Crystal Structure and Properties of Layered Pnictides BaCuSiPn (Pn = P, As).

Two novel layered compounds BaCuSiPn (Pn = P, As) adopting new structure types are reported. As revealed by single-crystal X-ray diffraction, both compounds are composed of unique Cu-Si-Pn layers featuring CuPn and SiPn structural motifs found in other archetypal pnictide materials. The stacking of the isostructural Cu-Si-Pn layers is different for phosphide and arsenide compounds. Synthesis from elements aided by synchrotron powder X-ray diffraction resulted in the obtainment of bulk powders with a minimized amount of admixtures. Experimentally measured physical properties of BaCuSiAs unexpectedly showed metal-like behavior at temperatures above 15 K, despite the fact that density functional theory calculations predict a small band gap of 0.4 eV. BaCuSiAs exhibits ultralow thermal conductivity, which can be explained by the combination of a layered crystal structure with alternating covalent and ionic bonding, which feature rattling of Cu atoms similar to that in tetrahedrites.