Discovery of a Superhard Iron Tetraboride Superconductor

Abstract

Single crystals of novel orthorhombic (space group Pnnm) iron tetraboride FeB4 were synthesized at pressures above 8 GPa and high temperatures. Magnetic susceptibility and heat capacity measurements demonstrate bulk superconductivity below 2.9 K. The putative isotope effect on the superconducting critical temperature and the analysis of specific heat data indicate that the superconductivity in FeB4 is likely phonon mediated, which is rare for Fe-based superconductors. The discovered iron tetraboride is highly incompressible and has the nanoindentation hardness of 62(5) GPa; thus, it opens a new class of highly desirable materials combining advanced mechanical properties and superconductivity.

Crystal structures, elastic properties, and hardness of high-pressure synthesized CrB2 and CrB4

“…However, after an exhausting search of the known structures for TMB x , the orthorhombic TMB 4 (TM = Cr, Mn, and Fe) is found to exhibit a 3D bonding network between TM and boron [15,17,27], which is structurally more favorable than hexagonal TMB 4 for producing mechanically isotropic borides. As expected, recent reports show that the orthorhombic FeB 4 and MnB 4 possess a superior nanoindentation hardness of 62 GPa and a Vickers hardness of H V = 37.4 GPa under a load of 9.8 g [13,14], approaching the superhard regime as defined by the asymptotic H V ≥ 40 GPa. Strikingly, both borides also exhibit intriguing superconducting and magnetic properties [13,14,16] .…”

“…As expected, recent reports show that the orthorhombic FeB 4 and MnB 4 possess a superior nanoindentation hardness of 62 GPa and a Vickers hardness of H V = 37.4 GPa under a load of 9.8 g [13,14], approaching the superhard regime as defined by the asymptotic H V ≥ 40 GPa. Strikingly, both borides also exhibit intriguing superconducting and magnetic properties [13,14,16] .…”

“…Since 2007, inspired by frontier research on ReB 2 by Chung et al [7], there has been a new surge of research interest in searching for ultra-incompressible and superhard transition-metal (TM) borides, nitrides, and carbides. These efforts have led to a series of new system including diborides (ReB 2 [8][9][10] and RuB 2 [10]), tetraborides (WB 4 [11,12], FeB 4 [13], MnB 4 [14,15], and CrB 4 [16][17][18]), nitrides (Re x N [19] and WN x [20]), and carbides (Re 2 C [21]). …”

Combinatorial search of superconductivity in Fe-B composition spreads

“…Recently, Gou et al have reported that FeB 4 in the oP10 ground state has been synthesized at pressures above 8 GPa and high temperature up to 2000 K. 7 The superconductivity at 3 K was verified by diamagnetism and the isotope effect, but the sample was too small for measuring the resistive transition. Given the small fraction of the volume which was found to exhibit superconducting behavior in our samples, there may also be a small fraction of superconducting oP10 FeB 4 in our film.…”

“…To get around this problem, one can synthesize bulk compounds under high pressure. 7 Another synthesis route is to make use of non-equilibrium thin film processes such as pulsed laser deposition and magnetron sputtering. In this study, we have utilized thin film co-sputtering composition spreads to search for possible superconducting phases in the Fe-B binary system.…”