SPring-8, the large synchrotron radiation facility

Date: 15:00-16：00, Jly 11, 2006

Place: HOUKOUKAN

Speaker: Dr. S. Vakhruschev (The Institute for Solid State Physics、Ioffe Physico-Technical Institute)

Title: Neutron and X-ray scattering study of Relaxor Ferroelectrics and ferroelectric and magnetic nanocomposits

Abatract: 
RELAXOR FERROELECTRICS
 Relaxor ferroelectrics were first discovered 50 years ago. Their name is related to the existence of the broad frequency dependent maximum in the e(T) dependence. These materials demonstrate giant dielectric susceptibility and record values of the electromechanic coupling. Recently substantial successes were achieved in the determination of the microscopic nature of the relaxors and I would like to summarize the main points: (i) The key concept for the understanding of the relaxors is the formation of the polar nanoregions (PNRs) at temperatures far above the position of the maximum of e: Tm. Characteristic size of the PNRs is from few to about a hundred nanometers and neutron and X-ray scattering are the most effective ways of their study. (ii) Formation of the PNRs is directly related to the condensation of the TO soft mode as it is evidenced by the neutron scattering. (iii)On cooling through the Tm either glass-like or nanodomain state is formed, depending on the cooling condition and exact material composition. Measurements of the neutron and SR scattering allow determining the average spatial parameters of the low-temperature state. The most prospective method to get detailed information on the nanodomain structure and dynamics is the coherent SR scattering technique, but no such results were reported yet.

FERROELECTRIC and MAGNETIC PHASE TRANSITIONS in CONFINEMENT
 It is known that finite-size effect results in the drastic changes of physical properties of dispersed materials. One can expect observed phenomena to become especially significant if the characteristic size of dispersed particles becomes comparable with correlation length of the order parameter critical fluctuation. There are different methods of preparation of such dispersed substances and one of them is an intrusion of materials into artificial or natural porous matrices that we have used for producing nanocomposite materials.

FERROELECTRIC NANOCOMPOSTES
 We have obtained samples of low-melting and water-soluble ferroelectrics such as KH₂PO₄, KD₂PO₄, Rochelle salt, NaNO₂ in porous glasses with average pore sizes from 3 nm to 20 nm, chrysotile asbestos, opals. The most complete and most intriguing results were obtained for the NaNO₂ nanocomposites. We have demonstrated, that as a result of the confinement results (in the case of narrow pores) first-order ferroelectric phase transition becomes continuous  In addition to the change of the order of the transition  we have found the giant growth of the thermal ionic displacements in the paraelectric state resulting and the formation  of the specific premelted state with the amplitude of the atomic vibration of the order of interatomic distance. This result obtained by the neutron diffraction measurements was confirmed by the NMR data.

MAGNEIC NANOCOMPOSITES
 MnO, CoO, Fe₂O₃ in porous glasses, asbestos and MCM-41 and SBA-15 silica matrices were obtained. Neutron and X-ray diffraction study together with ESR and direct magnetization measurements were carried out. It was demonstrated that the transition order changes as a function of the pore size and cluster morphology.

Organizer: AOKI (PHS 3140)