title: Vibration-reorientation dynamics, structural changes and its interrelation with the phase transition in polycrystalline [Cr(OC(NH₂)₂)₆](BF₄)₃.
abstract: Polycrystalline hexakis(urea-O)chromium(III) tetrafluoroborate possesses in the temperature range of 295-105 K one solid-solid phase transition at T(C) ≈ 255 K. Analysis of the band shapes associated with the ν(as)(CN) and δ(as)(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 (τ(R) ≈ 10(-12) s) reorientational motion of the protons from NH2 groups belonging to OC(NH2)2 (urea) ligands, which does not suddenly change at T(C). Moreover, splitting of the IR bands associated with the ν(as)(NH), ν(s)(NH) and ν(as)(BF)F2/ν(s)(CN) modes at T(C) 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 δ(s)(FBF)E, ν(s)(BF)A and ν(as)(BF)F2/ν(s)(CN) vibrational modes, indicated fast reorientation of the BF4(−) ions, which does not suddenly change at T(C), 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 T(C) was observed.
authors.affiliation: Faculty of Chemistry, Jagiellonian University, Ingardena 3, 30-060 Kraków, Poland. Electronic address: gorska@chemia.uj.edu.pl.
authors.forename: Natalia
authors.initials: N
authors.lastname: Górska
authors.affiliation: Faculty of Chemistry, Jagiellonian University, Ingardena 3, 30-060 Kraków, Poland.
authors.forename: Edward
authors.initials: E
authors.lastname: Mikuli
authors.affiliation: Faculty of Chemistry, Jagiellonian University, Ingardena 3, 30-060 Kraków, Poland.
authors.forename: Maria
authors.initials: M
authors.lastname: Łabanowska
authors.affiliation: Faculty of Chemistry, Jagiellonian University, Ingardena 3, 30-060 Kraków, Poland.
authors.forename: Magdalena
authors.initials: M
authors.lastname: Kurdziel
keywords: EPR; FT-IR; FT-RS; Hexakis(urea-O)chromium(III) tetrafluoroborate; Phase transition; Vibrational and reorientational dynamics