Superconductivity in High. Entropy Alloys.

Remi Leano..

PHYS241 ○ Fall 2019UC Merced

2 of 7

HEAs are alloys composed of five or more multi-principal elements, with equi- or near equi-atomic percentages. Usually, they have a single crystal structure and are based on 3d metals [1].

Jien-Wei Yeh at National Tsing Hua University, Hsinchu, Taiwan created the first HEA in 2004. He attributed the stability of the solid solution phase to the material’s high configurational entropy (random mixing of elements).

Brian Cantor at the University of Oxford, not knowing of Yeh’s work, published his own development of an HEA a few months later.

Figure 1. The illustrated concepts of random mixing of elements, as represented by the circles in di erent colors, in a multicomponent alloy, modeled as an ideal solution. Configurational entropy depends on the chemical composition [2].

[1] Chen, S. et al. Entropy 2018, 20, 937. [2] Ye, Y.F. et al. Materials Today 2016, 9, 6, 349. [3] Sun, L. & Cava, R.J. Phys. Rev. Materials 2019, 3, 090301.

3 of 7

The basic concept of high entropy alloys: maximize the configurational entropy of an alloy by increasing the number of alloying elements. This minimizes the Gibbs free energy in favor of a mono-phase material [3].

The configurational entropy of mixing dominates the thermodynamics of solidification, favoring the formation of a random solid solution against intermetallic compounds.

New studies aim to more closely estimate the mixing entropy [2]. Figure 2. Random mixing in which the configurational entropy of mixing of the alloy also depends on the atom

sizes and packing [2].

[2] Ye, Y.F. et al. Materials Today 2016, 9, 6, 349. [3] Sun, L. & Cava, R.J. Phys. Rev. Materials 2019, 3, 090301.

4 of 7

Figure 2. (A-D) Schematic representation of the BCC, CsCl, FCC and HCP lattices with randomly distributed atoms. Although the small unit cell BCC and CsCl crystal structures are highly related, (the latter has di erent distributions of atoms in the ½ ½ ½ and 000 positions while the former has the same distributions of atoms in the same positions) [3].

[3] Sun, L. & Cava, R.J. Phys. Rev. Materials 2019, 3, 090301.

BCC CsCl

FCC HCP

5 of 7[3] Sun, L. & Cava, R.J. Phys. Rev. Materials 2019, 3, 090301. [4] Kozelj, P. Phys. Rev. Lett. 2014, 113, 107001. [5] Ishizu, N. et al. Results in Physics 2019, 13, 1022752. [6] Lee, Y. et al. Physica C 2019, 566,13535204.

The first superconducting HEA was discovered in 2014 at the Stefan Institute and University of Ljubljana in Slovenia [4].

Newly discovered HEA superconductors (HEA-SCs) were published June 2019 at the Fukuoka Institute of Technology in Japan and November 2019, by the Cava group at Princeton University, New Jersey, US [5, 6].

International collaboration to study HEAs in general between researchers of the Erich Schmid Institute in Austria, and the U.S. Department of Energy (DOE) Lawrence Berkeley and Oak Ridge National Laboratories (Berkeley Lab and ORNL) [3].

Figure 3. HEAs on the cover of JMR in October 2018.

6 of 7[3] Sun, L. & Cava, R.J. Phys. Rev. Materials 2019, 3, 090301.

Type-A consist of the early transition metals and crystalize on a small unit cell BCC lattice.

Type-B mainly consist of the 5d transition metals, and crystallize on a larger-unit-cell cluster-based BCC lattice.

Type-C are composed of the early transition metals and the late transition metals and crystallize on a small cell CsCl-type lattice.

Type-D HEA superconductors crystallize on a HCP lattice and were recently discovered in 2019, there is currently limited data on type-D HEA-SC [3].

7 of 7[3] Sun, L. & Cava, R.J. Phys. Rev. Materials 2019, 3, 090301.

Figure 4. The HEA superconductors that crystallize on the small cell BCC or CsCl-type lattices have the highest transition temperatures. All TCs so far are limited to the sub-10-K range. The superconducting TCs of the HEAs so far found are intermediate between those of amorphous alloys and simple binary alloys, at a fixed VEC following a trend of increasing TC with decreasing disorder [3]. VEC also predicts type.

(Valence electron count, valence electrons per atom)

[1] Chen, S. et al. Entropy 2018, 20, 937.[2] Ye, Y.F. et al. Materials Today 2016, 9, 6, 349. ⭐[3] Sun, L. & Cava, R.J. Phys. Rev. Materials 2019, 3, 090301. ⭐[4] Kozelj, P. Phys. Rev. Lett. 2014, 113, 107001.[5] Ishizu, N. et al. Results in Physics 2019, 13, 1022752.[6] Lee, Y. et al. Physica C 2019, 566,13535204.

[ ⭐] Highly recommended review papers.