Charge transfer transitions in multiferroic BiFeO3 and related ferrite insulators

The optical response of the technologically interesting multiferroic ${\text{BiFeO}}_{3}$ and related complex iron oxides, having high N\'eel or Curie temperature, is studied in the wide spectral range from 0.6 up to 5.8 eV by means of spectroscopic ellipsometry. The investigated iron oxides have different crystal symmetry with ${\text{FeO}}_{6}$ octahedral and ${\text{FeO}}_{4}$ tetrahedral centers distorted to a certain degree. One of the two groups of materials includes ${\text{BiFeO}}_{3}$, ${\text{ErFeO}}_{3}$, ${\text{Y}}_{.95}{\text{Bi}}_{.05}{\text{FeO}}_{3}$, $\ensuremath{\alpha}{\text{-Fe}}_{2}{\text{O}}_{3}$, ${\text{Fe}}_{2\ensuremath{-}x}{\text{Ga}}_{x}{\text{O}}_{3}$, and ${\text{Fe}}_{3}{\text{BO}}_{6}$ in which iron ${\text{Fe}}^{3+}$ ions occupy only octahedral centrosymmetric or noncentrosymmetric positions and distortions range of 1\char21{}20 %. The second group includes ${\text{LiFe}}_{5}{\text{O}}_{8}$, ${\text{BaFe}}_{12}{\text{O}}_{19}$, ${\text{Sm}}_{3}{\text{Fe}}_{5}{\text{O}}_{12}$, and ${\text{Ca}}_{2}{\text{Fe}}_{2}{\text{O}}_{5}$ in which ${\text{Fe}}^{3+}$ ions occupy both octahedral and tetrahedral positions with a rising tetra/ortho ratio. We show that in the spectral range up to $\ensuremath{\sim}3.7\text{ }\text{eV}$, the optical response is dominated by $p\text{\ensuremath{-}}d$ charge transfer (CT) transitions, while at $Eg3.7\text{ }\text{eV}$ both $p\text{\ensuremath{-}}d$ and $d\text{\ensuremath{-}}d$ CT transitions are revealed. At variance with several previous investigations, we present a correct and unified assignment of different dipole-allowed and dipole-forbidden CT transitions. All the ferrites investigated are CT insulators with the band gap determined by a dipole-forbidden $p\text{\ensuremath{-}}d$ CT transition ${t}_{1g}\ensuremath{\rightarrow}{t}_{2g}$, forming a $\ensuremath{\sim}2.5\text{ }\text{eV}$ band on the tail of a strong 3.0 eV band assigned to dipole-allowed $p\text{\ensuremath{-}}d$ CT transitions ${t}_{2u}(\ensuremath{\pi})\ensuremath{\rightarrow}{t}_{2g}$ in octahedral ${\text{FeO}}_{6}$ centers. A noticeable enhancement of the optical response in ${\text{BiFeO}}_{3}$ at $\ensuremath{\sim}4\text{ }\text{eV}$ as compared with other related iron oxides is attributed to CT transitions within the Bi-O bonds. We report an observation of unexpected midinfrared CT bands in calcium ferrite ${\text{Ca}}_{2}{\text{Fe}}_{2}{\text{O}}_{5}$ and an enhanced structureless spectral weight in a wide range below the main CT bands in ${\text{BiFeO}}_{3}$ with a remarkable smearing of the fundamental absorption edge. All these anomalies are assigned to CT instabilities accompanied by a self-trapping of $p\text{\ensuremath{-}}d$ CT excitons and nucleation of electron-hole droplets. The optical detection of this CT instability agrees with the observation of a metal-insulator transition in bismuth ferrite.