Reversible Oxidation Quantified by Optical Properties in Epitaxial Fe2CrO4+δ Films on (001) MgAl2O4.

We report on the structural and optical properties of Fe2CrO4+δ epitaxial films grown by molecular beam epitaxy on MgAl2O4 (001) as a function of δ (average cation valence). The average Fe valence is linked to the out-of-plane lattice parameter and the extent of light absorption in the infrared spectral region. Over-oxidized films (0 < δ < 0.5) exhibit smaller lattice parameters and suppressed infrared absorption. The lattice parameter is found to differ for films of equivalent oxidation state but different thermal histories. We discuss the behavior of a novel infrared transition present at ∼0.6 eV in Fe2CrO4 films deposited at or above 400 °C. An optical transition found in all films at 0.9 eV independent of the synthesis temperature can be used to quantify the oxidation state of Fe2CrO4+δ. This research provides new insights into the atomic structure, optical processes, oxidation states, electronic structure, and application potential of Fe2CrO4+δ.

Effect of cation off-stoichiometry on optical absorption in epitaxial LaFeO3 films.

We have investigated the effect of La and Fe deficiency on the c-axis lattice parameter and optical absorption in La1-xFe1-yO3-δ films deposited by oxide MBE. La deficient films display minimal changes in optical and structural properties compared with stoichiometric LaFeO3, even with relatively large cation vacancy concentrations (∼22%). Fe deficient films show a linear increase in c-axis parameter with increasing Fe vacancy concentration. The optical absorption coefficient is reduced for Fe deficient films over the spectral range from 1.2 to 5.0 eV. This leads to a relatively simple and effective method to quantify the approximate Fe deficiency in LaFeO3. Minimal changes in optical and structural properties are observed when cation deficient films are exposed to strongly oxidizing (ozone annealing) conditions. This indicates a strong accompaniment of charge balancing oxygen vacancies to maintain a nominal Fe3+ valence state, acting to stabilize the band structure even in the presence of significant cation off-stoichiometry.

Raman scattering in La1-xSrxFeO3-δ thin films: annealing-induced reduction and phase transformation.

Raman scattering in thin film La0.2Sr0.8FeO3-δ on MgO(0 0 1) collected at 300 K after different stages of annealing at selected temperatures T (300 K < T < 543 K, to 10 h) and analysis reveal changes in spectral characteristics due to a loss of oxygen, onset of oxygen vacancy-induced disorder, and activation of Raman-inactive modes that are attributed to symmetry lowering. The interpretation is further supported by carrier transport measurements under identical conditions showing orders of magnitude increase in the resistivity induced by oxygen loss. After prolonged annealing in air, evolution of the spectrum signals the appearance of a possible topotactic transformation of the crystal structure from that of the rhombohedral ABO3 perovskites to that of Brownmillerite-like structure consisting of octahedrally and tetrahedrally coordinated Fe atoms.

Band structure and optical transitions in LaFeO3: theory and experiment.

The optical absorption properties of LaFeO(3) (LFO) have been calculated using density functional theory and experimentally measured from several high quality epitaxial films using variable angle spectroscopic ellipsometry. We have analyzed the calculated absorption spectrum using different Tauc models and find the model based on a direct-forbidden transition gives the best agreement with the ab initio band gap energies and band dispersions. We have applied this model to the experimental data and determine the band gap of epitaxial LFO to be ∼2.34 eV, with a slight dependence on strain state. This approach has also been used to analyze the higher indirect transition at ∼3.4 eV. Temperature dependent ellipsometry measurements further confirm our theoretical analysis of the nature of the transitions. This works helps to provide a general approach for accurate determination of band gaps and transition energies in complex oxide materials.

Control of functional responses via reversible oxygen loss in La1-xSrxFeO3-δ films.

La0.3 Sr0.7 FeO3-δ films undergo dramatic changes in electronic and optical properties due to reversible oxygen loss induced by low-temperature heating. This mechanism to control the functional properties may serve as a platform for new devices or sensors in which external stimuli are used to dynamically control the composition of complex oxide heterostructures.