Dielectric dispersions in pentakis methyl ammonium bismuthate single crystals. II. (CH₃NH₃)₅Bi₂Cl₁₁

Abstract

For pt.I see ibid., vol.4, p.2687 (1992). The dielectric dispersion of the permittivity epsilon c* in (CH3NH3)5Bi2Cl11 single crystals has been studied in the frequency range from 1 MHz to 20 GHz in the temperature range 150-400 K and from 18 to 40 GHz in the temperature range 300-360 K. Near the high-temperature ferroelectric phase transition (308 K) the dielectric behaviour is determined by the properties of two relaxators; a low-frequency relaxator in the megahertz region showing a critical slowing down and a high-frequency relaxator in the gigahertz region which is thermally activated. The contribution of the high-frequency relaxator vanishes near the temperature of the second phase transition at about 170 K where another contribution of a third relaxator with an extremely small strength appears. The origins of the observed dielectric contributions with respect to the molecular motion of the CH3NH3+ cations is discussed.