Raman studies of the temperature dependence of the internal vibrations of the ammonium ions in the mixed crystals K 1−x A x DP and R 1−x A x DP

• Published in: Low temperature physics (Woodbury, N.Y.) N.Y.), 2001-01, Vol.27 (1), p.61-68
• Main Authors: Van’kevich, A. V., Popkov, Yu. A., Taranova, I. A.
• Format: Article
• Language: English
• ISSN: 1063-777X; 1090-6517
• DOI: 10.1063/1.1344145

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 K 1−x ( NH 4 ) x H 2 PO 4 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 K 1−x ( NH 4 ) x H 2 PO 4 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 T N (68.5 and 80.5 K, respectively). Analogous processes are also observed in the isostructural crystal Rb 0.20 ( NH 4 ) 0.80 H 2 PO 4 .