M. Krupski

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.

Corundum‐filled resonator systems for high‐pressure and low‐temperature electron paramagnetic resonance studies

A cylindrical corundum‐filled resonator of the TE112‐type in the microwave X band, coupled to a coaxial line, is used in two systems designed for the high‐pressure, low‐temperature electron paramagnetic resonance (EPR) studies. The first one is used up to 800 MPa and over a temperature range of 80–400 K. The second permits EPR measurements down to 4 K in the pressure range below 400 MPa. Pressure and temperature can be simultaneously varied and controlled with the high accuracy needed to study spectral anomalies in the vicinity of the phase transitions in solids.

The temperature dependence of the pressure switching of Jahn–Teller deformation in the deuterated ammonium copper Tutton salt

Abstract The pressure switch of the Jahn–Teller deformation direction in (ND 4 ) 2 Cu(SO 4 ) 2 ·6D 2 O was investigated in the temperature range 130–320 K. Below 295 K, the new, pressure-induced phase, is stable under ambient pressure. Switching back is observed on heating to above 297 K. In the range 150–295 K a strong temperature dependence of the switching pressure (from 24 to 450 MPa) is observed. Below 150 K, the switching process is slow and a coexistence of two phases is observed. We conclude that the switch of the Cu(D 2 O) 6 complex deformation direction is the Jahn–Teller response to the changes in the hydrogen bond system.

Phase delay effect in a thermally homogeneous system heated periodically

The response of a thermally homogeneous system to a periodical heat inflow was studied. The results of the calculations and the experiment proved that the systems temperature changed periodically with a certain phase delay relative to the stimulus (heat influx) and dependent on the period Th and the systems relaxation time τ, according to the equation tan = 2ωτ, where ω = 2π/Th. This relationship can be used for determination of τ and the heat capacity of the system. The simple theory and experiment is a good example for understanding other relaxing processes of the exponential form and may be helpful to physics teachers at university level.

EPR Evidence of the F.C.C.–S.C. Phase Transition in Fullerene C60 under Hydrostatic Pressure

The EPR study of polycrystalline C 60 shows the evidence of a hydrostatic pressure effect (up to 700 MPa) on the g-value of intrinsic defects of polycrystalline C 60 , i.e. C + 60 . The pressure coefficient of the g-factor, (∂g/∂p) T , has different values in both f.c.c. and s.c. phases of C 60 . This result is explained by the large compressibility of the fullerene lattice.

High-pressure EPR studies of intermolecular interactions in solids

High-pressure technique as applied to studies of different problems in solid state physics by EPR spectroscopy is presented. An influence of hydrostatic pressure up to 500 MPa on a weak superexchange coupling between Cu(II) ions in monomeric and dimeric copper(II) compounds is described. Both decrease and increase in the coupling is observed under pressure depending on the crystal and molecular structure but in all cases a dominant contribution is due to a shortening of intermolecular distances, whereas molecular structure and molecular vibrations are not affected. In dimeric Cu(II) compounds with resolved fine structure of EPR spectra zero-field splitting is weakly or not at all dependent on pressure whereas a strong dependence on temperature exists. It is discussed as a result of vibronic ligand behaviour which is not volume dependent. For all cases complementary temperature dependences are presented and moreover a behaviour of EPR linewidth under pressure and temperature is shown. An influence of high pressure on spin exchange reactions in solutions and on the phase memory time in electron spin echo experiments is briefly described.

Temperature and pressure effects in EPR of copper phthalocyanine-iodine

Abstract An observed EPR line broadening with pressure and with lowering of temperature for Cu(Pc)I0. 2 was related to a volume dependence of the exchange coupling between copper (II) sites.

ESR Study of (TMTTF)2BF4 and TTF-TCNQ under Hydrostatic Pressure

Abstract We have measured the susceptibility, g-factor and linewidth of (TMTTF)2BF4, and TTF-TCNQ single crystals in the 80-300K temperature range up to 5 kbars of hydrostatic pressure in X-band. The decrease of the susceptibility and the increase in the 1inewidth on pressure is less expressed in (TMTTF)2 BF4, than In TTF-TCNQ. The g-factor does not depend on pressure in both cases.

Uniaxial stress system for electron paramagnetic resonance studies

A standard X-band electron paramagnetic resonance cavity has been adapted to measurements under uniaxial stress parallel to the external magnetic field direction. The system is applied to study single crystals under stress up to 100 MPa in the temperature range 100–400 K.

Phase lag in the amplitude modulated magnetic resonance

Abstract Response of a magnetic spin system to the amplitude modulated radiofrequency field is studied. Spin system is assumed to obey Bloch equations. For a spin system resonant at Larmor frequency ω 0 , calculations demonstrate how the phase lags Φ ⊥ and Φ z , of magnetization components M x , M y and M z occur and how sensitive they are to the relaxation properties of the system. Relative to Larmor frequency the angular frequency of the amplitude modulation, Ω⪡ω 0 , is very low, at the same time it is comparable to the relaxation rates of the spins. With reference to the analytical solution of a damped harmonic oscillator a simple formula tan Φ ⊥ =T 2 Ω describes the magnitude of the phase lag Φ ⊥ as a function of relaxation time T 2 .