High-Entropy Rare Earth Tetraborides

TL;DR

This study reports the first successful synthesis of six high-entropy rare earth tetraborides with a tetragonal structure, demonstrating their high density, hardness, and unique lattice distortions, expanding high-entropy ceramic materials.

Contribution

It introduces a new class of high-entropy borides with a tetragonal structure, synthesized via high-energy ball milling and spark plasma sintering, with detailed characterization.

Findings

Achieved >98% density in synthesized specimens

Determined microhardness of ~13-15 GPa

Observed anisotropic lattice distortion

Abstract

Six high-entropy rare earth tetraborides of the tetragonal UB4-prototyped structure have been successfully synthesized for the first time. The specimens are prepared from elemental precursors via high-energy ball mill and in-situ reactive spark plasma sintering. The sintered specimens are >98% in relative densities without detectable oxide impurities (albeit the presence of minor hexaborides in some compositions). No detectable secondary phase is observed in the composition (Y$_{0.2}$Nd$_{0.2}$Sm$_{0.2}$Gd$_{0.2}$Tb$_{0.2}$)B$_{4}$, which is proven homogeneous at both microscale and nanoscale. The Vickers microhardness are determined to be ~13-15 GPa at a standard indentation load of 9.8 N. A scientifically interesting observation is represented by the anisotropic lattice distortion from the rule-of-mixture averages. This work expands the family of high-entropy ceramics via fabricating a new class of high-entropy borides with a unique tetragonal quasi-layered crystal structure.