J. Vamvakas

Bio: J. Vamvakas is an academic researcher from Metz. The author has contributed to research in topics: Raman spectroscopy & Lattice (order). The author has an hindex of 2, co-authored 2 publications receiving 14 citations.

Anomalous raman spectrum in KNbO3

Abstract: The anomalous Raman spectrum of KNbO3 is analyzed in terms of one-and two-phonon densities of states based on a convenient lattice dynamical model. In particular the presence of stronq bands in cubic, tetranonal and orthorhombic phases is reinterpreted.

Infrared spectroscopy in KNbO3 through the successive ferroelectric phase transitions

Abstract: The temperature dependence of the infrared reflectivity spectra in KNbO3 is reported for the cubic, tetragonal and orthorhombic phases in a temperature range extending from 300 to 1200K. Spectra have been fitted with a model based on the factorised form of the dielectric function. The temperature dependence of the mode frequencies, the dampings and the oscillator strengths through the successive phase transitions is reported. The experimental data exhibit an unstable mode which decreases continuously in frequency upon cooling across the diverse transitions. The ionic effective charges, the spontaneous polarisation and the static dielectric constant are calculated from these data. All results are discussed in terms of a displacive/order-disorder crossover, which appears especially when the transition from the cubic to the tetragonal phase is approached. A particular phase transition mechanism is proposed, which leads to an understanding of the discrepancy between the experimental and the calculated values of the dielectric constant, the large damping of the soft mode, the incomplete softening of the unstable phonon and the special sequence of phase transitions.

Raman spectroscopy of (K,Na)NbO3 and (K,Na)1−xLixNbO3

Abstract: We report Raman spectroscopy in all four phases of a (K0.5+δNa0.5−δ)NbO3 (δ∼0.03) crystal and ((K0.5+δNa0.5−δ)1−xLix)NbO3 for x=0.02 and 0.03. The phase transitions in the Li-free crystal are homogeneous and hysteretic, with no mixed-phase region between the rhombohedral and orthorhombic phases. Raman spectra in the lowest-temperature phase in the Li-doped material differ significantly from those at x=0 and suggest a structural phase boundary at a Li concentration of less than 2%.

The Observation of B-Site Ordering by Raman Scattering in A(B′B″)O3 Perovskites

Abstract: Note: Bar Ilan Univ,Ramat Gan,Israel Minist Def,Haifa,IsraelG6408Times Cited:43Cited References Count:15 Reference LC-ARTICLE-1987-009doi:10.1366/0003702874449066 Record created on 2006-08-21, modified on 2017-05-10