From a simple equation of motion for a linear chain, the phonon amplitudes for mobile and cage ions are represented in terms of their ionic radius and atomic mass. Estimated values of the amplitude in superionic conductors are larger for the mobile ions than the cage ions. Based on this result, many binary and ternary ionic conductors are classified into superionic or normal-ionic conductors. The role of low-energy optical ͑LEO͒ phonon is to enhance the phonon amplitude and assist the jump of mobile ions. This is ascertained from the relationship between the frequency of the LEO phonon and the activation energy, which is similar to selfdiffusion in bcc metals. ͓S0163-1829͑97͒05141-2͔

Infrared reflectivity spectra of sintered crystal CdCr2S4 have been measured in the temperature range of 15–300 K. The frequencies of transverse and longitudinal phonon modes were determined from the spectra with the Kramers–Kronig relation. The temperature dependencies of those showed anomalously large shifts near the Curie temperature. The shifts for two modes on the higher-energy side increase and those for other two modes decrease with temperature. These shifts are described well by the terms of spin-correlation function and concluded to be originated from the spin-dependent force constant. The mode dependence of shifts is explained qualitatively from the relation between the linkage of super-exchange interaction and the direction of vibration of the ion. The effects of magnetic ordering were also observed on Szigeti effective charge, optical and static dielectric constants, and phonon damping constants. These are discussed from the viewpoint of the red shift of the absorption edge.

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The energy of phonon modes in the ␤ phase of AgI is estimated from the phonon modes of the ␥ phase since the Brillouin zone ͑B zone͒ of the former phase can be obtained by folding that of the latter. The idea of folding the Brillouin zone is extended to other binary and ternary superionic conductors ͑SIC's͒. Then the low-energy mode, which is observed for many SIC's at the center of the B zone, is assigned to an optical phonon at the lowest frequency ͑LEO phonon͒, originating from a transverse-zone-edge acoustic phonon. The proportionality of mode frequency with the inverse square root of mobile ion mass, the pressure and temperature dependencies of the mode frequency, and the close relationship of the LEO phonon to the low transition temperature are understood from the result of folding the B zone with respect to the specific crystal structure of the SIC's.

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