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Occurrence of BaCu2O2 in plasma-sprayed YBaCuO coatings
Volume 9, number
MATERIALS
10
OCCURRENCE
LETTERS
OF BaCu202 IN PLASMA-SPRAYED
June 1990
YBaCuO COATINGS
D. DUBE ‘, B. CHAMPAGNE a, P. LAMBERT a and Y. LE PAGE b a Industrial Materials Research Institute, National Research Councrl of Canada, 75 Boulevard de Mortagne, Boucherville, Quebec, Canada J4B 6 Y4 b Chemistry Division, National Research Councd of Canada, Ottawa, Canada KIA OR!? Received
16 March
1990
BaCu20z (tetragonal, a= 5.722, c= 10.064 A, space group 14,/amd, Z=4) was identified by X-ray powder diffraction in plasmasprayed YBaCuO coatings, together with Y203. YBaZCu,O,_d, Y,BaCuO, and BaCuOz. In atmospheric plasma spraying, YBaCuO particles are melted at very high temperature and undergo a rapid solidification under low oxygen pressure which leads to the formation of a large amount of BaCuzOz. This interpretation also explains the experimental results of previous investigations in which the X-ray lines of BaCu,OZ had been ascribed to “an unknown BaO-CuO phase” or to “a simple cubic phase”.
1. Introduction Phase equilibria and crystal chemistry in the YBaCuO system have been the subject of extensive investigations [ l-3 1. Nevertheless, the prediction of the phases produced in a given experiment is still not feasible in some cases. A factor that is often neglected is that this system is dependent upon the oxygen pressure. The recent discovery of the possible transformations of the so-called “l-2-3” compound in “2-4-7” and “ l-2-4” structural analogues at higher oxygen pressure and temperature is an example of this dependence [ 4-91. Since the equilibrium partial oxygen pressure of many cuprates at a given temperature increases with the formal copper valency [ 10,111, their stability domains are thus sensitive to the atmosphere during processing or synthesis. BaCuO, compounds have been synthesized with various oxygen content and values of x> 2 are only favored under higher oxygen pressure [ 1l- 16 1. Processing materials by solid state reaction or melting involving BaCuO, at higher temperature requires a high oxygen pressure to maintain this oxygen content [ 2,3 1. Moreover, phase equilibria of Ba-Cu-0 are not only related to oxygen pressure but also to specific processing conditions such as the soliditication rate and the density of materials. The present study indicates that normal practice in plasma spray-
ing of YBaCuO leads to a reduction of the formal copper valency during the deposition and to the formation of BaCuzOZ in as-sprayed coatings due to a modification of the equilibrium conditions. Furthermore, an excess of copper as used in this study, is also a promoting agent for increasing the amount of BaCu202.
2. Experimental YBazCu,O,_s powders ( + 1O-74 urn) were plasma sprayed under atmospheric pressure. The primary gas and the carrier gas were nitrogen and oxygen, respectively. An optimum power was selected to ensure complete melting of particles and to minimize the loss of copper by vaporization. The deposit characterization was performed by SEM and by X-ray powder diffraction. The powder diffraction pattern of BaCuzOz, which is not included in the JCPDS data base, was calculated from the structure results of Teske and MiillerBuschbaum [ 17 1, using the program DISPOW in the NRCVAX system of programs [ 18 1. The data shown in table 1, correspond to an isotropic mean-square amplitude of vibration of 0.01, 0.01 and 0.015 A’, respectively, for Ba, Cu and 0. Those typical values compensate for the absence of such parameters in 353
Volume 9, number Table 1 Calculated d-space
10
MATERIALS
LETTERS
June 1990
3. Results and discussion X-ray diffraction
(A)
4.974 3.153 2.8940 2.8610 2.5160 2.4871 2.4800 2.0346 2.0230 1.8988 I .8893 I .8740 1.7027 1.6581 1.5820 1.5766 1.5676 1.5495 1.4470
10 39 85 43 35 100 3 20 22 13 14 36 21
1.4346 1.4305 1.3845 1.3748 1.3027 1.2824 1.2795 1.2580
1.2534 1.2400 1.2301 1.1516 1.1425 1.1405 1.0953
12 6 3 10 5 3 10 5 5 3 6 3
powder pattern
of BaCuzOz
hkl
28’)
101 112 103 200 004 202 211 213 220 105 204 301 312 303 215 224 321 116 206 323 400 305 411 332 413 420 008 217 422 316 208 415 424 512
17.82 28.28 30.87 31.24 35.66 36.08 36.19 44.49 44.76 47.87 48.12 48.54 53.79 55.37 58.27 58.50 58.86 59.62 64.33 64.95 65.15 67.61 68.15 72.50 73.84 74.03 75.51 75.84 76.81 77.54 83.96 84.78 84.97 89.38
(deg)
The physical aspect of a typical as-sprayed coating is shown in fig. 1. The microstructure consists in the superposition of the usual lamellae due to the flattening of the particles caused by their impact with the substrate. Only a few unmelted particles in the form of spheres can be seen. It is worth noting that the lamellae are relatively thin which indicates that their velocity and their melting degree upon impact were high. Fig. 2 shows an X-ray powder diffraction pattern typical of as-sprayed coatings with the abovementioned process parameters. It can be entirely explained by the presence of the phases Y203 (JCPDS #25-1200), BaCuO, [ 191, YBa2Cu307_-d [20], BaCuzOz (table 1) and Y,BaCuO, [ 2 11. Rapid solidification in the YBaCuO has been the subject of numerous investigations, and the interpretation of X-ray diffraction patterns of asquenched materials [22-241 is still incomplete. A similar problem also arises in the characterization of X-ray diffraction patterns of as-deposited coatings by plasma spraying. Indeed, a rapid solidification occurs during deposition by plasma spraying leading to the same difficulties in the interpretation [25-271. For instance, McKittrick et al. [ 22,231 proposed to ascribe the powder lines of BaCuzOz to an “unknown simple cubic phase with a=7.01 A” which was also interpreted in the same way by Neiser et al. [ 27 ] and Sawano et al. [ 26 1.
‘) For Cu Ku radiation.
the structure report. They affect the calculated intensities in a minor way, and correspond to the observed dependence of the intensities on the Bragg angle. Cell parameters based on our experimental dspacings could have been refined. However, with the large intrinsic widths of those lines due to the poor crystallinity of the as-sprayed coatings, the numbers refined would have been less precise than the singlecrystal ones and not significantly different.
Fig. 1. SEM microstructure
354
of as-sprayed
YBaCuO
coating.
MATERIALS LETTERS
Votume 9, number IO
Two-theta
June 1990
(degrees)
Fig. 2. X-ray diffraction pattern of as-sprayed YBaCuO coating.
The present interpretation is almost consistent with the data of Sawano et al. [ 261, McKittrick et al. [22,23 1 and Neiser et al. [ 27 ] who reported 11 lines in their table III. Our table 2 explains the position and relative intensities for 8 of these lines. The line of medium intensity at d= 1.95 A in table III in Neiser et al. [ 271, which was not observed in the present study and other previous studies [ 22-251, should be mainly ascribed to the l-2-3 compounds Table 2
Observed powder patterns of as-sprayed YBaCuO coatings Present study
Neiser et al. 1271
d-space (A)
intensity
d-space (A)
intensity
4.97 3.14 2.87 2.49 2.03
VW m s s
VW
W W
1.89
m
1.70
W
4.95 3.14 2.87 2.49 2.03 1.87 1.70
1.66
W
1.66
1.57
m
W
W
vs s m m
resulting from incomplete melting during plasma spraying, as mentioned by Neiser et al. [ 27 ] and as illustrated in their fig. lf. We consider that the reflection with d=2.21 %,and intensity vw, which is described in a footnote [ 271 as “present but precise position and intensity obscured by overlapping line(s)” should be ascribed to the 2-l-l and l-2-3 compounds. The last line, with d=4.04 A corresponding to a Bragg angle of 22.00” is not visible in their fig. 1 [ 271, for which Bragg angles below 25.00 ’ are not shown, or in fig. 2 in Sawano et al. [ 26 1. We do not observe either this “very weak” line, which we thus consider to be unreliable. The above reinte~retation bears great similarity with the reinterpretation of the JCPDS pattern #I 961 1 as FezMo@a with space group P63mc and cell parameters a= 5.7732( 6), c= 10.0.542( 11) A (ref. [28]), which are qualitatively different from those proposed by Rusakov et al. [ 291, on the base of the powder pattern alone. This stresses the importance of the powder-pattern calculations for the numerous compounds for which the single-crystal structure is known, but for which no reliable observed pattern exists. It also emphasizes the difficulty of extracting
355
Volume 9, number 10
MATERIALS LETTERS
and inte~reting the diffraction pattern of a new phase from a mixture of phases, because of overlap of reflections.
4. Conclusions BaCu202 was identified by powder X-ray diffraction in plasma-sprayed YBaCuO coatings, together with Y203, YBa2Cu30,_8, Y2BaCuOS and BaCu02. The proportion of these constituents is largely dependent on the the oxygen pressure and on the temperature. The occurrence of BaCu20, and its relative proportion increases with processing conditions involving high melting tempe~ture at given oxygen pressure and high solidification rates as those found in plasma spraying. This interpretation also explains the experimental results of previous investigations in which the X-ray lines of BaCu202 had been ascribed to “an unknown BaO-CuO phase” or to “a simple cubic phase”.
References [ 1 ] R.S. Roth, K.L. Davis and J.R. Dennis, Advan. Ceram. Mater. 2 ( 1987) 303. [2] T. Aselage and K. Keefer, J. Mater. Res. 3 (1988) 1279. [3] D.M. de Leeuw, C.A.H.A. Mutsaers, C. Langereis, H.C.A. Smoorenburg and P.J. Rammers, Physica C 152 ( 1988) 39. [4] P. Mars&, R.M. Fleming, M.L. Mandich, A.M. DeSantolo, J. Kwo, M. Hong and L.J. Martinez-Miranda, Nature 334 (1988) 141. [ 5 ] J. Karpinski, E. Kaldis, E. Jiiek, S. Rusiecki and B. Bucher, Nature 336 ( 1988) 660. [6] J. Karpinski, E. Kaldis, S. Rusiecki, E. Jilek, P. Fischer, P. Bordet, C. Chailloux, .I. Chenavas, J.L. Hodeau and M. Marezio, J. Less-Common Met. 150 ( 1989) 129, ]7] D.E. Morris, N.G. Asmar, J.H. Nickel, R.L. Sid, J.Y.T. Wei and J.E. Post, to be published. [ 8 ] C. G6linas and 8. Champagne, Proceedings of the Materials Research Society 1989 Fall Meeting Symposium M: High Temperature Superconductors: Fundamental Properties and Novel Materials Processing, to be published.
June 1990
[9] K.P. Atwal, E.P. Kvam, R. Gronsky and D.E. Morris, Proceedings of the Materials Research Society 1989 Fall Meeting Symposium M: High Temperature Superconductors: Fun~ment~ Properties and Novel Materials Processing, to be published. [lo] D.M. de Leeuw, J. Less-Common Met. 150 ( 1989) 95. [ II] M. Arjomand and D.J. Machin, J. Chem. Sot. Dalton Trans. (1975) 1061. [ 121 WK. Wang-Ng, K.L. Davis and R.S. Roth, J. Am. Ceram. Sot. 71 ( 1988 ) C-64. 131 F. Abbattista, M. Vallino, C. Brisi and M. Lucco-Borlera, Mater. Res. Bull. 23 (1988) 1509. 141 I. Halasz, V. Fulop, 1. Kirschner and T. Porjesz, J. Crystal Growth 91 ( 1988) 444. 151 J.G. Thompson, J.D. Fitz Gerald, R.L. Withers, P.J. Barlow and J.S. Anderson, Mater. Res. Bull. 24 ( 1989) 505. 161 IJ. Straub, D. Krug, Ch. Ziegler, D. Schmeisser and W. Gopel, Mater. Res. Bull. 24 ( 1989) 68 1” [ 171 CL. Teske and H. Miiller-Buschbaum, Z. Naturlorsch. 27b (1972) 296. (181 E.J. Gabe,Y. IePage, J.P. Charland, EL. Leeand P.S. White, J. Appl. Cryst. 22 (1989) 384. [ 191 W. Wong-Ng, H.F. McMurdie, B. Paretzkin, Y. Zhang, K.L. Davis, C.R. Hubbard, A.L. Dragoo and J.M. Stewart, Powder Diff. 2 ( 1987) 257. [ 201 W. Wang-Ng, R.S. Roth, L.J. Swartzendruber, L.H. Bennett, C.K. Chiang, F. Beech and C.R. Hubbard, Advan. Ceram. Mater. 2 ( 1987) 565. (21] C. Michel and B. Raveau, J. Solid State Chem. 43 ( 1982) 73. [ 221 .I. McKittrick, L.Q. Chen, S. Sasayama, M.E. McHenry, G. Kalonji and R.C. G’Handley, Advan. Ceram. Mater. 2 (1987) 353. [23] J. McKittrick, S. Sasayama, M.E. McHenry, G. Kalonji and R.C. G’Handley, J. Appl. Phys. 65 (1989) 3662. [24] J. Sestak, M. Nevriva, E. Pollert, J. Hejtmanek, A. Triska and M. Simeckova, Thermochim. Acta 132( 1988 ) 35. [ 251 G.N. Heintze, R. M~Phe~on, D. Tolino and C. And~kidis, J. Mater. Sci. Letters 7 ( 1988) 25 1. [26] K. Sawano, M. Morita, K. Miyamoto, K. Doi, A. Hayashi, M. Murakami and S. Mats&a, Nippon Seramikkusu Kyokai Gakujutsu Ronbunshi 97 (1989) 1028. [ 27 ] R.A. Neiser, B. Gudmundsson, Y .M. Zhu and H. Herman, Proceedings of the International Thermal Spray Conference, London ( 1989), to be publi~ed. [28 ] Y. le Page and P. Strobel, Acta Cryst. B 38 (1982) 1265. [29] L.N. Rusakov, I.A. Novokhatskii, L.M. Lenev and A.A. Savinskaya, Dokl. Akad. Nauk SSSR 161 ( 1965) 4 10: Dokl. Chem. Tech. 161 (1965) 39.
MATERIALS
10
OCCURRENCE
LETTERS
OF BaCu202 IN PLASMA-SPRAYED
June 1990
YBaCuO COATINGS
D. DUBE ‘, B. CHAMPAGNE a, P. LAMBERT a and Y. LE PAGE b a Industrial Materials Research Institute, National Research Councrl of Canada, 75 Boulevard de Mortagne, Boucherville, Quebec, Canada J4B 6 Y4 b Chemistry Division, National Research Councd of Canada, Ottawa, Canada KIA OR!? Received
16 March
1990
BaCu20z (tetragonal, a= 5.722, c= 10.064 A, space group 14,/amd, Z=4) was identified by X-ray powder diffraction in plasmasprayed YBaCuO coatings, together with Y203. YBaZCu,O,_d, Y,BaCuO, and BaCuOz. In atmospheric plasma spraying, YBaCuO particles are melted at very high temperature and undergo a rapid solidification under low oxygen pressure which leads to the formation of a large amount of BaCuzOz. This interpretation also explains the experimental results of previous investigations in which the X-ray lines of BaCu,OZ had been ascribed to “an unknown BaO-CuO phase” or to “a simple cubic phase”.
1. Introduction Phase equilibria and crystal chemistry in the YBaCuO system have been the subject of extensive investigations [ l-3 1. Nevertheless, the prediction of the phases produced in a given experiment is still not feasible in some cases. A factor that is often neglected is that this system is dependent upon the oxygen pressure. The recent discovery of the possible transformations of the so-called “l-2-3” compound in “2-4-7” and “ l-2-4” structural analogues at higher oxygen pressure and temperature is an example of this dependence [ 4-91. Since the equilibrium partial oxygen pressure of many cuprates at a given temperature increases with the formal copper valency [ 10,111, their stability domains are thus sensitive to the atmosphere during processing or synthesis. BaCuO, compounds have been synthesized with various oxygen content and values of x> 2 are only favored under higher oxygen pressure [ 1l- 16 1. Processing materials by solid state reaction or melting involving BaCuO, at higher temperature requires a high oxygen pressure to maintain this oxygen content [ 2,3 1. Moreover, phase equilibria of Ba-Cu-0 are not only related to oxygen pressure but also to specific processing conditions such as the soliditication rate and the density of materials. The present study indicates that normal practice in plasma spray-
ing of YBaCuO leads to a reduction of the formal copper valency during the deposition and to the formation of BaCuzOZ in as-sprayed coatings due to a modification of the equilibrium conditions. Furthermore, an excess of copper as used in this study, is also a promoting agent for increasing the amount of BaCu202.
2. Experimental YBazCu,O,_s powders ( + 1O-74 urn) were plasma sprayed under atmospheric pressure. The primary gas and the carrier gas were nitrogen and oxygen, respectively. An optimum power was selected to ensure complete melting of particles and to minimize the loss of copper by vaporization. The deposit characterization was performed by SEM and by X-ray powder diffraction. The powder diffraction pattern of BaCuzOz, which is not included in the JCPDS data base, was calculated from the structure results of Teske and MiillerBuschbaum [ 17 1, using the program DISPOW in the NRCVAX system of programs [ 18 1. The data shown in table 1, correspond to an isotropic mean-square amplitude of vibration of 0.01, 0.01 and 0.015 A’, respectively, for Ba, Cu and 0. Those typical values compensate for the absence of such parameters in 353
Volume 9, number Table 1 Calculated d-space
10
MATERIALS
LETTERS
June 1990
3. Results and discussion X-ray diffraction
(A)
4.974 3.153 2.8940 2.8610 2.5160 2.4871 2.4800 2.0346 2.0230 1.8988 I .8893 I .8740 1.7027 1.6581 1.5820 1.5766 1.5676 1.5495 1.4470
10 39 85 43 35 100 3 20 22 13 14 36 21
1.4346 1.4305 1.3845 1.3748 1.3027 1.2824 1.2795 1.2580
1.2534 1.2400 1.2301 1.1516 1.1425 1.1405 1.0953
12 6 3 10 5 3 10 5 5 3 6 3
powder pattern
of BaCuzOz
hkl
28’)
101 112 103 200 004 202 211 213 220 105 204 301 312 303 215 224 321 116 206 323 400 305 411 332 413 420 008 217 422 316 208 415 424 512
17.82 28.28 30.87 31.24 35.66 36.08 36.19 44.49 44.76 47.87 48.12 48.54 53.79 55.37 58.27 58.50 58.86 59.62 64.33 64.95 65.15 67.61 68.15 72.50 73.84 74.03 75.51 75.84 76.81 77.54 83.96 84.78 84.97 89.38
(deg)
The physical aspect of a typical as-sprayed coating is shown in fig. 1. The microstructure consists in the superposition of the usual lamellae due to the flattening of the particles caused by their impact with the substrate. Only a few unmelted particles in the form of spheres can be seen. It is worth noting that the lamellae are relatively thin which indicates that their velocity and their melting degree upon impact were high. Fig. 2 shows an X-ray powder diffraction pattern typical of as-sprayed coatings with the abovementioned process parameters. It can be entirely explained by the presence of the phases Y203 (JCPDS #25-1200), BaCuO, [ 191, YBa2Cu307_-d [20], BaCuzOz (table 1) and Y,BaCuO, [ 2 11. Rapid solidification in the YBaCuO has been the subject of numerous investigations, and the interpretation of X-ray diffraction patterns of asquenched materials [22-241 is still incomplete. A similar problem also arises in the characterization of X-ray diffraction patterns of as-deposited coatings by plasma spraying. Indeed, a rapid solidification occurs during deposition by plasma spraying leading to the same difficulties in the interpretation [25-271. For instance, McKittrick et al. [ 22,231 proposed to ascribe the powder lines of BaCuzOz to an “unknown simple cubic phase with a=7.01 A” which was also interpreted in the same way by Neiser et al. [ 27 ] and Sawano et al. [ 26 1.
‘) For Cu Ku radiation.
the structure report. They affect the calculated intensities in a minor way, and correspond to the observed dependence of the intensities on the Bragg angle. Cell parameters based on our experimental dspacings could have been refined. However, with the large intrinsic widths of those lines due to the poor crystallinity of the as-sprayed coatings, the numbers refined would have been less precise than the singlecrystal ones and not significantly different.
Fig. 1. SEM microstructure
354
of as-sprayed
YBaCuO
coating.
MATERIALS LETTERS
Votume 9, number IO
Two-theta
June 1990
(degrees)
Fig. 2. X-ray diffraction pattern of as-sprayed YBaCuO coating.
The present interpretation is almost consistent with the data of Sawano et al. [ 261, McKittrick et al. [22,23 1 and Neiser et al. [ 27 ] who reported 11 lines in their table III. Our table 2 explains the position and relative intensities for 8 of these lines. The line of medium intensity at d= 1.95 A in table III in Neiser et al. [ 271, which was not observed in the present study and other previous studies [ 22-251, should be mainly ascribed to the l-2-3 compounds Table 2
Observed powder patterns of as-sprayed YBaCuO coatings Present study
Neiser et al. 1271
d-space (A)
intensity
d-space (A)
intensity
4.97 3.14 2.87 2.49 2.03
VW m s s
VW
W W
1.89
m
1.70
W
4.95 3.14 2.87 2.49 2.03 1.87 1.70
1.66
W
1.66
1.57
m
W
W
vs s m m
resulting from incomplete melting during plasma spraying, as mentioned by Neiser et al. [ 27 ] and as illustrated in their fig. lf. We consider that the reflection with d=2.21 %,and intensity vw, which is described in a footnote [ 271 as “present but precise position and intensity obscured by overlapping line(s)” should be ascribed to the 2-l-l and l-2-3 compounds. The last line, with d=4.04 A corresponding to a Bragg angle of 22.00” is not visible in their fig. 1 [ 271, for which Bragg angles below 25.00 ’ are not shown, or in fig. 2 in Sawano et al. [ 26 1. We do not observe either this “very weak” line, which we thus consider to be unreliable. The above reinte~retation bears great similarity with the reinterpretation of the JCPDS pattern #I 961 1 as FezMo@a with space group P63mc and cell parameters a= 5.7732( 6), c= 10.0.542( 11) A (ref. [28]), which are qualitatively different from those proposed by Rusakov et al. [ 291, on the base of the powder pattern alone. This stresses the importance of the powder-pattern calculations for the numerous compounds for which the single-crystal structure is known, but for which no reliable observed pattern exists. It also emphasizes the difficulty of extracting
355
Volume 9, number 10
MATERIALS LETTERS
and inte~reting the diffraction pattern of a new phase from a mixture of phases, because of overlap of reflections.
4. Conclusions BaCu202 was identified by powder X-ray diffraction in plasma-sprayed YBaCuO coatings, together with Y203, YBa2Cu30,_8, Y2BaCuOS and BaCu02. The proportion of these constituents is largely dependent on the the oxygen pressure and on the temperature. The occurrence of BaCu20, and its relative proportion increases with processing conditions involving high melting tempe~ture at given oxygen pressure and high solidification rates as those found in plasma spraying. This interpretation also explains the experimental results of previous investigations in which the X-ray lines of BaCu202 had been ascribed to “an unknown BaO-CuO phase” or to “a simple cubic phase”.
References [ 1 ] R.S. Roth, K.L. Davis and J.R. Dennis, Advan. Ceram. Mater. 2 ( 1987) 303. [2] T. Aselage and K. Keefer, J. Mater. Res. 3 (1988) 1279. [3] D.M. de Leeuw, C.A.H.A. Mutsaers, C. Langereis, H.C.A. Smoorenburg and P.J. Rammers, Physica C 152 ( 1988) 39. [4] P. Mars&, R.M. Fleming, M.L. Mandich, A.M. DeSantolo, J. Kwo, M. Hong and L.J. Martinez-Miranda, Nature 334 (1988) 141. [ 5 ] J. Karpinski, E. Kaldis, E. Jiiek, S. Rusiecki and B. Bucher, Nature 336 ( 1988) 660. [6] J. Karpinski, E. Kaldis, S. Rusiecki, E. Jilek, P. Fischer, P. Bordet, C. Chailloux, .I. Chenavas, J.L. Hodeau and M. Marezio, J. Less-Common Met. 150 ( 1989) 129, ]7] D.E. Morris, N.G. Asmar, J.H. Nickel, R.L. Sid, J.Y.T. Wei and J.E. Post, to be published. [ 8 ] C. G6linas and 8. Champagne, Proceedings of the Materials Research Society 1989 Fall Meeting Symposium M: High Temperature Superconductors: Fundamental Properties and Novel Materials Processing, to be published.
June 1990
[9] K.P. Atwal, E.P. Kvam, R. Gronsky and D.E. Morris, Proceedings of the Materials Research Society 1989 Fall Meeting Symposium M: High Temperature Superconductors: Fun~ment~ Properties and Novel Materials Processing, to be published. [lo] D.M. de Leeuw, J. Less-Common Met. 150 ( 1989) 95. [ II] M. Arjomand and D.J. Machin, J. Chem. Sot. Dalton Trans. (1975) 1061. [ 121 WK. Wang-Ng, K.L. Davis and R.S. Roth, J. Am. Ceram. Sot. 71 ( 1988 ) C-64. 131 F. Abbattista, M. Vallino, C. Brisi and M. Lucco-Borlera, Mater. Res. Bull. 23 (1988) 1509. 141 I. Halasz, V. Fulop, 1. Kirschner and T. Porjesz, J. Crystal Growth 91 ( 1988) 444. 151 J.G. Thompson, J.D. Fitz Gerald, R.L. Withers, P.J. Barlow and J.S. Anderson, Mater. Res. Bull. 24 ( 1989) 505. 161 IJ. Straub, D. Krug, Ch. Ziegler, D. Schmeisser and W. Gopel, Mater. Res. Bull. 24 ( 1989) 68 1” [ 171 CL. Teske and H. Miiller-Buschbaum, Z. Naturlorsch. 27b (1972) 296. (181 E.J. Gabe,Y. IePage, J.P. Charland, EL. Leeand P.S. White, J. Appl. Cryst. 22 (1989) 384. [ 191 W. Wong-Ng, H.F. McMurdie, B. Paretzkin, Y. Zhang, K.L. Davis, C.R. Hubbard, A.L. Dragoo and J.M. Stewart, Powder Diff. 2 ( 1987) 257. [ 201 W. Wang-Ng, R.S. Roth, L.J. Swartzendruber, L.H. Bennett, C.K. Chiang, F. Beech and C.R. Hubbard, Advan. Ceram. Mater. 2 ( 1987) 565. (21] C. Michel and B. Raveau, J. Solid State Chem. 43 ( 1982) 73. [ 221 .I. McKittrick, L.Q. Chen, S. Sasayama, M.E. McHenry, G. Kalonji and R.C. G’Handley, Advan. Ceram. Mater. 2 (1987) 353. [23] J. McKittrick, S. Sasayama, M.E. McHenry, G. Kalonji and R.C. G’Handley, J. Appl. Phys. 65 (1989) 3662. [24] J. Sestak, M. Nevriva, E. Pollert, J. Hejtmanek, A. Triska and M. Simeckova, Thermochim. Acta 132( 1988 ) 35. [ 251 G.N. Heintze, R. M~Phe~on, D. Tolino and C. And~kidis, J. Mater. Sci. Letters 7 ( 1988) 25 1. [26] K. Sawano, M. Morita, K. Miyamoto, K. Doi, A. Hayashi, M. Murakami and S. Mats&a, Nippon Seramikkusu Kyokai Gakujutsu Ronbunshi 97 (1989) 1028. [ 27 ] R.A. Neiser, B. Gudmundsson, Y .M. Zhu and H. Herman, Proceedings of the International Thermal Spray Conference, London ( 1989), to be publi~ed. [28 ] Y. le Page and P. Strobel, Acta Cryst. B 38 (1982) 1265. [29] L.N. Rusakov, I.A. Novokhatskii, L.M. Lenev and A.A. Savinskaya, Dokl. Akad. Nauk SSSR 161 ( 1965) 4 10: Dokl. Chem. Tech. 161 (1965) 39.