Conversion of a Defect Pyrochlore into a Double Perovskite via High-Pressure, High-Temperature Reduction of Te 6 .

High-pressure, high-temperature reaction conditions can be useful to stabilize metastable polymorphs of complex transition metal oxides. We successfully prepare a new defect pyrochlore Pb ₂ FeTeO _6.5 with B -site disordered Fe and Te cations under ambient conditions. Treatment of this material under 8 GPa and 950 °C results in a reductive transformation into the B -site cation-ordered double perovskite Pb ₂ FeTeO ₆ . Mössbauer and EELS spectroscopy confirm the iron cations are in the +3 oxidation state in both phases indicating that this transformation proceeds via reduction of the tellurium cations under apparently oxidizing conditions. This reaction demonstrates that for a suitably chosen system, it is possible to carry out chemical reactions under pressure in unexpected ways.