DFT-Chemical Pressure Analysis: Visualizing the Role of Atomic Size in Shaping the Structures of Inorganic Materials

Atomic size effects have long played a role in our empirical
understanding
of inorganic crystal structures. At the level of electronic structure
calculations, however, the contribution of atomic size remains difficult
to analyze, both alone and relative to other influences. In this paper,
we extend the concepts outlined in a recent communication to develop
a theoretical method for revealing the impact of the space requirements
of atoms: the density functional theory-chemical pressure (DFT-CP)
analysis. The influence of atomic size is most pronounced when the
optimization of bonding contacts is impeded by steric repulsion at
other contacts, resulting in nonideal interatomic distances. Such
contacts are associated with chemical pressures (CPs) acting upon
the atoms involved. The DFT-CP analysis allows for the calculation
and interpretation of the CP distributions within crystal structures
using DFT results. The method is demonstrated using the stability
of the Ca2Ag7 structure over the simpler CaCu5-type alternative adopted by its Sr-analogue, SrAg5. A hypothetical CaCu5-type CaAg5 phase is
found to exhibit large negative pressures on each Ca atom, which are
concentrated in two symmetry-related interstitial spaces on opposite
sides of the Ca nucleus. In moving to the Ca2Ag7 structure, relief comes to each Ca atom as a defect plane is introduced
into one of these two negative-pressure regions, breaking the symmetry
equivalence of the two sides and yielding a more compact Ca coordination
environment. These results illustrate how the DFT-CP analysis can
visually and intuitively portray how atomic size interacts with electronics
in determining structure, and bridge theoretical and experimental
approaches toward understanding the structural chemistry of inorganic
materials.