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 BaCu 4 As 2 , 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 Ba 2 Cu 18−x As 10 , BaCu 8 As 4 , BaCu 6 As 2 , and BaCu 2 As 2 shows that the characteristic electron–phonon scattering scale extracted from Bloch–Grüneisen fits follows the dimensionality and connectivity of the Cu–As polyhedral networks. BaCu 8 As 4 , 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.