Spectroscopy of structurally disordered hydrated iron fluoridotitanate in the regions of vibrational and electronic excitations

Dynamic processes of the water sublattice in FeTiF6·xH2O·yD2O crystal

“…The influence of D 2 O leads to the shift of ν s (TiF 6 Þ mode from 601 cm À1 in the hydrated sample to 611 cm À1 in a deuterated one. In a recent article, [12] we have shown that the width of 601 cm À1 Raman line in HITF remains practically constant upon cooling that indicates orientational order of TiF 6 octahedra. [22,23] In Figure 2a,c, one can see that in DITF, wavenumbers of 611 and 291 cm À1 modes practically linearly decrease with an increase in temperature.…”

“…A single anomaly on the DSC-signal reproducible in heating and cooling was detected, with a temperature hysteresis (ΔT ≈ 5 K), indicating a reversible phase transition of the first order. The phase transition temperature in the FeTiF 6 Á xH 2 O Á yD 2 O is 283 K. So, the replacement for H 2 O/D 2 O resulted in a light change of the transition temperature since in FeTiF 6 Á 6H 2 O it was 272 K. [12] The enthalpy and entropy of the phase transition in DITF were found from the DSC-data to be 2.1 J/g (710 J/ mol) and 2.5 J/(molÁK), respectively. The entropy change was more than 0.2R, which may indicate a significant change in the structure during the phase transition.…”

“…Raman scattering spectra of FeÁ6H 2 O were recently considered. [12] Stretching mode of the water molecule at 3500 cm À1 becomes split into the doublet below 100 K (Figure 5), which evidences on the ordering of FeO 6 octahedra with the reconstruction of hydrogen bonds O-H…F. Spectral profile of vibrations of FeÁ6D 2 O unit exhibits differences with that of FeÁ6H 2 O; at low temperatures, the multiplet of bands is observed in Figure 5; the parameters of Raman lines with the participation of D 2 O are presented in Table 1.…”

“…The system of hydrogen bonds O-H…F is significantly reconstructed with the phase transition. [12] In the room temperature (RT) structure, they are distributed into three groups: d(O-F) = 2.69, 2.75, and 2.83 Å. The O-F (2.75 Å) bonds link the TiF 6 and Fe(H 2 O) 6 octahedra into chains along the c axis, the O-F (2.69 Å) join the chains into layers parallel to the (110) plane, and the O-F bonds at 2.83 Å are realized between polyhedron atoms of different orientations.…”

“…Analysis of these results makes it possible to state that the phase transition in FeTiF 6 Á 6H 2 O is of a dynamic nature and is associated with a rotational ordering of iron-water octahedra with a decrease in temperature. [12] Contrariwise, the mobility of ½TiF 6 2À sublattice plays no critical role. However, such peculiarities like the manifestation of phase transition at 272 K in the Raman spectra only below 100 K, and the absence of narrowing of Raman line 601 cm À1 corresponding to the totally symmetric vibration A 1g ½TiF 6 2À , still draw attention to the crystal in question.…”