First-principles calculation of the stabilities of lithium garnet   compositions against hydration
  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. 
James R. Rustad
cond-mat.mtrl-sci