Capturing Phase Evolution during Solvothermal Synthesis of Metastable Cu₄O₃

The metastability of Cu 4 O 3 has long hindered the synthetic preparation of bulk samples with substantial crystallinity. The lack of suitable samples has thwarted the detailed understanding of the magnetic properties of Cu 4 O 3 and the ability to tune its properties. While Cu 4 O 3 was recently shown to form in solvothermal reactions, the results are unpredictable, and the crystals are small. We developed a new, more uniform synthesis technique using sealed fused silica tubes. Interrogation of the solid and liquid phases resulting from this reaction has shed more light on the kinetic evolution of copper-containing phases and the microstructural correlation between different precipitation products. We find that direct conversion of the intermediate phase Cu 2 (NO 3 )(OH) 3 to Cu 4 O 3 is a likely consequence of dimethylformamide (DMF) triggered in situ reduction. The optimal reduction environment should be more straightforward to attain given the improved reliability of our method, and it remains under investigation. We verify the formation of Cu 4 O 3 via X-ray diffraction, Raman microscopy, and SQUID magnetometry.