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Influence of crystal growth regimes on the structure and properties of Cu-intercalated Ta1+yS2
Physica C: Superconductivity and its applications 539 (2017) 35–39
Contents lists available at ScienceDirect
Physica C: Superconductivity and its applications journal homepage: www.elsevier.com/locate/physc
Influence of crystal growth regimes on the structure and properties of Cu-intercalated Ta1+ y S2 V. Antal∗, V. Kavecˇ anský, J. Kacˇ marcˇ ík, P. Diko Institute of Experimental Physics, Slovak Academy of Sciences, Watsonova 47, 04001 Košice, Slovakia
a r t i c l e
i n f o
Article history: Received 23 February 2017 Revised 17 May 2017 Accepted 30 June 2017 Available online 1 July 2017 Keywords: Dichalcogenides Superconductors TaS2 Intercalation
a b s t r a c t Ta1+ y S2 and Cu-intercalated Ta1+ y S2 crystals with high Cu concentration were grown by the chemical vapour transport method with different crystal growth regimes. Depending on the cooling process either 2H-type or 6R-type Cux Ta1+ y S2 crystals were grown. Transformation of 2H-Cux Ta1+ y S2 polytype crystals to 6R-Cux Ta1+ y S2 takes place during a low cooling process, whereas during air quenching 2H-type crystals result. It was shown that cooling process and additional annealing steps in crystal growth regimes have an essential influence on the structure of the samples and their physical properties such as superconductivity. The samples were examined by X-ray diffraction, energy dispersive X-ray spectroscopy and electrical resistivity measurements. © 2017 Published by Elsevier B.V.
1. Introduction Combinations of the transition metals from IVB, VB and VIB groups, such as Ta, Ti, Nb, Mo and others with S, Se and Te chalcogens, are well known in the scientific world as transition metal dichalcogenides (TMDC) and trichalcogenides (TMTC) [1,2]. These materials have been studied comprehensively for decades and interest in their investigation is still active. At present these materials are of great interest to physicists because of the possibility of their becoming superconductive and the presence of a charge density wave (CDW) transition [3]. For our investigations we choose TaS2 composition. Reaction of tantalum with sulphur vapour has been widely studied and it has been observed that this reaction takes place at a wide temperature range from about 450 °C up to 10 0 0 °C [2,4]. Different polytypes of TaS2 can be obtained [5] with the layered structure typical of TMDC [6] that indicates superconductivity and CDW. For example, the 2H-TaS2 polytype becomes superconductive below 0.8 K [7,8] and indicates CDW transition at about 78 K [9,10], the 6RTaS2 polytype becomes superconductive below 2.3 K and has two CDW transition temperatures at about 80 K and 325 K [11], and the 1T-TaS2 polytype does not display any superconductivity and has three CDW transition temperatures [12,13]. Intercalation [6,14], self–intercalation [5,15,16] or possible partial substitution of the transition metals in TMDC, as for example in the case of Ta sub-
∗
Corresponding author. E-mail address: [email protected] (V. Antal).
http://dx.doi.org/10.1016/j.physc.2017.06.007 0921-4534/© 2017 Published by Elsevier B.V.
stitution by Ni in TaS2 [17], has an influence on the fluctuation or inhibition of CDW transition and superconducting properties [18]. A lot of issues concerning the origin of superconductivity and CDW transition in intercalated TMDC have been reported. Beginning with the first principles of electronic structure calculations, whereby a change in the effective density of states (DOS) of the Fermi surface [19] and the carrier density were considered, charge transfer from intercalated atoms or molecules to the host layers of TMDC as well as electron-phonon coupling have been described [6,18,20,21]. However, the question of superconductivity and CDW transition in TMDC remains a subject for discussion and investigation. We concentrated our work on the preparation and investigation of Ta1+ y S2 crystals grown by the chemical vapour transport (CVT) method. After success preparation of Ta1+ y S2 crystals, Cu-intercalated Ta1+ y S2 crystals were prepared with three different crystal growth regimes. Our experiments confirmed that the additional annealing and cooling process of the crystal growth regime could have a substantial influence on the crystal structure of Cux Ta1+ y S2 and its physical properties. Also, we noted the presence of superconductivity in Cux Ta1+ y S2 crystals with a high concentration of Cu. 2. Experimental details The samples were prepared by the CVT method in a threezone tube furnace. Three evacuated quartz tubes were sealed with the same amount of TaS2 powder and Cu powder at a mol ratio of 1:0.5 and one quartz tube was sealed only with TaS2 powder. TaS2 powder, used as a precursor, was prepared by sintering
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V. Antal et al. / Physica C: Superconductivity and its applications 539 (2017) 35–39
Fig. 1. Scanning electron image of sample 1 that indicates the typical layered structure of transition metal dichalcogenides.
the necessary amount of high-purity Ta and S in an evacuated quartz tube at 900 °C for 96 h. As the transport agent we used iodine in the amount of about miodine /Vampule ≈ 5 mg ml−1 . Three different crystal growth regimes were used, but the first step in all regimes was the same. The first growth regime (Sample 1, Sample 2) Step 1: crystal growth for 10 days in 100 °C gradient (10 0 0 °C → 900 °C), between the source part and the growth part of the tube. Step 2: cooling down after Step 1 to 400 °C over about two days (cooling rate about 10 °C/h) and finally furnace cooling. The second growth regime (Sample 3) Step 1: the same as Step 1 in the first growth regime. Step 2: air quenching to room temperature by rapid removal of the quartz tube from the furnace. The third growth regime (Sample 4) Step 1: the same as Step 1 in the first growth regime. Step 2: the temperatures at both sides of the tube reduced by 100 °C and maintenance of this temperature for 22 h. Step 3: air quenching to room temperature by rapid removal of the quartz tube from the furnace. After growth, all samples were cleaned by ultrasonication in distilled water and ethanol. Prepared samples were analysed by scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDX). The X-ray diffraction analyses were performed by a Rigaku D/MAX Rapid II X-ray diffractometer with MoKα radiation. The ab in-plane electrical resistivity measurements as a function of temperature were obtained by the four-probe method at a temperature range of 0.4 K ≤ T ≤ 300 K.
were taken from different parts of the samples. It could be clearly seen that for all samples there was an excess of Ta. An excess of transition metals (self-intercalation) in TMDC was observed in early investigations, an excess caused by the presence of the metals in the Van der Waals gaps [5,16,22]. The EDX analyses of all samples confirmed that concentrations of Cu and Ta were not homogeneously distributed within the samples and small deviations in their concentrations were observed (Table 1). However, the EDX elemental mapping did not show any regions with Ta, S or Cu that would be associated with sharp changes of chemical composition of the crystals (Fig. 3). We may suppose that small deviations were caused by diffusive compositional macroscopic inhomogeneity on the level which cannot be detected by the EDX mapping. High concentrations of Cu were found in all Cux Ta1+ y S2 samples; the values vary, but have a similar concentration range. However, Ta concentration with essential excess was determined in samples 1 and 3–4 but sample 2 showed a reduced amount. The crystallographic cell parameters (Table 1) were determined by Rietveld refinement calculations from the X-ray patterns obtained at room temperature (Fig. 4). We can see that the lattice structure of sample 2 differs from that of the other samples. Samples 1 and 3–4 have hexagonal symmetry and a 2H-type crystal structure, whereas sample 2 has a 6R-type crystal structure and rhombohedral symmetry [22]. All samples were single-phase. Earlier investigations of 2H-TaS2 samples indicated that the aaxis and the c-axis were equal to 3.315 A˚ and 12.10 A˚ [5], 3.314 A˚ and 12.097 A˚ [23] or 3.316 A˚ and 12.070 A˚ [24], respectively. More recent results have reported an a-axis and c-axis of 3.31 A˚ and 12.08 A˚ [20,25], respectively, and these results are in close agreement with earlier investigations. The a-axis parameters of samples 1 and 3–4 are similar to published data for 2H-TaS2 samples, but because of the intercalation of Cu atoms in the Van der Waals gaps [25] elongation of the c-axis was evidenced in samples 3–4 in comparison with the 2H-TaS2 sample. If we compare the crystal lattice parameters of sample 2 with the literature data, where for the 6R-TaS2 polytype the a-axis and the c-axis are equal to 3.335 A˚ and 35.85 A˚ [5] or 3.335 A˚ and 35.622 A˚ [26], respectively, we can see slight decreasing of the aaxis and appreciable increasing of the c-axis. Similar behaviour by the crystal lattice parameters caused by Cu intercalation in 6RCux Ta1+ y S2 that corresponds to that of our sample 2 has been reviewed in the literature [15,22]. It is clear from the EDX and X-ray diffraction results (Table 1), that appreciable changes in the crystal structure and chemical composition of Cux Ta1+ y S2 samples were caused by additional annealing or different cooling processes after Step 1 of the crystal growth regime. If we compare sample 1 with sample 2, it seems that during the first growth regime sample 2 transformed from the 2H-type to the 6R-type during the cooling process. From these results we assume that the initial 2H-type structure formed in Step 1 of the crystal growth regime for samples 3–4 remained unchanged because of air quenching after the crystal growth. Also, we assume that additional annealing processes and different cooling rates after Step 1 of the crystal growth regimes could influence the Ta excess in the samples (Table 1), but it is not clear to us why there is a tendency towards high Ta excess in samples 1 and 3–4 and low Ta excess in sample 2.
3. Results and discussion 3.1. Chemical and structural analyses
3.2. Resistivity measurements
Scanning electron microscopy investigations confirmed the typical layered structure of the prepared samples (Fig. 1) and the chemical composition of the samples deduced from the EDX patterns (Fig. 2) is listed in Table 1. In order to determine more precisely the chemical composition of the samples, the EDX patterns
The electrical resistivity measurements of all prepared samples are shown in Fig. 5. As mentioned in introduction the superconducting transition temperature, Tc , for the 2H-TaS2 sample is about 0.8 K [7,8] and about 2.3 K for the 6R-TaS2 sample [11]. Superconducting transitions were not reported in early investigations
V. Antal et al. / Physica C: Superconductivity and its applications 539 (2017) 35–39
Fig. 2. The EDX patterns and macrographs (inserted photos) of sample 1 (a), sample 2 (b), sample 3 (c), and sample 4 (d).
Table 1 The crystallographic parameters and resistivity measurement results.
Chem.form. x y Cryst. syst. Space gr. ˚ a (A) ˚ c (A) V (A˚ 3 ) Z Tc,onset (K) TCDW (K)
Sample 1
Sample 2
Sample 3
Sample 4
Ta1+ y S2 – 0.25 ≤ y ≤ 0.34 Hexagonal P63/mmc 3.308(7) 12.09(3) 114.6(4) 2 2.62 72
Cux Ta1+ y S2 0.30 ≤ x ≤ 0.31 0.03 ≤ y ≤ 0.04 Rhombohedral R-3m 3.3046(18) 37.6053(6) 355.65(2) 6 0.71 –
Cux Ta1+ y S2 0.29 ≤ x ≤ 0.34 0.28 ≤ y ≤ 0.30 Hexagonal P63/mmc 3.3222(11) 12.9254(5) 123.553(8) 2 – –
Cux Ta1+ y S2 0.28 ≤ x ≤ 0.29 0.27 ≤ y ≤ 0.29 Hexagonal P63/mmc 3.32705(9) 12.9212(18) 123.866(5) 2 0.42 –
Fig. 3. The EDX elemental mapping of sample 1 (a) and sample 2 (b). Samples 3–4 have similar mapping to sample 2.
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V. Antal et al. / Physica C: Superconductivity and its applications 539 (2017) 35–39
Fig. 4. The X-ray diffraction patterns of sample 1 (a), sample 2 (b), sample 3 (c), and sample 4 (d). Upper lines indicate experementally obrained X-ray pattren (stars) and calculated pattern (solid red line); middle bars indicates calculated position of the peaks; bottom line indicates calculated difference between experimental and calculated patterns.
of Cux Ta1+ y S2 samples with high concentrations of Cu, x > 0.23 [15,22,27,28]. The onset transition temperature, Tc,onset , from the superconducting to normal state for sample 1 is about 2.62 K (Fig. 6), whereas the midpoint transition temperature, Tc, 0.5 , is about 1.52 K and the transition width (10%–90%) is about 0.82 K. Wagner et al. [21] explained increase in Tc above 1 K in 2H-TaS2 because of Ta excess or sub-1% impurity atoms present in the Van der Waals gaps. In 2H-Cux Ta1+ y S2 samples Tc is enhanced by up to 2.5 K because of initial effects such as changing of the interlayer coupling or disruption to the ordering of CDW. Further enhancement of Tc up to about 4.5 K is caused by electronic doping of the system. Decreasing Tc with increasing Cu content above optimal concentration has been explained by decreasing DOS [21]. Despite of Ta excess, sample 1 displays a kink on the electrical resistivity curve at around 72 K that attributes to the CDW transition, whereas any CDW transitions observed in samples 2 to 4 up to 300 K (Fig. 5). From these results it seems that sample 1 behaves like Cu0.01 TaS2
sample [21], but probably because of high Ta excess we have broad transition width. The electrical resistivity of sample 2 dropped down at about 0.71 K and that of sample 4 at about 0.42 K, corresponding to the superconducting transition. It was found that optimal concentrations of Cu in 2H-Cux Ta1+ y S2 samples, where CDW and superconductivity could coexist with the highest values of Tc , were observed at very low Cu concentrations of less than x ≤ 0.04 [17,21,25]. A lot of literature data indicates that the CDW transition is suppressed at lower temperatures or is inhibited by increasing Cu concentration and after complete inhibition of CDW the electrical resistivity curves of TMDC display a metallic behaviour. It was assumed that suppression of CDW is the result of the influence of intercalated atoms on the c-axis conduction in 2H-TaS2 that is reflected by increasing DOS at the Fermi surface with further enhancement of Tc [6,20]. As discussed in Section 3.1, sample 2 has 6R-TaS2 polytype structure that generally has higher Tc in comparison to 2H-TaS2 polytype structure. We suppose that additional annealing of sam-
V. Antal et al. / Physica C: Superconductivity and its applications 539 (2017) 35–39
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4. Conclusions In summary, it was found that different types of Cux Ta1+ y S2 structures were formed from the same starting powder precursors and caused by additional annealing processes or different cooling rates from the crystal growth regime. Transformation of the crystal structure from the 2H-type Cu-intercalated TaS2 to the 6Rtype could be obtained by slow cooling and initial crystal structure could be preserved by fast cooling. The obtained results showed that additional annealing or slow cooling could induce superconductivity in Cux Ta1+ y S2 samples with high Cu concentration up to about 30% of impurity. Acknowledgements
Fig. 5. Normalized temperature dependence of the electrical resistivity of all samples. TCDW indicates the CDW transition in sample 1.
This work was realized within the framework of the projects: Centre of Excellence of Advanced Materials with Nano- and Submicron Structure (ITMS 26220120019), Infrastructure Improving of Centre of Excellence of Advanced Materials with Nano- and Submicron Structure (ITMS 26220120035), New Materials and Technologies for Energetic (ITMS 26220220061), Research and Development of Second Generation YBCO Bulk Superconductors (ITMS 26220220041), APVV No. 0330-12, VEGA No. 2/0121/16, VEGA 2/0149/16, APVV-14-0605, Stefanik Project SK-FR-2013-0025, SAS Centre of Excellence: CFNT MVEP, PhysNet (ITMS 26110230097) and NANOKOP (ITMS 26110230061). References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15]
Fig. 6. The electrical resistivity in the vicinity of the superconducting transition temperature.
ple 4 in Step 2 of the crystal growth regime induces small structural changes (too small for detection by the X-ray and the EDX) that provoke superconductivity. Whereas, superconductivity in sample 2 was probably induced by transformation of the crystal structure from the 2H-type to the 6R-type during slow cooling down after Step 2 of the crystal growth regime and lower Ta excess in comparison to sample 4. Sample 3 did not display any transitions that could be associated with superconductivity in the temperature range from 0.4 K. It should be noted that superconducting properties of 6RCux Ta1+ y S2 polytype samples seem to be investigated less often than those of the 2H-Cux Ta1+ y S2 polytype.
[16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28]
D.E. Moncton, J.D. Axe, F.J. DiSalvo, Phys. Rev. B 16 (1977) 801. F. Lévy, H. Berger, J. Cryst. Growth 61 (1983) 61. T.E. Kidd, T. Miller, M.Y. Chou, T.-C. Chiang, Phys. Rev. Lett. 88 (2002) 226402. J.E. Dutrizac, J. Less Common Metals 85 (1982) 55. F. Jellinek, J. Less Common Metals 4 (1962) 9. L. Fang, et al., Phys. Rev. B 72 (2005) 014534. S. Nagata, T. Aochi, T. Abe, S. Ebisu, T. Hagino, Y. Seki, K. Tsutsumi, J. Phys. Chem. Solids 53 (1992) 1259. L.F. Mattheiss, Phys. Rev. B 8 (1973) 3719. G.A. Scholz, O. Singh, R.F. Frindt, A.E. Curzon, Solid State Commun. 44 (1982) 1455. J.M.E. Harper, T.H. Geballe, F.J. DiSalvo, Phys. Rev. B 15 (1977) 2943. E. Figueroa, et al., J. Solid State Chem. 114 (1995) 486. A. Spijkerman, J.L. de Boer, A. Meetsma, G.A. Wiegers, S. van Smaalen, Phys. Rev. B 56 (1997) 13757. C.B. Scruby, P.M. Williams, Philos. Mag. 31 (1975) 255. R.M. Fleming, R.V. Coleman, Phys. Rev. Lett. 34 (1975) 1502. S.I. Ali, S. Mondal, S.J. Prathapa, S. van Smaalen, S. Zörb, B. Harbrecht, Z. Anorg. Allg. Chem. 638 (15) (2012) 2625. Y. Gotoh, J. Akimoto, Y. Oosawa, J. Alloys Compd. 270 (1998) 115. X.D. Zhu, et al., Solid State Commun. 149 (2009) 1296. W.Z. Hu, G. Li, J. Yan, H.H. Wen, G. Wu, X.H. Chen, N.L. Wang, Phys. Rev. B 76 (2007) 045103. G. Wexler, A.M. Woolley, J. Phys. C 9 (1976) 1185. X. Zhu, et al., J. Phys. 21 (2009) 145701. K.E. Wagner, et al., Phys. Rev. B 78 (2008) 104520. B. Harbrecht, G. Kreiner, Z. Anorg. Allg. Chem. 572 (1989) 47. A. Meetsma, G.A. Wiegers, R.J. Haange, J.L. de Boer, Acta Crystallogr. C 46 (1990) 1598. L.E. Conroy, K.R. Pisharody, J. Solid State Chem. 4 (1972) 345. X.D. Zhu, et al., J. Cryst. Growth 311 (2008) 218. A.H. Thompson, Solid State Commun. 17 (1975) 1115. R. de Ridder, G. van Tendeloo, J. van Landuyt, D. van Dyck, S. Amelinckx, Phys. Stat. Sol. 37 (1976) 591. C. Ramos, A. Lerf, T. Butz, Hyperfine Interactions 61 (1990) 1209.
Contents lists available at ScienceDirect
Physica C: Superconductivity and its applications journal homepage: www.elsevier.com/locate/physc
Influence of crystal growth regimes on the structure and properties of Cu-intercalated Ta1+ y S2 V. Antal∗, V. Kavecˇ anský, J. Kacˇ marcˇ ík, P. Diko Institute of Experimental Physics, Slovak Academy of Sciences, Watsonova 47, 04001 Košice, Slovakia
a r t i c l e
i n f o
Article history: Received 23 February 2017 Revised 17 May 2017 Accepted 30 June 2017 Available online 1 July 2017 Keywords: Dichalcogenides Superconductors TaS2 Intercalation
a b s t r a c t Ta1+ y S2 and Cu-intercalated Ta1+ y S2 crystals with high Cu concentration were grown by the chemical vapour transport method with different crystal growth regimes. Depending on the cooling process either 2H-type or 6R-type Cux Ta1+ y S2 crystals were grown. Transformation of 2H-Cux Ta1+ y S2 polytype crystals to 6R-Cux Ta1+ y S2 takes place during a low cooling process, whereas during air quenching 2H-type crystals result. It was shown that cooling process and additional annealing steps in crystal growth regimes have an essential influence on the structure of the samples and their physical properties such as superconductivity. The samples were examined by X-ray diffraction, energy dispersive X-ray spectroscopy and electrical resistivity measurements. © 2017 Published by Elsevier B.V.
1. Introduction Combinations of the transition metals from IVB, VB and VIB groups, such as Ta, Ti, Nb, Mo and others with S, Se and Te chalcogens, are well known in the scientific world as transition metal dichalcogenides (TMDC) and trichalcogenides (TMTC) [1,2]. These materials have been studied comprehensively for decades and interest in their investigation is still active. At present these materials are of great interest to physicists because of the possibility of their becoming superconductive and the presence of a charge density wave (CDW) transition [3]. For our investigations we choose TaS2 composition. Reaction of tantalum with sulphur vapour has been widely studied and it has been observed that this reaction takes place at a wide temperature range from about 450 °C up to 10 0 0 °C [2,4]. Different polytypes of TaS2 can be obtained [5] with the layered structure typical of TMDC [6] that indicates superconductivity and CDW. For example, the 2H-TaS2 polytype becomes superconductive below 0.8 K [7,8] and indicates CDW transition at about 78 K [9,10], the 6RTaS2 polytype becomes superconductive below 2.3 K and has two CDW transition temperatures at about 80 K and 325 K [11], and the 1T-TaS2 polytype does not display any superconductivity and has three CDW transition temperatures [12,13]. Intercalation [6,14], self–intercalation [5,15,16] or possible partial substitution of the transition metals in TMDC, as for example in the case of Ta sub-
∗
Corresponding author. E-mail address: [email protected] (V. Antal).
http://dx.doi.org/10.1016/j.physc.2017.06.007 0921-4534/© 2017 Published by Elsevier B.V.
stitution by Ni in TaS2 [17], has an influence on the fluctuation or inhibition of CDW transition and superconducting properties [18]. A lot of issues concerning the origin of superconductivity and CDW transition in intercalated TMDC have been reported. Beginning with the first principles of electronic structure calculations, whereby a change in the effective density of states (DOS) of the Fermi surface [19] and the carrier density were considered, charge transfer from intercalated atoms or molecules to the host layers of TMDC as well as electron-phonon coupling have been described [6,18,20,21]. However, the question of superconductivity and CDW transition in TMDC remains a subject for discussion and investigation. We concentrated our work on the preparation and investigation of Ta1+ y S2 crystals grown by the chemical vapour transport (CVT) method. After success preparation of Ta1+ y S2 crystals, Cu-intercalated Ta1+ y S2 crystals were prepared with three different crystal growth regimes. Our experiments confirmed that the additional annealing and cooling process of the crystal growth regime could have a substantial influence on the crystal structure of Cux Ta1+ y S2 and its physical properties. Also, we noted the presence of superconductivity in Cux Ta1+ y S2 crystals with a high concentration of Cu. 2. Experimental details The samples were prepared by the CVT method in a threezone tube furnace. Three evacuated quartz tubes were sealed with the same amount of TaS2 powder and Cu powder at a mol ratio of 1:0.5 and one quartz tube was sealed only with TaS2 powder. TaS2 powder, used as a precursor, was prepared by sintering
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V. Antal et al. / Physica C: Superconductivity and its applications 539 (2017) 35–39
Fig. 1. Scanning electron image of sample 1 that indicates the typical layered structure of transition metal dichalcogenides.
the necessary amount of high-purity Ta and S in an evacuated quartz tube at 900 °C for 96 h. As the transport agent we used iodine in the amount of about miodine /Vampule ≈ 5 mg ml−1 . Three different crystal growth regimes were used, but the first step in all regimes was the same. The first growth regime (Sample 1, Sample 2) Step 1: crystal growth for 10 days in 100 °C gradient (10 0 0 °C → 900 °C), between the source part and the growth part of the tube. Step 2: cooling down after Step 1 to 400 °C over about two days (cooling rate about 10 °C/h) and finally furnace cooling. The second growth regime (Sample 3) Step 1: the same as Step 1 in the first growth regime. Step 2: air quenching to room temperature by rapid removal of the quartz tube from the furnace. The third growth regime (Sample 4) Step 1: the same as Step 1 in the first growth regime. Step 2: the temperatures at both sides of the tube reduced by 100 °C and maintenance of this temperature for 22 h. Step 3: air quenching to room temperature by rapid removal of the quartz tube from the furnace. After growth, all samples were cleaned by ultrasonication in distilled water and ethanol. Prepared samples were analysed by scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDX). The X-ray diffraction analyses were performed by a Rigaku D/MAX Rapid II X-ray diffractometer with MoKα radiation. The ab in-plane electrical resistivity measurements as a function of temperature were obtained by the four-probe method at a temperature range of 0.4 K ≤ T ≤ 300 K.
were taken from different parts of the samples. It could be clearly seen that for all samples there was an excess of Ta. An excess of transition metals (self-intercalation) in TMDC was observed in early investigations, an excess caused by the presence of the metals in the Van der Waals gaps [5,16,22]. The EDX analyses of all samples confirmed that concentrations of Cu and Ta were not homogeneously distributed within the samples and small deviations in their concentrations were observed (Table 1). However, the EDX elemental mapping did not show any regions with Ta, S or Cu that would be associated with sharp changes of chemical composition of the crystals (Fig. 3). We may suppose that small deviations were caused by diffusive compositional macroscopic inhomogeneity on the level which cannot be detected by the EDX mapping. High concentrations of Cu were found in all Cux Ta1+ y S2 samples; the values vary, but have a similar concentration range. However, Ta concentration with essential excess was determined in samples 1 and 3–4 but sample 2 showed a reduced amount. The crystallographic cell parameters (Table 1) were determined by Rietveld refinement calculations from the X-ray patterns obtained at room temperature (Fig. 4). We can see that the lattice structure of sample 2 differs from that of the other samples. Samples 1 and 3–4 have hexagonal symmetry and a 2H-type crystal structure, whereas sample 2 has a 6R-type crystal structure and rhombohedral symmetry [22]. All samples were single-phase. Earlier investigations of 2H-TaS2 samples indicated that the aaxis and the c-axis were equal to 3.315 A˚ and 12.10 A˚ [5], 3.314 A˚ and 12.097 A˚ [23] or 3.316 A˚ and 12.070 A˚ [24], respectively. More recent results have reported an a-axis and c-axis of 3.31 A˚ and 12.08 A˚ [20,25], respectively, and these results are in close agreement with earlier investigations. The a-axis parameters of samples 1 and 3–4 are similar to published data for 2H-TaS2 samples, but because of the intercalation of Cu atoms in the Van der Waals gaps [25] elongation of the c-axis was evidenced in samples 3–4 in comparison with the 2H-TaS2 sample. If we compare the crystal lattice parameters of sample 2 with the literature data, where for the 6R-TaS2 polytype the a-axis and the c-axis are equal to 3.335 A˚ and 35.85 A˚ [5] or 3.335 A˚ and 35.622 A˚ [26], respectively, we can see slight decreasing of the aaxis and appreciable increasing of the c-axis. Similar behaviour by the crystal lattice parameters caused by Cu intercalation in 6RCux Ta1+ y S2 that corresponds to that of our sample 2 has been reviewed in the literature [15,22]. It is clear from the EDX and X-ray diffraction results (Table 1), that appreciable changes in the crystal structure and chemical composition of Cux Ta1+ y S2 samples were caused by additional annealing or different cooling processes after Step 1 of the crystal growth regime. If we compare sample 1 with sample 2, it seems that during the first growth regime sample 2 transformed from the 2H-type to the 6R-type during the cooling process. From these results we assume that the initial 2H-type structure formed in Step 1 of the crystal growth regime for samples 3–4 remained unchanged because of air quenching after the crystal growth. Also, we assume that additional annealing processes and different cooling rates after Step 1 of the crystal growth regimes could influence the Ta excess in the samples (Table 1), but it is not clear to us why there is a tendency towards high Ta excess in samples 1 and 3–4 and low Ta excess in sample 2.
3. Results and discussion 3.1. Chemical and structural analyses
3.2. Resistivity measurements
Scanning electron microscopy investigations confirmed the typical layered structure of the prepared samples (Fig. 1) and the chemical composition of the samples deduced from the EDX patterns (Fig. 2) is listed in Table 1. In order to determine more precisely the chemical composition of the samples, the EDX patterns
The electrical resistivity measurements of all prepared samples are shown in Fig. 5. As mentioned in introduction the superconducting transition temperature, Tc , for the 2H-TaS2 sample is about 0.8 K [7,8] and about 2.3 K for the 6R-TaS2 sample [11]. Superconducting transitions were not reported in early investigations
V. Antal et al. / Physica C: Superconductivity and its applications 539 (2017) 35–39
Fig. 2. The EDX patterns and macrographs (inserted photos) of sample 1 (a), sample 2 (b), sample 3 (c), and sample 4 (d).
Table 1 The crystallographic parameters and resistivity measurement results.
Chem.form. x y Cryst. syst. Space gr. ˚ a (A) ˚ c (A) V (A˚ 3 ) Z Tc,onset (K) TCDW (K)
Sample 1
Sample 2
Sample 3
Sample 4
Ta1+ y S2 – 0.25 ≤ y ≤ 0.34 Hexagonal P63/mmc 3.308(7) 12.09(3) 114.6(4) 2 2.62 72
Cux Ta1+ y S2 0.30 ≤ x ≤ 0.31 0.03 ≤ y ≤ 0.04 Rhombohedral R-3m 3.3046(18) 37.6053(6) 355.65(2) 6 0.71 –
Cux Ta1+ y S2 0.29 ≤ x ≤ 0.34 0.28 ≤ y ≤ 0.30 Hexagonal P63/mmc 3.3222(11) 12.9254(5) 123.553(8) 2 – –
Cux Ta1+ y S2 0.28 ≤ x ≤ 0.29 0.27 ≤ y ≤ 0.29 Hexagonal P63/mmc 3.32705(9) 12.9212(18) 123.866(5) 2 0.42 –
Fig. 3. The EDX elemental mapping of sample 1 (a) and sample 2 (b). Samples 3–4 have similar mapping to sample 2.
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Fig. 4. The X-ray diffraction patterns of sample 1 (a), sample 2 (b), sample 3 (c), and sample 4 (d). Upper lines indicate experementally obrained X-ray pattren (stars) and calculated pattern (solid red line); middle bars indicates calculated position of the peaks; bottom line indicates calculated difference between experimental and calculated patterns.
of Cux Ta1+ y S2 samples with high concentrations of Cu, x > 0.23 [15,22,27,28]. The onset transition temperature, Tc,onset , from the superconducting to normal state for sample 1 is about 2.62 K (Fig. 6), whereas the midpoint transition temperature, Tc, 0.5 , is about 1.52 K and the transition width (10%–90%) is about 0.82 K. Wagner et al. [21] explained increase in Tc above 1 K in 2H-TaS2 because of Ta excess or sub-1% impurity atoms present in the Van der Waals gaps. In 2H-Cux Ta1+ y S2 samples Tc is enhanced by up to 2.5 K because of initial effects such as changing of the interlayer coupling or disruption to the ordering of CDW. Further enhancement of Tc up to about 4.5 K is caused by electronic doping of the system. Decreasing Tc with increasing Cu content above optimal concentration has been explained by decreasing DOS [21]. Despite of Ta excess, sample 1 displays a kink on the electrical resistivity curve at around 72 K that attributes to the CDW transition, whereas any CDW transitions observed in samples 2 to 4 up to 300 K (Fig. 5). From these results it seems that sample 1 behaves like Cu0.01 TaS2
sample [21], but probably because of high Ta excess we have broad transition width. The electrical resistivity of sample 2 dropped down at about 0.71 K and that of sample 4 at about 0.42 K, corresponding to the superconducting transition. It was found that optimal concentrations of Cu in 2H-Cux Ta1+ y S2 samples, where CDW and superconductivity could coexist with the highest values of Tc , were observed at very low Cu concentrations of less than x ≤ 0.04 [17,21,25]. A lot of literature data indicates that the CDW transition is suppressed at lower temperatures or is inhibited by increasing Cu concentration and after complete inhibition of CDW the electrical resistivity curves of TMDC display a metallic behaviour. It was assumed that suppression of CDW is the result of the influence of intercalated atoms on the c-axis conduction in 2H-TaS2 that is reflected by increasing DOS at the Fermi surface with further enhancement of Tc [6,20]. As discussed in Section 3.1, sample 2 has 6R-TaS2 polytype structure that generally has higher Tc in comparison to 2H-TaS2 polytype structure. We suppose that additional annealing of sam-
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4. Conclusions In summary, it was found that different types of Cux Ta1+ y S2 structures were formed from the same starting powder precursors and caused by additional annealing processes or different cooling rates from the crystal growth regime. Transformation of the crystal structure from the 2H-type Cu-intercalated TaS2 to the 6Rtype could be obtained by slow cooling and initial crystal structure could be preserved by fast cooling. The obtained results showed that additional annealing or slow cooling could induce superconductivity in Cux Ta1+ y S2 samples with high Cu concentration up to about 30% of impurity. Acknowledgements
Fig. 5. Normalized temperature dependence of the electrical resistivity of all samples. TCDW indicates the CDW transition in sample 1.
This work was realized within the framework of the projects: Centre of Excellence of Advanced Materials with Nano- and Submicron Structure (ITMS 26220120019), Infrastructure Improving of Centre of Excellence of Advanced Materials with Nano- and Submicron Structure (ITMS 26220120035), New Materials and Technologies for Energetic (ITMS 26220220061), Research and Development of Second Generation YBCO Bulk Superconductors (ITMS 26220220041), APVV No. 0330-12, VEGA No. 2/0121/16, VEGA 2/0149/16, APVV-14-0605, Stefanik Project SK-FR-2013-0025, SAS Centre of Excellence: CFNT MVEP, PhysNet (ITMS 26110230097) and NANOKOP (ITMS 26110230061). References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15]
Fig. 6. The electrical resistivity in the vicinity of the superconducting transition temperature.
ple 4 in Step 2 of the crystal growth regime induces small structural changes (too small for detection by the X-ray and the EDX) that provoke superconductivity. Whereas, superconductivity in sample 2 was probably induced by transformation of the crystal structure from the 2H-type to the 6R-type during slow cooling down after Step 2 of the crystal growth regime and lower Ta excess in comparison to sample 4. Sample 3 did not display any transitions that could be associated with superconductivity in the temperature range from 0.4 K. It should be noted that superconducting properties of 6RCux Ta1+ y S2 polytype samples seem to be investigated less often than those of the 2H-Cux Ta1+ y S2 polytype.
[16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28]
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