Topotactic Ion Exchange in a Three-Dimensional Close-Packed Trirutile Structure with an Octahedral Network

Topotactic ion exchange in open-framework solids and oxides with layered and tunnel structures has resulted in the formation of a variety of metastable functional materials that are inaccessible otherwise. These ion exchanges are primarily limited to the above structure types because of the presence of labile ions as loosely held charge-compensating cations/anions as in the framework or tunnel structures or the lability of the ions/charged motifs in interlayer galleries of layered oxides. While such topotactic exchanges are common in the above structure types, they are rare in the three-dimensional (3D) close-packed structures based solely on corner- and/or edge-connected polyhedral networks. Herein, we demonstrate divalent iron exchange in a close-packed all-octahedral-coordinated trirutile oxide. This has enabled the transformation of a near-ultraviolet-absorbing diamagnetic insulating oxide into a visible-light-active paramagnetic semiconductor. An ion exchange of this kind may open up avenues for the development of metastable functional oxides with a variety of other 3D structures and diverse properties.