Conventional high-temperature superconductivity in metallic, covalently bonded, binary-guest C-B clathrates

TL;DR

This study uses density functional theory to identify and analyze new binary-guest C-B clathrates with high-temperature superconductivity potential, highlighting the role of metal cation size and oxidation state in stability and Tc.

Contribution

The paper reports the discovery of 22 dynamically stable binary-guest C-B clathrates at ambient pressure, with one predicted to have a record high Tc of 88 K, expanding the known family of superconducting materials.

Findings

22 stable compounds identified at 1 atm

Tc can be tuned by metal oxidation state

KPbB6C6 predicted to have Tc of 88 K

Abstract

Inspired by the synthesis of XB3C3 (X= Sr, La) compounds in the bipartite sodalite clathrate structure, density functional theory (DFT) calculations are performed on members of this family containing up to two different metal atoms. A DFT-chemical pressure analysis on systems with X= Mg, Ca, Sr, Ba reveals that the size of the metal cation, which can be tuned to stabilize the B-C framework, is key for their ambient-pressure dynamic stability. High-throughput density functional theory calculations on 105 Pm-3 symmetry XYB6C6 binary-guest compounds (where X, Y are electropositive metal atoms) find 22 that are dynamically stable at 1 atmosphere, expanding the number of potentially synthesizable phases by 19 (18 metals and 1 insulator). The density of states at the Fermi level and superconducting critical temperature, Tc, can be tuned by changing the average oxidation state of the metal atoms, with Tc being highest for an average valence of +1.5. KPbB6C6, with an ambient-pressure Eliashberg Tc of 88 K, is predicted to possess the highest-Tc among the studied Pm-3n XB3C3 or Pm-3 XY B6C6 phases, and calculations suggest it may be synthesized using high-pressure high-temperature techniques then quenched to ambient conditions.