Thermodynamic Ground States of Multifunctional Metal Dodecaborides

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 B-12-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.