Tunability of TCF in Ruddlesden-Popper compounds

Ruddlesden-Popper phases (general formula, A/sub n+1/Ti/sub n/O/sub 3n+1/) are composed of perovskite blocks separated and sheared by rock-salt structured layers along 1/2 [111]. Members of the Ruddlesden-Popper family, based on Ca and Sr, have been fabricated for compositions where n=1-4 and /spl infin/ (pure perovskite). In Sr based compounds, TCF decreased from +1650MK/sup -1/ for pure SrTiO/sub 3/ to +140 for Sr/sub 2/TiO/sub 4/ (n=1), accompanied by a decrease in relative permittivity (k) from 190 to 37. A linear relationship was observed between k and TCF. However, TCF for Ca/sub 3/Ti/sub 2/O/sub 7/ was +50MK/sup -1/ (k=46) whereas Sr/sub 3/Ti/sub 2/O/sub 7/ had a value of +320MK/sup -1/ [158]. Electron diffraction patterns indicated that Ca/sub 3/Ti/sub 2/O/sub 7/ had undergone at least one octahedral tilt transition on cooling but that Sr/sub 3/Ti/sub 2/O/sub 7/ had the prototype I4/mmm, Ruddlesden-Popper symmetry. The tunability of TCF in Sr-based Ruddlesden-Popper phases is therefore explained in terms of k dilution arising from the incorporation of rock-salt structured layers in the lattice. Further tuning of TCF in Ca based compositions is attributed to the onset of octahedral tilting.