Raman studies of the temperature dependence of the internal vibrations of the ammonium ions in the mixed crystals K1−xAxDP and R1−xAxDP

Abstract

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.