INVESTIGADORES
MENDIVE Cecilia Beatriz
congresos y reuniones científicas
Título:
Raman studies on mullite-type PbMBO4 (M = Al, Ga, Mn, Fe)
Autor/es:
MURSHED M.M.; CURTI M.; HANSMANN D.; MENDIVE C.; GESING TH. M.
Lugar:
Warwick
Reunión:
Encuentro; The28th European Crystallography Meeting (ECM-28); 2013
Institución organizadora:
The European Crystallographic Association
Resumen:
The chemical composition, stereochemically active lone electron pairs of Pb2+ and the unique B?O bonding strength in the planar BO3 group play diverse roles for the thermal anisotropy, in particular for the negative thermal expansion in the a-axis of the mullite-type PbMBO4 (M = Al, Ga, Mn, Fe). The present study particularly emphasizes on temperature and pressure response of the Raman phonons of PbFeBO4, which helped explore the physical origin of the intrinsic anharmonicity leading to the thermal expansion anomaly. The bulk modulus, phonon frequency at 0 K and their response to pressure providing mode Grüneisen parameters (γi), were calculated by DFT. About 25% of the modes showed a non-linear correlation with pressure; thus the corresponding γi with positive as well as negative values. Marked phonon softening occurred in FeO6 and BO3, and hardening in PbO4 polyhedra. This behavior is not associated with the lattice anomalies but for possible spin-phonon couplings caused by phonon modulation of the superexchange integral at TNs between 120 K and 280 K, respectively for long and short range antiferromagnetic ordering. Selective phonon frequencies and their intrinsic line-widths were modeled using simplified Klemens model. Mode ω1183 requires high damping factors both for frequency and line-width. It demonstrates that the intrinsic anharmonicity for the thermal expansion mainly comes from the BO3 group. Alike many metal borates and glasses, the observed thermal expansion anomaly in essentially relates to the anharmonic vibrational properties of the associated borate groups. This observation further justifies the anharmonic term taken to the additive frame of the Debye-Einstein-Anharmonicity model to accurately describe the anomalous lattice thermal expansion of the relevant mullite-type materials [1].
