Anomalous Behavior of the Polar Soft Phonon of Hexagonal BaTiO_3 at Low Temperatures and Its Relation to Static Dielectric Constant

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Polar phonons h?rve been investigated in the hexagoraal BaTiO3 over a wide temperature rangeby hyper-Raman scattering. The lowest-freqtuency polar TO-phonon with Az.-species exhibitsa soft phonon behavior in the prototype hexagonal phase and shows a sjclitting into two peaksbelow the hexagonal-to-orthorhornbic phase transition tetnperature T. : 222 K. Tlae splitting ofthe polar soft phonon is caused by an accidental cottpling to the non-polar 7?z.-phonon. Havingthis in mind, we have estimated the static dielectric constant e. along the c-axis, from the hyper-Raunan spectrurn of the polar so['t phonon in terms of' the Krarners-Kronig relations, The resultshows a $1112111 bunup at T. and a rnonotonous increase below 200 K, whiclr reprodtrces well theanomalotrs temperature-dependent behavior of the reported dielectric constant. The saturatingtrend of the latter as the tezaaperature approaches 0 K, is also fotnnd to be primarily catrsed bythe above accidental cotrpling, not by qtranturaa effect.
1997-10-15