Superconducting YBa2Cu3Ox particulate produced by total consumption burner processing

Materials Science and Engineering, A124 (1990) 31-38

31

SuperconductingYna2Cu3OxParticulate Produced by Total Consumption Burner Processing BRIAN D. MERKLE, RICHARD N. KNISELEY, FREDERICK A. SCHMIDT and IVER E. ANDERSON Ames Laboratory, Iowa State University, Ames, IA 50011 (U.S.A.)

(Received September 26, 1988)

Abstract

This paper summarizes the results on the characterization of fine particulates of YBa2Cu 30x superconducting oxide produced by reacting an atomized nitrate solution containing yttrium: barium:copper in the atomic ratio 1:2:3 respectively, in an oxyhydrogen flame. The characterization of the resulting oxide compound includes microstructural analysis by optical microscopy and scanning electron microscopy (SEM), X-ray diffraction and magnetic susceptibility measurements. SEM reveals the primary particle size (about 0.1-0.3 /zm) and morphology of a substrate deposit, produced from solution concentrations ranging from 5 to 33 g 1-1. X-ray diffraction measurements of oxygen-annealed particulate reveal a crystal structure identical with that of the conventionally produced superconducting oxide. Magnetic susceptibility measurements using a superconducting quantum interference device magnetometer demonstrate that the material is superconducting with a Tc of about 93 K.

1. Introduction

Currently, a high level of international research activity is focused on the preparation and characterization of a new class of oxide superconducting materials with a Tc higher than the boiling point of liquid nitrogen, these materials being discovered by Bednorz and Muller [1], Chu et aL [2], and others. At present the majority of superconducting oxide materials are prepared using the conventional "ceramic" process of dry grinding and calcining of Y203, CuO and BaCO 3 powders [3-5]. The calcination involves a solid state reaction between the oxides, with the decomposition of BaCO 3 and the evolution of CO2 being associated with this reaction [6]. This material must be 0921-5093/90/$3.50

oxygen annealed after calcining to produce the fully oxygenated orthorhombic superconducting structure. The conventional process is prone to compositional inhomogeneities and microstructural discontinuities, i.e. porosity, grain growth and intergranular cracking. Other "solution" processes, i.e. coprecipitation, sol-gel synthesis and freeze drying [7-14], are geared toward alleviating these problems and producing more homogeneous structures. About 25 years ago there was considerable interest in the development of combustion heat sources, i.e. flames, for the production of free atoms for the analysis of materials by flame atomic emission and absorption techniques; Most of the flames used at that time were stoichiometric H2-O 2 because of their high temperatures. Unfortunately, free atoms of many elements were difficult to generate, especially those of the alkaline earth and rare earth elements, such as barium and yttrium. These elements form high temperature stable metal monoxides in the flame because their free energy of formation is significantly greater than the OH molecule, which is the principal competitor for oxygen in the flame [15]. In other words, the oxyhydrogen flame was undesirable for atomic emission spectrometry since many metal ions are less stable than the corresponding metal monoxides. Production of the rare earth-Ba-Cu oxide superconductor using an oxyhydrogen flame appeared logical because of the strong tendency for oxidation in the flame. Oxyhydrogen flames could be useful because the stoichiometric flame temperature is reasonably high, about 2600 °C [15], and the yttrium, barium and copper should be transformed to the corresponding oxides in the oxygen-rich flame. Oxidation of yttrium and barium in the oxyhydrogen flame is expected since the dissociation energies of the YO and © Elsevier Sequoia/Printed in The Netherlands

32 TABLE 1 Dissociation energies D 0 of some stable metal monoxides Molecule

YO BaO CuO OH

Dissociation energy

Dissociation energy

(eV)

(kcal)

7.3 5.7 4.1 4.4

168 131 95 101

INNER NOZZLE~ OUTER NOZZLE~

"~-HYDROGEN

BaO are much greater than OH, as shown in Table 1. On the contrary the copper competes less aggressively with hydrogen for the oxygen since their dissociation energies D O are similar (4.1 eV and 4.4 eV respectively). This oxygen competition should be overcome when excess oxygen is supplied to the flame and, in principle, the yttrium, barium, copper and hydrogen should all be oxidized. The excess oxygen could be supplied in a manner independent of the burner input gas by an oxygen sheath ring or by the entrainment of air. Although the reactions occurring in the flame are very complex, it was hoped that a properly mixed metal oxide of YBaeCu30 / would form during the residence time of the material in the hot flame. The ratios of the metal constituents in the mixed metal oxide would be, in principle, determined by the ratios present in the solutions. The flame technique for producing mixed metal oxide superconductors offers several advantages over the other techniques currently in use. The flame technique, since it involves production from solutions, eliminates the tedious grinding and calcining sequence involved in conventional oxide processing and allows the monitoring of the process by spectroscopic techniques. The flame process also may eliminate the need for additional processing chemicals used in other "solution" processes which can remain as impurities in the resulting oxide powder [11, 12]. The overall objective of this investigation is to develop a flame technique to produce homogeneous fine-sized particulate of oxide ceramics, particularly the high Tc superconductor YBa2Cu3Ox.

2. Experimental section

2.1. Burner

The total consumption burner (TCB), shown in Fig. 1, is basically a siphon-type spray nozzle with two concentric annular gas slits surrounding

\~OXYGEN

CAPILLARY

Fig. 1. Schematic representation of the TCB.

a central fluid feed tube [16]. Solution is drawn upward through the tube owing to a Bernoulli effect [17] which is induced by the oxygen gas flowing past the tube orifice. The oxygen flow also atomizes or "nebulizes" the solution upon exit from the tube. The outer channel provides the combustion gas, in this case hydrogen. The nebulized solution droplet enters the flame where it is dehydrated and melted and/or vaporized. An advantage of the TCB is that essentially all the aspirated solution enters the flame. This is not true for spray systems using exterior fluid feed arrangements. After ignition of the oxyhydrogen flame, both inlet gas flow rates were adjusted to suitable levels for particulate production, these being found to be 18 I min-1 for hydrogen gas and 15 I min- Lfor oxygen gas, as measured on precision rotameters. An oxygen sheath ring which fits concentrically around the total consumption burner body was supplied with oxygen gas at 10 1 min- ~. A flow of oxygen as a sheath gas around the flame promotes controlled oxidation by inhibiting air entrainment into the flame. The pertinent processing conditions are summarized in Table 2. 2.2. Substrate A schematic diagram of the experimental setup is shown in Fig. 2. A platinum substrate 7.6 cm x 7.6 cm x 0.0076 cm was mounted perpendicular to the flame on a positioning device which allowed centering of the flame impingement

33 TABLE 2

Typical process conditions

Total consumptionburner Hydrogen flow rate Oxygenflow rate Oxygensheath ring Oxygenflow rate Substrate-to-burner distance Substrate temperature Center 3.0 cm off center Solution

251 rain- ] 151 rain- l 101 min- J 15 cm 650 °C 550 °C YBa2Cu3 as nitrates 5, 10 and 33 g 1- i 2-10 ml min- ]

Concentration Uptake rate

t~emecou~e

I

su~t~ ~

/-'N ° " °~' °

s.s.

-s~m

I

I

I

aty~ln

burnlw

~-~x~

containing total metal ion concentrations ranging from 5 to 33 g 1-1. A peristaltic pump filled the sample beaker shown in Fig. 2, from the main solution supply, at the same rate that the solution was taken up through the capillary. Usually after deposition for 60 min the solution was removed and the hydrogen gas was shut off. The flame was extinguished and the deposit was enveloped in oxygen gas for about 5 rain while cooling to room temperature.

3.

Results

3.1. Solution droplets The nebulized aqueous solution droplet size was measured using a forward light scattering spectrometer probe courtesy of Gas Turbine Products Division, Delavan Corporation, West Des Moines, IA. The average droplet diameter of about 2 5 / z m was determined from the volume average calculated from the scattering data using a curve-fitting routine that assumes a single-mode gaussian distribution. Droplet samples for measurement were generated by nebulizing distilled-deionized water with the total consumption burner at oxygen flow rates ranging from 13.5 to 17.0 1 min -~, solution uptake rates of 2-8 ml min -1 and lift heights of 3.2-10.8 cm. These parameters were chosen to approximate the actual atomization conditions closely. Variation in the parameters over the chosen range did not produce any significant change in droplet size.

solution container copillory

Fig. 2. Schematic diagram of the experimental set-up for producing the mixed metal oxide particulate from solution.

pattern on the substrate. The burner-to-substrate distance was adjusted to 15 cm, for typical operation. Three chromel-alumel thermocouples were spot welded on the reverse side of the substrate, one at the center and two off center about 3 cm in opposite radial directions. The thermocouple outputs were recorded with a multipen chart recorder throughout the experiment.

2.3. Solution Pure nitrate salts of the metals yttrium, barium and copper were used to produce the solutions

3.2. Flame The introduction of a YBa2Cu 3 solution into the oxyhydrogen flame produced a yellowishgreen flame with a reddish crown. The top of the flame (reddish crown) touched the platinum substrate during deposition. The substrate temperature at the start of the deposition was about 650 °C and decreased to about 550 °C at the finish of the deposition. 3.3. Deposit A deposit 2 0 0 - 2 5 0 /zm thick was produced typically in about 1 h from a solution containing a total metal ion concentration of 5 g 1-1. The deposition process resulted in a "bullseye" pattern with a black circular center region about 3 cm in diameter, and a transition from blackbrown to brown expanding outward radially. Optical microscopy and scanning electron

34

Fig. 4. SEM micrograph of an as-deposited particulate from a 5 g 1- t solution using an uptake rate of 10 ml min- ~.

Fig. 5. SEM micrograph of an as-deposited particulate from a 33 g I ]solution using an uptake rate of 10 ml min--J.

Fig. 3. SEM micrographs of an as-deposited particulate on a flat platinum substrate: (a), (b) edge views; (c) a top view from the center of the deposit.

microscopy (SEM) of the center region revealed a deposit morphology consisting of columnar cells protruding toward the center of the deposit which was in the direction of the flow lines of the flame as it impinged upon the substrate. Each cell was made up of aggregates of uniformly sized particulate. Figure 3 depicts these cells and shows the particulate to be about 0.3/~m in size. T h e brown particulate from the outer deposition region was

about 0.1 /~m in diameter and was deposited as an aggregate layer without well-defined cells. Some larger-size (about 3 /~m) particulate was also present in the center region. Initial experiments were conducted to determine the effects of varying the solution concentration (5 and 33 g 1-1 ) at a constant 10 ml m i n - 1 uptake rate on deposit morphology. T h e morphologies of similar areas on these two deposits are shown in the SEM micrographs in Figs. 4 and 5. T h e particulate deposited on a fiat platinum substrate from a 5 g 1-1 solution is of a very fine (0.1/~m) agglomerated type. On the contrary, the micrographs of the 33 g 1-1 solution sample reveal splat-type deposits. 3.4. Particulate structure

T h e X-ray diffraction patterns of the asdeposited black particulate show a close resemblance to the X-ray diffraction pattern of a control sample as shown in Fig. 6. T h e control

35

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Fig. 8. X-ray diffraction pattern of the YBa2Cu~O, control is superimposed above the heat-treated particulate pattern.

40

i i i i i i i i i i i

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Fig. 7. X-ray diffraction pattern of the YBa2eu30 x control is superimposed above the as-deposited particulate pattern which exhibits predominantly BaCO 3 phase.

sample

consisted

of

a

pressed

pellet

of

Y B a 2 C u 3 0 x (the 123 phase) oxide produced by

the conventional process of dry grinding and calcining of the Y203, CuO and BaCO 3 powders [3]. The pelletized material was shown to have a Tc of 93 K. A distinct 123 phase pattern was present in our as-deposited sample except for the lack of double peaks near 2 0 = 47 ° and 58 °. This deviation from the 123 phase pattern can be attributed to the lack of full oxygenation of the 123 phase structure [18, 19]. Semiquantitative X-ray fluorescence measurements have verified that the concentration ratio of yttrium :barium: copper in the black as-deposited and heat-treated particulate follows the 1 : 2 : 3 atomic ratio. If process conditions are not precisely maintained, the black as-deposited particulate may exhibit an X-ray diffraction pattern which contains companion phases including BaCO3, YBa2Cu30 x (the 123 phase), BaCuO 2 (the 011 phase) and Y2BaCuO5 (the 211 phase). The X-ray diffraction patterns of these samples also reveal that the relative volume fraction of these

Fig. 9. SEM micrograph of a heat-treated particulate.

phases can vary from the predominantly oxygendeficient 123 phase to predominantly B a C O 3 phase, as shown in Figs. 6 and 7. In cases where the superconductor was not produced directly, i.e. the deposition resulted in a mixture of 123 and other phases, the asdeposited particulate was heat treated. A heattreated sample consisted of either particulate that was still adhered to the platinum substrate or particulate that was scraped from the substrate onto a platinum or alumina boat. Each particulate sample was heat treated in air or oxygen at 900 °C for about 16 h and furnace cooled at about 1.5 °C rain-1 to room temperature. Subsequent X-ray diffraction patterns showed only 123 phase, i.e. a heat-treated sample pattern is nearly identical with the 123 phase control pattern as shown in Fig. 8. Some heat-treated powder exhibited diffraction patterns of the 123 phase along with the 011 and 211 phases. After heat treatment the particulate assumes a plate-like shape with a considerable increase in particle size as Fig. 9 indicates.

36

3.5. Superconducting behavior The Meissner effect of both the as-deposited and the heat-treated TCB particulate in loose powder form was measured using a superconducting quantum interference device (SQUID) magnetometer. The particulate not containing X-ray evidence of the 123 phase exhibited no Meissner effect. The particulate exhibiting an X-ray diffraction pattern which contained the combination of 123, 011, 211 and B a C O 3 phases displayed a limited Meissner effect, owing to the limited 123 phase, with a superconducting transition temperature at about 93 K. The heat-treated TCB particulate and the heattreated control particulate, which contain only the 123 phase as shown in the X-ray diffraction patterns, both display a strong Meissner effect with a superconducting transition temperature at about 93 K as seen in Fig. 10. A difference between the magnetization of the heat-treated TCB and that of the control particulate may be attributed to the size difference of the superconducting 123 phase grains in the two types of particulate. The effective radius of the superconducting grains was derived from a mathematical model for both the heat-treated TCB particulate and the control paniculate from the magnetization data shown in Fig. 10 [20]. The calculations indicate that the effective radius of the superconducting grains of the heat-treated TCB particulate is approximately half the effective radius of the control particulate. Thus a reduction in the magnetization of the heat-treated TCB paniculate is consistent with a reduction in particle size between the heat-treated TCB and control data [20]. The normalized zero-field-cooled curves in Fig. 11 also display a superconductivity onset

which is less abrupt for the heat-treated TCB powder than for the control powder sample. The transition is known to sharpen with increased grain size and arises from the temperature dependence of the finite penetration depth of the magnetic field into the superconducting material [20]. 4. Discussion

In order to understand the variations in deposit morphology and product structures that have been observed in TCB processing, we must describe the general reaction path of a typical solution droplet as it travels through the flame. A nebulized solution droplet enters the flame w h e r e it is dehydrated, melted and/or vaporized. The resulting liquid and/or vapor are/is collected on a flat substrate or in a suitable collection device. The degree of melting and/or vaporization depend(s) upon many parameters, some of which include the initial dehydrated solid size, the residence time of the material in the flame and the flame temperature. Since the initial dehydrated solid size is a function of the initial nebulized solution droplet size and concentration, future investigations will be done to vary these parameters along with solution uptake rate. We shall monitor the changes in flame chemistry with an optical emission spectrometer and the change in deposit morphology, structure and properties with the characterization techniques described in this paper. As the initial studies indicate, one important parameter in the TCB process appears to be the solution concentration. In other words, the morphology of the as-deposited particulate

10 FC ZgC

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TCB

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ZFC

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ZFC -50 I0

, 30

, 50

, 70

, 90

TEMPERATURE~KI

Fig. 10. The Meissner effect, resulting from the application of a 50 Oe magnetic field, for heat-treated TCB and control samples of loose powder which exhibit a Tc of about 93 K.

-I

-10

•

, 3O

,

, 50

,

, 70

,

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TEMPERATtJ~Egq

Fig. 11. The zero-field-cooled Meissner effect of heattreated TCB and control samples of loose powder normalized at 10 K.

37

changed from agglomerates of discrete particles to splat-type deposits for concentrations of 5 g 1-1 and 33 g 1-1 respectively. The agglomerates of discrete particles appear to have been deposited from a flame plume that contains less liquid than the splat type of deposit. During deposition onto the flat substrate, the temperature decreases from about 650 to about 600 °C at the center and from about 550 to about 500 °C at a distance 2.5 cm off center. As the deposit thickens, the heat transfer to the reverse side of the substrate, where the thermocouple is attached, is reduced, thus lowering the measured temperature. Therefore the substrate temperature during deposition is not a direct but only a relative indicator of the deposition temperature. Since the introduction of the solution reduces the flame temperature relative to the pure oxyhydrogen flame, most of the aforementioned parameters should also affect the flame temperature. The flame is a complex reactive environment and future investigations will be geared toward developing the optimum particulate production parameters and collection methods. Experiments conducted with and without the excess oxygen supplied from the oxygen sheath ring produced as-deposited particulate containing at least a portion of 123 superconducting phase. This seems to indicate that there is enough oxygen present in the entrained air and the flame itself to produce the correct oxygen stoichiometry to form the 123 phase. However, the CO 2 and water vapor present in the air can react with the barium in the particulate after flame processing to form BaCO3 [21, 22]. This reaction would be accelerated owing to the large surface area of the fine particulate. To investigate this environmental degradation effect, an experiment in post-process air exposure time was conducted. A quartz sample tube was used to collect as-deposited particulate from the flat substrate immediately after the flame was extinguished and sealed to exclude air. Another sample was taken a few minutes later and also sealed. Debye-Scherrer X-ray patterns of these sealed samples indicated a relatively small fraction of BaCO3 in the first sample tube and predominantly BaCO 3 in the second. This indicates that BaCO 3 forms after deposition, upon cooling of the deposit. We are at present working on a technique to protect the as-deposited particulate from CO 2 and water vapor during the cooling. The development of multiple product phases

200+L i,i C¢ p-

I0~/ OII+L~

3E

I,-

211+L

925"C 123+011 +001

123+211

211 ÷ 200 ÷ 210

5YOI..54- BaO 3BaCuO +2Cu(

YBa2Cu30 x

Y2BaCuO!

Fig. 12. A section of an isopleth from the ternary phase diagram in the system YOLs-BaO-CuOx, that intersects the YBa2Cu3Ox and Y2BaCuO5 phases, adapted from Aselage and Keefer[6] with temperatures corrected for 1 bar O2. The positions of all lines are approximate and a small region is omitted for simplicity where BaCuO2 and YBa2Cu30, coexist with the ternary eutectic liquid. in the as-deposited particulate can be understood with the help of the isopleth that intersects the 123 and 211 phases as shown in Fig. 12 [3, 6]. In an equilibrium solidification process, a homogeneous liquid alloy would pass through two liquid plus solid phase fields before reaching the single-phase 123 compound. The variations in stoichiometry, i.e. microsegregation, caused by a normal solidification reaction path can produce various combinations and proportions of the equilibrium phases. When the molten flamereacted material solidifies, the first phase to solidify is 200 and then 2 l l . These yttriumcontaining compounds use up the yttrium before the 123 phase forms. This indeed may be the reason that a mixture of phases is seen in the X-ray diffraction results. Heat treatments of less than about 900 °C should allow the solid state transformations to occur and ideally the 123 phase should form from this mixture of phases. More extensive heat-treating studies are being undertaken to investigate this aspect. 5. Conclusion

This paper summarizes results on the characterization of fine particulate of YBa2Cu30 x superconducting oxides produced by reacting a nitrate solution containing yttrium, barium and copper in the atomic ratio 1:2:3 in an oxyhydrogen flame. This new TCB approach can produce a submicron particulate or coatings of the superconducting compound of controlled uniform stoichiometry. X-ray diffraction measurements of oxygen-annealed powders

38

reveal a crystal structure identical with the conventionally produced superconducting compound. Magnetic susceptibility measurements using a SQUID magnetometer demonstrate that the material is superconducting with a Tc of about 93 K. The results illustrate the effects of the flame process parameters on the particulate microstructure and superconducting properties. References 1 J. G. Bednorz and K. A. Muller, Z. Phys. B, 64 (1986) 189. 2 C.W. Chu, P. H. Hor, R. L. Meng, L. Gao, Z. J. Huang, Y. Q. Wang, M. K. V~, J. R. Ashburn and C. Y. Huang, Phys. Rev. Lett., 58 (1987) 911. 3 R.W. McCallum, J. Met., 41 (1989) 50. 4 M. Heuberger, A. Bhargava and R. L. Snyder, Mater. Lett., 5 (1987) 489. 5 D. Rotman, Ind. Chem., 8 (1987) 20. 6 T. Aselage and K. Keefer, J. Mater. Res., 3 (1988) 1279. 7 P. Barboux, J. M. Tarascon, L. H. Green, G. W. Hull and B. G. Bagley, J. Appl. Phys., 63 (1988) 2725. 8 H. H. Wang, K. D. Carlson, U. Geiser, R. J. Thron, H. C. I. Kao, M. A. Beno, M. R. Monagham, T. J. Allen, R. B. Proksch, D. I. Stupka, J. M. Williams, B. K. Flandermeyer and R. B. Poeppel, lnorg. Chem., 26 (1987) 1474. 9 A. M. Kini, U. Geiser, H. C. I. Kao, K. D. Carlson, H. H. Wang, M. R. Monagham and J. M. Williams, Inorg. Chem., 26 (1987) 1834. 10 B. Dunn, C. T. Chu, L. W. Zhow, J. R. Cooper and G. Gruner, Adv. Ceram. Mater., Suppl. 2 (1987) 343. 11 G. Kordas, K. Wu, U. S. Brahme, T. A. Friedman and D. M. Ginsberg, Mater. Lett., 5 (1987) 417.

12 M. A. Accibal, J. W. Draxton, A. H. Gabor, W. L. Gladfelter, B. A. Hassler and M. L. Mecartney, in C. J. Brinker, D. E. Clark and D. R. Ulrick (eds.), Better Ceramics Through Chemistry 111, Materials Research Society Syrup. Proc., Vol. 121, Materials Research Society, Pittsburgh, PA, 1988, p. 401. 13 M. Strasik, T. S. Luhman and N. G. Eror, SAMPE Q., 20 (1988) 11. 14 S. M. Johnson, M. I. Gusman, D. J. Rowcliffe, T. H. Gegalle and J. Z. Sun, Adv. Ceram. Mater., 2 (1987) 337. 15 J. A. Dean and T. C. Rains, Flame Emission and Atomic Absorption Spectrometry, Vol. 1, Marcel Dekker, New York, 1969, p. 189. 16 J. A. Dean and T. C. Rains, Flame Emission andAtomic Absorption Spectrometry, Vol. 2, Marcel Dekker, New York, 1971, p. 57. 17 I. E. Anderson and R. S. Figliola, Mod. Dev. Powder Metall., 20 (1988) 205. 18 P. K. Gallagher, H. M. O'Bryan, S. A. Sunshine and D. W. Murphy, Mater. Res. Bull., 22 (1987) 995. 19 E. M. Engler, R. B. Beyers, V. Y. Lee, A. Nazzal, G. Lim, S. S. P. Parkin, P. M. Grant, J, E. Vazquez, M. L. Ramirez and R. Jacowitz, in Z. Z. Gan, G. J. Cui, G. Z. Yang and Q. S. Yang (eds.), Proc. Beijing Int. Workshop on High Temperature Superconductivity, Belting, June 29-July 1, 1987, World Scientific, Singapore, 1987, p. 23. 20 J. R. Clem and V. G. Kogan, Jpn. J. Appl. Phys., 26 (1987) 1161. 21 T. Youqi, W. Nianzu, G. Linlin, L. Bingxiong, Z. Xiangmao, L. Zhengyi, Y. Daole, W. Ke, Z. Wenbin and W. Qingze, in Z. Z. Gan, G. J. Cui, G. Z. Yang and Q. S. Yang (eds.), Proc. Beifing Int. Workshop on High Temperature Superconductivity, Belting, June 29-July 1, 1987, World Scientific, Singapore, 1987, p. 213. 22 H. Fjellvag, P. Karen, A. Kjekshus, P. Kofstad and T. Norby, Acta Chem. Scand. A, 42 (1988) 178.