Thermal transfer in amorphous superionic Li2S

Abstract

Using atomic simulations, we characterize the scattering time, the nature, and the weight of thermal excitations in the example system Li 2 S from its amorphous solid state to its superionic and liquid states. For the amorphous solid state at 300 K, the vibration scattering time was found to range from a few femtoseconds to several picoseconds. As a result, both propagons and diffusons are the main heat carriers and contribute largely to the total thermal conductivity. The enhancement of scattering among vibrations and between vibrations and the free ions' flow due to the increase of temperature leads to a large reduction of the scattering times and of the acoustic vibrational thermal conductivity, i.e., 0.8 W/mK at 300 K to 0.56 W/mK in the superionic Li 2 S at 700 K. In this latter state, the thermal conductivity contributed by convection increases to half of the total, as a result of the cross correlation between the virial heat flux and the free ions' one in liquid Li 2 S at 1100 K. The vibrational scattering time can be large (∼1.5 ps) yet, and the vibrational conductivity is reduced to a still significant fraction highlighting the unexpected role of acoustic transverse and longitudinal vibrations. Our study provides a fundamental understanding of the thermal excitations at play in amorphous materials from solid to liquid, including the intermediate superionic phase.

A puzzle on isomeric configurations in and around $N=126$ closed shell

“…Similar behavior has been predicted for the isomers in neighboring N = 120, 122, 124 isotonic chains [12]. However, the recent B(E2) measurements of the 8 + isomeric state in 214,216 Th do not follow pure seniority estimates and require the mixing of f 7/2 orbital [14,15], compli-cating their corresponding configurations. Hence, more investigation into the nucleonic configurations for these 9/2 − , 8 + , and 21/2 − isotonic isomers is required.…”

“…Pure seniority results are able to explain the data until Z = 88 in both N = 124 and N = 126 isotones. But the role of configuration mixing (as suggested by GS * ) becomes crucial while explaining the recently measured 8 + isomers in 214,216 Th [14,15], where pure seniority would lead to a large B(E2) value. In contrast, it fits into the GS * trend leading to a much longer-lived isomer.…”

Origin of the weakly temperature-dependent thermal conductivity in ZIF-4 and ZIF-62

“…6(a As a result, the thermal conductivity contributed from the diffusons in ZIF-4 (ZIF-62) may increase with temperature when these nonpropagating vibrations in 0.2-0.3 and 0.5-0.8 THz (1.0-2.0 THz) transfer more thermal energy than the rest of the nonpropagating vibrations. Such a phenomenon has been also observed in superionic systems [55,56]. The temperature-independent thermal conductivity of ZIF-4 and ZIF-62 is then because of the similar variation (increasing or decreasing) rates of the thermal conductivities contributed from the propagating and nonpropagating vibrational modes.…”

Strongly Anharmonic Phonons and Their Role in Superionic Diffusion and Ultralow Thermal Conductivity of Cu7PSe6

“…The transverse ( T ) and longitudinal ( L ) components of the dynamical structure factor were calculated using [ 80 ] $$\[ \begin{array}{*{20}{c}}{{S_{T/L}}\left( {\left| {\vec{q}} \right|,E} \right) = \left\langle {\int_{0}^{\infty }{{{{\vec{j}}_{T{\rm{/}}L}}}}\left( {\left| {\vec{q}} \right|,t} \right)\cdot{{\vec{j}}_{T{\rm{/}}L}}\left( { - \left| {\vec{q}} \right|,0} \right)} \right\rangle {e^{iEt{\rm{/}}h}}dt}\end{array} \]$$Here E is the vibrational energy and $$\vec{q}$$ is a wavevector commensurate with the supercell dimensions. We consider the phonon mode velocity with transverse and longitudinal polarizations, $${\vec{j}_T}(|\vec{q}|,t)$$ and $${\vec{j}_L}(|\vec{q}|,t)$$ respectively, defined as [ 80 ] …”

“…To investigate the relative contributions of diffusions and propagons in Cu 7 PSe 6 , we performed spectral functional calculations over a 10 × 10 × 10 q-point mesh in the Brillouin zone, and investigated the presence of propagating quasiparticles in both longitudinal and transverse channels. The transverse (T) and longitudinal (L) components of the dynamical structure factor were calculated using [80] S q E j q t j q e dt , , • ,0…”