| Summary: | <p>Reaction between Na<sub>2</sub>FeSbO<sub>5</sub> and LiNO<sub>3</sub> at 300 °C yields the metastable phase Li<sub>2</sub>FeSbO<sub>5</sub> which is isostructural with the sodium “parent” phase (space group <em>Pbna</em>, <em>a</em> = 15.138(1) Å, <em>b</em> = 5.1440(3) Å, <em>c</em> = 10.0936(6) Å) consisting of an alternating stack of Li<sub>2</sub>Fe<sub>2</sub>O<sub>5</sub> and Li<sub>2</sub>Sb<sub>2</sub>O<sub>5</sub> sheets containing tetrahedral coordinated Fe<sup>3+</sup> and octahedrally coordinated Sb<sup>5+</sup>, respectively. Further reaction between Li<sub>2</sub>FeSbO<sub>5</sub> with NO<sub>2</sub>BF<sub>4</sub> in acetonitrile at room temperature yields LiFeSbO<sub>5</sub>, which adopts an orthorhombic structure (space group <em>Pbn</em>2<sub>1</sub>, <em>a</em> = 14.2943(4) Å, <em>b</em> = 5.2771(1) Å, <em>c</em> = 9.5610(3) Å) in which the LiFeO<sub>5</sub> layers have shifted on lithium extraction, resulting in an octahedral coordination for the iron cations. <sup>57</sup>Fe Mössbauer data indicate that the nominal Fe<sup>4+</sup> cations present in LiFeSbO<sub>5</sub> have disproportionated into a 1:1 combination of Fe<sup>3+</sup> and Fe<sup>5+</sup> centers which are ordered within the LiFeSbO<sub>5</sub> structural framework. It is widely observed that Fe<sup>4+</sup> centers tend to be unstable in delithiated Li–Fe–X–O phases currently proposed as lithium-ion battery cathode materials, so the apparent stability of highly oxidized Fe<sup>5+</sup> centers in LiFeSbO<sub>5</sub> is notable, suggesting cathode materials based on oxidizing Fe<sup>3+</sup> could be possible. However, in this instance, the structural change which occurs on delithiation of Li<sub>2</sub>FeSbO<sub>5</sub> prevents electrochemical cycling of this material.</p>
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