Dedicated to Professor Ewald Wicke on the occasion of his 80th birthdayComplete ionic conductivity spectra as well as quasielastic and inelastic neutron scattering spectra have been taken of solid silver bromide at various temperatures. High-amplitude vibrational move ments, essentially of the silver ions, contribute to both kinds of spectra. In particular, a conductivity maximum, located at about 500 GHz, reflects oscillations of individual silver ions along <111 > directions. -The microwave and millimetre-wave conductivities are dominated by a thermally ac tivated Debye-type relaxation process. The effect is consistently explained by the frequent hopping of silver ions from regular octahedral lattice sites into tetrahedral interstitial sites and back again, i.e., by the frequent creation and recombination of Frenkel pairs. -The effect is also responsible for the existence of thermally activated quasielastic components in the neutron scattering spectra. The width of the coherent quasielastic scattering shows that the forward-backward hopping of a silver ion is accompanied by fast correlated movements of ions in its immediate neighbourhood.C r e a tio n a n d R e c o m b in a tio n o f F r e n k e l D e fe c ts in A g B r
I. IntroductionSilver bromide, AgBr, is one of the most thoroughly studied solid electrolytes, see for instance [1]. It is a fast ionic conductor at temperatures well below its melting point, with a conductivity of 0.1 f t-1 cm-1 at about 625 K [2-4], The cation mobility in AgBr is caused by the thermally activated formation of Frenkel defects in the rocksalt structure, the Frenkel-pair formation enthalpy being about 1.15 eV [5,6]. On the basis of temperature dependent conductivity and tracer diffu sion data as well as model calculations, various migra tion enthalpies have been derived, the lowest applying to the collinear interstitialcy mechanism [7, 8], In 1961 Everett et al. [9] published microwave con ductivity data of AgBr, taken at 23 GHz. The conduc tivity was found to be significantly larger at 23 GHz than at 1 kHz [2][3][4], and the effect was attributed to a Debye-Falkenhagen type of relaxation [10,11] of the mobile silver ions. In an effort to substantiate the findings of Everett et al. we have performed conduc tivity measurements in a broad frequency range. While our earlier data were restricted to frequencies below 60 GHz [12], we now present complete conduc tivity spectra which also comprise the millimetre-wave and far-infrared frequency regimes, up to 10 THz. Our data corroborate the existence of a pronounced con ductivity dispersion at microwave frequencies. HowReprint requests to Prof. Dr. K. Funke. ever, as already stated in [12], they clearly do not bear the distinguishing marks of a Debye-Falkenhagentype jump relaxation process; these would include a power law frequency dependence of the conductivity with an exponent of less than one, see, e.g., [13,14], Instead, the observed frequency-dependent conduc tivity seems to follow a power law with an exponent of two, which is ty...
