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1

Electron Spin-Lattice Relaxation in Polymers and Crystals Related to Disorder and Structure Defects

100%

Hoffmann S., Hilczer W., Radczyk T., Polus I.

Acta Physica Polonica A

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2003

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vol. 103

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issue 4

373-385

EN

Temperature dependences (4-300 K) of the electron spin-lattice relaxation have been determined by electron spin echo technique for free radicals in two polymeric systems: phenol-formaldehyde resin and polyhydrazodisulphide. The dependences are described in terms of dynamics specific for amorphous systems involving two-level tunnelling states at low temperatures, exchange-coupled clusters of paramagnetic centres and local oscillators. Some universal temperature behaviour of the spin-lattice relaxation of amorphous systems is suggested, with a strong increase in relaxation rate with temperature at low temperatures and much weaker increase above 50 K with characteristic linear temperature dependence in a broad temperature range and cosech(Δ/kT) -type behaviour. It is also shown that the amorphous-type behaviour appears in low temperatures relaxation studies of single crystals but it is due to a non-uniform distribution of doped paramagnetic ions. Such behaviour we have found in Tutton salt crystals doped with Cu^{2+}, as well as for free radical centres produced by ionising irradiation in (NH_4)_3H(SeO_4)_2 and Li(N_2H_5)SO_4 single crystals, where extended phonons are suppressed around radiation damage centres suggesting a local amorphisation of the crystal structure.

2

Restricted Validity of the Two-State Model Describing a Vibronic EPR g-Factors Averaging in Cs_{2}Zn(SO_{4})_{2}·6H_{2}O Tutton Salt Crystals Doped with Cu^{2+} Ions

100%

Hoffmann S., Kaszyński R., Augustyniak M., Hilczer W.

Acta Physica Polonica A

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1999

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vol. 96

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issue 6

733-740

EN

Single crystals and powder EPR spectra of Cu^{2+} ions in Cs_{2}Zn(SO_{4})_{6}·6H_{2}O were recorded in the temperature range of 4.2-300 K and the g-factor temperature variations were determined. The g_{z} and g_{y} have the values of 2.443 and 2.134, respectively, at the rigid lattice limit below 20 K, and then continuously tend to an average value on heating. This vibronic averaging produced by reorientations of Cu(H_{2}O)_{6}-complexes between Jahn-Teller distorted octahedral configurations is described in terms of a two-state Silver-Getz model which is known as a good model for diamagnetic non-ammonium Tutton salts. We found, however, that this model is only a crude approximation in Cs_{2}Zn(SO_{4})_{6}·6H_{2}O below 150 K. Above this temperature the model works better and describes the vibronic dynamics between the two lowest energy potential wells in the adiabatic potential surface differing in the energy of δ_{12}=318(9) cm^{-1}=3.7 kJ/mol.

3

ESR and Electron Spin Echo Studies of Spin-Lattice Relaxation of Hydrazinium Radical in Li(N_{2}H_{5})O_{4} Single Crystal

100%

Morawski P., Hoffmann S. K., Hilczer W., Goslar J.

Acta Physica Polonica A

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1997

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vol. 91

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issue 6

1121-1129

EN

A hydrazinium ṄH-NH_{3}^{+} radical was identified by ESR in γ-irradiated Li(N_{2}H_{5})SO_{4} single crystal. Unpaired electron is strongly localized on the nitrogen atom as compared to hydrazinium radicals in other crystals. Angular variations of the multicomponent ESR spectra and computer calculations gave the isotropic hyperfine coupling parameters (in mT): a_{N1} = 1.93, a_{N2} = 0.80, a_{H1} = 2.50, a_{H1'} = 0.48 (NH_{3}), and a_{H2} = 1.40 (adjacent molecule). The radiation defects containing unpaired electrons do not influence much the protonic conductivity along the hydrazine molecule chains in the crystal. It confirms a tunelling mechanism of the conductivity activated by NH_{2}-group rotation. Spin-lattice relaxation time T_{1} varies strongly with temperature in the range from 100 ms (10 K) to 90 μs (170 K) and is governed by spectral diffusion (1/T_{1} ∝ T^{2}) and a Murphy-type mechanism (1/T_{1} ∝ cosech(Δ/T)) with Δ = 150 cm^{-1}. A distribution of the spin-relaxation times was observed with temperature independent dispersion parameter β = 0.76.

4

EPR and Spectral Studies of a Molecular and Crystal Structure of Cu (3,5-dimethylpyridine)_{3}(NO_{3})_{2}

88%

Hoffmann S., Goher M., Hilczer W., Goslar J., Hafez A.

Acta Physica Polonica A

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1994

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vol. 85

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issue 3

517-530

EN

X-band single crystal EPR, IR and UV-VIS data are used to deduce a molecular and crystal structure of a new compound with a chemical formula Cu(3,5-dimethylpyridine)_{3}(NO_{3})_{2}. EPR shows that four magnetically inequivalent molecules exist in the crystal unit cell of C_{4v} symmetry. IR spectra suggested that both monodentate and bidentate NO_{3}-groups exist in the crystal. A most probable Cu(II) complex geometry with d_{x²-y²} ground state is a square pyramid with three nitrogens from 3,5-dimethylpyridine molecules and one oxygen from monodentate nitrato group in a basal plane and an oxygen from NO_{3}-group in apical position. There exists a chemical contact between complexes as it is indicated by the exchange interaction with |J| ≤ 1 cm^{-1}. It suggests that square pyramid complexes are arranged in polymeric chains with semi-coordination of bidentate NO_{3}-group to the adjacent Cu(II) ion. An extended analysis of an exchange averaging effect between EPR lines from two and four Cu(II) sites is presented with equations relating molecular and crystal g-factors.

5

Electron Spin Echo Envelope Modulation Analysis of SeO¯_{3} Radical in (NH_{4})_{3}H(SeO_{4})_{2} Single Crystal

88%

Tritt-Goc J., Goslar J., Hilczer W., Hoffmann S., Augustyniak M.

Acta Physica Polonica A

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1993

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vol. 84

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issue 6

1131-1141

EN

A detailed computer analysis of the electron spin echo envelope modulations of SeO_{3} radical in a suitable orientation of (NH_{4})_{3}H(SeO_{4})_{2} single crystal is presented. It was found that the modulations are due to a weak dipolar coupling with nitrogens and protons of the only two neighbouring NH_{4} groups among the five NH_{4} groups surrounding SeO¯_{3} center. Isotropic dipolar coupling constant is 1.7 MHz for nitrogens and 0.9 MHz for protons. It was shown that thermal reorientations of NH_{4} groups observed by NMR have a negligible effect on the electron spin echo envelope modulation pattern but can be responsible for the same value of a^{H}_{iso} for all protons in a NH_{4} group. A good fit obtained between experimental spectra and theoretical calculations assuming nondisturbed crystal geometry indicates a small damage of the crystal by X-rays during the radical center formation.

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Electron Spin-Lattice Relaxation in Polymers and Crystals Related to Disorder and Structure Defects

Restricted Validity of the Two-State Model Describing a Vibronic EPR g-Factors Averaging in Cs_{2}Zn(SO_{4})_{2}·6H_{2}O Tutton Salt Crystals Doped with Cu^{2+} Ions

ESR and Electron Spin Echo Studies of Spin-Lattice Relaxation of Hydrazinium Radical in Li(N_{2}H_{5})O_{4} Single Crystal

EPR and Spectral Studies of a Molecular and Crystal Structure of Cu (3,5-dimethylpyridine)_{3}(NO_{3})_{2}

Electron Spin Echo Envelope Modulation Analysis of SeO¯_{3} Radical in (NH_{4})_{3}H(SeO_{4})_{2} Single Crystal