High-resolution XANES spectra of iron in minerals and glasses: structural information from the pre-edge region

Domains and domain walls are a fundamental property of interest in ferroelectrics, magnetism, ferroelastics, superconductors, and multiferroic materials. Unlike magnetic Bloch walls, ideal ferroelectric domain walls are well accepted to be only one to two lattice units wide, over which polarization and strain change across the wall. However, walls in real ferroelectrics appear to show unexpected property variations in the vicinity of domain walls that can extend over micrometer length scales. This chapter specifically reviews the local electrical, elastic, optical, and structural properties of antiparallel domain walls in the trigonal ferroelectrics lithium niobate and lithium tantalate. It is shown that extrinsic point defects and their clustering play a key role in the observed local wall structure and influence macroscale properties by orders of magnitude. The review also raises broader and yet unexplored fundamental questions regarding intrinsic widths, defect–domain wall interactions, and static versus dynamic wall structure.

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Defect–Domain Wall Interactions in Trigonal Ferroelectrics

A general model for the structural state of iron in a variety of silicate and aluminosilicate glass compositions in the systems Na2O-Al2O3-SiO2-Fe-O, CaO-Al2O3-SiO2-Fe-O, and MgO-Al2O3-SiO2-Fe-O is proposed. Quenched melts with variable Al/Si and NBO/T (average number of nonbridging oxygens per tetrahedrally coordinated cation), synthesized over a range of temperatures and values of oxygen fugacity, are analyzed with57Fe Mossbauer spectroscopy. For oxidized glasses with Fe3+/∑Fe>0.50, the isomer shift for Fe3+ is in the range ∼0.22–0.33 mm/s and ∼0.36 mm/s at 298 K and 77 K, respectively. These values are indicative of tetrahedrally coordinated Fe3−. This assignment is in agreement with the interpretation of Raman, luminescence, and X-ray,K-edge absorption spectra. The values of the quadrupole splitting are ∼0.90 mm/s (298 K and 77 K) in the Na-aluminosilicate glasses and compare with the values of 1.3 mm/s and 1.5 mm/s for the analogous Ca- and Mg-aluminosilicate compositions. The variations in quadrupole splittings for Fe3+ are due to differences in the degree of distortion of the tetrahedrally coordinated site in each of the systems. The values of the isomer shifts for Fe2+ ions in glasses irrespective of Fe3+/∑Fe are in the range 0.90–1.06 mm/s at 298 K and 1.0–1.15 mm/s at 77 K. The corresponding range of values of the quadrupole splitting is 1.75–2.10 mm/s at 298 K and 2.00–2.35 mm/s at 77 K. The temperature dependence of the hyperfine parameters for Fe2+ is indicative of noninteracting ions, but the values of the isomer shift are intermediate between those values normally attributable to tetrahedrally and octahedrally coordinated Fe2+. The assignment of the isomer-shift values of Fe2+ to octahedral coordination is in agreement with the results of other spectral studies. For reduced glasses (Fe3+/∑Fe≈<0.50), the value of the isomer shift for Fe3+ at both 298 K and 77 K increases and is linearly correlated with decreasing Fe3+/∑Fe in the range of
 
$$f_{O_2 } $$

 between 10−3 and 10−6 atm when a single quadrupole-split doublet is assumed to represent the absorption due to ferric iron. The increase in value of the isomer shift with decreasing
 
$$f_{O_2 } $$

 is consistent with an increase in the proportion of Fe3+ ions that are octahedrally coordinated. The concentration of octahedral Fe3+ is dependent on the
 
$$T - f_{O_2 } $$

 conditions, and in the range of log
 
$$f_{O_2 } $$

 between 10−2.0 and 10−5 a significant proportion of the iron may occur as iron-rich structural units with stoichiometry similar to that of inverse spinels such as Fe3O4, in addition to isolated Fe2+ and Fe3+ ions.

The structural state of iron in oxidized vs. reduced glasses at 1 atm: A 57 Fe Mössbauer study

Laboratory measurements of electrical conductivity of hydrous and dry silicic melts under pressure

Bioactivity of ferrimagnetic glass-ceramics in the system FeOFe2O3CaOSiO2

Mossbauer, ESR and magnetization measurements have been carried out on a basalt glass heat-treated at different temperatures (600, 650, 700, 800 and 900°C for 8 h). The as-annealed glass and the above five samples showed two-doublet Mossbauer spectra, while the last two samples also showed a six line magnetic hyperfine pattern at 300 K. At 4 K, the last four samples showed magnetic hyperfine patterns, while the as-annealed glass showed that there was already short range magnetic ordering present. High field Mossbauer data at 4 K showed that the surface spins are canted. The minimum quadrupole splitting and the maximum isomer shift around 700°C are related to the improved symmetry of the magnetite lattice. ESR spectra showed paramagnetic resonances at g = 4.3 and g = 2.0 for the first two samples, while the last four samples showed superparamagnetic resonance centred around g = 2.0 at 300 K. At lower temperatures, the 650 and 700°C samples showed ferrimagnetic resonance. Magnetization curves against H/T superpose well both at 300 and 77 K, showing the typical superparamagnetic behaviour of the small magnetite particles. The saturation magnetization (at 270 K) showed a sharp change around 700°C, showing the formation of magnetite. The magnetic structure of the small magnetite particles are discussed in terms of the above results.

Magnetic properties of a basalt glass and glass-ceramics

The magnetic behaviour of a basalt glass and glass-ceramic was studied by magnetization measurements between 4 and 800 K as a function of a wide range of magnetic fields (H) between 0 and 60 kG. For the as-annealed glass it was found that nearly all the iron ions behaved as paramagnetic ions. In the samples heat-treated at 700 and 900°C, the magnetization (M) values showed three magnetic components: paramagnetic Fe2+ ions, magnetite in a superparamagnetic and in a ferrimagnetic state. This confirmed our previous Mossbauer results. The superparamagnetic behaviour of the fine particles of magnetite was interpreted by Langevin's theory. From the M (H) and M(T) values we evaluated the percentage of each component as a function of temperature, the magnetization values in the saturated states, the mean particle diameter and the particle size distributions.

Superparamagnetism and ferrimagnetism of the small particles of magnetite in a silicate matrix

The resistivities and activation energies of borate glasses containing oxides of both V and Fe, and both V and Cu are lower than those of glasses containing the same amounts of each TM oxide separately. Optical and ESR spectra suggest that the proportions of ions in reduced valence states are not greatly affected by mixing the oxides. It is concluded that polaron-hopping between ions of different TM elements occurs readily in these mixed glasses.

Electrical conductivity of barium borate glasses containing mixed transition metal oxides

Ferroelectric materials such as lithium niobate and lithium tantalate show a non-linear hysteresis behaviour, which may be explained by dynamical system analysis. The behaviour of these ferroelectrics is usually explained by domains and domain wall movements. So, the spatial variation of the domain wall was studied previously in order to see its effect on the domain wall width in the context of the Landau-Ginzburg functional. In the present work, both temporal and spatial variations of polarization are considered, and by using the Euler-Lagrange dynamical equation of motion, a Klein-Gordon equation is derived by taking the ferroelectrics as a Hamiltonian system. An interaction has been considered between the nearest neighbour domains, which are stacked sideways in a parallel array with uniform polarization. This interaction term is associated with the spatial term and when this interaction is assumed to be zero, the spatial term vanishes, giving rise to a Duffing oscillator differential equation, which can be also studied by a dynamic system analysis.

https://iopscience.iop.org/article/10.1088/0953-8984/18/16/016/pdf

An approach to the Klein-Gordon equation for a dynamic study in ferroelectric materials.

DC and AC resistivities of some oxide glasses containing either Bi2O3 or selenium have been measured over a temperature range of -160 degrees C to 500 degrees C. The microstructure of bismuth-containing glasses consists of spherical metallic bismuth particles having diameters in the range 50 to 500 AA dispersed in a glass matrix. In the selenium containing glasses selenium crystals of dimensions varying between 50 and 2000 AA are present. The DC resistivity data at temperatures below 120 degrees C indicate that a tunnelling mechanism of charge carriers between the conducting granules is operative. The dielectric relaxation spectra of bismuth-containing glasses confirm this model of electron transport. The surface layers of the ion-exchanged and reduced samples of the selenium-containing glasses show a threshold switching from a low-resistance to a high-resistance state.

http://iopscience.iop.org/article/10.1088/0022-3727/10/15/011/pdf

A. K. Bandyopadhyay

Papers

Magnetic properties of a basalt glass and glass-ceramics

Superparamagnetism and ferrimagnetism of the small particles of magnetite in a silicate matrix

Electrical conductivity of barium borate glasses containing mixed transition metal oxides

An approach to the Klein-Gordon equation for a dynamic study in ferroelectric materials.

Electrical properties of oxide glasses containing bismuth and selenium granules