Zakład Fizyki Wysokich Ciśnień - publikacje

Abstract: Nuclear magnetic resonance (1H NMR and 19F NMR) measurements performed at 90-295K, inelastic incoherent neutron scattering (IINS) spectra and neutron powder diffraction (NPD) patterns registered at 22-190K, and X-ray powder diffraction (XRPD) measurements performed at 86-293K, provided evidence that the crystal of [Zn(NH3)4](BF 4)2 has four solid phases. The phase transitions occurring at: TC3=101K, TC2=117K and TC1=178K, 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 NH3 ligands and BF4 - anions (at TC3 and at TC2) and with the dynamical orientational order-disorder process ("tumbling") of tetrahedral [Zn(NH3)4]2+ and BF 4 - ions (at TC1). The crystal structure of [Zn(NH3)4](BF4)2 at room temperature was determined by XRPD as orthorhombic, space group Pnma (No. 62), a=10.523Å, b=7.892Å, c=13.354Å 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 TC1 phase transition. © 2003 Elsevier Inc. All rights reserved.