Open Access is an initiative that aims to make scientific research freely available to all. To date our community has made over 100 million downloads. It’s based on principles of collaboration, unobstructed discovery, and, most importantly, scientific progression. As PhD students, we found it difficult to access the research we needed, so we decided to create a new Open Access publisher that levels the playing field for scientists across the world. How? By making research easy to access, and puts the academic needs of the researchers before the business interests of publishers.
We are a community of more than 103,000 authors and editors from 3,291 institutions spanning 160 countries, including Nobel Prize winners and some of the world’s most-cited researchers. Publishing on IntechOpen allows authors to earn citations and find new collaborators, meaning more people see your work not only from your own field of study, but from other related fields too.
To purchase hard copies of this book, please contact the representative in India:
CBS Publishers & Distributors Pvt. Ltd.
www.cbspd.com
|
customercare@cbspd.com
The first purpose of this chapter is materials research on face-centered-cubic (fcc) high-entropy alloy (HEA) superconductors, which have not yet been reported. We have investigated several Nb-containing multicomponent alloys. Although we succeeded in obtaining Nb-containing samples with the dominant fcc phases, no superconducting signals appeared in these samples down to 3 K. The microstructure analyses revealed that all samples were multi-phase, but the existence of several new Nb-containing HEA phases was confirmed in them. The second purpose is the report of materials research on the Mn5Si3-type HEA superconductors. This hexagonal structure offers various intermetallic compounds, which often undergo a superconducting state. The Mn5Si3-type HEA is classified into the multisite HEA, which possesses the high degree of freedom in the materials design and is good platform for studying exotic HEA superconductors. We have successfully found a single-phase Mn5Si3-type HEA, which, however, does not show a superconducting property down to 3 K. The attempt of controlling the valence electron count was not successful.
Department of Electrical Engineering, Faculty of Engineering, Fukuoka Institute of Technology, Fukuoka, Japan
Naoki Ishizu
Department of Electrical Engineering, Faculty of Engineering, Fukuoka Institute of Technology, Fukuoka, Japan
Shusuke Hamamoto
Department of Electrical Engineering, Faculty of Engineering, Fukuoka Institute of Technology, Fukuoka, Japan
*Address all correspondence to: j-kitagawa@fit.ac.jp
1. Introduction
High-entropy alloys (HEAs) are a new class of materials and have attracted a great deal of attention [1, 2]. The concept of HEA was originally proposed for a face-centered-cubic (fcc), body-centered-cubic (bcc), or hexagonal-closed packing (hcp) structure. The most prominent feature of a HEA is that more than five elements, each having an atomic percentage between 5% and 35%, randomly occupy one crystallographic site (see also Figure 1(a)). This produces a large mixing entropy, and HEAs exhibit the combination of high yield strength and ductility [3], high strength at elevated temperatures [4], strong resistance to corrosion and oxidation [5], and so on. The high-entropy concept is extensively adapted in various materials such as oxides, chalcogenides, and halides [6, 7].
Figure 1.
Crystal structure of compounds with (a) fcc and (b) Mn5Si3-type structures. In (a), a multicolored ball means a random occupation by constituent elements.
One of the novelties of HEAs is a cocktail effect, which indicates an enhancement of physical properties beyond the simple mixture of those of components. For example, several bcc HEAs show superior mechanical properties compared to conventional hard materials. Another example is found in magnetic spinel oxide (Mg0.2Co0.2Ni0.2Cu0.2Zn0.2)Al2O4. The high-entropy type spinel oxide interestingly shows enhanced magnetic frustration [8]. The cocktail effect is also reported in the structural stability of high-entropy-type materials. A γ-type disilicate structure is stable from room temperature to 1900°C in (Gd1/6Tb1/6Dy1/6Tm1/6Yb1/6Lu1/6)2Si2O7. The outstanding thermal stability is ascribed to the high-entropy state at the rare-earth site [9]. The other novelty of HEAs is the tuning of physical properties via the change of microstructure. The manufacturing process of HEAs considerably affects their microstructures, which are often deeply related to their physical properties. Fe15Co15Ni20Mn20Cu30 shows a spinodal decomposition after the heat treatment [10]. The spinodally decomposed sample exhibits enhanced Curie temperature and magnetization compared to the homogenized single-phase sample. The tuning of magnetic properties is also reported in dual-phase HEAs [11, 12, 13].
One of the new research topics in HEA is the superconductivity found in 2014 [14]. Transition metal-based superconductors, forming simple crystalline structures, follow the so-called Matthias rule. When the superconducting critical temperature Tc is plotted as a function of valence electron count per atom (VEC), this rule shows broad peak structures at the specified VEC [15]. On the other hand, transition metal amorphous superconductors do not follow this rule and frequently show relatively high Tc values in the valley of the curve of the Matthias rule [16]. HEA superconductors with simple crystal structures have been found in bcc [17, 18, 19, 20, 21, 22] and hcp [23, 24, 25, 26]. The Tc vs. VEC plots of these HEAs seem to fall between a crystalline curve and an amorphous one [27, 28]. Thus, HEA superconductors will shed light on the study of the relationship between crystalline and amorphous compounds.
In the typical HEAs with fcc, bcc, or hcp structure, the superconductivity seems to appear in bcc or hcp HEAs. According to the classification by VEC, single-phase fcc HEA is stabilized for VEC larger than 8.0 [1, 2], where Tc would be substantially reduced in the VEC dependence of Tc observed in the Matthias rule. So it may be unrealistic to search for an fcc HEA superconductor. However, this is valuable to challenge because an fcc HEA superconductor would contribute to the deep understanding of HEA and/or the relationship between crystalline and amorphous compounds. In this chapter, we introduce our attempt at the search for an fcc HEA superconductor. Our strategy is to employ a rather high Tc element because the HEA superconductors reported to date contain superconducting elements [28]. We focused on the Nb-containing HEAs.
The concept of HEA is now used in superconducting materials with the crystal structures possessing multiple Wyckoff positions. For example, CsCl-type, α-Mn-type, A15, NaCl-type, σ-phase and CuAl2-type HEA superconductors are reported [29, 30, 31, 32, 33, 34, 35, 36, 37]. High degree-of-freedom in such a multisite HEA design would promote the investigations of multisite HEA superconductors. The second purpose of this chapter is the materials research on the hexagonal Mn5Si3-type HEAs, possessing multiple Wyckoff positions. Recently, several superconductors with the Mn5Si3-type—or its ordered derivative Ti5Ga4-type—structure have been found and attract much attention [38, 39, 40, 41, 42, 43, 44]. Besides, many intermetallic compounds are crystallizing into these crystal structures [45, 46]. Figure 1(b) shows the crystal structure of the Mn5Si3-type compound represented by M5X3. The space group is P63/mcm, and the M atoms occupy the 4d (for M1 atom) and 6 g (for M2 atom) Wyckoff positions and the X atom another 6 g one. The M2 atoms form a face-sharing octahedral chain along the c-axis. The X atoms also form another octahedron, which encloses the M1 atom forming a one-dimensional atomic chain along the c-axis.
In this chapter, we report the synthesis and characterization of the fcc and the Mn5Si3-type HEA samples. The measurement of AC magnetic susceptibility checked the superconducting state. We also present the phase analyses of both kinds of samples. Finally, the future direction of materials research on superconducting HEAs is mentioned.
All samples were synthesized by a home-made arc furnace in an Ar atmosphere. The constituent elements as listed in Table 1 were arc-melted on a water-cooled Cu hearth. The samples were turned over and melted several times. The Mn5Si3-type HEAs were annealed at 800°C for four days in evacuated quartz tubes.
Element
Supply company
Purity (%)
Crystal structure
Atomic radius (Å)
VEC
Zr
Soekawa Chemicals, Tokyo, Japan
99
A3 (hcp)
1.6025
4
Nb
Nilaco, Tokyo, Japan
99.9
A2 (bcc)
1.429
5
V
Kojundo Chemical Laboratory, Sakado, Japan
99.9
A2 (bcc)
1.316
5
Ru
Soekawa Chemicals, Tokyo, Japan
99.9
A3 (hcp)
1.3384
8
Ir
Furuya Metal, Tokyo, Japan
99.99
A1 (fcc)
1.3573
9
Rh
Soekawa Chemicals, Tokyo, Japan
99.9
A1 (fcc)
1.345
9
Pd
Tanaka Kinzoku Kogyo, Tokyo, Japan
99.9
A1 (fcc)
1.3754
10
Cu
Soekawa Chemicals, Tokyo, Japan
99.99
A1 (fcc)
1.278
11
Sc
Furuya Metal, Tokyo, Japan
99.9
A3 (hcp)
1.641
3
Ti
Nilaco, Tokyo, Japan
99.9
A3 (hcp)
1.4615
4
Ga
Kojundo Chemical Laboratory, Sakado, Japan
99.99
All
1.392
3
Si
Soekawa Chemicals, Tokyo, Japan
99.999
A4
1.153
4
Ge
Soekawa Chemicals, Tokyo, Japan
99.999
A4
1.24
4
Pt
Tanaka Kinzoku Kogyo, Tokyo, Japan
99.9
A1 (fcc)
1.387
10
Table 1.
Materials used in this study. The supply company, purity, crystal structure at room temperature, atomic radius [47], and VEC are also listed.
A powder X-ray diffractometer (XRD-7000 L, Shimadzu, Kyoto, Japan) with Cu-Kα radiation was employed to detect the X-ray diffraction (XRD) patterns of prepared samples. The microstructure of each sample was examined by a field emission scanning electron microscope (FE-SEM, JSM-7100F; JEOL, Akishima, Japan). The atomic compositions of the samples were checked by an energy dispersive X-ray (EDX) spectrometer equipped to the FE-SEM.
To confirm the diamagnetic signal due to the superconducting state, the temperature dependence of the AC magnetic susceptibility χac (T) was measured by a home-made system in a GM refrigerator (UW404, Ulvac cryogenics, Kyoto, Japan) between 3 and 300 K. The amplitude and the frequency of the AC field were 5 Oe and 800 Hz, respectively.
The starting compositions of prepared Nb-containing samples were determined, considering the conventional design rule [1, 2]: a δ-value less than 5% and a VEC larger than 8.0. To realize the requirements, fcc elements were predominantly used (see also Tables 1 and 2). The parameters of δ and VEC were calculated as follows:
No.
Sample
Composition of Phase I, II or III
δ
VEC
1
Cu20Nb15Pd25Rh30V10
3.52
8.65
Phase I
Cu8.3(8)Nb21.2(8)Pd21.6(4)Rh42.9(5)V6.0(5)
3.14
8.29
Phase II
Cu14.4(5)Nb12.6(5)Pd28(1)Rh26.4.(8)V18.6(5)
3.26
8.32
Phase III
Cu65(5)Nb2(1)Pd28(2)Rh2(1)V3(1)
3.56
10.38
2
Cu21Ir21Nb15Pd22Rh21
3.45
9.04
Phase I
Cu1.4(6)Ir36.4(8)Nb24.3(4)Pd10.1(9)Rh27.8(3)
2.52
8.16
Phase II
Cu8.5(7)Ir6(1)Nb12.5(6)Pd46(3)Rh27(2)
2.66
9.13
Phase III
Cu40(5)Nb4(1)Pd52(3)Rh4(1)
3.74
10.16
3
Cu21Nb15Pd22Rh21Zr21
8.00
7.99
Phase I
Cu19.3(3)Pd37.2(2)Rh19.2(4)Zr24.3(5)
8.30
8.54
Phase II
Cu7(1)Nb41(1)Pd8(1)Rh27(1)Zr17(1)
6.60
6.73
Phase III
Cu57(1)Pd13(1)Rh9(1)Zr21(1)
9.33
9.22
4
Cu20Nb15Pd24Rh25V10Zr6
5.62
8.34
Phase I
Cu14.5(3)Nb15.7(5)Pd31(1)Rh23.1(5)V7(1)Zr8.7(5)
5.98
8.26
Phase II
Cu15.0(5)Nb24(1)Pd18(1)Rh27.2(5)V15.8(8)
3.72
7.89
Phase III
Cu86(1)Pd14(1)
2.62
10.86
5
Cu40Nb20Pd30V10
4.44
8.9
Phase I
Cu21.1(2)Nb27.6(2)Pd39.7(7)V11 5(7)
4.05
8.25
Phase II
Cu89.2(5)Pd10.8(5)
2.35
10.89
6
Ir10Nb17Pd33Rh28Ru12
2.21
8.54
Phase I
Ir15.5(2)Nb17.7(8)Pd20.3(7)Rh29.7(2)Ru16.8(5)
2.30
8.33
Phase II
Ir4.0(6)Nb16.0(5)Pd51(1)Rh22(1)Ru7.0(7)
2.02
8.8
Table 2.
δ and VEC of Nb-containing samples and phases detected by EDX measurements.
δ=100×∑i=1nci1−rir¯2E1
and
VEC=∑i=1nciVECi,E2
where ci, ri, and VECi are the atomic fraction, the atomic radius, and the VEC of element i, respectively, and r¯ is the composition-weighted average atomic radius. The parameter δ means the degree of the atomic size difference among the constituent elements. The calculated parameters for the prepared samples are listed in Table 2, in which the samples are named as their starting compositions. Cu20Nb15Pd25Rh30V10, Cu21Ir21Nb15Pd22Rh21 and Ir10Nb17Pd33Rh28Ru12 fulfill the design requirements. To investigate the effect of larger δ values, two samples, including a Zr atom, were prepared.
Shown in Figure 2 is the XRD patterns of prepared samples. In the upper five samples, all containing Nb, Pd, and Cu atoms, Cu20Nb15Pd25Rh30V10 and Cu21Ir21Nb15Pd22Rh21 possess dominant fcc phases. On the other hand, the XRD patterns of Zr-containing samples (Cu21Nb15Pd22Rh21Zr21 and Cu20Nb15Pd24Rh25V10Zr6) cannot be characterized by fcc phases. These results suggest that Zr is unfavorable for the formation of an fcc structure. In order to further investigate the formation condition of the single fcc phase, the quaternary alloy Cu40Nb20Pd30V10 was synthesized. As shown in Figure 2, this sample exhibits two fcc phases with quite different lattice parameters. The XRD pattern of the sample with no Cu atom (see the bottom of Figure 2) can be explained by an fcc phase. The lattice parameters of all fcc phases were obtained by the least-square method [48, 49] and are shown in Figure 2.
Figure 2.
XRD patterns of Nb-containing samples. The origin of each pattern is shifted by an integer value.
Figures 3(a)–(f) display the SEM images of samples, all indicating multi-phases. In Cu20Nb15Pd25Rh30V10 (Figure 3(a)) and Cu21Ir21Nb15Pd22Rh21 (Figure 3(b)), three contrast phases I, II and III were detected. In each case, the brightest area (phase I) showed a dendritic morphology, which is surrounded by phase II with the median contrast. The darkest area (phase III) would be the precipitate that formed in the final solidification process. A part of Cu21Nb15Pd22Rh21Zr21 (Figure 3(c)) or Cu20Nb15Pd24Rh25V10Zr6 (Figure 3(d)) showed a eutectic-like structure formed by phase I and phase II (see, for example, the green elliptic closed-curve). As shown in Figure 3(e), Cu40Nb20Pd30V10 possesses two phases, both of which would be fcc phases taking into account the XRD results. Ir10Nb17Pd33Rh28Ru12 displays two contrast areas (see phases I and II in Figure 3(f)). The shape of the main phase has a dendritic-like morphology. The compositions of all phases determined by EDX are listed in Table 2.
Figure 3.
Back-scattered electron (15 keV) images of (a) Cu20Nb15Pd25Rh30V10, (b) Cu21Ir21Nb15Pd22Rh21, (c) Cu21Nb15Pd22Rh21Zr21, (d) Cu20Nb15Pd24Rh25V10Zr6, (e) Cu40Nb20Pd30V10, and (f) Ir10Nb17Pd33Rh28Ru12, respectively.
χac (T) measurements of all prepared samples suggested no superconductivity down to 3 K, which means that a Nb-containing fcc-HEA might be an inadequate strategy for searching fcc HEA superconductors. The appearance of superconductivity in a Nb-containing fcc compound might be a rare event because almost all Nb-based superconductors form bcc-related structures. NbN or NbC superconductor is a rare example, crystallizing into a NaCl-type structure related to the fcc structure [50, 51]. Although our results would be negative for the research of Nb-containing fcc HEA superconductors, it is to be noted that phase II in sample no. 1, phase I in sample no. 6 and possibly phases I and II in sample no. 4 are new members of HEA.
Here, we discuss the fcc phase stability, viewed from the parameters of δ and VEC, which is summarized in Table 2. We also calculate these parameters for the phases detected by EDX. The values of δ were very large in Cu21Nb15Pd22Rh21Zr21 and Cu20Nb15Pd24Rh25V10Zr6 due to the larger atomic radius of Zr, which would lead to no fcc phase in each sample. While δ is reduced in Cu20Nb15Pd25Rh30V10 and Cu21Ir21Nb15Pd22Rh21, each sample shows three phases, all with rather low δ values. In each case, going from phase I to phase III, the VEC value increases, which accompanies the decrease (increase) of the Nb (Cu) atomic fraction. This suggests that the combination of Nb and Cu is not recommended even with a reduced δ, because an Nb-rich phase and a Cu-rich phase are stabilized for a smaller VEC and a larger VEC, respectively. Probably due to this reason, quaternary Cu40Nb20Pd30V10 does not show a single fcc phase. We note here that the addition of Cu leads to the breakdown of single-phase fcc CoCrFeNi into two fcc phases. This is ascribed to the positive enthalpy of mixing between the Cu and several elements [52]. Ir10Nb17Pd33Rh28Ru12 with substantially suppressed δ and no Cu atom was expected to show a single fcc phase; however, two phases were detected in the sample. The detected phases possess reduced δ values and a similar VEC. Thus, it may not be easy to synthesize a single-phase Nb-containing fcc HEA.
3.2 Mn5Si3-type HEAs
We have prepared five Mn5Si3-type HEAs as listed in Table 3, and the XRD patterns are given in Figure 4. All XRD patterns are well indexed by the hexagonal Mn5Si3-type structure, and the determined lattice parameters are displayed in Figure 4. The SEM images of all samples are presented in Figures 5 and 6, and χac (T) of each sample is shown in Figure 7.
Chemical compositions of phases detected by EDX measurements for Mn5Si3-type HEA samples. VEC of each starting composition is also shown.
Figure 4.
XRD patterns of Mn5Si3-type HEAs. The simulation pattern is also shown. Each pattern is shifted by an integer value for clarity.
Figure 5.
Back-scattered electron (15 keV) images of (a) (NbScTiVZr)(GaGeSi), and (b) (Nb1.25Sc1.25Ti1.25Zr1.25)(Ge1.8Si1.2), respectively. The elemental mappings are also shown.
Figure 6.
Back-scattered electron (15 keV) images of (a) (Nb1.25Sc1.25Ti1.25Zr1.25)(Ge1.55Ru0.47Si0.98), (b) (Nb1.4ScTiZr1.6)(Ge1.6Pt0.3Si1.1), and (c) (Nb1.4ScTiZr1.6)(Ge1.6Ir0.3Si1.1), respectively.
Figure 7.
Temperature dependences of χac of Mn5Si3-type HEA samples.
We have started from (NbScTiVZr)(GaGeSi), which shows a diamagnetic signal (see Figure 7). However, as shown in Figure 5(a), the elemental mapping has revealed the inhomogeneous distribution of constituent elements, which is obviously signaled by the V atom. The atomic compositions determined by EDX are (Nb13.0(1)Sc15.5(1)Ti11.2(1)V4.6(2) Zr19.0(1))(Ga4.4(2)Ge19.3(1)Si13.1(1)) for the V-poor phase and Ga7(1)Ge6(1)Nb19(1)Sc8(1)Si7(1)Ti17(1)V28(1) Zr8(1) for the V-rich phase, respectively. The separately synthesized latter phase crystallizes into a bcc structure. This compound also shows the diamagnetic signal at approximately 5 K, which is identical to that of (NbScTiVZr)(GaGeSi). Therefore, (NbScTiVZr)(GaGeSi) would be an intrinsically normal state down to 3 K. The result of the chemical composition of the Mn5Si3-type phase in (NbScTiVZr)(GaGeSi) suggests the difficulty of incorporation of V and Ga atoms in a Mn5Si3-type HEA. Taking into account this experimental result, we have synthesized (Nb1.25Sc1.25Ti1.25Zr1.25)(Ge1.8Si1.2). As shown in Figure 4, the sample is almost single phase, which is also supported by homogeneous elemental mapping (see also Figure 5(b)). The determined atomic composition is (Nb17(1)Sc14(1)Ti16(1)Zr17(1))(Ge22(1)Si14(1)), which agrees well with the starting composition. While the single-phase Mn5Si3-type HEA is successfully obtained, the diamagnetic signal cannot be confirmed down to 3 K, as shown in Figure 7.
One of the conceivable reasons for no superconductivity in the samples mentioned above is that the VEC value is slightly less than the optimal value (see also Table 3). As pointed out in the review [28], multisite HEA superconductors follow the respective Matthias rule, which means the important role of the density of states at the Fermi level. The VEC values of Mn5Si3-type superconductors Zr5Sb3 and Zr5Ge2.5Ru0.5 are 4.375 and 4.25, respectively [40, 42], while the VEC value of (NbScTiVZr)(GaGeSi) or (Nb1.25Sc1.25Ti1.25Zr1.25)(Ge1.8Si1.2) is 4. Thus, aiming at increasing the VEC, we substituted Ru, Pt, or Ir atoms at the Si site of Mn5Si3-type HEA. The prepared samples were (Nb1.25Sc1.25Ti1.25Zr1.25)(Ge1.55Ru0.47Si0.98), (Nb1.4ScTiZr1.6)(Ge1.6Pt0.3Si1.1), and (Nb1.4ScTiZr1.6)(Ge1.6Ir0.3Si1.1) with the respective VEC value of 4.234, 4.275, and 4.238. In each sample, the main phase of XRD pattern is well characterized by the Mn5Si3-type structure (see Figure 4). However, the atomic composition, deviating from the starting one, as shown in Table 3, indicates that Ru, Pt, or Ir atoms cannot replace the atoms at the Si site. The SEM images of these samples show the precipitation of impurity phases at the grain boundaries of hexagonal-shaped main phases (see Figure 6(a)–(c)). χac (T) measurements of these Mn5Si3-type HEA do not show a superconducting signal down to 3 K.
We have carried out materials research on the fcc and the Mn5Si3-type HEA superconductors. In the study of fcc HEA superconductors, we employed the Nb element, taking into account that the inclusion of rather high Tc elements is advantageous. Although some Nb-containing samples showed dominant fcc phases, single-phase ones could not be obtained. While we have found several new Nb-containing HEA phases in the multi-phase samples, no superconductivity appeared in each HEA phase down to 3 K. Considering that the Nb element forms a bcc structure at room temperature, and there are only several examples of fcc-related Nb-based superconductors, the discovery of Nb-containing fcc HEA superconductors would be a hard task. For the Mn5Si3-type HEA samples, we have successfully confirmed the single phase in (Nb1.25Sc1.25Ti1.25Zr1.25)(Ge1.8Si1.2) with VEC = 4. However, this HEA compound does not show the diamagnetic signal down to 3 K. Some Mn5Si3-type superconductors possess the VEC value of 4.25 or 4.375. Thus Ru, Ir, or Pt element was substituted at the Si site to increase VEC, resulting in the unsuccessful attempt. We need a strategy to adjust the VEC value for the Mn5Si3-type HEAs.
The conclusions regarding the manufacturing process or microstructure in HEA superconductors are bulleted below.
If one wants to obtain a single-phase sample with bcc, hcp, or fcc type structure by the arc-melting method, it would be a rather hard task due to an appearance of secondary phase and/or of phase with a slightly different composition.
Eutectic HEAs receive much attention due to the rich functions arising from the microstructures [53]. In some cases, eutectic superconductors show enhanced superconducting critical temperatures. Therefore, the study of the eutectic phase in HEA superconductors might be interesting.
Mechanical alloying has been widely used to produce HEAs [54]. The mechanical alloying process is different from the arc-melting one. So this is another route to obtain single-phase HEA superconductors.
The formation of single-phase fcc HEA is realized at VEC larger than 8.0. According to the Matthias rule of transition metal alloys, Tc at such a large VEC value is substantially reduced. Therefore, the measurement of physical properties at much lower than 3 K is desired. Because the synthesis of quinary alloy with the single-phase might be a hard task, research on ternary or quarternary fcc multicomponent superconductor would be necessary. In the research area of HEAs, the CALPHAD (calculation of phase diagram) method is rapidly used for the prediction of HEAs or the study of the phase relation between HEAs and other alloys. If the thermodynamic data of various compounds in the present Nb-containing multicomponent systems are sufficiently collected, the CALPHAD method will elucidate the stability of a HEA in this system. Thus, our results will greatly assist in the evaluation of the CALPHAD method in the future.
Nb5Ir3O, crystallizing into the Ti5Ga4-type structure, which is the ordered derivative of Mn5Si3-type structure, is well known as a two-band superconductor [41]. By substituting Pt into Ir, the crossover to single-band superconductivity is observed [44], which is a rare phenomenon. This result promotes us to investigate multisite HEA superconductors for further search of the crossover phenomenon, and the high-entropy state may be a new route of controlling the superconducting band. Another interesting aspect of HEA is the cocktail effect. In bcc HEA superconductors, we have shown that the peculiar enhancement of Tc by introducing the high-entropy state is not observed [28]. It would be interesting to pursue the cocktail effect of Tc in multisite HEAs.
1.Gao M C, Yeh J W, Liaw P K, Zhang Y. High-entropy alloys: fundamentals and applications Swizerland: Springer; 2015.
2.Murty B S, Yeh J W, Ranganathan S, Bhattacharjee P P. High-Entropy Alloys. 2nd ed. Amsterdam: Elsevier; 2019.
3.Yeh J W, Chen S K, Lin S J, Gan J Y, Chin T S, Shun T T, Tsau C H, Chang S Y. Nanostructured high-entropy alloys with multiple principal elements: Novel alloy design concepts and outcomes. Adv. Eng. Mater. 2004;6:299-303. DOI: 10.1002/adem.200300567
4.Otto F, Yang Y, Bei H, George E P. Relative effects of enthalpy and entropy on the phase stability of equiatomic high-entropy alloys. Acta Mater. 2013;61:2628-2638. DOI: 10.1016/j.actamat.2013.01.042
5.Rost C M, Sachet E, Borman T, Moballegh A, Dickey E C, Hou D, Jones J L, Curtarolo S, Maria J P. Entropy-stabilized oxides. Nat. Commun. 2015;6:8485. DOI: 10.1038/ncomms9485
6.Musicό B L, Gilbert D, Ward T Z, Page K, George E, Yan J, Mandrus D, Keppens V. The emergent field of high entropy oxides: Design, prospects, challenges, and opportunities for tailoring material properties. APL Materials 2020;8:040912. DOI: 10.1063/5.0003149
7.Ying T, Yu T, Shiah Y-S, Li C, Li J, Qi Y, Hosono H. High-entropy van der Waals materials formed from mixed metal dichalcogenides, halides, and phosphorus trisulfides. J. Am. Chem. Soc. 2021;143:7042-7049. DOI: 10.1021/jacs.1c01580
8.Marik S, Singh D, Gonano B, Veillon F, Pelloquin D, Bréard Y. Enhanced magnetic frustration in a new high entropy diamond lattice spinel oxide. Scr. Mater. 2020;186:366-369. DOI: 10.1016/j.scriptamat.2020.04.027
9.Sun L, Luo Y, Ren X, Gao Z, Du T, Wu Z, Wang J. A multicomponent γ-type (Gd1/6Tb1/6Dy1/6Tm1/6Yb1/6Lu1/6)2Si2O7 disilicate with outstanding thermal stability. Mater. Res. Lett. 2020;8:424-430. DOI: 10.1080/21663831.2020.1783007
10.Rao Z, Dutta B, Körmann F, Lu W, Zhou X, Liu C, Kwiatkowski da Silva A, Wiedwald U, Spasova M, Farle M, Ponge D, Gault B, Neugebauer J, Raabe D, Li Z. Beyond solid solution high-entropy alloys: tailoring magnetic properties via spinodal decomposition. Adv. Funct. Mater. 2021;31:2007668. DOI: 10.1002/adfm.202007668
11.Quintana-Nedelcos A, Leong Z, Morley N A. Study of dual-phase functionalisation of NiCoFeCr-Alx multicomponent alloys for the enhancement of magnetic properties and magneto-caloric effect. Mater. Today: Energy 2021;20:100621. DOI: 10.1016/j.mtener.2020.100621
12.Jung C, Kang K, Marshal A, Gokuldoss Pradeep K, Seol J–B, Lee H M, Choi P–P. Effects of phase composition and elemental partitioning on soft magnetic properties of AlFeCoCrMn high entropy alloys. Acta Mater. 2019;171:31-39. DOI: 10.1016/j.actamat.2019.04.007
13.BaBa K, Ishizu N, Nishizaki T, Kitagawa J. Magnetic and transport properties of new dual-phase high-entropy alloy FeRhIrPdPt. Materials 2021;14:2877. DOI: 10.3390/ma14112877
14.Koželj P, Vrtnik S, Jelen A, Jazbec S, Jagličić Z, Maiti S, Feuerbacher M, Steurer W, Dolinšek J. Discovery of a superconducting high-entropy alloy. Phys. Rev. Lett. 2014;113:107001. DOI: 10.1103/PhysRevLett.113.107001
15.Matthias B T. Empirical relation between superconductivity and the number of valence electrons per atom. Phys Rev 1955;97:74-76. DOI: 10.1103/PhysRev.97.74
16.Collver M M, Hammond R H. Superconductivity in “amorphous” transition-metal alloy films. Phys Rev Lett 1973;30:92-95. DOI: 10.1103/PhysRevLett.30.92
17.von Rohr F O, Winiarski M J, Tao J, Klimczuk T, Cava R J. Effect of electron count and chemical complexity in the Ta-Nb-Hf-Zr-Ti high-entropy alloy superconductor. Proc. Natl. Acad. Sci. 2016;113:E7144-E7150. DOI: 10.1073/pnas.1615926113
18.von Rohr F O, Cava R J. Isoelectronic substitutions and aluminium alloying in the Ta-Nb-Hf-Zr-Ti high-entropy alloy superconductor. Phys. Rev. Mater. 2018;2:034801. DOI: 10.1103/PhysRevMaterials.2.034801
19.Marik S, Varghese M, Sajilesh K P, Singh D, Singh R P. Superconductivity in equimolar Nb-Re-Hf-Zr-Ti high entropy alloy. J. Alloys Compd. 2018;769:1059-1063. DOI: 10.1016/j.jallcom.2018.08.039
20.Ishizu N, Kitagawa J. New high-entropy alloy superconductor Hf21Nb25Ti15V15Zr24. Res. Phys. 2019;13:102275. DOI: 10.1016/j.rinp.2019.102275
21.Zhang X, Winter N, Witteveen C, Moehl T, Xiao Y, Krogh F, Schilling A, von Rohr F O. Preparation and characterization of high-entropy alloy (TaNb)1−x(ZrHfTi)x superconducting films. Phys. Rev. Res. 2020;2:013375. DOI: 10.1103/PhysRevResearch.2.013375
22.Harayama Y, Kitagawa J. Superconductivity in Al-Nb-Ti-V-Zr multicomponent alloy. J. Supercond. Nov. Magn. 2021 DOI: 10.1007/s10948-021-05966-z
23.Lee Y S, Cava R J. Superconductivity in high and medium entropy alloys based on MoReRu. Physica C 2019;566:1353520. DOI: 10.1016/j.physc.2019.1353520
24.Marik S, Motla K, Varghese M, Sajilesh K P, Singh D, Breard Y, Boullay P, Singh R P. Superconductivity in a new hexagonal high-entropy alloy. Phys. Rev. Mater. 2019;3:060602(R). DOI: 10.1103/PhysRevMaterials.3.060602
25.Liu B, Wu J, Cui Y, Zhu Q , Xiao G, Wu S, Cao G, Ren Z. Superconductivity in hexagonal Nb-Mo-Ru-Rh-Pd high-entropy alloys. Scr. Mater. 2020;182:109-113. DOI: 10.1016/j.scriptamat.2020.03.004
26.Liu B, Wu J, Cui Y, Zhu Q , Xiao G, Wu S, Cao G-h, Ren Z. Structural evolution and superconductivity tuned by valence electron concentration in the Nb-Mo-Re-Ru-Rh high-entropy alloys. J. Mater. Sci. Technol. 2021;85:11-17. DOI: 10.1016/j.jmst.2021.02.002
27.Sun L, Cava R J. High-entropy alloy superconductors: Status, opportunities, and challenges. Phys. Rev. Mater. 2019;3:090301. DOI: 10.1103/PhysRevMaterials.3.090301
28.Kitagawa J, Hamamoto S, Ishizu N. Cutting edge of high-entropy alloy superconductors from the perspective of materials research. Metals 2020;10:1078. DOI: 10.3390/met10081078
29.Stolze K, Tao J, von Rohr F O, Kong T, Cava R J. Sc−Zr−Nb−Rh−Pd and Sc−Zr−Nb−Ta−Rh−Pd high-entropy alloy superconductors on a CsCl-type lattice. Chem. Mater. 2018;30:906-914. DOI: 10.1021/acs.chemmater.7b04578
30.Stolze K, Cevallos F A, Kong T, Cava R J. High-entropy alloy superconductors on an α-Mn lattice. J. Mater. Chem. C 2018;6:10441-10449. DOI: 10.1039/C8TC03337D
31.Wu J, Liu B, Cui Y, Zhu Q , Xiao G, Wang H, Wu S, Cao G, Ren Z. Polymorphism and superconductivity in the V-Nb-Mo-Al-Ga high-entropy alloys. Sci. China Mater. 2020;63:823-831. DOI: 10.1007/s40843-019-1237-5
32.Mizuguchi Y. Superconductivity in high-entropy-alloy telluride AgInSnPbBiTe5. J. Phys. Soc. Jpn. 2019;88:124708. DOI: 10.7566/JPSJ.88.124708
33.Kasem M R, Hoshi K, Jha R, Katsuno M, Yamashita A, Goto Y, Matsuda T D, Aoki Y, Mizuguchi Y. 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;13:033001. DOI: 10.35848/1882-0786/ab7482
34.Yamashita A, Jha R, Goto Y, Matsuda T D, Aoki Y, Mizuguchi Y. An Efficient way of increasing the total entropy of mixing in high-entropy-alloy compounds: a case of NaCl-type (Ag,In,Pb,Bi)Te1-xSex (x = 0.0, 0.25, 0.5) superconductors. Dalton Trans. 2020;49:9118-9122. DOI: 10.1039/D0DT01880E
35.Liu B, Wu J, Cui Y, Zhu Q , Xiao G, Wu S, Cao G-h, Ren Z. Superconductivity and paramagnetism in Cr-containing tetragonal high-entropy alloys. J. Alloys Compd. 2021;869:159293. DOI: 10.1016/j.jallcom.2021.159293
36.Mizuguchi Y, Kasem M R, Matsuda T D. Superconductivity in CuAl2-type Co0.2Ni0.1Cu0.1Rh0.3Ir0.3Zr2 with a high-entropy-alloy transition metal site. Mater. Res. Lett. 2020:9;141-147. DOI: 10.1080/21663831.2020.1860147
37.Kasem M R, Yamashita A, Goto Y, Matsuda T D, Mizuguchi Y. Synthesis of high-entropy-alloy-type superconductors (Fe,Co,Ni,Rh,Ir)Zr2 with tunable transition temperature. J. Mater. Sci. 2021;56:9499-9505. DOI: 10.1007/s10853-021-05921-2
38.Cort B, Giorgi A L, Stewart G R. Low temperature specific heats of H(NbIrO) and R(NbPtO). J. Low. Temp. Phys. 1982;47:179-185. DOI: 10.1007/BF00682027
39.Waterstrat R M, Kuentzler R, Muller J. Structural instabilities and superconductivity in quasi-binary Mn5Si3-type compounds. J. Less Common Met. 1990;167:169-178. DOI: 10.1016/0022-5088(90)90302-Z
40.Lv B, Zhu X Y, Lorenz B, Wei F Y, Xue Y Y, Yin Z P, Kotliar G, Chu C W. Superconductivity in the Mn5Si3-type Zr5Sb3 system. Phys. Rev. B 2013;88:134520. DOI: 10.1103/PhysRevB.88.134520
41.Zhang Y, Wang B, Xiao Z, Lu Y, Kamiya T, Uwatoko Y, Kageyama H, Hosono H. Electride and superconductivity behaviors in Mn5Si3-type intermetallics. npj Quantum Materials 2017;2:45. DOI: 10.1038/s41535-017-0053-4
42.Li S, Liu X, Anand V, Lv B. Superconductivity from site-selective Ru doping studies in Zr5Ge3 compound. New J. Phys. 2018;20:013009. DOI: 10.1088/1367-2630/aa9ccd
43.Hamamoto S, Kitagawa J. Superconductivity in oxygen-added Zr5Pt3. Mater. Res. Express 2018;5:106001. DOI: 10.1088/2053-1591/aad9cf
44.Xu Y, Jöhr S, Das L, Kitagawa J, Medarde M, Shiroka T, Chang J, Shang T. Crossover from multiple- to single-gap superconductivity in Nb5Ir3−xPtxO alloys. Phys. Rev. B 2020;101:134513. DOI: 10.1103/PhysRevB.101.134513
45.Corbett J D, Garcia E, Guloy A M, Hurng W M, Kwon Y U, Leon-Escamilla E A. Widespread interstitial chemistry of Mn5Si3-type and related phases. Hidden impurities and opportunities. Chem. Mater. 1998;10:2824-2836. DOI: 10.1021/cm980223c
46.Kitagawa J, Hamamoto S. Superconductivity in Nb5Ir3−xPtxO. JPS Conf. Proc. 2020;30:011055. DOI: 10.7566/JPSCP.30.011055
47.Miracle D B, Senkov O N. A critical review of high entropy alloys and related concepts. Acta Mater. 2017;122:448-511. DOI: 10.1016/j.actamat.2016.08.081
48.Izumi F, Momma K. Three-dimensional visualization in powder diffraction. Solid State Phenom. 2007; 130:15-20. DOI: 10.4028/www.scientific.net/SSP.130.15
49.Tsubota M, Kitagawa J. A necessary criterion for obtaining accurate lattice parameters by Rietveld method. Sci. Rep. 2017;7:15381. DOI: 10.1038/s41598-017-15766-y
50.Giorgi A L, Szklarz E G, Storms E K, Bowman A L, Matthias B T. Effect of Composition on the superconducting transition temperature of tantalum carbide and niobium carbide. Phys. Rev. 1962;125:837-838. DOI: 10.1103/PhysRev.125.837
51.Matthias B T. Transition temperatures of superconductors. Phys. Rev. 1953;92:874-876. DOI: 10.1103/PhysRev.92.874
52.Otto F, Yang Y, Bei H, George E P. Relative effects of enthalpy and entropy on the phase stability of equiatomic high-entropy alloys. Acta Materialia 2013;61:2628-2638. DOI: 10.1016/j.actamat.2013.01.042
53.Lu Y, Dong Y, Guo S, Jiang L, Kang H, Wang T, Wen B, Wang Z, Jie J, Cao Z, Ruan H, Li T. A promising new class of high-temperature alloys: Eutectic high-entropy alloys. Sci. Rep. 2015;4:6200. DOI: 10.1038/srep06200
54.Vaidya M, Muralikrishna G M, Murty B S. High-entropy alloys by mechanical alloying: A review. J. Mater. Res. 2019;34:664-686. DOI: 10.1557/jmr.2019.37
Written By
Jiro Kitagawa, Naoki Ishizu and Shusuke Hamamoto
Submitted: 27 November 2020Reviewed: 29 July 2021Published: 17 August 2021
Open access peer-reviewed chapter
