J. Stankowski

Microwave resonators for EPR studies at high hydrostatic pressure

The sapphire cylindrical resonator with H112 wave directly coupled to the waveguide is the most advantageous of those hitherto investigated operating in the X band. It has a Q factor of 2500 and a very weak, linear pressure dependence of the resonance frequency with a coefficient of 6.5×10−4 kilobar−1.

Proton glass state in Rb1−x(NH4)xH2AsO4

Abstract A dielectric study of Rb1−x(NH4)xH2AsO4 - RADA mdash; mixed crystals has been made in a wide concentration range x. Both the real and the imaginary part of the dielectric constant were studied in the X-band between 5 K and 300 K and the phase diagram was determined. The proton glass state exists in the concentration range 0.1

Orbital dynamics of the CuXn complex in solids

Abstract The change in copper coordination from CuX 4 to CuX 5 leads to the quantum averaging of local field studied by g -factor in EPR experiment. Observed ‘liquid-like’ EPR spectra are discussed as the frustration between two equivalent CuX 5 orbital states: square pyramid | x 2 − y 2 〉 and trigonal bipyramid |3 z 2 − r 2 〉 configuration, related to the degeneracy of the ground orbital state. Orbital crossing point presents ideal outline of ‘chemically’ induced dynamic Jahn–Teller effect related to the fast exchange within the orbital states due to change in topology of the paramagnetic complex from rigid bonded CuX 4 in the lattice to CuX 5 with internal quantum dynamics.

Second-order effects and hyperfine structure of nitrogen and hydrogen in the EPR spectrum of glycine chelate with 63Cu2+ in triglycine sulphate

Abstract The angular dependence of the electron spin resonance of 63Cu2+ has been measured in triglycine sulphate and the spin-Hamiltonian parameters: g-factor, the hyperfine splitting, ACu, and the ligand hyperfine splittings, AN and AH, have been found. The angular dependence of spectra is found to obey Bleaneys equation for the energies of the spin states calculated to second order for the magnetic hyperfine interaction. In addition to the hyperfine structure arising from the nitrogen atoms, extra hyperfine lines produced as a result of the interaction of the unpaired electron with hydrogen atoms have been observed. Deuteration of triglycine sulphate by the replacement of the hydrogen atoms of the amine groups by deuterium proved that these protons are the responsible for the observed hydrogen hyperfine interaction.

EPR of Graphite and Fullerenes

Abstract Analysis of the EPR results for graphite and fullerenes has led to the development of a model for the g-factor in follerene based on the analogy between graphite and fullerene. Pressure dependence of g-factor in C60 powder confirms the validity of this model. 13C hyperfine splitting ofa0.36 mT in pristine fullerene is also reported. The g-factors of C60 −1 and C60 −3 in a solution and in a different K°C60 fullerides are presented. C60 +n states should be described by the spectroscopic splitting factor larger than that of a free spin.

Dielectric properties of TGS and TGFB monocrystals under high pressure

The Ps, Ec and Tc under high pressure in TGS and TGFB crystals in the range up to 25 kbar were investigated. At pressures up to 10-14 kbar the shift of the Curie point is linear with dTc/dp = 2.4°K/kbar. At higher pressures a nonlinear increase in Tc has been observed. It is associated with hindered rotation of the NH3 groups. The effect of pressure on the spontaneous polarization is greater than that resulting from a single displacement of TC vs. p.

Electron spin echo and EPR studies of paramagnetic centers in polycrystalline C60

Abstract Three different paramagnetic centers were detected by X-band EPR in C 60 powder sample below 60 K and identified as C 60 + radical ( g =2.0028), dimers of C 60 + radicals with zero field splitting D =0.00054 cm −1 , and conducting electrons ( g =2.0021) of graphite residuals. Only C 60 + EPR signals persist at room temperature and all spectral lines saturate easily for microwave powers higher than 50 mW. Two spin-lattice relaxation time T 1 and two phase memory times T M describe the system with T 1 =500 μ s and T M =2.6 μ s for C 60 + at 20 K. An anomaly in T 1 and T M is observed at about 55 K where the field-swept ESE spectrum is narrowest and where the anomaly in the dielectric permittivity is observed. It indicates an existence of a phase transition in this temperature range.

Proton glassy state of Rb1−x(NH4)xH2AsO4

Abstract We have presented the first experimental data of the temperature dependence of e′c and e″c for Rb1−x (NH4)x H2AsO4. The results are demonstrated by the presence of a glassy state in Rb1−x (NH4)x H2AsO4 for x = 0.31.

Radiospectroscopic studies of ferroelectric triglycine sulphate-like crystals

The possibility of using radio and microwave spectroscopy to study intermolecular interactions in ferroelectrics of the TGS family will be presented.Ferroelectric crystals (TGSe, TGS and TGFB) doped with Cr3+ chromium ions were investigated by epr as a function of temperature. The fine structure parameter D of the epr spectra near Tc is closely related to the macroscopic deformation measured by us for all TGS-type crystals.For the selenite the coordination of Cr3+ impurities was determined and the model of charge compensation was found. Anomalies of the D parameter at the phase transitions in TGSe, TGS and TGFB were observed. Effects of applied electric field on epr of ferroelectrics as well as the suggestion of the new kind of epr method with electric modulation-EM epr-will be discussed. This method gives possibilities of measurements of weak electric field effects in ferroelectric and nonferroelectric dielectric crystals.On the basis of the angular dependence of deuterium and proton nmr spectra for part...

Ferroelastic phase transitions in KSc(MoO4)2 monocrystals by electron paramagnetic resonance of Cr3+ ions

Abstract The phase transitions of chromium-doped KSc(MoO4)2 are studied by electron paramagnetic resonance (EPR) of Cr3+. The two transitions are apparent from a sixfold splitting of each of the resonance lines. The spin Hamiltonian parameters and the changes in direction of the principal axes of the Cr3+ complex crystal field in the two ferroelastic phases are reported. With regard to the temperature variations of the fine structure tensor a structural distortion model of the high-temperature phase is proposed, based on the motion of the oxygen atoms of the anion (MoO4)2-, constituting the environment of Sc3+. Below T1 = 258 K the distortion leads to a monoclinic phase of space group symmetry C2/m, and below T2 = 181 K to a monoclinic phase with the space group C2/c.