Iron valence in double-perovskite (Ba,Sr,Ca)2FeMoO6: Isovalent substitution effect

TL;DR

This study investigates how chemical pressure via A-site cation substitution in double-perovskite affects Fe valence, revealing that Fe valence can be fine-tuned and influences cation ordering and clustering.

Contribution

It demonstrates that Fe valence in double-perovskite can be adjusted through A-site cation size, affecting charge difference and cation order, using systematic Mossbauer spectroscopy.

Findings

Fe remains in mixed-valence state across A-site substitutions.

Increasing A-site cation size shifts Fe valence closer to II.

Higher Fe/Mo order correlates with Fe valence approaching II.

Abstract

In the Fe-Mo based B-site ordered double-perovskite, A2FeMoO6.0, with iron in the mixed-valence II/III state, the valence value of Fe is not precisely fixed at 2.5 but may be fine-tuned by means of applying chemical pressure at the A-cation site. This is shown through a systematic 57Fe Mossbauer spectroscopy study using a series of A2FeMoO6.0 [A = (Ba,Sr) or (Sr,Ca)] samples with high degree of Fe/Mo order, the same stoichiometric oxygen content and also almost the same grain size. The isomer shift values and other hyperfine parameters obtained from the Mossbauer spectra confirm that Fe remains in the mixed-valence state within the whole range of A constituents. However, upon increasing the average cation size at the A site the precise valence of Fe is found to decrease such that within the A = (Ba,Sr) regime the valence of Fe is closer to II, while within the A = (Sr,Ca) regime it is closer to the actual mixed-valence II/III state. As the valence of Fe approaches II, the difference in charges between Fe and Mo increases, and parallel with this the degree of Fe/Mo order increases. Additionally, for the less-ordered samples an increased tendency of clustering of the anti-site Fe atoms is deduced from the Mossbauer data.