Ag⁺dynamics in the superionic and liquid phases of Ag₂Se and Ag₂Te by coherent quasi-elastic neutron scattering

Abstract

The atomic motion of Ag+ ions in superionic and liquid Ag2Se and Ag2Te has been investigated by coherent quasi-elastic neutron scattering. The presence of a Q-dependent broadening of the quasielastic peak indicates that the Ag+ ion is mobile in the superionic phase of both materials. This motion takes place within a body-centred cubic sub-lattice of Se2- or Te2- ions respectively. Two Lorentzian components were required in order to fit the quasi-elastic broadening: a narrow peak that is attributed to the diffusion of the cations, and a much broader and less intense component that may be due to a localized motion of these ions. An asymmetry in the quasi-elastic peak indicates the presence of a low-energy excitation as reported for other superionic materials. The widths of the narrower Lorentzian showed the characteristic de Gennes narrowing at Q values close to the maximum in SAgAg(Q). A comparison of the widths of the quasi-elastic peak for the two liquids indicates a similar Ag+ diffusion coefficient but a difference in the mechanism of diffusion. The results for liquid Ag2Te indicate a jump-diffusion mechanism in comparison to those for Ag2Se that follow Fick's law.

Discovery of a Two-Dimensional Type I Superionic Conductor

“…37 However, in QENS studies of superionic α-Ag2Se, coherency effects were found to be negligible for Q 2 < 2 Å -2 (the maximum Q 2 value in our experiments), and the extracted diffusion coefficient from DQ 2 analysis matched values from ionic conductivity measurements and AIMD simulations. 33 Similarly, incoherent QENS analysis of the Cu + -ion SICs (Cu also mainly scatters neutrons coherently), α-Cu2Se 6 and α-CuCrSe2 38 also yielded diffusion coefficient values in good agreement with conductivity measurements and molecular dynamics simulations -again suggesting minimal coherency effects. Therefore, our approach is reasonable based on previous results and the agreement between our incoherent DQ 2 analysis and complementary AIMD simulations.…”

“…Similar QENS results were observed in α-Ag2Se where simple Fickian diffusion was also assigned. 33 In more complex jumpdiffusion models, e.g., the Chudley-Elliot model, 34 oscillatory behavior of Γ occurs with increasing Q 2 , associated with discrete jumps on a crystalline sublattice. Although the limited Q -range here means we cannot comment on the applicability of more complex models, this should not significantly affect the magnitude of the diffusion coefficient.…”

Discovery of a Two-Dimensional Type I Superionic Conductor

“…26 However, in QENS studies of superionic α-Ag2Se, coherency effects were found to be negligible for Q 2 < 2 Å -2 (the maximum Q 2 value in our experiments), and the extracted diffusion coefficient from DQ 2 analysis matched values from ionic conductivity measurements and AIMD simulations. 22 Similarly, incoherent QENS analysis of the Cu + -ion SICs (Cu also mainly scatters neutrons coherently), a-Cu2Se 6 and a-CuCrSe2 27 yielded diffusion coefficient values in good agreement with conductivity measurements and molecular dynamics simulations -again suggesting minimal coherency effects. Therefore, our approach is reasonable based on previous results and the agreement between our incoherent DQ 2 analysis and complementary AIMD simulations.…”

“…Similar QENS results were observed in a-Ag2Se, where simple Fickian diffusion was also assigned. 22 In more complex jump-diffusion models, e.g., the Chudley-Elliot model, 23 oscillatory behavior of G occurs with increasing Q 2 , associated with discrete jumps on a crystalline sublattice.…”

Single-crystalline Ag2Te nanorods prepared by room temperature sputtering of GeTe

“…The diffusion constant of monoclinic Ag 2 Te was reported to be around 10 −4 cm 2 s −1 at 110 °C [55]. The diffusion constant was three orders of magnitude lower than that of the high-temperature cubic phase, which is a well-known super ionic semiconductor [56,57], but the diffusion was still significant in microscale. Extrapolation assuming the Arrhenius law suggests ~ 10 μm 2 s −1 at RT.…”