Comparative Studies on the Phase Stability, Electronic Structure, and Topology of the Charge Density in the Li3XO4 (X = P, As, V) Lithium Orthosalt Polymorphs

Density functional theory (DFT) calculations within the generalized gradient approximation (GGA) were performed to study phase transitions in lithium orthosalts Li3XO4 (X = P, As, V). The equilibrium crystal structure parameters and the total energies of the Li3XO4 polymorphs were calculated for the beta-and gamma-phases. The relative lattice stabilities of the two polymorphs were examined in the light of various peculiarities at atomic scale. The beta-polymorph was systematically found to be the most stable one, agreeing well with experimental results. Using an all electron full potential method, we have investigated densities of states and the topology of charge density through the Bader's quantum theory of "atoms in molecules" (AIM) along with electrostatic energy density snaps in order to evaluate factors governing the stability of each polymorph. Higher stability of the beta-polymorph or differences in energy between beta- and gamma-phases according to the nature of X along with the experimental temperatures of phase transitions can be connected to several microscopic features, including differences in constraint within XO4 tetrahedron or chemical bonding characteristics. This new approach can be applied to other phase stability studies, including the family of Li2MSiO4, which is of great interest for cathode materials in lithium batteries.