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Synthetic studies of nanocrystalline composite oxides with the perovskite structure based on LaFeO3
Materials Letters 15 (1992) 175-179 North-Holland
Synthetic studies of nanocrystalline composite oxides with the perovskite structure based on LaFeO, Xi Li, Hengbin
Zhang,
Qingze
Jiao,
Shujia
Li and Muyu
Zhao
Department of Chemistry, Jilin University, Changchun 130023, China Received 1 April 1992
The precursors for synthesis of nanocrystalline composite oxides La, _$r,Fe, _$o,O, (x=0.2,0.4; y= 0.5,0.6) were prepared using the polyethylene glycol and citrate methods. After the solid-phase reaction at 550°C for 2 h, the mean diameters of the nanocrystalline grains were about 15 nm. The results indicate that the effects of the solid-phase reaction depend on the distribution of various components in the precursor and the conditions of preparing them. The polyethylene glycol method is superior in many respects to the citrate method.
1. Introduction
rials are easily obtained, pure.
Nanocrystalline materials have recently been developed for new types of solid materials [ l-41. Their defect structure and special properties [ 5-7 ] receive increasing attention by materials scientists. However, at present, most methods for synthesizing nanocrystalline materials are physical methods, such as evaporation and condensation [ 11. The synthesized nanocrystalline materials can be either metals, alloys or oxides. This limits the theoretical research and application of nanocrystal materials. Multicomponent oxides of conventional materials with the perovskite structure have been studied extensively in theory and for applications [8-l 01, but nanocrystalline multicomponent solid solutions have been seldom studied. Difficulties in synthesizing these materials have been encountered because the products not only should have crystalline grains with nanometer size, but must also be single phase. The latter requires that solid-phase reactions be thoroughly complete. In general, raising the reaction temperature and prolonging the calcining time can promote the process. However, the above two conditions would just make the crystalline grains grow. Our synthetic methods can solve these problems, and they have many advantages: the methods are simple, economic and easy to carry out, the starting mate0167-577x/92/$
and the products
are rather
2. Experimental Fe(N03)3.9H20, COG, Sr(N03)2, HNG, polyethylene glycol (molecular weight = 20000) and citric acid used for this study were all of analytical grade purity. Water used in the experiments was doubly distilled. The purity of Laz03 exceeded 99.9%. The precursor was calcined in a S/Y type furnace under an air atmosphere. The thermal effects of the calcining process were determined by using a System 7/4 differential thermal analyzer with a temperature rise rate of 1O”C/min under an air atmosphere. Xray diffraction analysis was carried out by use of a XD-3A diffractometer. The morphology and size of the crystalline grains were examined with a JEM2000FX transmission electron microscope. The distributions of the various components in the precursor were analyzed by use of a Hitachi X-650 electron scanning microscope with sodium ion probe.
3. Synthesis La203 was dissolved in the concentrated nitric acid. According to the stoichiometric relation of the reac-
OS.00 0 1992 Elsevier Science Publishers B.V. All rights reserved.
17.5
MATERIALS LETTERS
Volume 15, number 3
It relates to the synthetic method used to prepare the precursor and the conditions used in the synthetic processes. Fig. 1 shows two-dimensional model patterns of the component dist~bution in the precursor of an A(BB’)O+ype compound. Fig. la indicates the uniform distribution of different components in the precursor. In this case, different components only need a short diffusion distance to meet and react with other components, so that a lower temperature and a shorter time can be used to obtain the objective product, Conversely, when the distribution of the different components in the precursor, shown schematically in figs. lb and lc, is less uniform or not uniform, the components must pass through the interface between phases to meet and react with each other. This will require higher temperature and longer diffusion time for the solid-phase reaction to be completed. The crystalline grains will grow, and nanocrystals cannot be obtained. If the calcining temperature is not raised and the reaction time not long, mixed phases or intermediates, such as simple oxides, would be present [ 111. In fact, the more components in the product, the more difficult to obtain uniformity in the precursor. Thus, synthesizing single-phase nanocrystals with many components is difftcult. Fig. 2 shows DTA curves of the precursors obtained by the two synthetic methods, i.e. using polyethylene glycol and citric acid. The DTA curves indicate that there are no obvious thermal effects at above 360°C for x=0.2 and y=O.6. For samples C and D, obtained by use of citric acid, there are small
tants and x, y values of the objective products, given amounts of Fe(NO:,)3*9H,0, CO(NO~)~ and Sr( NO3 )2 were added successively into the above nitric acid solution, The mass of the added water was approximately equal to the mass of salts to form an aqueous solution. Polyethylene glycol ( 1.5 times the mass of water) was dissolved in the above solution while stirring. The system was dehydrated at a temperature within 60-90’ C while stirring. Finally, a gel with dark-red colour was obtained. The gel was further dehydrated under IR light to obtain the dried precursor. Similar to the above method, polyethylene glycol was replaced by citric acid. The added amount of citric acid slightly exceeded the mass of the salts. The system was dehydrated within the temperature range 60-80°C with stirring, Finally, a gel, red-black in colour, was obtained. The gel was further dehydrated under IR light or in the vacuum oven (the latter was controlled at 40°C and vacuum of about 1.33 Pa) to obtain the dried precursor. The precursors were calcined in a furnace at 500550°C for 2 h under an air atmosphere to get the objective products.
4. Results and discussion To complete the solid-phase reaction at the lower temperature and shorter time, i.e. to obtain a single phase of the objective product with a nanometer-size microstructure, the uniformity of the distribution of different components in the precursor is a key factor.
A2°3> B203,%03
b
a 0-A
a-8
November 1992
A2 03,
BaO3,B;Os
c 0-B’
Fig. 1. Two-dimensional model patterns of different distributions of various components in the precursors of an A( BB’)O, -type compound: (a) uniform, (b) less uniform and (c) not uniform.
176
MATERIALS LETTERS
Volume 15, number 3
3 60”~
Exo t Endo
I
0
342-c
I
100
I
200
I
L
I
1
300 400 500 600
,
700
(“cl Fig. 2. DTA curves for La,_~r,Fe,_,Co,0~ precursors. (A) x=0.2, y=O.6, using the polyethylene glycol method. (C) x=0.4, ~~0.5 and (D) x=0.2, y=O.$. usingthe citric acid method.
endothermic effects at 560 and 580°C. This result indicates that the thermal stability in this temperature range is poor, i.e. the reaction is not completed. In the DTA curves, the peak of the thermal effect of oxidative decomposition (reaction) is high, the distance between the initial and final lines of the peak at the data line is narrow. This indicates that the oxidative reaction of the precursor is easy and the time for completion of the solid-phase reaction is short. According to the XRD patterns in fig. 3, it can be seen that after calcining the precursor at 550°C for 2 h, sample B is single phase with the perovskite
November 1992
structure (tallied with JCPDS standard card 1% 148 ) , but in sample A there is a small amount of the 2Laz03*Sr0 phase (JCPDS standard card 22-1430) (20°~2&45”, ;1=1.5418 A). The difference between samples A and B is related mainly to the amount of polyethylene glycol in the preparation of the precursors, which will be discussed in the following. It can be seen from the XRD patterns of samples C and D in fig. 3 that although the products with the perovskite structure were basically obtained, there was a small amount of ZLa,O,.SrO in the samples which precursors were prepared with citric acid. Fig. 4 shows a photograph of the component distribution in the precursor for sample C by the electron probe microanalysis. The linear scanning result indicates that the distribution of the different components was comparatively uniform. Although the use of either polyethylene glycol (PEG) or citric acid yields nanocrystalline composite oxides, the mechanisms for gel formation are different. The citric acid method is often used to synthesize catalysts. The gel is a complex [ 121. Polyethylene glycol is a protecting agent; as soon as the colloidal particles form, they are covered immediately by polyethylene glycol molecular chains to avoid the coagulation due to the adsorption. After forming the gel, the steric effect of polyethylene glycol inhibited the contact among colloidal particles. Fig. 5 shows TEM photographs of nanoc~stalline materials La1 _Sr,Fe, _,Co,03 after calcining the precursors at 550°C for 2 h. The results indicate that the mean diameters of the crystalline grains obtained by use of PEG are smaller than those obtained with citric acid. The former is about 10 nm, the latter about 15 nm. Fig. 6 illustrates a possible role of PEG in the preparation process of the precursor when the added amounts of PEG are different [ 131. When the added amount of PEG is less than optimum, the molecular chains of the polymer play a bridging role, i.e. they adsorb many colloidal particles and inhibit the contact of the colloidal particles, but still provide the chance of contact for colloidal particles on adjacent chains (fig. 6a). When the amount of PEG added is optimum, PEG plays a wrapping and covering role. In this case, the contact of colloidal particles is prevented completely (fig. 6b). This is the reason for the difference in diffraction behavior between sam177
Volume i 5, number 3
MATERIALS LETTERS
I
I
20
30
I
40 (2 6)
November 1992
I
1
50
60
Fig. 3. XRD patterns of nanocrystalline materials La, _&,Fe, _,Co,03; the calcining condition and x, y values are also shown. The precursors of samples A and B were prepared with PEG, but the amount of PEG added for the latter is three times as large as that for the former. The precursors of samples C and D were prepared with citric acid.
Fig. 4. Photographs of electron probe microanalysis for the precursor for sample C (~,~Sr~.~Fe~,~Co~.508) Upper line: Fe, middle line: La, lower line: Sr.
ples A and B in fig. 3. But when more than the optimum PEG is added, the colloidal particles are covered by several layers of the polymer (fig. 6~). Because the adsorbed water in the covering layers is difficult to eliminate, the dried pure precursor powder is not easily obtained. When the molecular weight of PEG is too low, the short molecular chain cannot 178
cover the colloidal particles completely. Thus, PEG of low molecular weight is not suitable as a protecting agent. According to our experiments, the time when the PEG is added is also important. Only before the formation of the colloidal particles, will the addition of PEG be effective. If the larger colloidal particles are already formed, the product after calcination must be large crystalline grains. Although the synthetic procedure using citric acid is ripe for preparing conventional multicomponent composite oxides, the synthetic conditions for preparing nanocrystalline composite oxides must be strictly controlled. According to our experiments, the added amount of water, dehydration temperature and time are important factors affecting the quality of the product. Comparing the two methods, it is found that the synthesis time when PEG is used is shorter than when citric acid is used. In general, the size of crystalline grains obtained with PEG is smaller than when citric acid is used. But using the two methods, one can still obtain the nanoc~st~line composite oxides La, _$r,Fe, _,Co,,03 with the perovskite structure.
Volume 15,number 3
November 1992
a Fig. 5. TEM photographs of nanocrystalline materials LaI_~rxFel_,Co,.O,. (a) x=0.2, y=O.6; PEG used, calcined at 550°C for 2 h, and the sample corresponds to sample B in fig. 3. (b) x=0.4, y=O.5; citric acid used, calcined at 550°C for 2 h. (c) x=0.2, y=O.S; citric acid used, calcined at 550°C for 2 h.
References [ 1] H. Gleiter, J. Appl. Cryst. 24 ( 199 I ) 79.
b
0
-A,B;B’
Fig. 6. Schematic illustration of the proposed role of polyethylene glycol in nanometer-panicle formation. (a) Bridging; (b) covering; (c) multi-layer covering.
Acknowledgement This project was supported by the National Natural Science Foundation and Doctoral programme Fund of Higher Education of China.
[2] R. Birringer, Mater. Sci. Eng. A 117 (1987) 23. [ 31 R.W. Siegel and J.A. Eastman, Mater. Res. Sot. Symp. Proe. 132 (1989) 3. [4] X. Zhu, R. Bit-ringer, U. Herr and H. Gleiter, Phys. Rev. B 35 (1987) 9085. [ 5 ] F.H. Froes and C. Suryanarayana, J. Mater. 4 1 ( 1989) 12. [ 61 X. Li, F. Chi, B. XII and M. Zhao, J. Phys. D, in press. [ 7 ] X. Li, X. Liu, B. Xu and M. Zhao, J. Alloys Comp., in press. [ 81 M. Takano, J. Kawachi, N. Nakanishi and Y. Takeda, J. Solid State Chem. 39 ( 198 1) 75. [9]T.YuandY.Wu,Sci.ChinaB4(1988)351. [lo] J. Mizusaki, T. Swamoto, W.R. Cannon and H.K. Bowen, J. Am. Ceram. Sot. 66 (1983) 247. [ 11) X. Li, L. Zhang and M. Zhao, Acta Sci. Natur. Univ. Jilin 3 (1991) 85. [ 121 X. Li, H. Zhang, S. Li, F. Chi and M. Zhao, Mater. Chem. Phys., submitted for publication. [ 131 X. Li, B. Xu, Z. Wang, F. Chi and M. Zhao, Proceeding of the XIII AIRAPT International Conference on High Pressure Science and Technology, October ( 199 I ), Bangalore, India, J. Mater. Sci. Letters, in press.
179
Synthetic studies of nanocrystalline composite oxides with the perovskite structure based on LaFeO, Xi Li, Hengbin
Zhang,
Qingze
Jiao,
Shujia
Li and Muyu
Zhao
Department of Chemistry, Jilin University, Changchun 130023, China Received 1 April 1992
The precursors for synthesis of nanocrystalline composite oxides La, _$r,Fe, _$o,O, (x=0.2,0.4; y= 0.5,0.6) were prepared using the polyethylene glycol and citrate methods. After the solid-phase reaction at 550°C for 2 h, the mean diameters of the nanocrystalline grains were about 15 nm. The results indicate that the effects of the solid-phase reaction depend on the distribution of various components in the precursor and the conditions of preparing them. The polyethylene glycol method is superior in many respects to the citrate method.
1. Introduction
rials are easily obtained, pure.
Nanocrystalline materials have recently been developed for new types of solid materials [ l-41. Their defect structure and special properties [ 5-7 ] receive increasing attention by materials scientists. However, at present, most methods for synthesizing nanocrystalline materials are physical methods, such as evaporation and condensation [ 11. The synthesized nanocrystalline materials can be either metals, alloys or oxides. This limits the theoretical research and application of nanocrystal materials. Multicomponent oxides of conventional materials with the perovskite structure have been studied extensively in theory and for applications [8-l 01, but nanocrystalline multicomponent solid solutions have been seldom studied. Difficulties in synthesizing these materials have been encountered because the products not only should have crystalline grains with nanometer size, but must also be single phase. The latter requires that solid-phase reactions be thoroughly complete. In general, raising the reaction temperature and prolonging the calcining time can promote the process. However, the above two conditions would just make the crystalline grains grow. Our synthetic methods can solve these problems, and they have many advantages: the methods are simple, economic and easy to carry out, the starting mate0167-577x/92/$
and the products
are rather
2. Experimental Fe(N03)3.9H20, COG, Sr(N03)2, HNG, polyethylene glycol (molecular weight = 20000) and citric acid used for this study were all of analytical grade purity. Water used in the experiments was doubly distilled. The purity of Laz03 exceeded 99.9%. The precursor was calcined in a S/Y type furnace under an air atmosphere. The thermal effects of the calcining process were determined by using a System 7/4 differential thermal analyzer with a temperature rise rate of 1O”C/min under an air atmosphere. Xray diffraction analysis was carried out by use of a XD-3A diffractometer. The morphology and size of the crystalline grains were examined with a JEM2000FX transmission electron microscope. The distributions of the various components in the precursor were analyzed by use of a Hitachi X-650 electron scanning microscope with sodium ion probe.
3. Synthesis La203 was dissolved in the concentrated nitric acid. According to the stoichiometric relation of the reac-
OS.00 0 1992 Elsevier Science Publishers B.V. All rights reserved.
17.5
MATERIALS LETTERS
Volume 15, number 3
It relates to the synthetic method used to prepare the precursor and the conditions used in the synthetic processes. Fig. 1 shows two-dimensional model patterns of the component dist~bution in the precursor of an A(BB’)O+ype compound. Fig. la indicates the uniform distribution of different components in the precursor. In this case, different components only need a short diffusion distance to meet and react with other components, so that a lower temperature and a shorter time can be used to obtain the objective product, Conversely, when the distribution of the different components in the precursor, shown schematically in figs. lb and lc, is less uniform or not uniform, the components must pass through the interface between phases to meet and react with each other. This will require higher temperature and longer diffusion time for the solid-phase reaction to be completed. The crystalline grains will grow, and nanocrystals cannot be obtained. If the calcining temperature is not raised and the reaction time not long, mixed phases or intermediates, such as simple oxides, would be present [ 111. In fact, the more components in the product, the more difficult to obtain uniformity in the precursor. Thus, synthesizing single-phase nanocrystals with many components is difftcult. Fig. 2 shows DTA curves of the precursors obtained by the two synthetic methods, i.e. using polyethylene glycol and citric acid. The DTA curves indicate that there are no obvious thermal effects at above 360°C for x=0.2 and y=O.6. For samples C and D, obtained by use of citric acid, there are small
tants and x, y values of the objective products, given amounts of Fe(NO:,)3*9H,0, CO(NO~)~ and Sr( NO3 )2 were added successively into the above nitric acid solution, The mass of the added water was approximately equal to the mass of salts to form an aqueous solution. Polyethylene glycol ( 1.5 times the mass of water) was dissolved in the above solution while stirring. The system was dehydrated at a temperature within 60-90’ C while stirring. Finally, a gel with dark-red colour was obtained. The gel was further dehydrated under IR light to obtain the dried precursor. Similar to the above method, polyethylene glycol was replaced by citric acid. The added amount of citric acid slightly exceeded the mass of the salts. The system was dehydrated within the temperature range 60-80°C with stirring, Finally, a gel, red-black in colour, was obtained. The gel was further dehydrated under IR light or in the vacuum oven (the latter was controlled at 40°C and vacuum of about 1.33 Pa) to obtain the dried precursor. The precursors were calcined in a furnace at 500550°C for 2 h under an air atmosphere to get the objective products.
4. Results and discussion To complete the solid-phase reaction at the lower temperature and shorter time, i.e. to obtain a single phase of the objective product with a nanometer-size microstructure, the uniformity of the distribution of different components in the precursor is a key factor.
A2°3> B203,%03
b
a 0-A
a-8
November 1992
A2 03,
BaO3,B;Os
c 0-B’
Fig. 1. Two-dimensional model patterns of different distributions of various components in the precursors of an A( BB’)O, -type compound: (a) uniform, (b) less uniform and (c) not uniform.
176
MATERIALS LETTERS
Volume 15, number 3
3 60”~
Exo t Endo
I
0
342-c
I
100
I
200
I
L
I
1
300 400 500 600
,
700
(“cl Fig. 2. DTA curves for La,_~r,Fe,_,Co,0~ precursors. (A) x=0.2, y=O.6, using the polyethylene glycol method. (C) x=0.4, ~~0.5 and (D) x=0.2, y=O.$. usingthe citric acid method.
endothermic effects at 560 and 580°C. This result indicates that the thermal stability in this temperature range is poor, i.e. the reaction is not completed. In the DTA curves, the peak of the thermal effect of oxidative decomposition (reaction) is high, the distance between the initial and final lines of the peak at the data line is narrow. This indicates that the oxidative reaction of the precursor is easy and the time for completion of the solid-phase reaction is short. According to the XRD patterns in fig. 3, it can be seen that after calcining the precursor at 550°C for 2 h, sample B is single phase with the perovskite
November 1992
structure (tallied with JCPDS standard card 1% 148 ) , but in sample A there is a small amount of the 2Laz03*Sr0 phase (JCPDS standard card 22-1430) (20°~2&45”, ;1=1.5418 A). The difference between samples A and B is related mainly to the amount of polyethylene glycol in the preparation of the precursors, which will be discussed in the following. It can be seen from the XRD patterns of samples C and D in fig. 3 that although the products with the perovskite structure were basically obtained, there was a small amount of ZLa,O,.SrO in the samples which precursors were prepared with citric acid. Fig. 4 shows a photograph of the component distribution in the precursor for sample C by the electron probe microanalysis. The linear scanning result indicates that the distribution of the different components was comparatively uniform. Although the use of either polyethylene glycol (PEG) or citric acid yields nanocrystalline composite oxides, the mechanisms for gel formation are different. The citric acid method is often used to synthesize catalysts. The gel is a complex [ 121. Polyethylene glycol is a protecting agent; as soon as the colloidal particles form, they are covered immediately by polyethylene glycol molecular chains to avoid the coagulation due to the adsorption. After forming the gel, the steric effect of polyethylene glycol inhibited the contact among colloidal particles. Fig. 5 shows TEM photographs of nanoc~stalline materials La1 _Sr,Fe, _,Co,03 after calcining the precursors at 550°C for 2 h. The results indicate that the mean diameters of the crystalline grains obtained by use of PEG are smaller than those obtained with citric acid. The former is about 10 nm, the latter about 15 nm. Fig. 6 illustrates a possible role of PEG in the preparation process of the precursor when the added amounts of PEG are different [ 131. When the added amount of PEG is less than optimum, the molecular chains of the polymer play a bridging role, i.e. they adsorb many colloidal particles and inhibit the contact of the colloidal particles, but still provide the chance of contact for colloidal particles on adjacent chains (fig. 6a). When the amount of PEG added is optimum, PEG plays a wrapping and covering role. In this case, the contact of colloidal particles is prevented completely (fig. 6b). This is the reason for the difference in diffraction behavior between sam177
Volume i 5, number 3
MATERIALS LETTERS
I
I
20
30
I
40 (2 6)
November 1992
I
1
50
60
Fig. 3. XRD patterns of nanocrystalline materials La, _&,Fe, _,Co,03; the calcining condition and x, y values are also shown. The precursors of samples A and B were prepared with PEG, but the amount of PEG added for the latter is three times as large as that for the former. The precursors of samples C and D were prepared with citric acid.
Fig. 4. Photographs of electron probe microanalysis for the precursor for sample C (~,~Sr~.~Fe~,~Co~.508) Upper line: Fe, middle line: La, lower line: Sr.
ples A and B in fig. 3. But when more than the optimum PEG is added, the colloidal particles are covered by several layers of the polymer (fig. 6~). Because the adsorbed water in the covering layers is difficult to eliminate, the dried pure precursor powder is not easily obtained. When the molecular weight of PEG is too low, the short molecular chain cannot 178
cover the colloidal particles completely. Thus, PEG of low molecular weight is not suitable as a protecting agent. According to our experiments, the time when the PEG is added is also important. Only before the formation of the colloidal particles, will the addition of PEG be effective. If the larger colloidal particles are already formed, the product after calcination must be large crystalline grains. Although the synthetic procedure using citric acid is ripe for preparing conventional multicomponent composite oxides, the synthetic conditions for preparing nanocrystalline composite oxides must be strictly controlled. According to our experiments, the added amount of water, dehydration temperature and time are important factors affecting the quality of the product. Comparing the two methods, it is found that the synthesis time when PEG is used is shorter than when citric acid is used. In general, the size of crystalline grains obtained with PEG is smaller than when citric acid is used. But using the two methods, one can still obtain the nanoc~st~line composite oxides La, _$r,Fe, _,Co,,03 with the perovskite structure.
Volume 15,number 3
November 1992
a Fig. 5. TEM photographs of nanocrystalline materials LaI_~rxFel_,Co,.O,. (a) x=0.2, y=O.6; PEG used, calcined at 550°C for 2 h, and the sample corresponds to sample B in fig. 3. (b) x=0.4, y=O.5; citric acid used, calcined at 550°C for 2 h. (c) x=0.2, y=O.S; citric acid used, calcined at 550°C for 2 h.
References [ 1] H. Gleiter, J. Appl. Cryst. 24 ( 199 I ) 79.
b
0
-A,B;B’
Fig. 6. Schematic illustration of the proposed role of polyethylene glycol in nanometer-panicle formation. (a) Bridging; (b) covering; (c) multi-layer covering.
Acknowledgement This project was supported by the National Natural Science Foundation and Doctoral programme Fund of Higher Education of China.
[2] R. Birringer, Mater. Sci. Eng. A 117 (1987) 23. [ 31 R.W. Siegel and J.A. Eastman, Mater. Res. Sot. Symp. Proe. 132 (1989) 3. [4] X. Zhu, R. Bit-ringer, U. Herr and H. Gleiter, Phys. Rev. B 35 (1987) 9085. [ 5 ] F.H. Froes and C. Suryanarayana, J. Mater. 4 1 ( 1989) 12. [ 61 X. Li, F. Chi, B. XII and M. Zhao, J. Phys. D, in press. [ 7 ] X. Li, X. Liu, B. Xu and M. Zhao, J. Alloys Comp., in press. [ 81 M. Takano, J. Kawachi, N. Nakanishi and Y. Takeda, J. Solid State Chem. 39 ( 198 1) 75. [9]T.YuandY.Wu,Sci.ChinaB4(1988)351. [lo] J. Mizusaki, T. Swamoto, W.R. Cannon and H.K. Bowen, J. Am. Ceram. Sot. 66 (1983) 247. [ 11) X. Li, L. Zhang and M. Zhao, Acta Sci. Natur. Univ. Jilin 3 (1991) 85. [ 121 X. Li, H. Zhang, S. Li, F. Chi and M. Zhao, Mater. Chem. Phys., submitted for publication. [ 131 X. Li, B. Xu, Z. Wang, F. Chi and M. Zhao, Proceeding of the XIII AIRAPT International Conference on High Pressure Science and Technology, October ( 199 I ), Bangalore, India, J. Mater. Sci. Letters, in press.
179