Phase transitions, structural changes and molecular motions in [Zn(NH 3) 4](BF 4) 2 studied by neutron scattering, X-ray powder diffraction and nuclear magnetic resonance

Abstract

Nuclear magnetic resonance ( 1H NMR and 19F NMR) measurements performed at 90–295 K, inelastic incoherent neutron scattering (IINS) spectra and neutron powder diffraction (NPD) patterns registered at 22–190 K, and X-ray powder diffraction (XRPD) measurements performed at 86–293 K, provided evidence that the crystal of [Zn(NH 3) 4](BF 4) 2 has four solid phases. The phase transitions occurring at: T C3=101 K, T C2=117 K and T C1=178 K, as were detected earlier by differential scanning calorimetry (DSC), were connected on one hand only with an insignificant change in the crystal structure and on the other hand with a drastic change in the speed of the anisotropic, uniaxial reorientational motions of the NH 3 ligands and BF 4 − anions (at T C3 and at T C2) and with the dynamical orientational order–disorder process (“tumbling”) of tetrahedral [Zn(NH 3) 4] 2+ and BF 4 − ions (at T C1). The crystal structure of [Zn(NH 3) 4](BF 4) 2 at room temperature was determined by XRPD as orthorhombic, space group Pnma (No. 62), a=10.523 Å, b=7.892 Å, c=13.354 Å and Z=4. Unfortunately, it was not possible to determine the structure of the intermediate and the low-temperature phase. However, we registered the change of the lattice parameters and unit cell volume as a function of temperature and we can observe only a small deviation from near linear dependence of these parameters upon temperature in the vicinity of the T C1 phase transition.