High-pressure phase transition and properties of spinel (formula presented)

Abstract

X-ray photoelectron spectroscopy, magnetic measurements, and a single-crystal x-ray structure determination at normal pressure have shown that Jahn-Teller active manganese ions in (Formula presented) are present in one valence state (III) on the octahedral sites of the spinel structure. The high-pressure behavior of (Formula presented) was investigated up to 52 GPa using the energy-dispersive x-ray diffraction technique and synchrotron radiation. The structural first-order phase transition from the body-centered to primitive-tetragonal cell takes place at (Formula presented) The high-pressure phase is metastable down to normal pressure. The (Formula presented) ratio reduces from 1.62 to 1.10 above (Formula presented) and remains nearly pressure independent in the high-pressure phase. The transition is attributed to the changes in electron configuration of the (Formula presented) ions. According to the crystal field theory, the (Formula presented) electron of octahedrally coordinated (Formula presented) is either in the (Formula presented) orbital or in the (Formula presented) In the first configuration the (Formula presented) octahedron will be elongated and this is the case at normal pressure, while the second configuration gives the flattened octahedron. In the high-pressure phase some proportion of the (Formula presented) electrons of the (Formula presented) ions is moved to the (Formula presented) level, which is revealed as an abrupt fall of observed magnitude of the distortion of the bulk crystal above (Formula presented) © 1999 The American Physical Society.