Synthesis and μ-Raman scattering of Ruddlesden-Popper ceramics Sr3Ti2O7, SrLa2Al2O7 and Sr2LaAlTiO7

Ruddlesden-Popper Sr3Ti2O7, SrLa2Al2O7 and Sr2LaAlTiO7 ceramics were synthesized by microwave-assisted hydrothermal method or solid-state reactions. Single-phase, impurity free materials were obtained, which belong to the tetragonal I4/mmm (#139) space group. μ-Raman spectroscopy and group-theory calculations were carried out to study the vibrational features of these ceramics. For Sr3Ti2O7, first-order, second-order, defect-induced and infrared-active modes were simultaneously observed in the Raman spectra. It was possible to discern among these different types of modes, thus determining 9 of the 10 first-order modes predicted by group-theory. SrLa2Al2O7 and Sr2LaAlTiO7 ceramics presented complex Raman spectra, which were deconvoluted by Lorentzian curves. Eleven and fourteen Raman modes could be adjusted, respectively, in excellent agreement with the theoretical predictions, since 12 bands are expected for SrLa2Al2O7, while 14 Raman modes are predicted for Sr2LaAlTiO7. Polarized spectra were also obtained for Sr2LaAlTiO7 ceramics, which allowed us to discern between the A1g/B1g and the Eg modes. Our results complement the previous studies by infrared spectroscopy, and contribute to the knowledge of the optical vibrational behavior of these materials, fundamental to understand and improve their properties for microwave applications.