Metastable (Bi, M)2(Fe, Mn, Bi)2O6+x (M = Na or K) Pyrochlores from Hydrothermal Synthesis

The hydrothermal syntheses, structures, and magnetism of two new pyrochlore oxides of compositions (Na0.60Bi1.40)(Fe1.06Mn0.17Bi0.77)O-6.87 and (K0.24Bi1.51)(Fe1.07Mn0.15Bi0.78)O-6.86 are described. With preparation at 200 degrees C for 6 h in solutions of sodium or potassium hydroxide, the alkali metals introduced from these mineralizers are essential to the synthesis of the phases. The average long-range order of the pyrochlore structure, with space group Fd3(_)m, was investigated and refined against X-ray and neutron diffraction data, and it was shown that disorder is present in both the metal and coordinating oxygen positions, along with metal-mixing across both the A and B sites of the structure. XANES analysis confirms the presence of Mn4+ , mixed valence Bi3+ and Bi5+ , and Fe3+ , the last also verified by (57)Fe Mossbauer spectroscopy. Magnetic measurements show a lack of long-range magnetic ordering that is typical of geometrically frustrated pyrochlores. The observed glasslike interactions occur at low temperatures, with the onset temperature depending upon the magnitude of the applied external field. Variable temperature X-ray diffraction shows that these pyrochlores are metastable and collapse on heating at ca. 395 degrees C, which suggests that their formation by conventional solid-state synthesis would be impossible.