Synthesis of new high-entropy alloy-type Nb3 (Al, Sn, Ge, Ga, Si) superconductors
SSynthesis of new high-entropy alloy-type Nb (Al, Sn, Ge, Ga, Si) superconductors Aichi Yamashita , Tatsuma D. Matsuda , Yoshikazu Mizuguchi * 1. Department of Physics, Tokyo Metropolitan University, 1-1, Minami-osawa, Hachioji, 192-0397. *Corresponding author: Yoshikazu Mizuguchi. E-mail: [email protected]
Abstract
Studies on high-entropy alloy (HEA) superconductors have recently been increasing, particularly in the fields of materials science and condensed matter physics. To contribute to research on new HEA-type superconductors, in our study we synthesized polycrystalline samples of A15-type superconductors of Nb Al Sn Ge Ga Si ( Al Sn Ge Ga Si ( ΔC / γT c for the obtained samples were close to those reported for conventional Nb Sn family superconductors. Keywords: high-entropy alloy; new superconductor; A15-type superconductor Highlights ・ New high-entropy alloy (HEA)-type Nb M superconductors ( M : Al, Sn, Ge, Ga, Si) were synthesized. A superconducting transition was observed for Nb Al Sn Ge Ga Si at 11.0 K. ・ The HEA state at the M site of Nb M resulted in a lower T c .
1. Introduction
High-entropy alloys (HEAs) are typically defined as alloys containing at least five elements with concentrations between 5 and 35 at. % [1, 2], resulting in high configurational mixing entropy ( ΔS mix ), defined as ΔS mix = - R Σ i c i ln c i , where c i and R are the compositional ratio and the gas constants, respectively [2]. HEAs have recently attracted much attention in the fields of materials science and engineering because of their tunable properties as structural materials, such as excellent mechanical performance under extreme conditions [1, 2]. As HEA superconductors, simple alloys with bcc, α-Mn, CsCl, and hcp crystal structures have mainly been studied so far [3–14]. Among these, Ta Nb Hf Zr Ti with a superconducting transition temperature ( T c ) of 7.3 K, exhibits robustness in the superconducting state under extremely high pressures up to 190 GPa [4]. This suggests that HEA superconductors can be applied under extreme conditions and the study of HEA superconductors can be accelerated. Thus far, we have extended the concept of HEA to compounds [15–22], wherein one of the crystallographic sites is high-entropy alloyed. Recently, we reported layered superconductors with HEA-type crystallographic sites: HEA-type cuprate RE
123 (RE: rare earth elements) and RE O F BiS with a HEA-type rare earth (RE) site [15–17]. In a RE123-type cuprate, no degradation of T c was confirmed by increasing ΔS mix [15]. In addition, the emergence of bulk superconductivity has been observed in HEA-type RE O F BiS [16, 17]. Notably, crystal structural analysis revealed suppression of the local structural disorder, which corresponds to the improvement of the bulk nature of superconductivity in this system, with an increase in ΔS mix [17]. Based on these findings, the HEA effects in layered superconductors seem to work positively or at least less negatively. To investigate the effect of HEA on non-layered compounds, we also investigated NaCl-type metal chalcogenide uperconductors with high ΔS mix [20–22]. For instance, an HEA-type AgInSnPbBiTe superconductor with T c = 2.6 K was reported. In contrast to layered systems, NaCl-type HEA tellurides exhibited lower T c than low-entropy tellurides [21]. These results suggest that the effects of HEAs on the superconducting properties of compounds depend on their crystal structure and dimensionality. To further investigate the effects of HEAs on superconducting properties, we focused on A15-type Nb M ( M : Al, Sn, Ge, Ga, Si) compounds, which have a relatively high T c (above 18 K) and upper critical fields ( H c2 (0)) of approximately 30 T [23–33]. They are well known as practical materials for superconducting magnets higher than 10 T [33]. In this paper, we report on the syntheses and properties of new HEA-type Nb M superconductors, wherein the composition at the M site satisfies the compositional criterion of the HEA and achieves ΔS mix above 1.5 R .
2. Experimental details
Polycrystalline samples of Nb Al Sn Ge Ga Si ( Al Sn Ge Ga Si (
3. Results and discussion
The powder XRD patterns of Nb Al Sn Ge Ga Si ( Al Sn Ge Ga Si ( Si-type ( Pm _ m ) model. The lattice constant was estimated using Rietveld refinement and is plotted in Table 1. A schematic image of the refined crystal structure of sample Si were detected at 8.5% and 4.5% for samples Si is non-superconducting at T > 1 K. The compositions of (Al Sn Ge Ga Si ) and Nb (Al Sn Ge Ga Si ) by XRF measurements. The calculated ΔS mix for M site was 1.59 R and 1.50 R for M ( M = Al, Sn, Ge, Ga, Si) site is 1.0. Figures 2(a) and 2(b) show the temperature dependences of magnetization for M - H loop was observed for samples ΔM at each magnetic field, which is related to the critical current density ( J c ), for ρ ) for ρ for RRR = ρ / ρ ) of approximately 1.2 for both samples, which is often observed in conventional HEA superconductors [3–5, 20–22]. Compared to the superconducting transition of sample ΔT c ) of H = 0, 1, 2, 3, 5, 7, and 9 T) near the superconducting transition. The resistivity began to decrease at 9.0 and 11.0 K, which corresponds to the onset temperature ( T conset ), and it reached zero at 8.8 and 9.2 K ( T czero ) for T c decreased with an increase in magnetic field. Figure 3(d) shows the magnetic field-temperature phase diagrams for H c2 ( T = 0 K), the resistive midpoints ( T c ρ ) are plotted in Figure 3 (d). The H c2 is estimated as 10.4 and 13.3 T for H c2 ( T = 0 K) = -0.69* T c *(d H c2 /d T ) T = T c [36], which is used for superconductors in a dirty limit. To confirm the bulk nature of the observed superconductivity, the specific heat was measured for samples H = 0, 2, 3, 6, and 9 T) is summarized in Figures 4(a) and 4(b). A clear jump was observed below 11 K under 0 T, and the transition temperature shifted to a lower temperature with an increase in the magnetic field. Although a small jump still exists at 9 T in sample γ ) and the coefficient for the lattice specific heat contribution ( β ). The estimated γ and β were 18.9 and 0.16 for and 0.19 mJ/mol K for θ D ) was estimated to be 365 K for M system [30–32]. Figure 4(b) shows the electronic contributions of the specific heat ( C el ), which was calculated by subtracting the phonon contributions from the total specific heat. The clear jump in C el / T and the decrease in C el / T at low temperatures suggest that both samples are bulk superconductors. Interestingly, the superconducting transition observed from C el was relatively broad compared to pure Nb M systems [23, 24, 27]. A broad transition was also observed in n HEA-type Co Ni Cu Rh Ir Zr superconductor [19]. Therefore, the commonality in the broad specific heat jump at T c may indicate the possibility of a universal phenomenon for HEA-type superconductors. The nature of the broad transition may imply a semi-continuous evolution of the superconducting gap caused by the HEA effects. To reveal the origin of this phenomenon, further information from the specific heat analyses by functional parameter fitting or experiments, which can directly observe the superconducting gap structure, such as scanning tunneling microscopy (STM), is required. Although the origin of the broad transition is unclear, the present data are sufficient to confirm the emergence of bulk superconductivity in these samples. Compared to the broader specific heat transition of C el / T , ΔC sc / γT c was estimated as 1.64 for ΔC sc / γT c values are slightly greater than ΔC sc / γT c = 1.43, which is expected from conventional weak-coupling pairing [37], the present samples would be strong-coupling superconductors similar to the pure Nb M samples. Here, we discuss the effect of HEA on the superconducting states of the obtained samples. The present T c s are approximately half for those of the pure Nb M samples [30–32, 38]. The large decade of the T c s for the present samples can be related to the introduction of the short-range disorder of atomic ordering due to the HEA effect. There are two scenarios for the decrease in T c caused by the disorder. The first scenario is the decrease in the electronic density of state [38–41] and the second is the increase in Coulomb interaction due to electron localization [38, 42]. Considering the smaller γ values of the present sample than those for pure Nb M (Table 1), the reduction in the electronic density of state can explain the obtained results. In addition, considering the highly disordered (alloyed) situation for the present samples, we consider that the second scenario is also reasonable. In the second scenario, it is known that T c decreases with increasing Coulomb interaction due to electron localization in a sample with an electrical resistivity of 40 μΩ cm or higher [28, 36], that is, a sample having a large amount of disorder due to defects, impurities, or alloying. This is consistent with the resistivity values of approximately 150 μΩ cm for samples T c is not remarkable because of the disorder due to the long coherence length. In contrast, when the superconductors have a short coherence length such as Nb M , disorder, which potentially causes nanoscale phase separation and/or the formation of nanoscale domains, the superconducting pairing may be affected and hence result in a decrease in T c .
4. Conclusions
Herein, we have reported the synthesis and superconducting properties of new HEA-type superconductors, Nb Al Sn Ge Ga Si and Nb Al Sn Ge Ga Si . Polycrystalline samples were prepared using pure metals via arc melting. The composition of the obtained samples satisfied the definition of HEA. Superconducting transitions were observed at 9.0 and 11.0 K for H c2 (0) were estimated as 10.4 and 13.3 T. The bulk nature of superconductivity was confirmed from the specific heat jump at T c . Although the estimated Sommerfeld coefficient was slightly lower than that for pure Nb M , the Debye temperature and ΔC / γT c were close to those reported for pure Nb M . Even though the observed T c was almost half of that for pure Nb M , the discovery of superconductivity in HEA-type Nb M provides an additional pathway for exploring novel HEA-type superconductors and investigations on the relationship between the HEA effect and superconductivity in highly disordered compounds. Declaration of competing interests
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. cknowledgments
The authors thank M. Yamashita, M. Katsuno, Y. Goto, and O. Miura for their assistance.
This work was partly supported by JSPS-KAKENHI (Grant Nos. 18KK0076) and Tokyo Metropolitan Government Advanced Research (H31-1).
References [1]
J. W. Yeh, S. K. Chen, S. J. Lin, J. Y. Gan, T. S. Chin, T. T. Shun, et al., Nanostructured High‐Entropy Alloys with Multiple Principal Elements: Novel Alloy Design Concepts and Outcomes, Adv. Eng. Mater., (2004) 299-303. [2] M. H. Tsai and J. W. Yeh, High-Entropy Alloys: A Critical Review, Mater. Res. Lett. , (2014) 107. [3] P. Koželj, S. Vrtnik, A. Jelen, S. Jazbec, Z. Jagličić, S. Maiti, et al., Discovery of a Superconducting High-Entropy Alloy, Phys. Rev. Lett. , (2014) 107001. [4] J. Guo, H. Wang, F. von Rohr, Z. Wang, S. Cai, Y. Zhou, et al., Robust zero resistance in a superconducting high-entropy alloy at pressures up to 190 GPa, Proc. Natl. Acad. Sci. USA, , (2017) 13144. [5] F. O. von Rohr and R. J. Cava, Isoelectronic substitutions and aluminium alloying in the Ta-Nb-Hf-Zr-Ti high-entropy alloy superconductor, Phys. Rev. Mater., , (2018) 034801. [6] K. Y. Wu, S. K. Chen, and J. M. Wu, Superconducting in equal molar NbTaTiZr-based high-entropy alloys, Nat. Sci. J., , (2018) 110. [7] S. Liling and R. J. Cava, High-entropy alloy superconductors: Status, opportunities, and challenges, Phys. Rev. Mater., , (2019) 090301. [8] S. Vrtnik, P. Koželj, A. Meden, S. Maiti, W. Steurer, M. Feuerbacher, J. Dolinšek, Superconductivity in thermally annealed Ta-Nb-Hf-Zr-Ti high-entropy alloys, J. Alloys Compd. , (2017) 3530-3540. [9] S. Marik, K. Motla, M. Varghese, K. P. Sajilesh, D. Singh, Y. Breard, et al., Superconductivity in a new hexagonal high-entropy alloy, Phys. Rev. Mater., , (2019) 060602. 10] N. Ishizu and J. Kitagawa, New high-entropy alloy superconductor Hf Nb Ti V Zr , Results in Phys., , (2019) 102275. [11] Y. Yuan, Wu Y, Luo H, Wang Z, Liang X, Yang Z, Wang H, Liu X and Lu Z, Superconducting Ti Zr Nb Ta High-Entropy Alloy with Intermediate Electron-Phonon Coupling, Front. Mater., , (2018) 72. [12] K. Stolze, F. A. Cevallos, T. Kong, and R. J. Cava, High-entropy alloy superconductors on an α-Mn lattice, J. Mater. Chem. C, , (2018) 10441. [13] K. Stolze, J. Tao, F. O. von Rohr, T. Kong, and R. J. Cava, High-entropy alloy superconductors on an α-Mn lattice, Chem. Mater. , (2018) 906. [14] J. Kitagawa, S. Hamamoto and N. Ishizu, Cutting Edge of High-Entropy Alloy Superconductors from the Perspective of Materials Research, Metals, , (2020) 1078. [15] Y. Shukunami, A. Yamashita, Y. Goto, Y. Mizuguchi, Synthesis of RE123 high- T c superconductors with a high-entropy-alloy-type RE site, Physica C, , (2020) 1353623. [16] R. Sogabe, Y. Goto, Y. Mizuguchi, Superconductivity in REO0.5F0.5BiS2 with high-entropy-alloy-type blocking layers, Appl. Phys. Express, , (2018) 053102. [17] R. Sogabe, Y. Goto, T. Abe, C. Moriyoshi, Y. Kuroiwa, A. Miura, K. Tadanaga, Y. Mizuguchi, Improvement of superconducting properties by high mixing entropy at blocking layers in BiS -based superconductor REO F BiS , Solid State Commun., , (2019) 43. [18] Md. R. Kasem, A. Yamashita, Y. Goto, T. D. Matsuda, Y. Mizuguchi, Synthesis of high-entropy-alloy-type superconductors (Fe,Co,Ni,Rh,Ir)Zr with tunable transition temperature, arXiv:2011.05590. [19] Y. Mizuguchi, Md. R. Kasem, T. D. Matsuda, Superconductivity in CuAl -type Co Ni Cu Rh Ir Zr with a high-entropy-alloy transition metal site, arXiv:2009.07548. [20] Y. Mizuguchi, Superconductivity in High-Entropy-Alloy Telluride AgInSnPbBiTe , J. Phys. Soc. Jpn., , (2019) 124708. [21] Md. R. Kasem, K. Hoshi, R. Jha, M. Katsuno, A. Yamashita, Y. Goto, T. D. Matsuda, Y. Aoki, Y. izuguchi, Superconducting properties of high-entropy-alloy tellurides M-Te (M: Ag, In, Cd, Sn, Sb, Pb, Bi) with a NaCl-type structure, Appl. Phys. Express, , (2020) 033001. [22] A. Yamashita, R. Jha, Y. Goto, T. D. Matsuda, Yuji Aoki and Y. Mizuguchi, An efficient way of increasing the total entropy of mixing in high-entropy-alloy compounds: a case of NaCl-type (Ag,In,Pb,Bi)Te x Se x ( x = 0.0, 0.25, 0.5) superconductors, Dalton Trans., , (2020) 9118-9122. [23] M. N. Khlopkin, The specific heat of Nb Sn in magnetic fields up to 19 T,
Zh. Eksp. Teor. Fiz . , , (1986) 286-293. [24] G. R. Stewart, Bart Olinger, and L. R. Newkirk, Specific heat of A-15 Nb Si produced by explosive compression, Solid State Commun., (1981) 5-9. [25] Bart Olinger and L. R. Newkirk, Bulk A15, high T c Nb Si synthesized by shock compression, Solid State Commun., (1981) 613-617. [26] A. Junod, J. L. Jorda1 and J. Muller, Specific Heat of Nb Al Analyzed in the Eliashberg Formalism, Jpn. J. Appl. Phys . , , (1987) 911. [27] A. Junod, J. L. Jorda, M. Pelizzone, and J. Muller, Specific heat and magnetic susceptibility of nearly stoichiometric and homogeneous Nb Al, Phys. Rev. B, , (1984) 1189-1198. [28] A. Godeke, A review of the properties of Nb Sn and their variation with A15 composition, morphology and strain state, Supercond. Sci. Technol., , (2006) 68-80. [29] A. Junod, T. Jarlborg, and J. Muller, Heat-capacity analysis of a large number of A15-type compounds, Phys. Rev. B, , (1983) 1568. [30] H. B. Radousky, T. Jarlborg, G. S. Knapp, and A. J. Freeman, Assessment of theoretical determinations of the electron-phonon coupling parameter λ in metals and intermetallic compounds, Phys. Rev. B , , (1982) 1208. [31] G. R. Stewart, Superconductivity in the A15 structure, Physica C: Superconductivity and its Applications, , (2015) 28-35. [32] S. Foner, E. J. McNiff Jr., Upper critical fields of cubic and tetragonal single crystal and olycrystalline Nb3Sn in DC fields to 30 tesla, Solid State Commun., , (1981) 959-964. [33] J. J. Neumeier, W. J. Nellis, M. B. Maple, M. S. Torikachvili, K. N. Yang, J. M. Ferreira, et al., Metastable A15 phase Nb Si synthesized by high dynamic pressure, International Journal of High Pressure Research, , (1989) 267-289. [34] Izumi, F. & Momma, K. Three-dimensional visualization in powder diffraction. Solid State Phenom. , (2007) 15-20. [35] Momma, K. & Izumi, F., VESTA: a three-dimensional visualization system for electronic and structural analysis. J. Appl. Crystallogr., , (2008) 653-658. [36] N. R. Werthamer, E. Helfand, P. C. Hohemberg, Temperature and Purity Dependence of the Superconducting Critical Field, H c2 . III. Electron Spin and Spin-Orbit Effects, Phys. Rev., , (1966) 295. [37] J. Bardeen, L. Cooper, J. R. Schrieffer, Theory of Superconductivity, Phys. Rev., , (1957) 1175-1204. [38] H. Ihara, K. Togano, “Superconducting material” [Translated from Japanese.], University of Tokyo Press, (1987), pp.129-144, 19. [39] P. W. Anderson, K. A. Muttalib, and T. V. Ramakrishnan, Theory of the "universal" degradation of T c in high-temperature superconductors, Phys. Rev., , (1957) 117-120. [40] B. M. Klein, L. L. Boyer, D. A. Papaconstantopoulos, and L. F. Mattheiss, Self-consistent augmented-plane-wave electronic-structure calculations for the A15 compounds V X and Nb X, X=Al, Ga, Si, Ge, and Sn, Phys. Rev., , (1978) 6411-6438. [41] W. Weber, Electronic structure of tetragonal Nb Sn, Phys. Rev., , (1982) 2270-2284. [42] L. F. Mattheiss, and W. Weber, Electronic structure of cubic V Si and Nb Sn, Phys. Rev., , (1982) 2248-2269. Table 1. Parameters for these present samples and selected A15s with T c above 17 K. Chemical composition, configurational mixing entropy ( ΔS mix / R ), lattice constant ( a ), electronic specific heat coefficient ( γ ), transition temperature ( T c ), upper critical fields ( H c2(0) ), Debye temperature ( Θ D ), and electron-phonon interaction ( ΔC / γ * T c ) of A15-type superconductors. Sample ΔS mix / R a (Å) γ (mJ/mol K ) T c (K) H c2(0) (T) Θ D (K) ΔC / γ*T c Ref. Nb (Al Sn Ge Ga Si ) 1.59 5.183 18.9 9.0 10.4 364.8 1.64 This work Nb (Al Sn Ge Ga Si ) 1.50 5.192 19.3 11.0 13.3 347.7 1.67 This work Nb Al - 5.180 34.9~45.0 18.0 33.0 296 1.59~2.78 26,27,30, 31,38 Nb Sn - 5.289 35~52.4 18.0 29.0 208~290 1.30~1.7 30,31,38 Nb Ge - 5.125 30.3 21.8~23.9 38.0 302 1.64 30,31,38 Nb Ga - 5.171 46 19.8~20.2 34.1~35 280 1.7~1.74 31,33,38 Nb Si - 5.09 24~66.4 17.4~18.0 14.1~15.5 310~352 1.7 24,31,33, 38
Fig. 1 X-ray diffraction (XRD) patterns for the obtained samples M ( M = Al, Sn, Ge, Ga, Si). Fig. 2 Temperature dependences of magnetization for samples M - H - loop at 2.0 K (c, d). Fig. 3 Temperature dependences of electrical resistivity [ ρ ( T )] for samples ρ ( T ) to 0 K for samples H c2 for T c was estimated as the temperature at the resistive midpoint ( T c ρ ) (d). The dashed lines are the Werthamer-Helfand-Hohenberg (WHH) fitting results. Fig. 4 Temperature dependence of C / T for Al Sn Ge Ga Si at 0, 3, 6 and 9 T and Al Sn Ge Ga Si at 0, 2, 6 and 9 T (a, b). Temperature dependence of electronic specific heat C el / T for for