| Autor: |
Van'kevich, A. V., Popkov, Yu. A., Taranova, I. A. |
| Předmět: |
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| Zdroj: |
Low Temperature Physics; Jan2001, Vol. 27 Issue 1, p61, 8p |
| Abstrakt: |
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[sub 1-x](NH[sub 4])[sub x]H[sub 2]PO[sub 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[sub 1-x](NH[sub 4])[sub x]H[sub 2]PO[sub 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[sub N] (68.5 and 80.5 K, respectively). Analogous processes are also observed in the isostructural crystal Rb[sub 0.20](NH[sub 4])[sub 0.80]H[sub 2]PO[sub 4]. © 2001 American Institute of Physics. [ABSTRACT FROM AUTHOR] |
| Databáze: |
Complementary Index |
| Externí odkaz: |
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