E. V. Charnaya

Phenomenological model for the antiferroelectric phase transition in thin films and small particles

A phenomenological Landau model for the antiferroelectric phase transition in thin films and small cylindrical and spherical particles is developed. The boundary value problem was solved numerically for polarizations of polar sublattices and components of the susceptibility tensor. The spatial distribution of polarization and susceptibility and their temperature dependences were calculated. The enhancement of diagonal components of the susceptibility tensor compared to bulk was found. The size-dependent shift of the antiferroelectric phase transition temperature was obtained.

NMR studies of structure and ferroelectricity for Rochelle salt nanoparticles embedded in mesoporous sieves

NMR studies were carried out for Rochelle salt embedded in molecular sieves. 23 Na magic angle spinning (MAS) and multiple quantum (MQ) MAS NMR spectra revealed a complex structure of the confined crystalline material. The major part of particles within nanopores had a structure similar to that of bulk Rochelle salt. The 23 Na spin‐lattice relaxation times at various temperatures associated with this modification were also similar to those for bulk Rochelle salt and showed broad minima that corresponded to the ferroelectric and re-entrant phase transitions under nanoconfinement at temperatures just below the relevant transitions in bulk. This result suggests that the bulk-like modification within pores is ferroelectric in between. Fast spin relaxation in the rest of the confined material reflected high molecular mobility.

Effect of confined geometry on linear and nonlinear dielectric properties of triglycine sulfate near the phase transition

The temperature dependence of the linear permittivity and the third harmonic generation amplitude of nanocomposites representing nanoporous silica matrices (opal matrix and SBA-15) with triglycine sulfate embedded in pores has been studied in the region of the ferroelectric phase transition. A broadening of the phase transition and an increase its temperature in comparison with bulk triglycine sulfate have been revealed. The latter becomes more significant as the pore size decreases. It has been shown that the nonlinear dielectric properties of nanocomposites near the phase transition differ significantly from the properties of bulk triglycine sulfate.

Dielectric and calorimetric investigations of KNO3 in pores of nanoporous silica matrices MCM-41

The temperature dependences of the linear dielectric permittivity, the third harmonic amplitude, and the heat capacity of nanoporous silica matrices MCM-41 with cellular channels (3.7 and 2.6 nm in diameter) filled with KNO3 have been investigated in comparison with those obtained for bulk potassium nitrate. Measurements have been performed during heating and cooling in the range from room temperature to 463 K. Anomalies corresponding to structural phase transitions have been observed. A significant broadening of the temperature region of the existence of the ferroelectric phase III of potassium nitrate upon cooling has been revealed. This broadening increases with a decrease in the size of pores. It has been shown that, in the nanocomposites with potassium nitrate, the ferroelectric phase can also be formed during heating. The efficiency of observation of the third harmonic generation for studying nanocomposites with the ferroelectric phase has been demonstrated.

Nuclear magnetic resonance, resistance and acoustic studies of the melting-freezing phase transition of gallium in Vycor glass

The melting-freezing phase transition of gallium confined within Vycor glass was studied by NMR, resistance and acoustic techniques. A single although broadened 71Ga NMR line corresponding to melted gallium was observed in contrast to lineshapes found until now for liquid gallium in porous matrices. A difference between results obtained using the three methods was explained by formation of various confined solid gallium modifications. A depression of the freezing and melting phase transition temperatures and a pronounced hysteresis in the melting-freezing processes were found and are discussed. Heterogeneous nucleation was suggested to explain the dependence of crystallization on temperatures of pre-warming. Irreversible melting was observed for the second gallium modification.

Phase transitions for gallium microparticles in a porous glass

Abstract Studies of sound velocity and absorption supplemented with studies of heat capacity were carried out for the first time in the melting-freezing phase transition temperature range for Ga microparticles in a porous glass. The pronounced asymmetry of temperature behavior in cooling-heating processes was obtained, which was interpreted as the first order phase transition at cooling and the smeared phase transition at heating.

Acoustic studies of melting and freezing for mercury embedded into vycor glass

Acoustic technique was used to study the melting and freezing phase transitions of mercury embedded into Vycor glass. Both freezing and melting were found to be smeared and shifted noticeably to low temperatures compared to the melting point of bulk mercury, the offset of melting occurred at 223 K. The liquid skin model which was developed to treat melting in isolated metallic particles was used to explain the partly reversible character of melting for confined mercury and the difference between results obtained for longitudinal and transverse waves. The temperature dependence of the liquid skin thickness was calculated. Freezing was suggested to be driven by nucleation processes. It was shown that for confined mercury there is no unambiguous correspondence between pore sizes and temperatures of melting.

X-ray studies of the melting and freezing phase transitions for gallium in a porous glass

We use x-ray powder diffraction to study the melting and freezing processes for gallium within a porous glass. The only modification to solid gallium was found above 20 K, in contrast with previous x-ray studies of confined gallium. The size of gallium crystallites remained nearly constant during cooling and warming, while both the melting and freezing processes were smeared. The size of confined gallium crystallites was estimated as 22 nm, which is significantly larger than the pore size. The reasons for the phase-transition broadening and hysteresis between freezing and melting are discussed on the basis of the results obtained. @S0163-1829~98!02942-7#

Nuclear magnetic resonance and acoustic investigations of the melting - freezing phase transition of gallium in a porous glass

The melting - freezing phase transition of gallium in a porous glass was studied by NMR and acoustical techniques. A depression of the freezing and melting phase transition temperatures and a pronounced hysteresis in the melting - freezing processes were found. An intricate NMR lineshape for liquid gallium was observed with a temperature coefficient of the Knight shift more than twice that measured for the bulk melt. The results are discussed on the basis of the Gibbs - Thompson equation and by means of a geometric freezing model.

Dielectric parameters of mesoporous sieves filled with NaNO2

This paper reports on a dielectric study of MCM-41 molecular sieves with cellular channels of different sizes filled with the NaNO2 ferroelectric. The temperature dependences of the permittivity and electrical conductivity of sodium nitrite in cellular channels are calculated from experimental data on the permittivity and electrical conductivity of the composite. The calculations are performed using the relationships obtained for the hexagonal matrix with parallel cylindrical inclusions within pores. The observed increase in the conductivity of sodium nitrite in confined geometry at high temperatures is attributed to partial melting. It is shown that the increase in the permittivity of the composite is caused by Maxwell-Wagner relaxation processes.