Non-relativistic spin splitting in a triangular metal-excess magnet Fe$_{1+δ}$Sb

Non-relativistic spin-splitting (NRSS) antiferromagnets have recently emerged as an important class of magnetic materials that combine compensated magnetism with momentum-dependent spin splitting, offering new opportunities for spintronic applications. Here, we investigate a NiAs-type Fe$_{1+\delta}$Sb series ($\delta = 0.17$-0.30) using neutron diffraction, pair distribution function, magnetometry and density functional theory calculations. Neutron diffraction establishes that Fe$_{1+\delta}$Sb adopts a $120^\circ$ coplanar compensated magnetic order with a non-zero propagation vector $\mathbf{k}=(1/3,\,1/3,\,0)$. Increasing interstitial Fe suppresses the ordered magnetic moment while inducing local symmetry lowering, as revealed by pair distribution function refinements. Density functional theory predicts momentum-dependent spin splitting, dominated by an out-of-plane spin polarization with an odd-parity f-wave-like symmetry, establishing the material as a non-collinear NRSS antiferromagnet. Motivated by the structural similarities between Fe$_{1+\delta}$Sb and a known altermagnet CrSb, we further investigate their solid solution and find that Cr substitution at intermediate concentrations gives rise to a ferromagnetic component and a cluster spin-glass behavior. These results establish Fe$_{1+\delta}$Sb as a new platform for non-collinear NRSS antiferromagnetism and demonstrate metal interstitial and substitution as effective parameters for tuning the magnetic order and properties.