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Reflection spectra of various kinds of oxide glasses and fluoride glasses in the vacuum ultraviolet region
Journal of Non-Crystalline Solids 72 (1985) 39-50 North-Holland, Amsterdam
39
REFLECTION SPECTRA OF VARIOUS KINDS OF OXIDE GLASSES AND FLUORIDE GLASSES IN THE VACUUM ULTRAVIOLET REGION
Shinichiro HIROTA and Tetsuro IZUMITANI Hoya Corporation, Tokyo 196, Japan
Ryumyo ONAKA * Institute of Applied Physics, University of Tsukuba, Ibaraki 305, Japan Received 30 August 1984 Revised manuscript received 5 November 1984
Reflection spectra of silicate, borate, phosphate, fluorophosphate and fluoride glasses are studied in the spectral region of 2-13 eV in order to understand their dispersion behaviors in the visible region from the point of view of atomic structures. The absorption bands due to bridging oxygen ions or fluorine ions are found at 11.6 eV and 10.4-9.5 eV in silicate glasses, at 10.2 and 8.8 eV in borate glasses, at 9.5 eV in phosphate glasses, at 11.2 eV in fluorophosphate glasses and at 11 eV in fluorozirconate glasses. In silicate glasses, the bands due to nonbridging oxygen ions are found in the region 8.8-4.9 eV. They shift to lower energies with increasing ionic radius, in the order of Ca 2 +, Sr 2 + and Ba 2 +, for the glasses containing low valency cations, while they shift to higher energies with increasing ionic radius, in the order of Ti 4+, Zr 4+ and Th 4+ or in the order of Nb 5+ and Ta 5+, for the glasses containing high valency cations. In glasses containing large amounts of PbO, strong bands due to Pb 2+ ions appear in the lower energy regions of 6.3-5.6 eV and 5.2-4.7 eV.
1. Introduction
According to the Lorentz model, the dispersion (i.e. the refractive index, n, as a function of wavelength, ~) of a material in the transparent region is given by the following equation: n2--1=~"
1
1 '
(1)
J
x2
where Xj is the wavelength of the j t h fundamental absorption band, ~ is its oscillator strength, ~ is the number of relevant ions in a unit volume, and K * Present address: 2-26-20, Kyonancho, Musashino, Tokyo 180, Japan. 0022-3093/85/$03.30 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
S. Hirota et al. / Reflection spectra of oxide glasses
40
is a constant. The fundamental absorption bands consist of the absorption bands due to the electronic transitions in the ultraviolet region and the vibrational transitions in the infrared region. The absorption bands of oxide glasses containing modifier ions are usually composed of the electronic transitions due to bridging- and nonbridging-oxygen ions and in some cases due to additional cations such as Pb 2+ ions in the ultraviolet region. Eq. (1) is therefore rewritten as follows: n2
1
KNlfl
KN2f2
1
1
1 x2
1 x2
KN3f3
~- 1 ~ ~k2
3
KN4f4
1 ~ ' ~k2
~k2
(2)
~k2
where X1, fl and N~ refer to the absorption band due to the bridging oxygen ions, ?~2, f2 and N2 refer to the nonbridging oxygen ions, 2~3, f3 and N3 refer to the additional cations such as Pb 2+ ions, and ~4, f4 and N4 refer to the infrared absorption band. The fundamental absorptions originating from bridging and nonbridging oxygen ions, which are in most cases located in the vacuum ultraviolet region below 200 nm, have rarely been measured [1-3] probably because of technical difficulties in measurement. Therefore, only an empirical formula has been commonly used to describe the dispersion of optical glasses [4]. The wavelengths of the fundamental absorptions were estimated by fitting the dispersion curves measured to the dispersion formula (eq. (1)) using only one or two terms [5-8]. In the case of two terms, one term was for the ultraviolet absorption and the other term for the infrared absorption. The wavelength values obtained from eq. (1), however, may be quite different from the true absorption wavelengths. The actual measurements of the absorption bands in the range including the vacuum ultraviolet region are therefore inevitable for a correct understanding of the dispersion behaviors even in the visible region. Since the peaks in the reflection spectra are close to the absorption peaks, we may be able to discuss the absorption bands by using the reflection spectra. In the present study, the reflection spectra of various optical glasses are studied in the spectral range 2-13 eV for the purpose described above.
2. Experimental 2.1. Preparation of samples Silicate, borate, phosphate, fluorophosphate and fluoride glasses were prepared for the studies of reflection spectra. Reagent-grade SiO2, Na2CO 3, LiECO3, MgCO3, CaCO3, SrCO3, BaCO3, ZnO, PbO, AI(OH)3, Y203, La203, TiO2, ZrO2, ThO2, Nb205 and Ta205 were used as the starting materials of silicate glasses. Batches of these materials of about 50 g were melted in platinum crucibles at 1500-1550°C for 4-4.5 h. Batches of H3BO 3 and
S. Hirota et aL / Reflection spectra of oxide glasses
41
Ba(NO3) 2 were melted in the same way but at 1250°C for 1 h for borate glasses, and those of H3PO 4, BaCO 3 and AI(OH)3 were melted at 1200°C for 1 h for phosphate glasses. For fluorophosphate glasses, batches of P205, A1E~ a n d alkaline earth fluorides were melted at 9 0 0 - 9 5 0 ° C for 30 min in platinum crucibles with platinum covers. For fluoride glasses, batches of ZrF 4, BaF 2 and trivalent metal fluorides were melted at 850°C for 60 min in platinum crucibles with covers. Cast and then annealed samples were cut and ground. After that one surface of the samples was finely polished for the measurements of reflection spectra. 2.2. Optical measurements Since the absorption coefficients of the glasses are extremely high in the vacuum ultraviolet region, and are of the order of 105-106 cm -1, the fundamental absorptions were measured by means of reflection spectra. It is known that the reflection spectra are closely related with the absorption spectra [1], and each peak in the reflection spectra corresponds to an absorption band. A J o h n s o n - O n a k a type vacuum spectrometer [9] with a 50 cm concave grating was used for the optical measurements in the energy range from 6 to 13 eV. The apparatus is shown schematically in fig. 1. The light source was a
20
,
15
! '~
10 5
0
Concave grating '~
Monochromator
Light source ( Hydrogen discharge tube ) Entrance / slit \ . . - ~ ;\j
j Sodium salicylate coated fluorescent screen and Photomuttiplier
o
0
n- 0
o~-._~33.3 olJ12
10 8 6 Z, Photon energy (eV)
Fig. 1. Schematic diagram of the apparatus used. Fig. 2. Reflection spectra of (100-x)SiO2-xNa20 glasses;
x = 0, 20, 25, 29, 33.3and 36.5mol.%.
42
S. Hirota et al. / Reflection spectra of oxide glasses
hydrogen discharge lamp. The angle of incidence for the measurements of reflection spectra was about 10 ° throughout this study. An HTV R-106 photomultiplier with a sodium salicylate plate placed in front of its window was used as the detector. Measurements in the range from 2 to 6.3 eV were carried out with a Hitachi 340 spectrophotometer.
3. Results and discussion
3.1. Reflection spectra of silicate glasses 3.1.1. Alkali silicate glasses Fig. 2 shows the reflection spectra of the glasses of the SiO2-Na20 system. The spectrum of fused silica is characterized by two sharp bands at 11.6 eV (107 nm) and 10.3 eV (120 nm). This spectrum coincides well with those of Philipp [1] and Sigel [2]. The sharp 10.3 eV band has been identified as an excitonic transition [10-12]. The 11.6 eV band has been assigned to the interband transition from a nonbonding level to an antibonding level characteristic of the SiO4 tetrahedron, but there is no universal agreement at the present time. These two bands gradually decrease and shift slightly to lower energy by addition of Na20. A new band appears at 7.5 eV (165 nm) in the glass containing 29 mol.% N a 2 0 and becomes more distinct in the glass of 36.5 mol.% Na20. This weak band is attributed to the nonbridging oxygen ions produced by the presence of Na + ions. This formation of nonbridging oxygen ions increases with the amount of modifier cations. However, the band due to nonbridging oxygen ions was found to be very weak as compared with that due to bridging oxygen ions in the SiO2-Na20 glass system. The reflection spectrum of a glass containing Li2 ° was compared with that of a glass containing N a 2 0 by the amount of 36.5 mol.% alkali oxide. The shapes of the two spectra resemble each other closely as shown in fig. 3.
3.1.2. Alkaline earth silicate glasses The effect of alkaline earth oxide on the reflection spectrum was investigated by substitution of BaO for N a 2 0 in the S i O 2 - N a 2 0 - B a O system. As seen in fig. 4, four bands are observed in the glasses containing higher amounts of BaO. The two bands in the higher energy region do not much differ from the bands of fused silica and are probably due to bridging oxygen ions. The bands in the region 8.5-8.3 eV (145-149 nm) and 7.5-7.0 eV (166-178 nm) are likely due to nonbridging oxygen ions, which may be ascribed to the transitions of the nonbonding electrons in the nonbridging oxygen ions to the antibonding levels. The intensities of the bands due to nonbridging oxygen ions increase with increasing amount of BaO. Although they are very weak, these two bands seem also to exist in glasses of the Si02-Na20 system. The reflection spectra of some binary and ternary alkali silicate glasses have been measured in the wavelength region of 90 to 170 nm by Sigel [2]. The
S. Hirota et al. / Reflection spectra of oxide glasses
43
15
I
lO .~'~'~
J
1
._.~ O.C g
.,.,~1o+ '+~~~++ ._..__~_! I - +oo
;.... 12 10 8 6 4 Photon energy (eV}
~
10.0
"'+o°-~""~'~"v ~ +-~-2s.'r+7~_o -._._ o o
,+ ,i
+ 1
12 10 8 6 4 2 Photon energy (eV)
Fig. 3. Reflection spectra of 63.5 SIO2-36.5M20 glasses; M 2 0 = Li20 and Na20.
Fig. 4. Reflection spectra of 65 SiO2-(35-x)Na20-xBaO glasses; x = 0, 10, 17.5, 25 and 35 mol.%.
spectra of the glasses were characterized by three reflection peaks at approximately 145 nm (8.5 eV), 130 nm (9.3 eV) and 108 nm (11.5 eV) as shown in fig. 5. There is, however, no evidence of the peak observed in our spectra in the region of 160 to 180 nm (7.8 to 6.9 eV). There is a small difference between the spectra of Sigel and ours even in the spectra between 90 nm and 160 nm (13.8 eV and 7.8 eV). Fig. 6 shows the reflection spectra of glasses of the SiO2-SrO and SiO2-BaO systems. The two bands due to nonbridging oxygen ions are recognized in these systems, too. The effect of different kinds of alkaline earth oxide was also investigated in the S i O 2 - N a 2 0 - M O system, MO being MgO, CaO, SrO, BaO, ZnO or PbO. The amounts of SiO 2 and N a 2 0 were kept constant. Results are shown in fig. 7. The bands due to nonbridging oxygen ions shifted to lower energies of the order of CaO ~ SrO---, BaO. In the case of glass containing PbO, two much stronger bands probably due to the intraionic transitions of Pb 2+ ions appeared at 5.96 eV (208 nm) and 4.84 eV (256 nm). It is inferred that these bands are due to the ~S0 ~ ~P~ and 1So ~ 3p1 transition, respectively [13-15].
3.1.3. PbO containing glasses The relationship between the reflection spectra and the amount of PbO was further investigated for lead silicate optical glasses. The results are shown in fig. 8. The amounts of SiO 2 and PbO are expressed in mol.% in table 1. PbO-free K7 glass exhibits two bands at 11.7 eV (106 nm) and 10.2 eV (122 nm) due to bridging oxygen ions. With a decrease of SiO 2 and also an increase
S. Hirota et a L / Reflection spectra of oxide glasses
44
15 10
"~
~--s.r
----_ 35°/o
5 7-
1 Li20-2SiO 2
12
2
10
-
- '~..._\-'-v -B_2.29 0
0
~6
8 6
100
120
140
160
0
180
Wavelength (nm)
12
10
8
6
4
2
Photon energy (eV)
Fig. 5. Reflection spectra of two simple binary silicate glasses (from Sigel, J. Phys. Chem. Solids 32 (1971) 2373). Fig. 6. Reflection spectra of (100-x)SiO2-xMO glasses; MO = SrO and BaO, x = 29, 35 and 39 mol.%.
15~ 15
'
10
I
10
o
i
0
--
0
0
o
0
rr 0
=bC
\ 12
10
8
6
4
Photon energy (eV
12
10
8
/*
Photon energy (eV)
Fig. 7. Reflection spectra of 65 SiO2-10 N a 2 0 - 2 5 MO glasses; MO = MgO, CaO, SrO, BaO, ZnO and PbO. Fig. 8. Reflection spectra of lead silicate optical glasses.
45
S. Hirota el al. / Reflection spectra of oxide glasses
Table 1 Glass compositions, refractive indices and Abbe numbers of lead silicate optical glasses
K7 KF2 LF7 F2 SF8 SF4 SF6
Compositions (mol.%) SiO2 PbO Na20, K20 etc.
Refractive index nd
Abbe's number %
74.6 74.0 74.8 70.2 64.7 61.5 57.3
1.51112 1.52630 1.57501 1.62004 1.68893 1.75520 1.80518
60.5 51.1 41.5 36.3 31.2 27.5 25.5
0.0 4.1 12.8 18.3 27.4 35.3 40.5
25.4 21.9 12.4 11.5 7.9 3.2 2.2
of PbO, the intensities of the 11.7 eV and 10.2 eV bands decrease simultaneously. On the other hand, Pb 2÷ bands appear at 6.3 eV (197 nm) and 5.22 eV (238 nm) in KF2. The peaks of these bands shift to lower energies and their intensities markedly increase with increasing PbO. In SF6 which contains 40.5 mol.% PbO, the peaks lie at 5.55 eV (223 nm) and 4.68 eV (265 nm), and the band intensities are extremely high. The intensity of the 4.68 eV band increases more markedly than that of the 5.55 eV band. This result differs from the result of impurity doped alkali halide crystals [13,14] in which the lower energy band is weaker than the higher one. These strong bands in the low energy region indicate that the glass containing a large amount of PbO causes the high refractive index and the high dispersion (table 1). Besides these bands, two weak bands appear at 8.3 eV (149 nm) and 6.9 eV (181 nm) in F2, SF8, SF4 and SF6. These bands may be due to the formation of nonbridging oxygen ions. 3.1.4. A 120 ~ containing glasses Effects of the addition of A1203 t o silicate glasses on the reflection spectra were investigated in the SiO2-Na20-A1203 system. A120 3 was substituted for N a 2 0 at 65 mol.% SiO 2. Fig. 9 shows that the exciton band appearing at 9.8 eV in the A1203-free glass shifts to lower energies with increasing A120 3. In these cases A1203 is acting as the glass former. 3.1.5. Y,O 3 and LaeO 3 containing glasses Reflection spectra of Y203 and L a 2 0 3 containing glasses were investigated in the S i O 2 - N a 2 0 - Y z O 3 and S i O 2 - N a 2 0 - L a 2 0 3 systems. Y203 or L a 2 0 3 was substituted for N a 2 0 keeping the amount of SiO 2 at 60 mol.% as shown in fig. 10. The band in the region 10.7-11.6 eV is due to the bridging oxygen ions and the band at 7.2-7.6 eV seems to be due to nonbridging oxygen ions. It is not clear whether the band at around 8.6 eV is the shifted exciton band of SiO 4 or is due to nonbridging oxygen ions related to y 3 + or La 3+ ions, or due to overlapping of these two bands.
S. Hirota et al. / Reflection spectra of oxide glasses
46
islr
,
,
15 10
?s
rv"
0 12
10
8
6
4
Photon energy (eV)
2
12
10
I
8
6
~.
2
Photon energy (eV)
Fig. 9. Reflection spectra of 65 SiO2-(35-x)Na20-xA1203 glasses; x = 0, 5, 10 and 15 mol.%. Fig. 10. Reflection spectra of 60 S i O z - ( 4 0 - x ) N a 2 0 - x M 2 0 3 glasses; M203 = Y203 and La203, x = 6 and 12 mol.%.
3.1.6. TiO 2, ZrO 2 and ThO 2 containing glasses Reflection spectra of TiO 2, ZrO 2 and ThO 2 containing glasses were investigated in the S i O 2 - N a 2 0 - M O 2 system, where M O 2 m e a n s TiO 2, ZrO 2 and T h O z. TiO2, ZrO 2 and ThO 2 were substituted for N a 2 0 . As shown in fig. 11, besides the bands due to bridging and nonbridging oxygen ions, a strong band appears at around 5.3, 7.5 and 8.9 eV for Ti 4+, Zr 4+ and Th 4+ containing glass, respectively. The band shifts to higher energies in the order of TiO z ZrO 2 ~ ThO 2. These strong bands might be attributed to the electronic transition from nonbridging oxygen ions combined with Ti 4+, Zr 4+ and Th 4+ to the cations, or due to the formation of complex ions such as TiO 6. Besides these explanations there may be a possibility that these bands are due to the intracationic transition of an electron. Further investigation will be required. 3.1.7. Nb205 and Ta2Q containing glasses Reflection spectra of Nb205 and Ta205 containing glasses were investigated in the S i O 2 - N a 2 0 - M 2 0 5 system where N a 2 0 was substituted by Nb205 or Ta205. As shown in fig. 12, besides the bands due to bridging oxygen ions and to nonbridging oxygen ions, two strong bands characteristic of N b 5÷ or Ta 5+ were found. The two bands are observed at 5.9 eV and 5.0 eV in the Nb205 containing glasses, and at 6.6 eV and 5.6 eV in the Ta205 containing glasses. It is not clear whether these bands are due to nonbridging oxygen ions affected by N b 5+ and Ta 5÷ ions or due to the formation of complex ions. These bands have a tendency to shift to higher energies with increasing ionic radius of the modifier cations. As shown above, it is evident that the bands in the region 8.8-4.9 eV shift to lower energies with increasing ionic radius for the glasses containing low
S. Hirota et al, / Reflection spectra of oxide glasses
47
15
10
15
5
10
5
~o C EJ C
~o "s n--
o 0
0
~f2 10
8
6
Z,
2
12
Photon energy (eV) Fig. 11. R e f l e c t i o n s p e c t r a o f 60
10
8
6
Z,
Photon energy (eV)
Si02-(40-x)Na20-xMO 2 glasses; M O 2 = T i O 2, Z r O 2 a n d
T h O 2, x = 6 a n d 12 mol.%. Fig. 12. R e f l e c t i o n s p e c t r a of 60 S i O 2 - ( 4 0 - x ) N a 2 0 - x M 2 0 5 x = 6 a n d 12 mol.%.
glasses;
M205 = N b 2 0 5 a n d T a 2 0 5 ,
valency cations, while they shift to higher energies with increasing ionic radius for the glasses containing high valency cations. This tendency is the same as that of absorption wavelengths obtained from the calculation with the dispersion formula using one term [8].
3.2. Reflection spectra of borate and phosphate glasses 3.2.1. Borate glasses The reflection spectra of the B203-BaO system are shown in fig. 13. A relatively sharp band and a broad band are observed at 8.6 eV (144 nm) and 10.4 eV(ll9 nm), respectively, in the glass containing 16.1 mol.% BaO. The 10.4 eV band seems to shift to lower energies and its intensity increases with increasing concentration of modifier cations. Both of these bands may be due to bridging oxygen ions. A weak band appears at 7.7 eV (162 nm) for high concentration of BaO, which is considered to be due to nonbridging oxygen ions.
3.2.2. Phosphate glasses In the case of the P2Os-BaO system, a strong band is found at around 9.5 eV (131 nm) and a much weaker band at 7.8 eV (159 nm) as seen in fig. 14. These bands may be due to bridging and nonbridging oxygen ions, respectively.
48
S. Hirota et al. / Reflection spectra of oxide glasses 15
10
10
I
,J
8
6
5 A
o
~o ¢r
o 0
o
12 10
8
6
4
2
Photon energy (eV)
12 10
4
2
Photon energy (eV)
Fig. 13. Reflection spectra of (100-x)B203-xBaO glasses; x =16.1, 22.9, 29.0, 35.0 and 39.7 mol.%. Fig. 14. Reflection spectra of (100-x)P2Os-xBaO glasses; x = 20.0, 27.5, 35.0, 42.5 and 50.0 mol.%.
3.2.3. AleO3 containing phosphate glasses A1203 was substituted for BaO in the P2Os-BaO-A1203 system at the P205 concentration of 65 mol.%. The results are shown in fig 15. No band shift was observed in contrast to the SiO2-Na20-A1203 system. A very weak band seems to lie at 8.5 eV (146 nm) in the glasses with A1203 concentrations of more than 10 mol.%, which suggests that A13 + ions act as the modifier ion.
3.3. Reflection spectra of fluorophosphate and fluoride glasses 3. 3.1. Fluorophosphate glasses Reflection spectra of glasses of the PEOs-A1Fa-MF 2 system are shown in fig. 16 where MF 2 is an alkaline earth fluoride. Two bands are found at 11.3 eV (110 rim) and 10.1 eV (122 nm) in the glass containing 18.3 cationic % POE. s. The band at 11.3 eV becomes sharper both with decreasing P205 concentration and increasing fluoride concentration. The band may be assigned to an electronic transition in [A1Fr] 3- ions [16]. It is characteristic of fluorophosphate glasses that such an intense absorption band is found in the higher energy region than in the oxide glasses. The intensity of the 10.1 eV band decreases with decreasing P205 concentration. This band may be due to the bridging oxygen ions bonding to p5 + ions, although it lies at a higher energy as compared with that of P2Os-BaO glasses. The band due to nonbridging oxygen ions, which is found in the region from 7 to 9 eV in silicate, borate and phosphate glasses, was not found in these fluorophosphate glasses.
49
S. Hirota et al. / Reflection spectra of oxide glasses
15 10 5
15 10
/
~0
o~ 5
~0 Ct~
0
co 0
cY
k
i
i
12
10
8
6
4
12
Photon energy (eV}
10
8
6
Z,
Photon energy (eV)
Fig. 15. Reflection spectra of 65 P2Os-(35-x)BaO-xA1203 glasses; x = 0, 5, 10, 15 and 20 mol.%. Fig. 16. Reflection spectra of POzs-A1F3-MF2 glasses; (1) 18.3 PO2.5-33.5 AIF3-48.2 MF2, (2) 12.2 PO2.5-35.6 AIF3-52.2 MF2, (3) 6.1 PO2.5-39.7 AIF3-54.2 MF2 and (4) 3.1 PO2.5-42.7 AIF~-54.2 MF2.
3.3. 2. Fluoride glasses Reflection spectra of glasses of the ZrF4-BaF 2 and ZrFa-BaF2-MF 3 systems are shown in fig. 17 where MF 3 is GdF3, A1F3 or LaF3. In the case of fluoride glasses, three bands are observed. The 11 eV band may be due to an electronic transition in [ZrF8] 4- ions [17] and the band appearing at around 7.5 eV is likely due to the fluorine ions combined with Ba 2+ ions. The broad band at around 9.5 eV cannot be assigned yet.
15 10 !
0
0
12
10
I
8
6
4
Photon energy (eV)
2
Fig. 17. Reflection spectra of ZrF4-BaF2 and ZrF4-BaF2 - M F 3 glasses; (1) 60 ZrF4-40 BaF2, (2) 62.5 ZrF4-37.5 BaF2, (3) 57.4 ZrF4-34.6 BaF2-3.9 GdF3-4.0 AIF3 and (4) 60 ZrF4-20 BaF2-6 LaF3-14 NaF.
50
S. Hirota et al. / Reflection spectra of oxide glasses
4. Summary Reflection spectra for silicate, borate, phosphate, fluorophosphate and fluoride glasses were measured in the spectral region of 2 - 1 3 eV, and the following assignments were tentatively given to the peaks observed. (1) The bands due to bridging oxygen ions or fluorine ions are at 11.6 eV and 10.4-9.5 eV in silicate glasses, at 10.2 and 8.8 eV in borate glasses, at 9.5 eV in phosphate glasses, at 11.2 eV in fluorophosphate glasses and at 11 eV in fluorozirconate glasses. (2) In silicate glasses, the bands due to nonbridging oxygen ions are found in the region 8.8-4.9 eV. In the case of glasses containing low valency modifier cations, they shift to lower energies with increasing ionic radius of the cations, while in the case of glasses containing high valency cations, they shift to higher energies. (3) In glasses containing large amounts of PbO, strong bands appear in the lower energy regions of 6.3-5.6 eV and 5.2-4.7 eV, which seem to be due to the intraionic transitions of Pb 2+. (4) With the addition of A1203 the exciton b a n d of 9.8 eV shifts to lower energies in silicate glasses, which suggests that A13+ acts as the network former, while a weak b a n d appears at the low energy of 8.5 eV in phosphate glasses, which suggests that A13+ acts as the modifier ion. (5) F l u o r o p h o s p h a t e glasses with low P205 concentrations show a sharp b a n d at the high energy of 11.2 eV, which m a y be due to [A1F6] 3-. (6) Fluorozirconate glasses seem to have three bands. The 11 eV b a n d might be due to [ZrFs] 4- ions. The authors are grateful to Dr T. Mabuchi and Dr S. K i n n o of the University of T s u k u b a for their help with a part of the optical measurements.
References [1] H.R. Philipp, Solid St. Commun. 4 (1966) 73; J. Phys. Chem. Solids 32 (1971) 1935. [2] G.H. Sigel Jr, J. Phys. Chem. Solids 32 (1971) 2373. [3] T. Izumitani and S. Hirota, 2nd Int. Otto-Schott-Kolloquium (1983) p. 227 (Wiss. Ztschr. Friedrich-Schiller-Univ. Jena, Math.-Naturwiss. R., 32 Jg. 1983, H. 2/3). [4] Hoya Optical Glass Catalogue (Hoya Corporation); Schon Optical Glass Catalog No. 3050e (Jenaer Glaswerk Schott&Gen., Mainz), [5] F. Reitmayer, Glastechn. Ber. 34 (1961) 122. [6] K. Kordes, Glastechn. Ber. 38 (1965) 242. [7] S. Hirota and T. Izumitani, Yogyo-Kyokai-shi 84 (1976) 435. [8] S. Hirota and T. Izumitani, J. Non-Crystalline Solids 29 (1978) 109. [9] H. Saito, T. Mabuchi and R. Onaka, Sci. Light 16 (1967) 246. [10] E. Loh, Solid St. Commun. 2 (1964) 269. [11] A.R. Ruffa, Phys. Stat. Sol. 29 (1968) 605. [12] M.G. Reilly, J. Phys. Chem. Solids 31 (1970) 1041. [131 F. Seitz, J. Chem. Phys. 6 (1983) 150. [14] A. Fukuda, Sci. Light 13 (1964) 64. [15] A.J. Bourdillon, F. Khumalo and J. Bordas, Phil. Mag. B37 (1978) 731. [16] D. Ehrt, M. Krauss, C. Erdmann and W. Vogel, Z. Chem. 22 (1982) 315. [17] Y. Kawamoto and T. Horisaka, J, Non-Crystalline Solids 56 (1983) 39.
39
REFLECTION SPECTRA OF VARIOUS KINDS OF OXIDE GLASSES AND FLUORIDE GLASSES IN THE VACUUM ULTRAVIOLET REGION
Shinichiro HIROTA and Tetsuro IZUMITANI Hoya Corporation, Tokyo 196, Japan
Ryumyo ONAKA * Institute of Applied Physics, University of Tsukuba, Ibaraki 305, Japan Received 30 August 1984 Revised manuscript received 5 November 1984
Reflection spectra of silicate, borate, phosphate, fluorophosphate and fluoride glasses are studied in the spectral region of 2-13 eV in order to understand their dispersion behaviors in the visible region from the point of view of atomic structures. The absorption bands due to bridging oxygen ions or fluorine ions are found at 11.6 eV and 10.4-9.5 eV in silicate glasses, at 10.2 and 8.8 eV in borate glasses, at 9.5 eV in phosphate glasses, at 11.2 eV in fluorophosphate glasses and at 11 eV in fluorozirconate glasses. In silicate glasses, the bands due to nonbridging oxygen ions are found in the region 8.8-4.9 eV. They shift to lower energies with increasing ionic radius, in the order of Ca 2 +, Sr 2 + and Ba 2 +, for the glasses containing low valency cations, while they shift to higher energies with increasing ionic radius, in the order of Ti 4+, Zr 4+ and Th 4+ or in the order of Nb 5+ and Ta 5+, for the glasses containing high valency cations. In glasses containing large amounts of PbO, strong bands due to Pb 2+ ions appear in the lower energy regions of 6.3-5.6 eV and 5.2-4.7 eV.
1. Introduction
According to the Lorentz model, the dispersion (i.e. the refractive index, n, as a function of wavelength, ~) of a material in the transparent region is given by the following equation: n2--1=~"
1
1 '
(1)
J
x2
where Xj is the wavelength of the j t h fundamental absorption band, ~ is its oscillator strength, ~ is the number of relevant ions in a unit volume, and K * Present address: 2-26-20, Kyonancho, Musashino, Tokyo 180, Japan. 0022-3093/85/$03.30 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
S. Hirota et al. / Reflection spectra of oxide glasses
40
is a constant. The fundamental absorption bands consist of the absorption bands due to the electronic transitions in the ultraviolet region and the vibrational transitions in the infrared region. The absorption bands of oxide glasses containing modifier ions are usually composed of the electronic transitions due to bridging- and nonbridging-oxygen ions and in some cases due to additional cations such as Pb 2+ ions in the ultraviolet region. Eq. (1) is therefore rewritten as follows: n2
1
KNlfl
KN2f2
1
1
1 x2
1 x2
KN3f3
~- 1 ~ ~k2
3
KN4f4
1 ~ ' ~k2
~k2
(2)
~k2
where X1, fl and N~ refer to the absorption band due to the bridging oxygen ions, ?~2, f2 and N2 refer to the nonbridging oxygen ions, 2~3, f3 and N3 refer to the additional cations such as Pb 2+ ions, and ~4, f4 and N4 refer to the infrared absorption band. The fundamental absorptions originating from bridging and nonbridging oxygen ions, which are in most cases located in the vacuum ultraviolet region below 200 nm, have rarely been measured [1-3] probably because of technical difficulties in measurement. Therefore, only an empirical formula has been commonly used to describe the dispersion of optical glasses [4]. The wavelengths of the fundamental absorptions were estimated by fitting the dispersion curves measured to the dispersion formula (eq. (1)) using only one or two terms [5-8]. In the case of two terms, one term was for the ultraviolet absorption and the other term for the infrared absorption. The wavelength values obtained from eq. (1), however, may be quite different from the true absorption wavelengths. The actual measurements of the absorption bands in the range including the vacuum ultraviolet region are therefore inevitable for a correct understanding of the dispersion behaviors even in the visible region. Since the peaks in the reflection spectra are close to the absorption peaks, we may be able to discuss the absorption bands by using the reflection spectra. In the present study, the reflection spectra of various optical glasses are studied in the spectral range 2-13 eV for the purpose described above.
2. Experimental 2.1. Preparation of samples Silicate, borate, phosphate, fluorophosphate and fluoride glasses were prepared for the studies of reflection spectra. Reagent-grade SiO2, Na2CO 3, LiECO3, MgCO3, CaCO3, SrCO3, BaCO3, ZnO, PbO, AI(OH)3, Y203, La203, TiO2, ZrO2, ThO2, Nb205 and Ta205 were used as the starting materials of silicate glasses. Batches of these materials of about 50 g were melted in platinum crucibles at 1500-1550°C for 4-4.5 h. Batches of H3BO 3 and
S. Hirota et aL / Reflection spectra of oxide glasses
41
Ba(NO3) 2 were melted in the same way but at 1250°C for 1 h for borate glasses, and those of H3PO 4, BaCO 3 and AI(OH)3 were melted at 1200°C for 1 h for phosphate glasses. For fluorophosphate glasses, batches of P205, A1E~ a n d alkaline earth fluorides were melted at 9 0 0 - 9 5 0 ° C for 30 min in platinum crucibles with platinum covers. For fluoride glasses, batches of ZrF 4, BaF 2 and trivalent metal fluorides were melted at 850°C for 60 min in platinum crucibles with covers. Cast and then annealed samples were cut and ground. After that one surface of the samples was finely polished for the measurements of reflection spectra. 2.2. Optical measurements Since the absorption coefficients of the glasses are extremely high in the vacuum ultraviolet region, and are of the order of 105-106 cm -1, the fundamental absorptions were measured by means of reflection spectra. It is known that the reflection spectra are closely related with the absorption spectra [1], and each peak in the reflection spectra corresponds to an absorption band. A J o h n s o n - O n a k a type vacuum spectrometer [9] with a 50 cm concave grating was used for the optical measurements in the energy range from 6 to 13 eV. The apparatus is shown schematically in fig. 1. The light source was a
20
,
15
! '~
10 5
0
Concave grating '~
Monochromator
Light source ( Hydrogen discharge tube ) Entrance / slit \ . . - ~ ;\j
j Sodium salicylate coated fluorescent screen and Photomuttiplier
o
0
n- 0
o~-._~33.3 olJ12
10 8 6 Z, Photon energy (eV)
Fig. 1. Schematic diagram of the apparatus used. Fig. 2. Reflection spectra of (100-x)SiO2-xNa20 glasses;
x = 0, 20, 25, 29, 33.3and 36.5mol.%.
42
S. Hirota et al. / Reflection spectra of oxide glasses
hydrogen discharge lamp. The angle of incidence for the measurements of reflection spectra was about 10 ° throughout this study. An HTV R-106 photomultiplier with a sodium salicylate plate placed in front of its window was used as the detector. Measurements in the range from 2 to 6.3 eV were carried out with a Hitachi 340 spectrophotometer.
3. Results and discussion
3.1. Reflection spectra of silicate glasses 3.1.1. Alkali silicate glasses Fig. 2 shows the reflection spectra of the glasses of the SiO2-Na20 system. The spectrum of fused silica is characterized by two sharp bands at 11.6 eV (107 nm) and 10.3 eV (120 nm). This spectrum coincides well with those of Philipp [1] and Sigel [2]. The sharp 10.3 eV band has been identified as an excitonic transition [10-12]. The 11.6 eV band has been assigned to the interband transition from a nonbonding level to an antibonding level characteristic of the SiO4 tetrahedron, but there is no universal agreement at the present time. These two bands gradually decrease and shift slightly to lower energy by addition of Na20. A new band appears at 7.5 eV (165 nm) in the glass containing 29 mol.% N a 2 0 and becomes more distinct in the glass of 36.5 mol.% Na20. This weak band is attributed to the nonbridging oxygen ions produced by the presence of Na + ions. This formation of nonbridging oxygen ions increases with the amount of modifier cations. However, the band due to nonbridging oxygen ions was found to be very weak as compared with that due to bridging oxygen ions in the SiO2-Na20 glass system. The reflection spectrum of a glass containing Li2 ° was compared with that of a glass containing N a 2 0 by the amount of 36.5 mol.% alkali oxide. The shapes of the two spectra resemble each other closely as shown in fig. 3.
3.1.2. Alkaline earth silicate glasses The effect of alkaline earth oxide on the reflection spectrum was investigated by substitution of BaO for N a 2 0 in the S i O 2 - N a 2 0 - B a O system. As seen in fig. 4, four bands are observed in the glasses containing higher amounts of BaO. The two bands in the higher energy region do not much differ from the bands of fused silica and are probably due to bridging oxygen ions. The bands in the region 8.5-8.3 eV (145-149 nm) and 7.5-7.0 eV (166-178 nm) are likely due to nonbridging oxygen ions, which may be ascribed to the transitions of the nonbonding electrons in the nonbridging oxygen ions to the antibonding levels. The intensities of the bands due to nonbridging oxygen ions increase with increasing amount of BaO. Although they are very weak, these two bands seem also to exist in glasses of the Si02-Na20 system. The reflection spectra of some binary and ternary alkali silicate glasses have been measured in the wavelength region of 90 to 170 nm by Sigel [2]. The
S. Hirota et al. / Reflection spectra of oxide glasses
43
15
I
lO .~'~'~
J
1
._.~ O.C g
.,.,~1o+ '+~~~++ ._..__~_! I - +oo
;.... 12 10 8 6 4 Photon energy (eV}
~
10.0
"'+o°-~""~'~"v ~ +-~-2s.'r+7~_o -._._ o o
,+ ,i
+ 1
12 10 8 6 4 2 Photon energy (eV)
Fig. 3. Reflection spectra of 63.5 SIO2-36.5M20 glasses; M 2 0 = Li20 and Na20.
Fig. 4. Reflection spectra of 65 SiO2-(35-x)Na20-xBaO glasses; x = 0, 10, 17.5, 25 and 35 mol.%.
spectra of the glasses were characterized by three reflection peaks at approximately 145 nm (8.5 eV), 130 nm (9.3 eV) and 108 nm (11.5 eV) as shown in fig. 5. There is, however, no evidence of the peak observed in our spectra in the region of 160 to 180 nm (7.8 to 6.9 eV). There is a small difference between the spectra of Sigel and ours even in the spectra between 90 nm and 160 nm (13.8 eV and 7.8 eV). Fig. 6 shows the reflection spectra of glasses of the SiO2-SrO and SiO2-BaO systems. The two bands due to nonbridging oxygen ions are recognized in these systems, too. The effect of different kinds of alkaline earth oxide was also investigated in the S i O 2 - N a 2 0 - M O system, MO being MgO, CaO, SrO, BaO, ZnO or PbO. The amounts of SiO 2 and N a 2 0 were kept constant. Results are shown in fig. 7. The bands due to nonbridging oxygen ions shifted to lower energies of the order of CaO ~ SrO---, BaO. In the case of glass containing PbO, two much stronger bands probably due to the intraionic transitions of Pb 2+ ions appeared at 5.96 eV (208 nm) and 4.84 eV (256 nm). It is inferred that these bands are due to the ~S0 ~ ~P~ and 1So ~ 3p1 transition, respectively [13-15].
3.1.3. PbO containing glasses The relationship between the reflection spectra and the amount of PbO was further investigated for lead silicate optical glasses. The results are shown in fig. 8. The amounts of SiO 2 and PbO are expressed in mol.% in table 1. PbO-free K7 glass exhibits two bands at 11.7 eV (106 nm) and 10.2 eV (122 nm) due to bridging oxygen ions. With a decrease of SiO 2 and also an increase
S. Hirota et a L / Reflection spectra of oxide glasses
44
15 10
"~
~--s.r
----_ 35°/o
5 7-
1 Li20-2SiO 2
12
2
10
-
- '~..._\-'-v -B_2.29 0
0
~6
8 6
100
120
140
160
0
180
Wavelength (nm)
12
10
8
6
4
2
Photon energy (eV)
Fig. 5. Reflection spectra of two simple binary silicate glasses (from Sigel, J. Phys. Chem. Solids 32 (1971) 2373). Fig. 6. Reflection spectra of (100-x)SiO2-xMO glasses; MO = SrO and BaO, x = 29, 35 and 39 mol.%.
15~ 15
'
10
I
10
o
i
0
--
0
0
o
0
rr 0
=bC
\ 12
10
8
6
4
Photon energy (eV
12
10
8
/*
Photon energy (eV)
Fig. 7. Reflection spectra of 65 SiO2-10 N a 2 0 - 2 5 MO glasses; MO = MgO, CaO, SrO, BaO, ZnO and PbO. Fig. 8. Reflection spectra of lead silicate optical glasses.
45
S. Hirota el al. / Reflection spectra of oxide glasses
Table 1 Glass compositions, refractive indices and Abbe numbers of lead silicate optical glasses
K7 KF2 LF7 F2 SF8 SF4 SF6
Compositions (mol.%) SiO2 PbO Na20, K20 etc.
Refractive index nd
Abbe's number %
74.6 74.0 74.8 70.2 64.7 61.5 57.3
1.51112 1.52630 1.57501 1.62004 1.68893 1.75520 1.80518
60.5 51.1 41.5 36.3 31.2 27.5 25.5
0.0 4.1 12.8 18.3 27.4 35.3 40.5
25.4 21.9 12.4 11.5 7.9 3.2 2.2
of PbO, the intensities of the 11.7 eV and 10.2 eV bands decrease simultaneously. On the other hand, Pb 2÷ bands appear at 6.3 eV (197 nm) and 5.22 eV (238 nm) in KF2. The peaks of these bands shift to lower energies and their intensities markedly increase with increasing PbO. In SF6 which contains 40.5 mol.% PbO, the peaks lie at 5.55 eV (223 nm) and 4.68 eV (265 nm), and the band intensities are extremely high. The intensity of the 4.68 eV band increases more markedly than that of the 5.55 eV band. This result differs from the result of impurity doped alkali halide crystals [13,14] in which the lower energy band is weaker than the higher one. These strong bands in the low energy region indicate that the glass containing a large amount of PbO causes the high refractive index and the high dispersion (table 1). Besides these bands, two weak bands appear at 8.3 eV (149 nm) and 6.9 eV (181 nm) in F2, SF8, SF4 and SF6. These bands may be due to the formation of nonbridging oxygen ions. 3.1.4. A 120 ~ containing glasses Effects of the addition of A1203 t o silicate glasses on the reflection spectra were investigated in the SiO2-Na20-A1203 system. A120 3 was substituted for N a 2 0 at 65 mol.% SiO 2. Fig. 9 shows that the exciton band appearing at 9.8 eV in the A1203-free glass shifts to lower energies with increasing A120 3. In these cases A1203 is acting as the glass former. 3.1.5. Y,O 3 and LaeO 3 containing glasses Reflection spectra of Y203 and L a 2 0 3 containing glasses were investigated in the S i O 2 - N a 2 0 - Y z O 3 and S i O 2 - N a 2 0 - L a 2 0 3 systems. Y203 or L a 2 0 3 was substituted for N a 2 0 keeping the amount of SiO 2 at 60 mol.% as shown in fig. 10. The band in the region 10.7-11.6 eV is due to the bridging oxygen ions and the band at 7.2-7.6 eV seems to be due to nonbridging oxygen ions. It is not clear whether the band at around 8.6 eV is the shifted exciton band of SiO 4 or is due to nonbridging oxygen ions related to y 3 + or La 3+ ions, or due to overlapping of these two bands.
S. Hirota et al. / Reflection spectra of oxide glasses
46
islr
,
,
15 10
?s
rv"
0 12
10
8
6
4
Photon energy (eV)
2
12
10
I
8
6
~.
2
Photon energy (eV)
Fig. 9. Reflection spectra of 65 SiO2-(35-x)Na20-xA1203 glasses; x = 0, 5, 10 and 15 mol.%. Fig. 10. Reflection spectra of 60 S i O z - ( 4 0 - x ) N a 2 0 - x M 2 0 3 glasses; M203 = Y203 and La203, x = 6 and 12 mol.%.
3.1.6. TiO 2, ZrO 2 and ThO 2 containing glasses Reflection spectra of TiO 2, ZrO 2 and ThO 2 containing glasses were investigated in the S i O 2 - N a 2 0 - M O 2 system, where M O 2 m e a n s TiO 2, ZrO 2 and T h O z. TiO2, ZrO 2 and ThO 2 were substituted for N a 2 0 . As shown in fig. 11, besides the bands due to bridging and nonbridging oxygen ions, a strong band appears at around 5.3, 7.5 and 8.9 eV for Ti 4+, Zr 4+ and Th 4+ containing glass, respectively. The band shifts to higher energies in the order of TiO z ZrO 2 ~ ThO 2. These strong bands might be attributed to the electronic transition from nonbridging oxygen ions combined with Ti 4+, Zr 4+ and Th 4+ to the cations, or due to the formation of complex ions such as TiO 6. Besides these explanations there may be a possibility that these bands are due to the intracationic transition of an electron. Further investigation will be required. 3.1.7. Nb205 and Ta2Q containing glasses Reflection spectra of Nb205 and Ta205 containing glasses were investigated in the S i O 2 - N a 2 0 - M 2 0 5 system where N a 2 0 was substituted by Nb205 or Ta205. As shown in fig. 12, besides the bands due to bridging oxygen ions and to nonbridging oxygen ions, two strong bands characteristic of N b 5÷ or Ta 5+ were found. The two bands are observed at 5.9 eV and 5.0 eV in the Nb205 containing glasses, and at 6.6 eV and 5.6 eV in the Ta205 containing glasses. It is not clear whether these bands are due to nonbridging oxygen ions affected by N b 5+ and Ta 5÷ ions or due to the formation of complex ions. These bands have a tendency to shift to higher energies with increasing ionic radius of the modifier cations. As shown above, it is evident that the bands in the region 8.8-4.9 eV shift to lower energies with increasing ionic radius for the glasses containing low
S. Hirota et al, / Reflection spectra of oxide glasses
47
15
10
15
5
10
5
~o C EJ C
~o "s n--
o 0
0
~f2 10
8
6
Z,
2
12
Photon energy (eV) Fig. 11. R e f l e c t i o n s p e c t r a o f 60
10
8
6
Z,
Photon energy (eV)
Si02-(40-x)Na20-xMO 2 glasses; M O 2 = T i O 2, Z r O 2 a n d
T h O 2, x = 6 a n d 12 mol.%. Fig. 12. R e f l e c t i o n s p e c t r a of 60 S i O 2 - ( 4 0 - x ) N a 2 0 - x M 2 0 5 x = 6 a n d 12 mol.%.
glasses;
M205 = N b 2 0 5 a n d T a 2 0 5 ,
valency cations, while they shift to higher energies with increasing ionic radius for the glasses containing high valency cations. This tendency is the same as that of absorption wavelengths obtained from the calculation with the dispersion formula using one term [8].
3.2. Reflection spectra of borate and phosphate glasses 3.2.1. Borate glasses The reflection spectra of the B203-BaO system are shown in fig. 13. A relatively sharp band and a broad band are observed at 8.6 eV (144 nm) and 10.4 eV(ll9 nm), respectively, in the glass containing 16.1 mol.% BaO. The 10.4 eV band seems to shift to lower energies and its intensity increases with increasing concentration of modifier cations. Both of these bands may be due to bridging oxygen ions. A weak band appears at 7.7 eV (162 nm) for high concentration of BaO, which is considered to be due to nonbridging oxygen ions.
3.2.2. Phosphate glasses In the case of the P2Os-BaO system, a strong band is found at around 9.5 eV (131 nm) and a much weaker band at 7.8 eV (159 nm) as seen in fig. 14. These bands may be due to bridging and nonbridging oxygen ions, respectively.
48
S. Hirota et al. / Reflection spectra of oxide glasses 15
10
10
I
,J
8
6
5 A
o
~o ¢r
o 0
o
12 10
8
6
4
2
Photon energy (eV)
12 10
4
2
Photon energy (eV)
Fig. 13. Reflection spectra of (100-x)B203-xBaO glasses; x =16.1, 22.9, 29.0, 35.0 and 39.7 mol.%. Fig. 14. Reflection spectra of (100-x)P2Os-xBaO glasses; x = 20.0, 27.5, 35.0, 42.5 and 50.0 mol.%.
3.2.3. AleO3 containing phosphate glasses A1203 was substituted for BaO in the P2Os-BaO-A1203 system at the P205 concentration of 65 mol.%. The results are shown in fig 15. No band shift was observed in contrast to the SiO2-Na20-A1203 system. A very weak band seems to lie at 8.5 eV (146 nm) in the glasses with A1203 concentrations of more than 10 mol.%, which suggests that A13 + ions act as the modifier ion.
3.3. Reflection spectra of fluorophosphate and fluoride glasses 3. 3.1. Fluorophosphate glasses Reflection spectra of glasses of the PEOs-A1Fa-MF 2 system are shown in fig. 16 where MF 2 is an alkaline earth fluoride. Two bands are found at 11.3 eV (110 rim) and 10.1 eV (122 nm) in the glass containing 18.3 cationic % POE. s. The band at 11.3 eV becomes sharper both with decreasing P205 concentration and increasing fluoride concentration. The band may be assigned to an electronic transition in [A1Fr] 3- ions [16]. It is characteristic of fluorophosphate glasses that such an intense absorption band is found in the higher energy region than in the oxide glasses. The intensity of the 10.1 eV band decreases with decreasing P205 concentration. This band may be due to the bridging oxygen ions bonding to p5 + ions, although it lies at a higher energy as compared with that of P2Os-BaO glasses. The band due to nonbridging oxygen ions, which is found in the region from 7 to 9 eV in silicate, borate and phosphate glasses, was not found in these fluorophosphate glasses.
49
S. Hirota et al. / Reflection spectra of oxide glasses
15 10 5
15 10
/
~0
o~ 5
~0 Ct~
0
co 0
cY
k
i
i
12
10
8
6
4
12
Photon energy (eV}
10
8
6
Z,
Photon energy (eV)
Fig. 15. Reflection spectra of 65 P2Os-(35-x)BaO-xA1203 glasses; x = 0, 5, 10, 15 and 20 mol.%. Fig. 16. Reflection spectra of POzs-A1F3-MF2 glasses; (1) 18.3 PO2.5-33.5 AIF3-48.2 MF2, (2) 12.2 PO2.5-35.6 AIF3-52.2 MF2, (3) 6.1 PO2.5-39.7 AIF3-54.2 MF2 and (4) 3.1 PO2.5-42.7 AIF~-54.2 MF2.
3.3. 2. Fluoride glasses Reflection spectra of glasses of the ZrF4-BaF 2 and ZrFa-BaF2-MF 3 systems are shown in fig. 17 where MF 3 is GdF3, A1F3 or LaF3. In the case of fluoride glasses, three bands are observed. The 11 eV band may be due to an electronic transition in [ZrF8] 4- ions [17] and the band appearing at around 7.5 eV is likely due to the fluorine ions combined with Ba 2+ ions. The broad band at around 9.5 eV cannot be assigned yet.
15 10 !
0
0
12
10
I
8
6
4
Photon energy (eV)
2
Fig. 17. Reflection spectra of ZrF4-BaF2 and ZrF4-BaF2 - M F 3 glasses; (1) 60 ZrF4-40 BaF2, (2) 62.5 ZrF4-37.5 BaF2, (3) 57.4 ZrF4-34.6 BaF2-3.9 GdF3-4.0 AIF3 and (4) 60 ZrF4-20 BaF2-6 LaF3-14 NaF.
50
S. Hirota et al. / Reflection spectra of oxide glasses
4. Summary Reflection spectra for silicate, borate, phosphate, fluorophosphate and fluoride glasses were measured in the spectral region of 2 - 1 3 eV, and the following assignments were tentatively given to the peaks observed. (1) The bands due to bridging oxygen ions or fluorine ions are at 11.6 eV and 10.4-9.5 eV in silicate glasses, at 10.2 and 8.8 eV in borate glasses, at 9.5 eV in phosphate glasses, at 11.2 eV in fluorophosphate glasses and at 11 eV in fluorozirconate glasses. (2) In silicate glasses, the bands due to nonbridging oxygen ions are found in the region 8.8-4.9 eV. In the case of glasses containing low valency modifier cations, they shift to lower energies with increasing ionic radius of the cations, while in the case of glasses containing high valency cations, they shift to higher energies. (3) In glasses containing large amounts of PbO, strong bands appear in the lower energy regions of 6.3-5.6 eV and 5.2-4.7 eV, which seem to be due to the intraionic transitions of Pb 2+. (4) With the addition of A1203 the exciton b a n d of 9.8 eV shifts to lower energies in silicate glasses, which suggests that A13+ acts as the network former, while a weak b a n d appears at the low energy of 8.5 eV in phosphate glasses, which suggests that A13+ acts as the modifier ion. (5) F l u o r o p h o s p h a t e glasses with low P205 concentrations show a sharp b a n d at the high energy of 11.2 eV, which m a y be due to [A1F6] 3-. (6) Fluorozirconate glasses seem to have three bands. The 11 eV b a n d might be due to [ZrFs] 4- ions. The authors are grateful to Dr T. Mabuchi and Dr S. K i n n o of the University of T s u k u b a for their help with a part of the optical measurements.
References [1] H.R. Philipp, Solid St. Commun. 4 (1966) 73; J. Phys. Chem. Solids 32 (1971) 1935. [2] G.H. Sigel Jr, J. Phys. Chem. Solids 32 (1971) 2373. [3] T. Izumitani and S. Hirota, 2nd Int. Otto-Schott-Kolloquium (1983) p. 227 (Wiss. Ztschr. Friedrich-Schiller-Univ. Jena, Math.-Naturwiss. R., 32 Jg. 1983, H. 2/3). [4] Hoya Optical Glass Catalogue (Hoya Corporation); Schon Optical Glass Catalog No. 3050e (Jenaer Glaswerk Schott&Gen., Mainz), [5] F. Reitmayer, Glastechn. Ber. 34 (1961) 122. [6] K. Kordes, Glastechn. Ber. 38 (1965) 242. [7] S. Hirota and T. Izumitani, Yogyo-Kyokai-shi 84 (1976) 435. [8] S. Hirota and T. Izumitani, J. Non-Crystalline Solids 29 (1978) 109. [9] H. Saito, T. Mabuchi and R. Onaka, Sci. Light 16 (1967) 246. [10] E. Loh, Solid St. Commun. 2 (1964) 269. [11] A.R. Ruffa, Phys. Stat. Sol. 29 (1968) 605. [12] M.G. Reilly, J. Phys. Chem. Solids 31 (1970) 1041. [131 F. Seitz, J. Chem. Phys. 6 (1983) 150. [14] A. Fukuda, Sci. Light 13 (1964) 64. [15] A.J. Bourdillon, F. Khumalo and J. Bordas, Phil. Mag. B37 (1978) 731. [16] D. Ehrt, M. Krauss, C. Erdmann and W. Vogel, Z. Chem. 22 (1982) 315. [17] Y. Kawamoto and T. Horisaka, J, Non-Crystalline Solids 56 (1983) 39.