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Superconductivity at 22K and 10K in the quaternary borocarbides RPd4BCX(R = Y,Lu)
~ ) Pergamon
Solid State Communications, Vol. 92, No. 4, pp. 341-344, 1994 Elsevier Science Ltd Printed in Great Britain 0038-1098(94)00573-7 0038-1098(94)$7.00+ .00
SUPERCONDUCTIVITY AT 22 K AND 10K IN THE QUATERNARY BOROCARBIDES RPd4BCx (R = Y, Lu) Zakir Hossain@, L.C. Gupta@, Chandan Mazumdar*, R. Nagarajan@, S.K. Dhar@, C. Godart+, $, C. Levy-Clement#,$, B.D. Padalia* and R. Vijayaraghavan@ @Tam Institute of Fundamental Research, Bombay 400 005, India. *Indian Institute of Technology, Bombay 400 076, India. +UPR-209, C.N.R.S., Meudon Cedex, F-92195, France. #Laboratoire de Physique des Solides, C.N.R.S., Meudon Cedex, F-92195, France. (Received by J. Joffrin, March 23, 1994 Revised May 10, June 20, 1994) Recently we reported superconductivity in Y-Ni-B-C system, the first known quaternary superconducting system. One of the superconducting materials reported by us had the nominal chemical composition YNi4BC0. 2. We examine, in this paper, effect of replacing nickel by palladium "which is an obvious and natural step for us since we believe that the delectrons are carriers of superconductivity in these materials. Our samples with the nominal composition YPd4BCx (0.2 < x < 1) have a superconducting minority phase with transition onset temperature as high as 22 K. Some of the samples show another superconducting transition at ~10 K, indicating one more superconducting phase in this system. We have also investigated LuPd4BC0. 2, (Lu being the end member of the rare earth series) which also shows existence of a superconducting phase with T e ~10 K.
1. INTRODUCTION Our discovery of superconductivity in the quaternary system Y-Ni-B-C [1-4] assumes importance due to the fact that such a system allows greater flexibility in terms of vast number of combinations of elements to try and examine for superconductivity. It is pertinent to recall here that with a similar flexibility, high T c - 160 K have been attained in the copper oxide superconductors. With the trends of results described below, there is a great potential for realising intermetallics with higher Tc's. Such materials should prove to be technologically important also. HighT c superconductivity in intermetallics is generally observed in materials that have clusters of atoms, for instance, as in the ternary rare earth rhodium-borides and molybdenum-snlphides. Although structural details of all the possible superconducting phases of the quaternary systems have yet to be worked out, it seems to us that these materials have sheets of the transition metal atoms instead of clusters, as shown for example in the case of LuNi2B2C [5]. All these aspects would give a new direction to the investigation of the phenomenon of superconductivity. This discovery came about as we were in search of suitable materials containing transition metals other than copper, for example nickel, where we could possibly expect supemonductivity. This was because, like many workers in the area of high T c superconductivity, we had been intrigued by the fact that high-Tc superconductivity is $ Presently at Tata Institute of Fundamental Research, Bombay 400 005, India
observed in cuprate systems only. Other transition metal oxides do not exhibit high-Tc phenomenon. For this purpose, we turned our attention to intermetallies. We had already studied several rare earth-nickelsilieides for their physical properties. As borides hold a high promise for the existence of superconductivity, we considered rare earth niekel-borides for our studies and focused on nickel-based ternary borides RENi4B (RE = rare earth atoms) [6]. We found that CeNi4B [7] and SmNi4B [8] exhibit interesting properties: the former is a moderate heavy fermion system, with the linear coefficient of electronic specific heat, = 30 nO/mole K 2 and the latter is a ferromagnet having anomalously high magnetic ordering temperature {Tm of SmNi4B (-- 39 K) is higher than T m of GdNi4B (= 36 K) [8]; according to the de Gennes scaling, T m of SmNi4B should have been less than I0 K}. Thus the members of this series already showed very interesting physical properties. Y-atoms do not carry magnetic moment and in many intermetallics, magnetic moment of nickel atoms is quenched due to the filling of the d-holes. We were motivated to study magnetic and transport properties of YNi4B. We observed a sharp and substantial drop of resistance at ~12 K in this material. Associated with the drop in resistance was also a diamagnetic response indicating the presence of superconductivity with T c -12 K. In certain batches of the material we observed onset of superconductivity at a temperature as high as 15 K [3]. The superconducting fraction, however, was quite low -23%. 'We added 0.2 atomic fraction of carbon to the material. Superconducting fraction of the material got enhanced considerably as a consequence. We also varied 341
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the composition. These results showed that we had a quaternary YoNi-B-C borocarbide superconducting phase with T c = 12 K in our multiphase system [4]. Here we present our results on YPd4BC x (0 < x < 1), LuPd4BC0. 2 and ScPd4BC0. 2. While the work was in progress, Cava et al. reported studies on their multiphase material with starting composition YPd5B3C x [9].
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All the samples (in batches of -1 g.) of the composition YPd4BC x were prepared by melting high purity elements Y (99.9%), Pd (99.9%), B (99.8%) and C (99.7%) in an arc furnace in a protective atmosphere of flowing argon. We also synthesised LuPd4BC0. 2 and ScPd4BC0. 2 in which Lu and Sc metals having 99.9% purity were used. Starting materials of Y, Sc, Lu and B were in the form of pieces, Pd was in the form of wire and carbon was in the form of graphite felt. First a material with nominal composition YPd4B was prepared. After the first melt, weight was checked and any loss was compensated by adding requisite amount of B in the subsequent melting. The material was melted four times, flipping it each time for homogenlsation. To this ingot, carbon was added and melted twice. Similar procedure was adopted for Sc and Lu analogues. One batch of YPd4BC0. 2 (batch-2) (--0.5 g.) was prepared by melting pellet of powders of starting materials (except Y which was taken in very small pieces). We infer from their powder x-ray diffraction (XRD) patterns that all these materials are multiphase systems. 3. RESULTS 3a. YTTRIUM-BASED COMPOUNDS Figures 1-3 (see their insets) shows resistance, measured using four-probe method, of YPd4BC0. 2 (batch 1 and 2) and YPd4BC0. 5 as a function of temperature. These figures show the data over a limited range of temperature (T < 35 K); however, the measurements have been carried out up to 300 K. Resistance of all the sample is only weakly temperature dependent, with the resistance ratio R(300 K)/(R(25 K) ~ 1.2 for Y-based samples and -2.5 for Lu -based sample. This is typical of materials that have considerable disorder (chemical and/or structural). In such cases, scattering of conduction electrons due to disorder dominates over phonon induced scattering. YPd4BC0. 2 (batch-l) exhibits a drop of resistance at 21 K (inset Fig. 1). Within a width of temperature interval 4 K, resistance drops by about 50% of its value above 21 K and then remains nearly constant. We associate this drop of resistance to the onset of a superconducting transition (see below results of our magnetic measurements). It is important to point out here that in our initial studies on YNi-B system [1] also, only a sharp drop in resistance -- and not zero resistance - was observed at the superconducting transition temperature. That the resistance does not go to zero at and below T c represents the fact that there are regions along the conduction path which remain normal down to the lowest temperature of these measurements. Magnetic measurements, using a SQUID magnetometer (Quantum Design, U.S.A.) were carried out
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Fig. 1. Temperature dependence of magnetic susceptibility of YPd4BC0. 2 (batch-l) at a field of 20 (3 both at zero field cooled (ZFC) and field cooled (FC) conditions. The solid lines connecting the data points are guides to the eye. The inset shows DC electrical resistance (R) of the sample as a function of temperature. as a function of temperature in a field of 20 G, both in zero field cooled (ZFC) and field cooled (FC) conditions (Fig. 1). These measurements show diamagnetic response at 22 K, confirming superconductivity. The zero field cooled diamagnetic susceptibility value at 5 K is about 2% of that expected for perfect diamagnetism. Field cooled diamagnetic susceptibility exhibits Meissner effect. The ratio of Meissner signal to the diamagnetic shielding signal at 5 K is about 0.5. In an intermetallic, T c = 22 K (or T c = 22.6K in Ref. 8) is really a very high superconducting transition temperature. It is higher than the highest T c reported in bulk samples of any other intermetallic systems investigated so far. It must be pointed out that we did not detect superconductivity down to 4.2 K in a sample having nominal composition YPd4B. Similarly, we have also examined a sample of nominal composition Pd4BC0. 2 and Pd2B0.5C0.5, for any possible superconductivity down to 4.2 K. Again, no superconducting transition was observed. Examination of powder x-ray diffraction pattern of YPd4BC x (0, 0.2, 0.5 and 1.0) reveals that there are no diffraction lines corresponding to free carbon in materials with x -- 0.2 and 0.5. However, two lines corresponding to carbon are visible in the case of x -- 1.0. These results clearly indicate the quaternary nature of the superconducting phase in our sample of YPd4BC0. 2. YPd4BC0. 2 (batch -2) (prepared by melting a pellet of powder of starting materials) shows a superconducting transition near 10 K (Fig. 2), but no transition around 22 K. The zero field cooled diamagnetic susceptibility value at 5 K is about 0.06% of that expected for perfect diamagnetism. Field cooled diamagnetic susceptibility exhibits Meissner effect. The ratio of Meissner signal to the diamagnetic shielding signal at 5 K is about 0.7. This shows that the superconducting phase in the material is quite sensitive to the conditions of preparation. YPd4BC0. 5 exhibits a superconducting transition near 22 K (onset of resistance drop 22.5 K; onset of
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In YPd4BC we observed the superconducting transition at the higher temperature, ~22 K, only. Strength of the diamagnetic response in this material is comparable to that observed in YPd4BC0. 2 (batch-2). In all our samples reported here, superconducting fraction (as estimated from diamagnetic response) is very small. Thus, the intensity of the XRD lines due to the superconducting phase would be too small to be detected in the XRD pattern. We, therefore, refrain from commenting on the crystallographic aspects of the superconducting phase in our materials. Nevertheless, we note that the XRD pattern of the materials prepared by the two different methods, indicated above, are similar except for difference in the intensity of the lines.
3b. Lu- AND Sc-BASED COMPOUNDS Fig. 2. Temperature dependence of magnetic susceptibility of YPd4BC0. 2 (batch-2) at a field of 20 G both at zero field cooled (ZFC) and field cooled (FC) conditions. The solid lines connecting the data points are guides to the eye. The inset shows DC electrical resistance (R) of the sample as a function of temperature.
diamagnetic transition 21 K) similar to that observed in the sample YPd4BC0. 2 (batch-l) (Fig. 3). Another superconducting transition is observed at 9 K also (Fig. 3). The zero field cooled diamagnetic susceptibility value at 5 K is about 0.1% of that expected for perfect diamagnetism. Field cooled diamagnetic susceptibility exhibits Meissner effect. The ratio of Meissner signal to the diamagnetic shielding signal is about 0.5. A small difference between field cooled and zero field cooled susceptibility is seen in the temperature region 22 K-26 K (Fig. 3). More careful measurements are needed before this is attributed to effects of superconductivity.
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To study the effect of variation of the rare earth element on the superconducting properties in this system, we have examined samples of the compound LuPd4BC0. 2. Inset of Fig. 4 shows temperature dependence of resistance of this material. A drop in resistance at 9 K is clearly visible. Diamagnetic response of magnetic susceptibility confirms the superconducting nature of the transition (Fig. 4). We do not observe 22 K transition in this material. The zero field cooled diamagnetic susceptibility value at 5 K is about 9.2% of that expected for perfect diamagnetism. Field cooled diamagnetic susceptibility exhibits Meissner effect. The ratio of Meissner signal to the diamagnetic shielding signal at 5 K is about 0.5. We have made preliminary studies on Sc analogue also. In this material there is an indication of a superconducting transition near 10 K; efforts are in progress to study it further. As remarked earlier, it may be noted that rare earth free samples do not show superconductivity.
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Fig. 3. Temperature dependence of magnetic susceptibility of YPd4BC0. 5 at a field of 20 G both at zero field cooled (ZFC) and field cooled (FC) conditions. The solid lines connecting the data points are guides to the eye. The inset shows DC electrical resistance (R) of the sample as a function of temperature.
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Fig. 4. Temperature dependence of magnetic susceptibility of LuPd4BC0. 2 at a field of 20 G both at zero field cooled (ZFC) and field cooled (FC) conditions. The solid lines connecting the data points are guides to the eye. The inset shows DC electrical resistance (R) of the sample as a function of temperature.
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SUPERCONDUCTIVITY AT 22K AND 10K 4. DISCUSSION
The fact that the superconducting phase in all these materials is only a minority phase (-1%) does not reduce significance of the results presented here. The reason is that in a quaternary material, the probability of getting the right phase in a substantial measure is rather small if the nominal composition of the material deviates from the one that corresponds to that of the required right phase. As T c (22 K) of our muitiphase sample YPd4BC0. 2 is quite close to that in multiphase YPd5B3C0. 3 [9], it is possible that the phase responsible for superconductivity may be the same in both the sample. Experiments are in progress to examine this feature. Fraction of the fight phase in a quaternary system depends rather critically on the relative proportions of the constituent elements (we varied only the concentration of carbon in this work) and the preparative conditions. We believe that basically this is the reason why discovery of superconductivity in quaternary systems was delayed by so many years. It is also to be mentioned here that this will be the pattern of research in new quaternary superconductors; namely, detection of weak signals of superconductivity to start with and then identifying the fight phase responsible for superconductivity. To conclude, our quaternary material with chemical composition YPd4BC x exhibits superconductivity with Tc
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as high as 22 K. That there are two superconducting transitions in this system, suggests that either there are at least two crystaliographieally distinct phases that are responsible for the two superconducting transitions or the two transitions arise due to a variation of relative proportion of the constituents within the same phase. We consider the second possibility to be rather remote, though we do not rule it out completely, as in that case there would have been a rather broad transition and not two distinct transitions. One of the most urgent tasks towards understanding these materials is to determine the precise chemical compositions and the crystal structures of these phases. Our results show that there are at least two other structures which are likely to show superconductivity, apart from the presently known LuNi2B2C [5,10], which enhances the potential for new superconductors in this family. Our own efforts indicate that these materials crystallise in more than one structure. We had already pointed this out in our earlier publication on these materials [4].
Acknowledgements We thank S.K. Paghdar for • assistance in the experiments and K.V. Gopalakrishnan for assistance in SQUID measurements. Part of this work was performed under Project No. 509-1 of Indo-French Centre for Promotion of Advanced Research, New Delhi (India).
REFERENCES [1] Chandan Mazumdar, R. Nagarajan, C. Godart, L.C. Gupta, M. Latroebe, S.K. Dhar, C. Levy-Clement, B.D. Padalia and R. Vijayaraghavan, Solid State Comm. 87, 413-416 (1993). [2] Chandan Mazumdar, R. Nagarajan, C. Godart, L.C. Gupta, M. Latroehe, S.K. Dhar, C. Levy-Clement, B.D. Padalia and R. Vijayaraghavan, Paper presented at the 20th International Conference on Low Temperature Physics (LT20), Eugene, Oregon, U.S.A, Aug. 4-11, 1993. Results with and without carbon were presented. [3] Chandan Mazumdar, R. Nagarajan, C. Godart, L.C. Gupta, M. Latroche, S.K. Dhar, C. Levy-Clement, B.D. Padalia and R. Vijayaraghavan, Poster No. KE56, presented at the International Conference on Strongly Correlated Electron Systems (SCES 93), San Diego, California, U.S.A, Aug. 16-19, 1993. Results with and without carbon were presented. [4] R. Nagarajan, Chandan Mazumdar, Zakir Hossain, S.K. Dhar, K.V. Gopalakrishnan, L.C. Gupta, C. Godart, B.D. Padalia and R. Vijayaraghavan, Phys. Rev. Lett. 72, 274-277 (1994). [5] T. Siegrist, H.W. Zandbergen, R.J. Cava, J.J. Krajewski and W.F. Peck Jr, Nature 367, 254 (1994).
[6] Chandan Mazumdar, R. Nagarajan, L.C. Gupta, B.D. Padalia and R. Vijayaraghavan, Proc. Solid State Physics Symposium of Dept. Atomic Energy, India, (Jan. 1-4, 1991) Vol. 33C, 265 (1991). [7] Chandan Mazumdar, R. Nagarajan, S.K. Dhar, L.C. Gupta, B.D. Padalia and R. Vijayaraghavan, Proc. Solid State Physics Symposium of Dept. Atomic Energy, India, (Dec. 21-24, 1991) Vol. 34C, 241 (1991). [8] Chandan Mazumdar, R. Nagarajan, L.C. Gupta, B.D. Padalia and R. Vijayaraghavan, 37th Annual Conference on Magnetism and Magnetic Materials, Dec. 1-4 (1992), Houston, Texas, U.S.A., Abstract No. BPI4. [9] R.J. Cava, H. Takagi, B. Batlogg, H.W. Zandbergen, J.J. Krajewski, W.F. Peck Jr, R.B. van Dover, R.J. Felder, T. Siegrist, K. Mizuhashi, J.O. Lee, H. Eisaki, S.A. Carter and S. Uchida, Nature 367, 146 (1994). [10] R.J. Cava, H. Takagi, H.W. Zandbergen, J.J. Krajewski, W.F. Peck Jr, T. Siegrist, B. Batlogg, R.B. van Dover, R.J. Felder, K. Mizuhashi, J.O. Lee, H. Eisaki, and S. Uchida, Nature 367, 252 (1994).
Solid State Communications, Vol. 92, No. 4, pp. 341-344, 1994 Elsevier Science Ltd Printed in Great Britain 0038-1098(94)00573-7 0038-1098(94)$7.00+ .00
SUPERCONDUCTIVITY AT 22 K AND 10K IN THE QUATERNARY BOROCARBIDES RPd4BCx (R = Y, Lu) Zakir Hossain@, L.C. Gupta@, Chandan Mazumdar*, R. Nagarajan@, S.K. Dhar@, C. Godart+, $, C. Levy-Clement#,$, B.D. Padalia* and R. Vijayaraghavan@ @Tam Institute of Fundamental Research, Bombay 400 005, India. *Indian Institute of Technology, Bombay 400 076, India. +UPR-209, C.N.R.S., Meudon Cedex, F-92195, France. #Laboratoire de Physique des Solides, C.N.R.S., Meudon Cedex, F-92195, France. (Received by J. Joffrin, March 23, 1994 Revised May 10, June 20, 1994) Recently we reported superconductivity in Y-Ni-B-C system, the first known quaternary superconducting system. One of the superconducting materials reported by us had the nominal chemical composition YNi4BC0. 2. We examine, in this paper, effect of replacing nickel by palladium "which is an obvious and natural step for us since we believe that the delectrons are carriers of superconductivity in these materials. Our samples with the nominal composition YPd4BCx (0.2 < x < 1) have a superconducting minority phase with transition onset temperature as high as 22 K. Some of the samples show another superconducting transition at ~10 K, indicating one more superconducting phase in this system. We have also investigated LuPd4BC0. 2, (Lu being the end member of the rare earth series) which also shows existence of a superconducting phase with T e ~10 K.
1. INTRODUCTION Our discovery of superconductivity in the quaternary system Y-Ni-B-C [1-4] assumes importance due to the fact that such a system allows greater flexibility in terms of vast number of combinations of elements to try and examine for superconductivity. It is pertinent to recall here that with a similar flexibility, high T c - 160 K have been attained in the copper oxide superconductors. With the trends of results described below, there is a great potential for realising intermetallics with higher Tc's. Such materials should prove to be technologically important also. HighT c superconductivity in intermetallics is generally observed in materials that have clusters of atoms, for instance, as in the ternary rare earth rhodium-borides and molybdenum-snlphides. Although structural details of all the possible superconducting phases of the quaternary systems have yet to be worked out, it seems to us that these materials have sheets of the transition metal atoms instead of clusters, as shown for example in the case of LuNi2B2C [5]. All these aspects would give a new direction to the investigation of the phenomenon of superconductivity. This discovery came about as we were in search of suitable materials containing transition metals other than copper, for example nickel, where we could possibly expect supemonductivity. This was because, like many workers in the area of high T c superconductivity, we had been intrigued by the fact that high-Tc superconductivity is $ Presently at Tata Institute of Fundamental Research, Bombay 400 005, India
observed in cuprate systems only. Other transition metal oxides do not exhibit high-Tc phenomenon. For this purpose, we turned our attention to intermetallies. We had already studied several rare earth-nickelsilieides for their physical properties. As borides hold a high promise for the existence of superconductivity, we considered rare earth niekel-borides for our studies and focused on nickel-based ternary borides RENi4B (RE = rare earth atoms) [6]. We found that CeNi4B [7] and SmNi4B [8] exhibit interesting properties: the former is a moderate heavy fermion system, with the linear coefficient of electronic specific heat, = 30 nO/mole K 2 and the latter is a ferromagnet having anomalously high magnetic ordering temperature {Tm of SmNi4B (-- 39 K) is higher than T m of GdNi4B (= 36 K) [8]; according to the de Gennes scaling, T m of SmNi4B should have been less than I0 K}. Thus the members of this series already showed very interesting physical properties. Y-atoms do not carry magnetic moment and in many intermetallics, magnetic moment of nickel atoms is quenched due to the filling of the d-holes. We were motivated to study magnetic and transport properties of YNi4B. We observed a sharp and substantial drop of resistance at ~12 K in this material. Associated with the drop in resistance was also a diamagnetic response indicating the presence of superconductivity with T c -12 K. In certain batches of the material we observed onset of superconductivity at a temperature as high as 15 K [3]. The superconducting fraction, however, was quite low -23%. 'We added 0.2 atomic fraction of carbon to the material. Superconducting fraction of the material got enhanced considerably as a consequence. We also varied 341
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the composition. These results showed that we had a quaternary YoNi-B-C borocarbide superconducting phase with T c = 12 K in our multiphase system [4]. Here we present our results on YPd4BC x (0 < x < 1), LuPd4BC0. 2 and ScPd4BC0. 2. While the work was in progress, Cava et al. reported studies on their multiphase material with starting composition YPd5B3C x [9].
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All the samples (in batches of -1 g.) of the composition YPd4BC x were prepared by melting high purity elements Y (99.9%), Pd (99.9%), B (99.8%) and C (99.7%) in an arc furnace in a protective atmosphere of flowing argon. We also synthesised LuPd4BC0. 2 and ScPd4BC0. 2 in which Lu and Sc metals having 99.9% purity were used. Starting materials of Y, Sc, Lu and B were in the form of pieces, Pd was in the form of wire and carbon was in the form of graphite felt. First a material with nominal composition YPd4B was prepared. After the first melt, weight was checked and any loss was compensated by adding requisite amount of B in the subsequent melting. The material was melted four times, flipping it each time for homogenlsation. To this ingot, carbon was added and melted twice. Similar procedure was adopted for Sc and Lu analogues. One batch of YPd4BC0. 2 (batch-2) (--0.5 g.) was prepared by melting pellet of powders of starting materials (except Y which was taken in very small pieces). We infer from their powder x-ray diffraction (XRD) patterns that all these materials are multiphase systems. 3. RESULTS 3a. YTTRIUM-BASED COMPOUNDS Figures 1-3 (see their insets) shows resistance, measured using four-probe method, of YPd4BC0. 2 (batch 1 and 2) and YPd4BC0. 5 as a function of temperature. These figures show the data over a limited range of temperature (T < 35 K); however, the measurements have been carried out up to 300 K. Resistance of all the sample is only weakly temperature dependent, with the resistance ratio R(300 K)/(R(25 K) ~ 1.2 for Y-based samples and -2.5 for Lu -based sample. This is typical of materials that have considerable disorder (chemical and/or structural). In such cases, scattering of conduction electrons due to disorder dominates over phonon induced scattering. YPd4BC0. 2 (batch-l) exhibits a drop of resistance at 21 K (inset Fig. 1). Within a width of temperature interval 4 K, resistance drops by about 50% of its value above 21 K and then remains nearly constant. We associate this drop of resistance to the onset of a superconducting transition (see below results of our magnetic measurements). It is important to point out here that in our initial studies on YNi-B system [1] also, only a sharp drop in resistance -- and not zero resistance - was observed at the superconducting transition temperature. That the resistance does not go to zero at and below T c represents the fact that there are regions along the conduction path which remain normal down to the lowest temperature of these measurements. Magnetic measurements, using a SQUID magnetometer (Quantum Design, U.S.A.) were carried out
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Fig. 1. Temperature dependence of magnetic susceptibility of YPd4BC0. 2 (batch-l) at a field of 20 (3 both at zero field cooled (ZFC) and field cooled (FC) conditions. The solid lines connecting the data points are guides to the eye. The inset shows DC electrical resistance (R) of the sample as a function of temperature. as a function of temperature in a field of 20 G, both in zero field cooled (ZFC) and field cooled (FC) conditions (Fig. 1). These measurements show diamagnetic response at 22 K, confirming superconductivity. The zero field cooled diamagnetic susceptibility value at 5 K is about 2% of that expected for perfect diamagnetism. Field cooled diamagnetic susceptibility exhibits Meissner effect. The ratio of Meissner signal to the diamagnetic shielding signal at 5 K is about 0.5. In an intermetallic, T c = 22 K (or T c = 22.6K in Ref. 8) is really a very high superconducting transition temperature. It is higher than the highest T c reported in bulk samples of any other intermetallic systems investigated so far. It must be pointed out that we did not detect superconductivity down to 4.2 K in a sample having nominal composition YPd4B. Similarly, we have also examined a sample of nominal composition Pd4BC0. 2 and Pd2B0.5C0.5, for any possible superconductivity down to 4.2 K. Again, no superconducting transition was observed. Examination of powder x-ray diffraction pattern of YPd4BC x (0, 0.2, 0.5 and 1.0) reveals that there are no diffraction lines corresponding to free carbon in materials with x -- 0.2 and 0.5. However, two lines corresponding to carbon are visible in the case of x -- 1.0. These results clearly indicate the quaternary nature of the superconducting phase in our sample of YPd4BC0. 2. YPd4BC0. 2 (batch -2) (prepared by melting a pellet of powder of starting materials) shows a superconducting transition near 10 K (Fig. 2), but no transition around 22 K. The zero field cooled diamagnetic susceptibility value at 5 K is about 0.06% of that expected for perfect diamagnetism. Field cooled diamagnetic susceptibility exhibits Meissner effect. The ratio of Meissner signal to the diamagnetic shielding signal at 5 K is about 0.7. This shows that the superconducting phase in the material is quite sensitive to the conditions of preparation. YPd4BC0. 5 exhibits a superconducting transition near 22 K (onset of resistance drop 22.5 K; onset of
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35
In YPd4BC we observed the superconducting transition at the higher temperature, ~22 K, only. Strength of the diamagnetic response in this material is comparable to that observed in YPd4BC0. 2 (batch-2). In all our samples reported here, superconducting fraction (as estimated from diamagnetic response) is very small. Thus, the intensity of the XRD lines due to the superconducting phase would be too small to be detected in the XRD pattern. We, therefore, refrain from commenting on the crystallographic aspects of the superconducting phase in our materials. Nevertheless, we note that the XRD pattern of the materials prepared by the two different methods, indicated above, are similar except for difference in the intensity of the lines.
3b. Lu- AND Sc-BASED COMPOUNDS Fig. 2. Temperature dependence of magnetic susceptibility of YPd4BC0. 2 (batch-2) at a field of 20 G both at zero field cooled (ZFC) and field cooled (FC) conditions. The solid lines connecting the data points are guides to the eye. The inset shows DC electrical resistance (R) of the sample as a function of temperature.
diamagnetic transition 21 K) similar to that observed in the sample YPd4BC0. 2 (batch-l) (Fig. 3). Another superconducting transition is observed at 9 K also (Fig. 3). The zero field cooled diamagnetic susceptibility value at 5 K is about 0.1% of that expected for perfect diamagnetism. Field cooled diamagnetic susceptibility exhibits Meissner effect. The ratio of Meissner signal to the diamagnetic shielding signal is about 0.5. A small difference between field cooled and zero field cooled susceptibility is seen in the temperature region 22 K-26 K (Fig. 3). More careful measurements are needed before this is attributed to effects of superconductivity.
~
' ' ' ' 1 ' ' ' ' 1
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To study the effect of variation of the rare earth element on the superconducting properties in this system, we have examined samples of the compound LuPd4BC0. 2. Inset of Fig. 4 shows temperature dependence of resistance of this material. A drop in resistance at 9 K is clearly visible. Diamagnetic response of magnetic susceptibility confirms the superconducting nature of the transition (Fig. 4). We do not observe 22 K transition in this material. The zero field cooled diamagnetic susceptibility value at 5 K is about 9.2% of that expected for perfect diamagnetism. Field cooled diamagnetic susceptibility exhibits Meissner effect. The ratio of Meissner signal to the diamagnetic shielding signal at 5 K is about 0.5. We have made preliminary studies on Sc analogue also. In this material there is an indication of a superconducting transition near 10 K; efforts are in progress to study it further. As remarked earlier, it may be noted that rare earth free samples do not show superconductivity.
' ' ' ' 1 ' ' ' ' 1 ' ' ' ' 1 ' ' ' ' 1 ' ' ' ' 1 ' ' ' ' 1
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v
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.......
o
lo
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5
I0 15 20 Temperature
20
,-..
30
,.
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35
Fig. 3. Temperature dependence of magnetic susceptibility of YPd4BC0. 5 at a field of 20 G both at zero field cooled (ZFC) and field cooled (FC) conditions. The solid lines connecting the data points are guides to the eye. The inset shows DC electrical resistance (R) of the sample as a function of temperature.
0
5
, ....
I0 15 20 Temperature
, .... 25 (K)
,..,.I 30
35
Fig. 4. Temperature dependence of magnetic susceptibility of LuPd4BC0. 2 at a field of 20 G both at zero field cooled (ZFC) and field cooled (FC) conditions. The solid lines connecting the data points are guides to the eye. The inset shows DC electrical resistance (R) of the sample as a function of temperature.
344
SUPERCONDUCTIVITY AT 22K AND 10K 4. DISCUSSION
The fact that the superconducting phase in all these materials is only a minority phase (-1%) does not reduce significance of the results presented here. The reason is that in a quaternary material, the probability of getting the right phase in a substantial measure is rather small if the nominal composition of the material deviates from the one that corresponds to that of the required right phase. As T c (22 K) of our muitiphase sample YPd4BC0. 2 is quite close to that in multiphase YPd5B3C0. 3 [9], it is possible that the phase responsible for superconductivity may be the same in both the sample. Experiments are in progress to examine this feature. Fraction of the fight phase in a quaternary system depends rather critically on the relative proportions of the constituent elements (we varied only the concentration of carbon in this work) and the preparative conditions. We believe that basically this is the reason why discovery of superconductivity in quaternary systems was delayed by so many years. It is also to be mentioned here that this will be the pattern of research in new quaternary superconductors; namely, detection of weak signals of superconductivity to start with and then identifying the fight phase responsible for superconductivity. To conclude, our quaternary material with chemical composition YPd4BC x exhibits superconductivity with Tc
Vol. 92, No. 4
as high as 22 K. That there are two superconducting transitions in this system, suggests that either there are at least two crystaliographieally distinct phases that are responsible for the two superconducting transitions or the two transitions arise due to a variation of relative proportion of the constituents within the same phase. We consider the second possibility to be rather remote, though we do not rule it out completely, as in that case there would have been a rather broad transition and not two distinct transitions. One of the most urgent tasks towards understanding these materials is to determine the precise chemical compositions and the crystal structures of these phases. Our results show that there are at least two other structures which are likely to show superconductivity, apart from the presently known LuNi2B2C [5,10], which enhances the potential for new superconductors in this family. Our own efforts indicate that these materials crystallise in more than one structure. We had already pointed this out in our earlier publication on these materials [4].
Acknowledgements We thank S.K. Paghdar for • assistance in the experiments and K.V. Gopalakrishnan for assistance in SQUID measurements. Part of this work was performed under Project No. 509-1 of Indo-French Centre for Promotion of Advanced Research, New Delhi (India).
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[6] Chandan Mazumdar, R. Nagarajan, L.C. Gupta, B.D. Padalia and R. Vijayaraghavan, Proc. Solid State Physics Symposium of Dept. Atomic Energy, India, (Jan. 1-4, 1991) Vol. 33C, 265 (1991). [7] Chandan Mazumdar, R. Nagarajan, S.K. Dhar, L.C. Gupta, B.D. Padalia and R. Vijayaraghavan, Proc. Solid State Physics Symposium of Dept. Atomic Energy, India, (Dec. 21-24, 1991) Vol. 34C, 241 (1991). [8] Chandan Mazumdar, R. Nagarajan, L.C. Gupta, B.D. Padalia and R. Vijayaraghavan, 37th Annual Conference on Magnetism and Magnetic Materials, Dec. 1-4 (1992), Houston, Texas, U.S.A., Abstract No. BPI4. [9] R.J. Cava, H. Takagi, B. Batlogg, H.W. Zandbergen, J.J. Krajewski, W.F. Peck Jr, R.B. van Dover, R.J. Felder, T. Siegrist, K. Mizuhashi, J.O. Lee, H. Eisaki, S.A. Carter and S. Uchida, Nature 367, 146 (1994). [10] R.J. Cava, H. Takagi, H.W. Zandbergen, J.J. Krajewski, W.F. Peck Jr, T. Siegrist, B. Batlogg, R.B. van Dover, R.J. Felder, K. Mizuhashi, J.O. Lee, H. Eisaki, and S. Uchida, Nature 367, 252 (1994).