Crystal Structure and Physical Properties of the Cage Compound Hf(2)B(2–2δ)Ir(5+δ)

[Image: see text] Hf(2)B(2–2δ)Ir(5+δ) crystallizes with a new type of structure: space group Pbam, a = 5.6300(3) Å, b = 11.2599(5) Å, and c = 3.8328(2) Å. Nearly 5% of the boron pairs are randomly replaced by single iridium atoms (Ir(5+δ)B(2–2δ)). From an analysis of the chemical bonding, the crystal structure can be understood as a three-dimensional framework stabilized by covalent two-atom B–B and Ir–Ir as well as three-atom Ir–Ir–B and Ir–Ir–Ir interactions. The hafnium atoms center 14-atom cavities and transfer a significant amount of charge to the polyanionic boron–iridium framework. This refractory boride displays moderate hardness and is a Pauli paramagnet with metallic electrical resistivity, Seebeck coefficient, and thermal conductivity. The metallic character of this system is also confirmed by electronic structure calculations revealing 5.8 states eV(–1) fu(–1) at the Fermi level. Zr(2)B(2–2δ)Ir(5+δ) is found to be isotypic with Hf(2)B(2–2δ)Ir(5+δ), and both form a continuous solid solution.