Anomalous pressure dependence of the Lamb-Mössbauer factor in a spin crossover system

The dependence of the f-factor on temperature and on pressure up to 1500 bar has been measured in the spin crossover compound [Fe(2-pic-ND ₂ ) ₃ ]cl ₂ ·EtOD. In the transition region around T _c =146.7 K the pressure dependence of f shows an anomalous increase, which can be explained on the basis of a theoretical model for spin transitions developed by us.     

Time integral and time differential Mössbauer measurements on [57Co/Mn(bipy)3](PF6)2

The Mössbauer emission spectra of nucleogenic iron(II) complexes with a low spin (LS) ground state show two metastable iron(II) high spin (HS) states at low temperatures. In order to identify these metastable HS states, the compound [⁵⁷Co/Mn(bipyridine)₃](PF₆)₂ has been studied by time differential Mössbauer emission spectroscopy (TDMES) and optical lifetime measurements of excited electronic states in the corresponding Fe-doped Mn compound. The lifetime of one of the HS states of the nucleogenic iron(II) determined by TDMES has been measured to be the same as the lifetime of the laser-excited iron(II) electronic state.     

The debye-waller factor in spincrossover molecular crystals: a mössbauer study on [FexZn1−x(ptz)6](BF4)2

In a first approximation the Debye-Waller factor (DWF) of molecular crystals is factorized into a molecular and a lattice part:f=f _m f _l. In the case of spincrossover compounds there is the unique possibility to measure differences of both parts in the two spin states by switching the spin states with the LIESST effect. Measurements of the DWF depending on temperature, -ray direction and spin state were performed with⁵⁷Fe-Mössbauer spectroscopy on three single crystals (R¯3) of the spincrossover system [Fe_xZn_1–x(ptz)₆](BF₄)₂ (ptz=1-propyltetrazole), a concentrated sample withx=1 and diluted ones withx=0.30 andx=0.005. The DWF decreases from 10 to 300 K by a factor of 100 and its anisotropyf _/f increases from 1 to 5.4. The differences of the DWF in the low spin and the high spin state (after LIESST) for the diluted and the concentrated samples were measured at low temperatures ( 50 K). From these data both the Grüneisen constant of the lattice _G=2.9 and the lowest IR active intramolecular frequency (45 cm^–1 (HS), 58 cm^–1 (LS)) have been determined. The contribution of the DWF from the acoustic modes of the lattice is calculated from the elastic constants of rhombohedral [Fe _x Zn_1–x (ptz)₆](BF₄)₂. The contribution of the optical lattice vibrations is estimated. The influence of different DWF in the two spin states on the evaluation of the fraction of molecules in either spin state from Mössbauer area data is discussed.In partial fulfillment of his thesis.     

Spin transition in [Fe(phen)2(NCS)2] and [Fe(bipy)2(NCS)2]: Hysteresis and effect of crystal quality

Detailed magnetic susceptibility measurements on the polycrystalline complexes [Fe(phen)₂(NCS)₂] (phen = 1.10-phenanthroline) and [Fe(bipy)₂(NCS)₂] (bipy = 2,2′-bipyridine) have revealed a narrow hysteresis in both systems indicative of a first-order nature of the spin transition ⁵T_2g(O_h) ? ¹ A_tg(O_h). The crystal quality, in particular crystal defects (through preparation or grinding), have been shown to influence strongly the spin transition behaviour.     

A time-differential Mössbauer emission spectrometer with high efficiency and high time resolution

A high-efficiency, high rate-capability, computer-based measuring setup for Time-Differential Mössbauer Spectroscopy was constructed. Applying NaI(Tl) and plastic scintillators for the detection of the 14.4 keV and 122 keV γ-radiation respectively, a superior timing (2, 3 ns FWHM) was achieved by a “fast-fast-ultra fast” timing method developed for that purpose.     

The mass composition of cosmic rays according to the data obtained using the TUNKA-25 setup

The data obtained using the Cherenkov TUNKA-25 extensive air shower (EAS) installation and the refined method were used to obtain the energy dependence of the average EAS depth maximum. The EAS distributions over the depth of the maximum were obtained for the energies of 5 × 10¹⁵ and 2 × 10¹⁶ eV. An analysis of the shape of distributions indicates that the model of interaction of the nuclei that yields the highest position of the maximum (QGSJET-01) is preferable. If this model is used to account for the maximum’s position, one finds that 〈InA〉 = 1.9±0.2 and helium is dominant in the mass composition at the energy of 5 × 10¹⁵ eV. A sharp increase in the mean mass of the composition is observed at energies in excess of 10¹⁶ eV.     

Aftereffects in zeolite‐encapsulated 57Co‐complexes

Tris(2,2'-bipyridyl)-⁵⁷Co^II and bis(2,2':6',2'-terpyridine)-⁵⁷Co^II complexes were synthesised in the supercages of zeolite‐Y in order to study the effect of molecular isolation on the aftereffects of the ⁵⁷Co(EC)⁵⁷Fe decay. As compared to the regular crystalline salts of the complex ions where, according to the emission Mössbauer spectra, the most abundant species is low‐spin Fe^II, the molecular isolation in the zeolite resulted in a larger fraction of low‐spin Fe^III and a varying amount of high‐spin Fe²⁺ species. In the investigated temperature range, 20 K to 295 K, the majority of the changes was observed above 80 K. In the case of tris (2,2'-bipyridyl)-⁵⁷Co^II-Y, the most characteristic change occurred in valence states, while for bis (2,2':6',2'-terpyridine)-⁵⁷Co^II‐Y, the temperature dependence of the spin states was more prominent. The change in the low spin valence states is explained partly by donor-acceptor properties of the zeolite lattice. The variation in the high spin fraction is explained by radiation damage of the ligand sphere and/or fragmentation of the complex ion followed by incomplete recombination in the supercage. Molecular isolation itself did not seem to increase the chance of fragmentation (as a consequence of charge neutralization following Auger ionization) of these highly conjugated complex molecules.     

The cooperative spin transition in [FexZn1 − x(ptz)6](BF4)2: I. Elastic properties — an oriented sample rotation study by Brillouin spectroscopy

The complete set of seven elastic constants of the Fe(II) spincrossover compound [Fe(ptz)₆] (BF₄)₂ (ptz = 1-propyltetrazole) with crystal symmetry R3i was measured by Brillouin spectroscopy at 300K. The measurement on the plate-like single crystals were performed using an oriented sample rotation technique and the combination of two scattering geometries. For comparison the elastic constants of two isostructural compounds were also measured at 300K: the analogous perchlorate spin-crossover compound [Fe(ptz)₆] (ClO₄)₂ and the zinc complex [Zn(ptz)₆] (BF₄)₂. The knowledge of the elastic constants is neccessary for the calculation of the elastic interaction between the spincrossover molecules, and this is the first time that the complete set of elastic constants of a spincrossover compound has been measured directly.