Spectral parameters of the Raman scattering band corresponding to intrinsic vibration ν2(A1) of the PO4 tetrahedron in mixed crystals of K1−xAxDP with various ammonium concentrations are studied in the temperature range 4.2–300 K. Abrupt changes in frequency and half-width of the band are observed during transitions to ordered ferroelectric (x=0.00, 0.02, 0.04, 0.08) or antiferroelectric (x=0.74,0.82) phases. The decrease in the band half-width during ordering indicates the participation of excitations responsible for the corresponding states in relaxation processes. The transition to the structural glass phase (x=0.22, 0.32, 0.53) is not accompanied with sharp changes in the band parameters, but the band half-width at low temperatures is smaller than the expected value for totally disordered paraelectric phase.
In the paper, the temperature behavior of the vibrational spectrum of the [Formula: see text] crystal has been analyzed by Raman scattering. At a temperature of about 70 K the appearance of new lines was detected in the Raman spectra. The appearance of additional lines is associated with the fixation of the torsional vibration of the ethylenediamine chelate ring. Such a fixation testifies to the implementation of an order–disorder structural phase transition in these crystals. A model of the mechanism of the phase transition in the [Formula: see text] crystal involving the chelate ring of ethylenediamine is proposed.
A comparative analysis of the changes in the fluctuation conductivity and characteristics of the superconducting state of YBa2Cu3O7–δ single crystals caused by various types of defects is carried out. These defects appeared due to irradiation with high-energy electrons (radiation doses from 1.4 to 8.8 1018 cm–2), changes in oxygen deficiency (0.08 ≤ δ ≤ 0.23) due to annealing at different temperatures, or doping with praseodymium (Y1–zPrzBa2Cu3O7−δ, 0 ≤ z ≤ 0.5 at optimal oxygen concentration). It is shown that the introduction of such defects leads to a significant expansion of the temperature range of the existence of excess conductivity, and upon doping with praseodymium, it also leads to the appearance of a thermally activated deflection on the temperature dependence of the electrical resistance. The effect of such defects on the superconducting transition temperature, Tc, and the coherence length along the c axis, ξc(0), is studied. In particular, ξc(0) more than quadruples (at z = 0.43), while the 2D-3D crossover point shifts towards higher temperatures. Possible reasons for the suppression of superconductivity in YBa2Cu3O7–δ upon irradiation with fast electrons and the qualitatively different temperature dependences of its resistivity in the basal plane, ρab (T), are discussed.
This work investigated the vibrational spectrum of the deuterated organometallic crystal Сu(еn) [Formula: see text] in a wide range of frequencies and temperatures. Results indicated that deuteration leads to a change of hydrogen not only in the water molecule, but also in the [Formula: see text] molecule of the ethylenediamine ring. It was found that deuteration of the crystal disrupts the phase transition observed in the pure crystal.
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