Symmetry of ferroelectric phase of SrTi18O3 determined by ab initio calculations

Substitution of more than 33% of the naturally abundant O-16 in strontium titanate SrTiO3 by O-18 causes the system to become ferroelectric at low temperatures. The ferroelectricity has been observed via susceptibility measurements, but to date the details of the ferroelectric phase and the phase transition are unclear. Using ab initio density functional theory and lattice-dynamics calculations, we find that the stable structure of the ferroelectric phase is orthorhombic with Ima2 symmetry. The Ima2 point group is noncentrosymmetric and the proposed structure exhibits an electric dipole moment of (0.57 0 0) e angstrom. The Ima2 symmetry is consistent with the limited structural details that are reported using neutron diffraction and Raman spectroscopy.