Sb^IV, an Unusual Player in 2D Spintronic Devices

We present a first-principles study of ultrathin films of the perovskite Rb₂SbCl₆. Density-functional theory calculations combined with Bader charge analysis show that antimony adopts the Sb⁴⁺ oxidation state, and density-functional perturbation theory confirms structural stability for multilayer configurations. The Rb-terminated bilayer displays nodal-line semimetal behavior, characterized by an "egg-shaped" valence- and conduction-band topology that permits sign-reversal doping. The computed Seebeck coefficient is approximately 30 μV K^-1 at 100 K and trends toward zero at 300 K, while the spin Hall conductivity is about 10^-2 Ω^-1 cm^-1, comparable to values reported for other two-dimensional materials. As the number of layers increases, charge disproportionation arises, with Sb(III) localized at the surface and Sb(V) in inner layers. These results relate structural stability, charge localization, and reduced dimensionality in Rb-terminated Rb₂SbCl₆ multilayers and indicate inverse-sign thermoelectric and spin Hall responses relevant for spin-charge interconversion in spin-based transistors and nonvolatile memory devices.