Magnetic Properties Tuning via Broad Range Site Deficiency in Square Net Material UCu_xBi₂

HfCuSi₂-type pnictogen compounds have recently been shown to be a versatile platform for designing materials with topologically nontrivial band structures. However, these phases require strict control over the electron count to tune the Fermi level, which can only be achieved in compositions with A²⁺M²⁺Pn₂ and A³⁺M⁺Pn₂ (A = lanthanides, M = transition metals, Pn = pnictogens P-Bi) charge distribution. While such lanthanide compounds have been thoroughly studied as candidate magnetic topological materials, their heavy element analogs with uranium and bismuth remain largely underexplored. In this report, we present the synthesis of UCu_xBi₂ single crystals and study their magnetic properties. Detailed structural analysis revealed that flux-grown crystals always form as a site-deficient UCu_xBi₂ composition, where x varies between 0.20 and 0.64. Magnetic property measurements revealed a dependence of the magnetic coupling on the Cu site deficiency, linearly changing the Néel temperature from 51 K for UCu_0.60Bi₂ to 118 K for UCu_0.30Bi₂. Moreover, higher Cu concentration promotes a metamagnetic transition in highly magnetically anisotropic UCu_0.60Bi₂ single crystals. We show that DFT calculations can successfully model site deficiency in the UCu_xSb₂ and UCu_xBi₂ systems. This work paves the way toward using the site deficiency to tune the Fermi level in more ubiquitous A³⁺M²⁺_xPn₂ phases, which previously have not been considered topological candidate materials due to unfavorable electron count.