49 research outputs found
Ising-Like Model For Ferroelectric Phase Transitions In Tetragonal Tungsten Bronze Compounds
The two-component anisotropic Ising-like model was proposed to model the ferroelectric phase transitions in Tetragonal Tungsten Bronze (TTB) compounds. Using the mean-field approach we reconstructed the phase diagram of the TTB system and showed that depending on he relative strength of the interaction parameters there ferroelectric states are possible: (i) the state with y-directed polarization that corresponds to the tetragonal ferroelectric phase in TTB compounds, (ii) the state with x-directed polarization that corresponds to the orthorhombic TTB phase and (iii) Â the state with superposition of x- and y- polarization components that can correspond to the not yet discovered mixed phase.The two-component anisotropic Ising-like model was proposed to model the ferroelectric phase transitions in Tetragonal Tungsten Bronze (TTB) compounds. Using the mean-field approach we reconstructed the phase diagram of the TTB system and showed that depending on he relative strength of the interaction parameters there ferroelectric states are possible: (i) the state with y-directed polarization that corresponds to the tetragonal ferroelectric phase in TTB compounds, (ii) the state with x-directed polarization that corresponds to the orthorhombic TTB phase and (iii) Â the state with superposition of x- and y- polarization components that can correspond to the not yet discovered mixed phase
Proton conductivity in Al-stevensite pillared clays
Fine stevensite mineral fraction (< 2 m) has been extracted from natural Moroccan ghassoulite clay. Thereafter, it has been pillared by Al13 polycations species. Physico-chemical characterization, performed using X-ray diffraction (XRD), thermal analysis (TG-TDA) and Scanning Electron Microscopy (SEM) equipped with X-ray Energy Dispersion (XED) analysis, has proved that stevensite pillaring has been successfully achieved. Electrical impedance measurements, carried out onto samples before and after pillaring operation, have shown an increase in proton conductivity for pillared with respect to pristine stevensite clay mineral.Fine stevensite mineral fraction (< 2 m) has been extracted from natural Moroccan ghassoulite clay. Thereafter, it has been pillared by Al13 polycations species. Physico-chemical characterization, performed using X-ray diffraction (XRD), thermal analysis (TG-TDA) and Scanning Electron Microscopy (SEM) equipped with X-ray Energy Dispersion (XED) analysis, has proved that stevensite pillaring has been successfully achieved. Electrical impedance measurements, carried out onto samples before and after pillaring operation, have shown an increase in proton conductivity for pillared with respect to pristine stevensite clay mineral
Elaboration and characterization by XRD and SEM of the glass-ceramics in BaTiO3 – NaPO3 system
There is a strong need for low cost glass-ceramic materials whose high chemical stability and dielectric properties can be engineered flexibly in bulk and fiber forms. Therefore we have manufactured transparent ferroelectric glass-ceramics consisting of barium titanate (BT) crystallites in a chain metaphosphate glass matrix. Several glass compositions, in the series (1-x) NaPO3 – x BaTiO3 have been prepared by a conventional melt quenching method. The compositions have been selected on the basis of chemical stability. X-ray diffraction (XRD) and scanning electron microscopy (SEM) indicate the formation of small particle size of BaTiO3 in the glass network when its amount is above 4 mol%.There is a strong need for low cost glass-ceramic materials whose high chemical stability and dielectric properties can be engineered flexibly in bulk and fiber forms. Therefore we have manufactured transparent ferroelectric glass-ceramics consisting of barium titanate (BT) crystallites in a chain metaphosphate glass matrix. Several glass compositions, in the series (1-x) NaPO3 – x BaTiO3 have been prepared by a conventional melt quenching method. The compositions have been selected on the basis of chemical stability. X-ray diffraction (XRD) and scanning electron microscopy (SEM) indicate the formation of small particle size of BaTiO3 in the glass network when its amount is above 4 mol%
Phase diagram of TTB ferroelectric compounds Pb1−xK2xNb2O6
Substitution of Pb with K in the PbNb2O6 phases leads to a new composition of solid solution with chemical composition Pb1−xK2xNb2O6 with x ranging from 0 to 0.34 in steps of 0.05. Ferroelectric ceramics were synthesized using solid state reaction between the corresponding oxides and carbonates. Powders are pressed and heated into ceramics and their compaction is about 92%. The tetragonal tungsten bronze (TTB) structure at room temperature was confirmed by X-ray diffraction (XRD). The temperature dependence of dielectric permittivity was measured from 35 to 600°C in the 20-106 Hz frequency range. Transition temperature decreases with the lead concentration while from x=0, with Tc=600°C and to x=0.3 with Tc=388°C.  These measurements permit to present a basical phase diagram of this family compound showing the evolution of the characteristic transition temperature Tc versus temperature. The behaviour of Tc is in agreement with theoretical study of the ferroelectric phase transition in TTB using Monte Carlo (MC) simulation. The dielectric properties of these ceramics are similar to those obtained on a single crystal and illustrate the quality of preparative conditions.Substitution of Pb with K in the PbNb2O6 phases leads to a new composition of solid solution with chemical composition Pb1−xK2xNb2O6 with x ranging from 0 to 0.34 in steps of 0.05. Ferroelectric ceramics were synthesized using solid state reaction between the corresponding oxides and carbonates. Powders are pressed and heated into ceramics and their compaction is about 92%. The tetragonal tungsten bronze (TTB) structure at room temperature was confirmed by X-ray diffraction (XRD). The temperature dependence of dielectric permittivity was measured from 35 to 600°C in the 20-106 Hz frequency range. Transition temperature decreases with the lead concentration while from x=0, with Tc=600°C and to x=0.3 with Tc=388°C.  These measurements permit to present a basical phase diagram of this family compound showing the evolution of the characteristic transition temperature Tc versus temperature. The behaviour of Tc is in agreement with theoretical study of the ferroelectric phase transition in TTB using Monte Carlo (MC) simulation. The dielectric properties of these ceramics are similar to those obtained on a single crystal and illustrate the quality of preparative conditions
Mean field theory and Monte Carlo simulation of Phase transitions and Magnetic Properties of a tridimensional Fe7S8 Compound
The structural, electronic and magnetic properties of Fe7S8 material have
been studied within the framework of the ab-initio calculations, the mean field
approximation (MFA) and Monte Carlo simulation (MCS). Our study shows that two
forms of the iron atoms, Fe2+ with spin S=2, and Fe3+ with spin {\sigma}=5/2
are the most probable configurations. A mixed Ising model with ferromagnetic
spin coupling between Fe2+ and Fe3+ ions and between Fe3+ and Fe3+ ions, and
with antiferromagnetic spin coupling between Fe2+ ions of adjacent layers has
been used to study the magnetic properties of this compound. We demonstrated
that the magnetic phase transition can be either of the first or of the second
order, depending on the value of the exchange interaction and crystal field.
The presence of vacancies in every second iron layer leads to incomplete
cancellation of magnetic moments, hence to the emergence of the ferrimagnetism.
Anomalies in the magnetization behavior have been found and compared with the
experimental results.Comment: 18 pages, 14 Figures, 4 Table