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Substitution of more than 33% of the naturally abundant 16O in strontium titanate SrTiO3 by 18O 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) eÅ. The Ima2 symmetry is consistent with the limited structural details that are reported using neutron diffraction and Raman spectroscopy. [ABSTRACT FROM AUTHOR]

We calculate the spectrum of internal vibrations of a single ferrocene Fe(C[sub 5]H[sub 5])[sub 2] molecule using ab initio density functional theory (without free parameters) and compare this with inelastic neutron scattering data on ferrocene in the solid state at 28 K. Due to the good agreement, we can assign each vibrational mode to each observed peak in the neutron spectrum and so remove ambiguities existing in the literature. There is also consistency between the calculated potential energy of a single ferrocene molecule for different orientations, φ, of the two cyclopentadienyl C[sub 5]H[sub 5] rings with respect to each other, which shows a potential barrier of 0.9 kcal/mol, and electron diffraction, and between the calculated shallow minimum at φ=9 deg and x-ray diffraction. © 2000 American Institute of Physics. [ABSTRACT FROM AUTHOR]