New insights on the structural and optical-vibration properties of noncentrosymmetric lanthanides pyrogermanates

Abstract

A complete series of noncentrosymmetric Ln2Ge2O7 materials (Ln = lanthanides) were obtained by the molten-salts processing route. Triclinic (P1 space group) as well as tetragonal (P41212 space group) structures were observed, respectively, for the La–Eu compounds (larger ions) and for the Gd–Lu ones (smaller ions). The optical-vibration properties of both groups of samples were well investigated by Raman spectroscopy besides group-theory calculations. All the predicted phonon modes were discerned for the triclinic polar phase, while the most significant phonon modes of the tetragonal phase were determined. Beside the well-known classical dependence of the phonon wavenumbers with the ionic radii of the lanthanide (hardening of the higher-frequency modes from La to Lu), it was observed an opposite softening of the low-frequency modes within the triclinic phase (La–Eu samples). This result could be related to the contribution of the increasing lanthanide masses, added to the high lattice anharmonicity near the phase boundary between the triclinic and tetragonal phases, which can be a consequence of the strong changes in the coordination numbers of the Ln-polyhedra. Once the polar triclinic phase and the noncentrosymmetric tetragonal structure are thermally stable, and owing to the low-temperature magnetic features already shown by of non-cubic Ln2Ge2O7 compounds, we believe that some of these materials could present magnetoelectric or related properties.