Fe³⁺ optical activity in the THz due to the rare earth smaller ion size in RFeO₃ (R= Er, Lu)

We report on Fe³⁺ induced RFeO₃ (R= Er, Lu) absorptions and magnetic behavior up to 7 T measured at the THz BESSY II coherent source. The proven lattice ductility of distorted perovskites has profound implications for enabling perovskite B site crystal field (CF) symmetry forbidden Fe³⁺ transitions of the ⁶A₁ multiplet. Fe³⁺ ground state Zeeman split is a distinctive feature of the LuFeO₃ THz spectra that additionally weighs to the spin modes only picture in LaFeO_3, that closer to cubic, prevents Fe³⁺ (CF) transitions becoming optically active. Although both compounds share the P_bnm (D^2h₁₆) space group, the Fe³⁺ site point group in LuFeO₃ is altered by the ligand distortions due to Lu³⁺ smaller ionic radius in a cage with only six Lu-O nearest neighbors with non-centrosymmetric subtle deformations in Fe³⁺-O ligands. These Fe³⁺ CF Zeeman branching in LuFeO₃ has a counterpart in the ErFeO₃ Er³⁺ (⁴I_15/2) multiplet now strongly biased due to iron - rare earth exchanges and populatin deviations. The iron absorption shows as a distinctive Boson peak in near normal FIR reflectivity. The local induced minute lattice displacements underly the possibility of assigning non-centrosymmetry that when associated to the intrinsic oxygen electronic anisotropy allows ferroelectric spontaneous polarization