The free energy density of a crystal: calcite (CaCO3) as a case of study

A new calculation metihodology for determining the static energy density at 0 K of each layer forming the slab (i.e., how the static energy density varies within the slab) is proposed. This work is the continuation of a previous one in which the vibrational free energy density of a slab was estimated. Now, it becomes possible to determine the free energy density profile of a slab delimited by any (hkl) surface and temperature, by adding the static contribution to the vibrational one. Moreover, it is now possible to estimate the surface free energy of a crystal face without calculating the free energy of the bulk. Finally, this model can be extended to the calculation of the free energy of the interface between (i) two identical crystals in a twinning relationship and (ii) two different crystals in an epitaxial relationship. The model was applied to the (10.4) (twinned and untwinned), (10.0) and (01.2) slabs of calcite (CaCO3): their free energy density profiles were calculated, as well as their surface/interface free energies. To the best of my knowledge, this is the first time that it is evaluated how a planar defect like a surface/interface affects the thermodynamic behavior of the underlying layers forming the crystal.