Towards a Single Chemical Model for Understanding Lanthanide Hexaborides

Abstract

Lanthanide hexaborides (LnB6) have disparate and often anomalous properties, from structurally homogeneous mixed valency, to superconductivity, spectral anomalies, and unexplained phase transitions. It is unclear how such a diversity of properties may arise in the solids of identical crystal structures and seemingly very similar electronic structures. Building on our previous model for SmB6 (mixed valent, with a peak in specific heat, and pressure induced magnetic phase transitions), we present a unifying dynamic bonding model for LnB6 that explains simultaneously EuB6 (possessing an anomalous peak in specific heat at low T, magnetic phase transitions, and no mixed valency), YbB6 (mixed valent topological insulator), and rather ordinary LaB6. We show that Ln can engage in covalent bonding with boron, and, in some members of the LnB6 family, also easily access alternative bonding states through the electron–phonon coupling. The accessibility, relative energetics, and bonding nature of the states involved dictate the properties.

Why Yttrium Hexaboride Exhibits a Much Higher Superconducting Tc than Near-Identical Lanthanum Hexaboride

“…Our previous studies showed that these B-B bonds constitute the most significant B-B interactions in rare-earth hexaborides. [6,9] We aim to reveal how the eigenstates (orbitals) comprising the valence bands of the studied hexaborides physically couple to these vibrations. From the PDOS in Figures 1d and 1e, we see that boron 2s and 2p orbitals and yttrium 4d orbitals comprise the Fermi surface of YB6, while boron 2s and 2p orbitals and lanthanum 5f orbitals comprise that of LaB6.…”

“…While experimental and computational studies have been conducted to characterize the Fermi level electronic structure [3,12,[37][38][39][40][41] of YB6 and LaB6 and their phonon modes [41][42][43][44][45][46][47][48] contributing to EPC, the linkage of the two -how frontier electronic eigenstates physically couple to these lattice vibrations -has yet to be established, as has been done with other prominent phonon-mediated superconductors. [26,27,49] Prior studies of our group have quantitatively and qualitatively explained properties of metal hexaborides through the lens of chemical bonding [6,9,50] , and in this study, we extend this capability to superconducting hexaborides by analyzing changes in this bonding under phonon-induced lattice deformations.…”

“…Unfortunately, any changes in band structure due to post-DFT electron correlation (band renormalization) proved too computationally expensive to compute within the G0W0 approximation, which does not require choice of Hubbard parameters as in DFT+U. Electron correlation has been shown to be crucial to properties of localized, non-conducting metal hexaborides deeper in the 4f block [6,9] , but can be considered negligible for the itinerant electronic structure of YB6 and LaB6. This is also consistent with our benchmarking showing no need for a van-der-Waals correction and the minimal effects of localizing 4f orbitals with a Hubbard correction.…”

“…All sharing an ambient crystal structure (Pm3m space group) of a metal surrounded by a network of boron octahedra, these ceramic compounds present themselves vastly differently as semiconductors [1][2][3] , Kondo insulators [4][5][6] , magnetically ordered materials [7,8] , and more. [2,[9][10][11][12][13][14][15] Lanthanum hexaboride, though primarily exploited in hot cathodes for its high electron emissivity, is also known to be a low-temperature, Bardeen-Cooper-Schrieffer (BCS, i.e phonon-mediated) superconductor at temperatures below 0.45 K. [16] Isostructural yttrium hexaboride, possessing a near-identical phonon spectrum, a similar lattice constant (4.155 Å for LaB6, 4.098 Å for YB6), and yttrium being one row directly above lanthanum in the periodic table, is a significantly better BCS superconductor at a critical temperature (Tc) of up to 8.4 K. [17][18][19] This notably makes it the only reported superconducting hexaboride with a Tc reachable with 4 He cryogenics. [20,21] It also possesses the second highest superconducting Tc of any metal boride [22] , the highest being that of the unusual [23] and thoroughly studied MgB2 [24][25][26][27][28] , the highest-Tc "conventional" (i.e.…”

Why Yttrium Hexaboride Exhibits a Much Higher Superconducting Tc than Near-Identical Lanthanum Hexaboride

“…All sharing an ambient crystal structure (Pm3m space group) of a metal surrounded by a network of boron octahedra, these ceramic compounds present themselves vastly differently as semiconductors [1][2][3] , Kondo insulators [4][5][6] , magnetically ordered materials [7,8] , and more. [2,[9][10][11][12][13][14][15] Lanthanum hexaboride, though primarily exploited in hot cathodes for its high electron emissivity, is also known to be a low-temperature, Bardeen-Cooper-Schrieffer (BCS, i.e phonon-mediated) superconductor at temperatures below 0.45 K. [16] Isostructural yttrium hexaboride, possessing a near-identical phonon spectrum, a similar lattice constant (4.155 Å for LaB6, 4.098 Å for YB6), and yttrium being one row directly above lanthanum in the periodic table, is a significantly better BCS superconductor at a critical temperature (Tc) of up to 8.4 K. [17][18][19] This notably makes it the only reported superconducting hexaboride with a Tc reachable with 4 He cryogenics. [20,21] It also possesses the second highest superconducting Tc of any metal boride [22] , the highest being that of the unusual [23] and thoroughly studied MgB2 [24][25][26][27][28] , the highest-Tc "conventional" (i.e.…”