Peierls distortion, magnetism, and high hardness of manganese tetraboride

Abstract

We report crystal structure, electronic structure, and magnetism of manganese tetraboride, MnB 4 , synthesized under high-pressure, high-temperature conditions. In contrast to superconducting FeB 4 and metallic CrB 4 , which are both orthorhombic, MnB 4 features a monoclinic crystal structure. Its lower symmetry originates from a Peierls distortion of the Mn chains. This distortion nearly opens the gap at the Fermi level, but despite the strong dimerization and the proximity of MnB 4 to the insulating state, we find indications for a sizable paramagnetic effective moment of about 1.7 μ B /f.u., ferromagnetic spin correlations, and, even more surprisingly, a prominent electronic contribution to the specific heat. However, no magnetic order has been observed in standard thermodynamic measurements down to 2 K. Altogether, this renders MnB 4 a structurally simple but microscopically enigmatic material; we argue that its properties may be influenced by electronic correlations.

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] .…”

“…However, our later synthesis shows that CrB 4 does not form at all at a lower pressure of 8 GPa and temperature of 1200 °C for 1 hour, which is similar to the case of FeB 4 with reported formation pressures of 8-18 GPa [13]. But for MnB 4 , it can be synthesized at a lower pressure of 3 GPa [14].…”

“…In addition, the detailed structure of CrB 4 is an unsettled issue between orthorhombic symmetries of Immm and Pnnm [17]. A similar structure controversy has also been reported in MnB 4 [14,28]. In this work, for the first time, we synthesized CrB 4 through a solid-state reaction between chromium and boron under high-pressure (p)/hightemperature (T) conditions.…”

“…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]). …”