Polyhedral Connectivity and Emergent Electronic Behavior in Ba–Cu–As Pnictides

Ternary transition metal pnictides offer a rich landscape of structural and bonding diversity, where varying transition metal to pnictogen ratios yield polyhedral motifs, particularly tetrahedra and trigonal pyramids, and complex crystallographic arrangements. A central challenge is to understand how the dimensionality of polyhedral networks dictates the interplay between lattice dynamics and charge carrier behavior in complex quantum materials. In this work, we investigate the structure–property correlations in the Ba/Cu/As system through comprehensive structure, transport and Fermi surface studies on high-quality single crystals of five distinct phases, including the three-dimensional frameworks Ba₂Cu_18-xAs₁₀ and BaCu₈As₄, and the layered structures BaCu₆As₂, BaCu₄As₂, and BaCu₂As₂. Temperature-dependent resistivity reveals metallic behavior and follows Bloch–Grüneisen 3D scaling model in all phases. Notably, a distinct low-field cusp-like magnetoresistance in BaCu₈As₄ is reminiscent of weak antilocalization, indicative of quantum interference effects. Furthermore, BaCu₄As₂ shows high magnetoresistance and undergoes a structural phase transition from rhombohedral to triclinic symmetry at 225 K, leading to a reconstruction of the Fermi surface, as evidenced by an anomaly in electrical and thermodynamic study. High magnetic field quantum oscillation studies reveal both BaCu₈As₄ and BaCu₄As₂ host low effective mass carriers, associated with 3D Fermi pockets. The coexistence of trigonal-pyramidal and tetrahedral motifs in these two phases underpins their exotic properties, in contrast with the conventional metallicity of phases containing only tetrahedral networks.