Role of Defects in the Paramagnetism of Fe-doped Cs$_{2}$AgBiBr$_{6}$ Double Perovskite

Transition-metal doping enables the introduction of spin functionality into halide double perovskites, while simultaneously modifying optical properties. Here, we combine controlled single-crystal growth, optical characterization, comprehensive electron paramagnetic resonance (EPR) spectroscopy, and first-principles modeling to identify the microscopic nature of Fe-related centers in Fe-doped Cs$_{2}$AgBiBr$_{6}$. Single crystals with nominal Fe$^{3+}$ concentrations up to 15% in the precursor stage were grown using a controlled-cooling method, yielding reproducible Fe incorporation up to 0.1% w.r.t. Bi, without secondary phases. Despite this low concentration, Fe doping introduces electronic states that influence optical absorption and photoluminescence. EPR measurements reveal an S = 5/2 Fe$^{3+}$-related center whose anisotropy follows the cubic-to-tetragonal phase transition below 120 K. Angular-dependent EPR resolves two configurations of this nearly axial spin center, with principal axes rotated by 90$^\circ$ and aligned with the $a/b$ plane of the tetragonal lattice. Density-functional calculations attribute these centers to impurity-vacancy complexes, most likely Fe$_{\rm Bi}$-V$_{\rm Br}$, that stabilise in a basal configuration of the low-temperature phase. This approach resolves vacancy-coupled defect orientations, narrowing possible models to Fe$^{3+}$-vacancy complexes and establishing them as stable, orientation-sensitive spin probes of structural symmetry in halide double perovskites, while providing a microscopic basis for tuning their magnetic and optical responses.