High-Pressure Evolution of Unexpected Chemical Bonding and Promising Superconducting Properties of YB₆

Abstract

As the prototype of MB6 (M = alkaline-earth and rare-earth metals) compounds, YB6 possesses the highest superconducting critical temperature in this family at the ambient pressure. Here, we performed a first principle exploration on the chemical bonding states and the corresponding electronic properties of YB6 at high pressure. Two phases with Cmcm and I4/mmm space groups are predicted using CALYPSO method, energetically more stable than the previously proposed structures. The B covalent network is eventually evolved from B6 octahedron in the cubic Pm-3m phase to B24 unit in I4/mmm phase. The calculated electron–phonon coupling parameters show that the contribution of B is significantly increased by the high-pressure effect in Cmcm and I4/mmm structure, contrary to that in Pm-3m structure. Further calculations of electron–phonon coupling indicate that the high-pressure phases are likely superconducting with the major contribution by boron phonon vibration.

Electron-phonon coupling superconductivity in two-dimensional orthorhombic MB6 (M=Mg,Ca,Ti,Tang1, Liu2, Luo3 et al. 2020Phys. Rev. Materials250110View full textAdd to dashboardCite

“…The Tc of our studied materials, except for hex-TiB6, is greater than that of ort-AlB6 [35]. Compared with bulk YB6 [44,74], 2D hex-YB6 has larger N(EF) and EPC constant, which leads to a larger Tc. The different arrangement of atoms in the crystal cell results in the difference of superconductivity between 2D and bulk YB6.…”

Pressure-induced evolution of structures and promising superconductivity of ScB6

“…Transition metal borides usually display interesting physical changes due to transition metal elements with more free valency electrons than alkaline-earth-metals. Li et al 13 predicted the superconductivity of YB 6 possibly attributed to boron phonon vibration under pressure. Scandium, as a typical transition metal element, is a perfect candidate to obtain a detailed comprehension of the superconducting properties of borides.…”

Electron counting and high pressure phase transformations in metal hexaborides

“…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.…”

“…High pressure computational and experimental work has been carried out on MB 6 (M = Ca, Sr, Ba, Y), principally in the solid state physics community, frequently involving computational structure searches. 10,[24][25][26][27] Structure searching for CaB 6 25,27 and YB 6 , 10 using the MAISE and CALPYSO programs respectively, both identified a transition sequence from the P m 3m ambient-pressure structure to a Cmcm structure, and then to an I4/mmm structure.…”