Evolution of boron clusters in iron tetraborides under high pressure: semiconducting and ferromagnetic superhard materials

Abstract

We investigated the high-pressure structures and properties of iron tetraborides (FeB 4 ) using a combination of an ab initio high-throughput search and a particle-swarm optimization algorithm for crystal structure prediction. We found that, under compression, the boron sublattice in FeB 4 from the buckled boron layer first polymerizes into B 4 tetrahedral clusters and then forms cubo-octahedral B 12 clusters. At 55 GPa, the orthorhombic crystal structure with a Pnnm space group (58-FeB 4 ) transforms into a tetragonal I4 1 /acd structure (142-FeB 4 ), which is stable within a wide pressure range up to 695 GPa. Then, a cubic Im 3m phase (229-FeB 4 ) emerges at higher pressures up to at least 1 TPa. The computed Vicker's hardnesses of 58-, 142-, and 229-FeB 4 are 61.58, 47.44, and 50.87 GPa, respectively. All of them can be considered as superhard materials. Compared to the previously reported 58-FeB 4 as a superhard superconductor, the B 4 tetrahedral cluster-based 142-FeB 4 is a superhard semiconductor with an indirect band gap of 1.34 eV. The pressure-induced metal-to-semiconductor transition can be related to a unique Fe-B-B three-center covalent bond. Moreover, 229-FeB 4 , which is composed of cubooctahedral B 12 clusters, is ferromagnetic with a magnetic moment of 0.929m B per Fe atom at ambient pressure. The magnetic moment will decrease rapidly with increasing pressure and be completely quenched as pressure exceeds 40 GPa. The pressure-induced evolution of boron cluster units not only adds new features to boron chemistry, but also gives rise to novel superhard semiconductors or ferromagnetic materials. Moreover, our results may inspire further experimental and theoretical interest in designing new materials using clusters as pseudo-atoms with expected properties.

Superhard Semiconducting Phase of Osmium Tetraboride Stabilizing at 11 GPa

“…Intriguingly, recent works have also predicted high pressure phases of FeB 4 . A metal-to-semiconductor phase transition with a transition pressure of 48–55 GPa has been predicted in this material, and the predicted semiconducting phases have I 4 1 / acd and P 4 2 / nmc symmetries. − The superconducting temperature of the ambient phase of FeB 4 was reported at around 2.9 K by both theoretical and experimental studies. , For OsB 4 , this material has been predicted at ambient pressure as a hard metallic Pmmn phase with a hardness of 28 GPa and suggested to have a high-pressure phase with MoB 4 -type structure ( P 6 3 / mmc ) at 33.7 GPa . However, the previous theoretical high-pressure study of OsB 4 was limited to a few selected structures from the literature.…”

“…Intriguingly, Figure 4b shows that the PDOS of the P4 2 /nmc phase at 15 GPa has a semiconducting feature similar to those of the P4 2 /nmc and I4 1 /acd phases predicted in FeB 4 . 17,18 According to Figure 5a, the band structure displays an indirect band gap in which the valence band maximum (VBM) and conduction band minimum (CBM) are at different wave vectors. The band gap of the P4 2 /nmc phase at 15 GPa is 2.02 and 2.90 eV, as calculated by using GGA-PBE and the hybrid functional of HSE06, 40 respectively.…”

Structural variety beyond appearance: high-pressure phases of CrB4in comparison with FeB4

“…This phase is found to be stable up to 300 GPa. Under further compression, another phase transition above 695 GPa was reported by Jiang et al 28 This new cubic Im% 3m phase was predicted to be stable up to 1 TPa. As we know, the neighboring CrB 4 has similar structural and metallic features under ambient conditions.…”

Structural Phase Transitions, Electronic Properties, and Hardness of RuB4 under High Pressure in Comparison with FeB4 and OsB4

“…4,5 Its high-pressure phases (the tetragonal structures with space groups of I4 1 /acd and P4 2 / nmc) are the semiconducting and hard/superhard materials. 6,10 Recently, the structural transitions sequence of osmium tetraboride (OsB 4 ) has been presented up to 300 GPa. 11,12 At 11 GPa, the P4 2 /nmc phase emerges in OsB 4 similarly resembling FeB 4 .…”

“…For instance, iron tetraboride (FeB 4 ) exhibits superhard and superconducting properties. , Its high-pressure phases (the tetragonal structures with space groups of I 4 1 / acd and P 4 2 / nmc ) are the semiconducting and hard/superhard materials. , Recently, the structural transitions sequence of osmium tetraboride (OsB 4 ) has been presented up to 300 GPa. , At 11 GPa, the P 4 2 / nmc phase emerges in OsB 4 similarly resembling FeB 4 . Both FeB 4 and OsB 4 undergo a metal-to-semiconductor phase transition.…”