 kHz in the 125 Te MAS NMR measurements. Telluric acid Te(OH)6 was used as a secondary standard whose chemical shift was d 692.2 ppm with reference to (CH3 )2 Te [27].
2.3.
125
Te static NMR spectra
The principal components of the chemical shift tensors, d1 , d2 and d3 were estimated by ®tting the calculated theoretical spectra to the experimental static NMR spectra as described in a previous paper [28]. The isotropic chemical shift diso static , the chemical shift anisotropy Dd and the asymmetry parameter g were also determined by using the following de®nitions [29,30]: diso static
d 1 d2 d3 ; 3
1
Dd d3 ÿ
d1 d2 ; 2
2
S. Sakida et al. / Journal of Non-Crystalline Solids 243 (1999) 13±25
g
d 2 ÿ d1 : d3 ÿ diso static
15
3
The principal components of the chemical shift tensors can be determined based on the following relation. d3 ÿ diso static P d1 ÿ diso static P d2 ÿ diso static : 4 3. Results The 125 Te MAS NMR spectra of 30Li2 O á 70TeO2 á pFe2 O3 (p 0, 0.05, 0.1, 0.2, 0.3, 0.4 and 0.5) glasses are shown in Fig. 1. It is clear that the isotropic chemical shift does not change with the addition of a small amount of Fe2 O3 . This result was also demonstrated experimentally for several tellurite crystals in a previous paper [28]. Since the peaks of all glasses appeared at 2000 50 ppm, the structural change from the addition of a small amount of Fe2 O3 was considered to be negligible. This result enabled us to measure 125 Te NMR spectra at a pulse delay of 2.5 s, which was remarkably shortened compared to 20 s without Fe2 O3 . The S/N ratio of spectra containing 0.1±0.3 mol% Fe2 O3 was consequently higher than that without Fe2 O3 . Since the S/N ratio of spectra containing 0.3 mol% Fe2 O3 was the highest, 125 Te NMR spectra were measured for TeO2 and M2 O± TeO2 (M Li, Na, K, Rb and Cs) glasses containing 0.3 mol% Fe2 O3 . Fig. 2 shows the 125 Te MAS NMR spectra of Li2 O±TeO2 glasses. Whereas MAS NMR spectra of tellurite crystals (Fig. 5 in Ref. [28]) were composed of an isotropic chemical shift and a number of spinning side bands (SSB), all the spectra of glasses consisted of one broad peak without splitting in the range of 500±2800 ppm, which was obviously due to variations in site symmetry. Accordingly, isotropic chemical shifts could not be obtained from MAS NMR spectra of tellurite glasses. The intensity of the peaks in the range of 500±1800 ppm decreased with increasing Li2 O content, suggesting that the local structure around Te atoms in glasses changes with the Li2 O content. Fig. 3 shows the 125 Te static NMR spectra of Li2 O±TeO2 glasses together with those of a-TeO2
Fig. 1. 125 Te MAS NMR spectra of 30Li2 O á 70TeO2 á pFe2 O3 (p 0, 0.05, 0.1, 0.2, 0.3, 0.4 and 0.5) glasses. x 0: 1.0 ls pulse length, 20 s pulse delay, 4325 scans. Others: 2.5 ls pulse length, 2.5 s pulse delay, 1000 scans.
and Li2 TeO3 crystals. All the spectra of glasses consisted of one broad peak without splitting in the range of 500 to 2800 ppm and the line pro®les were very similar to those of the MAS spectra shown in Fig. 2. The line pro®les of the 125 Te static and MAS NMR spectra of M2 O±TeO2 (M Na, K, Rb and Cs) glasses which are not shown here were basically similar to those of Li2 O±TeO2 glasses. The similarity of the line pro®les between 125 Te MAS and static spectra have been observed in the
16
S. Sakida et al. / Journal of Non-Crystalline Solids 243 (1999) 13±25
Fig. 2. 125 Te MAS NMR spectra of xLi2 O á (100 ) x)TeO2 (x 0, 10, 20, 30 and 33.3) glasses.
tellurite glasses as shown in Figs. 2 and 3. The reason is discussed below. It is generally known that the intervals between neighboring peaks depend on sample spinning speed, and the intensity and the number of SSBs decreases with increasing sample spinning speed. In the case of tellurite glasses, a speed of more than 290 kHz is required to eliminate most of the SSB's. In other words, the chemical shift anisotropy of tellurite glasses is too large to be eliminated at a speed of about 5±6 kHz employed in this work, and therefore the SSB
Fig. 3. 125 Te static NMR spectra of xLi2 O á (100 ) x)TeO2 (x 0, 10, 20, 30 and 33.3) glasses and a-TeO2 and Li2 TeO3 crystals.
peaks should have hardly decreased in intensity or number. 4. Discussion 4.1. 125 Te MAS and static NMR spectra and local structure of tellurite glasses In a previous paper [28], the local structure around Te atoms has been classi®ed into ®ve
S. Sakida et al. / Journal of Non-Crystalline Solids 243 (1999) 13±25
17
Fig. 4. Classi®cation of structural units of (1) TeO3 type which consists of (i) isolated TeO3 type and (ii) terminal TeO3 type, (2) TeO31 type, and (3) TeO4 type which consists of (i) a-TeO2 type and (ii) b-TeO2 type.
groups based on the structural units present in tellurite crystals as illustrated in Fig. 4. These are (1) TeO3 type which consists of an isolated TeO3 type and a terminal TeO3 type, (2) TeO31 type and (3) TeO4 type which consists of an a-TeO2 type and a b-TeO2 type. An isolated TeO3 type means that the TeO3 tp structural unit is present as a free anion; the terminal TeO3 type means that the TeO3 tp has one corner which is connected with a neighboring TeOn polyhedron; the TeO31 type means that the TeO3 tp has an additional O atom at a remote distance from 0.22 to 0.25 nm. Furthermore, the a-TeO2 type contains TeO4 tbp structural units which are connected by sharing their corners, and the b-TeO2 type contains Te2 O6 units consisting of two edge-shared TeO4 tbps. The MAS spectra of TeO2 and M2 O±TeO2 glasses are too broad to obtain an isotropic chemical shift. Therefore, further quantitative analysis based on the MAS spectra is impossible. To solve this problem, we turned to the 125 Te static NMR technique for this work. Fig. 5 plots the relationship between the asymmetry parameter g and the absolute value of the chemical shift anisotropy |Dd| for the structural units as found in various tellurite crystals as obtained in a previous paper [28]. A region determined by both g and |Dd|, which corresponds to each structural unit
Fig. 5. Plots of the g vs. the |Dd| for tellurite glasses. Pro®le 1 is the peak on the basis of TeO2 glass. Pro®le 2 was determined from M2 O±TeO2 (M Li, Na, K, Rb and Cs) glasses.
18
S. Sakida et al. / Journal of Non-Crystalline Solids 243 (1999) 13±25
shown in Fig. 4, is depicted in this g-|Dd| diagram. This diagram is useful for the examination of the local structure of tellurite glasses. Once the principal components of the chemical shift tensor, d1 , d2 and d3 , are estimated from the static spectrum of TeO2 glass (x 0), the isotropic chemical shift diso static , Dd and g are calculated from Eqs. (1)± (3). The result is listed in Table 1. The Dd and g of TeO2 glass are )1285 ppm and 0.74, respectively, and are plotted in the g-|Dd| diagram (Fig. 5). Since the point determined from both Dd and g of TeO2 glass is located just in the region of a-TeO2 type, it can be said that TeO2 glass is composed of TeO4 tbps of a-TeO2 type, that is, the TeO4 tbps are connected with each other by sharing corners. As clearly seen from Fig. 3, the line pro®le of 125 Te static NMR spectra of Li2 O±TeO2 glasses varies with increasing alkali oxide content and is getting close to that of the Li2 TeO3 crystal containing only a TeO3 tp as a structural unit. This suggests that the formation of TeO3 tp takes place on addition of Li2 O to TeO2 glass and the content increases with increasing Li2 O concentration. It seems reasonable to assume that M2 O±TeO2 glasses are mainly composed of TeO4 tbp and TeO3 tp as structural units, their fractions depending on composition. To con®rm this assumption, the experimental 125 Te static NMR spectra of alkali tellurite glasses are deconvoluted by using two spectral components: Pro®le 1 is due to TeO4 tbp and Pro®le 2 due to TeO3 tp. An example of line pro®le deconvolution performed for Li2 O±TeO2 glasses is shown in Fig. 6. Here the principal components of the chemical shift tensor, d1 , d2 and d3 , for TeO2 glass are used as the initial values for Pro®le 1. It was assumed for convenience that the shape of Pro®le 1 was the same as that for the TeO2 glass and that of Pro®le 2 was independent of glass composition. That is, the parameters |Dd| and g of Pro®les 1 and 2 of the present alkali tellurite glasses were assumed to be unchanged although their positions were not restricted. The values obtained by the spectral deconvolution for M2 O±TeO2 glasses are summarized in Table 1. The NMR parameters, g and Dd, of Pro®le 2 obtained for respective glass systems are shown in Fig. 5. It is interesting to note that all
Fig. 6. Simulated and experimental 125 Te static NMR spectra of xLi2 O á (100 ) x)TeO2 (x 0, 10, 20, 30 and 33.3) glasses.
the values fall in the area which is assigned to the structural unit TeO3 tp of terminal TeO3 type. This result supports the previous assumption that alkali tellurite glasses mainly consist of TeO4 tbp and TeO3 tp. Fig. 7 plots the fractions of TeO4 tbp (N4 ) and TeO3 tp (N3 ) as a function of M2 O content in the M2 O±TeO2 glasses. N3 increases and N4 decreases almost linearly with increasing M2 O content. Since the changes of N3 and N4 for each M2 O±TeO2
S. Sakida et al. / Journal of Non-Crystalline Solids 243 (1999) 13±25
19
Table 1 Chemical shift parameters used in the simulated spectra, chemical shift anisotropy and asymmetry parameter Glass x/mol%
Pro®le 1 (TeO4 ) d1
d2
Pro®le 2 (TeO3 ) d3
diso
Dd
g
d1
d2
d3
diso
Dd
g
Area/%
620
1477
)1285
0.74 100
)
)
)
)
)
)
0
720 520 610 690
1577 1377 1467 1547
)1285 )1285 )1285 )1285
0.74 0.74 0.74 0.74
79 58 35 28
2290 2290 2190 2210
2070 2070 1970 1990
1270 1270 1170 1190
1877 1877 1777 1797
)910 )910 )910 )910
0.36 0.36 0.36 0.36
21 42 65 72
640 610 620 750
1497 1467 1477 1607
)1285 )1285 )1285 )1285
0.74 0.74 0.74 0.74
78 57 35 27
2310 2470 2270 2250
2100 2260 2060 2040
1430 1590 1390 1370
1947 2107 1907 1887
)775 )775 )775 )775
0.41 0.41 0.41 0.41
22 43 65 73
xK2 O á (100)x)TeO2 glasses x 10 2350 1720 x 20 2190 1560 x 25 2240 1610
750 590 640
1607 1447 1497
)1285 )1285 )1285
0.74 80 0.74 58 0.74 47
2380 2340 2370
2150 2110 2140
1460 1420 1450
1997 1957 1987
)815 )815 )815
0.42 0.42 0.42
20 42 53
xRb2 O á (100)x)TeO2 x 10 2340 x 20 2300 x 25 2150
glasses 1710 1670 1520
740 700 550
1597 1557 1407
)1285 )1285 )1285
0.74 80 0.74 59 0.74 48
2210 2370 2310
2030 2190 2130
1400 1560 1500
1880 2040 1980
)730 )730 )730
0.37 0.37 0.37
20 41 52
xCs2 O á (100)x)TeO2 glasses x 10 2310 1680 x 15 2320 1690
710 720
1567 1577
)1285 )1285
0.74 78 0.74 69
2460 2390
2240 2170
1600 1530
2100 2030
)765 )765
0.45 0.45
22 31
TeO2 glass 2220 1590 xLi2 O á (100)x)TeO2 glasses x 10 2320 1690 x 20 2120 1490 x 30 2210 1580 x 33.3 2290 1660 xNa2 O á (100)x)TeO2 x 10 2240 x 20 2210 x 30 2220 x 33.3 2350
glasses 1610 1580 1590 1720
Area/%
All the values except g and area have a unit of ppm. The errors of d1 , d2 and d3 are 10 ppm. The errors of area are 2 %.
4.2. Structural model for tellurite glasses
glass system are almost the same, the structures of M2 O±TeO2 glasses are similar to one another. The broken curves in Fig. 7 denote the calculated amounts of TeO4 tbp remaining and TeO3 tp produced when the added M2 O is completely used to transform TeO4 into TeO3 . The calculated values of N3 and N4 can be given by Eqs. (5) and (6), respectively.
A model presenting a structural change on addition of a network modifying oxide to TeO2 glass is proposed by Himei et al. [22]. When the transformation of TeO4 tbp to TeO3 tp occurs, the following equilibrium reactions may be established in the glass melts:
N3 % 200x= 100 ÿ x;
5
TeO4=2 $ O2=2 Te@O;
7
N4 % 100 ÿ N3 %;
6
O3=2 Te±Oÿ $ O1=2 Te @OAOÿ
8
where x is the M2 O content (mol%). As seen in Fig. 7, the discrepancy between the experimental and theoretical values at the same composition increases with the increase of M2 O content, indicating that all the alkali oxides added are not used to form TeO3 tp.
where TeO4=2 and O2=2 Te@O units are represented as TeO2 , and O3=2 Te±Oÿ . The O1=2 Te(@O)±Oÿ unit makes up the basic structural units present in typical tellurite crystals, represented as 1/2[Te2 O5 ]2ÿ . Then the vitri®cation reaction can be described as
20
S. Sakida et al. / Journal of Non-Crystalline Solids 243 (1999) 13±25
MO±TeO2 (M Ba and Zn) glasses from the intensity ratio of the corresponding Raman peaks and discussed the glass structure on the basis of the model studied above. However, since their method does not give unique values of b and c, the molar ratios of the structural units in tellurite glasses cannot be determined. According to a molecular orbital calculation on clusters modeling TeO2 glass by Uchino et al. [31], it is pointed out that if the O2=2 Te O unit exists, a peak would appear near 900 cmÿ1 in the Raman spectrum. Such a peak, however, does not appear near 900 cmÿ1 in the experimental Raman spectra of tellurite glasses. Therefore, it can safely be said that the O2=2 Te O unit does not exist, that is, c 0. Eq. (9) is rewritten using c 0 and a 1 ) b: yO2ÿ TeO4=2 ! 2yf 1 ÿ bO3=2 TeOÿ bO1=2 Te @OAOÿ g 1 ÿ 2yTeO4=2 :
11
The ratio [TeO3 ]/[TeO4 ] is described as follows Fig. 7. Plots of the fraction of N4 (top) and N3 (bottom) vs. M2 O content in the M2 O±TeO2 (M Li, Na, K, Rb and Cs) glasses. The broken curves denote the theoretical curves (see text).
yO2ÿ TeO4=2 ! 2yfaO3=2 TeAOÿ bO1=2 Te @OAOÿ g 9 cyO2=2 Te@O 1 ÿ 2y ÿ cyTeO4=2 ; where a; b and c are the fractions and a + b 1. Eq. (9) holds only when 0 6 y 6 0.5. When y > 0.5, Eq. (9) cannot be used since (1 ) 2y ) cy) < 0. [O2=2 Te(±O)2 ]2ÿ (TeO4 tbp with two non-bridging oxygens (NBO)) and isolated (TeO3 tp) are considered to be hardly TeO2ÿ 3 formed when 0 6 y 6 0.5. The molar ratio of TeO3 /TeO4 is thus represented by Eq. (10) as a function of b, c and y TeO3 2b cy : TeO4 1 ÿ 2b cy
10
Himei et al. [22] estimated the molar ratio of TeO3 / TeO4 for R2 O±TeO2 (R Li, Na and K) and
TeO3 2yb : TeO4 1 ÿ 2yb
12
The y value is known from the glass composition. Since the ratio [TeO3 ]/[TeO4 ] is directly obtained in this NMR work, Eq. (12) gives the b values. The fractions of TeO4 tbp without a non-bridging oxygen (NBO) [TeO4=2 (N04 )], TeO4 tbp with an NBO [O3=2 Te±Oÿ (Nÿ 4 )] and TeO3 tp with two NBO's [O1=2 Te(@O)±Oÿ (Nÿ 3 )] can be calculated from the ÿ ÿ b values. The values of N04 , Nÿ 4 , N3 , N4 / ÿ + N ) and the average coordination number (Nÿ 3 4 of Te atoms Nave in M2 O±TeO2 glasses are summarized in Table 2. As clearly seen from this table, all these values are not sensitive to alkali oxide and show similar composition dependences. In Table 3 the average coordination numbers Nave of Te atoms in xLi2 O á (100 ) x)TeO2 glasses obtained by NMR (this work) are compared with those obtained by EXAFS [12,24] and RDF [12]. Although the Nave values have a tendency to decrease with increasing Li2 O content, the composition change of Nave is not systematic except in the present result. Therefore, it can be said that the Nave derived from the present NMR study appears
S. Sakida et al. / Journal of Non-Crystalline Solids 243 (1999) 13±25
21
Table 2 ÿ ÿ ÿ ÿ The percentage of the fractions of several tellurite structural units; N04 , Nÿ 4 and N3 , the ratio N4 /(N3 + N4 ), and the average coordination number Nave of Te atoms Glass x/mol%
N04 /%
Nÿ 4 /%
Nÿ 3 /%
ÿ ÿ Nÿ 4 /(N3 + N4 )
TeO2 glass xLi2 O á (100)x)TeO2 glasses x 10 x 20 x 30 x 33.3
100.0
0.0
0.0
Nave
)
4.0
77.8 50.0 14.3 0.0
1.2 8.0 20.7 28.0
21.0 42.0 65.0 72.0
0.06 0.16 0.24 0.28
3.8 3.6 3.4 3.3
xNa2 O á (100)x)TeO2 glasses x 10 x 20 x 30 x 33.3
77.8 50.0 14.3 0.0
0.2 7.0 20.7 27.0
22.0 43.0 65.0 73.0
0.01 0.14 0.24 0.27
3.8 3.6 3.4 3.3
xK2 O á (100)x)TeO2 glasses x 10 x 20 x 25
77.8 50.0 33.3
2.2 8.0 13.7
20.0 42.0 53.0
0.10 0.16 0.21
3.8 3.6 3.5
xRb2 O á (100)x)TeO2 glasses x 10 x 20 x 25
77.8 50.0 33.3
2.2 9.0 14.7
20.0 41.0 52.0
0.10 0.18 0.22
3.8 3.6 3.5
xCs2 O á (100)x)TeO2 glasses x 10 x 15
77.8 64.7
0.2 4.3
22.0 31.0
0.01 0.12
3.8 3.7
Table 3 The average coordination number Nave of Te atoms in xLi2 O á (100)x)TeO2 glasses Glass x/mol%
Nave NMR [This work]
TeO2 glass xLi2 O á (100)x) TeO2 glasses x5 x 10 x 15 x 20 x 25 x 30 a
4.0
a
) 3.8 ) 3.6 ) 3.3
EXAFS [24]
EXAFS [12]
RDF [12]
4.0
)
)
3.8 3.4 3.3 3.2 3.1 3.1
) ) 3.7 4.0 3.8 3.8
) ) 3.7 3.8 3.6 3.4
The value for TeO2 glass is assumed to be four.
more reliable than those obtained by EXAFS and RDF. ÿ Variations of N04 , Nÿ 4 and N3 with the Li2 O content in the Li2 O±TeO2 glass system are shown in Fig. 8 as an example. N04 decreases, while Nÿ 4 and Nÿ 3 increase with increasing Li2 O content. It is
interesting to note that the initial addition of M2 O to TeO2 glass results in the formation not of Nÿ 4 ÿ but of Nÿ 3 , and the N4 increases rapidly above 20 mol% M2 O. This behavior can be explained as follows. Owing to the resonance between the Te±Oÿ and
22
S. Sakida et al. / Journal of Non-Crystalline Solids 243 (1999) 13±25
Fig. 8. Plots of the fractions of several tellurite structural units; ÿ N04 , Nÿ 4 and N3 vs. composition in the Li2 O±TeO2 glasses.
Te@O bonds, the O1=2 Te(@O)±Oÿ unit seems more stable than the O3=2 Te±Oÿ unit and therefore the former unit is the main product at a low M2 O content. As the content of the O1=2 Te(@O)±Oÿ unit increases with increasing M2 O content, a repulsion between the lone pairs of electrons in the
O1=2 Te(@O)±Oÿ units increases markedly, and the relative stability of the O3=2 Te±Oÿ units may increase. The O3=2 Te±Oÿ unit has two variations with one NBO at an axial or an equatorial position since they are formed with equal probability on addition of M2 O to TeO2 as shown in Fig. 9. Recently, Uchino et al. [32,33] have reported that the trio of three-centered orbitals (bonding, nonbonding and antibonding orbitals) of which the electronic con®guration is shown in Fig. 10 is formed in the Oax ±Te±ax O bond of TeO4 tbp unit. When the Te±ax O bond has an NBO, excess electrons in the non-bonding orbital should have an increased opportunity to move into the antibonding orbital just above it, making the Oax ±Te±ax O bond unstable. By contrast, since the Te±eq O bond consists of r bondings formed by sp2 hybrid orbitals from Te5s;5p and O2p atomic orbitals, the formation of an equatorial NBO merely raises the energy level. Therefore, it can be said that the O3=2 Te±Oÿ unit with one NBO is present most probably as the O3=2 Te±eq Oÿ unit and the O3=2 Te±ax Oÿ units are easily transformed into the O1=2 Te(@O)±Oÿ (TeO3 tp) units. 4.3. Mechanism for the structural change of tellurite glasses Although a mechanism for the structural change of the tellurite glasses is proposed by
Fig. 9. A model showing a modi®cation of TeO2 glass network by M2 O.
S. Sakida et al. / Journal of Non-Crystalline Solids 243 (1999) 13±25
Fig. 10. The electronic con®guration of a Te atom in TeO4 tbp after Uchino et al. [32,33].
Yoko et al. [16] and Sekiya et al. [20], the structural unit of O1=2 Te(@O)±Oÿ which has been experimentally con®rmed to be present in this work has not been taken into account. We propose a new mechanism for the structural change which includes the structural unit of O1=2 Te(@O)± Oÿ . As illustrated in Fig. 11, when two MO1=2 are added to TeO2 glass containing deformed spirals formed by sharing the corners of TeO4 tbps as in a-TeO2 (step I), they break the Te±eq Oax )Te linkage to form two O3=2 Te±Oÿ units having Te±eq Oÿ and Te±ax Oÿ bonds. That is, O3=2 Te±eq Oÿ and O3=2 Te±ax Oÿ units are formed (step II). The mechanism of formation of the O1=2 Te(@O)±Oÿ unit from the O3=2 Te±eq Oÿ and O3=2 Te±ax Oÿ units is as follows: 1. Since the increase of the electron-donating ability in Te(1)±eq Oÿ (1) bond in O3=2 Te(1)±eq Oÿ and the formation of a Te(3)±ax Oÿ (2) bond in O3=2 Te(3)±ax Oÿ both make the charge around Te atoms excessively negative, a Te(1)±ax O1=2 (6) bond in O3=2 Te(1)±eq Oÿ and a Te(3)±ax O1=2 (4) bond in O3=2 Te(3)±ax Oÿ are weakened, resulting in the elongation of Te±ax O1=2 bonds and Te(3)- - -ax O1=2 (4) (Te(1)- - -ax O1=2 (6) bonds). 2. Simultaneously with (1), the Te(4)±ax O1=2 (5) bond in Te(4)O4=2 and the Te(2)±ax O1=2 (3) bond in Te(2)O4=2 are elongated, forming Te(4)- - -ax O1=2 (5) and Te(2)- - -ax O1=2 (3) bonds, respectively (step III). 3. The Te(1)- - -ax O1=2 (6) and the Te(4)- - -ax O1=2 (5) bonds formed at step III are cleaved by the transfers of electron, forming a O1=2 Te(1)(@O)±Oÿ (step IV). Similarly, the
23
Te(3)- - -ax O1=2 (4) and the Te(2)- - -ax O1=2 (3) bonds are cleaved by the transfers of electron, forming a O1=2 Te(3)(@O)±Oÿ (step IV). Then, although deformed spirals are formed, the deformation can be relaxed by the increase in the degree of freedom of a glass network. Thus, Te±eq Oax ±Te linkages are broken by the addition of a network modifying oxide and the constrains of the network are reduced in tellurite glasses. Therefore, tellurite glasses are easily vitri®ed by the addition of a network modifying oxide. However, the excessive addition of a network modifying oxide reverses the process and makes vitri®cation at a high M2 O composition dicult. 5. Conclusion The 125 Te static and MAS NMR spectra have been measured for TeO2 and M2 O±TeO2 (M Li, Na, K, Rb and Cs) glasses containing a trace of Fe2 O3 . The following conclusions were obtained. 1. The addition of Fe2 O3 allows the measurement of 125 Te NMR spectra at a pulse delay of 2.5 s, which was remarkably shortened compared to 20 s without Fe2 O3 . The S/N ratio of spectra containing 0.3 mol% Fe2 O3 was the highest. 2. TeO2 glass has a structure in which the TeO4 tbps are connected by sharing corners as in aTeO2 . 3. The fraction of TeO3 tp (N3 ) increases and that of TeO4 tbp (N4 ) decreases almost linearly with increasing M2 O content. The structures of M2 O±TeO2 glasses with the same composition are very similar to one another. 4. The fraction of TeO4 tbp without a non-bridging oxygen (NBO) [TeO4=2 (N04 )] decreases, and the fractions of TeO4 tbp with an NBO atom [O3=2 Te±Oÿ (Nÿ 4 )] and TeO3 tp with two NBO atoms [O1=2 Te(@O)±Oÿ (Nÿ 3 )] increase with increasing M2 O content. 5. The ratio of O3=2 Te±Oÿ (TeO4 tbp) to O1=2 Te(@O)±Oÿ (TeO3 tp) increases with increasing M2 O content. This is attributed to the repulsion between the lone pairs of electrons of excessive O1=2 Te(@O)±Oÿ tp groups. 6. When M2 O is added to the tellurite glass, TeO4=2 (TeO4 tbp) changes to O3=2 Te±Oÿ
24
S. Sakida et al. / Journal of Non-Crystalline Solids 243 (1999) 13±25
Fig. 11. Mechanism for the structural change induced by the addition of MO1=2 component to the network composed of TeO4 tbp's.
S. Sakida et al. / Journal of Non-Crystalline Solids 243 (1999) 13±25
(TeO4 tbp) ®rst, and then O3=2 Te±Oÿ (TeO4 tbp) to O1=2 Te(@O)±Oÿ (TeO3 tp) depending on the composition. Acknowledgements The authors thank Professor F. Horii, Mrs K. Omine and Dr H. Kaji of Kyoto University for their helpful advice and assistance in the NMR measurements. They also thank Dr T. Uchino of Kyoto University for Fig. 10 obtained on the basis of the ab initio molecular orbital calculation on clusters modeling TeO2 glass. One of the authors (T.Y.) also acknowledges a grant by the Asahi Glass Foundation. References [1] H. Nasu, O. Matsusita, K. Kamiya, H. Kobayashi, K. Kubodera, J. Non-Cryst. Solids 124 (1990) 275. [2] S. Tanabe, K. Hirao, N. Soga, J. Non-Cryst. Solids 122 (1990) 79. [3] J.E. Stanworth, Nature 169 (1952) 581. [4] J.E. Stanworth, J. Soc. Glass Tech. 36 (1952) 217T. [5] H. Burger, W. Vogel, V. Kozhukharov, Infrared Phys. 25 (1985) 395. [6] A.K. Yakhkind, J. Am. Ceram. Soc. 49 (1966) 670. [7] M.J. Redman, J.H. Chen, J. Am. Ceram. Soc. 50 (1967) 523. [8] G.W. Brady, J. Chem. Phys. 24 (1956) 477. [9] G.W. Brady, J. Chem. Phys. 27 (1957) 300. [10] T. Yoko, H. Takeuchi, K. Kamiya, K. Tanaka, The 28th Symposium on Glass Abstracts, Tokyo, (1987) 67. [11] Y. Dimitriev, V. Dimitrov, E. Gatev, E. Kashchieva, H. Petkov, J. Non-Cryst. Solids 95&96 (1987) 937.
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