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 57Fe Mössbauer spectroscopy. Spectroscopic analyses reveal that the appropriate ionic model of this compound at room temperature is Sr2+Cu∼2.4+3Fe∼3.7+4O12. The valence states continuously transform to Sr2+Cu∼2.8+3Fe∼3.4+4O12 upon cooling to ∼200 K, followed by a charge disproportionation transition into the Sr2+Cu∼2.8+3Fe3+∼3.2Fe5+∼0.8O12 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 (3Cu2+ + 4Fe3.75+ → 3Cu3+ + 4Fe3+) in Ln3+Cu2+3Fe3.75+4O12 (Ln = trivalent lanthanide ions).