Correlation of Ionic Conductivity with Intersurface Phenomena in Superionic-Conductor-Superlattice

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A computer simulation by a molecular dynamics method at constant volume has been performed to a model system which is composed of accumulating two different ionic conductors :・・・AgI-Ag_2S-AgI-Ag_2S・・・. Along the c-axis (z-axis), AgI-parts are compressed and Ag_2S-parts are stretched. About 3% of Ag ions in Ag_2S-parts have been transfered to AgI-parts. The self-diffusion coefficient of Ag ions in AgI-parts decreases by 40-50 % compared with that in a single AgI-system, while the diffusion coefficient of Ag ions in Ag_2S-parts increases by 30 % compared with that in a single Ag_2S-system. The ionic conductivity of the superlattice-system is larger than that of either material of the AgI-system or Ag_2S-system. The calculation suggests the possibility of the existence of a new material which has a larger ionic conductivity.In this calculation, the newtonian equations of motion of the particles in a fixed MD cell of volume V have been solved numerically. The total energy E has been conserved, and thus the ensemble generated by the simulation is the microcanonical or (E,V,N) ensemble. But these conditions of simulation are not the same as those normally encountered in experiments. The usual experiments are performed under conditions of constant temperature and constant pressure or (T,P,N) conditions. Then we are now performing the MD calculation at constant pressure to the superionic superlattice-system, with also considering the electronic polarizability of anions.