Superconductivity in (Hg,Tl) (Ba,Sr) CaCuOx

BG Physica C 242 (1995) 221-227

EI~qEVIER

Superconductivity in (Hg,T1) (Ba,Sr)CaCuOx W.L. Lechter a, L.E. Toth a, M.S. Osofsky a,., E.F. Skelton b, A.R. Drews b, C.C. Kim b, B. Das a, S.B. Qadri b, A.W. Webb b, R.J. Soulen Jr. a a Materials Science and Technology Division, Naval Research Laboratory, Washington, DC 20375-5000, USA b Condensed Matter and Radiation Sciences Division, Naval Research Laboratory, Washington, DC 20375-5000 USA

Received 3 August 1994; revised manuscript received 18 November 1994

Abstract

Several members of a new family of (Hg,TI) ( Ba,Sr ) Ca~_2CunOxhigh-temperature superconductors have been synthesized at elevated pressures and temperatures. These compounds, which are analogs to the Hg-Ba-Ca and TI-Ba-Ca layered cuprates, are multi-phased and have superconducting-transition temperatures which exceed 100 K. Incorporation of Hg appears to stabilize several of the TI compounds, including a double-layer T1/Sr system in a manner similar to the role that Pb plays in the TI/ Sr and Bi/Sr systems.

1. Introduction

The highest known superconducting transition temperature (To) at ambient pressure occurs in the system HgBa2Can_ ~CunOy [ 1,2 ]. The next highest set of transition temperatures exists in the TIBaCaCuO layered cuprate family [ 3,4 ]. For the former compounds, as many as four C u - O layers have been made, but are accompanied by only one H g - O layer in a unit cell [5]. For the latter compounds, the maximum number of C u - O layers is four, but both single and double T1-O phases exist and they have higher transition temperatures. Attempts to increase Tc in these compounds by doping has not succeeded: Substituting Sr for Ba significantly reduces Tc in the T1 system [ 6 ] and apparently has a similar effect in the Hg system. Recently there was a report of a Hg/T1 intergrowth forming a unit cell wherein a 1201/Hg was combined with a 2201/T1 [7] with a Tc about half that of either end member. Very recently, Hur * Corresponding author.

et al. [8 ] reported the synthesis of Hgo.sTlo.sBa2 (Cal.72Sr0.2s) Cu3Oy,with a single Hg/ T1 layer and T¢ values of 128-132 K. Bryntse [9] reported a (Hg,T1)2Ba2CaCu2Oy material with a Tc of 100 K, and Sun et al. [ 10 ] reported a Tc as high as 140 K in (Hg, T1)Ba2Ca2Cu3Oy. We initiated a systematic effort to synthesize mixed Hg and T1 cuprate compounds with the hope of producing several new intergrowth structures with high superconductive-transition temperatures. We used a hot isostatic press ( H I P ) [6,1 1 ] to prepare samples since it had proved successful in synthesizing compounds which were difficult to fabricate by other means. We had originally applied the HIP technique as a means to contain toxic materials such as TI compounds during synthesis and subsequently found the process to be useful in the preparation of T1/Sr layered cuprates [ 6,11 ]. In addition to offering the element of safety, this technique allows the operator to select a broader range of reaction temperatures and pressures than are normally accessible in conventional processing. More recently we showed that high-

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Tc Hg compounds could also be fabricated by the HIP [12] process where the externally applied pressure served to contain the high Hg pressures developed during the reaction. We also demonstrated that bulk devices such as shields and test bars could be fabricated directly in a HIP. In addition we used a tetrahedral press to increase the pressure during synthesis. In this report we describe our success in synthesizing compounds with nominal compositions corresponding to the intergrowth formulas Tl~tor2)Hg(Ba(Sr) )4Ca2~Cu2+2nOx. For n = 0 , this formula corresponds to a layer sequence of T 1 0 - T 1 0 (Ba,Sr)O-CuO2-(Ba,Sr)O-HgO-(Ba,Sr)O-CuO2(Ba,Sr)O-T10-T10-(Ba,Sr)O-CuO2 (Ba,Sr)OHgO-(Ba, Sr)O-CuO2(Ba,Sr)O. Martin et al. [7] have synthesized the n = 0 Ba compound and found that it has a 42.2/kX 3.86/~ unit cell.

while the pressure was increased to 210 MPa. After holding the sample under these conditions for about 3 h, the sample was cooled following the procedure described above. After processing and removal from the stainlesssteel container, the gold foil was peeled off the sample. No evidence for a reaction between the stainless steel and gold, or between the gold and the sample was found. We were also successful in synthesizing these compounds at much higher pressures ( ~ 6 GPa) and at higher temperatures in a tetrahedral press, the details of those efforts will be reported elsewhere [ 13 ].

3. Results 3. I. C o m p o s i t i o n a l analyses

2. Experimental details Precursors of the Ba, Sr, Ca, and Cu oxides were prepared from BaCO3, SrCO 3, CaCO3 and CuO by alternate calcination at 900°C in air for 16 h and grinding of the products in a mortar and pestle. After two or three cycles the precursors were then mixed in a mortar and pestle with HgO and T1203. These mixed powders were placed in a pouch of gold foil, which was then sealed inside a thin-walled stainless steel container under vacuum. Boron nitride was used as a release agent between the gold and stainless steel. After leak checking the containers, the enclosed samples were placed in the HIP chamber. The system was purged and then pressurized to about 15 MPa with argon gas, then heated to and held at 300°C for about 30 min. The gas pressure was then increased to about 50 MPa and the temperature was increased to 850 ° C. During heating, the pressure in the chamber increased to about 110 MPa. Using a piston-type compressor, the chamber was then pressurized to 160 MPa. The sample was held under these conditions for about 30 min after which time the power to the furnace was shut off. When the furnace cooled to about 300°C, the remaining gas pressure was released. The complete shut-down cycle lasted about 15 min. In an alternate procedure, which produced denser samples, the chamber was pressurized at room temperature to 15 MPa and the sample was then heated to 900°C

The composition of the samples was determined using SEM with EDS, optical microscopy, X-ray fluorescence and chemical ICP analyses. Due to the volatility of Hg and the fact that many of the Tc values of the (Hg,T1) compounds are comparable to those found for the pure T1 compounds, we carried out a study to determine the extent to which the Hg was actually incorporated into the superconducting material. These efforts included bulk determinations by commercial chemical laboratories, SEM (EDS) of the samples and X-ray fluorescence of the container materials. Optical microscopy of polished sections showed the samples to be very dense and consisting of mostly homogeneous grains in the shape of platelets a few ~m on a side and 1-3 ~m thick. When the reflected light from a polarized source was viewed through a second polarizer oriented perpendicular to the first, most grains displayed a golden-brown color characteristic of the superconducting layered cuprates [ 14,15 ]. Some regions of the samples (10-30% of the volume) were composed of grains that did not display the yellow color, despite having an otherwise similar morphology. Finally, small regions of unreacted material were also visible. Imaging in the SEM clearly revealed the unreacted masses, thereby corroborating the optical observations described above, but no contrast was evident between the gold and non-gold colored grains distin-

IV..L. Lechter et al. / Physica C 242 (1995) 221-227

guished in the optical microscopy. EDS analysis of grains for chemical composition showed clear evidence for the incorporation of Hg. Compositional analysis indicated a TI: Hg ratio of about 4: 1, though the partial overlap of the Hg and T1 L lines and the volatility of Hg make precise measurements difficult. To determine the homogeneity of the Hg and T1, an X-ray dot map was made of a region containing several large grains. No variation of Hg or T1 composition was visible within the grains. Earlier EDS analyses of T1/Hg/Sr samples also showed that the TI and Hg were uniformly distributed [ 14 ]. Grains of Ca or Ba cuprates were also found. Samples were also sent to a commercial laboratory for ICP analyses. These results showed that the Hg and T1 content of the reacted bulk samples was very nearly equal to that of the nominal composition of the starting material [ 16 ], indicating that very little Hg could have escaped from the sample. Furthermore, examination of the gold and stainless-steel containment pouches by XRF showed no evidence of Hg contamination. 3.2. X-ray structural analyses

Standard polycrystalline X-ray diffraction methods were used to identify the crystal structures of the phases present in each of the samples. A portion of each specimen was ground into a fine powder and immersed in a dilute solution of Duco ® cement dissolved in acetone. This was then deposited on a polished (100) face of a Si single crystal with a surface area of approximately 1 cm 2. As the solution evaporated, a random distribution of crystallites adhered to the (100) surface. The Si crystal coated with the powder sample was mounted on an automated powder diffractometer coupled to a rotating-anode X-ray tube; the sample was illuminated with Cu Kct radiation and the scattered radiation was measured with a graphite monochromator. The (100) reciprocal lattice vector of the Si was tilted slightly off axis to avoid inteference between the (h00) reflections of the Si and the sample peaks. The use of single crystal Si as the holder of the powder sample reduced background radiation. Scans were typically made in 0.02 ° steps from 0.5 ° to 100 °. A typical spectrum is shown in Fig. 1 for the HgTIBa4Ca2Cu4Ox sample. The measured spectra were compared to those of

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20(o) Fig. 1. X-raydiffraction spectrumof the HgTlBa4Ca2Cu4Oxsampie. Only prominent peaks have been indexed. the known structures. Interpretation of the data is complicated by the fact that none of our synthesis efforts yielded a single phase product. The X-ray patterns for what are believed to be the dominant phases were analyzed and the a- and c-axis parameters were determined. Most of the patterns could be interpreted as originating from samples containing a mixture of two or more of the known Tl-structures, "2212", "2223", and "2234". All diffraction patterns showed a-axes of about 3.8 ]L which is approximately the same value for all the known Hg and T1 compounds. The c-axis parameters, though, were inconsistent with the nominal chemistry ofintergrowth structures, phase separated end members of T1 and Hg compounds, or T1 compounds alone. The last case was considered as a likely consequence in the event of loss of all the Hg. Intergrowths of Hg and TI structures would result in large c-axis lattice parameters that may be observable in XRD if growth conditions were optimum. In three samples, one produced in the tetrahedral press and two prepared in the HIP, we found a strong peak at about 1.7 ° in 20, corresponding to a d spacing of about 53 A. This result could not be reproduced, however, on other samples prepared by either the tetrahedral press or by the HIP. Satellite peaks around the low-angle peaks of the major phases of samples produced in the HIP were also sometimes observed. Such peaks might indicate some intergrowth behavior. Further structural analysis using TEM is in progress. The results of the analyses of the X-ray diffraction

W.L. Lechter et al. / Physica C 242 (1995) 221-227

224

spectra are summarized in Table 1. For most samples the spectrum was matched with a known structure which was expected from the n o m i n a l composition. HgT1Sr4Ca2Cu40~ is such an example: here a computer indexing program identified the known structure for TISr2CaCu20x (" 1212" ) which was consistent with the nominal composition of the components used to synthesize it. In other cases, the spectra best matched a structure that was inconsistent with the nominal composition. These inconsistent results may be explained by phase separation, intergrowths with disordered stackings of Hg- and Tl-like unit cells, or substitution of Hg into sites within a T1 superconductor structure. For T12HgSr4Ca2CuaOx and T12HgSraCaaCu6Ox a pattern similar to the "2212" structure was observed, but the c-axis was about 3% smaller, i.e., ~ 28.4 A instead of 29.5 /~. This could correspond to a distorted 2212-TI structure ( 1 4 / m m m ), but with Ba lattice parameters. There is, however, no known 2212T1/Sr phase. Tai et al. [ 17 ] were able to synthesize T1PbSr2CaCu20~ (c-axis = 24 A) with a small fraction of 2223 in their sample (c-axis 29/~). It is possible that the addition of Hg stabilizes a new comp o u n d with a 2212 crystal structure and a c-axis of

28.4 A, or that the structure is an intergrowth o f a Hg1212 with a T1-1223 (c-axis ~ 2 8 A).

3.3. Superconducting properties The superconducting-transition temperatures were determined from plots of the resistance versus temperature, R (T). Four-probe AC resistance measurements were performed in which a current with a peakto-peak amplitude of 140 laA and a frequency of 486 Hz was applied to the sample. Wires were bonded to the samples using In solder. R ( T ) curves were measured on cooling and warming to ensure that thermal offsets did not skew the determination of Tc. All of the (Hg/T1) layered cuprates were found to be superconducting by this method. The samples containing Ba always had Tc values above 100 K, which were invariably higher than their Sr analogs. R ( T ) curves for several samples of HgTI(1 or E)Ba4Ca(E or 4)Cut4o, 6)Ox measured prior to any post-processing anneals are plotted in Fig. 2. The HgT1Ba4Ca2Cu4Ox, HgTIEBagCa2Cu4Ox, and HgTl2Ba4Ca4Cu6Ox samples display two resistive transitions: onsets at about 130 K and l l0 K and zero resistance at 95-1 l0 K. These transitions are entered in Table 1 as To, s~t and

Table 1 Compound

Structure(s) a

a (A); c (A)

Toner(K)

TO(K)

TN b

HgTIBa4Ca2Cu40~ HgTlBa4Ca4Cu60~ HgT12Ba4Ca2Cu4Ox

"2223" "2223" "2212" "2223" "2234" "2223" "2234" "2212"

3.85_+0.005; 35.85_+0.05 3.85_+0.02; 36.1 _+0.4 3.85_+0.004; 29.5_+0.03 3.85_+0.01; 35.81 _+0.25 3.86_+0.01; 41.8_+0.3 3.86-+0.003; 35.86_+0.07 3.86_+0.01; 42.3_+0.4 3.86_+0.007; 29.60_+0.07

110 126 125, 113

106 119 1l0

122", IllY, 65+

123-120c, 130 a

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see discussion in text ~ 3.79 _+0.03; 28.3_+0.1 3.82+0.06; 29.6_+1.1 3.83_+.0.05;28.6_+0.5

130-112e 129-90 r 110-100, 50 90, 50 98

853 804 95, 40 40 85

110", 97:, 50+ 90", 50+ 105°, 98:, 60+

110, 100

70

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HgTl2Ba4Ca4Cu6Ox HgT12Ba4Ca4Cu6Ox HgTISr4Ca2Cu4Ox HgTISr4Ca4Cu60~ HgTl2Sr4CazCu4Ox HgTIzSr4Ca4Cu60x

• Structures are listed in descendingorder of relative abundance. b TNvalues are assignedrough fractions of the susceptibilitychangeas noted: (*) smallest; (:) intermediate ( < 10%); ( + ) largest. c Four samplesthat were hot isostaticallypressed without subsequent heat treatments. d The same samplesafter a low-temperatureanneal ( ~ 200°C). e Prepared by reaction inside a sealed, heavy-walledquartz tube and reacted at 850°C. f The quartz tube samplesafter low-temperatureanneals ( ~ 200 °C). s X-ray data unavailable for this sample.

W.L. Lechter et al. / Physica C 242 (1995) 221-227 1.2 1

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Fig. 3. Resistance (normalized to the room-temperature resistance) vs. temperaturefor a sample of HgTI2Ba4Ca4Cu60~prior to and after two different post-processingreduction anneals. To ( c o l u m n s 4 and 5, respectively). The HgT1Ba4Ca4Cu6Ox sample had an onset at 126 K and R = 0 at 119 K. A subsequent anneal o f that sample at 5 0 0 ° C for 100 h in a v a c u u m improved the properties slightly (see Fig. 3). Annealing another piece o f the original sample at 2 5 0 ° C for 20 h in an Ar atmosphere improved the transition considerably. Fig. 4 shows the best results to date for this system, including samples made in the tet press [ 13 ]. The best sample had an onset at 132 K and R = 0 at 127 K. Fig. 5 shows R ( T ) measurements on HgTl(]/2)Sr4CatE/4)Cut4/6)Ox samples prior to any anneals. These samples have transition onsets at 50 and 100 K. The extrapolation o f the R ( T ) curves to T = O shows a rather large residual resistance either because o f the grain boundaries or because the material

HgTISr~Ca~Cu40= I HgTI2Sr,CaaCu,O,

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W.L. Lechter et al. / Physica C 242 (1995) 221-227

226

is underdoped. Fig. 6 shows two samples of HgT1Sr4Ca2Cu4Ox, nominally made under the same processing conditions. Sample 1 is metallic with a T~ of 50 K while the resistance of Sample 2 increases slightly as the temperature decreases, but has a Tc of 100-110 K. As shown in Fig. 6 the transition sharpened and the normal-state resistance became more metallic with subsequent vacuum anneals. The DC magnetic magnetic moment m (T) of the samples was also measured as a function of temperature using a Quantum Design TM MPMS SQUID system. The moments of small pieces of the samples were recorded in a magnetic field of 10 Oe in both field-cooled and zero-field-cooled situations. An example of the behavior seen is shown in Fig. 7 for a sample of HgTl2Ba4CaaCu6Ox (fourth entry in Table 1). As in the R ( T ) measurements, more than one transition was observed. The various transitions observed for the samples are recorded as multiple entries in the last column of Table 1 which is labeled as TN. These temperatures are not identified with a particular phenomenon such as an onset temperature or a temperature where full diamagnetism occurs because these features are not uniquely defined by the measurements. Superscripts are given in Table 1 which do serve to distinguish the magnitudes of the susceptibility change observed at each transition. 0.0005

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field-cooled trace (upper) and zero-field-cooledtrace (lower). Inset: expanded temperature display near the onset of superconductivity.

There is reasonably good agreement between the onset temperature determined from the R(T) curves and the temperature at which diamagnetism first appears. Correlations between any other transitions identified by R (T) or Z(T) are not clearcut.

4. Discussion

We have used a hot isostatic press to synthesize several Hg/Ba and T1/Ba (calcium) cuprates which had Tc values which are intermediate between those for the Hg/Ba and Tl/Ba compounds. This observation applies for both the single and double-layer T1 cuprates. Thus far we do not have conclusive evidence for the formation of the higher-order intergrowth structures of Hg/T1 compounds. EDS, though, clearly shows that the superconducting materials formed contain both T1 and Hg. These materials have superconductive transition temperatures greater than 100 K that are significantly higher than those reported by Martin et al. [7] on a 1201/2201 Hg/T1 intergrowth. While only a limited number of intergrowth compositions were explored, we believe that there is reasonable evidence for the existence of new ( H g / T l ) BaCaCuO compounds with Tc values above 100 K. For Hg/T1/Sr compositions, superconductivity was found in crystal structures corresponding to both single and double T1-O layers. When the predominant structure was 1212, corresponding to a 1:1 ratio of Hg and Tl, the T~ at R = 0 was either 40 K, or nearly 100 K. It is probable that the higher transitions are for a 1223-like structure. Higher T¢ values were also observed in samples with double T1-O layers, 2212, with R - - 0 at 70 to 80 K and onsets greater than 100 K. Without the addition of Hg, double layers of TIP are not found in T1/Sr cuprates. In fact, without the addition of some stabilizing element such as Pb or Hg, Tl/Sr cuprates are difficult to form. T¢ values in the (T1,Hg)Sr compounds are not as high as found in the T1/Pb/Sr cuprates; T l / P b / S r 1223 has an onset of 125 K [ 18 ], whereas the highest transition observed for T1/Hg/Sr was 114 K. These high onset temperatures ( > 100 K) indicate that Hg/T1/Sr cuprates are potentially interesting and useful materials.

W.L. Lechter et al. / Physica C242(1995)221-227

Acknowledgements We thank Ken Killian for his help in all aspects of running the HIP and for sealing many samples. We also acknowledge the financial support of the Naval Research Laboratory, the Office of Naval Research and the Advanced Research Projects Agency. One of us (ARD) acknowledges the support of the National Research Council for through a postdoctoral fellowship. We all thank Donald U. Gubser for his continuing interest in this project.

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[7]C. Martin, M. Huve, G. Van Tendeloo, A. Maignan, C. Michel, M. Hervieu and B. Raveau, Physica C 212 (1993) 274. [8] N.H. Hur, N.H. Kim, K.W. Lee, Y.K. Park and J.C. Park, Mater. Res. Bull. 29 (1994) 959. [ 9] I. Bryntse, Physica C 226 (1994) 184. [10]G.F. Sun, K.W. Wong, B.R. Xu, T. Xin and D.F. Lu, preprint. [11 ] W.L. Lechter, M.S. Osofsky, E.F. Skelton and L.E. Toth, United States Patent No. 5, 120, 704 ( 1992 ). [ 12 ] W. Lechter, L. Toth, M. Osofsky, J. Schwartz, J. Kessler and C. Wolfers, Physica C, submitted. [ 13 ] Complete details of samples prepared in the tetrahedral press will be given in a separate paper. [14]H.A. Hoff, W.L. Lechter and L.E. Toth, J. Scanning Microscopy 13 ( 1991 ) 265. [15]H.A. Hoff, M.S. Osofsky, W.L. Lechter, L.E. Toth, M. Rubinstein, T.A. Vanderah, B.N. Das, L.E. Richards, R.J. Soulen Jr., S.A. Wolf and C.S. Pande, Physica C 162-164 (1989) lllS; H.A. Hoff, M. Rubinstein, M.S. Osofsky, A.K. Singh, L.E. Richards, W.L. Lechter, L.E. Toth, B.N. Das and C.S. Pande, J. Supercond. 2 (1989) 351. [ 16 ] Galbraith Laboratories, Inc., Knoxville, TN. [ 17] M.F. Tai, W.N. Wang and H.C.C. Ku, Jpn. J. Appl. Phys. 27 (1988) L2287. [ 18 ] M.A. Subramanian, C.C. Torardi, J. Gopalakrishnan, P.L. Gai, J.C. Calabrese, T.R. Askew, R.B. Flippen and A.W. Sleight, Science 242 (1988) 249.