Controlled-valence properties of La1-xSrxFeO3 and La1-xSrxMnO3 studied by soft-x-ray absorption spectroscopy.

TLDR: In this paper, a comparison between the transition-metal 2p spectra and atomic-multiplet calculations is used to determine the 3d count of holes induced by substitution for both series are located in states of mixed metal 3d--oxygen 2p character.

Abstract: The controlled-valence properties of ${\mathrm{La}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Sr}}_{\mathit{x}}$${\mathrm{FeO}}_{3}$ and ${\mathrm{La}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Sr}}_{\mathit{x}}$${\mathrm{MnO}}_{3}$ are studied by means of soft-x-ray absorption spectroscopy. A comparison between the transition-metal 2p spectra and atomic-multiplet calculations is used to determine the 3d count. The O 1s spectrum is used to characterize changes in unoccupied states that contain oxygen p character. The results indicate that the holes induced by substitution for both series are located in states of mixed metal 3d--oxygen 2p character. The ground state of ${\mathrm{LaFeO}}_{3}$ is mainly 3${\mathit{d}}^{5}$ and becomes 3${\mathit{d}}^{5}$L (where L denotes a ligand hole) in the ${\mathrm{La}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Sr}}_{\mathit{x}}$${\mathrm{FeO}}_{3}$ series for low Sr concentration. The main component of the ground state of ${\mathrm{LaMnO}}_{3}$ is 3${\mathit{d}}^{4}$ and becomes a mixture of 3${\mathit{d}}^{3}$ and 3${\mathit{d}}^{4}$L in the ${\mathrm{La}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Sr}}_{\mathit{x}}$${\mathrm{MnO}}_{3}$ series. The trends in controlled- valence properties of similar oxides across the transition-metal series can be rationalized within the framework of the Zaanen-Sawatzky-Allen model.

Figures

FIG. 8. Schematic diagram of the Mn 3d levels in SrMn03. The levels are split by exchange interactions 6, „ into majority (spin-up) and minority (spin-down) states. Each state is additionally split by the octahedral crystal field 10Dq into t2g (threefold degenerate) and eg (twofold degenerate) orbitals. The Mn 3d electrons are in a high-spin configuration, tzg ( A2g) The majority eg and minority t,g orbitals are almost degenerate because, in this case, 6, „ is only slightly larger than 10Dq.

FIG. 1. Fe 2p x-ray absorption spectra of Lal Sr„Fe03for various Sr concentrations. The vertical arrows indicate the appearance of a different component for high Sr contents.

FIG. 9. Simulations of the Fe 2p x-ray absorption spectra for various initial state symmetries. The spectra were calculated assuming a 3d ground state with 10Dq =1.8 eV. The spectra are shown in order of increasing initial state energy; the groundstate symmetry of LaFe03 is A&g. For high Sr contents, the 3d' part of the 3d'L ground state becomes a mixture of different

FIG. 2. (a) Comparison between the Fe 2p x-ray absorption spectrum of LaFe03 (dots) and a simulation calculated assuming a 3d' ground state, with 10Dq = 1.8 eV (solid line). (b) Cornparison between the Fe 2p x-ray absorption spectrum of SrFe03 (dots) and a simulation calculated assuming a 3d ground state, with 10Dq = 1.8 eV (solid line).

FIG. 4. Schematic diagram of the Fe 3d levels in LaFe03 ~ The levels are split by exchange interactions 5,„ into majority (spin-up) and minority (spin-down) states. Each state is additionally split by the octahedral crystal field, 10aq, into t2g (threefold degenerate) and eg (twofold degenerate) orbitals. The Fe 3d electrons are in a high-spin configuration, t,'g eg ( A ]g ), because 6,„&10Dq.

FIG. 3. 0 ls x-ray absorption spectra of La&,Sr„Fe03for various Sr concentrations. The shaded region in the spectrum of LaFeO, indicates bands of primarily Fe 3d character. The vertical arrows signal a growing prepeak induced by the substitution of La'+ by Sr +. The dashed line illustrates how the La Sd related structure in LaFe03 evolves into Sr 4d bands across the series.