Conventional high-temperature superconductivity in metallic covalently bonded binary-guest C-B clathrates

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Conventional high-temperature

superconductivity in metallic, covalently

bonded, binary-guest C-B clathrates

Nisha Geng,†Katerina P. Hilleke,†Li Zhu,‡Xiaoyu Wang,†Timothy A.

Strobel,¶and Eva Zurek∗,†

†Department of Chemistry, State University of New York at Buffalo, Buffalo, NY 14260-3000, USA

‡Department of Physics, Rutgers University, Newark, NJ 07102, USA

¶Earth and Planets Laboratory, Carnegie Institution of Washington, Washington, DC 20011, USA

E-mail: ezurek@buffalo.edu

arXiv:2210.05042v2 [cond-mat.supr-con] 2 Dec 2022

Abstract

Inspired by the synthesis of XB3C3(X= Sr, La) compounds in the bipartite sodalite

clathrate structure, density functional theory (DFT) calculations are performed on members

of this family containing up to two different metal atoms. A DFT-chemical pressure analy-

sis on systems with X= Mg, Ca, Sr, Ba reveals that the size of the metal cation, which can

be tuned to stabilize the B-C framework, is key for their ambient-pressure dynamic stabil-

ity. High-throughput density functional theory calculations on 105 P m¯

3symmetry XY B6C6

binary-guest compounds (where X, Y are electropositive metal atoms) ﬁnd 22 that are dynam-

ically stable at 1 atmosphere, expanding the number of potentially synthesizable phases by 19

(18 metals and 1 insulator). The density of states at the Fermi level and superconducting critical

temperature, Tc, can be tuned by changing the average oxidation state of the metal atoms, with

Tcbeing highest for an average valence of +1.5. KPbB6C6, with an ambient-pressure Eliash-

berg Tcof 88 K, is predicted to possess the highest-Tcamong the studied P m¯

3n XB3C3or

P m¯

3XY B6C6phases, and calculations suggest it may be synthesized using high-pressure

high-temperature techniques then quenched to ambient conditions.

Introduction

The advent of the high-pressure superconducting hydrides has renewed interest in conventional

superconductors, demonstrating that their critical temperatures (Tcs) may approach room tem-

perature.1The discovery of many of these compounds was theory driven, highlighting that ﬁrst

principles-based methods – consisting of crystal structure prediction (CSP) searches and electron-

phonon coupling calculations – can identify promising superconducting materials for future syn-

theses.2–4 One of the structure types that has emerged to be conducive for superconductivity, ini-

tially pinpointed theoretically within hydrides containing an electropositive metal,5,6 is an Im¯

symmetry XH6superhydride. This structure type is based on a bcc packing of face-sharing

X@H24 truncated octahedra with six square and eight hexagonal faces, in which the hydrogenic

lattice, isostructural with the sodalite-type clathrate, is stuffed with alkaline-earth or rare-earth

metal atoms. Theory has identiﬁed many stable compounds possessing this motif – and recently a

number of them have been synthesized under high pressure and their Tcs have been measured (e.g.,

CaH6[215 K at 172 GPa,7210 K at 160 GPa8], YH6[220 K at 183 GPa,9224 K at 166 GPa10],

and (La,Y)H6, with a transition circa 237 K at 183 GPa tentatively attributed11).

Achieving high-temperature superconductivity, even room-temperature superconductivity, is

therefore no longer the ‘holy grail’. But, because none of the predicted or synthesized high-Tc

superhydrides are stable (or even metastably recoverable) at ambient pressure, the immediate chal-

lenge is to ﬁnd light-element-based structural analogues or derivatives that could be metastable

at 1 atm. One class of materials that are actively being considered12–19 can be constructed from

the XH6superhydride lattices by replacing the framework hydrogen atoms by carbon and boron

atoms (some examples are illustrated in Fig. 1). In these light-element hexahydride analogues the

clathrate lattice is held together by strong covalent B-C bonds, which are metallized via electron

transfer from an encapsulated metal atom with the appropriate valence. Vibrations of the covalently

bonded metallic lattice induces electron-phonon coupling, resulting in superconductivity. Indeed,

efﬁcient electron-phonon coupling has been predicted and observed in various sp2/sp3covalent

materials, with the most famous example being MgB2.20–24

Figure 1: Chemical pressure (CP) schemes of simple B-C clathrates in the bipartite sodalite P m¯

structure at ambient pressure: (a) MgB3C3, (b) CaB3C3, (c) SrB3C3and (d) BaB3C3. Chemical

pressures are represented by atom-centered spherical harmonic functions in which the magnitude

of the CP in a particular direction is represented by the size of the lobes and their color (white =

positive and black = negative). A scalebar representing the magnitude of the CP lobes is included.

The large negative pressures on the Mg atom demonstrate a poor ﬁt inside the surrounding boro-

carbide cage, which improves (with a concomitant decrease in the negative CPs) as the size of the

metal atoms increase from Ca to Sr to Ba. Metal atoms are denoted in purple, and B/C atoms

emanate green/yellow stick bonds.

Pure carbon clathrates have yet to be synthesized, and geometric constraints place strict limi-

tations on the atoms that could potentially be stuffed into their cages.25–28 However, calculations

have suggested that such materials can be stabilized by substituting some of the carbon atoms with

boron and inserting small cations into the borocarbide framework.29 Boron and carbon are the

lightest elements that can form strong covalent bonds, and materials based on these elements are

known to be good candidates for phonon mediated superconductivity at atmospheric conditions

(e.g., MgB2,Tc=39 K,30 boron doped diamond Tc=4 K for a doping level of 2.5%,31 B-doped

Q-carbon with a Tcas high as 55 K32). P m¯

3nSrB3C3is the ﬁrst member of the family of borocar-

bide analogues of the clathrate hexahydrides to be predicted computationally via CSP techniques.12

This compound, in the bipartite sodalite (Type-VII clathrate) structure, was computed to be ther-

modynamically stable between 50-200 GPa12 with a Tcpredicted to be as high as 43 K at 1 atm.13

Subsequently, SrB3C3was synthesized at 57 GPa and quenched to ambient conditions in an inert

atmosphere, and evidence for the superconducting transition was recently observed.12,13 Not long

after, an isotypic lanthanum phase, LaB3C3, was synthesized at milder pressure and quenched to

ambient conditions, where its HSE06 bandgap was computed to be 1.3 eV.14

The synthesis of SrB3C3and LaB3C3inspired numerous theoretical investigations of related

materials. The superconducting mechanism in the hole-conductor P m¯

3nSrB3C3was shown to

result from the strong coupling between the sp3σ-bonding bands and boron-associated Egmodes

with a Tcof 40 K, as estimated via solution of the Eliashberg equations.15 In fact, the Tccalculated

for all of the P m¯

3n XB3C3alkaline earth analogues (X=Ca, Sr, Ba) ranged from 40-50 K.13,15–17

P m¯

3nScB3C3was dynamically unstable, however following the imaginary eigenvectors resulted

in a non-centrosymmetric Ama2structure with a spontaneous polarization that was large compared

with other well known ferroelectric materials.19 The superconducting properties of a few borocar-

bide clathrates stuffed with two different metal atoms have also been studied theoretically.16–18 In

the silicon analogues, P m¯

3nRbB3Si3was thermodynamically stable with respect to the elemental

phases between 7-35 GPa, and it remained metastable at 1 atm with an estimated Tcof 14 K.33

CSP coupled with high-throughput calculations have uncovered analogous clathrate cages, but

with inequivalent C:B ratios, identifying I4/mmm CaB2C4and SrB4C2stoichiometry structures

as superconductors with Tcs of 2 and 19 K, respectively, while SrB2C4and BaB2C4were predicted

to possess superior mechanical properties with Vickers Hardnesses, Hvs, of 44 and 41 GPa, re-

spectively,34 though they were not on the convex hull.12 We note that metal-doped clathrates with

pure carbon frameworks are also predicted to exhibit high-Tcsuperconductivity23,24 (for example

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Conventionalhigh-temperaturesuperconductivityinmetallic,covalentlybonded,binary-guestC-BclathratesNishaGeng,yKaterinaP.Hilleke,yLiZhu,zXiaoyuWang,yTimothyA.Strobel,{andEvaZurek,yyDepartmentofChemistry,StateUniversityofNewYorkatBuffalo,Buffalo,NY14260-3000,USAzDepartmentofPhysics,RutgersUniversity,N...

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Conventional high-temperature superconductivity in metallic covalently bonded binary-guest C-B clathrates

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