Design and synthesis of clathrate ${\mathrm{LaB}}_8$ with superconductivity

Boron-based clathrate materials, typically with three-dimensional networks of B atoms, have tunable properties through the substitution of guest atoms, but the tuning of B cages themselves has not yet been developed. By combining a crystal structural search with the laser-heated diamond anvil cell technique, we successfully synthesized a new B-based clathrate boride, ${\mathrm{LaB}}_8$, at $\sim 108\mathrm{GPa}$ and $\sim 2100\mathrm{K}$. The novel structure has a B-richest cage, with 26 B atoms encapsulating a single La atom. ${\mathrm{LaB}}_8$ demonstrates phonon-mediated superconductivity with an estimated transition temperature of 14 K at ambient pressure, mainly originating from the electron-phonon coupling of B cage. The replacement of La with alkaline earth metals can remarkably elevate the transition temperature. This work creates a prototype platform for subsequent investigation on tunable electronic properties through the choice of captured atoms.

Figure 1

(a) Thermodynamic stability of various La-B compounds with respect to elements La and B at selected pressures. The energetically stable phases are shown using solid symbols connected by solid lines on the convex hull. (b) Pressure-composition phase stability diagram of the La-B system.

Figure 2

(a) The crystal structures of clathrate ${\mathrm{LaB}}_8$. Each cage consists of 12 twisted rhombi and six twisted hexagons, with a single La atom at the center. (b) Experimental XRD data (black points) of cell-3 collected at 131 GPa with Rietveld refinement (red line) of the R-$3m$ ${\mathrm{LaB}}_8$ and Fm-$3m$ MgO phase. The two-dimensional cake image is presented above the integrated pattern. Unidentified reflections are indicated by asterisks.

Figure 3

(a) Experimental XRD patterns of cell-2 during decompression in the pressure range of 66–108 GPa. (b) Experimental EOS (solid black line) with ${\mathrm{B}}_0$ = 194(2) GPa, ${\mathrm{B}}_0^{\prime }$ = 4.0 (fixed) and calculated EOS (dashed lines) with ${\mathrm{B}}_0$ = 188(1) GPa (DFT-PBE), ${\mathrm{B}}_0^{\prime }$ = 4.0 (fixed) for R-$3m$ ${\mathrm{LaB}}_8$. Experimental data for the samples are shown as asterisk, rhombus, and square points. The third-order Birch-Murnaghan equation was fitted to the PV data.

Figure 4

(a) The electronic band structure and projected DOS for R-$3m$ ${\mathrm{LaB}}_8$ at 0 GPa. (b) Phonon dispersion relations, projected PHDOS, and Eliashberg spectral function for of ${\mathrm{LaB}}_8$ at 0 GPa. The size of the red dots represents the magnitude of the EPC.