Interstitial Nature of Mn²⁺ Doping in 2D Perovskites

Halide perovskites doped with magnetic impurities (such as the transition metals Mn²⁺, Co²⁺, Ni²⁺) are being explored for a wide range of applications beyond photovoltaics, such as spintronic devices, stable light-emitting diodes, single-photon emitters, and magneto-optical devices. However, despite several recent studies, there is no consensus on whether the doped magnetic ions will predominantly replace the octahedral B-site metal via substitution or reside at interstitial defect sites. Here, by performing correlated nanoscale X-ray microscopy, spatially and temporally resolved photoluminescence measurements, and magnetic force microscopy on the inorganic 2D perovskite Cs₂PbI₂Cl₂, we show that doping Mn²⁺ into the structure results in a lattice expansion. The observed lattice expansion contrasts with the predicted contraction expected to arise from the B-site metal substitution, thus implying that Mn²⁺ does not replace the Pb²⁺ sites. Photoluminescence and electron paramagnetic resonance measurements confirm the presence of Mn²⁺ in the lattice, while correlated nano-XRD and X-ray fluorescence track the local strain and chemical composition. Density functional theory calculations predict that Mn²⁺ atoms reside at the interstitial sites between two octahedra in the triangle formed by one Cl^- and two I^- atoms, which results in a locally expanded structure. These measurements show the fate of the transition metal dopants, the local structure, and optical emission when they are doped at dilute concentrations into a wide band gap semiconductor.