Vibration-reorientation dynamics, structural changes and its interrelation with the phase transition in polycrystalline [Cr(OC(NH2)2)6](BF4)3

•One solid phase transition was indicated in [Cr(urea)6](BF4)3 at TC ≈ 255 K.•Reorientational dynamics was studied by analysing the IR, RS and EPR spectra.•Fast reorientation of NH2 groups and BF4− ions is not interrupted by the transition.•The phase transition at TC is connected with the crystal structure symmetry lowering.

Polycrystalline hexakis(urea-O)chromium(III) tetrafluoroborate possesses in the temperature range of 295–105 K one solid–solid phase transition at TC ≈ 255 K. Analysis of the band shapes associated with the νas(CN) and δas(NH2) vibrational modes of the Fourier transform infrared absorption (FT-IR) spectra, registered in the temperature range of 295–10 K, indicated existence of fast (τR ≈ 10− 12 s) reorientational motion of the protons from NH2 groups belonging to OC(NH2)2 (urea) ligands, which does not suddenly change at TC. Moreover, splitting of the IR bands associated with the νas(NH), νs(NH) and νas(BF)F2/νs(CN) modes at TC indicated that this phase transition is associated with a change of crystal structure. Similar analysis of the Raman scattering bands (FT-RS), associated with the δs(FBF)E, νs(BF)A and νas(BF)F2/νs(CN) vibrational modes, indicated fast reorientation of the BF4− ions, which does not suddenly change at TC, and additionally confirmed structural character of this phase transition. Results obtained from vibrational spectroscopy measurements are compatible with that obtained by electron paramagnetic resonance spectroscopy (EPR) measurements in function of temperature, where rapid narrowing of the EPR line in the vicinity of the TC was observed.

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