Raman and infrared spectroscopic studies of LaTaTiO6 polymorphs

X-ray diffraction, Raman scattering and infrared spectroscopy were used to investigate the orthorhombic-to-monoclinic polymorphic transformation of LaTaTiO6 ceramics, induced at high temperatures. For sake of comparison, LnTaTiO6 ceramics for various lanthanides (Ln), including La, were prepared by the solid-state reaction method. Phase-pure orthorhombic (Pnma space group) and monoclinic (C2/c space group) structures were obtained for LaTaTiO6 at sintering temperatures of 1300 °C and 1400 °C, respectively. The Raman spectrum of the orthorhombic LaTaTiO6 appeared to be very close to that of PrTaTiO6, which crystallizes within the aeschynite structure. Raman and infrared spectra of monoclinic LaTaTiO6 ceramics are reported for the first time. Polarized Raman spectroscopy allowed determining (and assigning) all the non-polar modes of the material into the C2/c monoclinic phase. Infrared reflectivity spectra revealed the main polar modes that contribute to the dielectric response. The intrinsic dielectric parameters obtained by this procedure match well with those found by microwave techniques in ceramics prepared at relatively high sintering temperatures - which must belong to the described monoclinic structure.