For decades, borides have been primarily studied as crystallographic oddities. With such a wide variety of structures (a quick survey of the Inorganic Crystal Structure Database counts 1253 entries for binary boron compounds!), it is surprising that the applications of borides have been quite limited despite a great deal of fundamental research. If anything, the rich crystal chemistry found in borides could well provide the right tool for almost any application. The interplay between metals and the boron results in even more varied material's properties, many of which can be tuned via chemistry. Thus, the aim of this review is to reintroduce to the scientific community the developments in boride crystal chemistry over the past 60 years. We tie structures to material properties, and furthermore, elaborate on convenient synthetic routes toward preparing borides.

Rediscovering the Crystal Chemistry of Borides.

The Raman spectra of numerous dodecaborides have been measured on high-quality single crystals at ambient conditions with high spectral resolution and signal-to-noise ratio. Besides the strong Raman-active modes, numerous Raman-inactive modes occur in the spectra, indicating distortions of the structures. Ab initio calculation of the phonon spectra on ZrB(12) excellently agrees with the experimental results. Force constants are theoretically calculated and force parameters are estimated from the Raman frequencies. The influence of the surface range on the Raman spectra is evident. The different isotopic effects (virtual crystal approximation, the polarization effect and the effect of isotopic disorder) on the phonon frequencies are determined, separated and discussed.

https://iopscience.iop.org/article/10.1088/0953-8984/23/6/065403/pdf

Raman scattering and isotopic phonon effects in dodecaborides

Structural and mechanical properties of several rare-earth diborides were systematically investigated by first principles calculations. Specifically, we studied XB2, where X = Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, and Lu in the hexagonal AlB2, ReB2, and orthorhombic OsB2-type structures. The lattice parameters, bulk modulus, bond distances, second order elastic constants, and related polycrystalline elastic moduli (e.g., shear modulus, Young's modulus, Poisson's ratio, Debye temperature, sound velocities) were calculated. Our results indicate that these compounds are mechanically stable in the considered structures, and according to "Chen's method", the predicted Vickers hardness shows that they are hard materials in AlB2- and OsB2-type structures.

http://iopscience.iop.org/article/10.1088/1674-1056/22/4/046202/pdf

Structural and mechanical stability of rare-earth diborides

Previous efforts toward the synthesis of materials with superior mechanical properties (eg, superhardness, high bulk modulus, and high shear modulus) have focused on binary metal boride structures with short covalent bonds In this paper, we discuss the methods for enhancing the intrinsic and extrinsic hardening factors commonly used to achieve these properties As the realm of binary metal borides has all but been exhausted, there is a need to move to ternary and higher borides as well as to achieve greater morphological control at the nanoscale The use of ternary and higher borides as starting materials provides a multitude of diverse compositional and structural combinations These compounds merit further investigation as they offer the potential for the optimization of mechanical properties

/pdf/perspective-superhard-metal-borides-a-look-forward-4fvhin8jls.pdf

Perspective: Superhard metal borides: A look forward

The new ternary actinide metal diborides U 2 MoB 6 , U 2 ReB 6 , U 2 OsB 6 , URuB 4 and UOsB 4 were prepared and found to crystallize with either the Y 2 ReB 6 or the ThMoB 4 type of structure. LuRuB 4 and LuOsB 4 crystallize with the YCrB 4 type of structure. In a ternary series of solid solutions YRh 3 B 1− x (0 ⩽ x ⩽ 0.45 at 1400 °C), boron was found to stabilize a Cu 3 Au type of structure. The superconductivity of the new uranium compounds and of a series of ternary transition metal borides was investigated; no superconductivity was observed for temperatures as low as 1.3–1.5 K. The cubic perovskite or filled Cu 3 Au structure is discussed as a type which is very unfavorable for the occurrence of superconductivity.

New ternary transition metal borides containing uranium and rare earth elements

Borides with a composition LnB2 and LnB12 have been reported [1,2] for the smaller members of the lanthanide series. Diborides have been prepared [3–10] for Ln = Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, Y, Sc and dodecaborides are known [5,6,11–16] for Ln = Tb, Dy, Ho, Er, Tm, Yb, Lu, Y, Sc. The diborides are isomorphous, with an AlB2-type (C32) structure. This structure is hexagonal (P6/mmm) and consists of alternating layers of metal atoms and boron atoms. The metal atoms form close-packed (36) layers and the boron atoms, hexagonal (63) layers. The layers are arranged such that boron atoms are positioned over the faces of the triangles formed by the metal atoms. The dodecaborides (except SCB12) each assume a UB12-type(D2f) structure. This structure is cubic (Fm3m) with metal atoms at fcc positions and each metal atom surrounded by a B24 cubo-octahedron. The tetragonal structure of ScB12 is described by Matkovich, et al. [16].

David M. Cannon

Papers

High pressure syntheses of SmB2 and GdB12