Abstract
Raman scattering spectroscopy is used to monitor the transition of reduced niobium oxyfluoride Nb2O2F3 single crystals from their high-temperature monoclinic (I2/a) phase into the low-temperature triclinic (P1) phase at Tc≈90K due to charge disproportionation of (Nb-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 cm-1. Strikingly, the kinetics of the monoclinic-to-triclinic structural transformation and the volume fraction of corresponding phases below Tc 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 TN≈ 49 K.
| Original language | English |
|---|---|
| Article number | 094306 |
| Journal | Physical Review B |
| Volume | 97 |
| Issue number | 9 |
| DOIs | |
| State | Published - Mar 28 2018 |
Funding
This work was supported in part by the R. A. Welch Foundation (E-1297), the state of Texas through the Texas Center for Superconductivity, and the U.S. AFOSR.