Space‒time inhomogeneity of the electron flow in pyroelectric X-ray sources

The operating conditions of X-Ray sources based on LiNbO₃ crystals are investigated during heating-cooling cycles. It is demonstrated that the radiation intensity is unstable. The radiation is accompanied by electrical breakdowns in the Z plane of the crystals and emission of photon packets not described by the Poisson distribution. Visualization of the electron beam through the grid electrode by a luminescent screen showed that the electron beam is not uniform in the Z-plane of the crystal and greatly changes with temperature. It is found that, under definite conditions, the numerous redistributions of the intense emission zones occurred between different areas at Z-surface of the crystal. Possible reasons for the observed effects are examined. The obtained data are important for creating pyroelectric X-ray and neutron sources presuming the usage of strong electric fields whose strength reaches 10⁵ V/cm.     

Nature of Local Symmetry Violations of Ions in the Magnetic Subsystem of Magnetoplumbite Crystal,According to Raman Scattering Spectroscopy Data

The structure of an M-type lead hexaferrite (magnetoplumbite) crystal is studied via the diffraction of synchrotron radiation. The crystal model contains two iron ions positions with dynamic disorder. The dynamics of the crystal lattice in the temperature range of 80 to 775 K is studied by Raman scattering. Particular attention is given to the discussing the nature of the local disorder of iron ions in the trigonal bipyramidal environment of oxygen ions.     

Features of the Jahn-Teller transition in Ni1 − x Co x Cr2O4 solid solutions

The structure of Ni_{1 ? x} Co _x Cr₂O₄ solid solutions has been investigated by synchrotron X-ray powder diffraction in the temperature range of 90–350 K for compositions x = 0, 0.005, 0.01, 0.015, 0.02, 0.1, 0.5, and 0.8, and their structures have been refined using the Rietveld analysis. The T-x phase diagram for solid solutions in the studied range of temperatures and concentrations has been constructed based on the obtained data. The revealed structural phase transitions have been explained from the viewpoint of the crystal field theory and the Jahn-Teller cooperative effect. Distortions of polyhedra, which are the cause of the structural phase transition, have been evaluated. The applicability of the Landau phenomenological theory and the possibility of using the magnitude of spontaneous strains as the order parameter have been discussed.     

Synthesis of pisiferol revisited; control of stereochemistry in an intramolecular Diels-Alder reaction

Thermolysis of 4(X=H) gives 6a and 6b(X=H) in a ratio of 1:4 whereas 4 (X=SO₂Ph) gives more of the trans-ring junction product 6a(X=SO₂Ph) suitable for the synthesis of pisiferol [ratio 6a : 6b(X=SO₂Ph) = 1.5:1].