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Determination of gallium coordination in cesium gallate glasses by high-resolution pulsed NMR
122
Journal
of Non-Crystalline
Solids 94 (1987) 122-132 North-Holland, Amsterdam
DETERMINATION OF GALLIUM COORDINATION IN CESIUM GALLATE GLASSES BY HIGH-RESOLUTION PULSED NMR * Jianhui ZHONG Deportment Received Revised
o/Physics,
and P.J. BRAY Brown
Universig:
17 November 1986 manuscript received 9 June
Providence,
RI 02912.
USA
1987
High-resolution “Ga and “‘Ga NMR spectra of glasses in the system xCs,O.(lx)GazO, with x = 0.3-0.7 have been studied at 91.5 MHz and 72.0 MHz respectively. Absorption peaks corresponding to the tetrahedrally and octahedrafly coordinated gallium sites can be unambiguously distinguished in both the static and magic-angle sample spinning (MASS) spectra. Assignment of spectral features to the different gallium atomic coordinations has been done in light of the close similarity in chemical and coordination properties between gallium and aluminum, and by comparison with gallium compounds of known gallium coordinations. The spin-lattice relaxation time r, and spin-spin relaxation time T2 of the Ccoordinated and 6-coordinated gallium sites were measured: the existence of different relaxation times helps in distinguishing the two different sites. Close similarities are found with the results of “Al NMR studies of similar systems.
1. Introduction Group III of the periodic table contains three typical metallic elements aluminum, gallium and indium. They are closely related to each other as shown by similarities in their metallurgy and in the chemical properties of the elements and their derivatives. Due to the importance of aluminum in some major fields of industrial chemistry and its favorable NMR characteristics high sensitivity and large chemical shift range - 27Al has continued to be the most extensively studied element of this triad.But it was found recently that in some glassforming systems the glassforming range is larger in the gallate than in the aluminate [1,2] case. Thus gallium is a better candidate for the structural study of the glasses formed with elements in the group. It has also been shown that the gallate glasses have a high optical transmission up to even longer wavelengths in the infrared than aluminate glasses [2]. It is thus of practical importance and theoretical interest to explore the physical and chemical properties as well as the coordination structure of gallate glass systems. The structure of alkali aluminosilicate glasses has been the subject of considerable controversy. Most people working with the system agree that both tetrahedrally and octahedrally coordinated aluminum exist in the glasses. * This research was supported by the Materials is funded by the National Science Foundation.
0022-3093/87/$03.50 0 Elsevier Science Publishers (North-Holland Physics Publishing Division)
Research
B.V.
Laboratory
at Brown
University
which
J. Zhong,
P.J. Bray
/ Gallium
coordinnrion
in cesium
gallare
glasses
123
The major difference in the different models lies in the composition at which the 6-coordinated aluminum appears (when the molar ratio of aluminum/alkali > 1.0, as given by Day and Rindone [3], or as early as the initial addition of alumina to alkali silicate glasses as given by Shelby [4] and Hunold and Bruckner [5]. The group of Whichard and Day [l] and the group of Kokubo, Inaka and Sal&a [2] separately found that the glassforming range for gallate glass can be as large as up to 70 mol% of gallium oxide. But their studies of the system with IR spectroscopy resulted in different conclusions and provided consequently different structural models. The former found that in the CaO-Ga,O, system with 37.5 to 62 mol% GazO, the gallium ion is presented in both 4-coordination and 6-coordination in all the samples. The latter found on the other hand that in the systems (Na ,O, K,O or Cs,O)-Ta,O,-Ga,O, and (SrO or BaO)-Ta,O,-Ga ,O, all the gallium ions are tetrahedrally coordinated in the glasses. The structural role of aluminum ions in the binary alkali aluminate glasses should serve as a guide for the roles of alumina in the ternary alkali aluminosilicate glasses. The limited glassforming range for the alkali aluminate glasses, however, restricts extensively study of the binary system. Fortunately, the discovery of the larger glassforming range of alkali gallate glasses and the close similarities of aluminum and gallium provide another opportunity to explore the glass system. The shapes of the property/composition curves for both glass systems are similar, as discovered by Piguet et al. [6] in their extensive study of the transformation range behavior of lithium aluminosilicate and galliosilicate glasses. It follows that the structural models used to explain the property variations will be quite similar for the glasses formed in both binary systems. Study of the structure of alkali gallate glasses should, therefore, yield additional insight regarding the structure of alkali aluminate glasses and alkali aluminosilicate glasses. This paper presents the results of the high-resolution solid-state NMR study of the cesium gallate glass system with the gallium oxide content ranging up to 70 mol%. The study is based on “Ga and 69Ga static and MASS spectra at room temperature, and relaxation time measurements (spin-lattice relaxation time T, and spin-spin relaxation time T,). The NMR signals provide direct evidence for the existence of both 4-coordinated and 6-coordinated gallium atoms and changes of their ratio as a function of composition of the glass system. Comparisons are made between the gallate and aluminate glasses and, not surprisingly, close similarities are found between them, in agreement with other studies. 2. Experimental 2.1. Sample preparation Powder mixtures prepared from reagent grade gallium oxide and cesium carbonate were put into a platinum crucible and melted in an electric furnace
124
J. Zhong,
P.J. By
/ Gallium
coordination
in cesium
gallare
glasses
at 150 “C for about half an hour. During the whole melting process the crucible was covered with a piece of platinum foil to prevent the batch materials from evaporating. The melts together with the platinum crucible were quickly quenched in cold water and the samples plus the crucible were carefully weighted before and after melting. By assuming that all the CO? has boiled off during melting, and therefore subtracting the expected weight loss of CO,, we found the net weight loss to be less than 3.0 wt% for the sample with GazO,/CszO = 3 : 7, which can be expected to have the largest weight loss through evaporation of Cs,O. Since it has been reported in the Literature that in the alkali borates of high alkali content there is a CO, retention problem [7], the assumption that all the CO, boils off in the preparation of the cesium gallate samples needs to be verified. The CO, content is hard to determine accurately with NMR due to the low natural abundance and sensitivity of 13C and “0. Hence the intensities of the ‘j3Cs NMR resonance signal (the total area under the resonance peak, which is proportional to the cesium content in samples of normalized molar content) were measured for the samples with the highest and lowest cesium contents. The ratio of the intensities agrees very well with the cesium content ratio expected for the samples (within 1.5%). All the samples were analysed with X-ray diffraction spectroscopy and verified to have no evidence of devitrification except for the sample with Ga,O,/Cs,O = 3 : 7 which is partially devitrified. All samples were ground to fine powders and sealed at the melting site in teflon vials and stored in a dessicator for future use. 2.2. NMR study A Bruker MSL300 high-power NMR spectrometer was used to look at ‘I Ga and 69Ga NMR signals at 91.5 MHz and 72.0 MHz respectively. To ensure that no artifacts from the spectrometer exist, a simple 90” pulse sequence and a solid echo (QUADECHO), with the interval between the irradiating pulse and the observing pulse a little larger than the system ringdown time, were used to observe the NMR signals; no distinguishable difference was found. (To observe only the central transition, a 45” pulse was actually used). The spin-lattice relaxation time T, and spin-spin relaxation time T2 were studied with an inverse-recovery (180~r-90-acquisition) pulse sequence and a 90+90r-echo pulse sequence, respectively. For selected samples, MASS was used to observe the 69Ga signal. The maximum spinning rate used was 4.95 kHz.
3. Results and discussion The static spectra of “Ga for the samples with a variable Gaz03/Cs20 ratio (as indicated) are plotted in fig. 1. The maximum intensities for all of the samples are normalized and the high-frequency side is to the left. 1M Ga(NO,),
J. Zhong,
P.J. Bray
/ Gallium
coordination
in cesium
gallare
GA
I
I
I
I
I
'
glasses
125
c5
I
I
-1000
1000 pOpm
Fig. 1. “Ga
static
spectrum
for samples
with
different
Ga,O,/Cs,O
ratios
at 91.53
MHz.
solution is commonly used by chemists as the “Ga chemical shift reference. But in our experiments, since a GaAs polycrystalline sample gives a very strong and sharp spectrum which facilitates determination of the proper pulse length for solids, all spectra were scaled to the GaAs spectrum. A later measurement determined that the peak for 1M Ga(NO,), is about 37 ppm to the high-frequency side of the GaAs peak. Two separated peaks can be unambiguously observed at S = O-15 and 6 = - 150 ppm respectively for the glass samples with the Ga,O,/Cs,O ratio around one, and it can be seen clearly that the intensity of the peak at lower frequency increases compared with the peak at higher frequency as the gallium content increases. Chemical shift ranges for tetrahedral and octahedral galliums are given by Brevard and Granger [8] as S (tetrahedral) = - 500- + 600 ppm and 6 (octahedral) = - 200- - 50 ppm in the scale we used. Due to the overlapping of the chemical shift ranges, the coordination of gallium cannot be uniquely determined by measuring the chemical shift of the observed responses. But it has been shown that the chemical shift range of gallium follows the same trends as aluminum[9] and there have been numerous studies of the chemical shift range of aluminum [lo-121 in glasses and solutions [13]. It has become a
126
J. Zhong.
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/ Gallium
coordinarion
in cesium
gallare
glasses
well established fact that the isotropic “Al chemical shifts in oxide glasses depend mainly on the Al-O coordination, and for Al-O tetrahedra shifts of 55-80 ppm to higher frequency were measured in contrast to those of Al-O octahedra [lo]. Similar conclusions have been drawn for aluminum in solutions in ref. [13]. This would suggest that the high-frequency “Ga peak (on the left in fig. 1) originates from 4-coordinated gallium, while the low-frequency peak is from 6-coordinated gallium. Spectra of several compounds with known gallium coordination were also studied in order to aid the identification of gallium coordinations for the two separated peaks in the glass system. Gaz(SO,)j, which has an octahedral gallium environment, was found to have a sharp resonance peak at 6 = - 215 ppm. GaCl, in tap water, with molecular structure M,X, and a tetrahedral gallium environment, has a resonance peak at 6 = 27 ppm. The GaAs polycrystalline sample, used as a reference in this study (6 = 0), also has a
GaAs
I
800
-800
0 Ppm
Fig. 2. “Ga static spectrum spectra for a 1M Ga(NO,),
for the sample with GazO,/Cs,O = 7: 3 at 91.53 MHz. together with solution. polycrystalline GaAs, GaCI, in tap water and a Ga2(S0,), polycrystalline sample.
J. Zhong,
P.J. By
/ Gallium
coordination
in cesium
gallute
glussses
127
tetrahedral gallium environment. Spectra of the above gallium compounds together with the spectrum of the Ga,0,/Cs20 = 7 : 3 glass sample and for 1M Ga(NO), are plotted in fig. 2. According to the literature, cu-Ga,O, has only tetrahedral gallium, while ,&Ga,O, has both tetrahedral and octahedral gallium sites. But as pointed out by Marezio and Remeika [14], under normal conditions for synthesis only fl-Ga,O, is formed. The transformation p + (Ycan be performed by subjecting P-Ga,O, to 44 kbar at 1000°C. We were no able to find a commercial source as supplier of cy-Ga 2O,. A study of polycrystalline /3-Ga 2O, shows that the spectrum is a complicated mixture of features from both quadrupolar and chemical shift interactions. Analysis of this spectrum must await acquisition of spectra at other magnetic fields. In light of the close similarities in physical and chemical properties and microscopic structures between aluminate and gallate glasses, and the chemical shift data presented above, the peak on the high-frequency side (on the left) can be attributed to GaO, tetrahedral units, and the peak on the low-frequency side (on the right) can be assigned to cationic, 6-coordinated gallium units. The appearance of 6-coordinated gallium units at a Ga zO,/CsZO ratio of less than one suggests the beginning of the role of gallium oxide as a glass modifier at about this composition. Separation between the peaks from 4-coordinated and 6-coordinated gallium ions is the same in all the samples with two peaks, and is about 140 ppm, larger than the value for aluminum in most aluminate glasses (about 55-80 ppm as given by refs. [lo-121). The peak assignments according to the gallium coordination can be further confirmed with relaxation time studies, as discussed later in this paper. The “Ga static spectrum is less informative than the “Ga static spectrum. Selected spectra are plotted in fig. 3; a GaAs crystal was again taken as the reference. Although the overall center of the spectrum can be seen to shift toward the low-frequency side, and the resonance line becomes a little asymmetric as the gallium oxide concentration increases, no clearly separated features can be identified. The magic-angle sample spinning technique was therefore utilized in an effort to suppress the dipolar interaction, chemical shift anisotropy, and part of the quadrupolar interaction. The MASS spectrum for the sample with a Ga,O,/Cs,O ratio 7 : 3 is given in fig. 4(a) together with the corresponding static spectrum for comparison. The spinning rate used is 4.90 kHz. The spinning sidebands are sorted out as indicated by the circle and cross marks on the expanded plot of the same MASS spectrum in fig. 4(b). The crosses denote features from 6-coordinated gallium, and the circles identify features from 4-coordinated gallium. The linewidth is indeed much narrower in the mass spectrum, and the features of contributions from two different gallium sites can be seen. Separation between the peaks from 6-coordinated and 4-coordinated galliums is about 27 ppm, or 1.95 kHz, much smaller than the separation between the two peaks in the “Ga spectrum. The existence of many spinning sidebands indicates that there are strong quadrupole or anisotropic chemical shift interactions in the samples. The 69Ga
128
J. Zhong,
P.J. Brq
/ Gallium
coordination
in cesium
gallare
glasses
=A
Fig. 3. 69Ga static
spectrum
for samples
with
different
GazO,/CszO
ratios
/
cs
at 72.05
MHz.
isotope has a larger quadrupole moment and lower resonance frequency as compared to “Ga, and consequently the shift of the center of gravity of the spectrum due to the second-order quadrupole interaction is larger [15]. If we assume that the separation of the two peaks in the “Ga spectrum is totally due to the shift by the second-order quadrupole interaction, then the shift should be about 40 kHz or 440 ppm, which is about 20 times larger than the experimental result for the 69Ga resonance. The discrepancy presumably arises from the effects of the isotropic chemical shift in the glass system under study. The “Ga spectra obtained by use of a spin-echo pulse sequence with T = 10, 100, 200, 300 and 400 ~LSfor the sample with a Gaz0,/Cs20 ratio equal to 7 : 3 is plotted in fig. 5. It can be seen that when 7 is greater than 300 ps, the peak on the high-frequency side totally disappears. To see the effect more clearly, the spectra for 7 equal to 10 ps and 300 ~LSare again plotted in fig. 6 with normalized intensity (the spectrum with 7 = 300 ps has been multiplied by 6.6 to compensate for the decay during the echo). The difference spectrum, calculated by subtracting the normalized spectrum with 7 = 300 ps from the normalized spectrum with 7 = 10 ps, is also shown at the bottom of fig. 6. The magnitude of T, for the peak on the high-frequency side could not be determined accurately from the spin-echo experiment because of the weak
J. Zhong,
P.J. Bray
/ Gallium
coordination
in cesium
gallare
glasses
129
a
-1200
-600
0
600
PPm
b
200
0
-200
-400
PPm Fig. 4. 69Ga static and MASS spectra for the sample Sidebands for the tetrahedral and octahedral galliums ( X ) respectively.
with Ga,O,/Cs,O = 7: 3 at 72.05 MHz. are indicated by a circle (0) and a cross
intensity and the fast decay of the echo. However, the fact that the peak has essentially vanished at 7 = 300 ps suggests that T, for the site is less than 300 ps. The magnitude of T, calculated by fitting the exponentially decaying
J. Zhong.
130
‘w”-
800
P.J.
---
Bray
-~
400
/ Gallium
coordination
in cesium
-”
-
0
-400
gallare
.-
”
glasses
“-----”
-800
PPm Fig.
5. “Ga
static spectrum for the sample with Ga,03/Cs20 experiment. 7 is the time between the irradiating
= 7: 3 obtained in a spin-echo pulse and the echo.
intensity [15] for the peak on the low frequency side is about 400 ps. A T, study for the same sample shows the values of 2-, for the peaks on the high-frequency side and low-frequency side are 8 ms and 6 ms respectively. The same measurements were made on the sample for which the Ga 20,/Cs20 ratio is 3 : 7. Only one peak was observed, at the position of the peak on the high-frequency side, as in fig. 1. T, and T, values are close to the values of the sample with the Ga,O,/Cs,O = 7 : 3 (Tl equals 6 ms and T, is less than 300 PI. As discussed in detail by Fukushima and Roeden [16], accurate T, and T, measurements of quadrupolar nuclei are tricky at best since the Zeeman levels will no longer be equally spaced and it is impossible to define a unique relaxation time, because the nuclear relaxation will be a complex sum of relaxations between the different levels. Bearing this in mind, the actual values of Tl and T,, obtained from the above measurements are not trustworthy or significant in themselves. However, they do provide an additional means for identifying the existence of more than one distinct gallium coordination site
J. Zhong,
2000
P.J. Bray
1000
/ Gallium
coordination
0
in cesium
-1000
gallare
-
glasses
131
2000
Pm Fig. 6. “Ga
static
with different tions.
spectrum for the sample with Ga,0,/Cs20 = 7 : 3. Two sites can be identified by the difference in the two spectra of different 7 values.
relaxation times in samples with high gallium oxide concentra-
4. Conclusion The results of this study suggest that NMR is an effective tool in studying gallium-containing glass systems. Separated features can be identified without magic-angle sample spinning in the case of “Ga, and with the help of MASS in the case of 6gGa. Larger separations between the features from 4-coordinated gallium in the “Ga spectra make “Ga superior to *‘Al for NMR studies. Relaxation time studies can further distinguish signals from galliums in different coordination environments and will be useful in the study of ion hopping and other motional processes in the glass. In the cesium gallate glasses, almost all gallium atoms act as network glass formers when the Ga/Cs ratio is small (less than 3 : 7) i.e., the galliums are in 4-coordination. But when the Ga/Cs ratio increases, the excess gallium goes into the glass system as 6-coordination ions.
J. Zhong,
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P.J. Bray
/ Gallium
coordination
in cesium
gal/ate
glasses
References [l] [2] [3] [4] [5] [6] [7] [S] [9] [lo] [ll] [12] [13] [14] [15] [16]
G. Whichard and D.E. Day, J. Non-Cryst. Solids 66 (1984) 477. T. Kokubo, Y. Inaka and S. Sakka, in press. D.E. Day and G.E. Rindone. J. Am. Ceram. Sot. 45 (1962) 489. J.E. Shelby, J. Appl. Phys. 49 (1978) 588. V.K. Hunold and R. Bruckner. Glastechn. Ber. 53 (1980) 169. J.L. Piguet, J.C. Lapp and J.E. Shelby, J. Am. Ceram. Sot. 68 (1985) 326. S.W. Martin and C. Austen Angell, J. Am. Ceram. Sot. 67 (1984) C-148. C. Brevard and P. Granger, Handbook of High Resolution NMR (Wiley, New York, 1981). R. Harris and B. Mann, NMR and Periodic Table (Academic Press, New York, 1978) p. 285. D. Muller, W. Gessner, H.J. Behrens and G. ScheIIer, Chem. Phys. Lett. 51 (1981) 59. D. Muller, G. Berger, I. Grunze, G. Ladwig, E. HaIIas and U. Haubenreisser, J. Phys. Chem. Glasses 24 (1983) 37. G. Engelhardt, M. Nofz, K. Forkel, F.G. Wihsmann, M. Magi, A. Samoson and E. Lippmaa, Phys. Chem. Glasses 26 (1985) 157. J.J. Delpuech, NMR of Newly Accessible Nuclei, Vol. 2, ed. P. Larszlo (Academic Press, New York, 1983) p. 153. M. Marezio and J.P. Remeika, J. Chem. Phys. 46 (1967) 1862. A. Abragam, Principles of Nuclear Magnetism (Oxford Univ. Press, Oxford, 1983). E. Fukushima and S.B.W. Roeden, Experimental Pulse NMR, A Nuts and Bolts Approach (Addison-Wesley, New York, 1981).
Journal
of Non-Crystalline
Solids 94 (1987) 122-132 North-Holland, Amsterdam
DETERMINATION OF GALLIUM COORDINATION IN CESIUM GALLATE GLASSES BY HIGH-RESOLUTION PULSED NMR * Jianhui ZHONG Deportment Received Revised
o/Physics,
and P.J. BRAY Brown
Universig:
17 November 1986 manuscript received 9 June
Providence,
RI 02912.
USA
1987
High-resolution “Ga and “‘Ga NMR spectra of glasses in the system xCs,O.(lx)GazO, with x = 0.3-0.7 have been studied at 91.5 MHz and 72.0 MHz respectively. Absorption peaks corresponding to the tetrahedrally and octahedrafly coordinated gallium sites can be unambiguously distinguished in both the static and magic-angle sample spinning (MASS) spectra. Assignment of spectral features to the different gallium atomic coordinations has been done in light of the close similarity in chemical and coordination properties between gallium and aluminum, and by comparison with gallium compounds of known gallium coordinations. The spin-lattice relaxation time r, and spin-spin relaxation time T2 of the Ccoordinated and 6-coordinated gallium sites were measured: the existence of different relaxation times helps in distinguishing the two different sites. Close similarities are found with the results of “Al NMR studies of similar systems.
1. Introduction Group III of the periodic table contains three typical metallic elements aluminum, gallium and indium. They are closely related to each other as shown by similarities in their metallurgy and in the chemical properties of the elements and their derivatives. Due to the importance of aluminum in some major fields of industrial chemistry and its favorable NMR characteristics high sensitivity and large chemical shift range - 27Al has continued to be the most extensively studied element of this triad.But it was found recently that in some glassforming systems the glassforming range is larger in the gallate than in the aluminate [1,2] case. Thus gallium is a better candidate for the structural study of the glasses formed with elements in the group. It has also been shown that the gallate glasses have a high optical transmission up to even longer wavelengths in the infrared than aluminate glasses [2]. It is thus of practical importance and theoretical interest to explore the physical and chemical properties as well as the coordination structure of gallate glass systems. The structure of alkali aluminosilicate glasses has been the subject of considerable controversy. Most people working with the system agree that both tetrahedrally and octahedrally coordinated aluminum exist in the glasses. * This research was supported by the Materials is funded by the National Science Foundation.
0022-3093/87/$03.50 0 Elsevier Science Publishers (North-Holland Physics Publishing Division)
Research
B.V.
Laboratory
at Brown
University
which
J. Zhong,
P.J. Bray
/ Gallium
coordinnrion
in cesium
gallare
glasses
123
The major difference in the different models lies in the composition at which the 6-coordinated aluminum appears (when the molar ratio of aluminum/alkali > 1.0, as given by Day and Rindone [3], or as early as the initial addition of alumina to alkali silicate glasses as given by Shelby [4] and Hunold and Bruckner [5]. The group of Whichard and Day [l] and the group of Kokubo, Inaka and Sal&a [2] separately found that the glassforming range for gallate glass can be as large as up to 70 mol% of gallium oxide. But their studies of the system with IR spectroscopy resulted in different conclusions and provided consequently different structural models. The former found that in the CaO-Ga,O, system with 37.5 to 62 mol% GazO, the gallium ion is presented in both 4-coordination and 6-coordination in all the samples. The latter found on the other hand that in the systems (Na ,O, K,O or Cs,O)-Ta,O,-Ga,O, and (SrO or BaO)-Ta,O,-Ga ,O, all the gallium ions are tetrahedrally coordinated in the glasses. The structural role of aluminum ions in the binary alkali aluminate glasses should serve as a guide for the roles of alumina in the ternary alkali aluminosilicate glasses. The limited glassforming range for the alkali aluminate glasses, however, restricts extensively study of the binary system. Fortunately, the discovery of the larger glassforming range of alkali gallate glasses and the close similarities of aluminum and gallium provide another opportunity to explore the glass system. The shapes of the property/composition curves for both glass systems are similar, as discovered by Piguet et al. [6] in their extensive study of the transformation range behavior of lithium aluminosilicate and galliosilicate glasses. It follows that the structural models used to explain the property variations will be quite similar for the glasses formed in both binary systems. Study of the structure of alkali gallate glasses should, therefore, yield additional insight regarding the structure of alkali aluminate glasses and alkali aluminosilicate glasses. This paper presents the results of the high-resolution solid-state NMR study of the cesium gallate glass system with the gallium oxide content ranging up to 70 mol%. The study is based on “Ga and 69Ga static and MASS spectra at room temperature, and relaxation time measurements (spin-lattice relaxation time T, and spin-spin relaxation time T,). The NMR signals provide direct evidence for the existence of both 4-coordinated and 6-coordinated gallium atoms and changes of their ratio as a function of composition of the glass system. Comparisons are made between the gallate and aluminate glasses and, not surprisingly, close similarities are found between them, in agreement with other studies. 2. Experimental 2.1. Sample preparation Powder mixtures prepared from reagent grade gallium oxide and cesium carbonate were put into a platinum crucible and melted in an electric furnace
124
J. Zhong,
P.J. By
/ Gallium
coordination
in cesium
gallare
glasses
at 150 “C for about half an hour. During the whole melting process the crucible was covered with a piece of platinum foil to prevent the batch materials from evaporating. The melts together with the platinum crucible were quickly quenched in cold water and the samples plus the crucible were carefully weighted before and after melting. By assuming that all the CO? has boiled off during melting, and therefore subtracting the expected weight loss of CO,, we found the net weight loss to be less than 3.0 wt% for the sample with GazO,/CszO = 3 : 7, which can be expected to have the largest weight loss through evaporation of Cs,O. Since it has been reported in the Literature that in the alkali borates of high alkali content there is a CO, retention problem [7], the assumption that all the CO, boils off in the preparation of the cesium gallate samples needs to be verified. The CO, content is hard to determine accurately with NMR due to the low natural abundance and sensitivity of 13C and “0. Hence the intensities of the ‘j3Cs NMR resonance signal (the total area under the resonance peak, which is proportional to the cesium content in samples of normalized molar content) were measured for the samples with the highest and lowest cesium contents. The ratio of the intensities agrees very well with the cesium content ratio expected for the samples (within 1.5%). All the samples were analysed with X-ray diffraction spectroscopy and verified to have no evidence of devitrification except for the sample with Ga,O,/Cs,O = 3 : 7 which is partially devitrified. All samples were ground to fine powders and sealed at the melting site in teflon vials and stored in a dessicator for future use. 2.2. NMR study A Bruker MSL300 high-power NMR spectrometer was used to look at ‘I Ga and 69Ga NMR signals at 91.5 MHz and 72.0 MHz respectively. To ensure that no artifacts from the spectrometer exist, a simple 90” pulse sequence and a solid echo (QUADECHO), with the interval between the irradiating pulse and the observing pulse a little larger than the system ringdown time, were used to observe the NMR signals; no distinguishable difference was found. (To observe only the central transition, a 45” pulse was actually used). The spin-lattice relaxation time T, and spin-spin relaxation time T2 were studied with an inverse-recovery (180~r-90-acquisition) pulse sequence and a 90+90r-echo pulse sequence, respectively. For selected samples, MASS was used to observe the 69Ga signal. The maximum spinning rate used was 4.95 kHz.
3. Results and discussion The static spectra of “Ga for the samples with a variable Gaz03/Cs20 ratio (as indicated) are plotted in fig. 1. The maximum intensities for all of the samples are normalized and the high-frequency side is to the left. 1M Ga(NO,),
J. Zhong,
P.J. Bray
/ Gallium
coordination
in cesium
gallare
GA
I
I
I
I
I
'
glasses
125
c5
I
I
-1000
1000 pOpm
Fig. 1. “Ga
static
spectrum
for samples
with
different
Ga,O,/Cs,O
ratios
at 91.53
MHz.
solution is commonly used by chemists as the “Ga chemical shift reference. But in our experiments, since a GaAs polycrystalline sample gives a very strong and sharp spectrum which facilitates determination of the proper pulse length for solids, all spectra were scaled to the GaAs spectrum. A later measurement determined that the peak for 1M Ga(NO,), is about 37 ppm to the high-frequency side of the GaAs peak. Two separated peaks can be unambiguously observed at S = O-15 and 6 = - 150 ppm respectively for the glass samples with the Ga,O,/Cs,O ratio around one, and it can be seen clearly that the intensity of the peak at lower frequency increases compared with the peak at higher frequency as the gallium content increases. Chemical shift ranges for tetrahedral and octahedral galliums are given by Brevard and Granger [8] as S (tetrahedral) = - 500- + 600 ppm and 6 (octahedral) = - 200- - 50 ppm in the scale we used. Due to the overlapping of the chemical shift ranges, the coordination of gallium cannot be uniquely determined by measuring the chemical shift of the observed responses. But it has been shown that the chemical shift range of gallium follows the same trends as aluminum[9] and there have been numerous studies of the chemical shift range of aluminum [lo-121 in glasses and solutions [13]. It has become a
126
J. Zhong.
P.J.
Bra.v
/ Gallium
coordinarion
in cesium
gallare
glasses
well established fact that the isotropic “Al chemical shifts in oxide glasses depend mainly on the Al-O coordination, and for Al-O tetrahedra shifts of 55-80 ppm to higher frequency were measured in contrast to those of Al-O octahedra [lo]. Similar conclusions have been drawn for aluminum in solutions in ref. [13]. This would suggest that the high-frequency “Ga peak (on the left in fig. 1) originates from 4-coordinated gallium, while the low-frequency peak is from 6-coordinated gallium. Spectra of several compounds with known gallium coordination were also studied in order to aid the identification of gallium coordinations for the two separated peaks in the glass system. Gaz(SO,)j, which has an octahedral gallium environment, was found to have a sharp resonance peak at 6 = - 215 ppm. GaCl, in tap water, with molecular structure M,X, and a tetrahedral gallium environment, has a resonance peak at 6 = 27 ppm. The GaAs polycrystalline sample, used as a reference in this study (6 = 0), also has a
GaAs
I
800
-800
0 Ppm
Fig. 2. “Ga static spectrum spectra for a 1M Ga(NO,),
for the sample with GazO,/Cs,O = 7: 3 at 91.53 MHz. together with solution. polycrystalline GaAs, GaCI, in tap water and a Ga2(S0,), polycrystalline sample.
J. Zhong,
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/ Gallium
coordination
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gallute
glussses
127
tetrahedral gallium environment. Spectra of the above gallium compounds together with the spectrum of the Ga,0,/Cs20 = 7 : 3 glass sample and for 1M Ga(NO), are plotted in fig. 2. According to the literature, cu-Ga,O, has only tetrahedral gallium, while ,&Ga,O, has both tetrahedral and octahedral gallium sites. But as pointed out by Marezio and Remeika [14], under normal conditions for synthesis only fl-Ga,O, is formed. The transformation p + (Ycan be performed by subjecting P-Ga,O, to 44 kbar at 1000°C. We were no able to find a commercial source as supplier of cy-Ga 2O,. A study of polycrystalline /3-Ga 2O, shows that the spectrum is a complicated mixture of features from both quadrupolar and chemical shift interactions. Analysis of this spectrum must await acquisition of spectra at other magnetic fields. In light of the close similarities in physical and chemical properties and microscopic structures between aluminate and gallate glasses, and the chemical shift data presented above, the peak on the high-frequency side (on the left) can be attributed to GaO, tetrahedral units, and the peak on the low-frequency side (on the right) can be assigned to cationic, 6-coordinated gallium units. The appearance of 6-coordinated gallium units at a Ga zO,/CsZO ratio of less than one suggests the beginning of the role of gallium oxide as a glass modifier at about this composition. Separation between the peaks from 4-coordinated and 6-coordinated gallium ions is the same in all the samples with two peaks, and is about 140 ppm, larger than the value for aluminum in most aluminate glasses (about 55-80 ppm as given by refs. [lo-121). The peak assignments according to the gallium coordination can be further confirmed with relaxation time studies, as discussed later in this paper. The “Ga static spectrum is less informative than the “Ga static spectrum. Selected spectra are plotted in fig. 3; a GaAs crystal was again taken as the reference. Although the overall center of the spectrum can be seen to shift toward the low-frequency side, and the resonance line becomes a little asymmetric as the gallium oxide concentration increases, no clearly separated features can be identified. The magic-angle sample spinning technique was therefore utilized in an effort to suppress the dipolar interaction, chemical shift anisotropy, and part of the quadrupolar interaction. The MASS spectrum for the sample with a Ga,O,/Cs,O ratio 7 : 3 is given in fig. 4(a) together with the corresponding static spectrum for comparison. The spinning rate used is 4.90 kHz. The spinning sidebands are sorted out as indicated by the circle and cross marks on the expanded plot of the same MASS spectrum in fig. 4(b). The crosses denote features from 6-coordinated gallium, and the circles identify features from 4-coordinated gallium. The linewidth is indeed much narrower in the mass spectrum, and the features of contributions from two different gallium sites can be seen. Separation between the peaks from 6-coordinated and 4-coordinated galliums is about 27 ppm, or 1.95 kHz, much smaller than the separation between the two peaks in the “Ga spectrum. The existence of many spinning sidebands indicates that there are strong quadrupole or anisotropic chemical shift interactions in the samples. The 69Ga
128
J. Zhong,
P.J. Brq
/ Gallium
coordination
in cesium
gallare
glasses
=A
Fig. 3. 69Ga static
spectrum
for samples
with
different
GazO,/CszO
ratios
/
cs
at 72.05
MHz.
isotope has a larger quadrupole moment and lower resonance frequency as compared to “Ga, and consequently the shift of the center of gravity of the spectrum due to the second-order quadrupole interaction is larger [15]. If we assume that the separation of the two peaks in the “Ga spectrum is totally due to the shift by the second-order quadrupole interaction, then the shift should be about 40 kHz or 440 ppm, which is about 20 times larger than the experimental result for the 69Ga resonance. The discrepancy presumably arises from the effects of the isotropic chemical shift in the glass system under study. The “Ga spectra obtained by use of a spin-echo pulse sequence with T = 10, 100, 200, 300 and 400 ~LSfor the sample with a Gaz0,/Cs20 ratio equal to 7 : 3 is plotted in fig. 5. It can be seen that when 7 is greater than 300 ps, the peak on the high-frequency side totally disappears. To see the effect more clearly, the spectra for 7 equal to 10 ps and 300 ~LSare again plotted in fig. 6 with normalized intensity (the spectrum with 7 = 300 ps has been multiplied by 6.6 to compensate for the decay during the echo). The difference spectrum, calculated by subtracting the normalized spectrum with 7 = 300 ps from the normalized spectrum with 7 = 10 ps, is also shown at the bottom of fig. 6. The magnitude of T, for the peak on the high-frequency side could not be determined accurately from the spin-echo experiment because of the weak
J. Zhong,
P.J. Bray
/ Gallium
coordination
in cesium
gallare
glasses
129
a
-1200
-600
0
600
PPm
b
200
0
-200
-400
PPm Fig. 4. 69Ga static and MASS spectra for the sample Sidebands for the tetrahedral and octahedral galliums ( X ) respectively.
with Ga,O,/Cs,O = 7: 3 at 72.05 MHz. are indicated by a circle (0) and a cross
intensity and the fast decay of the echo. However, the fact that the peak has essentially vanished at 7 = 300 ps suggests that T, for the site is less than 300 ps. The magnitude of T, calculated by fitting the exponentially decaying
J. Zhong.
130
‘w”-
800
P.J.
---
Bray
-~
400
/ Gallium
coordination
in cesium
-”
-
0
-400
gallare
.-
”
glasses
“-----”
-800
PPm Fig.
5. “Ga
static spectrum for the sample with Ga,03/Cs20 experiment. 7 is the time between the irradiating
= 7: 3 obtained in a spin-echo pulse and the echo.
intensity [15] for the peak on the low frequency side is about 400 ps. A T, study for the same sample shows the values of 2-, for the peaks on the high-frequency side and low-frequency side are 8 ms and 6 ms respectively. The same measurements were made on the sample for which the Ga 20,/Cs20 ratio is 3 : 7. Only one peak was observed, at the position of the peak on the high-frequency side, as in fig. 1. T, and T, values are close to the values of the sample with the Ga,O,/Cs,O = 7 : 3 (Tl equals 6 ms and T, is less than 300 PI. As discussed in detail by Fukushima and Roeden [16], accurate T, and T, measurements of quadrupolar nuclei are tricky at best since the Zeeman levels will no longer be equally spaced and it is impossible to define a unique relaxation time, because the nuclear relaxation will be a complex sum of relaxations between the different levels. Bearing this in mind, the actual values of Tl and T,, obtained from the above measurements are not trustworthy or significant in themselves. However, they do provide an additional means for identifying the existence of more than one distinct gallium coordination site
J. Zhong,
2000
P.J. Bray
1000
/ Gallium
coordination
0
in cesium
-1000
gallare
-
glasses
131
2000
Pm Fig. 6. “Ga
static
with different tions.
spectrum for the sample with Ga,0,/Cs20 = 7 : 3. Two sites can be identified by the difference in the two spectra of different 7 values.
relaxation times in samples with high gallium oxide concentra-
4. Conclusion The results of this study suggest that NMR is an effective tool in studying gallium-containing glass systems. Separated features can be identified without magic-angle sample spinning in the case of “Ga, and with the help of MASS in the case of 6gGa. Larger separations between the features from 4-coordinated gallium in the “Ga spectra make “Ga superior to *‘Al for NMR studies. Relaxation time studies can further distinguish signals from galliums in different coordination environments and will be useful in the study of ion hopping and other motional processes in the glass. In the cesium gallate glasses, almost all gallium atoms act as network glass formers when the Ga/Cs ratio is small (less than 3 : 7) i.e., the galliums are in 4-coordination. But when the Ga/Cs ratio increases, the excess gallium goes into the glass system as 6-coordination ions.
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glasses
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