Temperature‐dependent Raman scattering study of Fe 3 O 2 BO 3 ludwigite

Raman scattering measurements were carried out for Fe 3 O 2 BO 3 and Ni 2 FeO 2 BO 3 ludwigite crystals in the temperature range from 15 to 300 K. At room temperature ludwigite has an orthorhombic structure belonging to space group $D_{2h}^{9}$ . Attempts made to record polarized Raman spectra were successful, and we were able to identify most of the phonons belonging to A 1 g species. A careful analysis of the spectra shows that the observed Raman modes do not exhibit variations either in wavenumber or intensity or both with temperature, indicating that the $D_{2h}^{9}$ phase does not undergo any structural modification in the temperature range investigated. Also, we observed for Fe 3 O 2 BO 3 ludwigite the appearance of a continuous scattering background in the phonon spectrum for temperatures below 200 K . This background, which is due to electronic transitions between Fe 2+ and Fe 3+ ions, is related to the charge delocalization effect. Copyright © 2002 John Wiley & Sons, Ltd.