Origin of the low-energy mode in superionic conductors

Abstract

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.

Phonon spectra of Cu_{6}PS_{5}Br superionic ferroelastic: experimental and theoretical studies

“…Figure 1 illustrates the temperature studies of the low-frequency range (ω < 100 cm −1 ) of Cu 6 PS 5 Br crystal Raman spectra. It should be noted that the studies of the low-frequency spectral range are most interesting for solid electrolytes, since the low-energy mode observed in phonon spectra is their typical characteristic, generally referred to as the low-energy optical (LEO) phonon [22,23]. The role of the LEO phonon consists in the increase of the vibration amplitude and in supporting the mobile ion hopping, i. e. assisting the realization of an effective ion transport and superionic conductivity.…”

Phonon Dispersion in Superionic Copper Selenide: Observation of Soft Phonon Modes in Superionic Phase Transition

“…The effect of folding of the BZ has direct relationship to the suggestion of Wakamura who considers that the low-energy mode in -AgI originates from the zone-edge acoustic phonons in -AgI because of folding of BZ. 13). However, the important difference in the case of the Cu 1.8 Se compound is that the ordering process and the folding of the BZ are driven by a soft mode, that is a second order transition compared to a first order transition as found in AgI.…”

Фононные спектры, однофононные и двухфононные плотности состояний UO-=SUB=-2-=/SUB=- и PuO-=SUB=-2-=/SUB=-

“…Уширение полосы оптических фононов приводит к тому, что численные значения частот в верхней части спектра существенно отличаются от значений в нижней части -появляются так называемые низкоэнергетические опти-ческие фононы (НОФ). Эти фононы играют особую роль в суперионной проводимости, как это отмечалось в ряде работ [32][33][34]. НОФ имеют большие значения коэффи-циентов затухания вблизи температуры суперионного перехода, в работах [33,34] отмечается, что само появ-ление НОФ определяется спецификой кристаллической структуры, в частности наличием высокосимметричных подрешеток или " генетической" высокосимметричной структуры.…”