RESUMO
We report detailed optical experiments on the layered compound α-RuCl3 focusing on the THz and sub-gap optical response across the structural phase transition from the monoclinic high-temperature to the rhombohedral low-temperature structure, where the stacking sequence of the molecular layers is changed. This type of phase transition is characteristic for a variety of tri-halides crystallizing in a layered honeycomb-type structure and so far is unique, as the low-temperature phase exhibits the higher symmetry. One motivation is to unravel the microscopic nature of THz and spin-orbital excitations via a study of temperature and symmetry-induced changes. The optical studies are complemented by thermal expansion experiments. We document a number of highly unusual findings: A characteristic two-step hysteresis of the structural phase transition, accompanied by a dramatic change of the reflectivity. A complex dielectric loss spectrum in the THz regime, which could indicate remnants of Kitaev physics. Orbital excitations, which cannot be explained based on recent models, and an electronic excitation, which appears in a narrow temperature range just across the structural phase transition. Despite significant symmetry changes across the monoclinic to rhombohedral phase transition and a change of the stacking sequence, phonon eigenfrequencies and the majority of spin-orbital excitations are not strongly influenced. Obviously, the symmetry of a single molecular layer determines the eigenfrequencies of most of these excitations. Only one mode at THz frequencies, which becomes suppressed in the high-temperature monoclinic phase and one phonon mode experience changes in symmetry and stacking. Finally, from this combined terahertz, far- and mid-infrared study we try to shed some light on the so far unsolved low energy (<1 eV) electronic structure of the ruthenium 4d 5 electrons in α-RuCl3.
RESUMO
We report the synthesis of an air-stable nonporous coordination compound based on iron(II) centers, formate anions, and a 4,4'-bipyrazole (H2BPZ) ligand. Upon thermal treatment, a porous metal-organic framework (MOF) formed due to decomposition of the incorporated formate anions. This decomposition step and the following structural changes constituted a single-crystal to single-crystal transformation. The resulting [Fe(BPZ)] framework contained tetrahedrally coordinated iron(II) metal centers. The framework was sensitive toward oxidation by molecular oxygen even at temperatures of 183 K, as followed by oxygen sorption measurements and a color change from colorless to metallic black. The semiconductor properties of the oxidized material were studied by diffuse reflectance UV/vis/NIR spectroscopy and dielectric spectroscopy.
RESUMO
The pentanuclear Co(II) complex [Co5Tp*4(Me2bta)6] containing N-donor ligands (5,6-dimethyl benzotriazolate; Me2bta6) and N-donor capping ligands (tris(3,5-dimethyl-1-pyrazolyl)borate; Tp*) was prepared by a simple and efficient ligand exchange reaction from [Co5Cl4(Me2bta)6] and tetra-n-butyl ammonium tris(3,5-dimethyl-1-pyrazolyl)borate. Compared to the precursor complex [Co5Cl4(Me2bta)6], which contains one Co(II) ion in octahedral and four Co(II) ions in tetrahedral coordination geometry, the title compound features all five Co(II) ions in an octahedral coordination environment while keeping a high complex symmetry. This results in modified properties including improved solubility and distinct magnetic behavior as compared to the precursor complex. The molecular structure and phase purity of the compound was verified by XRPD, UV-vis, ESI-MS, IR, and NMR measurements. Thermal stability of the compound was determined via TGA. The magnetic properties of here reported novel complex [Co5Tp*4(Me2bta)6] as well as its precursor [Co5Cl4(Me2bta)6] were examined in detail via ESR and SQUID measurements, which indicated weak anti-ferromagnetic exchange interactions between high-spin Co(II) centers at T < 20 and 50 K, respectively.
