High-pressure synthesis and crystal structure of the mixed valent iron borate Fe8B15O28(OH)8

The new mixed valent iron borate Fe8B15O28(OH)8 was synthesized under high-pressure/high-temperature conditions of 12 GPa and 850 °C in a multianvil device (Walker-type module). It crystallizes with four formula units in the tetragonal space group I41/a with the structural parameters a = 1142.3(2) and c = 1537.4(2) pm (V = 2.006(1) nm3, R1 = 0.0586, wR2 = 0.0721 (all data)). Fe8B15O28(OH)8 is composed exclusively from corner-sharing BO4 tetrahedra and can be described as two identical interpenetrating three-dimensional networks that are built up from B11O28 building blocks and interconnecting BO2O(OH) units. The FeII and FeIII ions in FeII5FeIII3B15O28(OH)8 are located in the interspaces between the two networks. 57Fe Mössbauer, Raman, and infrared spectroscopic data are reported.

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