Phase transitions and molecular motions in [Zn(NH3)4](BF4)2 studied by nuclear magnetic resonance, infrared and Raman spectroscopy

Middle infrared absorption, Raman scattering and proton magnetic resonance relaxation measurements were performed for [Zn(NH3)4](BF4) in order to establish relationship between the observed phase transitions and reorientational motions of the NH3 ligands and BF4− anions. The temperature dependence of spin-lattice relaxation time (T1(1H)) and of the full width at half maximum (FWHM) of the bands connected with ρr(NH3), ν2(BF4) and ν4(BF4) modes in the infrared and in the Raman spectra have shown that in the high temperature phase of [Zn(NH3)4](BF4)2 all molecular groups perform the following stochastic reorientational motions: fast (τR≈10−12 s) 120° flips of NH3 ligands about three-fold axis, fast isotropic reorientation of BF4− anions and slow (τR≈10−4 s) isotropic reorientation (“tumbling”) of the whole [Zn(NH3)4]2+ cation. Mean values of the activation energies for uniaxial reorientation of NH3 and isotropic reorientation of BF4− at phases I and II are ca. 3 kJ mol−1 and ca. 5 kJ mol−1, respectively. At phases III and IV the activation energies values for uniaxial reorientation of both NH3 and of BF4− equal to ca. 7 kJ mol−1. Nearly the same values of the activation energies, as well as of the reorientational correlation times, at phases III and IV well explain existence of the coupling between reorientational motions of NH3 and BF4−. Splitting some of the infrared bands at TC2=117 K suggests reducing of crystal symmetry at this phase transition. Sudden narrowing of the bands connected with ν2(BF4), ν4(BF4) and ρr(NH3) modes at TC3=101 K implies slowing down (τR⪡10−10 s) of the fast uniaxial reorientational motions of the BF4− anions and NH3 ligands at this phase transition.