“Soft” reactions, which alter local chemistry and structure while retaining the basic lattice arrangement, are important for synthesizing solids with novel properties. For example, new layered oxides LaNiO2 and SrFeO2 were prepared by reduction of the corresponding perovskites at temperatures as low as 1908C.[3, 4] Precursors for “soft” chemistry are usually prepared at ambient pressure and sometimes under moderate gas pressures, for example, 10–30 MPa of oxygen gas was used to produce fully oxidized LaNiO3 and SrFeO3 perovskites for the latter reactions. “Hard” high-pressure (1–20 GPa) conditions can lock instabilities, such as unusual oxidation states or coordination environments, into dense phases that are metastable when recovered to ambient conditions, thus leading to unusual properties, such as intermetallic charge transfer and negative thermal expansion in LaCu3Fe4O12 and BiNiO3. Hence, it is attractive to explore combined “hard–soft” routes to novel materials by partially relieving the instability of a high-pressure precursor through post-synthesis modification.
“Hard–Soft” Synthesis of SrCrO3− δ Superstructure Phases
