No abstract

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.

The Raman spectra in the frequency region corresponding to the internal vibrations of the ammonium ions are investigated in the temperature range 4.2–300 K for the mixed crystals K1−x(NH4)xH2PO4 with different ammonium concentrations (x=0.22, 0.32, 0.53, 0.74, and 0.82). Analysis of the temperature dependence of the frequencies and half-widths of the internal vibration lines of the ammonium ions confirms that in the paraelectric phase the position of the ammonium ions in the crystal lattice is not fixed: they execute hops with a change in the position of the center of mass. The activation energies of these reorientation processes are determined. It is found that at temperatures of the order of 110–120 K for the K1−x(NH4)xH2PO4 crystals of all concentrations a correlated fixing of the ammonium ion in the lattice occurs, leading to the formation of an antiferroelectric cluster structure. For the crystals with concentrations x=0.22, 0.32, and 0.53 this structure is preserved in the structural glass phase down to liquid helium temperatures, while for the crystals with x=0.74 and 0.82 it is preserved to the temperatures of the antiferroelectric phase transition TN (68.5 and 80.5 K, respectively). Analogous processes are also observed in the isostructural crystal Rb0.20(NH4)0.80H2PO4.

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