Thermodynamic Ground States of Multifunctional Metal Dodecaborides

Abstract

A large class of metal dodecaborides (MB12) is currently raising great expectations as multifunctional materials, but their refined structures are not fully resolved, which severely limits the understanding of structure–property relationships. Here, we report that the tetragonal tI26 structure is the thermodynamic ground state of ScB12, and we predict the tetragonal YB12, ZrB12, and HfB12 to be metastable, whereas the cubic cF52 structure is the high-temperature phase of ScB12 and represents the thermodynamic ground state of YB12, ZrB12, and HfB12. Crystal structures based on experimental synchrotron data are reported for tetragonal ScB12 and cubic YB12, and high-temperature X-ray data prove the phase transformation into cubic ScB12. In both types of crystal structures, the most prominent feature is that the boron atoms are linked into a rigid three-dimensional network of interconnected, empty B12-cuboctahedra with metal atoms in large cages in form truncated octahedra consisting of 24 boron atoms. It is the uniqueness of these configurations that causes unusual functionalities, i.e., the coexistence of high hardness, low density, and good electrical conductivity. Furthermore, we elucidate that these physical properties are of electronic origins. These findings not only resolve the longstanding structural puzzle of this family of MB12 but also provide crucial insights into the underlying nature of their remarkable properties.

Synthesis and Characterization of Single-Phase Metal Dodecaboride Solid Solutions: Zr1–xYxB12 and Zr1–xUxB12

“…26 Metal dodecaborides can crystallize in two different structures: cubic-UB12 ( 3 ̅ ) and tetragonal-ScB12 (I4/mmm) (Figure 1). 1,21,26,31 The majority of metals form the cubic phase 1,21 and only Sc, as well as solid solutions based on ScB12 (at <5 at.% secondary metal addition), crystallizes in the tetragonal structure. 26,[31][32][33] The dodecaboride structure can be thought of as a face-centered cubic (FCC), or body-centered tetragonal (BCT) for ScB12, lattice of 12-coordinate metal atoms, each surrounded by a 24 boron atom cuboctahedron cage.…”

“…1,21,26,31 The majority of metals form the cubic phase 1,21 and only Sc, as well as solid solutions based on ScB12 (at <5 at.% secondary metal addition), crystallizes in the tetragonal structure. 26,[31][32][33] The dodecaboride structure can be thought of as a face-centered cubic (FCC), or body-centered tetragonal (BCT) for ScB12, lattice of 12-coordinate metal atoms, each surrounded by a 24 boron atom cuboctahedron cage. Whether a metal can form a dodecaboride phase is primarily determined by the size of the metal in a 12 coordinate environment; generally, the metal has to have a 12-coordinate radius in between that of zirconium (1.603 Å) and yttrium (1.801 Å).…”

Radial X‐Ray Diffraction Study of Superhard Early Transition Metal Dodecaborides under High Pressure

“…Each metal atom is located at the center of a cuboctahedral cluster (blue in color) formed by 24 borons. Smaller clusters (black), formed by 12 borons, can be found at the edge centers and body centers . Note that the occurrence of cuboctahedral clusters in WB 4 is at most one for every four metal atoms, whereas it is one per metal atom in MB 12 structures.…”

Design and Synthesis of Clathrate LaB8 with Superconductivity

“…To date, B-based clathrate structures have existed only in two types of MB 12 compound, namely I4/mmm ScB 12 and Fm-3m UB 12 , which include sodalite-like B 24 cages with metal atoms at the center of each B cage [4]. With different guest atoms, MB 12 exhibits properties such as super-hardness (above 40 GPa for ZrB 12 [5] and Zr 0.5 Y 0.5 B 12 [6]), superconductivity (at 4.7 K for YB 12 and 5.8 K for ZrB 12 [7]), and oxidation resistance (at ∼695 • C for Y 0.5 Sc 0.5 B 12 [8]), making them of broad interest for industrial application. Studies of B-based clathrate materials seem to have focused mainly on guest-atom substitution [6,9].…”