Interfacial Segregation in Perovskites: II, Experimental Evidence

Experimental evidence determined by scanning transmission electron microscopy, scanning Auger electron spectroscopy, and lattice parameter measurements is reported for interfacial segregation in pure and doped perovskite materials, e.g., BaTiO₃ and SrTiO₃. The segregation behavior for isovalent dopants (e.g., Sr²⁺, Ca²⁺, or Pb²⁺ at a Ba²⁺ site) is mainly associated with a strain energy contribution from ionic misfit. However, both strain energy and space charge contributions are important in determining the segregation behavior of aliovalent ions (e.g., La³⁺ or Nd³⁺ at a Ba²⁺ site and Fe³⁺ or Ni³⁺ at a Ti⁴⁺ site). Segregation depths for aliovlent dopants are typically 15 to 20 nm thick. Besides dopants, constituent alkaline-earth ions also segregate to interfaces. Their segregation behavior is promoted by high temperatures, low partial pressures of oxygen, and the presence of acceptors. The results are explained in terms of a proposed theory of segregation presented in part I of the present series.     

Effect of Divalent Cation Additives on the γ-Al2O3-to-α-Al2O3 Phase Transition

The effect on the γ-Al₂O₃-to-α-Al₂O₃ phase transition of adding divalent cations was investigated by differential thermal analysis, X-ray diffractometry, and surface-area measurements. The cations, Cu²⁺, Mn²⁺, Co²⁺, Ni²⁺, Mg²⁺, Ca²⁺, Sr²⁺, and Ba²⁺, were added by impregnation, using the appropriate nitrate solution. These additives were classified into three groups, according to their effect: (1) those with an accelerating effect (Cu²⁺ and Mn²⁺), (2) those with little or no effect (Co²⁺, Ni²⁺, and Mg²⁺), and (3) those with a retarding effect (Ca²⁺, Sr²⁺, and Ba²⁺). The crystalline phase formed by reaction of the additive with γ-Al₂O₃ at high temperature was a spinel-type structure in groups (1) and (2) and a magnetoplumbite-type structure in group (3). In groups (2) and (3), a clear relationship was found between the transition temperature and the difference in ionic radius of Al³⁺ and the additive (Δ r ): The transition temperature increased as Δ r increased. This result indicates that additives with larger ionic radii are more effective in suppressing the diffusion of Al³⁺ and O²⁻ in γ-Al₂O₃, suppressing the grain growth of γ-Al₂O₃, and retarding the transformation into α-Al₂O₃.     

Optical and Magnetic Properties of Some Transition Metal Ions in Barium Phosphate Glass

In barium phosphate glass containing first-row transition metal ions, the ions appear to possess normal oxidation states: Cr³⁺, Mn³⁺, Fe^2+,3+, Co²⁺, Ni²⁺, and Cu²⁺. Room temperature and 77°K spectra for Cr³⁺-, Mn³⁺-, Fe^2+,3+-, Ni²⁺-, and Cu²⁺-containing glasses can be interpreted, on the, basis of octahedrally coordinated metal ions, whereas the spectrum of the Co²⁺ glass correlates with a tetrahedrally coordinated metal ion. Magnetic moments for the glasses are similar to those for the spin-only values of the various metal ions.     

Defect Chemistry of BaTiO3 with Additions of CaTiO3

The substitution of up to 5% Ca²⁺ for Ba²⁺ in BaTiO₃ results in a shift in the oxygen pressure dependence of the equilibrium electrical conductivity that is in the same direction as that caused by addition of acceptor impurities such as Al³⁺ or Ca²⁺ substituted for Ti⁴⁺. In contrast to the latter effect, however, the shape of the conductivity plot is not changed, the conductivity value at the conductivity minimum is not affected, and the amount of the shift increases with decreasing temperature of measurement. It is shown that the shift is primarily due to an increase in the enthalpy of reduction and a decrease in the enthalpy of oxidation as increasing amounts of Ba²⁺ are replaced by Ca²⁺.     

Origin of Grain-Boundary Diffusion in MgO

Preferential diffusion of Ni²⁺ and Co²⁺ along grain boundaries was observed in certain bicrystals of MgO. This enhancement is attributed to impurity segregation at the boundary. The identified impurities responsible for the effect are the principal impurities in the single-crystal MgO: Ca, Si, and Fe. No enhancement was observed in any bicrystal prepared above 1300°C, a temperature similar to that at which studies of the mechanical properties of MgO have implied a reabsorption of impurity precipitates into solid solution. It is concluded that enhanced grain-boundary diffusion of cations in MgO is an extrinsic, rather than an intrinsic, property of the boundary.     

Selective Cation Exchange in Substituted Tobermorites

Selective cation exchange in tobermorites with three levels of Al³⁺ and Na⁺ substitution for Si⁴⁺ has been investigated. The cation exchange selectivity for a tobermorite with 2 mol% Al³⁺ and 0 mol% Na⁺ substitution increased as follows: Mg²⁺ > Ba²⁺ > Sr²⁺. Cation exchange in the above tobermorite is postulated to take place mainly from edge and planar surface sites and apparently from interlayer Ca²⁺ sites. Tobermorites with 10 and 15 mol% Al³⁺ (and Na⁺) substitution have additional Na⁺ exchange sites in the interlayers and show the following selectivity: Ba²⁺ > Sr²⁺ > Mg²⁺. The cation exchange selectivity in the substituted tobermorites can be explained by the hydrated radii of cations and the steric limitations of the ion exchange cavity in tobermorite; the latter was determined by ²⁷Al and ²⁹Si magic angle spinning nuclear magnetic resonance spectroscopy. These basic selective cation exchange studies of substituted tobermorites are of relevance in nuclear waste treatment and disposal.     

Characterization and Microwave Dielectric Properties of M2+Nb2O6 Ceramics

This paper details the investigation of the quality factor ( Q ), dielectric permittivity (ɛ_r) and temperature coefficient of resonant frequency (τ_f) of the TE_01δ mode of the columbite binary niobate ceramics, with the formula MNb₂O₆ where M=²⁺ cation, in relation to their degree of sintering, microstructure and phase composition. The ceramics were made from a mixed oxide preparative route and fired over a range of temperatures from 800° to 1400°C, and most formed the columbite structure. A comprehensive study was made of the niobates containing the transition metal cations M=Mn²⁺, Co²⁺, Ni²⁺, Cu²⁺, and Zn²⁺, and the group II metal cations M=Mg²⁺, Ca²⁺, Sr²⁺, and Ba²⁺. All columbite niobates were found to have ɛ_r between 17 and 22 and negative τ_f values between –45 and –76 ppm/°C, and ZnNb₂O₆, MgNb₂O₆, CaNb₂O₆, and CoNb₂O₆ had high Q f values of 84 500, 79 600, 49 600, and 41 700 GHz, respectively. The Q f of MgNb₂O₆ was found to rise to over 95 000 GHz when heated at 1300°C for 50 h.     

Glass Formation in the System SiO2–PbO Containing Transition-Metal Oxides

Glass-forming regions, valence states, and viscosities in SiO₂–PbO systems containing various transition-metal oxides as a third component were investigated. The glasses were prepared by melting in an open atmosphere. The glass-forming regions ranged as follows: MnO≡ZnO > FeO_1.5>NiO. The ratios Fe²⁺/(Fe²⁺+ Fe³⁺) and Mn³⁺/ (Mn³⁺+ Mn²⁺) in the glasses were determined by chemical analysis. The Fe²⁺/ (Fe²⁺+ Fe³⁺) ratio in SiO₂–PbO–FeO_1.5 glasses ranged from 0.016 to 0.050. The Mn³⁺/ (Mn³⁺+ Mn²⁺) ratio in SiO₂–PbO–MnO glasses ranged from 0.056 to 0.30. The fraction of manganese (III) ions in the glasses varies considerably with the glass composition. The effects of transitionmetal oxides on the viscosity are discussed.     

Evaluation of Lattice Site and Valence of Mn and Fe in Polycrystalline PbTiO3 by Electron Spin Resonance and Thermogravimetry

The oxides of Mn and Fe or combinations of La₂O₃ with these oxides were incorporated into PbTiO₃ under defined thermodynamic conditions. The substitution reaction of these additives was examined gravimetrically. At low partial pressures of O₂ and PbO vapor, Mn²⁺ and Fe²⁺ are formed at the A sites, and Mn³⁺ and Fe³⁺ enter both A and B sites. At higher partial pressures of PbO, Mn⁴⁺ is most probably formed at the B sites.
The Mn²⁺, Mn⁴⁺, and Fe³⁺ were determined by ESR.     

Segregation of Aliovalent Solutes Adjacent Surfaces in MgO

The segregation of ferrous iron, ferric iron, chromium, and scandium solutes adjacent {100} surfaces of MgO crystals in the single-phase region was studied by ion microprobe mass spec-troscopy. Excess concentration in a region near the surface was found for Fe³⁺, Cr³⁺, and Sc³⁺, but none for Fe²⁺. This segregation is the type predicted as necessary to balance the negative surface charge.     

Oxidation-Reduction Equilibria in Molten Na2O.2SiO2 Glass

The oxidation-reduction equilibria in molten glasses having a soda/silica ratio of 1/2 and containing small amounts of variable-valence ions of the transition elements titanium, vanadium, iron, cobalt, and nickel, the post-transition elements tin and antimony, and the rare-earth element cerium were obtained by equilibrating the melts with various atmospheres. The simple mass expression
was always applicable. In the expression, n is the number of electrons involved in the valence change of the metal M. The value of n is 1 for all systems except for the ions of antimony and tin where n is 2. The ions found are those generally accepted as existing in glass melts which are Ti³⁺, Ti⁴⁺; Fez+, Fe³⁺; Ce³⁺, Ce⁴⁺; Mn²⁺, Mn³; Co²⁺, Co³⁺; Ni²⁺, Ni³; Sb³⁺, Sb⁵⁺; and Sn²⁺, Sn⁴⁺. Two mass action expressions were needed for vanadium-containing glasses to describe the equilibria between 5+, 4+, and 3 f species.     

Strengthening of Alkali Halides by Divalent-Ion Additions

Single crystals of NaCl, NaBr, KCl, and KBr containing divalent additions of Ca²⁺, Sr²⁺, and Ba²⁺ were tested mechanically. In the solution-treated condition, the yield strength, σ _v , as determined from compression testing in a 〈100〉 direction is essentially dependent on the concentration of the dopant only and is independent of the species of either the dopant or of the host material. All crystals soften on aging, with the exception of the NaCl:Ca²⁺, NaBr:Ca²⁺, and NaBr:Sr²⁺ systems. In addition, correlation was good between σ _v , and the Knoop hardness number, H , obtained by indentation with the long axis of the indenter aligned in 〈100〉. The equation is of the form σ _v = C ( H–H ₀), where C ≅ 0.21 for all four halide families and H₀ is near the hardness value of the pure halides. Furthermore, H ₀≅5×10⁻³E₁₁₁, the Young's modulus in the 〈111〉 direction. Hence σ _v ≅0.21 H –10⁻³E₁₁₁.     

Effect of Substituent Ions on Martensitic Transformation Temperatures in Dicalcium Silicate Solid Solutions

Five types of Ca₂SiO₄ solid solutions, doped with either P⁵⁺, Ge⁴⁺, Fe³⁺, Mg²⁺, or Ba²⁺, were prepared and examined by high-temperature X-ray diffractometry up to 800°C. The starting and finishing temperatures were determined for the α'_L-to-β martensitic transformation during cooling and the reverse (β-to-α'_L) transformation during heating. These four types of transformation temperatures for the preparations doped with either P⁵⁺ or Ge⁴⁺ steadily decreased with increasing substituted fraction. The effect of the substitution on the decrease for each transformation temperature was quantitatively evaluated by Δ T / x , where Δ T is the difference in the transformation temperatures between the solid solutions and pure Ca₂SiO₄, and x represents the fraction substituted for Si⁴⁺ or Ca²⁺ in the α'_L-phase structure. The evaluated value for the substitution of P⁵⁺ was more than 3 times that of Ge⁴⁺. The effect of the substituent ions mentioned above, together with Na⁺ and Sr²⁺, on the lowering of the starting temperature of the α'_L-to-β transformation was principally determined by the differences in the ionic radius between the interchanging cations.     

Redox-Controlled Solubility of Palladium in a Borosilicate Glass Melt

Palladium establishes the Pd²⁺–Pd⁰ redox equilibrium in a reference alkali borosilicate melt. Most of the palladium exists as the insoluble metal, with only a small fraction of the palladium present as the divalent palladium ion dissolved in the melt. The relative reduction potential of this Pd²⁺–Pd⁰ redox couple in the reference glass melt is greater than such easily reduced couples as Ni³⁺–Ni²⁺ and Mn³⁺–Mn²⁺. Palladium metal in glass melts containing appropriate amounts of nickel and sulfur readily partitions into an immiscible nickel sulfide phase; palladium also tends to associate with tellurium in insoluble particulates.     

Vibrational Spectra of Hydrothermally Prepared Hydroxyapatites

Apatites can be used as bioceramic materials for tooth and bone implants. Infrared (4000 to 400 cm⁻¹) and laser-Raman spectra were obtained for hydrothermally prepared hydroxyapatites with the general formula X₁₀(PO₄)₆(OH)₂ where X= Ca²⁺ Sr²⁺, Ba²⁺, or Pb²⁺ Fundamental vibrational modes were identified and empirical band assignments made. Analysis of the vibrational spectra led to the reassignment of the v₂ mode to a higher wave-number, lower-intensity band (e.g. ∼470 cm⁻¹ for Ca hydroxyapatite). Effects of solid-state interaction were noted in the vibrational spectra and interpreted by factor-group analysis. An "effective" factor group C_6h, which neglects the locations of the OH⁻ ions, best describes the vibrational spectra of the hydroxyapatites.     

EXAFS Study of Cation Distribution in Nickel Aluminate Ferrites

A new method for analyzing the cation distribution of spinel crystals by extended X-ray absorption fine-structure spectroscopy has been developed to overcome the disadvantage of X-ray diffraction, which cannot readily distinguish among different transition-metal ions. EXAFS analysis was conducted on Ni²⁺ and Fe³⁺ ions in nickel aluminate ferrites, and the cation distributions obtained were in good agreement with values obtained by other methods. The ratio of tetrahedral sites occupied by Ni²⁺ ions decreases with increasing iron content. Fe³⁺ ions distribute nearly equally to both tetrahedral and octahedral sites. Apparently, not only crystal-field stabilization energy but also ion size affects cation distribution.     

Room-Temperature Preparation of Crystallized Ba1-χSrχWO4 Solid-Solution Films by an Electrochemical Method

Films of a complete series of solid-solution oxides, Ba^1-χSr^χWO⁴ (O ≥ X ≥ 1), have been prepared on tungsten substrates in an electrolytic solution containing Ba²⁺ and Sr²⁺ ions by an electrochemical method at room temperature (25°C). The composition X could easily be controlled by the concentrations of Ba and Sr species in the starting solutions. X-ray diffraction and X-ray photoelectron spectroscopy analyses showed that a complete series of well-crystallized solid-solution oxides was formed even at room temperature.     

Influence of Iron Solute on the Structure of Small-Angle [001] Twist Boundaries in Magnesium Oxide

The structure of small-angle [001] twist boundaries in pure and Fe-doped MgO was examined by electron imaging techniques in order to study the influence of solute on grain-boundary structure. In addition, the effect of the oxidation state of the Fe on the boundary structure was also examined. The [001] twist boundaries were produced in the form of bicrystals, by the hot-pressing of cleaved single crystals of pure MgO and MgO containing Fe. In pure MgO the [001] twist boundaries produced with small plastic strains are planar and contain a square array of screw dislocations with Burgers vectors of the type a /2<110>. In the presence of solute the original interface becomes wavy and dissociates into subboundaries. Distorted hexagonal dislocation networks as well as square arrays of screw dislocations are often observed in Fe-doped MgO. These observations are evidence that [001] twist boundaries in MgO whose production was accompanied by small amounts of plastic deformation undergo a structural transformation induced by the presence of Fe solute. The influence of the Fe oxidation state on boundary structure is less well understood. It appears that when the [001] twist boundaries are produced using crystals containing a substantial amount of Fe³⁺ ions, the resulting boundary structure is more likely to contain distorted hexagonal dislocation networks than when the boundary is produced from crystals containing predominantly Fe²⁺ ions.     

The Effect of Sr Substitution on Phase Formation and Magnetic Properties of Y-type Hexagonal Ferrite

The effect of Sr substitution in Y-type hexagonal ferrite of composition Ba_{2– x} Sr _x Zn_1.2Co_0.4Cu_0.4Fe₁₂O₂₂ was investigated. x varied from 0 to 2.0. The samples were prepared by the solid-state reaction method. Phase formation was characterized by powder X-ray diffraction. The microstructure was observed via scanning electron microscopy and magnetic properties were measured using an impedance analyzer and vibrating sample magnetometer (VSM). Results reveal that Sr²⁺ cannot completely substitute for Ba²⁺ ions in Y-type hexagonal ferrites. The maximum solubility of Sr in the Y-type ferrite is about 1.8. With Sr above 1.8, the lattice mismatch induced by the difference in ionic radii of Ba²⁺ and Sr²⁺ ions prevents the formation of Y-type phase. While Sr content is equal or less than 1.0, the magnetic properties may be enhanced. As x >1.0, because of the internal stress resulting from the lattice mismatch, demagnetizing field significantly degrades initial permeability.     

Influence of Several Metal lons on the Gelation Activation Energy of Silicon Tetraethoxide

The effects of nine metal cations (Li⁺, Na⁺, Mg²⁺, Ca²⁺, Sr²⁺, Cu²⁺, Al³⁺, La³⁺, and Y³⁺) on silica gel formation have been investigated by studying the hydrolysis and polycondensation of silicon tetraethoxide (TEOS) in the presence of metal nitrates. The influence of the water: TEOS mole ratio, metal ion concentration, and the reaction temperature has been investigated. The overall activation energy for gel formation has been determined from the temperature dependence of the time of gelation for each system. The activation energy for network formation is 54.5 kJ/mol. The gel formation time as well as the activation energy sharply increases in the presence of Cu²⁺, Al³⁺, La³⁺, and Y³⁺. In contrast, the presence of Li⁺, Na⁺, Mg²⁺, Ca²⁺, or Sr²⁺ lowers the gelation time but has no appreciable effect on the activation energy. This difference may be attributed to the participation or nonparticipation of the metal ions in the formation of the three-dimensional polymeric network during polycondensation. The concentration of metal ion (Mg²⁺, Ca²⁺, Y³⁺) or the water: TEOS mole ratio had no appreciable effect on the gelation activation energy. A simple test has been proposed to determine whether a metal ion would act as a network intermediate or modifier in silica and other glassy networks.