Crystal structures and compressibility of novel iron borides Fe2B7 and FexB50 synthesized at high pressure and high temperature
Graphical abstract
Crystal structures of novel iron borides, Fe2B7 and FexB50 (x=1.01(1), 1.04(1), 1.32(1)).
Introduction
Metal borides are an important class of compounds having a number of remarkable properties like superconductivity (MgB2, FeB4 [1], [2]), low compressibility (OsB2, MnB4, FeB4, WB4 [2], [3], [4], [5]), and high hardness (tungsten borides, FeB4 [2], [6]). Therefore, synthesis of novel metal borides and investigation of their properties are of great interest for materials science and technology. Theoretical calculations can help in a search for the compounds with a combination of beneficial properties. Recently Kolmogorov et al. [7] predicted the existence of a superconducting iron tetraboride, FeB4. Later on Bialon et al. [8] calculated the formation conditions of tetraborides and suggested that the phases are stabilized by high pressure. We have successfully synthesized FeB4 using multianvil apparatuses and demonstrated its impressive mechanical properties like high hardness of 62(5) GPa and very low compressibility with the bulk modulus of 252(5) GPa [2]. We propose that the low compressibility originates from short boron–boron covalent bonds oriented along the b crystallographic axis that makes the structure of FeB4 in corresponding direction as incompressible as diamond [2]. Similar anisotropy in compressibility is found in MnB4 structurally close to FeB4. Recent high-pressure high-temperature (HPHT) synthesis of novel cobalt boride Co5B16[9] suggests variety of structurally related borides with other transition metals.
We mentioned formation of other iron borides, Fe2B7 and Fe1.04(1)B50, along with FeB4 upon HPHT synthesis of the latter in our previous work [2]. According to theoretical calculations [10], Fe2B7 is metastable up to 30 GPa. The authors of Ref. [10] particularly emphasized that without a priori experimental knowledge ab initio prediction of the compound with such stoichiometry and a large unit cell “would have been no less than an act of clairvoyance”. In the current work we present a detailed description of the crystal structures of Fe2B7 and FexB50 (x=1.01(1), 1.04(1), 1.32(1)) and the behavior of Fe2B7 and FexB50 (x=1.04(1), 1.32(1)) under compression to about 50 GPa.
Section snippets
Sample preparation
Single crystals of Fe2B7 and FexB50 were grown using a powder of β-boron (purity of 99.9995 at%, grain size of <1000 μm, Chempur Inc.) and iron wire (purity of 99.99+%, 0.5 mm and 1 mm diameter, Alfa Aesar) as starting materials.
High-pressure high-temperature synthesis of Fe2B7 was carried out in a multianvil apparatus using a 1200-ton Sumitomo hydraulic press installed at the Bayerisches Geoinstitut (BGI, Bayreuth, Germany). Starting materials placed in h-BN capsule and pressurized to 15 GPa were
Crystal structure of Fe2B7
The X-ray diffraction data for Fe2B7 and some experimental details are presented in Tables 1 and 2. The unit cell of Fe2B7 is orthorhombic (Pbam, a=16.9699(15) Å, b=10.6520(9) Å, c=2.8938(3) Å). The crystal structure of Fe2B7 represents a new structure type and no isostructural compounds have been reported so far. The structure of Fe2B7 can be described based on a rigid covalent framework of boron atoms which have from 3 to 5 boron neighbors. Boron–boron distances in the network range between
Discussion
The five crystalline boron allotropes, α-, β-, γ-, δ-, ε- [21], [23], are rather similarly composed. All their structures contain B12 icosahedra connected through covalent B–B bonds. Compressibility data were reported only for α-, β-, and γ-B [24], [25], [26], [27], [28], all allotropes were found to contract almost uniformly in all directions.
Compressibility of FexB50 gives a rough approximation for the compressibility of δ-B phase, so far unknown. The δ-B can be grown only as tiny ~2–5 µm
Conclusions
Our study of Fe2B7 and FexB50 demonstrates how differences in crystal structure influence the high-pressure behavior of iron borides. FexB50 is composed of B12 icosahedra connected through boron and iron atoms. Such a network contracts easier upon compression due to the presence of large voids. Contrary, Fe2B7 has no pronounced voids in its structure and, similarly to MnB4 and FeB4, it is poorly compressible. Moreover, Fe2B7 together with MnB4 demonstrate the highest value of bulk modulus among
Acknowledgments
The work was supported by the German Research Foundation (DFG). N.D. thanks DFG for financial support through the Heisenberg Program and the DFG Project DU 954-8/1. H.G. gratefully acknowledges financial support of the Alexander von Humboldt Foundation.
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