Optical properties of Ln2Ti2O7 (with Ln = La to Lu) thin films grown on (110)-SrTiO3 substrates by pulsed laser deposition

Ln2Ti2O7 thin films with Ln = La to Lu were successfully grown on (110)-SrTiO3 substrates by pulsed laser deposition. The microstructure and optical properties of the entire Ln2Ti2O7 series were investigated by X-Ray diffraction, atomic force microscopy, scanning transmission electron microscopy coupled with energy-dispersive X-ray spectroscopy and spectroscopic ellipsometry. For large ionic radii (i.e. Ln3+ = La3+ to Eu3+), the monoclinic/layered-perovskite structure is obtained with epitaxial growth. For smaller ionic radii (i.e. Ln3+ = Tb3+ to Lu3+), the cubic/pyrochlore structure is privileged. We have also observed the growth of a Gd2Ti2O7 polymorphic phase between the stability limit of the two previously described structures. The optical band gap increases when the ionic radius of Ln3+ decreases. Refractive index (n) and extinction coefficient (k) have been calculated over a wavelength range of 260–1500 nm. The low value of the extinction coefficient indicates that these materials are transparent in the visible range and can therefore be used in electro-optics systems in the case of the layered-perovskite structure and more widely as emitting materials in solid-state lasers optically pumped. [Display omitted] •Growth of Ln2Ti2O7 (Ln = La to Lu) thin films on (110)-oriented SrTiO3 substrates was successfully performed by PLD.•For Ln2Ti2O7 thin films, the boundary between layered-perovskite/pyrochlore structures has been highlighted.•Optical indices and optical band gap were determined using the modelling of ellipsometric measurements.•The low value of extinction coefficient indicates that these oxides are transparent in the visible range.