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The rare-earth metal carbide halide superconductors RE2C2X2 (RE=Y, La; X=Cl, Br, I)
Physica C 317–318 Ž1999. 456–459
The rare-earth metal carbide halide superconductors RE 2 C 2 X 2 žRE s Y, La; X s Cl, Br, I / R.K. Kremer ) , K. Ahn, R.W. Henn 1, Hj. Mattausch, W. Schnelle 2 , A. Stolovits 3, A. Simon Max-Planck-Institut fur Heisenbergstraße 1, D-70569 Stuttgart, Germany ¨ Festkorperforschung, ¨
Abstract Superconductivity in layered yttrium carbide bromides and iodides with transition temperatures ranging up to 11.6 K is achieved by adjusting the Br:I, ratio to f 1:3 in phases of Y2 C 2 ŽBr,I. 2 . In our contribution, we compile basic physical and chemical properties of Y2 C 2 X 2 ŽX s Cl, Br, I. and present new results of the La-based phases La 2 C 2 X 2 ŽX s Br, I.. q 1999 Elsevier Science B.V. All rights reserved. Keywords: Rare-earth metal; Superconductivity; Carbide halide
1. Introduction The discovery of high-Tc superconductivity ŽSC. in the layered oxocuprates again focused considerable effort onto the search for SC in other systems with layered crystal structures. So, SC in electrondoped layered Hf and Zr nitride halides, up to 25.5 K could be achieved recently w1–3x. The crystal structure of the undoped Hf and Zr nitride halides bears some analogy to the structures of the layered rareearth metal ŽRE. carbide halide phases, RE 2 C 2 X 2
) Corresponding author. E-mail: [email protected] 1 New address: Fachbereich Materialwissenschaft, Technische Universitat ¨ Darmstadt, Petersenstr. 23, D-64287 Darmstadt, Germany. 2 New address: Max-Planck-Institut fur ¨ Chemische Physik fester Stoffe, Pirnaer Landstr. 176, D-01257 Dresden, Germany. 3 Permanent address: Institute of Physics, Tartu University, Riia 142, EE-2400 Tartu, Estonia.
ŽRE s Y, La; X s Cl, Br, I., for which we reported bulk SC up to 10 K in the Y- based phases some time ago w4–6x. Here, we shall review some essential properties of the Y2 C 2 X 2 superconductors and report new results for the currently investigated Labased phases, La 2 C 2 X 2 ŽX s Br, I..
2. Experimental The layered RE carbide halides crystallize in the Gd 2 C 2 Br2 structure types w7,8x. Therein, C–C units are located in octahedrally coordinated voids of close-packed metal atom double layers. The metal atom double layers are sandwiched by layers of halogen atoms. Such slabs are connected via the van der Waals interaction. Different stacking sequences Ž1s- and 3s-stacking variants. are found, which differ in the number of slabs necessary to complete a unit cell ŽFig. 1..
0921-4534r99r$ - see front matter q 1999 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 4 5 3 4 Ž 9 9 . 0 0 0 9 8 - 2
R.K. Kremer et al.r Physica C 317–318 (1999) 456–459
457
found somewhat lower than in the Y-based phases at f 7 K w13x.
3. Results and discussion
Fig. 1. Perspective view of the crystal structures of RE 2 C 2 X 2 Ž1s-stacking variant, left and 3s-stacking variant, right.. X, RE and C atoms are drawn with decreasing diameter, respectively. Stacking sequences and unit cells are outlined. ŽA, B, C s X; a , b, gsRE; a, b, c s carbon..
In contrast to the Hf and Zr nitride halides, the phases RE 2 C 2 X 2 are metallic without any intercalation of electron donorsracceptors into the van der Waals gap. However, doping with alkali metals into the van der Waals gap was shown to be possible and an increase of Tc , e.g., of Y2 C 2 Br2 by 1.2 K could be achieved w9,10x. The transition temperatures of mixed halogen phases Y2 C 2 ŽX,X X . 2 ŽX,X X s Cl, Br, I. were found to vary monotonically with the mean halogen radius irrespective of the stacking variant being present in the crystal structure of the particular sample w5,9x. This finding gives strong evidence that the properties of the X–Y–C 2 –Y–X slabs themselves are, in the first place, of importance for the SC in the RE carbide halides. The highest Tc in the RE 2 C 2 X 2 system of 11.6 K which could be reached so far, was obtained by adjusting the Br:I ratio to f 1:3 in phases of Y2 C 2 ŽBr, I. 2 w9,10x. Table 1 summarizes the Tc and the upper critical fields Žwhere available. of some RE 2 C 2 X 2 superconductors. A first exploration of the SC properties of La 2 C 2 X 2 ŽX s Br, I., 4 as well as of halogen mixed phases La 2 C 2 ŽX,X X . 2 has been carried out by Ahn et al. w13x. As in the Y2 C 2 ŽX,X X . 2 phases, Tc in the La 2 C 2 ŽX,X X . 2 phases varies monotonically with the mean halogen radius. The maximum for Tc was
In the following, we present results of some recent experiments performed to gain more insight into the properties of the La 2 C 2 X 2 ŽX s Br, I. superconductors. Details of the sample preparation and the experimental conditions shall be described in a forthcoming publication w12x. La 2 C 2 Br2 Ž1s-type. was reported by Ahn et al. to become a superconductor below 6.2 K ŽTconset . but a very broad transition was found in the DC magnetic susceptibility. Fig. 2 displays a series of resistivity and DC magnetization measurements on a new sample which indicate that Tc of La 2 C 2 Br2 is rather close to 7.07Ž5. K. This marked increase of Tc , which is paralleled by the very sharp resistance and susceptibility transitions to SC, has to be attributed to a distinctly improved sample homogeneity, very likely due to a reduced C deficiency. It could be achieved by using La 2 C 3 rather than elementary C powder in the synthesis of this sample. C deficiency viz. partial substitution of the C 2 group by single C atoms has been shown to reduce Tc very effectively, e.g., in case of YC 2 w14x and also Y2 C 2 I 2 , while Y2 C 2 Br2 appeared to be less susceptible to C 2rC substitution and the Tc of the samples of Y2 C 2 Br2 was largely insensitive to a variation of preparational conditions w5x. The C deficiency in the La 2 C 2 Br2 samples may be tentatively estimated to be a few
Table 1 Transition temperatures Tc and upper critical fields of some Y2 C 2 X 2 and La 2 C 2 X 2 phases Compound Y2 C 2 Cl 2 Y2 C 2 Br2 Y2 C 2 I 2 La 2 C 2 Br2a 1s-La 2 C 2 I b2 3s-La 2 C 2 I b2 a
La 2 C 2 Cl 2 could not be synthesized.
2.3 5.04 Ž1. 9.97 Ž2. 7.07 Ž5. 1.52 Ž2. 1.72 Ž2.
m 0 Hc2 ŽT.
Reference
– 3.0 Ž2. 11.8 Ž2. 4 Ž1. – –
w5x w11x w11x This work w12x, this work w12x, this work
Tc of 6.2 K is reported in Ref. w13x. For a discussion of the origin of the different Tc in the different stacking sequences, see text.
b 4
Tc ŽK.
458
R.K. Kremer et al.r Physica C 317–318 (1999) 456–459
Fig. 2. Electrical resistivity of a sample of La 2 C 2 Br2 Ža. in zero-field, 1 T and 4 T applied external magnetic field. Žb. DC magnetic susceptibility measured in an external field of 1 mT Župper curve: fc, lower curve: 2fc susceptibility..
percent or less since high resolution neutron powder diffraction experiments ŽFig. 3. on a sample
Fig. 3. Observed Žv ., refined Žfull line, Rietveld method. and difference pattern of a sample of La 2 C 2 Br2 ŽTc collected at 1.5 K at the high resolution powder diffractometer D1A at the ILL ŽGrenoble... Vertical bars indicate the positions of the reflections used to simulate the pattern. The broad maximum in the background at low angles is due to diffuse scattering from the quartz glass ampule the sample was contained in.
of La 2 C 2 Br2 ŽTc s 6.6 K. —within the limits of the sensitivity of this method—failed to reveal any occupation of the octahedra centres by single C atoms. In contrast to Y2 C 2 I 2 , samples of La 2 C 2 I 2 show a markedly lower Tc . Surprisingly, there is a difference in Tc for the samples with different stacking sequences ŽFig. 4., which is unprecedented in the Y-based phases. There, the Tc of mixed halide samples, Y2 C 2 ŽX,X X . 2 , smoothly pass over two induced changes of the stacking sequence without showing any non-monotonic steps. The difference in Tc for the 1s- and 3s-La 2 C 2 I 2 samples could hint to an additional sensitivity of the electronic conditions in the I–La–C 2 –La–I slabs to packing effects in the halogen atom double layer. However, we find that synthesis of pure 3s-type samples of La 2 C 2 I 2 can only be achieved by addition of a considerable excess of carbon while 1s-type samples, with some addition of the 3s-type phase, result when carbon excess is reduced and the educts are composed close to the ratio required by stoichiometry. This finding seems to indicate that the 1s-type samples should rather be characterized by a carbon deficiency, La 2 C 2yx I 2 . Since Rietveld refinements of high-resolution neutron powder patterns of both samples of Fig. 4 w12x gave no evidence of any carbon non-stoichiometry, we estimate the carbon deficiency to be of the order of a few percent or less. Such a slight sub-stoichiometric carbon content may easily be achieved by a substitution of C 24y groups by isoelec-
Fig. 4. AC-susceptibility of a sample of 1s-La 2 C 2 I 2 Ž`. and 3s-La 2 C 2 I 2 Žq..
R.K. Kremer et al.r Physica C 317–318 (1999) 456–459
tronic C 4y anions without noticeably altering the crystal structure. In fact, as experienced in the case of Y2 C 2 I 2 , partial replacement of C 2 by C 1 leads to a marginal decrease of the lattice parameters only, while it may induce a marked reduction of Tc w5x.
4. Conclusion In summary, the Y and La carbide halide superconductors form an interesting new family of layered superconductors. The transition temperatures range up to 11.6 K and to f 7 K in the Y and La based phases, respectively. C deficiency, even of the order of a few percentage appears to be detrimental to the superconducting properties of these phases. By exploring various synthesis routes and after a careful optimization, we were able to obtain phase pure products, now also of the La-based phases, which exhibit sharp transitions to superconductivity and complete flux exclusion. A comparison with the properties of the superconducting RE-carbides allow conclusions as to the role of the structural anisotropy on the superconducting properties of the RE carbide halides.
459
References w1x S. Yamanaka, K. Hotehama, H. Kawaji, Nature 392 Ž1998. 580. w2x H. Kawaji, K. Hotehama, S. Yamanaka, Chem. Mater. 9 Ž1997. 2127. w3x S. Yamanaka, H. Kawaji, K. Hotehama, M. Ohashi, Adv. Mater. 8 Ž1996. 771. w4x A. Simon, Hj. Mattausch, R. Eger, R.K. Kremer, Angew. Chem. Int. Ed. Engl. 30 Ž1991. 1188. w5x A. Simon, A. Yoshiasa, M. Backer, R.W. Henn, C. Felser, ¨ R.K. Kremer, Hj. Mattausch, Z. Anorg. Allg. Chem. 622 Ž1996. 123. w6x R.W. Henn, W. Schnelle, R.K. Kremer, A. Simon, Phys. Rev. Lett. 77 Ž1996. 374. w7x U. Schwanitz-Schuller, A. Simon, Z. Naturforsch. B 40 ¨ Ž1985. 710. w8x Hj. Mattausch, R.K. Kremer, R. Eger, A. Simon, Z. Anorg. Allg. Chem. 609 Ž1992. 7. w9x M. Backer, Thesis, University Stuttgart, 1997. ¨ w10x M. Backer, A. Simon, R.K. Kremer, Hj. Mattausch, R. ¨ Dronskowski, J. Rouxel, Angew. Chem. Int. Ed. Engl. 35 Ž1996. 752. w11x R.W. Henn, Thesis, University Karlsruhe ŽTH., 1996. w12x K. Ahn, B. Gibson, R.K. Kremer, Hj. Mattausch, A. Stolovits, A. Simon, submitted. w13x K. Ahn, Hj. Mattausch, A. Simon, Z. Anorg. Allg. Chem. 623 Ž1997. 619. w14x Th. Gulden, R.W. Henn, O. Jepsen, R.K. Kremer, W. Schnelle, A. Simon, Phys. Rev. B 56 Ž1997. 9021.
The rare-earth metal carbide halide superconductors RE 2 C 2 X 2 žRE s Y, La; X s Cl, Br, I / R.K. Kremer ) , K. Ahn, R.W. Henn 1, Hj. Mattausch, W. Schnelle 2 , A. Stolovits 3, A. Simon Max-Planck-Institut fur Heisenbergstraße 1, D-70569 Stuttgart, Germany ¨ Festkorperforschung, ¨
Abstract Superconductivity in layered yttrium carbide bromides and iodides with transition temperatures ranging up to 11.6 K is achieved by adjusting the Br:I, ratio to f 1:3 in phases of Y2 C 2 ŽBr,I. 2 . In our contribution, we compile basic physical and chemical properties of Y2 C 2 X 2 ŽX s Cl, Br, I. and present new results of the La-based phases La 2 C 2 X 2 ŽX s Br, I.. q 1999 Elsevier Science B.V. All rights reserved. Keywords: Rare-earth metal; Superconductivity; Carbide halide
1. Introduction The discovery of high-Tc superconductivity ŽSC. in the layered oxocuprates again focused considerable effort onto the search for SC in other systems with layered crystal structures. So, SC in electrondoped layered Hf and Zr nitride halides, up to 25.5 K could be achieved recently w1–3x. The crystal structure of the undoped Hf and Zr nitride halides bears some analogy to the structures of the layered rareearth metal ŽRE. carbide halide phases, RE 2 C 2 X 2
) Corresponding author. E-mail: [email protected] 1 New address: Fachbereich Materialwissenschaft, Technische Universitat ¨ Darmstadt, Petersenstr. 23, D-64287 Darmstadt, Germany. 2 New address: Max-Planck-Institut fur ¨ Chemische Physik fester Stoffe, Pirnaer Landstr. 176, D-01257 Dresden, Germany. 3 Permanent address: Institute of Physics, Tartu University, Riia 142, EE-2400 Tartu, Estonia.
ŽRE s Y, La; X s Cl, Br, I., for which we reported bulk SC up to 10 K in the Y- based phases some time ago w4–6x. Here, we shall review some essential properties of the Y2 C 2 X 2 superconductors and report new results for the currently investigated Labased phases, La 2 C 2 X 2 ŽX s Br, I..
2. Experimental The layered RE carbide halides crystallize in the Gd 2 C 2 Br2 structure types w7,8x. Therein, C–C units are located in octahedrally coordinated voids of close-packed metal atom double layers. The metal atom double layers are sandwiched by layers of halogen atoms. Such slabs are connected via the van der Waals interaction. Different stacking sequences Ž1s- and 3s-stacking variants. are found, which differ in the number of slabs necessary to complete a unit cell ŽFig. 1..
0921-4534r99r$ - see front matter q 1999 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 4 5 3 4 Ž 9 9 . 0 0 0 9 8 - 2
R.K. Kremer et al.r Physica C 317–318 (1999) 456–459
457
found somewhat lower than in the Y-based phases at f 7 K w13x.
3. Results and discussion
Fig. 1. Perspective view of the crystal structures of RE 2 C 2 X 2 Ž1s-stacking variant, left and 3s-stacking variant, right.. X, RE and C atoms are drawn with decreasing diameter, respectively. Stacking sequences and unit cells are outlined. ŽA, B, C s X; a , b, gsRE; a, b, c s carbon..
In contrast to the Hf and Zr nitride halides, the phases RE 2 C 2 X 2 are metallic without any intercalation of electron donorsracceptors into the van der Waals gap. However, doping with alkali metals into the van der Waals gap was shown to be possible and an increase of Tc , e.g., of Y2 C 2 Br2 by 1.2 K could be achieved w9,10x. The transition temperatures of mixed halogen phases Y2 C 2 ŽX,X X . 2 ŽX,X X s Cl, Br, I. were found to vary monotonically with the mean halogen radius irrespective of the stacking variant being present in the crystal structure of the particular sample w5,9x. This finding gives strong evidence that the properties of the X–Y–C 2 –Y–X slabs themselves are, in the first place, of importance for the SC in the RE carbide halides. The highest Tc in the RE 2 C 2 X 2 system of 11.6 K which could be reached so far, was obtained by adjusting the Br:I ratio to f 1:3 in phases of Y2 C 2 ŽBr, I. 2 w9,10x. Table 1 summarizes the Tc and the upper critical fields Žwhere available. of some RE 2 C 2 X 2 superconductors. A first exploration of the SC properties of La 2 C 2 X 2 ŽX s Br, I., 4 as well as of halogen mixed phases La 2 C 2 ŽX,X X . 2 has been carried out by Ahn et al. w13x. As in the Y2 C 2 ŽX,X X . 2 phases, Tc in the La 2 C 2 ŽX,X X . 2 phases varies monotonically with the mean halogen radius. The maximum for Tc was
In the following, we present results of some recent experiments performed to gain more insight into the properties of the La 2 C 2 X 2 ŽX s Br, I. superconductors. Details of the sample preparation and the experimental conditions shall be described in a forthcoming publication w12x. La 2 C 2 Br2 Ž1s-type. was reported by Ahn et al. to become a superconductor below 6.2 K ŽTconset . but a very broad transition was found in the DC magnetic susceptibility. Fig. 2 displays a series of resistivity and DC magnetization measurements on a new sample which indicate that Tc of La 2 C 2 Br2 is rather close to 7.07Ž5. K. This marked increase of Tc , which is paralleled by the very sharp resistance and susceptibility transitions to SC, has to be attributed to a distinctly improved sample homogeneity, very likely due to a reduced C deficiency. It could be achieved by using La 2 C 3 rather than elementary C powder in the synthesis of this sample. C deficiency viz. partial substitution of the C 2 group by single C atoms has been shown to reduce Tc very effectively, e.g., in case of YC 2 w14x and also Y2 C 2 I 2 , while Y2 C 2 Br2 appeared to be less susceptible to C 2rC substitution and the Tc of the samples of Y2 C 2 Br2 was largely insensitive to a variation of preparational conditions w5x. The C deficiency in the La 2 C 2 Br2 samples may be tentatively estimated to be a few
Table 1 Transition temperatures Tc and upper critical fields of some Y2 C 2 X 2 and La 2 C 2 X 2 phases Compound Y2 C 2 Cl 2 Y2 C 2 Br2 Y2 C 2 I 2 La 2 C 2 Br2a 1s-La 2 C 2 I b2 3s-La 2 C 2 I b2 a
La 2 C 2 Cl 2 could not be synthesized.
2.3 5.04 Ž1. 9.97 Ž2. 7.07 Ž5. 1.52 Ž2. 1.72 Ž2.
m 0 Hc2 ŽT.
Reference
– 3.0 Ž2. 11.8 Ž2. 4 Ž1. – –
w5x w11x w11x This work w12x, this work w12x, this work
Tc of 6.2 K is reported in Ref. w13x. For a discussion of the origin of the different Tc in the different stacking sequences, see text.
b 4
Tc ŽK.
458
R.K. Kremer et al.r Physica C 317–318 (1999) 456–459
Fig. 2. Electrical resistivity of a sample of La 2 C 2 Br2 Ža. in zero-field, 1 T and 4 T applied external magnetic field. Žb. DC magnetic susceptibility measured in an external field of 1 mT Župper curve: fc, lower curve: 2fc susceptibility..
percent or less since high resolution neutron powder diffraction experiments ŽFig. 3. on a sample
Fig. 3. Observed Žv ., refined Žfull line, Rietveld method. and difference pattern of a sample of La 2 C 2 Br2 ŽTc collected at 1.5 K at the high resolution powder diffractometer D1A at the ILL ŽGrenoble... Vertical bars indicate the positions of the reflections used to simulate the pattern. The broad maximum in the background at low angles is due to diffuse scattering from the quartz glass ampule the sample was contained in.
of La 2 C 2 Br2 ŽTc s 6.6 K. —within the limits of the sensitivity of this method—failed to reveal any occupation of the octahedra centres by single C atoms. In contrast to Y2 C 2 I 2 , samples of La 2 C 2 I 2 show a markedly lower Tc . Surprisingly, there is a difference in Tc for the samples with different stacking sequences ŽFig. 4., which is unprecedented in the Y-based phases. There, the Tc of mixed halide samples, Y2 C 2 ŽX,X X . 2 , smoothly pass over two induced changes of the stacking sequence without showing any non-monotonic steps. The difference in Tc for the 1s- and 3s-La 2 C 2 I 2 samples could hint to an additional sensitivity of the electronic conditions in the I–La–C 2 –La–I slabs to packing effects in the halogen atom double layer. However, we find that synthesis of pure 3s-type samples of La 2 C 2 I 2 can only be achieved by addition of a considerable excess of carbon while 1s-type samples, with some addition of the 3s-type phase, result when carbon excess is reduced and the educts are composed close to the ratio required by stoichiometry. This finding seems to indicate that the 1s-type samples should rather be characterized by a carbon deficiency, La 2 C 2yx I 2 . Since Rietveld refinements of high-resolution neutron powder patterns of both samples of Fig. 4 w12x gave no evidence of any carbon non-stoichiometry, we estimate the carbon deficiency to be of the order of a few percent or less. Such a slight sub-stoichiometric carbon content may easily be achieved by a substitution of C 24y groups by isoelec-
Fig. 4. AC-susceptibility of a sample of 1s-La 2 C 2 I 2 Ž`. and 3s-La 2 C 2 I 2 Žq..
R.K. Kremer et al.r Physica C 317–318 (1999) 456–459
tronic C 4y anions without noticeably altering the crystal structure. In fact, as experienced in the case of Y2 C 2 I 2 , partial replacement of C 2 by C 1 leads to a marginal decrease of the lattice parameters only, while it may induce a marked reduction of Tc w5x.
4. Conclusion In summary, the Y and La carbide halide superconductors form an interesting new family of layered superconductors. The transition temperatures range up to 11.6 K and to f 7 K in the Y and La based phases, respectively. C deficiency, even of the order of a few percentage appears to be detrimental to the superconducting properties of these phases. By exploring various synthesis routes and after a careful optimization, we were able to obtain phase pure products, now also of the La-based phases, which exhibit sharp transitions to superconductivity and complete flux exclusion. A comparison with the properties of the superconducting RE-carbides allow conclusions as to the role of the structural anisotropy on the superconducting properties of the RE carbide halides.
459
References w1x S. Yamanaka, K. Hotehama, H. Kawaji, Nature 392 Ž1998. 580. w2x H. Kawaji, K. Hotehama, S. Yamanaka, Chem. Mater. 9 Ž1997. 2127. w3x S. Yamanaka, H. Kawaji, K. Hotehama, M. Ohashi, Adv. Mater. 8 Ž1996. 771. w4x A. Simon, Hj. Mattausch, R. Eger, R.K. Kremer, Angew. Chem. Int. Ed. Engl. 30 Ž1991. 1188. w5x A. Simon, A. Yoshiasa, M. Backer, R.W. Henn, C. Felser, ¨ R.K. Kremer, Hj. Mattausch, Z. Anorg. Allg. Chem. 622 Ž1996. 123. w6x R.W. Henn, W. Schnelle, R.K. Kremer, A. Simon, Phys. Rev. Lett. 77 Ž1996. 374. w7x U. Schwanitz-Schuller, A. Simon, Z. Naturforsch. B 40 ¨ Ž1985. 710. w8x Hj. Mattausch, R.K. Kremer, R. Eger, A. Simon, Z. Anorg. Allg. Chem. 609 Ž1992. 7. w9x M. Backer, Thesis, University Stuttgart, 1997. ¨ w10x M. Backer, A. Simon, R.K. Kremer, Hj. Mattausch, R. ¨ Dronskowski, J. Rouxel, Angew. Chem. Int. Ed. Engl. 35 Ž1996. 752. w11x R.W. Henn, Thesis, University Karlsruhe ŽTH., 1996. w12x K. Ahn, B. Gibson, R.K. Kremer, Hj. Mattausch, A. Stolovits, A. Simon, submitted. w13x K. Ahn, Hj. Mattausch, A. Simon, Z. Anorg. Allg. Chem. 623 Ž1997. 619. w14x Th. Gulden, R.W. Henn, O. Jepsen, R.K. Kremer, W. Schnelle, A. Simon, Phys. Rev. B 56 Ž1997. 9021.