Structure-controlled quantum magnetotransport in Ba-Cu-As pnictide single crystals.

The relationship between local coordination, framework connectivity, and lattice scattering, electronic structure, and quantum transport in complex pnictides remains poorly understood. Here, we show that Cu-As polyhedral architecture provides a structural route to tuning scattering and quantum magnetotransport in Ba-Cu-As pnictide single crystals. The clearest example is BaCu4As2, which undergoes a first-order transition near 225 K from a trigonal to a triclinic structure. This symmetry lowering converts the Cu-As framework into a more distorted mixed-coordination network and is accompanied by a sharp resistive anomaly, thermal hysteresis, heat-capacity feature, Hall-response changes, and reconstruction of the low-temperature electronic structure. High-field measurements further reveal light carriers associated with small three-dimensional Fermi pockets in the reconstructed phase. A broader comparison with Ba2Cu18-xAs10, BaCu8As4, BaCu6As2, and BaCu2As2 shows that the characteristic electron-phonon scattering scale extracted from Bloch-Grüneisen fits follows the dimensionality and connectivity of the Cu-As polyhedral networks. BaCu8As4, a three-dimensional mixed-coordination framework, additionally exhibits low-field magnetoconductance consistent with weak-antilocalization-like behavior and symmetry-indicator evidence suggestive of nontrivial topology. These results underscore the Ba-Cu-As family as a platform in which Cu coordination, framework dimensionality, and metal or pnictogen bonding provide chemical handles for controlling metallic scattering, Fermi-surface reconstruction, and quantum magnetotransport.