Towards a Single Chemical Model for Understanding Lanthanide Hexaborides

Abstract

Supporting information for this article is given via a link at the end of the document.

Electron counting and high pressure phase transformations in metal hexaborides

“…This may tell us that the B 6 octahedron is the principal covalent building block of the P m 3m structure and that its delocalized bonding must be considered carefully when studying hexaborides with complex transport properties. 12,13 The B 6 octahedra contract under pressure, as expected, but the fraction of the volume of the cell that they occupy increases as P increases, which means that they are less compressible than other parts of the cell. We note that for all three compounds, the transition away from the P m 3m structure occurs when the octahedron occupies 3.6% of the cell, even though the absolute octahedral volumes and bond lengths are different.…”

“…We will test this hypothesis on the MB6 hexaborides (M = Ca, Sr, Ba) as they have been shown to undergo structural phase transitions under pressure. [10][11][12][13] Pressure is a powerful probe of structure-property relationships in solid state materials, and has come under special attention due to reports of high-temperature superconductivity in metal hydrides. [14][15][16][17] Metal hexaborides MB 6 have been synthesized for multiple s-block and f -block metals, and display varied electron transport properties: superconductivity (YB 6 ), 18 topological Kondo insulation (SmB 6 ), 13,19 and complex magnetism (CeB 6 , EuB 6 ).…”

“…23 Structural transformations under pressure have also been studied experimentally and theoretically, particularly for s-block hexaborides. [10][11][12][13] To understand the relationships between structure and transport properties, it is important to understand the morphological changes of metal hexaborides under pressure.…”

Electron Counting and High-Pressure Phase Transformations in Metal Hexaborides

“…The exo - bonds are two-center two-electron “normal” covalent bonds, which is why they are shorter than the more delocalized endo - bonds, but it is remarkable that the shorter bonds, generally assumed to be stronger, are more easily compressed than the longer bonds. This may tell us that the B 6 octahedron is the principal covalent building block of the Pm 3̅ m structure and that its delocalized bonding must be considered carefully when studying hexaborides with complex transport properties. , The B 6 octahedra contract under pressure, as expected, but the fraction of the volume of the cell that they occupy increases as P increases, which means that they are less compressible than other parts of the cell. We note that for all three compounds, the transition away from the Pm 3̅ m structure occurs when the octahedron occupies 3.6% of the cell, even though the absolute octahedral volumes and bond lengths are different.…”

“…Pressure is a powerful probe of structure–property relationships in solid-state materials and has come under special attention due to reports of high-temperature superconductivity in metal hydrides. − Metal hexaborides MB 6 have been synthesized for multiple s-block and f-block metals, and display varied electron transport properties: superconductivity (YB 6 ), topological Kondo insulation (SmB 6 ), , and complex magnetism (CeB 6 , EuB 6 ). , High-pressure experiments, in conjunction with electronic structure calculations, have been used to study magnetism in EuB 6 and GdB 6 , and metal–insulator transitions in SmB 6 and YB 6 . Structural transformations under pressure have also been studied experimentally and theoretically, particularly for s-block hexaborides. − To understand the relationships between structure and transport properties, it is important to understand the morphological changes of metal hexaborides under pressure.…”