  Experimental data on the structure of ferroelectric oxide Bismuth Titanate
suggests two different kinds of structures i.e. orthorhombic and monoclinic. We
have performed density functional theory (DFT) based first principle
calculations to determine the ground state structure of bismuth titanate, based
on experimentally observed monoclinic and orthorhombic phases of Bi4Ti3O12.
Both of these phases are optimized to zero pressure and lattice parameters were
determined as a = 5.4370 Ang, b = 5.4260 Ang, c = 32.6833 Ang and Z = 4 for
structure with space group B2cb and a = 5.4289 Ang, b = 5.4077 Ang, c = 32.8762
Ang, b = 90.08 deg and Z = 4 for the structure with space group B1a1 . Static
and dynamic calculations show that the monoclinic structure with the space
group B1a1 is the ground state structure. It is noted that small difference in
the energies of both structures could be a factor behind experimental
observation of either of the structure.
