Long and short range view of BiFeO3 temperature dependent lattice dynamics

NESTOR E. MASSA; LEIRE DEL CAMPO; DOMINGOS DE SOUSA MENESES; PATRICK ECHEGUT; GILBERTO F.L. FABBRIS; GUSTAVO AZEVEDO; MARíA JESúS MARTíNEZ-LOPE; JOSé ANTONIO ALONSO

Biarritz, Francia

Congreso; 7th International Conference on Inorganic Materials; 2010

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  We present the temperature dependent lattice behavior of BiFeO3 as seen by two techniques with different scale and acquisition times. While for low temperatures we confirm a soft phonon obeying a power law with exponent â=0.25 (result of strong lattice fluctuations), in the high temperature regime, far infrared reflectivity and emissivity suggest that not only thermal fluctuations but off-stoichiometry chemical disorder are taking place partially reducing long-range order. Strong overlapping, merging, and internal mode band smearing, implies tripling in an idealized high temperature paraelectric cubic phase above Tc ~1090 K. On the other hand, X-ray absorption near edge structure, with longer acquisition times, shows Fe3+ turning into Fe2+ while the Bi edge energy downshifts under heating in a very complex configuration. Our high temperature structural measurements reveal defect induced irreversible lattice changes in an environment lacking of long-range coherence due to BiFeO3 metastability. By contrast long scale far infrared wavelengths, and shorter acquisition times, do not suggest that local symmetry breaking only yielding above Tc three very broad bands as it would be if the cubic phase Pmcm (Z=1) is adopted in a macroscopic view.