Stabilization of superconducting LnPt2B2C by partial substitution of gold for platinum

Physica C 226

( 1994) I TO-I 7-l

Stabilization of superconducting LnPt2B2C by partial substitution of gold for platinum R.J. Cava a,*, B. Batlogg a, J.J. Krajewski a, W.F. Peck Jr. ‘, T. Siegrist ‘I,R.M. Fleming ;I, S. Carter ‘, H. Takagi b, R.J. Felder a, R.B. van Dover ‘. L.W. Rupp Jr. ’ uA7.~6 7 Bell LaboraioricJb, .lurra~~ Iirli. .VJ 0’9’4. 1 ‘,C1 ” Deparrrneni o.fApplwd Phys/c.\. ~‘~~II.P~sI~~~ q/ ToXjw. 7b!gx 113,

.luputr

Received 22 March 1991

Abstract The quaternary intermetallic compounds LnPt,B,C‘ (Ln = La. Pr and \r’ ) WK recently shown to be superconducting at ~t’mperatures up to 10.5 K. Annealing conditions were not found. however. which yielded single-phase material. Here it is reported that the partial substitution of Au for Pt in LnF%,B>C yields essentially single-phase polycrqstalline material under annealing conditions readily accessible bq conventional techniques. This is accomplished through arcmelting LnPt, 5-\u,, hB2C. a composltion slightly superstoichiometric in transition metal. and annealing at 1100 C in Ta foil: a smali amount of melt phase formed in the anneal is wicked away from the sample leaving good-quality material brhind.

1. Introduction

The chemistry and physics of copper oxide-based superconductors has been studied extensively since 1986. .4lthough research on intermetallic superconductors has continued as an undercurrent during that time, there has been little progress in discovering ne\h high-T, intermetallic phases. Recently, however. we reported superconductivity at 23 K in a new type 01 intermetallic system. based on yttrium, palladium. boron and carbon, equivalent to the highest T, e\.cr reported for an intermetallic compound [ I 1. Superconductivity has also been recently reported for YNi-B-C [2] andLnNi,B2C (Ln=Lu (T,=16.6 K), Y, Tm, Er and Ho) [ 31. and the crystal structure of the latter materials was determined [ 41. The layered crystal structures, and the presence of the late transition metals Ni and Pd, generally displaying magnetl

Corresponding author

0921-4534/94/$07.00

ism in intermetaliic compounds. make the occurrence of superconductivit! in these materials of considerable interest. Expanding on those results. ac have found [ 51 that superconductivity also occurs for related intermetallic compounds based on the Sd transition metal Pt when combined with the larger rare earths. For materials of nominal composition LnPtZBIC, for Ln = La, Pr and Y. T,‘s are approximately 10 K. 6 K. and 10 K. respectively. WC could not. however, devise appropriate annealing conditions within an accessible temperature range to obtain good-quality single-phase polycrystalline matcrial, an important step needed for more detailed characterization of the superconducting phase and comparison with the other members of the growing family of boride-carbide superconductors. Based on the suggestion that an increased electron count might increase T, [ 61. WCsynthesized materials of the type LnPtz.. ,M,B$I’. for M = Cu. Ag and Au, and found. for the case of Au, grcatl), improved phase purity un-

0 1994 Elsevier Science R.V. 111rights reserved

.SSD10921-4534(94)00195-L

171

R.J. Cava et al. / Physica C 226 (1994) 170-l 74

der easily accessible annealing conditions. The synthetic method is described here, as are initial structural and physical characterization.

2. Synthesis and structural characterization Starting materials were lanthanide metal shavings or sublimed dendrites (99.99%), Pt and Au foil (99.99%) and coarse C (99.99%) and B (99.6%) powder. Samples of 0.75 g total weight were first pressed into 0.25 inch diameter pellets. They were then arcmelted under Ar on a standard water-cooled copper hearth. They were melted three times, with the button turned over between each melt. The La analog was studied in detail to determine the best synthetic conditions. Two types of nominal stoichiometry were investigated, LaPt, _x+yA~,BZC and B 2C , with 01x10.5 and 01y10.3. LaPtr-,Au,+, The arcmelted buttons were wrapped in Ta foil, sealed in evacuated quartz tubes, and annealed at temperatures between 1050 and I 175 ‘C for several days. Although satisfactory results were obtained for both types of starting stoichiometry for 0.3 Ix< 0.5 and 0.1 I ~50.2, when considering both phase purity and superconducting properties, the best materials were of composition LaPt,.SAu,,sB2C, 5% rich in noblemetal content. Reproducibly good phase-purity polycrystalline material, (with 3% or less impurity phase estimated from powder X-ray diffraction patterns) was obtained by annealing buttons of LaPt L.5A~0.6B2C at 1050’ C for one night and then rewrapping, resealing and annealing at 1100°C for several nights. A small amount of melt phase formed during the anneals and was stuck to the Ta foil; this was discarded. The remaining button was fine-grained polycrystalline material and was not stuck to the foil. The formation of the melt phase is likely to accelerate reaction kinetics allowing the relatively low temperature anneal to be effective to obtain good phase purity. This is also probably why LaPt,,,B,C annealed at 1275’ C gave the best phase purity in our earlier study [ 51; the partially gold-substituted material has considerably better physical properties. Estimation of the Pt :Au ratio by EDAX in a nominally LaPt L.5Au0.6B2C post-annealed button revealed a Pt :Au ratio somewhat larger than that in a binary alloy standard of stoichiometry 0.75Pt:0.25Au, suggesting a phase

composition LaPt,.,Au0.JB2C. The same synthetic method was also found to give good-quality materials for the rare earths Ce, Pr, and Nd; the instability of YPt,B,C [ 5 ] was slightly improved by the partial gold substitution, but single-phase materials were not obtained. The phase purity of the LnPt,.5Au0.6B2C samples prepared as described was checked by conventional powder X-ray diffraction using a high intensity Cu Ku source. Fig. 1. shows the powder X-ray diffraction pattern for nominal LaPt,.SAu,,,eB2C. The majority of the peaks are indexed, as shown, on the body-centered tetragonal all of the type found for LnNi2B2C [4], in this case with cell dimensions uo= 3.87 and co= 10.75 A. The largest non-indexable peak in Fig. 1 is approximately 3% of the intensity of the strongest peak of the min phase. A similar purity is obtained for larger rare earths in LnPt,.5Au0.sB2C for Ln=Ce, Pr, and Nd as well, but for Sm and Y, the other rare-earths studied, the phase is present but the purity is poor, indicating a decreasing stability of the phase with decreasing rare earth size. Table 1 presents the crystallographic cell dimensions, obtained by least squares fits to the positions of 15 powder diffraction lines, of a few large rare-earth variants of

8000

4000 z P

002 I 101

“O

114 ZOO 102;

Jw_+JL& 103

3j E a 0

0 20

.s

30

40

I

E

Z C

50

8000 -

a = 3.87

0.1 Ka

c = 10.75 Tetragonal

60

70

80

A

90

29 (degrees)

Fig. 1. Powder X-ray sample of composition

diffraction pattern, Cu Ku radiation, LaPt,,5Auo.6B2C annealed at 1100°C.

of

Table I Crystallographic cell parameters phases of nominal composition I IOO’C. We have a body-centered

of LnNi2B2C structure tbpc LnPt, Juo 6B,C. annealed at tetragonal cell

Ln

a0 (A)

(‘0 ( .4)

La Ce Pr Nd

3.8729(S) 3.8512(6) 3.8358(4) 3.8287(9)

10.7401( 15) 10.7209(20) 10.7442( 13) 10.7540(‘5)

0.005

-~--

I

* ._~_

r(“l-“-

__---3__..11.~

I

LaPt, 5A~, ,Ei,C nomlnal 1100°C

t

.--

Anneal

--

-+--., I

-0.2 1 4

6

IO

a Temperature

12

(K)

Fig. 2. Superconductmg transItIon for LaP1, sAuo.aBZC. Data are for warming after cooling in the earth’s field (shielding) and for cooling in a field of 10 Oc.

LnPt, sAuo,6B,C. The u-axis dimensions shoB a monotonic decrease with decreasing rare-earth size. but the c-axis is anomalously small for the Cc analog. Unlike the case for CeNi,B,C [ 41. where both u and (‘are anomalous, a conclusions about the valence state of Ce cannot easily be drawn from the unit-cell dimensions.

3. Physical properties Superconducting properties of polycrystalline materials were measured on a commercial SQUID magnetometer in fields of 5-20 Oe. Fig. 2 shows the magnetically measured superconducting transition for essentially single-phase LaPt ,.sAuo 6B2C after annealing at 1 1OO’C. The sample cooled in the earth’s field and warmed in 10 Oe showed a magneticshielding signal approximately equal, when corrected

for demagnetization effects, to 100%of that expected for perfect diamagnetism. Fig. 3 shows the superconducting transition for a variety of La( Pt. Au j2B2C samples. The data are shown for field cooling only. and show flux expulsion equivalent to up to 10% of that expected for perfect diamagnetism, as has been observed for other members of the boride-carbide family. The sharpness of the transition for two of the samples is characteristic of materials with strong flux pinning. Also suggested by the data is that a slight increase in 7; is observed. approximately 0.2 K, for annealing at 1175’C as opposed to 1 lOO’C, although this could also be due to a small difference in the Ptratio. Finally that although to-Au note LaPt,.,Auo5B2C had comparable phase purity to LaPt, 5A~0.6B2C. its superconducting transition is broader and at a slightly lower temperature. Fig. 4 shows the magnetically measured superconducting transition for essentially single-phase P~P~,.,.Au,~6BZC. showing very similar characteristics to the La analog. CePt,.5Au0.6B,C and NdPt &u~.~B~C also yielded essentially single-phase polycrystalline material b> this method. Both were found to be non-superconducting at temperatures above 1.8 K. For the case ot Nd this is not surprising due to the increased rareearth magnetism. For the case of Ce, which would be expected to have a 7, between the La and Pr analogs for Ce’+. the reason for the supression of T, is not clear, unless even small amounts of hybridization of Ce states at E,arc bad for superconductivity. The

173

R.J. Cava et al. / Physica C 226 (I 994) 170-l 74

z

0

.o, ‘2 -0.02

/

300

0.02

prpt, ,s%6Bzc

250 -

nominal 1100°C Anneal

I

I I I

i

I

NdPt, ,Au,,,B,C nominal 1100°C Anneal

5 200

: .” s -0.04 -0 w Z -0.06 E z = -0.08

E $ 150 .E. ,x - 100

50

0

-0.1 L 0

4

2

8

6

_I

0

200

100

300

400

Temperature (K)

Temperature (K)

Fig. 4. Superconducting transition for PrPtI.SAu0.e,B2C.Data are for cooling in a field of 10 Oe.

Fig. 6. Temperature-dependent magnetic susceptibility for NdPtl.5Au0.6BZCbetween 5 and 300 K, measured in an applied field of 10 kOe. 1601,,,,,,,,~,,,,,,~~,~

CePt,.&uO,B,C

,\,,,,,

-

nominal 1100°C Anneal

.60

OL 0

I

I

100

I

I

200

I

300

Temperature (K)

100

150

200

Temperature (K)

Fig. 5. Temperature-dependent magnetic susceptibility for CePt,.5Au0.6B2Cbetween 5 and 300 K measured in an applied field of 10 kOe.

Fig. 7. Temperature-dependent resistivity for essentially singlephase polycrystalline sample of LaPt, 5A~0.6B2C.Inset: detail of the superconducting transition.

magnetic susceptibilities of these materials measured over a wide range of temperatures are shown in Figs. 5 and 6. For the Ce and Nd analogs, high-temperature fits to the Curie-Weiss law yield moments of 2.5 & 0.05,~~ and 3.6-t O.l,&,, respectively. These moments are what is expected for trivalent rare earths. Further study of CePt,.SAu0,6B2C will be needed to clarify the interaction of the rare-earth moment and the conduction electrons which leads to the supression of superconductivity. Finally, the temperature-dependent resistivity of a polycrystalline sample of LaPtl,SAu0.6B,C is pre-

sented in Fig. 7. The resistivity ratio, p300/p4.2, is seen to be relatively low. In addition, the resistivity just above T,, approximately 50 uficm, is considerably larger than is observed for LnNi2B2C [ 31, approximately 5 @cm. The observed resistivity can be either intrinsic, as the Au-Pt solid solution would be expected to introduce considerable disorder scattering, or extrinsic, due to possible poor grain boundaries in the very tine-grained polycrystalline material. Detailed interpretation of p( T) is likely best to await the growth of single crystals of sufftcient size for electrical measurement.

174

R.J. Cavu et al. / Ph.vwa C‘ 226 (1994) I70- I ‘4

4. Conclusions

The quaternary intermetallic boride carbides arc a new family of superconductors which will be the subject of considerable study in the future. For the Ni based analogs. excellent-quality small single crystals and polycrystalline material can be synthesized. For the pure Pt based analogs, however, conditions could not be found in the initial study [ 51 which yielded single-phase polycrystalline materials under commonly available annealing conditions. We have reported here a set of reproducible synthetic conditions for partially substituted Pt-Au based material which yield essentially single-phase polycrystalline samples: gold greatly reduces the annealing temperatures required for synthesis. The superconducting critical temperature did not significantly increase as suggested [6] by band-structure calculations, but the electron count change was only 0.3e/formula, and other mitigating factors, such as the Pt-Au solid solution disorder, could be operating. With good-quality materials, more detailed study of these 5d metal based superconductors and comparison to the 3d

based versions can now be made. .4s of this writtng. the influence of transition metal magnetism on the superconductivity is still not clear: comparison of the details of the behavior of the 3d. Ni based. and 5d. Pt, Au based superconductors in the same structure type should be illuminating.

References [ I ] K.J. C‘ava. H. Takagl. B. Batlogg. H.W. Zandbergen. J.J KraJowski. W.F. Peck Jr.. K.B. \an Dover. R.J. Felder. 1. Siegrist. K. Miruhashl, J.O. Lee. H. Eisakl, SA. Carter and S. L:chida, Nature ( London ) 367 ( 1994 ) 146. [2] R. Nagarajan. C. Mazumdar. Z. Hossain. S.K. Dhar. K.1’ Golpakrishnan, L.C. Gupta. C. Godan. B.D. Padalia and R Vljayaraghavan, Ph)s. Rev. Lett. 72 ( 1994) 174. [ 3] R.J. Cava. H. Takagi, H.W. Zandbergen, J.J. Krajewskl. W.F. Peck Jr.. T. Slegrist. B. Batlogg. R.B. van Dover. R.J. Felder. K. Miruhashi. J.O. Lee. H. Eisakl and S. Uchida. Nature (London) 367 ( 1994) 252. [4] T. Siegrist. H.W. Zandbergen. R.J. Cava J.J. KraJewkl and W.F. Peck Jr.. Nature (London) 367 ( 1994) 254. [ 51R.J. Cava. B. Batlogg. T. Siegrist. J.J. Krajewskl. W.F. Peck Jr.. S. Carter, R.J. Fcldcr. H. Takagi and R.B. van Do\er. Phqs. Re\. B. submltted. [ 61 L.F. Mattheiss. submitted.