  A series of density functional electronic structure calculations were carried
out to better understand the crystallographic factors governing the stability
of LinA3B2O12 lithium garnet phases against hydration. The reaction studied is
H2O + LinA3B2O12 = LiOH + HnA3B2O12. Most of the compositions are stable
against pure water; the main driving force for instability in the atmosphere is
the reaction of lithium hydroxide with CO2 to make lithium carbonate. The
calculated hydration resistance scales with the Pauling bond valence on the
oxygen atom contributed by the coordinating A and B ions. In the unexchanged
Li-garnets, this bond valence must be balanced by lithium, so there is also a
good overall correlation of hydration stability with lithium stoichiometry (n):
hydration resistance increases in the order Li8-garnet < Li7-garnet <
Li6-garnet < Li5-garnet < Li3-garnet. Only Li3A3B2O12 garnets have proton
exchange energies sufficiently positive to overcome the decomposition energy of
lithium hydroxide into lithium carbonate + water; the n=3 garnets are predicted
to be stable to hydration under atmospheric conditions, in agreement with
observations (Galven et al. Chem. Mater. 2012 24, 3335-3345). At a given
lithium ion stoichiometry, hydration resistance is greater for A, B ions having
smaller ionic radii.
