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This work reports a computer modeling comparison of two solid electrolyte materials -- Li3PO4 and Li3PS4 in terms of the bulk structures, Li ion mobilities, and their interface properties with vacuum and with Li metal. The simulations show that for some forms of Li3PS4, Li ions can move from a host lattice site to an interstitial site with negligible net energy change. This favorable process for the formation of vacancy-interstitial pairs, is undoubtedly important for the structural and ion migration properties of $\beta$-Li3PS4. Our model results for the idealized interfaces between the electrolytes and Li metal show that Li3PO4/Li interfaces are stable, while Li3PS4/Li interfaces are not. We find that a Li3PS4 surface in the presence of a small amount of Li relaxes to a complicated structure involving broken P-S bonds. On the other hand, the computer models show that a very thin film of Li2S on the Li3PS4 surface can provide a protective buffer layer to stabilize the interface between the Li3PS4 electrolytes and the Li metal anode.