Raman scattering and far infrared reflectivity of Ba{sub 5}Ta{sub 4}O{sub 15} and Ba{sub 5}Nb{sub 4}O{sub 15{minus}x} (0.0{le}x{le}0.56)
- Univ. Nacional de La Plata (Argentina)
- Univ. Nacional de Cordoba (Argentina). Dept. de Fisicoquimica
- Consejo Superior de Investigaciones Cientificas, Madrid (Spain). Inst. de Ciencia de Materiales
- Division Fisica del Solido, Buenos Aires (Argentina)
Quite often the literature cites superconductors with high critical temperature as having an anharmonic lattice close to a structural phase transition. It is of interest then to find materials, with distorted perovskite structure, that reveal bare phonon behavior expected for such environment. Here the authors discuss layered Ba{sub 5}Nb{sub 4}O{sub 15} and Ba{sub 5}Ta{sub 4}O{sub 15{minus}x} (0.0{le}x{le}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{sub 6} sublattice. As consequence, one expect strong anharmonicities that translate into several subtle anomalous shapes and wide phonon band profiles in the low temperature Raman and infrared spectra. They will see that while a centrosymmetric D{sub 3d}{sup 3}-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 Ba{sub 5}Nb{sub 4}O{sub 15}. Weaker structure on the band lower frequency side suggest 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 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{sub 5}Nb{sub 4}O{sub 15}, are understood as local modes due to unrelaxed portions of the lattice that may be related to changes the phonon damping in infrared reflectivity spectra. It is also noteworthy to point 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.
- Sponsoring Organization:
- CONICET (Argentina); Comision Interministerial de Ciencia y Tecnologia (Spain)
- OSTI ID:
- 538214
- Report Number(s):
- CONF-960163-; ISBN 0-8194-2070-0; TRN: 97:018597
- Resource Relation:
- Conference: Photonics West `96: conference on quantum well and superlattice physics VI, San Jose, CA (United States), 27 Jan - 2 Feb 1996; Other Information: PBD: 1996; Related Information: Is Part Of Spectroscopic studies of superconductors. Part A: Infrared and Raman spectra; Bozovic, I.; Marel, D. van der [eds.]; PB: 337 p.; Proceedings/SPIE, Volume 2696
- Country of Publication:
- United States
- Language:
- English
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