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 Fe-57 Mossbauer spectroscopy. Spectroscopic analyses reveal that the appropriate ionic model of this compound at room temperature is Sr2+Cu3 similar to 2.4+Fe4 similar to 3.7+O12. The valence states continuously transform to Sr2+Cu3 similar to 2.84+Fe similar to 3.2+3Fe similar to 0.85+O12 upon cooling to similar to 200 K, followed by a charge disproportionation transition into the (Sr2+CU similar to 2.8)+3Fe similar to 3.23+Fe similar to 0.85+O12 valence state at similar to 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 (Cu-3(2+) + 4Fe(3.75+) -> 3Cu(3+) + 4Fe(3+)) in Ln(3+)Cu(3)(2+)Fe(4)(3.75+)O(12) (Ln = trivalent lanthanide ions).