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

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• 作者：Luke M. Daniels ; Helen Y. Playford ; Jean-Marc Gren猫che ; Alex C. Hannon ; Richard I. Walton
• 刊名：Inorganic Chemistry
• 出版年：2014
• 出版时间：December 15, 2014
• 年：2014
• 卷：53
• 期：24
• 页码：13197-13206
• 全文大小：586K
• ISSN：1520-510X

文摘

The hydrothermal syntheses, structures, and magnetism of two new pyrochlore oxides of compositions (Na_0.60Bi_1.40)(Fe_1.06Mn_0.17Bi_0.77)O_6.87 and (K_0.24Bi_1.51)(Fe_1.07Mn_0.15Bi_0.78)O_6.86 are described. With preparation at 200 掳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虆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 ⁵⁷Fe M枚ssbauer 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 掳C, which suggests that their formation by conventional solid-state synthesis would be impossible.