Structural, spectroscopic and theoretical studies of selected Ag(II) fluoride systems in the solid state

This doctoral thesis describes the research conducted on a range of AgII/F materials using various experimental and theoretical tools. Its goals were focused on the structural, spectroscopic and magnetic properties of silver(II) fluorides of low magnetic dimensionality. The thesis commences with a theoretical introduction that covers key concepts related to research on silver(II) systems, like magnetic interactions in the solid-state or crystal lattice dynamics. Theoretical basics of experimental and theoretical methods employed within the work are also discussed. The introduction ends with a literature review of hypothetical and known AgII materials. The “simplest” i.e. binary silver(II) compound – AgF2, was analysed using ellipsometry and RIXS to determine its electronic excitations. The ellipsometry measurements resulted in a spectrum similar to that of La2CuO4, with d-d and charge-transfer excitations clearly present. Measurement of the RIXS spectrum allowed for the assignment of d-d transitions and confirmed the charge-transfer insulating character of AgF2, with the charge-transfer gap of ca. 3.4 eV. Infrared and Raman spectra of MAgF3 (M = K, Rb, Cs) were successfully assigned using theoretical tools reaching a good agreement between theory and experiment. Throughout the work, a low-temperature polymorph of RbAgF3 was proposed, isostructural with KAgF3. Low-temperature X-ray and Raman experiments of RbAgF3 down to 80 K, led to a conclusion, that the possible phase transaction should be searched for below that temperature and likely in the liquid helium regime. Low-temperature neutron diffraction and μSR experiments allowed for the determination of low-temperature magnetic order in KAgF3, which is present below the ordering temperature of 29.26(2) K. This material has an A-type antiferromagnetic order, with magnetic moments oriented along c axis with the magnitude of 0.488(2) μB confirming theoretical predictions. Higher temperature (ca. 30–66 K) spiral magnetic phases are also proposed here based on quantum mechanical calculations. Quasi-1D system [AgF][BF4] was analysed using SQUID magnetometry and photoacoustic spectroscopy to experimentally measure its intra-chain magnetic exchange constant J. Magnetometry results have shown unexpected susceptibility drop at low temperatures (< 5 K). Field-induced phase transition was also observed at the critical field of 5.5 T. Calculated superexchange constant (–106.72(33) meV or –121.02(84) meV) was smaller in absolute value than the theoretically predicted value of –298 meV. Reasons might be sample specific and these values constitute lower bound of the actual J value. Photoacoustic spectroscopy data showed a feature at 0.46 eV that translates to the superexchange constant of –251.3 meV. The theoretically predicted structure of other quasi-1D system was also described in this work. [AgF][AuF4] is postulated to be isostructural with [CuF][AuF4] and to exhibit straight Ag–F chains together with extremely short Ag–F bonds. The superexchange constant along these chains was predicted to exceed –250 meV, while the calculated secondary superexchange constant does not exceed 2 meV. Two unsuccessful attempts at a synthesis of novel systems are also reported here. Mixed alkali metal NaRbAg2F6 attempts have shown undesired phase separation based on X-ray diffraction. Experiments aiming for the synthesis of AgII/LnIII/F (Ln = Eu, Ce, Gd, Yb) systems were also shown to lead to phase separation.