Structural, optical and vibrational properties of self-assembled Pb(n)(+1)(Ti(1−x)Fe(x))(n)O(3n+1−δ) Ruddlesden-Popper superstructures

Bulk crystals and thin films of PbTi(1−x)Fe(x)O(3−δ) (PTFO) are multiferroic, exhibiting ferroelectricity and ferromagnetism at room temperature. Here we report that the Ruddlesden-Popper phase Pb(n)(+1)(Ti(1−x)Fe(x))(n)O(3n+1−δ) forms spontaneously during pulsed laser deposition of PTFO on LaAlO(3) substrates. High-resolution transmission electron microscopy, x-ray diffraction and x-ray photoemission spectroscopy were utilised to perform a structural and compositional analysis, demonstrating that [Image: see text] and [Image: see text]. The complex dielectric function of the films was determined from far-infrared to ultraviolet energies using a combination of terahertz time-domain spectroscopy, Fourier transform spectroscopy, and spectroscopic ellipsometry. The simultaneous Raman and infrared activity of phonon modes and the observation of second harmonic generation establishes a non-centrosymmetric point group for Pb(n)(+1)(Ti(0.5)Fe(0.5))(n)O(3n+1−δ), a prerequisite for (but not proof of) ferroelectricity. No evidence of macroscopic ferromagnetism was found in SQUID magnetometry. The ultrafast optical response exhibited coherent magnon oscillations compatible with local magnetic order, and additionally was used to study photocarrier cooling on picosecond timescales. An optical gap smaller than that of BiFeO(3) and long photocarrier lifetimes may make this system interesting as a ferroelectric photovoltaic.