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Light scattering in halide glasses at high temperatures: anomalous scattering and nanoscale inhomogeneities
J O U R N A L OF
Journal of Non-Crystalline Solids 161 (1993) 157-160 North-Holland
I~[1~?~1~ |_T_T~aI~~ . l l ~ l l V l r l i l [ 1 tl I I U l I ~ I h tlVlalA;
Light scattering in halide glasses at high temperatures: anomalous scattering and nanoscale inhomogeneities J o h n S c h r o e d e r , M i c h a e l A. W h i t m o r e 1, M a r k u s R. Silvestri, S u s a n t a K. S a h a a n d C o r n e l i u s T. M o y n i h a n Departments of Physics and Materials Engineering and Center for Glass Science and Technology, Rensselaer Polytechnic Institute, Troy, N Y 12180-3590, USA
The intrinsic light scattering behavior of several halide glasses has been studied as a function of temperature from room temperature to beyond the glass transition temperature, Tg. The Landau-Placzek ratios and the Brillouin shifts and widths were obtained as a function of temperature. Anomalous scattering results were found in the glass transition region both for the Landau-Placzek ratios and the Brillouin shifts. These results are interpreted as evidence of the physical origin of the non-exponential character of structural relaxation kinetics and their relationship to thermally induced nanoscale inhomogeneities.
1. Introduction In recent years studies of the structural relaxation process in the glass transition region have attained considerable interest. Previous works have shown anomalous scattering behavior in this temperature range [1-4]. This manifests itself, for example, in a maximum in the light scattering intensity versus temperature curves during heating. The glassy state is not well understood and there is no generally accepted microscopic theory. Moynihan and Schroeder [5,9] showed that effects associated with non-exponential structural relaxation kinetics can be related to variations in nanoscale inhomogeneities which are responsible then for anomalous scattering. Light scattering may be described in terms of the L a n d a u - P l a c zek (LP) ratio, which is by definition the intensity
1 Present address: MIT-Lincoln Laboratories, Concord, MA, USA. Correspondence to: Dr J, Schroeder, Departments of Physics and Center for Glass Science and Technology, Rensselaer Polytechnic Institute, Troy, NY 12180-3590, USA. Tel: + 1-518 276 8408. Telefax: + 1-518 276 6680.
of the central Rayleigh line divided by the total intensity of the frequency shifted Brillouin lines. Brillouin lines are due to adiabatic pressure fluctuations and are therefore expected to vary monotonically with temperature [6]. The Rayleigh line, on the other hand, may be identified with the entropy fluctuations, which will in turn be sensitive to structural rearrangements in the glass transition region [6]. Hence, the L a n d a u - P l a c z e k ratio will reflect these changes as well. In this work we report some additional recent measurements of anomalous scattering during heating through the glass transition region for two halide glasses, ZBLAN20 and ZBLA.
2. Experimental aspects The two ZrF4-based heavy metal fluoride glasses, Z B L A and ZBLAN20, used in this study were prepared in our laboratory and at Galileo Electro-Optics Corporation, as described in ref. [7]. The glass transition temperatures of Z B L A and ZBLAN20 are respectively 306°C and 260°C. Their compositions are given in refs. [3,8].
0022-3093/93/$06.00 © 1993 - Elsevier Science Publishers B.V. All rights reserved
158
J. Schroeder et al. / Light scattering in halide glasses at high temperatures
To achieve a narrow linewidth of about 20 MHz in the incident light beam, an etalon was placed into the argon-ion laser, which was operating at 488 nm. The scattered light was collected at 90 ° and analyzed with a three-pass F a b r y - P e r o t interferometer housed in a thermally stabilized box. The light then, after passing a spatial and spike filter, was detected by an ITT FW-130 cooled ( - 20°C) photomultiplier tube (PMT) with a dark count of approximately 0.5 counts/s. A Burleigh DAS-1 system drove the F a b r y - P e r o t and collected the signals from the PMT, which were later transferred to an MS-DOS computer system for further analysis. The samples were placed on a glass plate and positioned in a temperature-controlled molybdenum furnace with stability of about + I°C. The temperature inside the furnace was monitored with a chromel-alumel thermocouple positioned just beneath the sample holder.
3. Results Figure 1 is a typical Rayleigh-Brillouin spectrum. R1 and R2 are the Rayleigh lines, and B1 and B2 are the corresponding Brillouin lines. B1 and B2 are the anti-stokes and Stokes components, respectively. The variations in the Landau-Placzek ratio with temperature during heating of the ZBLAN20 glass are shown in fig. 2. There is a gradual decrease in the magnitude of the LP ratio up to the glass transition region, followed by a sharp increase with a local maxi-
80
"~ a. ~ 6070
ZBLAN20 o
,
~
_z 50 40 300
!
!
400
500
Temp(K)
3
ZBLA
R2
_..a•R B1 B2
o 1
0
.~,lkl[
10
20
30
40
50
Frequency [GHz]
Fig. 1. Typicalspectra at 100°C.
60
600
Fig. 2. Landau-Placzek ratio versus temperature during heating of ZBLAN20. The line is drawn as a guide for the eye.
mum behavior which replicates that observed in our earlier study [1] on a different ZBLAN20 specimen. This maximum is also observed in the LP ratio vs. temperature heating curve for the Z B L A glass depicted in fig. 3. Similar anomalous behavior has been observed in Bokov and Andreev's study of light scattering from B 2 0 3 glass [4] and in Golubkov's small angle X-ray scattering experiments on B 2 0 3 glass [2]. Figure 4 shows the longitudinal Brillouin shift of ZBLAN20 as a function of temperature during heating in which we observe a marked change in slope near Tg. While heating through the glass transition region, we also observed anomalous oscillations in the Brillouin peak linewidths. The linewidths taken directly from the spectra are shown as a function of temperature in fig. 5 for Z B L A glass. Behavior of this sort has been observed as well in earlier
30
1
~
ZBLA
o 25
o~
~" ..a
20 15 500
,
!
540
580
620
Temp(K) Fig. 3. Landau-Placzek
ratio of Z B L A
as a function of
temperature during heating. The line is drawn as a guide for the eye.
J. Schroeder et al. / Light scattering in halide glasses at high temperatures
tions. Concentration fluctuations freeze in at temperatures well above Tg so that their contributions can be neglected in the present study [6]. Considering only density fluctuations (entropy fluctuations), the Landau-Placzek ratio, provided that the glass transition region is excluded, can be written as
19
ZBLAN20
17
16 200
, 300
, 400
RLp = ( Tf/T)(KT,o( Tf) - Ks:( T) )pV2L:, 500
600
TemDfK) Fig. 4. Longitudinal Brillouin shift of Z B L A N 2 0 as a function of temperature during heating. The line is drawn as a guide for the eye.
3.5
ZBLA = 3.0
Tg
N 2.5
500
,
~ -
550
159
,
600
Temp(K) Fig. 5. Total Brillouin linewidth as a function of temperature during heating. T h e line is drawn as a guide for the eye.
experiments [10] on other specimens and is quite reproducible.
4. Discussion
The anomalous behavior shown in the previous results are discussed in terms of structural relaxation, density fluctuations and relaxation times. The behavior of the Landau-Placzek ratio above and below Tg is explained by variations in density fluctuations (i.e., entropy fluctuations). A relationship between density fluctuations and the distribution of structural relaxation times has been developed by Moynihan and Schroeder [5]. Here the distribution of relaxation times are explained in terms of independently relaxing nanoregions. Intrinsic light scattering from glasses is due to microscopic density and concentration fluctua-
where KT,0(Tf) is the low frequency isothermal compressibility at the fictive temperature, Tf (configurational temperature), Ks:(T) = (pV2:) -] = C ~ 1, the adiabatic compressibility, and T is the lattice or phonon temperature [6]. Adiabatic fluctuations in density (propagating pressure fluctuations) give rise to the Brillouin lines. The behavior of the Landau-Placzek ratio up to Tg is attributed primarily to changes in the phonon temperature and to a lesser extent to the adiabatic compressibility (Ks:(T)) in agreement with the above equation. Above Tg, changes in the isothermal compressibility KT,0(Tf) b e c o m e dominant due to variations in the entropy fluctuations caused by structural changes. We have observed anomalous intensity peaks in the Landau-Placzek ratio for ZBLAN20 and ZBLA glasses that occur in and near the glass transition region. These anomalous peaks may be attributed to the emergence of non-uniform regions with structures different from the surrounding disordered lattice [2,5]. Here the disordered lattice is simply the volume surrounding the nonuniform region. This non-uniform region has a structure that corresponds to that of the disordered lattice at a higher temperature. The intensity rise may be attributed to an increase in the distance between the electron densities of these regions and the disordered lattice [2,5]. At temperatures in the glass transition region, a growth in the magnitude of the density fluctuations is caused by structural relaxations of these nanoscale inhomogeneities whereas in the glassy state, below Tg, the nanoscale inhomogeneities are frozen-in and rearrangements are impossible. These nanoregions behave as the disordered lattice at a higher temperature [2]. Therefore, the nanoregions are causing an additional increase in the Rayleigh intensity which produces the anoma-
160
J. Schroeder et al. / Light scattering in halide glasses at high temperatures
lous peak in the Landau-Placzek ratio curves. This 'additional intensity' drops with further increase in the temperature as the transition from the glassy state to the melt reaches equilibrium values (Tf = T) and, in addition, at a high enough temperature there is an uncoupling of propagating and non-propagating modes. The changing structure may lower the coordination number of the Zr 4+ complexes, affecting the sound velocity such that both the isothermal and adiabatic compressibilities are affected. From fig. 4 it is evident that this effect manifests itself in a decrease of the Brillouin shift as we traverse It may also be possible to explain the behavior of the Brillouin linewidth at temperatures in the glass transition region by the presence of these nanoregions. The natural Brillouin linewidth is inversely proportional to the phonon lifetime and the presence of nanoregions may cause fluctuations in the phonon lifetimes [11].
5. Conclusion
Rayleigh and Brillouin scattering techniques have been applied to ZBLAN20 and ZBLA fluoride glasses during heating through the glass transition region. The Landau-Placzek ratios, the longitudinal BriUouin shifts and widths have been measured. Anomalous behavior is observed in the glass transition region for the Landau-Placzek ratio and Brillouin linewidths. In light of the model discussed in refs. [5,10], these results can
be interpreted as new evidence as to the physical origin of the non-exponential character of structural relaxation kinetics in terms of nanoscale inhomogeneities. This research was supported in part by the National Science Foundation (NSF)-USA under Grant No. MRG-DMR-88-01004.
References [1] J. Schroeder, L.G. Hwa, X.S. Zhao, L. Busse and I. Aggarwal, Mater. Sci. Forum 67&68 (1991) 471. [2] V.V. Golubkov, Sov. J. Glass Phys. and Chem. 15 (1989) 280. [3] J. Schroeder, L.G. Hwa, X.S. Zhao, L. Busse and I. Aggarwal, in: Proc. 6th Int. Halide Glass Conference, Clausthal, Germany, Oct. 1989; Also see: Luu-Gen Hwa, PhD thesis, Rensselaer Polytechnic Institute (1989). [4] N.A. Bokov and N.S. Andreev, Sov. J. Glass Phys. and Chem. 15 (1991) 243. [5] C.T. Moynihan and J. Schroeder, J. Non-Cryst. Solids 160 (1993) 52. [6] J. Schroeder, in: Treatise on Materials Science and Technology, Vol. 12, ed. M. Tomozawa and R.H. Doremus, (Academic Press, New York, 1977) p. 157. [7] Y. Nakao and C.T. Moynihan, Mater. Sci. Forum 67&68 (1991) 187. [8] J. Schroeder, S.K. Saha, M.R. Silvestri, M. Lee and C.T. Moynihan, these Proceedings, p. 173. [9] C.T. Moynihan and J. Schroeder, these Proceedings, p. 148. [10] M.A. Whitmore, MS thesis, Rensselaer Polytechnic Institute (1992). [11] L.G. Hwa, J. Schroeder and X.S. Zhao, J. Opt. Soc. B6 (1989) 833.
Journal of Non-Crystalline Solids 161 (1993) 157-160 North-Holland
I~[1~?~1~ |_T_T~aI~~ . l l ~ l l V l r l i l [ 1 tl I I U l I ~ I h tlVlalA;
Light scattering in halide glasses at high temperatures: anomalous scattering and nanoscale inhomogeneities J o h n S c h r o e d e r , M i c h a e l A. W h i t m o r e 1, M a r k u s R. Silvestri, S u s a n t a K. S a h a a n d C o r n e l i u s T. M o y n i h a n Departments of Physics and Materials Engineering and Center for Glass Science and Technology, Rensselaer Polytechnic Institute, Troy, N Y 12180-3590, USA
The intrinsic light scattering behavior of several halide glasses has been studied as a function of temperature from room temperature to beyond the glass transition temperature, Tg. The Landau-Placzek ratios and the Brillouin shifts and widths were obtained as a function of temperature. Anomalous scattering results were found in the glass transition region both for the Landau-Placzek ratios and the Brillouin shifts. These results are interpreted as evidence of the physical origin of the non-exponential character of structural relaxation kinetics and their relationship to thermally induced nanoscale inhomogeneities.
1. Introduction In recent years studies of the structural relaxation process in the glass transition region have attained considerable interest. Previous works have shown anomalous scattering behavior in this temperature range [1-4]. This manifests itself, for example, in a maximum in the light scattering intensity versus temperature curves during heating. The glassy state is not well understood and there is no generally accepted microscopic theory. Moynihan and Schroeder [5,9] showed that effects associated with non-exponential structural relaxation kinetics can be related to variations in nanoscale inhomogeneities which are responsible then for anomalous scattering. Light scattering may be described in terms of the L a n d a u - P l a c zek (LP) ratio, which is by definition the intensity
1 Present address: MIT-Lincoln Laboratories, Concord, MA, USA. Correspondence to: Dr J, Schroeder, Departments of Physics and Center for Glass Science and Technology, Rensselaer Polytechnic Institute, Troy, NY 12180-3590, USA. Tel: + 1-518 276 8408. Telefax: + 1-518 276 6680.
of the central Rayleigh line divided by the total intensity of the frequency shifted Brillouin lines. Brillouin lines are due to adiabatic pressure fluctuations and are therefore expected to vary monotonically with temperature [6]. The Rayleigh line, on the other hand, may be identified with the entropy fluctuations, which will in turn be sensitive to structural rearrangements in the glass transition region [6]. Hence, the L a n d a u - P l a c z e k ratio will reflect these changes as well. In this work we report some additional recent measurements of anomalous scattering during heating through the glass transition region for two halide glasses, ZBLAN20 and ZBLA.
2. Experimental aspects The two ZrF4-based heavy metal fluoride glasses, Z B L A and ZBLAN20, used in this study were prepared in our laboratory and at Galileo Electro-Optics Corporation, as described in ref. [7]. The glass transition temperatures of Z B L A and ZBLAN20 are respectively 306°C and 260°C. Their compositions are given in refs. [3,8].
0022-3093/93/$06.00 © 1993 - Elsevier Science Publishers B.V. All rights reserved
158
J. Schroeder et al. / Light scattering in halide glasses at high temperatures
To achieve a narrow linewidth of about 20 MHz in the incident light beam, an etalon was placed into the argon-ion laser, which was operating at 488 nm. The scattered light was collected at 90 ° and analyzed with a three-pass F a b r y - P e r o t interferometer housed in a thermally stabilized box. The light then, after passing a spatial and spike filter, was detected by an ITT FW-130 cooled ( - 20°C) photomultiplier tube (PMT) with a dark count of approximately 0.5 counts/s. A Burleigh DAS-1 system drove the F a b r y - P e r o t and collected the signals from the PMT, which were later transferred to an MS-DOS computer system for further analysis. The samples were placed on a glass plate and positioned in a temperature-controlled molybdenum furnace with stability of about + I°C. The temperature inside the furnace was monitored with a chromel-alumel thermocouple positioned just beneath the sample holder.
3. Results Figure 1 is a typical Rayleigh-Brillouin spectrum. R1 and R2 are the Rayleigh lines, and B1 and B2 are the corresponding Brillouin lines. B1 and B2 are the anti-stokes and Stokes components, respectively. The variations in the Landau-Placzek ratio with temperature during heating of the ZBLAN20 glass are shown in fig. 2. There is a gradual decrease in the magnitude of the LP ratio up to the glass transition region, followed by a sharp increase with a local maxi-
80
"~ a. ~ 6070
ZBLAN20 o
,
~
_z 50 40 300
!
!
400
500
Temp(K)
3
ZBLA
R2
_..a•R B1 B2
o 1
0
.~,lkl[
10
20
30
40
50
Frequency [GHz]
Fig. 1. Typicalspectra at 100°C.
60
600
Fig. 2. Landau-Placzek ratio versus temperature during heating of ZBLAN20. The line is drawn as a guide for the eye.
mum behavior which replicates that observed in our earlier study [1] on a different ZBLAN20 specimen. This maximum is also observed in the LP ratio vs. temperature heating curve for the Z B L A glass depicted in fig. 3. Similar anomalous behavior has been observed in Bokov and Andreev's study of light scattering from B 2 0 3 glass [4] and in Golubkov's small angle X-ray scattering experiments on B 2 0 3 glass [2]. Figure 4 shows the longitudinal Brillouin shift of ZBLAN20 as a function of temperature during heating in which we observe a marked change in slope near Tg. While heating through the glass transition region, we also observed anomalous oscillations in the Brillouin peak linewidths. The linewidths taken directly from the spectra are shown as a function of temperature in fig. 5 for Z B L A glass. Behavior of this sort has been observed as well in earlier
30
1
~
ZBLA
o 25
o~
~" ..a
20 15 500
,
!
540
580
620
Temp(K) Fig. 3. Landau-Placzek
ratio of Z B L A
as a function of
temperature during heating. The line is drawn as a guide for the eye.
J. Schroeder et al. / Light scattering in halide glasses at high temperatures
tions. Concentration fluctuations freeze in at temperatures well above Tg so that their contributions can be neglected in the present study [6]. Considering only density fluctuations (entropy fluctuations), the Landau-Placzek ratio, provided that the glass transition region is excluded, can be written as
19
ZBLAN20
17
16 200
, 300
, 400
RLp = ( Tf/T)(KT,o( Tf) - Ks:( T) )pV2L:, 500
600
TemDfK) Fig. 4. Longitudinal Brillouin shift of Z B L A N 2 0 as a function of temperature during heating. The line is drawn as a guide for the eye.
3.5
ZBLA = 3.0
Tg
N 2.5
500
,
~ -
550
159
,
600
Temp(K) Fig. 5. Total Brillouin linewidth as a function of temperature during heating. T h e line is drawn as a guide for the eye.
experiments [10] on other specimens and is quite reproducible.
4. Discussion
The anomalous behavior shown in the previous results are discussed in terms of structural relaxation, density fluctuations and relaxation times. The behavior of the Landau-Placzek ratio above and below Tg is explained by variations in density fluctuations (i.e., entropy fluctuations). A relationship between density fluctuations and the distribution of structural relaxation times has been developed by Moynihan and Schroeder [5]. Here the distribution of relaxation times are explained in terms of independently relaxing nanoregions. Intrinsic light scattering from glasses is due to microscopic density and concentration fluctua-
where KT,0(Tf) is the low frequency isothermal compressibility at the fictive temperature, Tf (configurational temperature), Ks:(T) = (pV2:) -] = C ~ 1, the adiabatic compressibility, and T is the lattice or phonon temperature [6]. Adiabatic fluctuations in density (propagating pressure fluctuations) give rise to the Brillouin lines. The behavior of the Landau-Placzek ratio up to Tg is attributed primarily to changes in the phonon temperature and to a lesser extent to the adiabatic compressibility (Ks:(T)) in agreement with the above equation. Above Tg, changes in the isothermal compressibility KT,0(Tf) b e c o m e dominant due to variations in the entropy fluctuations caused by structural changes. We have observed anomalous intensity peaks in the Landau-Placzek ratio for ZBLAN20 and ZBLA glasses that occur in and near the glass transition region. These anomalous peaks may be attributed to the emergence of non-uniform regions with structures different from the surrounding disordered lattice [2,5]. Here the disordered lattice is simply the volume surrounding the nonuniform region. This non-uniform region has a structure that corresponds to that of the disordered lattice at a higher temperature. The intensity rise may be attributed to an increase in the distance between the electron densities of these regions and the disordered lattice [2,5]. At temperatures in the glass transition region, a growth in the magnitude of the density fluctuations is caused by structural relaxations of these nanoscale inhomogeneities whereas in the glassy state, below Tg, the nanoscale inhomogeneities are frozen-in and rearrangements are impossible. These nanoregions behave as the disordered lattice at a higher temperature [2]. Therefore, the nanoregions are causing an additional increase in the Rayleigh intensity which produces the anoma-
160
J. Schroeder et al. / Light scattering in halide glasses at high temperatures
lous peak in the Landau-Placzek ratio curves. This 'additional intensity' drops with further increase in the temperature as the transition from the glassy state to the melt reaches equilibrium values (Tf = T) and, in addition, at a high enough temperature there is an uncoupling of propagating and non-propagating modes. The changing structure may lower the coordination number of the Zr 4+ complexes, affecting the sound velocity such that both the isothermal and adiabatic compressibilities are affected. From fig. 4 it is evident that this effect manifests itself in a decrease of the Brillouin shift as we traverse It may also be possible to explain the behavior of the Brillouin linewidth at temperatures in the glass transition region by the presence of these nanoregions. The natural Brillouin linewidth is inversely proportional to the phonon lifetime and the presence of nanoregions may cause fluctuations in the phonon lifetimes [11].
5. Conclusion
Rayleigh and Brillouin scattering techniques have been applied to ZBLAN20 and ZBLA fluoride glasses during heating through the glass transition region. The Landau-Placzek ratios, the longitudinal BriUouin shifts and widths have been measured. Anomalous behavior is observed in the glass transition region for the Landau-Placzek ratio and Brillouin linewidths. In light of the model discussed in refs. [5,10], these results can
be interpreted as new evidence as to the physical origin of the non-exponential character of structural relaxation kinetics in terms of nanoscale inhomogeneities. This research was supported in part by the National Science Foundation (NSF)-USA under Grant No. MRG-DMR-88-01004.
References [1] J. Schroeder, L.G. Hwa, X.S. Zhao, L. Busse and I. Aggarwal, Mater. Sci. Forum 67&68 (1991) 471. [2] V.V. Golubkov, Sov. J. Glass Phys. and Chem. 15 (1989) 280. [3] J. Schroeder, L.G. Hwa, X.S. Zhao, L. Busse and I. Aggarwal, in: Proc. 6th Int. Halide Glass Conference, Clausthal, Germany, Oct. 1989; Also see: Luu-Gen Hwa, PhD thesis, Rensselaer Polytechnic Institute (1989). [4] N.A. Bokov and N.S. Andreev, Sov. J. Glass Phys. and Chem. 15 (1991) 243. [5] C.T. Moynihan and J. Schroeder, J. Non-Cryst. Solids 160 (1993) 52. [6] J. Schroeder, in: Treatise on Materials Science and Technology, Vol. 12, ed. M. Tomozawa and R.H. Doremus, (Academic Press, New York, 1977) p. 157. [7] Y. Nakao and C.T. Moynihan, Mater. Sci. Forum 67&68 (1991) 187. [8] J. Schroeder, S.K. Saha, M.R. Silvestri, M. Lee and C.T. Moynihan, these Proceedings, p. 173. [9] C.T. Moynihan and J. Schroeder, these Proceedings, p. 148. [10] M.A. Whitmore, MS thesis, Rensselaer Polytechnic Institute (1992). [11] L.G. Hwa, J. Schroeder and X.S. Zhao, J. Opt. Soc. B6 (1989) 833.