Structural variations, relationships and properties of M2B metal borides

Abstract

Metal borides represent a class of materials with a large variety of crystal structures, yet their physical and catalytic properties are overwhelmingly understudied. In this review, we present the structural variations, relationships and properties of metal borides with a metal-to-boron ratio (M:B) of 2:1, which have over 130 known phases (from binaries to quaternaries) that crystallize with 21 structure types. While most of these structure types contain isolated boron atoms only, B−B-bonds (d(B−B)< 2.0 Å) are observed in nine of them (less than half). Even though several coordination environments for boron atoms are found, the trigonal prismatic coordination (CN = 6(+3), present in seventeen structure types) is by far the most common followed by the square anti-prismatic coordination (CN = 8). These trigonal prisms can be undistorted like in the CeCo3B2-type, where boron consequently resides in the exact center, but also distorted prisms are found for example in the Co2Si-type. Stronger distortions like in the ZrCo3B2-type can lead to the formation of short B−B-bonds (d(B−B)< 2.0 Å). When B-centered trigonal prisms share common rectangular faces, bonding between neighboring boron occurs. Ni3ZnB2 and Ti1+xRh2–x+yIr3–yB3 are examples with zigzag B4 fragments and in Ti1+xOs2−xRuB2 trigonal planar B4 fragments are found. In the case of the MoAlB-type the rectangular faces are shared leading to a zigzag B-chain.Except for the anti-MoS2-type (Pt2B, existence is still debated), the unit cell length of the short axis in these structures is always close to 3.0 Å, which corresponds to the height of a typical B[M6] prism. In the case of URu3B2 and ZrCo3B2 the short axis is doubled respectively tripled due to their superstructure in comparison to the related CeCo3B2-type.Even though interesting magnetic and superconducting properties have been reported in many phases (especially in the CeCo3B2-type) as well as some fascinating catalytic behaviors (such as in CuAl2-type phases), more must be done in the future to capitalize on the structural diversity from this 2:1 borides composition space.