Hans-Rudolf Hagemann

Porous and Dense Magnesium Borohydride Frameworks: Synthesis, Stability, and Reversible Absorption of Guest Species†

A highly porous form of Mg(BH4)2 (see picture; Mg green, BH4 blue, unit cells shown in red) reversibly absorbs H2, N2, and CH2Cl2. At high pressures, this material transforms into an interpenetrated framework that has 79 % higher density than the other polymorphs. Mg(BH4)2 can act as a coordination polymer that has many similarities to metal–organic frameworks.

Lithium boro-hydride LiBH4: II. Raman spectroscopy

Abstract Polycrystalline LiBH 4 has been studied by Raman spectroscopy in the temperature interval 295–412 K and the frequency range 2700–130 cm −1 . The Raman active modes are consistent with the presence of a (BH 4 ) − ion having a distorted tetrahedral configuration. As the temperature is increased the sudden disappearance of mode splitting points to the onset of a structural phase transition that leads to a higher local symmetry of the (BH 4 ) − tetrahedron. The transition occurs at ∼384 K, is of first-order and has a hysteresis of about 8 K. A strong and discontinuous broadening of bands remaining after the transition suggests the onset of large vibrational amplitudes of the (BH 4 ) − tetrahedra about their trigonal axis.

Raman spectra of single crystal CuO

Polarized Raman measurements on single crystals of CuO yield the symmetry assignments of the three predicted Raman active lattice modes : 297cm−1 (Ag, 344cm−1 (Bg) and 629cm−1 (Bg). These results are compared to literature data, including IR spectra. Our measurements confirm at low temperature the appearance of an additional Raman band around 240 cm−1. The temperature dependence of the linewidth of the Ag mode presents an anomalous behavior near the magnetic phase transition, suggesting the possible presence of magnon-phonon couplings in the antiferromagnetic phase.

Electron-phonon interaction and charge carrier mass enhancement in SrTiO3

We report a comprehensive THz, infrared and optical study of Nb-doped SrTiO3 as well as dc conductivity and Hall effect measurements. Our THz spectra at 7 K show the presence of an unusually narrow (<2 meV) Drude peak. For all carrier concentrations the Drude spectral weight shows a factor of three mass enhancement relative to the effective mass in the local density approximation, whereas the spectral weight contained in the incoherent midinfrared response indicates that the mass enhancement is at least a factor two. We find no evidence of a particularly large electron-phonon coupling that would result in small polaron formation.

Raman studies of reorientation motions of [BH4]− anionsin alkali borohydrides

Raman spectra of the alkali borohydride series MBH4 (M = Li, Na, K, Rb, Cs) have been measured as a function of temperature in the range 300–540 K. For the cubic modification of M = Na, K, Rb and Cs, the analysis of the Raman line widths suggests that the energy barrier of reorientation of the [BH4] − anions decreases as a function of cation size in the sequence Na: 12.1(5), K: 9.2(4), Rb: 8.8(3) and Cs: 8.2(4) kJ/mol. For the hexagonal high temperature modification of LiBH 4, the data suggest two energy barriers of reorientation at ∼5 and ∼60 kJ/mol, respectively.

Pronounced Electrochemical Amphotericity of a Fused Donor-Acceptor Compound : A Planar Merge of TTF with a TCNQ-Type Bithienoquinoxaline

Narrowing the gap: A compactly fused π‐conjugated molecule combines a high‐lying HOMO with a low‐lying LUMO (Eox−Ered=0.52 eV) and a fairly low‐lying LUMO+1 on the bridging unit, giving rise to strong optical charge‐transfer transitions. A facile electron transfer has been observed by EPR and NMR spectroscopies.

Solvent and Spectral Effects in the Ultrafast Charge Recombination Dynamics of Excited Donor-Acceptor Complexes

The charge recombination dynamics of excited donor-acceptor complexes consisting of hexamethylbenzene (HMB), pentamethylbenzene (PMB), and isodurene (IDU) as electron donors and tetracyanoethylene (TCNE) as electron acceptor in various polar solvents has been investigated within the framework of the stochastic approach. The model accounts for the reorganization of intramolecular high-frequency vibrational modes as well as for the solvent reorganization. All electron-transfer energetic parameters have been determined from the resonance Raman data and from the analysis of the stationary charge transfer absorption band, while the electronic coupling has been obtained from the fit to the charge recombination dynamics in one solvent. It appears that nearly 100% of the initially excited donor-acceptor complexes recombine in a nonthermal (hot) stage when the nonequilibrium wave packet passes through a number of term crossings corresponding to transitions toward vibrational excited states of the electronic ground state. Once all parameters of the model have been obtained, the influence of the dynamic solvent properties (solvent effect) and of the carrier frequency of the excitation pulse (spectral effect) on the charge recombination dynamics have been explored. The main conclusions are (i) the model provides a globally satisfactory description for the IDU/TCNE complex although it noticeably overestimates the spectral effect, (ii) the solvent effect is quantitatively well described for the PMB/TCNE and HMB/TCNE complexes but the model fails to reproduce their spectral effects, and (iii) the positive spectral effect observed with the HMB/TCNE complex cannot be described within the framework of two-level models and the charge redistribution in the excited complexes should most probably be taken into account.

Lattice anharmonicity and structural evolution of LiBH4: an insight from Raman and X-ray diffraction experiments

New in situ Raman and synchrotron X-ray diffraction data (between 300 and 400 K) in conjunction with separate temperature-dependent Raman data (between 7 and 400 K) are presented. The low-frequency Raman spectra show good agreement with theoretical values obtained previously using periodic DFT calculations. The temperature-dependent spectra reveal the presence of significant anharmonicity of librational modes neither predicted theoretically nor noted in previous experiments. The splitting of the internal deformation mode υ 2 (of E symmetry in the free ion) decreases continuously with increasing temperature, but drops abruptly at the first-order orthorhombic to hexagonal phase transition observed at 381 K. The temperature dependence of the linewidth of the internal deformation mode υ 2 reveals coupling to reorientational motions of the borohydride ion in the orthorhombic phase. The thermal evolution of both crystal structure and vibration frequencies agree with the phase diagram suggested by the Landau theory.

Europium doped BaMgF4, an EPR and optical investigation

Eu2+ was introduced into pure and oxygen codoped BaMgF4 single crystals. A detailed EPR study of this ion (S=7/2) was realized on both types of systems. The result is that only one spectrum was observed involving a strong crystal field. The associated site symmetry of the impurity is Cs. It occupies very closely a Barium lattice site as was established by correlating the EPR results with those of a refined X-ray structure analysis on a Ba0.8Eu0.2MgF4 single crystal realized in our laboratory. The oxygen codoped crystals exhibited this same Eu2+ EPR spectrum (the only one). Optical emission and excitation experiments were performed between 13 000 and 53 000 cm−1. The results due to the Eu2+ impurity are given and discussed qualitatively within the 4f7⇔4f65d1 scheme.

Experimental Raman scattering investigation of phonon anharmonicity effects in

Experiments in which the Raman linewidth was measured as a function of temperature (7-1183 K) and pressure (0-400 bar) were performed on the (111) and (100) planes of single crystals of the cubic anti-fluorite . The temperature dependence of the lattice constant was determined by x-ray diffraction (11-295 K). From these results and published Brillouin scattering data for this host, the volume thermal expansion coefficient as a function of temperature was obtained as well as the isothermal compressibility and the isothermal Raman mode Gruneisen parameter. Using the thermodynamic approach within the quasi-harmonic approximation, we show that below 400 K the volume effects describe well the temperature dependence of the Raman linewidth whereas above this temperature there are direct anharmonic effects appearing. Above approximately 850 K new Raman lines appear that are and E polarized.