Far Infrared Reflection Spectra of AgCl, AgBr, and AgI at Low Temperatures

Abstract

The reflection spectra of AgCl, AgBr, and AgI are measured at liquid hydrogen, liquid nitrogen, and room temperatures. For AgCl, they show distinctly two components. For AgBr and AgI, the structure, though complex, is not resolved. Nevertheless, in all cases, except for AgI, at low temperature, the Lorentz analysis needs two oscillators. The stronger one v(1) corresponds to the TO transition at point Gamma, the second one v(2), about an order of magnitude smaller, corresponds to a TA + TO addition band for the NaCl type structure, and to a TA + LA for the zinc blende or wurtzite structure, confirming preliminary measurements made by a completely different experimental approach (transmission of thin evaporated films instead of reflection on bulk crystals). They are, however, slightly distorted towards high frequencies. The proposed frequencies for the TO branch at point Gamma are, for AgCl: 105 cm(-1) (290 degrees K), 114 cm(-1), (80 degrees K), 119 cm(-1) (25 degrees K); for AgBr: 79 cm(-1) (290 degrees K), 84 cm(-1) (80 degrees K), 85 cm(-1) (25 degrees K), and for AgI: 103 cm(-1) (290 degrees K), 106 cm(-1) (80 degrees K), 106 cm(-1) (25 degrees K). The oscillator strengths and the damping of both oscillators are studied vs temperature. A large decrease of the damping is still observed between liquid nitrogen and liquid hydrogen temperatures. The refractive index and the absorption index are determined in the whole ir for the first time at liquid hydrogen, liquid nitrogen, and room temperatures, showing large variations, particularly a high transparency in the very far ir at liquid hydrogen temperature. The maximum values of n and k range from 2 to 10. The real and the imaginary parts of the dielectric constants are also computed for the whole ir range at room and lower temperatures down to liquid hydrogen temperature.

Long‐Wavelength Optical Phonons of AgBrxCl1−x Mixed Crystals

“…I n this model the mode behaviour of the mixed crystal can be predicted from macroscopic parameters of the pure end members. Since the static and high-frequency dielectric constants from our KK analysis were not of sufficient, accuracy, we took for both AgBr and AgCl from [6]. Using then our values for the L O and TO long-wavelength frequencies the corresponding E~ was computed by means of the LydanneSachs-Teller relation.…”

Silver Chloride (AgCl), Silver Bromide (AgBr), Silver Iodide (AgI)

“…In the far-IR region, Hadni et al [6] reported the optical constants determined by a K-K analysis at room temperature (shown in Table V), 80 K, and 25 K. The experimental procedure was the same as in the case of AgC1, but the samples were not single crystals; they were pellets. Two Lorentz oscillators were also needed for AgBr; the stronger one corresponds to the TO mode and the weaker one to the TA + TO mode (see Table II).…”

“…In the far-IR, Hadni et al [6] reported the effective optical constants shown in Table VI obtained by K-K analysis. The experiment was the same one as in the case of AgC1, except that the samples were not single crystals, but pellets.…”

“…Hadni et al [6] measured the far-IR reflectivity of a single crystal at room temperature, 80 K, and 25 K by means of a grating spectrometer. The optical constants shown in Table IV were obtained by a K-K analysis.…”

Jahn‐Teller Effect on Transition Metal Ions in Silver Halides III. Interpretation

“…the J T splitting AE increases with increasing temperature. I n Table 3 the measured shifts AE(300 " K ) / A E ( 2 0 OK) are compared with the shifts calculated with (4.1) using the values A w~o = wT0(20 O K ) -wTo(3o0 OK) given byHadiii et al [19]. I n consideration of the uncertainty of the assumption AoE/o, = = AcuTO/cuTO i t seems justified to conclude that the weak temperature dependence of AE is caused by that of me.…”