Raman scattering and far-infrared reflectivity of Ba5Ta4O15-x and Ba5Nb4O15-x (0.0<x<0.56)

Nestor E. Massa; Silvina Pagola; Raul E. Carbonio; Jose Antonio Alonso; Isidoro Rasines; Griselda Polla; Gabriela Leyva

doi:10.1117/12.241768

10 June 1996 Raman scattering and far-infrared reflectivity of Ba5Ta4O15-x and Ba₅Nb₄O_15-x (0.0

Nestor E. Massa, Silvina Pagola, Raul E. Carbonio, Jose Antonio Alonso, Isidoro Rasines, Griselda Polla, Gabriela Leyva

Proceedings Volume 2696, Spectroscopic Studies of Superconductors; (1996) https://doi.org/10.1117/12.241768
Event: Photonics West '96, 1996, San Jose, CA, United States

Abstract

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 Ba₅Nb₄O₁₅ and Ba₅Ta₄O_15-x (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)O₆ 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_3d³- P3m1 space 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 Ba₅Nb₄O₁₅. 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 Ba₅Nb₄O₁₅, 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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Nestor E. Massa, Silvina Pagola, Raul E. Carbonio, Jose Antonio Alonso, Isidoro Rasines, Griselda Polla, and Gabriela Leyva "Raman scattering and far-infrared reflectivity of Ba5Ta4O15-x and Ba₅Nb₄O_15-x (0.0https://doi.org/10.1117/12.241768

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