Tunability of TCF in Ruddlesden-Popper compounds

Ruddlesden-Popper phases (general formula, A_n+1Ti_n O_3n+1) are composed of perovskite blocks separated and sheared by rock-salt structured layers along ½[111]. Members of the Ruddlesden-Popper family, based on Ca and Sr, have been fabricated for compositions where n=1-4 and ∞ (pure perovskite). In Sr based compounds, TCF decreased from +1650MK^-1 for pure SrTiO₃ to +140 for Sr₂TiO₄ (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 Ca₃ Ti₂O₇ was +50MK^-1 (k=46) whereas Sr₃Ti₂O₇ had a value of +320MK^-1 (158). Electron diffraction patterns indicated that Ca₃Ti₂O₇ had undergone at least one octahedral tilt transition on cooling but that Sr₃Ti₂ O₇ 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