Microscopic Origin of Random Singlet Behavior in B-site Disordered Spin-1/2 Perovskite BaCu₁/3Nb₂/3O₃ Revealed by EXAFS and Thermodynamics

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We report a combined structural and thermodynamic study of the ABO₃-type disordered perovskite BaCu₁/₃Nb₂/₃O₃ (BCNO), whose B site is jointly occupied by Cu and Nb in the 1:2 ratio. Using synchrotron powder x-ray diffraction (XRD) and x-ray absorption fine structure (XAFS) spectroscopy, we investigate the microscopic nature of Cu²⁺/Nb⁵⁺ disorder on the pseudo-cubic B-sublattice and its relation to the emergent random-singlet (RS) behavior evidenced at low temperatures. While XRD reveals no long-range Cu/Nb ordering and average site occupancy consistent with stoichiometry, XAFS reveals a peculiar local chemical order characterized by preferential heteroatomic Cu:Nb correlations. This local arrangement strongly suppresses direct Cu:Cu linkages, despite the Cu concentration being close to the percolation threshold of a cubic lattice. The resulting exchange network explains the absence of spin-glass freezing or long-range magnetic order in the presence of substantial antiferromagnetic interactions, as indicated by a Curie-Weiss temperature ΘCW≈ -50 K. Instead, the magnetic susceptibility χ(T) and specific heat cₚ(T) exhibit power-law behavior and characteristic single-parameter T/H scaling over broad temperature and magnetic-field ranges, consistent with random-singlet phenomenology. Notably, at very low temperatures, the specific heat behavior transitions from T¹⁻γ (γ≈ 0.6 from the T/H scaling) in zero-field to a T-linear dependence under high field, indicating a crossover to a distinct low-energy regime whose microscopic origin remains to be established.