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A new member of the iodate family, fe io 3, has recently been discovered to undergo a significant pressure driven structural phase transition at 15-22 GPa. Its unique high-pressure behaviour has been investigated by powder X-ray diffraction and infrared microspectroscopy measurements in the far-IR spectral range, combined with density functional theory calculations.
For FTIR, the transmission spectra of amorphous fe io 3 and a-La(IO3)3 nanocrystals were measured on an FT/IR Jasco spectrometer in the 4000-500 cm-1 range with KBr as sample carrier. Using multiple peak fit techniques, the vibrational frequencies of 25 phonons were identified. The results are summarized in Table 1 and compared with frequencies reported in the literature.
We have found that the low-frequency phonons soften under compression, with a change in frequency-pressure dependence, similar to observations for isostructural phase transitions in other iodates (e.g., Fe(IO3)3 at 3.6 and 8.8 GPa). However, whereas these phonons are primarily associated with pure translational or librational movements of the iodine atoms in the coordination polyhedra, these fe io 3 phonons exhibit a more complex behavior, possibly associated with structural changes in the IO3 polyhedra that result in a loss of symmetry and the generation of additional modes that can be assigned to internal vibrations of these ions.
The crystal structure of a-La(IO3)3 consists of large LaO9 polyhedra connected by isolated asymmetric IO3 polyhedra, forming a distorted trigonal-pyramidal geometry. The vibrational modes can be correlated with the atomic movements of these polyhedra, and a total of 51 phonons are expected according to group theory (117A’ + 117A”). In addition to the Raman-active vibrational modes, a-La(IO3)3 exhibits a large number of NLO modes.