Effect of electron count and chemical complexity in the Ta-Nb-Hf-Zr-Ti high-entropy alloy superconductor

Abstract

High-entropy alloys are made from random mixtures of principal elements on simple lattices, stabilized by a high mixing entropy. The recently discovered body-centered cubic (BCC) Ta-Nb-Hf-Zr-Ti highentropy alloy superconductor appears to display properties of both simple crystalline intermetallics and amorphous materials; e.g., it has a well-defined superconducting transition along with an exceptional robustness against disorder. Here we show that the valence electron count dependence of the superconducting transition temperature in the high-entropy alloy falls between those of analogous simple solid solutions and amorphous materials and test the effect of alloy complexity on the superconductivity. We propose high-entropy alloys as excellent intermediate systems for studying superconductivity as it evolves between crystalline and amorphous materials.high-entropy alloys | superconductivity | disordered metals A lloys are among the most relevant materials for modern technologies. Conventional alloys typically consist of one principal element, such as the iron in steel, plus one or more dopant elements in small proportion (e.g., carbon in the case of steel) that enhance a certain property of interest; the properties are based on the modification of those of the principal element. In sharp contrast, highentropy alloys (HEAs) are composed of multiple principal elements that are all present in major proportion, with the simple structures observed attributed to the high configurational entropy of the random mixing of the elements on their lattice sites (1). Thus, the concept of a "principal element" becomes irrelevant. The elements in HEAs arrange on simple lattices with the atoms stochastically distributed on the crystallographic positions; HEAs are commonly referred to as metallic glasses on an ordered lattice ( Fig. 1 A and B). The properties of HEAs arise as a result of the collective interactions of the randomly distributed constituents (2, 3). There is no strict definition, but HEAs are typically composed of four or more major elements in similar concentrations. By applying this concept, several new alloys with simple body-centered cubic (BCC), hexagonal closest-packing (HCP), or face-centered cubic (FCC) structures have been realized (2, 3, 4). The HEAs compete for thermodynamic stability with crystalline intermetallic phases with smaller numbers of elemental constituents (5). Therefore, one central concept of designing these alloys is to understand the interplay between mixing entropy ΔS mixing and phase selection. Considering the large number of metals in the periodic table, the total number of possible HEA compositions is virtually unlimited.In addition to their structural and chemical diversity, HEAs can display novel, highly tunable properties such as, for example, excellent specific strength (6, 7), superior mechanical performance at high temperatures (8), and fracture toughness at cryogenic temperatures (9, 10), making them promising candidates for new applications. Simple niobium-titanium-based binary al...

Change of electronic properties on transition from high-entropy to Ni-rich (TiZrNbCu)1−xNi alloys

“…We also show that superconductivity in our alloys as well as that in crystalline HEAs [18,21,23] and binary transition metal alloys (e.g. [43]) is determined by their electronic structure and shows universal variation with the band (bare) density of states.…”

“…These measurements have shown that the coefficient γ of the electronic contribution to LTSH, which is proportional to dressed DOS at E F , N γ (E F ) structural change obey the Dynes-Varma correlation [58] which seems valid for all disordered transition metal alloys, both crystalline (e.g., [21,23]) and amorphous [43]. As already noted in [24], since N γ (E F ) is enhanced by the electron-phonon interaction, it cannot be used to prove that the changes in figure 4 for x > 0.35 are due to change in N 0 (E F ).…”

“…3 ref. [58]; squares are from [18,21]; stars are from [23]; up-triangles are from [60]; diamonds are from [45,61]; down-triangles are from [62]. [55,56].…”

“…However, present research of HEAs seems somewhat uneven, with the main effort invested in the development of new structural materials, thus focused on their mechanical properties, microstructure and thermal stability (e.g., [5,14,17]). Meanwhile, the research aimed at their electronic structures and properties, both experimental [5,18,19,20,21,22,23,24] and theoretical [5,25,26,27,28,29], is in spite of their potential as functional materials [5] still insufficient. The lack of insight into their electronic structure which in metallic systems determines almost all their properties (e.g., [30,31]) hinders the conceptual understanding of both crystalline (c-) and amorphous (a-) HEAs [19,24].…”

Pressure effects on the electronic structure and superconductivity of(TaNb)0.67(HfZrTi)0.33high entropy alloy

“…It is a type-II superconductor with the transition temperature of T c = 7.3 K. Experimental data as well as theoretical calculations [10] suggest conventional mechanism of superconductivity with a relatively strong electron phonon coupling parameter λ ∼ 1. Several other examples of * wiendlocha@fis.agh.edu.pl superconducting HEAs were later reported [11][12][13], however, the TaNbHfZrTi family is still the most investigated one [9,10,14,15]. When the atomic concentration is slightly changed to Ta 33.5 Nb 33.5 Hf 11 Zr 11 Ti 11 [15] [denoted as (TaNb) 0.67 (HfZrTi) 0.33 or TNHZT in short], superconducting transition temperature slightly increases to 7.7 K. This alloy also hosts a cubic body-centered crystal structure, with the lattice parameter of 3.34Å.…”