Observation of anomalously increasing phonon damping constant in theβphase of the fast-ionic conductorAg3SI

Abstract

The correlation of phonon frequency co, and phonon damping yj to the ionic conductivity cr has been observed from the temperature dependence of infrared-reflectivity spectra of a Ag3SI single crystal. Two reststrahlen bands and a rapid increase of reflectivity in low-frequency region were observed above T, (=157 K), the P-y phase transition temperature, and described by the dispersion relation due to the phonons and the diffusing ions. The yj exhibits the drastic change at T, and has an anomalously large magnitude above T, . The shifts of y, and co, also showed appreciable changes at about 250 K. Furthermore, the toi of the low-frequency mode in the P phase increases with increasing temperature. We can qualitatively interpret these results from the disorder arrangement of the Ag ion above T, and the coupling of mobile ions above 250 K with phonons near the Brillouinzone boundary.

Ion conduction in proton- and related defect (super) ionic conductors: Mechanical, electronic and structure parameters

“…The second is a broadened band shape observed in many IHCs [17,[35][36][37]57], SPCs [39,[70][71][72][73], β-alumina [40,41,74], Li-ion conductors [2,42], YSZ [43,59,60], and NdGaO 3 [75], and also rapid band broadening of the host lattice mode with increasing temperature observed in SICs [8][9][10][11]15,34,76]. Both the behaviors are typical evidence for the fourth-order anharmonicity.…”

“…For the E ac value of LiN 2 H 5 SO 4 , we employ the value measured by Kreuer et al [32], instead of 0.07 eV assigned by Frech and Brown, since they obtained it from the half width of the vibration mode without taking the fourth-order term [33], in spite of considerable magnitude of that term as predicted from SICs [8][9][10]34] and IHCs [35][36][37][38]. 7) or Eq.…”

“…4 since the compound plotted near B-or Acurve takes large or small fourth-order anharmonicity of the vibration mode and also the first-or second-order phase transition [8][9][10][11]16,27,[34][35][36][37][38][70][71][72][73][74]. Rb 3 H(SeO 4 ) 2 plotted on the position between A-and Bcurves takes the mixture of the first-and second-order phase transitions [46,47].…”

“…For SICs, Brüesch et al considered the coupling of shear-type vibration mode with movable ion [8]. Wakamura et al noticed close relation of the phonon frequency of optical mode to E ac value [9,10], and understood it qualitatively from double minimum potential produced with the third-and fourth-order phonon-phonon couplings [11].…”

“…For those conductors, significant relations of the vibration modes and of the electronic energy state near the band gap on conduction ion have been discussed by several researchers [8][9][10][11][12][13][14][15][16][17][18][19][20][21][22]. For SICs, Brüesch et al considered the coupling of shear-type vibration mode with movable ion [8].…”