Stability and superstructural ordering of alkali-triel-pnictide clathrates A$_8$T$_{27}$Pn$_{19}$

TL;DR

This study uses high-throughput density functional theory to analyze stability, electronic properties, and superstructural ordering in alkali-triel-pnictide clathrates, revealing key factors influencing their stability and structure.

Contribution

It provides new insights into stability trends, electronic influences, and superstructural ordering in ATPn clathrates, including the role of spin-orbit effects and synthesis attempts.

Findings

Ionization potential of guest atoms affects stability and rattler behavior.

Targeted synthesis yielded new compounds but not the desired clathrates.

Spin-orbit effects are crucial for stability in heavy-element clathrates.

Abstract

Clathrates are a class of inclusion compounds that offer various useful and surprising phenomena, including superconductivity, thermoelectricity, and the potential for high-density ion storage. Stability conditions within the Alkali-Triel-Pnictide A$_8$T$_{27}$Pn$_{19}$ family of unconventional clathrates are investigated with high-throughput density functional theory calculations, establishing trends in formation energy, structural and electronic properties. Electronic structure calculations and first-principles molecular dynamics simulations show that the ionization potential of guest alkaline atoms strongly influences the stability of electron-exact clathrates and affects their rattler behavior. Targeted reactive synthesis from elemental precursors is attempted, resulting in two novel ternary compounds. However, the targeted clathrate phases are not obtained. Further analysis reveals that the stability of ATPn clathrate compounds containing heavy elements, such as bismuth, depends strongly on spin-orbit effects, which are often neglected in high-throughput studies that compute formation energies. Finally, chemically induced superstructural ordering is described in relation to Wyckoff sites in the prototypical type-I clathrate unit cell.