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Evidence for impurity phase superconductivity in EuMo6S8 under pressure
Solid State Communications, Vol. 42, No. 11, pp. 819-822, 1982. Printed in Great Britain.
0038--1098/82/230819--04503.00/0 Pergamon Press Ltd.
EVIDENCE FOR IMPURITY PHASE SUPERCONDUCTIVITY IN EuMo6Ss UNDER PRESSURE R.W. McCallum, W.A. Kalsbach, T.S. Radhakrishnan* and F. Pobell lnstitut for Festk6rperforschung, Kernforschungsanlage, D-5170 Jiilich, West Germany and R.N. Shelton t and P. Klavins Ames Laboratory - USDOE ¢ and Department of Physics, Iowa State University, Ames, IO 50011, U.S.A.
(Received 19 January 1982 by P.H. Dederichs) Magnetization measurements in a static field as a function of temperature (2.5-20 K) and pressure to 14 kbar are reported for a series of well characterized samples of EuMo6+x Ss-y. From these d.c. magnetization data, we find no evidence for bulk superconductivity in any of our samples, although some of them exhibit a strong diamagnetic anomaly in the a.c. susceptibility under pressure, similar to that reported in the literature. An explanation for this anomaly is presented in terms of the presence of a granular superconducting impurity on the grain boundaries of the Chevrel phase compound EuMo6Ss. RECENTLY, Chu and co-workers [1,2] and Harrison
et al. [3] have reported the observation of superconductivity with an onset of the resistive transition up to 11 K in EuxMo6Ss when hydrostatic pressures of greater than approximately 7 kbar are applied. The resistive transitions are typically broad with widths of the order of 2 - 3 K in the 6 - 1 0 kbar range and are narrowing to 1 K at higher pressures. Harrison and co-workers [3,4] state that "susceptibility measurements indicate that the superconductivity is a bulk effect" in their samples, whereas Chu and co-workers [1,2] noted that less than 30% of the bulk samples was superconducting at 18 kbar. While these reports are in general agreement, the details of the observations vary and the presence of superconductivity under pressure is at variance with some samples we have previously studied [5]. In order to clarify the situation, we performed d.c. magnetization measurements on four well characterized samples of EuMo6.xSs_~. All samples investigated in this study were prepared by standard powder metallurgical techniques [6] starting from either EuS, Mo and S, or EuS, Mo2~6S3 and Mo. *On leave from Reactor Research Centre, Kalpakkam 603 012, India. tA portion of the research by R.N. Shelton was performed as a guest at the Institut fiir Festk6rperforschung, KFA, D-5170 J01ich, West Germany. ~tOperated for the U.S. Department of Energy by Iowa State University under contract No. W-7405-Eng-82. This research was supported by the Director of Energy Research, Office of basic Energy Sciences.
The second method eliminated the need for elemental sulfur. In some instances the Mo powder was reduced in a flow of H2 gas at 1000°C prior to the synthesis of the Chevrel phase. The four samples investigated in this work are listed in Table 1, along with nominal compositions and impurity phases detected by X-ray diffraction analysis and lSlEu M6ssbauer effect measurements. Since the strongest Mo line in the X-ray spectrum is coincident with a Chevrel phase line, a careful intensity analysis is necessary to determine the presence of a small amount of elemental molybdenum. In addition, care must be taken since a significant fraction of the Eu may easily be incorporated in an impurity phase of less than 2% of the total sample. It should also be noted that independent of the starting composition, the X-ray diffraction patterns for all our Chevrel phase compounds are the same within the measurement resolution. When the impurity phases present in the sample are quantitatively analyzed using the X-ray and M6ssbauer effect data it appears that the Chevrel phase always forms stoichiometrically, i.e. Eux.oMo6Ss, independent of starting composition. The susceptibility of all samples in this study was measured over the temperature range 1.2 to 20 K and at hydrostatic pressures up to 18 kbar in a piston-cylinder clamp. All pressures were determined at low temperature by means of a superconducting tin or lead manometer. Details of these techniques are described in detail elsewhere [7]. Two of the samples (Nos. 1,2) showed no diamagnetic signal in the a.c. measurements at any pressure. For sample No. 4, we observed a reproducible diamagnetic anomaly at
819
820
IMPURITY PHASE SUPERCONDUCTIVITY IN EuMo6Sa 12
Table I No. 1
2 3 4
Vol. 42, No. 1 t
Compound
Impurity phase(s)
Measurements
EuMo6.3S8 Mo2.o6Sa (5-10%) Me ( < 5%) EuMo6Sa Eu202S (< 5%) EuMo6ST.6 MoS2 !(5-10%) Me ( < 5%) EuMo6S7 Eu~O2S ( < 2%) Me ( < 5%)
a,
10
0 0
M, AC
0
<
AC M, AC*, DC M, AC*, DC
~e- 6 ._~ if) i
o"EuMo6S7" 112Kbar ,," Eu Mo6S76"129 Kbor ,,'" Eu Mo6S76"15.3 Kber
~ 2 i
Nominal compositions: M = M6ssbauer effect; AC = a.c. susceptibility under hydrostatic pressure, * = diamagnetic signal observed; DC = d.c. magnetization at ambient and hydrostatic pressure. The formulas give the normal relative starting concentrations. •
.
• ,. •
. ,. •
.
.
,
,
i
,
200 400 Measuring Frequency (Hz)
600
Fig. 2. Amplitude of the a.c. susceptibility signal as a function of measuring frequency for samples Nos. 3,4.
normalized to the size of the superconducting transition of the Sn manometer, decreased sharply as the measuring frequency was decreased and is shown in Fig. 2. When using a.c. susceptibility to investigate super(No3)31 t 8 • (Ret conductivity, one measures the screening currents • (Ref. 3 ) induced on the surface of the sample rather than the I (Ref 1 ) Meissner effect. For a bulk superconductor the screening ~6 of the sample is complete and independent of frequency. If, on the other hand, the observed signal is due to super4 conducting loops on the surface or along grain boundaries, the screening is less effective at low frequencies 2 and the signal decreases as the frequency is reduced. Our measurements show that this is the case for our EuMo6ST~ sample. 01 2 ¼ 6 8 10 12 14 1'6 18 20 For the two samples (Nos. 3,4) which show a diaPRESSURE (Kbor) magnetic signal in the a.c. susceptibility, we also perFig. l. "Superconducting transitions" reported for formed low field d.c. magnetization measurements in a various samples of compounds synthesized from commercial SQUID magnetometer [8] at ambient europium-molybdenum-sulfide. Open symbols for our samples represent a~c.-inductively measured signals, while pressure. The inverse molar susceptibility X~ for these filled symbols represent literature values for resistive samples is plotted vs temperature in Fig. 3; it is quantitransitions. Error bars indicate the width of the transition. tatively similar to literature data [9, 10]. The data for each sample can be fitted over the entire temperature range, 5.6-316 K, by a Curie-Weiss law, yielding a pressures above 8 kbar whose behaviour is in reasonsmall, negative Curie-Weiss temperature with absolute able agreement with the published reports [ 1-3 ]. For value less than 1 K. The analysis of the susceptibility for sample No. 3, we observed a significantly sharper sample No. 3 yields an effective moment, #en (60 mK width) diamagnetic anomaly. The data from (7.85 + 0.15)/an, which is equal within experimental the latter two samples are presented in Fig. 1 along error to the value (7.94#B) predicted for the Hund's rule with literature values [ 1,3 ] for comparison. It is noteground state of Eu 2+ with gj = 2 and J = 7/2. The worthy that the amplitude of the diamagnetic anomaly divalent character of essentially all Eu ions in this sample is a strongly increasing function of pressure in the range is confirmed by X-ray analysis which indicated no other below 12 kbar and remains constant above 12 kbar. This Eu-compounds, and by MOssbauer effect measurements pressure dependence of the signal is qualitatively similar which indicated that more than 98% of the Eu in this to the results reported by Chu and co-workers [1,2]. sample is in the divalent state. For sample No. 4, for For samples No. 3 and 4, we performed a study of the which the X-ray analysis indicates some Eu202S (< 2%), frequency dependence of the amplitude of the diamagwe obtain/aen = (7.35 -+ 0.15)#n. This effective moment netic anomaly at fixed pressures. The amplitude, 12
o
EuMo6S~
Vol. 42, No. 11 ,
/.0
821
IMPURITY PHASE SUPERCONDUCTIVITY IN EuMo6S8 ,
,
,
,
* EuMo~S~
X [cmJ/m°le)~
~ ¢
/" /
,
/
(a) " EuNo6S 7 "
9-'/'/~ -
/
/
-~ "5 E
1.3
1.2
X
1A I
2C
f
E to
lC
1',oo °°~'o 50 % 2 03'o t,' ,oo ~o
,I
s'o
1~0
~0
200
TEMPERATURE (K)
2~0
I
I
)
I
7'2
7~
(b)
X~'lc'~/m°'el-'
3~0
Fig. 3. Inverse molar susceptibility for sample No. 3 (o) and sample No. 4 (o) between 5.6 and 316 K. is explained by a spatial mixture of "~ 86% Eu 2÷ and 14% Eu 3÷, attributing the Eu 3÷ to the oxysulfide impurity phase. This interpretation agrees with lSlEu M6ssbauer effect data on this sample, showing a value of 84% for the Eu 2+ fraction. The inclusion of 15% of the Eu in Eu202S is in reasonable agreement with the X-ray results. Thus the combination of magnetization measurements, M6ssbauer effect data and X-ray diffraction analysis clearly indicates that all Eu in the (very probably stoichiometric) Chevrel phase is divalent. This result is inconsistent with several reports [9, 10] where a spatially inhomogeneous valence mixture of Eu in the Chevrel phase was assumed in order to explain the susceptibility data. In order to establish whether or not the two EuMo6S8 samples which exhibit the diamagnetic signal in the a.c. susceptibility exhibit bulk superconductivity, we performed d.c. magnetization measurements as a function of temperature and pressure. These measurements were performed in our susceptometer with a dummy clamp located in one coil of the SQUID flux transformer and the pressure clamp containing the sample and the Sn manometer in the other coil. The measurements were made in the rest field of the 5 T magnet which is given by the manufactuerer as 0.5 mT. The resolution of the experiment was such that a signal due to 1% of the transition of the 40 mg Sn manometer sample could be readily distinguished from the background. Each Chevrel phase sample was slightly larger than the Sn manometer and was measured over the temperature range 2 . 5 - 2 0 K. Sample No. 4 was investigated at 0.0, 4.0 and 12.1 kbar, while the pressure for the runs with sample No. 3 was 11.7 and 14.3 kbar. Although these pressures include the range of the observed signal in the a.c. susceptibility (see Fig. 1), no d.c. transition or Meissner effect was detected down to below the 1%
-5 E
0.02
E
0
..
<] - 0.02
616
618
7'o T (K)
Fig. 4. (a) Total measured susceptibility for 2.5 g sample of EuMo6S7 between 6.5 and 7.5 K. (b) Difference in the susceptibility between the measured values ( - - ) and the paramagnetic behavior of the Chevrel phase EuM%Sa (-- -) as a function of temperature. level. While at first glance this would seem to be a definitive determination of the lack of bulk superconductivity in these materials, this is unfortunately not true. Since these samples are extreme Type II superconductors, the observable Meissner effect can be drastically reduced due to flux trapping in the sample. For example d.c. magnetization measurements at ambient pressure on small, (0.1 mm) 3, single crystals of PbMo6S8 yielded an 8% flux expulsion, a powdered sample of the same material showed an 18% effect, and for superconducting Euo.sYbo.sMo63Sa the signal was less than 1%. The magnetization measurements under pressure did however reveal that the Eu retrained its magnetic moment consistent with M6ssbauer measurements under pressure at 4.2 K which showed no significant change in the isomer shift within the resolution of the measurements (-- 12.8 -+ 0.2 mm sec -1 at 0 kbar, -- 12.3 -+ 0.2 mm sec -1 at 16 kbar) [ 11]. This clearly demonstrates that there is no change of the Eu valence under pressure. In order to determine if the "superconductivity" observed in a.c. measurements on EuMo6Sa may be due to a superconducting impurity phase, sensitive d.c. magnetization studies of a large ('~ 2.5 g) sample were performed. The resulting data in both raw form and after subtracting the paramagnetic contribution of the Eu 2+ ions in the Chevrel phase are shown in Fig. 4. At a m b i e n t pressure a superconducting transition is clearly visible at 7.0 K in this measurement. We are unable to unambiguously state the origin of this transition. But it is known that molybdenum in granular form exhibits a superconducting transition at temperatures as high as 6.4 K [12]. In addition, Koepke and Bergmann [13] found transition temperatures of 8.2 K for amorphous
822
IMPURITY PHASE SUPERCONDUCTIVITY IN EuMo6S8
Mo. In addition it should be recalled that we cannot rule out the presence of MoC or MoN in some samples. These materials can easily explain transition temperatures up to 14 K [14]. In granular materials, fluctuations due to the small dimensions of the grains may produce decreases in resistivity well above Tc. Since our samples contain some free molybdenum it seems probable that the observed zero pressure superconductivity is due to this Mo. While the anomaly in the d.c. signal corresponds to only 0.1% of the excess Mo, it is known that due to flux trapping and penetration depth effects granular superconductors also exhibit greatly reduced Meissner effects. Whatever the exact composition of the superconducting impurity phase, the presence of superconducting grains mixed with the EuMo6S8 can easily explain the observed properties of "EuMo6S8" under pressure. All of the anomalous properties attributed to EuMo6S8 [ 1-3 ] have been observed at ambient pressure in granular materials [ 15]. When superconducting grains are contained in a non-metallic medium, the overlap of the superconducting electron wave functions of adjacent grains depends exponentially on the separation of the grains. At ambient pressure, the material is possibly composed of isolated superconducting paths through the sample resulting in a broad resistive transition and a screening of the a.c. measuring field. Since our grains are situated in a Chevrel phase host which has a large compressibility [ 16], this overlap should be very sensitive to external pressure. When a granular superconductor is placed in a metallic host, the proximity effect strongly depresses Te. Thus granular Mo in EuMo6Se8, which unlike the semiconductor EuMo6S8 is metallic, should show no superconductivity, consistent with what has been reported. In conclusion we note that we have investigated a number of samples of "EuMo6Ss" under pressure. Although X-ray diffraction analysis of these samples shows that the crystallographic parameters of the Chevrel phase component are identical within the resolution of the measurement in all samples, half of them show no signs of superconductivity at all. In the two samples which exhibit a shielding signal in a.c. susceptibi/ity measurements, the pressure and frequency dependence of the amplitude of the signal and the results of our d.c. magnetization measurements indicate that the reported "superconductivity in EuMo6Ss" is not occurring in this ternary phase. The observed diamagnetic anomaly in the a.c. susceptibility and the resistive transition into the superconducting state are very probably due to the presence of a superconducting impurity phase. We have detected the presence of such a
Vol. 42, No. I !
phase at ambient pressure through sensitive d.c. magnetization measurements. The phase is probably a granular molybdenum based material and we propose that the reported superconducting properties at high pressure [1-4] are due to this phase. Acknowledgements - The work of Mr F. Claasen in preparing some of the samples and the excellent technical assistance of Mr W. Zander are most gratefully acknowledged. The X-ray diffraction measurements on our samples by Dr C. Freibrug was essential to this work. R.N. Shelton expresses his appreciation for the hospitality shown him during his stay at the IFF, KFA Jtilich. REFERENCES 1.
2.
3.
4. 5. 6. 7. 8. 9. 10.
11. 12. 13. 14. 15.
16.
C.W. Chu, S.Z. Huang, C.H. Lin, R.L. Meng, M.K. Wu & P.H. Schmidt, Phys. Rev. Lett. 46, 276 (1981); (a.c.-suscept. and a.c.-resistivity at 26 Hz). C.W. Chu, S.Z. Huang, C.H. Lin, R.L. Meng, M.K. Wu & P.H. Schmidt, Ternary Superconductors (Edited by G.K. Shenoy, B.D. Dunlap & F.Y. Fradin), p. 103. North-Holland, New York (1981). D.W. Harrison, K.C. Lim, J.D. Thompson, C.Y. Huang, P.D. Hambourger & H.L. Luo, Phys. Rev. Lett. 46,280 (1981); a.c.-suscept, and a.c.resistivity. K.C. Lin, J.D. Thompson, D.W. Harrison, C.Y. Huang & J.L. Luo, J. Appl. Phys. 52, 2177 (1981). R.N. Shelton & A.R. Moodenbaugh, Phys. Rev. B24, 2863 (1981). R.N. Shelton, R.W. McCallum & H. Adrian,Phys. Lett. 56A, 213 (1976). T.F. Smith, J. Low Temp. Phys. 6, 171 (1972). S.H.E. Corporation Variable Temperature Susceptometer. M. Pellizone, A. Treyvaud, P. Spitzli & 0. Fischer, J. Low Temp. Phys. 29,453 (1977). D.C. Johnston & R.N. Shelton, aT. Low Temp. Phys. 26,561 (1977); J.D. Thompson, M.P. Maley, C.H. Huang, J .O. Willis, J.L. Smith & H.L. Luo, J. Low Temp. Phys. 43,243 (1981). M.M. Abd-Elmeguid & H. Micklitz, to be published. J.J. Hanak, J.I. Gittleman, J.P. Pellicane & S. Bozowski, Phys. Lett. 30A, 201 (1969). R. Koepke & G. Bergmann, Solid State Commun. 19,435 (1976). J.M. Vandenberg & B.T. Matthias, Mat. Res. Bull. 9, 1085 (1974). G. Deutscher, O. Entin-Wohlman, M. Rappaport & Y. Shapira, Inhomogeneous Superconductors - 1979 (Edited by D.U. Gubser, T.L. Francavilla, S.A. Wolf & J.R. Leibowitz), p. 23. American Institute of Physics, New York (1980), and other references in this Conferece Proceedings. A.W. Webb & R.N. Shelton, J. Phys. F: Metal Phys. 8,261 (1978).
0038--1098/82/230819--04503.00/0 Pergamon Press Ltd.
EVIDENCE FOR IMPURITY PHASE SUPERCONDUCTIVITY IN EuMo6Ss UNDER PRESSURE R.W. McCallum, W.A. Kalsbach, T.S. Radhakrishnan* and F. Pobell lnstitut for Festk6rperforschung, Kernforschungsanlage, D-5170 Jiilich, West Germany and R.N. Shelton t and P. Klavins Ames Laboratory - USDOE ¢ and Department of Physics, Iowa State University, Ames, IO 50011, U.S.A.
(Received 19 January 1982 by P.H. Dederichs) Magnetization measurements in a static field as a function of temperature (2.5-20 K) and pressure to 14 kbar are reported for a series of well characterized samples of EuMo6+x Ss-y. From these d.c. magnetization data, we find no evidence for bulk superconductivity in any of our samples, although some of them exhibit a strong diamagnetic anomaly in the a.c. susceptibility under pressure, similar to that reported in the literature. An explanation for this anomaly is presented in terms of the presence of a granular superconducting impurity on the grain boundaries of the Chevrel phase compound EuMo6Ss. RECENTLY, Chu and co-workers [1,2] and Harrison
et al. [3] have reported the observation of superconductivity with an onset of the resistive transition up to 11 K in EuxMo6Ss when hydrostatic pressures of greater than approximately 7 kbar are applied. The resistive transitions are typically broad with widths of the order of 2 - 3 K in the 6 - 1 0 kbar range and are narrowing to 1 K at higher pressures. Harrison and co-workers [3,4] state that "susceptibility measurements indicate that the superconductivity is a bulk effect" in their samples, whereas Chu and co-workers [1,2] noted that less than 30% of the bulk samples was superconducting at 18 kbar. While these reports are in general agreement, the details of the observations vary and the presence of superconductivity under pressure is at variance with some samples we have previously studied [5]. In order to clarify the situation, we performed d.c. magnetization measurements on four well characterized samples of EuMo6.xSs_~. All samples investigated in this study were prepared by standard powder metallurgical techniques [6] starting from either EuS, Mo and S, or EuS, Mo2~6S3 and Mo. *On leave from Reactor Research Centre, Kalpakkam 603 012, India. tA portion of the research by R.N. Shelton was performed as a guest at the Institut fiir Festk6rperforschung, KFA, D-5170 J01ich, West Germany. ~tOperated for the U.S. Department of Energy by Iowa State University under contract No. W-7405-Eng-82. This research was supported by the Director of Energy Research, Office of basic Energy Sciences.
The second method eliminated the need for elemental sulfur. In some instances the Mo powder was reduced in a flow of H2 gas at 1000°C prior to the synthesis of the Chevrel phase. The four samples investigated in this work are listed in Table 1, along with nominal compositions and impurity phases detected by X-ray diffraction analysis and lSlEu M6ssbauer effect measurements. Since the strongest Mo line in the X-ray spectrum is coincident with a Chevrel phase line, a careful intensity analysis is necessary to determine the presence of a small amount of elemental molybdenum. In addition, care must be taken since a significant fraction of the Eu may easily be incorporated in an impurity phase of less than 2% of the total sample. It should also be noted that independent of the starting composition, the X-ray diffraction patterns for all our Chevrel phase compounds are the same within the measurement resolution. When the impurity phases present in the sample are quantitatively analyzed using the X-ray and M6ssbauer effect data it appears that the Chevrel phase always forms stoichiometrically, i.e. Eux.oMo6Ss, independent of starting composition. The susceptibility of all samples in this study was measured over the temperature range 1.2 to 20 K and at hydrostatic pressures up to 18 kbar in a piston-cylinder clamp. All pressures were determined at low temperature by means of a superconducting tin or lead manometer. Details of these techniques are described in detail elsewhere [7]. Two of the samples (Nos. 1,2) showed no diamagnetic signal in the a.c. measurements at any pressure. For sample No. 4, we observed a reproducible diamagnetic anomaly at
819
820
IMPURITY PHASE SUPERCONDUCTIVITY IN EuMo6Sa 12
Table I No. 1
2 3 4
Vol. 42, No. 1 t
Compound
Impurity phase(s)
Measurements
EuMo6.3S8 Mo2.o6Sa (5-10%) Me ( < 5%) EuMo6Sa Eu202S (< 5%) EuMo6ST.6 MoS2 !(5-10%) Me ( < 5%) EuMo6S7 Eu~O2S ( < 2%) Me ( < 5%)
a,
10
0 0
M, AC
0
<
AC M, AC*, DC M, AC*, DC
~e- 6 ._~ if) i
o"EuMo6S7" 112Kbar ,," Eu Mo6S76"129 Kbor ,,'" Eu Mo6S76"15.3 Kber
~ 2 i
Nominal compositions: M = M6ssbauer effect; AC = a.c. susceptibility under hydrostatic pressure, * = diamagnetic signal observed; DC = d.c. magnetization at ambient and hydrostatic pressure. The formulas give the normal relative starting concentrations. •
.
• ,. •
. ,. •
.
.
,
,
i
,
200 400 Measuring Frequency (Hz)
600
Fig. 2. Amplitude of the a.c. susceptibility signal as a function of measuring frequency for samples Nos. 3,4.
normalized to the size of the superconducting transition of the Sn manometer, decreased sharply as the measuring frequency was decreased and is shown in Fig. 2. When using a.c. susceptibility to investigate super(No3)31 t 8 • (Ret conductivity, one measures the screening currents • (Ref. 3 ) induced on the surface of the sample rather than the I (Ref 1 ) Meissner effect. For a bulk superconductor the screening ~6 of the sample is complete and independent of frequency. If, on the other hand, the observed signal is due to super4 conducting loops on the surface or along grain boundaries, the screening is less effective at low frequencies 2 and the signal decreases as the frequency is reduced. Our measurements show that this is the case for our EuMo6ST~ sample. 01 2 ¼ 6 8 10 12 14 1'6 18 20 For the two samples (Nos. 3,4) which show a diaPRESSURE (Kbor) magnetic signal in the a.c. susceptibility, we also perFig. l. "Superconducting transitions" reported for formed low field d.c. magnetization measurements in a various samples of compounds synthesized from commercial SQUID magnetometer [8] at ambient europium-molybdenum-sulfide. Open symbols for our samples represent a~c.-inductively measured signals, while pressure. The inverse molar susceptibility X~ for these filled symbols represent literature values for resistive samples is plotted vs temperature in Fig. 3; it is quantitransitions. Error bars indicate the width of the transition. tatively similar to literature data [9, 10]. The data for each sample can be fitted over the entire temperature range, 5.6-316 K, by a Curie-Weiss law, yielding a pressures above 8 kbar whose behaviour is in reasonsmall, negative Curie-Weiss temperature with absolute able agreement with the published reports [ 1-3 ]. For value less than 1 K. The analysis of the susceptibility for sample No. 3, we observed a significantly sharper sample No. 3 yields an effective moment, #en (60 mK width) diamagnetic anomaly. The data from (7.85 + 0.15)/an, which is equal within experimental the latter two samples are presented in Fig. 1 along error to the value (7.94#B) predicted for the Hund's rule with literature values [ 1,3 ] for comparison. It is noteground state of Eu 2+ with gj = 2 and J = 7/2. The worthy that the amplitude of the diamagnetic anomaly divalent character of essentially all Eu ions in this sample is a strongly increasing function of pressure in the range is confirmed by X-ray analysis which indicated no other below 12 kbar and remains constant above 12 kbar. This Eu-compounds, and by MOssbauer effect measurements pressure dependence of the signal is qualitatively similar which indicated that more than 98% of the Eu in this to the results reported by Chu and co-workers [1,2]. sample is in the divalent state. For sample No. 4, for For samples No. 3 and 4, we performed a study of the which the X-ray analysis indicates some Eu202S (< 2%), frequency dependence of the amplitude of the diamagwe obtain/aen = (7.35 -+ 0.15)#n. This effective moment netic anomaly at fixed pressures. The amplitude, 12
o
EuMo6S~
Vol. 42, No. 11 ,
/.0
821
IMPURITY PHASE SUPERCONDUCTIVITY IN EuMo6S8 ,
,
,
,
* EuMo~S~
X [cmJ/m°le)~
~ ¢
/" /
,
/
(a) " EuNo6S 7 "
9-'/'/~ -
/
/
-~ "5 E
1.3
1.2
X
1A I
2C
f
E to
lC
1',oo °°~'o 50 % 2 03'o t,' ,oo ~o
,I
s'o
1~0
~0
200
TEMPERATURE (K)
2~0
I
I
)
I
7'2
7~
(b)
X~'lc'~/m°'el-'
3~0
Fig. 3. Inverse molar susceptibility for sample No. 3 (o) and sample No. 4 (o) between 5.6 and 316 K. is explained by a spatial mixture of "~ 86% Eu 2÷ and 14% Eu 3÷, attributing the Eu 3÷ to the oxysulfide impurity phase. This interpretation agrees with lSlEu M6ssbauer effect data on this sample, showing a value of 84% for the Eu 2+ fraction. The inclusion of 15% of the Eu in Eu202S is in reasonable agreement with the X-ray results. Thus the combination of magnetization measurements, M6ssbauer effect data and X-ray diffraction analysis clearly indicates that all Eu in the (very probably stoichiometric) Chevrel phase is divalent. This result is inconsistent with several reports [9, 10] where a spatially inhomogeneous valence mixture of Eu in the Chevrel phase was assumed in order to explain the susceptibility data. In order to establish whether or not the two EuMo6S8 samples which exhibit the diamagnetic signal in the a.c. susceptibility exhibit bulk superconductivity, we performed d.c. magnetization measurements as a function of temperature and pressure. These measurements were performed in our susceptometer with a dummy clamp located in one coil of the SQUID flux transformer and the pressure clamp containing the sample and the Sn manometer in the other coil. The measurements were made in the rest field of the 5 T magnet which is given by the manufactuerer as 0.5 mT. The resolution of the experiment was such that a signal due to 1% of the transition of the 40 mg Sn manometer sample could be readily distinguished from the background. Each Chevrel phase sample was slightly larger than the Sn manometer and was measured over the temperature range 2 . 5 - 2 0 K. Sample No. 4 was investigated at 0.0, 4.0 and 12.1 kbar, while the pressure for the runs with sample No. 3 was 11.7 and 14.3 kbar. Although these pressures include the range of the observed signal in the a.c. susceptibility (see Fig. 1), no d.c. transition or Meissner effect was detected down to below the 1%
-5 E
0.02
E
0
..
<] - 0.02
616
618
7'o T (K)
Fig. 4. (a) Total measured susceptibility for 2.5 g sample of EuMo6S7 between 6.5 and 7.5 K. (b) Difference in the susceptibility between the measured values ( - - ) and the paramagnetic behavior of the Chevrel phase EuM%Sa (-- -) as a function of temperature. level. While at first glance this would seem to be a definitive determination of the lack of bulk superconductivity in these materials, this is unfortunately not true. Since these samples are extreme Type II superconductors, the observable Meissner effect can be drastically reduced due to flux trapping in the sample. For example d.c. magnetization measurements at ambient pressure on small, (0.1 mm) 3, single crystals of PbMo6S8 yielded an 8% flux expulsion, a powdered sample of the same material showed an 18% effect, and for superconducting Euo.sYbo.sMo63Sa the signal was less than 1%. The magnetization measurements under pressure did however reveal that the Eu retrained its magnetic moment consistent with M6ssbauer measurements under pressure at 4.2 K which showed no significant change in the isomer shift within the resolution of the measurements (-- 12.8 -+ 0.2 mm sec -1 at 0 kbar, -- 12.3 -+ 0.2 mm sec -1 at 16 kbar) [ 11]. This clearly demonstrates that there is no change of the Eu valence under pressure. In order to determine if the "superconductivity" observed in a.c. measurements on EuMo6Sa may be due to a superconducting impurity phase, sensitive d.c. magnetization studies of a large ('~ 2.5 g) sample were performed. The resulting data in both raw form and after subtracting the paramagnetic contribution of the Eu 2+ ions in the Chevrel phase are shown in Fig. 4. At a m b i e n t pressure a superconducting transition is clearly visible at 7.0 K in this measurement. We are unable to unambiguously state the origin of this transition. But it is known that molybdenum in granular form exhibits a superconducting transition at temperatures as high as 6.4 K [12]. In addition, Koepke and Bergmann [13] found transition temperatures of 8.2 K for amorphous
822
IMPURITY PHASE SUPERCONDUCTIVITY IN EuMo6S8
Mo. In addition it should be recalled that we cannot rule out the presence of MoC or MoN in some samples. These materials can easily explain transition temperatures up to 14 K [14]. In granular materials, fluctuations due to the small dimensions of the grains may produce decreases in resistivity well above Tc. Since our samples contain some free molybdenum it seems probable that the observed zero pressure superconductivity is due to this Mo. While the anomaly in the d.c. signal corresponds to only 0.1% of the excess Mo, it is known that due to flux trapping and penetration depth effects granular superconductors also exhibit greatly reduced Meissner effects. Whatever the exact composition of the superconducting impurity phase, the presence of superconducting grains mixed with the EuMo6S8 can easily explain the observed properties of "EuMo6S8" under pressure. All of the anomalous properties attributed to EuMo6S8 [ 1-3 ] have been observed at ambient pressure in granular materials [ 15]. When superconducting grains are contained in a non-metallic medium, the overlap of the superconducting electron wave functions of adjacent grains depends exponentially on the separation of the grains. At ambient pressure, the material is possibly composed of isolated superconducting paths through the sample resulting in a broad resistive transition and a screening of the a.c. measuring field. Since our grains are situated in a Chevrel phase host which has a large compressibility [ 16], this overlap should be very sensitive to external pressure. When a granular superconductor is placed in a metallic host, the proximity effect strongly depresses Te. Thus granular Mo in EuMo6Se8, which unlike the semiconductor EuMo6S8 is metallic, should show no superconductivity, consistent with what has been reported. In conclusion we note that we have investigated a number of samples of "EuMo6Ss" under pressure. Although X-ray diffraction analysis of these samples shows that the crystallographic parameters of the Chevrel phase component are identical within the resolution of the measurement in all samples, half of them show no signs of superconductivity at all. In the two samples which exhibit a shielding signal in a.c. susceptibi/ity measurements, the pressure and frequency dependence of the amplitude of the signal and the results of our d.c. magnetization measurements indicate that the reported "superconductivity in EuMo6Ss" is not occurring in this ternary phase. The observed diamagnetic anomaly in the a.c. susceptibility and the resistive transition into the superconducting state are very probably due to the presence of a superconducting impurity phase. We have detected the presence of such a
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phase at ambient pressure through sensitive d.c. magnetization measurements. The phase is probably a granular molybdenum based material and we propose that the reported superconducting properties at high pressure [1-4] are due to this phase. Acknowledgements - The work of Mr F. Claasen in preparing some of the samples and the excellent technical assistance of Mr W. Zander are most gratefully acknowledged. The X-ray diffraction measurements on our samples by Dr C. Freibrug was essential to this work. R.N. Shelton expresses his appreciation for the hospitality shown him during his stay at the IFF, KFA Jtilich. REFERENCES 1.
2.
3.
4. 5. 6. 7. 8. 9. 10.
11. 12. 13. 14. 15.
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