Title: Prediction of Stable Ground-State Lithium Polyhydrides under High Pressures,
Abstract: Hydrogen-rich compounds are important for understanding the dissociation of
dense molecular hydrogen, as well as searching for room temperature
Bardeen-Cooper-Schrieffer (BCS) superconductors. A recent high pressure
experiment reported the successful synthesis of novel insulating lithium
polyhydrides when above 130 GPa. However, the results are in sharp contrast to
previous theoretical prediction by PBE functional that around this pressure
range all lithium polyhydrides (LiHn (n = 2-8)) should be metallic. In order to
address this discrepancy, we perform unbiased structure search with first
principles calculation by including the van der Waals interaction that was
ignored in previous prediction to predict the high pressure stable structures
of LiHn (n = 2-11, 13) up to 200 GPa. We reproduce the previously predicted
structures, and further find novel compositions that adopt more stable
structures. The van der Waals functional (vdW-DF) significantly alters the
relative stability of lithium polyhydrides, and predicts that the stable
stoichiometries for the ground-state should be LiH2 and LiH9 at 130-170 GPa,
and LiH2, LiH8 and LiH10 at 180-200 GPa. Accurate electronic structure
calculation with GW approximation indicates that LiH, LiH2, LiH7, and LiH9 are
insulative up to at least 208 GPa, and all other lithium polyhydrides are
metallic. The calculated vibron frequencies of these insulating phases are also
in accordance with the experimental infrared (IR) data. This reconciliation
with the experimental observation suggests that LiH2, LiH7, and LiH9 are the
possible candidates for lithium polyhydrides synthesized in that experiment.
Our results reinstate the credibility of density functional theory in
description H-rich compounds, and demonstrate the importance of considering van
der Waals interaction in this class of materials.