Role of Iron in Calcium Phosphate Glasses

Some physical properties okf phosphate glasses con-taining up to about 26 mol% Fe₂O₂ were studied. Pronounced changes in properties were observed at compositions containingabout 6, 10, and 13 mol% Fe₂O₃. The X-ray diffraction spectra of devitrified (heat-treated) samples showed new compounds near these compositions. Electron spin resonance and optical studies confirmed the presence of Fe³⁺ and Fe²⁺ in both 4- and 6-coordination. An increase in total iron in these samples was associated with a decrease in the ratios Fe²⁺ 4-coordinated/Fe³⁺ 6-coordinated 6-coordinated and Fe³⁺ 4-coordinated/Fe³⁺ 6-coordinated up to about 2.0 mol% Fe₂O₃, as shown by the intensity of the optical absorption bands at about 2.0 and 1.0 μm and by the intensity of the ESR lines at g⋍4.2 and 2.0, respectively. Samples containing up to 4.3 mol% Fe₂O₃ showed an increase in Fe³⁺ concentration and a decrease in Fe²⁺ concentration after gamma irradiation. The electrical conductivity and activation energy decreased sharply with increasing Fe₂O₃ content.     

A Study of Conductivity in the Sr(Fe1−xTix)O3−δ System

The conductivity and conductivity-temperature characteristics of the Sr(Fe_1−xTi_xO_3−δ system fired in air are studied by means of Mössbauer spectroscopy at room temperature and at 78 K. On the basis of quantitative analysis of the Fe⁴⁺ content in the solid solution, the concentrations of oxygen vacancies are calculated and the relationship between Fe⁴⁺ content and conductivity is found. It is also found that the turning points of conductivity and material constant B, as well as Fe⁴⁺ and the percentage of oxygen vacancies, occur at x=0.7 on the curve. When x=0.7, Fe³⁺ (II), which has never been reported before, disappears and the lattice parameters no longer vary with x. Besides Fe⁴⁺, Fe³⁺(II) is also a factor affecting the conductivity of this system.     

Phase Diagram of the System BaO-Fe2O3

The phase diagram of the system BaO-Fe₂0₃ was determined by X-ray diffraction, melting-point measurement, and microscopic methods. Since the reduction of Fe³⁺ to Fe²⁺ was observed by chemical analysis in the samples heated at high temperature, especially in molten samples, the samples were heated at 1 atmosphere pressure of oxygen in the temperature region in which the liquid was in equilibrium; 1 atmosphere pressure of oxygen was su5cient to restrain the reduction of Fe³⁺. In the temperature region of solid-solid equilibrium, the dissociation was not observed even when the samples were heated in air. BaO - 6Fe₂O₃ formed a solid solution with BaO.Fe₂0₃. The BaO:Fe₂0₃ ratio of the solid solution was BaO.4.5Fe₂0₃ at 1350°C. and BaO. 5.0Fe₂0_a at 800°C. The precipitation micro-structures of each primary solid solution were observed.     

Redox State of Iron and Its Related Effects in the CaO–P2O5–Fe2O3 Glasses

Iron exists in Fe²⁺ and Fe³⁺ states in CaO–P₂O₅–Fe₂O₃ glasses and they impart characteristic optical absorption bands that allow analysis of relative proportion of the two species. This communication reports the redox states of glasses melted under air, argon, and oxygen atmospheres and relates them to the dissolution rates. The dissolution rate was found to be related to the redox state and it is lowered if the glass is melted under oxidizing atmospheres.     

Co2+-Substitution Effect in Ce-TZP/La M-type Hexaferrite Composites

A Ce-TZP/platelike La(Co(Fe_0.9Al_0.1)₁₁)O₁₉ composite was synthesized in situ while sintering from a mixture of Ce-TZP, La(Fe_0.9Al_0.1)O₃, Fe₂O₃, Al₂O₃, and CoO powders. Platelike La(Co(Fe_0.9Al_0.1)₁₁)O₁₉ crystals were grown in a dense Ce-TZP matrix after sintering at temperatures of 1200°–1350°C. The temperature range for sintering Ce-TZP/La(Fe,Al)₁₂O₁₉ composites was expanded widely by substituting Co²⁺ ions for Fe²⁺ ions in its structure. The highest value of the bending strength of the Ce-TZP/La(Co(Fe_0.9Al_0.1)₁₁)O₁₉ composites was 880 MPa, which was higher than that of the Ce-TZP/La(Fe,Al)₁₂O₁₉ composite (780 MPa) and Ce-TZP (513 MPa). The saturation magnetization of the Ce-TZP/La(Co(Fe_0.9Al_0.1)₁₁)O₁₉ composite was a constant value of 7.7 emu/g after the composite was sintered at 1200°–1350°C.     

Influence of Cr and Fe on Formation of α-Al2O3 from γ-Al2O3

The effect of Cr and Fe in solid solution in γ-Al₂O₃ on its rate of conversion to α-Al₂O₃ at 1100°C was studied by X-ray diffraction. The δ form of Al₂O₃ was the principal intermediate phase produced from both pure γ-Al₂O₃ and that containing Fe³⁺ in solid solution, although addition of Fe greatly reduced crystallinity. Reflectance spectra and magnetic susceptibilities showed that Cr exists as Cr⁶⁺ in γ-Al₂O₃ and as Cr³⁺ in α-Al₂O₃, with θ-Al₂O₃ as the intermediate phase. The intermediates formed rapidly, and the rates of their conversion to α-Al₂O₃ were increased by 2 and 5 wt% additions of Fe and decreased by 2 and 4 wt% additions of Cr. An approximately linear relation observed between α-Al₂O₃ formation and decrease in specific surface area was only slightly affected by the added ions. This relation can be explained by a mechanism in which the sintering of δ- or θ-Al₂O₃, within the aggregates of their crystallites, is closely coupled with conversion of cubic to hexagonal close packing of O^2- ions by synchro-shear.     

Incorporation of Aliovalent Dopants into the Bismuth-Layered Perovskite-Like Structure of BaBi4Ti4O15

The solid solubility of the aliovalent dopants Fe³⁺ and Nb⁵⁺ in the BaBi₄Ti₄O₁₅ compound, a member of the family of Aurivillius bismuth-based layer-structure perovskites, has been studied using quantitative wavelength-dispersive spectroscopic microanalysis (SEM/EPMA) in combination with X-ray powder diffractometry (XRPD). The samples with nominal (starting) compositions corresponding to the chemical formulas BaBi₄Ti_{4–4 X} Fe_{4 X} O₁₅ and BaBi₄Ti_{4–4 X} Nb_{4 X} O₁₅ were prepared by hot forging a mixture of BaTiO₃ and Bi₄Ti₃O₁₂ with additions of Fe₂O₃ or Nb₂O₅ followed by a long annealing at 1100°C. The study showed that an excess charge introduced into the structure by the substitution of Ti⁴⁺ ions with aliovalent dopants was preferentially compensated by a change in the ratio of Ba²⁺ to Bi³⁺ ions in the host structure according to the general formulas of the solid solutions Ba_{1–4 X} Bi_{4+4 X} Ti_{4–4 X} Fe^'_{4 X} O₁₅ and Ba_{1+4 X} Bi_{4–4 X} Ti_{4–4 X} Nb^·_{4 X} O₁₅.     

Kinetics of Cation Redisribution in Ferrospinels

In situ electrical resistivity measurements were employed to follow the kinetics of cation redistribution in ferrospinels, x Fe₃O₄–(1 – x )MeFe₂O₄ (Me = Co, Mg, Mn, or Ni) or x Fe₃O₄–FeMe₂O₄ (Me = Al) with x ≥ 0.2. Relaxation temperatures (at 20°C/min heating rate) were established and kinetic parameters—activation energies and time constants—were determined for each cation. These parameters were insensitive to grain size, cation ratio, and oxygen nonstoichiometry. The mechanism was shown to be a local "homogeneous" point defect reaction in contrast to the "heterogeneous" nucleation and growth mechanism reported to take place in ferrispinels lacking Fe²⁺. Electron hopping between Fe²⁺ and Fe³⁺ in ferrospinels apparently screens the excess ionic charge associated with the diffusing species and enables homogeneous point defect reactions with lower attendant activation energies to take place.     

Equilibrium Studies of Fe in Alkali Phosphate Glasses

The Fe²⁺-Fe³⁺ equilibrium in binary Na₂O-P₂O₅, glasses was studied by equilibrating glass melts at different temperatures in air. The enthalpy change (δH) of the reaction 1/2Fe₂O₃⇌FeO+1/4O₂ was calculated for 4 glasses. The results indicate that (1) the equilibrium shifts toward the oxidized state as the Na₂O content of the glass increases (plots of log ([Fe²⁺]/[Fe³⁺]) vs mol% alkali were linear) and (2) Δ H values for glasses of different composition are nearly equal but differ from the standard (calculated) value for the reaction. The experimental ΔH values were nearly equal to that for the reaction FePO₄→1/3 Fe₃(PO₄)²+ 1/6P₂O₅+1/4O₂, indicating that Fe forms phosphate or polyphosphate configurations in the Na₂O-P₂O₅ glasses. In certain of the glasses studied a faint-pink solid precipitated; its X-ray diffraction pattern indicated that its principal component is crystalline Na₂Fe¹¹¹P₃O₁₀.     

Role of Donor and Acceptor Ions in the Dielectric Loss Tangent of (Zr0.8Sn0.2)TiO4 Dielectric Resonator Material

The role of donor and acceptor ions in the dielectric loss tangent of (Zr,Sn)TiO₄ was investigated at frequencies from 0.1 kHz to 9 GHz. When a specimen was doped with Fe₂O₃, the loss tangent increased drastically at microwave frequencies. This increase is explained by a large concentration of Fe³⁺ ions and oxygen vacancies in the grains. When the specimen was doped with Ta₂O₅ and then annealed in a reducing atmosphere, electrical resistivity decreased and tan δ increased extremely at low frequencies but trivially at microwave frequencies. The high microwave Q value of Tadoped specimens is explained by assuming the Ta ions in the crystal to be tetravalent. Dielectric loss is determined predominantly by space charge polarization at low frequencies and by the disordered charge distribution at microwave frequencies.     

Behavior of Paramagnetic Iron during the Thermal Transformations of Kaolinite

Kaolinites with various degrees of structural order and iron content were heated and subsequently analyzed via electron paramagnetic resonance. Iron was present in two different states in the heated materials, either as dilute structural Fe³⁺ ions or in concentrated Fe³⁺ phases. During metakaolinization, the environment of dilute Fe³⁺ ions changed, following modifications of the Al³⁺ coordination, and the Fe³⁺ concentration increased. With the breakdown of metakaolinite, the diffusion of Fe³⁺ ions induced their exsolution in superparamagnetic iron-rich domains (Fe³⁺ clusters in γ-Al₂O₃ and/or Fe³⁺ oxide nanophases), which produced a decrease in the dilute Fe³⁺ concentration. The subsequent breakdown of γ-Al₂O₃ and the formation of mullite made the dilute Fe³⁺ concentration increase again, because of the incorporation of Fe³⁺ ions in the mullite structure.     

Oxidation Equilibrium of Iron in Borosilicate Glass

Ferrous/ferric equilibria were determined in alkali-alkaline-earth borosilicate glass as a function of temperature, oxygen partial pressure, and glass composition. Expected linear relations are found between log(Fe²⁺/Fe³⁺) and log( p O₂) or 1/ T . The slopes of the correlation with 10g( p O₂) are near the expected value of -0.25, but are found to decrease with increasing temperature. Reaction enthalpies determined from the correlation with 1/ T of –100 to –116 kJ/mol are similar to those reported for other silicate glasses. The ferroudferric equilibrium is not dependent on total iron content in the range 0.5 to 0.09% Fe₂O₃. More reducing conditions are required at lower temperatures to stabilize the amber chromophore. The ferroudferric equilibria are correlated to the number of bridging and nonbridging oxygen ions in the glass. The results suggest that the oxidation-reduction reaction can be written as: Fe²⁺+ (¹/₄)O₂+ (³/₂)O^2-= FeO₂⁻     

Grain Growth in CaO-Stabilized ZrO2 in the Presence of a Liquid Phase

Calcia-stabilized zirconia, CSZ (7.5 wt% CaO), with impurities of A1₂O₃, SiO₂, MgO, Fe₂O₃, and TiO₂, was sintered in air at 1783 K for times (t) up to 230 h. The microstructure consisted of grains of CSZ with small amounts of pores and a CaO-Al₂O₃-SiO₂ eutectic. Grain diameters grew in proportion to t^1/3. The results are consistent with a mechanism in which grain growth is limited by diffusion of Zr⁴⁺ ions through the liquid eutectic.     

Thermal Grooving of MgO and Al2O3

High-purity polycrystalline MgO and Al₂O₃ were thermally grooved at 1500° and 1600°C. Accurate techniques were developed for following the growth of a single groove. For high-purity samples growth kinetics were essentially similar to those reported in the literature but were determined to be controlled by volume diffusion. Specimens for thermal grooving were prepared from Al₂O₃ to which transition metal oxides (Fe₂O₃₉, MnO, and TiO₂), which are known to accelerate shrinkage and sintering of Al₂O₃ powder compacts, had been added; the rate of groove growth was increased remarkably by minor amounts of these additives. Control of partial pressure indicated that Fe²⁺ and Ti⁴⁺ are the species active in promoting groove growth. Substantial evidence was found for volume diffusion as the mechanism controlling groove formation.     

Factors Affecting the Preparation of Sr2Fe2−xMoxO6

Samples of Sr₂Fe_{2− x} Mo _x O₆were prepared by solid-state reaction in air and 5%H₂–95%N₂. X-ray diffractometry was used to identify the phases and evaluate the lattice parameters. It is found that molybdenum ions can dissolve in the SrFeO₃ even if the sample is heated in air but the solubility is limited. The solubility can be enhanced by heating the sample in low oxygen partial pressure, which is attributed to the larger ionic radii of Fe³⁺ and Mo⁵⁺ than that of Fe⁴⁺. The degradation of Sr₂Fe_{2− x} Mo _x O₆in water and air is also reported.     

Electron Paramagnetic Resonance Investigation of Acceptor Centers in Pb(Zr,Ti)O3 Ceramics

The nature of extrinsic point defects in lead zirconate titanate (PZT) ceramics of various compositions prepared by solution chemistry has been explored. Using electron paramagnetic resonance (EPR), several impurity sites have been identified in the as-received materials, which include Fe³⁺oxygen vacancy (V_O^‥) complex and isolated Cu²⁺ ions; both of these ions are incorporated into the lattice by replacing the Ti(Zr) ion. A Fe₃₊V_O^‥ complex serves as a sensitive probe of the local crystalline field environment of the ceramic; the symmetry of this defect is roughly correlated with its phase diagram as the composition is varied from PbTiO₃ to PbZrO₃. The Fe³⁺V_O^‥ complex experiences tetragonal, rhombic, or orthorhombic symmetry as the composition is varied from PbTiO₃ to PbZrO₃. As the composition of the Cu²⁺ ion is varied, it appears as though the addition of Zr, and not necessarily a change in phase, is largely responsible in determining the local environment of this acceptor impurity. Also, the Cu²⁺ resonance parameters weakly reflect the relative Ti–O(Zr–O) bond covalency in the perovskite lattice.     

Diffusion of Iron and Nickel in Magnesium Oxide Single Crystals

Diffusion kinetics and mechanisms were studied in the Fe_xO-MgO (vacuum), NiO-MgO (vacuum and air), and Fe₂0₃-MgO (air) systems. In the Fe_xO-MgO system, Fe entered MgO by a redox reaction; the diffusivity and activation energy depended on concentration. In the NiO-MgO system in air the diffusivity depended on concentration and the activation energy did not; in vacuum both the diffusivity and activation energy were concentration-independent. In the Fe₂O₃-MgO system in the MgO phase the activation energy and diffusivity did not depend on concentration. Because of impurities, the diffusion results were for the extrinsic region. Formation of trivalent ions and consequent chemically created vacancies in the Fe_xO-MgO and NiO-MgO (air) systems resulted in the concentration dependence of diffusivity. Concentration dependence of activation energy in the Fe_xO-MgO system is associated with structural changes due to a change with concentration of the Fe³⁺ octahedraI/Fe³⁺ tetrahedral ratio. In the Fe₂O₃-MgO system structural changes do not occur during diffusion because this ratio remains constant.     

Ion Exchange in Alkali Layers of Potassium β -Ferrite ((1 + x)K2O · 11Fe2O3) Single Crystals

The potassium ions in potassium β-ferrite ((1 + x)K₂O ·11Fe₂O₃) crystals were exchanged with Na⁺, Rb⁺, Cs⁺, Ag⁺, NH₄⁺, and H₃O⁺ in molten nitrates or in concentrated H₂SO₄. On the other hand, spinel and hexagonal ferrites were formed by soaking the crystals in the melt of divalent salts. The crystals of K⁺, Rb⁺, and Cs⁺β-ferrites decomposed to form α-Fe₂O₃ at high temperatures of 800° to 1100°C. In addition, H₃O⁺, NH₄⁺, and Ag⁺β-ferrites decomposed to form α-Fe₂O₃ at relatively low temperatures of 350° to 650°C, in accordance with the stabilities of the inserted ions. The electrical properties of some β-ferrites were measured.     

Effect of Fe4+ in the System SrFeO3-SrTiO3

Perovskites of the system SrFeO₃-SrTiO₃ were prepared, and measurements were made of their magnetic and electrical behavior. Chemical analysis showed that the percentage of Fe⁴⁺ varied from 72.5% for SrFeO_2.86 to about zero for Sr(Fe_0.1Ti_0.9)O_2.95; the remainder of the iron was in the Fe³⁺ state and electrical balance was achieved by oxygen loss. Sr(Fe_{1- x} Ti _x )O₃ was antiferromagnetic between x = 0 and x = 0.9, with a Néel temperature below 60°K. A parasitic ferromagnetic component developed when these compounds were cooled in a magnetic field, the magnitude of this component being dependent on the cooling field. The conductivity of these perovskites ranged from 10⁻⁸ ohm⁻¹ cm⁻¹ for x = 1.0 to 10⁻² for x = 0.0 and showed a marked change at x = 0.8. The break corresponded to a change in slope of the lattice parameter and the disappearance of Fe⁴⁺. The Fe⁴⁺ content depended on the heat treatment and atmosphere during formation.     

Effect of Oxygen Partial Pressure on the Formation of Metastable Phases from an Undercooled YbFeO3 Melt Using an Aerodynamic Levitator

The Yb₂O₃–Fe₂O₃ system was studied to investigate the effect of oxygen partial pressure on the formation of metastable phases over a wide range of oxygen partial pressures from 10⁵ to 10⁻¹ Pa. Two kinds of metastable phases, with space groups of P 6₃ cm and P 6₃/ mmc , were found through rapid solidification of an undercooled YbFeO₃ melt in an atmosphere with reduced P o₂. The crystal structure of the as-solidified samples changed from orthorhombic Pbnm to hexagonal P 6₃ cm and P 6₃/ mmc with decreasing P o₂. X-ray diffractometric and scanning electron microscopic results confirmed the existence of various phases in the as-solidified samples. The stabilities of each phase were studied by annealing the bulk sample in the thermogravimetric–differential thermal analysis (TG-DTA) furnace up to 1673 K, and the equilibrium phase diagram was constructed for the Yb–Fe–O system at 1473 K. TG analysis showed an increase of the sample mass during annealing and revealed that the existence of Fe²⁺, which has an ionic radius larger than that of Fe³⁺, decreases the tolerance factor and therefore destabilizes the perovskite structure.