CONICET | Buscador de Institutos y Recursos Humanos

Quite often the literature cites superconductors with high critical temperature as having an anharmonic lattice close to a structural phase transition. It is then of interest to find materials, with distorted perovskite structure, that reveal bare phonon behavior expected for such an environment. Here we discuss layered Ba Nb O and Ba Ta O (0.0 less than or equal to x less than or equal to 0.56) that have four Nb/Ta ions per formula unit. This implies that there is an empty octahedra that in the lattice avoids direct face sharing of the (Nb/Ta)O sublattice. As consequence, one expects strong anharmonicities that translate into several subtle anomalous shapes and wide phonon band profiles in the low temperature Raman and infrared spectra. We see that while a centrosymmetric D P3m1space group is deduced for both compounds from neutron diffraction measurements, narrow splits of a symmetric stretching Raman mode hint small local departures of that symmetry in Ba Nb O . Weaker structure on the band lower frequency side suggests that in their understanding one has to consider arguments that indicate that the anharmonic contributions to the lattice potential energy are so large that they cannot be treated only by perturbation theory, i.e., there is a lifting of the degeneracy of phonon transitions in the harmonic approximation. Defect induced modes, that are Raman active in Ba Nb O , are understood as local modes due to unrelaxed portions of the lattice that may be related to changes of the phonon damping in infrared reflectivity spectra. It is also noteworthy to point out that the replacement Nb by Ta induces a hardening of most phonons. This is reflected in an increase of frequencies in spite of a heavier Ta ion.

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