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The influence of actinides on the superconducting properties of molybdenum chalcogenides
0038-1098/82/070569-02502.00/0 Pergamon Press
Solid State Communications, Vol. 41, No. 7, pp. 569-570. 1982. Printed in Great Britain
THE INFLUENCE OF ACTINIDES ON THE SUPERCONDUCTING PROPERTIES OF MOLYBDENUM CHALCOGENIDES N.E. Alekseevskii,* A.V. Mitin,* C. Bazan, B. Greri and L. Folcik 1 International Laboratory of High Magnetic Fields and Low Temperatures, Pr6chnika 95, 53-529 Wroctaw, Poland
(Received 18 August 1981 by A. Blandin) The results of the investigations of molybdenum chalcogenides: UxPbl_xMo6S8, UxPbl_xMo6.4Ss, ThxPbl_xMo6S8, UxLal_xMo6S8 with actinides as a component, are presented. As a rule the decrease of superconducting parameters with x was observed. Possible influence of magnetic interaction in this system is noted. MOLYBDENUM CHALCOGENIDES of the type AxMo6Ch 8 where Ch denotes chalcogen: S, Se or Te and A-metal atom, present a class of substances with very exciting features. Among them are superconductors with the record high critical magnetic field reaching 50 T, magnetically ordered systems including so-called re-entrant superconductors, semiconductors and so on. Addition of small quantities of other elements to the ternary molybdenum chalcogenides (TMC) may drastically change their properties [ 1,2]. In some cases enhancement of superconducting parameters was observed. In TMC with rare earths some interesting properties were found [3]. Therefore the interest arose to investigate the influence of small additions of actinides on the TMC properties. In the present work were investigated: UxVbl_xMo6Ss, UxPDt-xM06.4Ss, ThxPbl-xMo6Ss, UxLal_xMO6Ss. The samples were prepared by direct synthesis described in [4]. For various concentrations x there have been measured: critical temperatures, specific resisitivity Pn near To, and the dependence of the critical magnetic field on temperature. The lattice constants and the temperature dependence of the magnetic susceptibility were determined for some systems too. The magnetic field was generated by Bitter type coils [5]. The magnetic susceptibility was measured with a string magnetometer [6]. In the chalcogenides: UMo6Ss, UMo6Se8 and UMo6Tes no superconductivity above 1.7 K was observed, and U M o 6 . 4 S 8 w a s not superconducting down to 0.4 K. The electrical resistivity of all these systems * Permanent address: Institute of Physical Problems, Moscow, Vorobyovskoe shosse 2, USSR. t Permanent address: Institute of Low Temperatures and Structure Research, P1. Katedralny 1, Wrodaw, Poland. 569
Tc (x} O.E ~ Tc (o"~"0.6
~
~
.
?
0.402 I
0.1
I
0.2
l
0.3 X
I
04
I
0 5
Fig. 1. The concentration dependence of Te(x)/Te(O ). o, UxPbl_xMO6S8 , Te(0) = 13.9; [], ThxPbl_xM%S8, Te(0) = 13.9;v, UxPb1_xM%.4Ss, Te(0) = 13.0; A, UxPbl_xM%.4Ss, Te(0) = 11.2; O, UxLal-xMo6Ss, Te(0) = 7.0. As Tc the temperature was assumed at which r/rn = 0.5. exhibited semiconducting character. In the temperature dependence of the magnetic susceptibility some peculiarities were observed, which may be attributed to the magnetic ordering. For all investigated quaternary compounds, the critical temperature decreases with concentration x. In Fig. 1 is presented the reduced critical temperature Te(x)/Te(O) as a function of actinide concentration x. Te(O) equals 13.9 K for PbMo6S8. The transition width ATe, marked in Fig. 1 by vertical lines, increases with x. The lattice constants of UxVbl_xMo6S8 have been determined and thereafter the volume of the primitive hexagonal cell V was evaluated. It was stated that V diminishes with x in accordance with the results of [2]. The initial slope dHe2/dT near Te decreases with concentration x for UxPbl_xM06.4S 8 but it practically
570
INFLUENCE OF ACTINIDES OF MOLYBDENUM CHALCOGENIDES
does not change for UxLal_xMo6Ss, whereas He2 reduces in both cases. Magnetic susceptibility measurements performed for TMC doped with actinides have shown that at 4.2 K X is about 10 -6 emu g-~. In the chalcogenides doped with uranium some singularities near 60 K were detected, similar to that observed in ternary uranium chalcogenides.* Such anomalies were not found in thorium doped chalcogenides, e.g., in ThxPbl_xMo6S 8 with 0.1 ~< x ~< 0.5 the susceptibility obeys Curie-Weiss law, still being lower than 10 -6 emu g-~. As a rule, Sommerfeld constant 7, estimated from dHe2/dT and Pn data diminishes with actinide concentration. For instance in the system UxPbl_xMo6.4S8, 7 = 95 -+ 10 and 68 -+ 7 mJ mole -1K -2 for x = 0.1 and 0.2 respectively. It is not excluded that the decrease of 7 and therefore N(0) may influence the diminution of the critical temperature. As follows from Fig. 1 the rate of decrease of the reduced critical temperature Te(x)/Te(O) is nearly the same for all investigated compounds of P b - M o - S series, apart from their different Te(0) values. For UxLal_xMO6S8 the decrease is much smaller. Comparing the results obtained here with those relating to ternary molybdenum chalcogenides with rare earths [3], it should be marked that whereas the rare earths addition involves in some cases the increase of the critical field (for example, in Pbl_xEuxM06Ss)the addition of actinides leads to the depression of critical
* The presence of small amounts of magnetic uranium phases like US, UOS (see [7]) which might influence the X(T) dependence cannot be excluded.
Vol. 41, No. 7
parameters in all investigated systems. This different influence of rare earths and actinides on the critical parameters of TMC may be related to the peculiarities of the magnetic interactions. To explain the role of such interactions in TMC doped with actinides, additional investigations are necessary, which will be performed in the nearest time.
Acknowledgements - The authors feel indebted to express the acknowledgement to Professor W. Trzebiatowski for his interest in the work and for the helpful discussion, to N.M. Dobrovolsky and T. Mydlarz for assistance in measurements. REFERENCES 1. 2.
3. 4. 5. 6, 7.
N.E. Alekseevskii, A.V. Mitin, C. Bazan, N.M. Dobrovolskii & B. Ronczka, J. Exp. Teor. Phys. 74, 384 (1978). M. Sergent, R. Chevrel, C. Rossel & 0. Fischer, J. Less-Common Metals 58,179 (1978). O. Fischer, Appl. Phys. 16, I (1978). N.E. Alekseevskii, N.M. Dobrovolskii & V.I. Tsebro, Soy. Phys: J. Exper. Teor. Phys. Lett. 20, 50 (1974). K. Troinar, Proc. Magnet. Technology Con£ MT6, p. 410. Bratislava (1977). N.E. Alekseevskii, E.P. Krasnopiorov & V.C. Nazin, DAN USSR 197, 814 (1971). W. Trzebiatowski, Ferromagnetic Materials, (Edited by E.P. Wohlfarth), Vol. 1, p. 415. NorthHolland, Amsterdam (1980).
Solid State Communications, Vol. 41, No. 7, pp. 569-570. 1982. Printed in Great Britain
THE INFLUENCE OF ACTINIDES ON THE SUPERCONDUCTING PROPERTIES OF MOLYBDENUM CHALCOGENIDES N.E. Alekseevskii,* A.V. Mitin,* C. Bazan, B. Greri and L. Folcik 1 International Laboratory of High Magnetic Fields and Low Temperatures, Pr6chnika 95, 53-529 Wroctaw, Poland
(Received 18 August 1981 by A. Blandin) The results of the investigations of molybdenum chalcogenides: UxPbl_xMo6S8, UxPbl_xMo6.4Ss, ThxPbl_xMo6S8, UxLal_xMo6S8 with actinides as a component, are presented. As a rule the decrease of superconducting parameters with x was observed. Possible influence of magnetic interaction in this system is noted. MOLYBDENUM CHALCOGENIDES of the type AxMo6Ch 8 where Ch denotes chalcogen: S, Se or Te and A-metal atom, present a class of substances with very exciting features. Among them are superconductors with the record high critical magnetic field reaching 50 T, magnetically ordered systems including so-called re-entrant superconductors, semiconductors and so on. Addition of small quantities of other elements to the ternary molybdenum chalcogenides (TMC) may drastically change their properties [ 1,2]. In some cases enhancement of superconducting parameters was observed. In TMC with rare earths some interesting properties were found [3]. Therefore the interest arose to investigate the influence of small additions of actinides on the TMC properties. In the present work were investigated: UxVbl_xMo6Ss, UxPDt-xM06.4Ss, ThxPbl-xMo6Ss, UxLal_xMO6Ss. The samples were prepared by direct synthesis described in [4]. For various concentrations x there have been measured: critical temperatures, specific resisitivity Pn near To, and the dependence of the critical magnetic field on temperature. The lattice constants and the temperature dependence of the magnetic susceptibility were determined for some systems too. The magnetic field was generated by Bitter type coils [5]. The magnetic susceptibility was measured with a string magnetometer [6]. In the chalcogenides: UMo6Ss, UMo6Se8 and UMo6Tes no superconductivity above 1.7 K was observed, and U M o 6 . 4 S 8 w a s not superconducting down to 0.4 K. The electrical resistivity of all these systems * Permanent address: Institute of Physical Problems, Moscow, Vorobyovskoe shosse 2, USSR. t Permanent address: Institute of Low Temperatures and Structure Research, P1. Katedralny 1, Wrodaw, Poland. 569
Tc (x} O.E ~ Tc (o"~"0.6
~
~
.
?
0.402 I
0.1
I
0.2
l
0.3 X
I
04
I
0 5
Fig. 1. The concentration dependence of Te(x)/Te(O ). o, UxPbl_xMO6S8 , Te(0) = 13.9; [], ThxPbl_xM%S8, Te(0) = 13.9;v, UxPb1_xM%.4Ss, Te(0) = 13.0; A, UxPbl_xM%.4Ss, Te(0) = 11.2; O, UxLal-xMo6Ss, Te(0) = 7.0. As Tc the temperature was assumed at which r/rn = 0.5. exhibited semiconducting character. In the temperature dependence of the magnetic susceptibility some peculiarities were observed, which may be attributed to the magnetic ordering. For all investigated quaternary compounds, the critical temperature decreases with concentration x. In Fig. 1 is presented the reduced critical temperature Te(x)/Te(O) as a function of actinide concentration x. Te(O) equals 13.9 K for PbMo6S8. The transition width ATe, marked in Fig. 1 by vertical lines, increases with x. The lattice constants of UxVbl_xMo6S8 have been determined and thereafter the volume of the primitive hexagonal cell V was evaluated. It was stated that V diminishes with x in accordance with the results of [2]. The initial slope dHe2/dT near Te decreases with concentration x for UxPbl_xM06.4S 8 but it practically
570
INFLUENCE OF ACTINIDES OF MOLYBDENUM CHALCOGENIDES
does not change for UxLal_xMo6Ss, whereas He2 reduces in both cases. Magnetic susceptibility measurements performed for TMC doped with actinides have shown that at 4.2 K X is about 10 -6 emu g-~. In the chalcogenides doped with uranium some singularities near 60 K were detected, similar to that observed in ternary uranium chalcogenides.* Such anomalies were not found in thorium doped chalcogenides, e.g., in ThxPbl_xMo6S 8 with 0.1 ~< x ~< 0.5 the susceptibility obeys Curie-Weiss law, still being lower than 10 -6 emu g-~. As a rule, Sommerfeld constant 7, estimated from dHe2/dT and Pn data diminishes with actinide concentration. For instance in the system UxPbl_xMo6.4S8, 7 = 95 -+ 10 and 68 -+ 7 mJ mole -1K -2 for x = 0.1 and 0.2 respectively. It is not excluded that the decrease of 7 and therefore N(0) may influence the diminution of the critical temperature. As follows from Fig. 1 the rate of decrease of the reduced critical temperature Te(x)/Te(O) is nearly the same for all investigated compounds of P b - M o - S series, apart from their different Te(0) values. For UxLal_xMO6S8 the decrease is much smaller. Comparing the results obtained here with those relating to ternary molybdenum chalcogenides with rare earths [3], it should be marked that whereas the rare earths addition involves in some cases the increase of the critical field (for example, in Pbl_xEuxM06Ss)the addition of actinides leads to the depression of critical
* The presence of small amounts of magnetic uranium phases like US, UOS (see [7]) which might influence the X(T) dependence cannot be excluded.
Vol. 41, No. 7
parameters in all investigated systems. This different influence of rare earths and actinides on the critical parameters of TMC may be related to the peculiarities of the magnetic interactions. To explain the role of such interactions in TMC doped with actinides, additional investigations are necessary, which will be performed in the nearest time.
Acknowledgements - The authors feel indebted to express the acknowledgement to Professor W. Trzebiatowski for his interest in the work and for the helpful discussion, to N.M. Dobrovolsky and T. Mydlarz for assistance in measurements. REFERENCES 1. 2.
3. 4. 5. 6, 7.
N.E. Alekseevskii, A.V. Mitin, C. Bazan, N.M. Dobrovolskii & B. Ronczka, J. Exp. Teor. Phys. 74, 384 (1978). M. Sergent, R. Chevrel, C. Rossel & 0. Fischer, J. Less-Common Metals 58,179 (1978). O. Fischer, Appl. Phys. 16, I (1978). N.E. Alekseevskii, N.M. Dobrovolskii & V.I. Tsebro, Soy. Phys: J. Exper. Teor. Phys. Lett. 20, 50 (1974). K. Troinar, Proc. Magnet. Technology Con£ MT6, p. 410. Bratislava (1977). N.E. Alekseevskii, E.P. Krasnopiorov & V.C. Nazin, DAN USSR 197, 814 (1971). W. Trzebiatowski, Ferromagnetic Materials, (Edited by E.P. Wohlfarth), Vol. 1, p. 415. NorthHolland, Amsterdam (1980).