Valence transitions in negative thermal expansion material SrCu₃Fe₄O₁₂

The valence states of a negative thermal expansion material, SrCu3Fe4O12, are investigated by X-ray absorption and (57)Fe Mössbauer spectroscopy. Spectroscopic analyses reveal that the appropriate ionic model of this compound at room temperature is Sr(2+)Cu(~2.4+)3Fe(~3.7+)4O12. The valence states continuously transform to Sr(2+)Cu(~2.8+)3Fe(~3.4+)4O12 upon cooling to ~200 K, followed by a charge disproportionation transition into the Sr(2+)Cu(~2.8+)3Fe(3+)(~3.2)Fe(5+)(~0.8)O12 valence state at ~4 K. These observations have established the charge-transfer mechanism in this compound, and the electronic phase transitions in SrCu3Fe4O12 can be distinguished from the first-order charge-transfer phase transitions (3Cu(2+) + 4Fe(3.75+) → 3Cu(3+) + 4Fe(3+)) in Ln(3+)Cu(2+)3Fe(3.75+)4O12 (Ln = trivalent lanthanide ions).