Lattice dynamical probe of phase transformations in niobium oxyfluoride Nb2O2F3

Abstract

Raman scattering spectroscopy is used to monitor the transition of reduced niobium oxyfluoride ${\mathrm{Nb}}_{2}{\mathrm{O}}_{2}{\mathrm{F}}_{3}$ single crystals from their high-temperature monoclinic (I2/a) phase into the low-temperature triclinic ($P\overline{1}$) phase at ${T}_{c}\phantom{\rule{4pt}{0ex}}\ensuremath{\approx}90\phantom{\rule{4.pt}{0ex}}\text{K}$ due to charge disproportionation of (${\mathrm{Nb}\text{\ensuremath{-}}\mathrm{Nb})}^{7+}$ dimers and creation of crystallographically nonequivalent dimers with long and short Nb-Nb bonds. The group-theoretic analysis of lattice vibrations is performed, and the assignment of observed phonon lines to the specific lattice eigenmodes of the two phases is achieved based on a comparison with the results of density functional lattice dynamics calculations. Nb-Nb dimers are found to possess stretching vibrational frequencies as high as 382 ${\mathrm{cm}}^{\ensuremath{-}1}$. Strikingly, the kinetics of the monoclinic-to-triclinic structural transformation and the volume fraction of corresponding phases below ${T}_{c}$ are shown to strongly depend upon the sample cooling rate. Fast cooling results in a ``freezing'' of the high-temperature monoclinic phase and allows us to observe a spin-ordered state below ${\mathrm{T}}_{N}\ensuremath{\approx}$ 49 K.