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Brillouin scattering study of elastic properties of superionic (AgI)x(AgPO3)1−x glasses
Volume 107A, number 4
28 January 1985
PHYSICS LETTERS
BRILLOUIN SCATTERING STUDY OF ELASTIC PROPERTIES OF SUPERIOMC (AgI)x(AgP03)1 _-x GLASSES L. BORJESSON and L.M. TORELL Department
of Physics, Chalmers University of Technology,
S-41 2 96 Gothenburg,
Sweden
Received 5 September 1984 Revised manuscript received 19 November 1984
Longitudinal and transverse phonon frequencies in superionic (AgI)X(AgP03)1_, glasses for x = 0.3, 0.4 and 0.5 have been measured by Brillouin scattering. The hypersonic velocities as a function of temperature were determined along with the elastic constants. The velocity data extrapolate linearly to those of a-AgI in support of a local cluster model.
During the past decade there has been a considerable interest in the dynamics of superionic conductors or solid electrolytes; a group of materials with a definite crystalline structure in which the conducting ions can more of less freely move around. Among them (YAgI often serves as the archetype with a high conductivity (“1 R-l cm-l) in a temperature range 420828 K due to mobile Ag+ in the solid bee iodine structure [ 11. Recently relatively large values of ionic conductivity (-10e2 Cl-l cm-l) at room temperature have been reported for amorphous electrolytes like borate and phosphate glasses doped with AgI [2-41. The growing attention focused on superionic glasses is to a large extent due to their technological advantages and different experimental techniques have been applied to understand how the glass network is modified when AgI is introduced [3-81. It has been suggested that AgI is mainly coordinated by the matrix oxygen ions [9], while other authors propose that the glass network is not affected chemically but that dissolved AgI tends to reproduce on the local level microdomains of the superionic phase of pure AgI [ 5,6, 81. In support of the last mentioned model recent optical absorption studies report an absorption which shifts linearly at the increasing of the AgI content and extrapolates to the value of cr-AgI [6]. The aim of the present work concerning the system (AgI)X(&P03)~_X is to investigate if the acoustic behaviour of the systems can be extrapolated to that of pure cw-AgIwhich 190
has previously been studied in this laboratory [lo]. This implies measurements in which not only the AgI concentration is to be gradually changed but also the temperature, since CY-AgIonly exists at elevated temperatures. In the following a Brillouin scattering study of the temperature dependence of longitudinal and transverse sound velocities in AgPO3 doped with 0.3, 0.4 and 0.5 AgI respectively is presented. As far as we know this is the first Brillouin scattering study reported for superionic glasses. Brillouin scattering is a well recognized tool for determining structural changes. In Brillouin scattering from an amorphous solid the principal components of the scattered light are the Rayleigh line (R) centered at the incident frequency uo, a pair of dopplershifted lines (L) due to longitudinal phonons in the material and a weaker doublet (T), which arises from the presence of transverse phonons. Fig. 1 presents a typical spectrum of the (AgI)X(AgP03)l_X systems; two pairs of longitudinal modes are shown since the interferometer is adjusted for overlapping orders in order to resolve the transverse modes. The frequency shifts vL and VT are related to the longitudinal and transverse sound velocities, uL and u, by uL = vLk/2n
,
VT = &&/2n ,
(0 (2)
where k is the wave vector of the phonon given by 0.3759601/85/$03.30 0 Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
Volume 107A, number 4
PHYSICS LETTERS
28 January 1985
_ 2.9
FL.7
E 2.8 a Q2.7 z? :: 2.6 T ’ 2.5 15 10
5
0
Frequency
5
I
0:x=0.3 o:x=_o.* A: X=0.5
-
10 15
GHz
300
350 Tern perature
Fig. 1. Brillouin scattering spectrum of (AgI)o.4(AgPOs)o.e at room temperature. The spectral free range of the interferometer is 21.54 GHz, working at overlapping orders.
k = (4?m/Ao)
sin(e/2),
400
( K1
Fig. 2. The temperature dependence of the longitudinal velocity in (AgI)x(AgPOa)l_x glasses, 0: x = 0.3, o: x = 0.4 and A: x = 0.5. Solid lines represent linear tits and dash-dotted lines the glass transition temperatures. The dashed line is from ultrasonic measurements of (AgI)&AgPO& 1151.
(3)
where ho is the wavelength of the incident light, 8 is the scattering angle and n is the refractive index of the medium. In the present study light scattered at 90’ was collected using an experimental arrangement essentially the same as that described previously [ 111. The light source is a single mode Kr-ion laser operating at 647.1 nm with an output power of about 160 mW. The scattered light was analyzed by a triple-passed piezo-electrically scanned Fabry-Perot interferometer with an electronic feedback system to adjust for any nonlinearities of the piezo-elements during a scan. The interferometer was adjusted for several spectral free ranges to optimize the resolution at every temperature; the overall finesse of the system was ~40. The detector system includes a cooled ITT-FW 130 photomultiplier and photon counting equipment coupled to a multichannel analyzer for storage of the data. The samples, prepared elsewhere [ 12,131, were in a cylindrical shape (@ - 6 mm) of a height -10 mm and were selected to be optically homogeneous and transparent. The circular surfaces were polished before every measurement since they served as inlet and outlet windows for the laser beam. During the measurements the glass ‘samples were situated in a cylindrical pyrex cell (a - 7 mm) with an optically flat window fused to the bottom of the cell. Refractive index matching oil filled the gap between the sample and the cell. This turned out to be essential not only to avoid refractions but also to prevent Ag from being developed on the entrance surface of the sample and block-
ing the laser beam. This often occurred when the materials were exposed to the laser beam in an oxygen atmosphere. The cell was placed in a thermostat constructed with several light ports sealed by fused quartz plates of optical quality, which allowed right angle light scattering to be collected. The temperature was measured by a thermocouple close to the wall of the scattering cell, the temperature stability being better than *OS” during a measurement. Brillouin spectra were obtained in a range starting from room temperature up to the glass transition temperatures; Tg = 398, 375 and 356 K for x = 0.3, 0.4 and 0.5, respectively, in the (AgI)x(AgP03)1_x system. From the observed frequency shifts the sound velocities could be calculated by using eqs. (1) and (2) and present data for the longitudinal mode are shown in fig. 2. The transverse modes were too weak in intensity to give accurate values for the shift as a function of temperature; room temperature results are given in table 1. As can be seen from fig. 2 the longitudinal velocities decrease with increasing amount of AgI as well as with increasing temperature. The temperature dependences are in close agreement for the three systems and linear fits are given by x = 0.3,
(4)
uL = 3049 - 1. lOOT(K) ms-l ,
x = 0.4 ,
(5)
uL = 2898 - l.l05T(K)
x = 0.5 ,
(6)
uL = 3201 - 1.2843(K)
ms-l
,
ms-l ,
191
Volume 107A, number 4
PHYSICS LETTERS
28 January 1985
Table 1 Results of elastic properties from Brillouin scattering measurements on (AgI),(AgPO$r_
X glasses at room temperature.
Molar fraction AgI
longitudinal shift LJL(GHz) transverse shift q- (GHz) longitudinal velocity VL ( lo3 ms-‘) transverse velocity q- ( lo3 ms-‘) density a) p ( lo3 kg mS3) index of refraction a) n elastic constant CL (10’ Nmo2) elastic constant CT (10’ Nmw2) Young’s modulus Y (10’ Nme2) bulk modulus B (10’ Nme2) Poissons ratio (I
0.3
0.4
0.5
11.78 5.09 2.84 1.23 4.98 1.90 40.2 7.5 20.8 30.2 0.39
11.53 4.91 2.13 1.16 5.20 1.93 38.7 7.0 19.5 29.4 0.39
11.04 4.11 2.51 1.11 5.41 1.97 35.6 6.6 18.4 26.7 0.39
a) Ref. [14].
with correlations better than 0.9. Preliminary ultrasonic velocity measurements on (AgI)4(AgP03)6 [ 1.51 are shown in fig. 2 as a dashed line and exhibit the same temperature dependence as do the present hypersonic velocities. From the values of the longitudinal and transverse elastic wave velocities the constants which characterize the glass can be calculated, i.e. the elastic constants, CL and CT, Young’s modulus Y, shear modulus cc, bulk modulus B and Poisson’s ratio u by using the relations CL=&
(7)
CT=&&
(8)
y = CT ](3CL - 4CT)/(C,
- cT)] 3
(9)
p=C,,
(10)
B=CL-:CT,
(11)
u= Y/2/J-
1.
(12)
The corresponding data for the present systems based on room temperature measurements are given in table 1. Compared to optical glasses [ 161 the sound velocities of the (AgI)X(AgP03)l_X systems are low and for example CL is only about one half the value for SiO,, which reflects a less rigid structure for these systems. The ratio of the transverse to longitudinal velocity, r+/uL, for the present systems is 0.43 which 192
is lower than the average ratio of 0.59 * 0.03 found in optical glasses indicating a slightly lower ratio between the shear and compressional restoring forces. From fig. 2 it follows that the longitudinal velocity decreases with the addition of AgI. This is also the case for the transverse velocity as can be seen in table 1. The velocity data thus confirm that the structure becomes less rigid with increasing AgI content, which is also consistent with the decreasing glass transition temperatures. It is of interest to investigate if the glass matrix itself is changed by the introduction of AgI or if species of AgI merely stay in separated clusters or microdomains in the host matrix network. We would therefore like to raise the following question: How does the response of the structure of high conducting AgI doped glasses to high frequency stresses compare with that of the superionic structure of pure AgI, i.e. cu-AgI? The hypersonic velocities of cw-AgIhave recently been investigated in this laboratory and at a temperature of 563 K the longitudinal velocity was found to be in the range 1680-1970 ms-l depending on crystal orientation [lo]. The corresponding values for the transverse velocity are in the range 720-960 ms-I. By using the temperature dependences obtained in the present study for the longitudinal velocity for the investigated AgI concentrations, the longitudinal velocities of the present glass systems can be compared to those of superionic AgI at 563 K. Such a comparison is shown as a solid line in fig. 3, and it is
Volume 107A, number 4
PHYSICS LETTERS
Molorfroction 41
Fig. 3. Plot of longitudinal (vL) and transverse (vy) velocity versus AgI concentration for (AgI),(AgPOa) t_ x glasses (a: present results) and for (AgI),(AgzO-2B2Oa)t_, glasses (o: ultrasonic data of ref. [7]). The velocity ranges at x = 1 for or-AgI are from ref. [lo]. Solid lines represent data at 536 K. Dashed lines represent room temperature data.
seen that the “glass” data extrapolate within uncertainty limits to that of o-AgI. However, since all AgI glass compositions have about the same temperature dependence of UL (mean value =1.2 ms-l K-l) it is more interesting to assign the same temperature dependence to the CY-AgIdata, and extrapolate vL of cu-AgI to room temperature (the change in the velocity range is small, see fig. 3). Then the wider ranging and more rapidly varying room temperature velocity data of Chiodelli et al. on (AgI)X(Ag20-2B20&_X glasses can be included in the comparison of our room temperature glasses and cu-AgI behaviour. This broader comparison is also shown in fig. 3 (dashed lines). A linear fit, with a correlation of 0.99, of the ultrasonic data of Chiodelli et al. crosses the &AgI range at 2150 ms-l , i.e. within the experimental accuracy at the center of the range (2147 ms-l). For the transverse mode the temperature dependence was not revealed in the spectra, since the error in the frequency shifts probably is the same or larger than the variation of I+ in the limited temperature range in which the systems remain in the glassy state. In general, the temperature dependence of VT is considerably smaller than for vL and moreover, the intensity of the transverse peak is weak so the error in locating its position is larger, thus making extrapolations to higher temperature unfeasible. However, the elastic properties of the systems are comparable to a
28 January 1985
previously investigated glass, Ca(NO&KN03, for which Tg = 333 K, vL = 3303 ms-l, r+ = 1527 ms-l and.with the same temperature dependence of vL (1.2 ms-l K-1) 1171. The temperature dependence of 9 for Ca(NOj)2KNOj is reported to be 0.38 ms-l K-‘. In the temperature range of the present study the corresponding transverse velocity decrease would be less than 3%, i.e. within the experimental accuracy. In the following a temperature dependence of 0.38 ms-l K-l is used to compare the room temperature transverse velocities of the AgI-glasses with the data of cu-AgI obtained at 563 K. The results are shown in fig. 3. The accordance of both longitudinal and transverse velocities in (AgI),.(AgPO~)l_x glasses with the ar-AgI characteristics seen in fig. 3 is quite impressive. Moreover, comparisons with previous reports on (AgI),JAgzO2B203)1_x glasses suggest that the elastic properties of the high conducting Agl-glasses in general can be extrapolated to superionic cr-AgI. This indicates that the AgI soluted in the host glass matrix tends to reproduce a situation similar to that of a-AgI, which also seems to be the case for dilute glasses. The authors are indebted to CA. Angell and C. Liu, Purdue University, for providing sample materials as well as refractive index and density data, and to R Bogue and RJ. Sladek also of Purdue University for permission to use their preliminary ultrasonic data. J.P. Malugani, Universite de Franche-Corn& is also gratefully acknowledged for some of the sample prep arations. The work has been supported financially by Stiftelsen Lars Hiertas Minne and the Swedish Natural Research Council. References [ I] A. Kvist and A. Josephson, Z. Naturforsch. 239 (1968) 625. [2] J.P. Malugani, A. Wasniewski, M. Doreau and G. Robert, Mater. Res. BuIl 13 (1978) 427. [3] T. Minami, T. Shimizu and M. Tanaka, Solid State Ionics 9/10 (1983) 577. [4] G. Chiodalh, A. Magistris, M. Villa and 1. Bjorkstam, J. Non-Cryst. Solids 5 l(l982) 143. [S] A. Fontana, G. Mariotto, E. Cazzanelli, G. Carini, M. Cutroni and M. Federico, Phys. Rev. Lett. 43 (1983) 209. [6] G. Dalba, A. Fontana, P. Fornasini, G. Mariotto, M.R. Masullo and F. Rocca, Solid. State Ionics 9/10 (1983) 597. [7 ] G. Garini, M. Curtoni, M. Federico, G. Galli and G. Tripodo, J. NonCryst. Solids 56 (1983) 393.
193
Volume
107A, number
4
PHYSICS
[S] J.P. Mahrgani and R. Mercier, Solid State Ionics 13 (1984) 529. [9] G. Chiodalli, A. Magistris and A. Shiraldi, Electrochem. Acta 24 (1979) 203. [ 10 ] R. Aronsson, L. Bijjesson and L.M. Torell, Solid State Ionics 8 (1983) 147. [ll] R. Aronsson, H.E.G. Knape and L.M. Torell, J. Chem. Phys. 77 (1982) 677. [ 12 ] C. Liu, Purdue University.
194
28 January
LETTERS [ 131 J.P. Malugani,
Universite
de Franche-Corn&?. private communication. R. Bogue and R.J. Sladek, Purdue University, private communication. D. Heiman, D.S. Hamilton and R.W. Hellwarth, Phys. Rev. B19 (1979) 6583. L.M. Tore11 and R. Aronsson, J. Chem. Phys. 78 (1983) 1121.
[ 141 C.A. Angell, Purdue University, [15] [16] [17]
1985
28 January 1985
PHYSICS LETTERS
BRILLOUIN SCATTERING STUDY OF ELASTIC PROPERTIES OF SUPERIOMC (AgI)x(AgP03)1 _-x GLASSES L. BORJESSON and L.M. TORELL Department
of Physics, Chalmers University of Technology,
S-41 2 96 Gothenburg,
Sweden
Received 5 September 1984 Revised manuscript received 19 November 1984
Longitudinal and transverse phonon frequencies in superionic (AgI)X(AgP03)1_, glasses for x = 0.3, 0.4 and 0.5 have been measured by Brillouin scattering. The hypersonic velocities as a function of temperature were determined along with the elastic constants. The velocity data extrapolate linearly to those of a-AgI in support of a local cluster model.
During the past decade there has been a considerable interest in the dynamics of superionic conductors or solid electrolytes; a group of materials with a definite crystalline structure in which the conducting ions can more of less freely move around. Among them (YAgI often serves as the archetype with a high conductivity (“1 R-l cm-l) in a temperature range 420828 K due to mobile Ag+ in the solid bee iodine structure [ 11. Recently relatively large values of ionic conductivity (-10e2 Cl-l cm-l) at room temperature have been reported for amorphous electrolytes like borate and phosphate glasses doped with AgI [2-41. The growing attention focused on superionic glasses is to a large extent due to their technological advantages and different experimental techniques have been applied to understand how the glass network is modified when AgI is introduced [3-81. It has been suggested that AgI is mainly coordinated by the matrix oxygen ions [9], while other authors propose that the glass network is not affected chemically but that dissolved AgI tends to reproduce on the local level microdomains of the superionic phase of pure AgI [ 5,6, 81. In support of the last mentioned model recent optical absorption studies report an absorption which shifts linearly at the increasing of the AgI content and extrapolates to the value of cr-AgI [6]. The aim of the present work concerning the system (AgI)X(&P03)~_X is to investigate if the acoustic behaviour of the systems can be extrapolated to that of pure cw-AgIwhich 190
has previously been studied in this laboratory [lo]. This implies measurements in which not only the AgI concentration is to be gradually changed but also the temperature, since CY-AgIonly exists at elevated temperatures. In the following a Brillouin scattering study of the temperature dependence of longitudinal and transverse sound velocities in AgPO3 doped with 0.3, 0.4 and 0.5 AgI respectively is presented. As far as we know this is the first Brillouin scattering study reported for superionic glasses. Brillouin scattering is a well recognized tool for determining structural changes. In Brillouin scattering from an amorphous solid the principal components of the scattered light are the Rayleigh line (R) centered at the incident frequency uo, a pair of dopplershifted lines (L) due to longitudinal phonons in the material and a weaker doublet (T), which arises from the presence of transverse phonons. Fig. 1 presents a typical spectrum of the (AgI)X(AgP03)l_X systems; two pairs of longitudinal modes are shown since the interferometer is adjusted for overlapping orders in order to resolve the transverse modes. The frequency shifts vL and VT are related to the longitudinal and transverse sound velocities, uL and u, by uL = vLk/2n
,
VT = &&/2n ,
(0 (2)
where k is the wave vector of the phonon given by 0.3759601/85/$03.30 0 Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
Volume 107A, number 4
PHYSICS LETTERS
28 January 1985
_ 2.9
FL.7
E 2.8 a Q2.7 z? :: 2.6 T ’ 2.5 15 10
5
0
Frequency
5
I
0:x=0.3 o:x=_o.* A: X=0.5
-
10 15
GHz
300
350 Tern perature
Fig. 1. Brillouin scattering spectrum of (AgI)o.4(AgPOs)o.e at room temperature. The spectral free range of the interferometer is 21.54 GHz, working at overlapping orders.
k = (4?m/Ao)
sin(e/2),
400
( K1
Fig. 2. The temperature dependence of the longitudinal velocity in (AgI)x(AgPOa)l_x glasses, 0: x = 0.3, o: x = 0.4 and A: x = 0.5. Solid lines represent linear tits and dash-dotted lines the glass transition temperatures. The dashed line is from ultrasonic measurements of (AgI)&AgPO& 1151.
(3)
where ho is the wavelength of the incident light, 8 is the scattering angle and n is the refractive index of the medium. In the present study light scattered at 90’ was collected using an experimental arrangement essentially the same as that described previously [ 111. The light source is a single mode Kr-ion laser operating at 647.1 nm with an output power of about 160 mW. The scattered light was analyzed by a triple-passed piezo-electrically scanned Fabry-Perot interferometer with an electronic feedback system to adjust for any nonlinearities of the piezo-elements during a scan. The interferometer was adjusted for several spectral free ranges to optimize the resolution at every temperature; the overall finesse of the system was ~40. The detector system includes a cooled ITT-FW 130 photomultiplier and photon counting equipment coupled to a multichannel analyzer for storage of the data. The samples, prepared elsewhere [ 12,131, were in a cylindrical shape (@ - 6 mm) of a height -10 mm and were selected to be optically homogeneous and transparent. The circular surfaces were polished before every measurement since they served as inlet and outlet windows for the laser beam. During the measurements the glass ‘samples were situated in a cylindrical pyrex cell (a - 7 mm) with an optically flat window fused to the bottom of the cell. Refractive index matching oil filled the gap between the sample and the cell. This turned out to be essential not only to avoid refractions but also to prevent Ag from being developed on the entrance surface of the sample and block-
ing the laser beam. This often occurred when the materials were exposed to the laser beam in an oxygen atmosphere. The cell was placed in a thermostat constructed with several light ports sealed by fused quartz plates of optical quality, which allowed right angle light scattering to be collected. The temperature was measured by a thermocouple close to the wall of the scattering cell, the temperature stability being better than *OS” during a measurement. Brillouin spectra were obtained in a range starting from room temperature up to the glass transition temperatures; Tg = 398, 375 and 356 K for x = 0.3, 0.4 and 0.5, respectively, in the (AgI)x(AgP03)1_x system. From the observed frequency shifts the sound velocities could be calculated by using eqs. (1) and (2) and present data for the longitudinal mode are shown in fig. 2. The transverse modes were too weak in intensity to give accurate values for the shift as a function of temperature; room temperature results are given in table 1. As can be seen from fig. 2 the longitudinal velocities decrease with increasing amount of AgI as well as with increasing temperature. The temperature dependences are in close agreement for the three systems and linear fits are given by x = 0.3,
(4)
uL = 3049 - 1. lOOT(K) ms-l ,
x = 0.4 ,
(5)
uL = 2898 - l.l05T(K)
x = 0.5 ,
(6)
uL = 3201 - 1.2843(K)
ms-l
,
ms-l ,
191
Volume 107A, number 4
PHYSICS LETTERS
28 January 1985
Table 1 Results of elastic properties from Brillouin scattering measurements on (AgI),(AgPO$r_
X glasses at room temperature.
Molar fraction AgI
longitudinal shift LJL(GHz) transverse shift q- (GHz) longitudinal velocity VL ( lo3 ms-‘) transverse velocity q- ( lo3 ms-‘) density a) p ( lo3 kg mS3) index of refraction a) n elastic constant CL (10’ Nmo2) elastic constant CT (10’ Nmw2) Young’s modulus Y (10’ Nme2) bulk modulus B (10’ Nme2) Poissons ratio (I
0.3
0.4
0.5
11.78 5.09 2.84 1.23 4.98 1.90 40.2 7.5 20.8 30.2 0.39
11.53 4.91 2.13 1.16 5.20 1.93 38.7 7.0 19.5 29.4 0.39
11.04 4.11 2.51 1.11 5.41 1.97 35.6 6.6 18.4 26.7 0.39
a) Ref. [14].
with correlations better than 0.9. Preliminary ultrasonic velocity measurements on (AgI)4(AgP03)6 [ 1.51 are shown in fig. 2 as a dashed line and exhibit the same temperature dependence as do the present hypersonic velocities. From the values of the longitudinal and transverse elastic wave velocities the constants which characterize the glass can be calculated, i.e. the elastic constants, CL and CT, Young’s modulus Y, shear modulus cc, bulk modulus B and Poisson’s ratio u by using the relations CL=&
(7)
CT=&&
(8)
y = CT ](3CL - 4CT)/(C,
- cT)] 3
(9)
p=C,,
(10)
B=CL-:CT,
(11)
u= Y/2/J-
1.
(12)
The corresponding data for the present systems based on room temperature measurements are given in table 1. Compared to optical glasses [ 161 the sound velocities of the (AgI)X(AgP03)l_X systems are low and for example CL is only about one half the value for SiO,, which reflects a less rigid structure for these systems. The ratio of the transverse to longitudinal velocity, r+/uL, for the present systems is 0.43 which 192
is lower than the average ratio of 0.59 * 0.03 found in optical glasses indicating a slightly lower ratio between the shear and compressional restoring forces. From fig. 2 it follows that the longitudinal velocity decreases with the addition of AgI. This is also the case for the transverse velocity as can be seen in table 1. The velocity data thus confirm that the structure becomes less rigid with increasing AgI content, which is also consistent with the decreasing glass transition temperatures. It is of interest to investigate if the glass matrix itself is changed by the introduction of AgI or if species of AgI merely stay in separated clusters or microdomains in the host matrix network. We would therefore like to raise the following question: How does the response of the structure of high conducting AgI doped glasses to high frequency stresses compare with that of the superionic structure of pure AgI, i.e. cu-AgI? The hypersonic velocities of cw-AgIhave recently been investigated in this laboratory and at a temperature of 563 K the longitudinal velocity was found to be in the range 1680-1970 ms-l depending on crystal orientation [lo]. The corresponding values for the transverse velocity are in the range 720-960 ms-I. By using the temperature dependences obtained in the present study for the longitudinal velocity for the investigated AgI concentrations, the longitudinal velocities of the present glass systems can be compared to those of superionic AgI at 563 K. Such a comparison is shown as a solid line in fig. 3, and it is
Volume 107A, number 4
PHYSICS LETTERS
Molorfroction 41
Fig. 3. Plot of longitudinal (vL) and transverse (vy) velocity versus AgI concentration for (AgI),(AgPOa) t_ x glasses (a: present results) and for (AgI),(AgzO-2B2Oa)t_, glasses (o: ultrasonic data of ref. [7]). The velocity ranges at x = 1 for or-AgI are from ref. [lo]. Solid lines represent data at 536 K. Dashed lines represent room temperature data.
seen that the “glass” data extrapolate within uncertainty limits to that of o-AgI. However, since all AgI glass compositions have about the same temperature dependence of UL (mean value =1.2 ms-l K-l) it is more interesting to assign the same temperature dependence to the CY-AgIdata, and extrapolate vL of cu-AgI to room temperature (the change in the velocity range is small, see fig. 3). Then the wider ranging and more rapidly varying room temperature velocity data of Chiodelli et al. on (AgI)X(Ag20-2B20&_X glasses can be included in the comparison of our room temperature glasses and cu-AgI behaviour. This broader comparison is also shown in fig. 3 (dashed lines). A linear fit, with a correlation of 0.99, of the ultrasonic data of Chiodelli et al. crosses the &AgI range at 2150 ms-l , i.e. within the experimental accuracy at the center of the range (2147 ms-l). For the transverse mode the temperature dependence was not revealed in the spectra, since the error in the frequency shifts probably is the same or larger than the variation of I+ in the limited temperature range in which the systems remain in the glassy state. In general, the temperature dependence of VT is considerably smaller than for vL and moreover, the intensity of the transverse peak is weak so the error in locating its position is larger, thus making extrapolations to higher temperature unfeasible. However, the elastic properties of the systems are comparable to a
28 January 1985
previously investigated glass, Ca(NO&KN03, for which Tg = 333 K, vL = 3303 ms-l, r+ = 1527 ms-l and.with the same temperature dependence of vL (1.2 ms-l K-1) 1171. The temperature dependence of 9 for Ca(NOj)2KNOj is reported to be 0.38 ms-l K-‘. In the temperature range of the present study the corresponding transverse velocity decrease would be less than 3%, i.e. within the experimental accuracy. In the following a temperature dependence of 0.38 ms-l K-l is used to compare the room temperature transverse velocities of the AgI-glasses with the data of cu-AgI obtained at 563 K. The results are shown in fig. 3. The accordance of both longitudinal and transverse velocities in (AgI),.(AgPO~)l_x glasses with the ar-AgI characteristics seen in fig. 3 is quite impressive. Moreover, comparisons with previous reports on (AgI),JAgzO2B203)1_x glasses suggest that the elastic properties of the high conducting Agl-glasses in general can be extrapolated to superionic cr-AgI. This indicates that the AgI soluted in the host glass matrix tends to reproduce a situation similar to that of a-AgI, which also seems to be the case for dilute glasses. The authors are indebted to CA. Angell and C. Liu, Purdue University, for providing sample materials as well as refractive index and density data, and to R Bogue and RJ. Sladek also of Purdue University for permission to use their preliminary ultrasonic data. J.P. Malugani, Universite de Franche-Corn& is also gratefully acknowledged for some of the sample prep arations. The work has been supported financially by Stiftelsen Lars Hiertas Minne and the Swedish Natural Research Council. References [ I] A. Kvist and A. Josephson, Z. Naturforsch. 239 (1968) 625. [2] J.P. Malugani, A. Wasniewski, M. Doreau and G. Robert, Mater. Res. BuIl 13 (1978) 427. [3] T. Minami, T. Shimizu and M. Tanaka, Solid State Ionics 9/10 (1983) 577. [4] G. Chiodalh, A. Magistris, M. Villa and 1. Bjorkstam, J. Non-Cryst. Solids 5 l(l982) 143. [S] A. Fontana, G. Mariotto, E. Cazzanelli, G. Carini, M. Cutroni and M. Federico, Phys. Rev. Lett. 43 (1983) 209. [6] G. Dalba, A. Fontana, P. Fornasini, G. Mariotto, M.R. Masullo and F. Rocca, Solid. State Ionics 9/10 (1983) 597. [7 ] G. Garini, M. Curtoni, M. Federico, G. Galli and G. Tripodo, J. NonCryst. Solids 56 (1983) 393.
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4
PHYSICS
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