Electronic Conductivity, Seebeck Coefficient, and Defect Structure of LaFeO3

Electronic conductivity and Seebeck coefficients of LaFeO₃ were measured as a function of temperature (1000° to 1400°C) and P ( O ₂) (10⁵ to 10⁻¹³ Pa). Electronic conduction was found to be n-type in the lower P ( O₂ ) range, and p -type in a higher P(O₂) range. The calculated carrier mobilities suggest a hopping-type conduction mechanism. The carrier concentrations were calculated as a function of P ( O₂ ) and the defect structure was described. It was found that the electrical properties of LaFeO₃ are determined not only by the concentration of oxygen vacancies, but also by the La/Fe ratio.     

Electrical Conduction in Single-Crystal Thallic Oxide: I, Crystals "As-Grown" from the Vapor in Air

Thallic oxide, "T1₂O₃," has been shown to be a degenerate n -type semiconductor with resistivity varying from 60 to 150 μΩ-cm over the range 4° to 900°K. The carrier concentration was 7 × 10²⁰ cm⁻³ and is temperature independent. Room-temperature Hall mobility was 105 cm² V⁻¹ s⁻¹, increasing to 130 cm² V⁻¹ s⁻¹ below 70°K. Donor states were shown to be native defects, probably oxygen vacancies.     

High-Temperature Hall Measurements on BaSnO3Ceramics

Simultaneous Hall and conductivity measurements were performed in situ between 650° and 1050°C on n-type semiconducting BaSnO₃ceramics. The variation of the Hall mobility and the Hall carrier density as a function of oxygen partial pressure between 10² and 10⁵ Pa and of temperature was investigated. At temperatures below 900°C the conductivity exhibits a dependence on temperature and oxygen partial pressure which is mainly determined by variations of the Hall mobility. Above 900°C most of the significant dependence is due to a variation in carrier density. Furthermore, a simple defect model assuming doubly ionized oxygen vacancies and acceptor impurities is discussed for BaSnO₃.     

Electronic Conductivity of La-Doped Ceria Ceramics

Lanthanum-doped ceria powder with a composition Ce_0.8La_0.2O_1.9 was prepared by heating the oxalate solid solution (Ce_0.8La_0.2)₂(C₂O₄)₃ at 873 K in air. As-prepared powder was densified to 96%–97% relative density by sintering in air at 1773 K for 4 h. The electronic current of the disk sample was measured in a temperature range from 773 to 1113 K by the direct current polarization method using a Hebb–Wagner ion-blocking cell. A linear relationship, which was theoretically predicted, was measured between log σ_e (electronic conductivity) and E (applied voltage) in the applied voltage range of 0.2–1.0 V. The σ_e was proportional to P _O2^−1/4.3≈ P _O2^−1/4.6 in the oxygen partial pressure range of 10⁻²–10⁻⁸ Pa, and to P _O2^−1/6.7≈ P _O2^−1/7.1 in the oxygen partial pressure range of 10^−7.5–10⁻²² Pa. The apparent activation energy of the electronic conduction was 1.87–1.94 eV. The hole conduction was also measured in the P o₂ range of 10²–10⁵ Pa. The transport number of oxide ion was 0.96–1.00 at 773–1113 K under an oxygen partial pressure of 10⁻⁵ Pa.     

Point Defects in Optical Ceramics: High-Temperature Absorption Processes in Lanthana-Strengthened Yttria

The optical transmission of transparent polycrystaliine La₂O₃ strengthened Y₂O₃ was measured in both the near-ultraviolet and infrared regions at temperatures between 20° and 1400°C. The absorption remains low until about 900°C, but rises almost exponentially thereafter. The magnitude of this increase is a function of the oxygen partial pressure (Po₂) in the ambient atmosphere. The temperature-dependent absorption is lowest when the Po₂ is between 10⁻¹¹ and 10⁻⁸ atm (1 atm = 10⁵ Pa), representing the range in which the concentration of free carriers (conduction band electrons and valence band holes), generated by stoichiometty-related point defects (oxygen interstitiais and vacancies), is minimized. The temperature dependence is significantly greater in the uitraviolet than in the infrared, but the optimal Po ₂ range is the same. The absorption behavior can be described in terms of a simple phenomenological model involving carriers thermally liberated from defect states within the band gap of the material. Various aspects of the model and their experimental implications are discussed.     

Formation of Hexagonal Metastable Phases from an Undercooled LuFeO3 Melt in an Atmosphere with Low Oxygen Partial Pressure

Controlling the oxygen partial pressure ( P _O2) in the ambient atmosphere is an important parameter for material processing because the transition metal ion changes its valence depending on P _O2. In the present study, containerless solidification of the LuFeO₃ melt, where the undercooling level can be treated as another experimental parameter, was carried out in order to explore the unidentified metastable phases under the controlled P _O2 using an aerodynamic levitator. Decreasing P _O2 down to 1 × 10³ Pa, the unidentified phase was solidified from the undercooled melt. The X-ray diffractometry results after annealing at 1 × 10³ Pa showed the peak profile of the stable perovskite LuFeO₃ phase, suggesting that this unidentified phase was thermodynamically metastable. Thermogravimetric analysis showed that the mass of the sample solidified at P _O2= 1 × 10³ Pa significantly increased, suggesting that the formation of the metastable phases might be related to the presence of Fe²⁺ ions.     

Deviation from Stoichiometry in Cr2O3 at High Oxygen Partial Pressures

The deviation from stoichiometry, δ, in Cr₂−δO₃ was measured by a tensivolumetric method in the high pO₂ range of ≊10⁴ to 10⁴ Pa at 1100°C. The value of δ, or chromium vacancy concentration, was≊9×10⁻⁵ mol/mol Cr₂O₃ in air for Cr₂O₃ with 99.999% purity. The chemical diffusion coefficient, DT, determined from equilibration data was ≊4.6× cm²·s⁻¹ at 1100°C for pO₂= 2.2 ×10¹ Pa. The self-diffusion coefficient of Cr ions was calculated from and δ and found to be≊1.6×10-17 cm²-s⁻¹, in good agreement with recently measured values.     

Oxygen Self-Diffusion in Magnesium- or Titanium-Doped Alumina Single Crystals

Oxygen Setf-diffusion coefficients have been measured in single crystals of N₂O₃ doped with Mg or Ti under an oxygen partial pressure of 20 kPa in the temperature range 1400° to 1700°C. Diffusion coefficients in Mg-doped crystals obey the equation D_o (cm²/s) = 1 × 10¹¹∼ (1×10¹³) exp[−915 ± 50(kJ/mol)/ RT ]. The diffusivity of oxygen in Ti-doped A1₂O₃ is lower than Mg-doped A1₂O₃. A vacancy mechanism explains these results.     

Vapor Pressure of Barium and Copper Components in YBa2Cu3O7−x Superconductor Ceramics

Purified air is passed over a specimen of YBa₂Cu₃O_{7– x} at 890°C; the vaporized substances are condensed in a pure alumina tube, then subjected to inductively controlled plasma analysis. Vapor pressure values of 2.5 × 10⁻⁵ Pa for BaO( g ), 1.2 × 10⁻⁴ Pa for Cu( g ), and 2.2 × 10⁻⁵ Pa for CuO( g ) are obtained under 2.1 × 10⁴ Pa (0.21 bar) of oxygen pressure. No Y vapor is detected at this temperature.     

Thermal Expansion of the Low-Temperature Form of BaB2O4

Thermal expansion of the low-temperature form of BaB₂O₄ (β-BaB₂O₄) crystal has been measured along the principal crystallographic directions over a temperature range of 9° to 874°C by means of high-temperature X-ray powder diffraction. This crystal belongs to the trigonal system and exhibits strongly anisotropic thermal expansions. The expansion along the c axis is from 12.720 to 13.214 Å (1.2720 to 1.3214 nm), whereas it is from 12.531 to 12.578 Å (1.2531 to 1.2578 nm) along the a axis. The expansions are nonlinear. The coefficients A, B , and C in the expansion formula L _t = L ₀(1 + At + Bt ²+ Ct ³) are given as follows: a axis, A = 1.535 × 10⁻⁷, B = 6.047 × 10⁻⁹, C = -1.261 × 10⁻¹²; c axis, A = 3.256 × 10⁻⁵, B = 1.341 × 10⁻⁸, C = -1.954 × 10⁻¹²; and cell volume V, A = 3.107 × 10⁻⁵, B = 3.406 × 10⁻⁸, C = -1.197 × 10⁻¹¹. Based on α _t = (d L _t /d t )/ L ₀, the thermal expansion coefficients are also given as a function of temperature for the crystallographic axes a , c , and cell volume V.     

Electrical Properties of Gallium-Doped Zinc Oxide Films Prepared by RF Sputtering

Ga-doped polycrystalline ZnO films on glass substrates were prepared by sputtering the targets, which had been prepared by sintering disks consisting of ZnO powder and various amounts of Ga₂O₃, to investigate the effects of gallium doping and sputtering conditions on electrical properties. Optimizing the RF power density, argon gas pressure, and gallium content, transparent Ga-doped ZnO films with resistivity less than 10⁻³Ω·cm were obtained. Electron concentrations for undoped and Ga-doped ZnO films were on the order of 10¹⁸ and 10²¹/cm³, respectively. The Ga-doped ZnO films became degenerate when the electron concentration exceeded ∼ 10¹⁹/cm³, and the optical band gap increased with increasing carrier concentration because of the increase of Fermi energy in the conduction band.     

Energy Transfer Process for the Blue Up-Conversion in Calcium Aluminate Glasses Doped with Tm3+ and Nd3+

Excitation of Tm³⁺ to ³ H ₄ using the 791 nm pump source showed the frequency up-converted blue emission (∼480 nm) due to the Tm³⁺:¹ G ₄→³ H ₆ transition in Tm³⁺/Nd³⁺ codoped CaO·Al₂O₃ glasses. Intensity and lifetime changes with rare-earth concentrations suggested the efficient energy transfer of Tm³⁺:³ H ₄→ Nd³⁺:⁴ F _5/2 and Nd³⁺:⁴ F _3/2→ Tm³⁺:¹ G ₄. The latter transfer enabled Tm³⁺ to reach its ¹ G ₄ level, and the blue emission became possible through the ¹ G ₄→³ H ₆ transition. Quantitative analysis with rate equations proved that these two transitions were the most efficient among all the possible energy transfer routes between Tm³⁺ and Nd³⁺. Calculated up-conversion efficiency of the Tm³⁺/Nd³⁺ combination in CaO·Al₂O₃ glass was 6.6 × 10⁻³, and it was ∼4 orders of magnitude larger than those reported for other oxide glasses.     

Thermodynamic Study of the Cu-Sr-O System

The Gibbs energies of formation of the inter-oxide compounds in the Cu-Sr-O system have been obtained from solid-state electrochemical measurements in the temperature range 900 to 1300 K. Cells employing yttria-stabilized zirconia or single-crystal calcium fluoride as electrolyte were used in studies of SrCu₂O₂, Sr₂CuO₃, SrCuO₂, and "Sr₃Cu₅O_{8+ z} ." The oxygen potential vs stoichiometry dependence was studied, particularly for this last compound, by thermogravimetry at a few selected temperatures in the oxygen partial pressure interval 1 × 10¹ to 1 × 10⁵ Pa. Based on the results of emf measurement, thermogravimetric study, and X-ray powder diffraction, phase relations in this system were calculated.     

Epitaxial Aluminum-Doped Zinc Oxide Thin Films on Sapphire: II, Defect Equilibria and Electrical Properties

The electrical transport properties of epitaxial ZnO films grown on different orientations of sapphire substrates have been measured as a function of partial pressure of oxygen. After equilibration, the carrier concentration is found to change from a p ^-1/4_O2 to a p ^-3/8_O2 dependence with increasing oxygen partial pressure. The partial pressure dependence is shown to be consistent with zinc vacancies being the rate-controlling diffusive species. In addition, the carrier concentration in ZnO films grown on A-, C-, and M-plane sapphire are the same but that of R-plane sapphire is systematically lower. Electron Hall mobility measurements as a function of carrier concentration for all the substrate orientations exhibit a transition from "single-crystal" behavior at high carrier concentrations to "polycrystalline" behavior at low carrier concentrations. This behavior is attributed to the effective height of potential barriers formed at the low-angle grain boundaries in the epitaxial ZnO films. The trap density at the grain boundaries is deduced to be 7 × 10¹²/cm². The electron mobility, at constant carrier concentration, varies with the substrate orientation on which the ZnO films were grown. The difference is attributed to the difference in dislocation density in the films produced as a result of lattice mismatch with the different sapphire orientations.     

High-Temperature Conductivity and Creep of Polycrystalline AI2O3 Doped with Fe and/or Ti

Partial ionic and electronic dc conductivities and compressional creep rate were measured for hot-pressed poly crystalline AI₂O₃ made from AI-isopropoxide (AI₂O₃(II)). The undoped material was found to contain 1.5×10¹⁸ cm⁻³ fixed valency acceptors (Mg). Properties of undoped material and material doped with Fe or Ti were investigated as a function of grain size, dopant concentration, oxygen pressure, and temperature. No fast ionic conduction along grain boundaries is found in either acceptor- or donor-dominated material. Absolute values of self-diffusion coefficients calculated from conductivity and creep indicate that both effects are limited by migration of AI, involving V _AI"in donor-, AI," in acceptor-dominated material. In creep, oxygen is transported along grain boundaries in a neutral form (O_i^p). The p_O2 dependence of σ _t and σ _h are as expected on the basis of a defect model. That of creep is weaker for reasons that are not entirely clear. An ionic conductivity with low activation energy, observed at low temperature, is attributed to the presence of AI-silicate second phase.     

Thermal Expansion of the Hexagonal (6H) Polytype of Silicon Carbide

The thermal expansion of the hexagonal (6H) polytype of α-SiC was measured from 20° to 1000°C by the X-ray diffraction technique. The principal axial coefficients of thermal expansion were determined and can be expressed for that temperature range by second-order polynomials: α¹¹= 3.27 × 10^–6+ 3.25 × 10^–9T – 1.36 × 10^–12 T ² (1/°C), and ş₃₃= 3.18 × 10^–6+ 2.48 × 10^–9 T – 8.51 × 10^–13 T ² (1/°C). The σ₁₁ is larger than α₃₃ over the entire temperature range while the thermal expansion anisotropy, the δş value, increases continuously with increasing temperature from about 0.1 × 10^–6/°C at room temperature to 0.4 × 10^–6/°C at 1000°C. The thermal expansion and thermal expansion anisotropy are compared with previously published results for the (6H) polytype and are discussed relative to the structure.     

Conduction and Dielectric Loss Mechanisms in β-Alumina and Glass: A Discussion Based on the Paired Interstitialcy Model

A paired interstitialcy model is used as a basis for qualitative comparisons of conductivity and dielectric phenomena in β-alumina crystals and in glass. Thus, the increase in the conductivity of sodium silicate glasses with increasing Na₂O activity can be explained if the concentration of (Na₂*)²⁺ interstitial pairs increases with increased polarizability of O^2- ions, expressed in terms of the optical basicity parameter, Δ. Similarly, the occurrence of the pronounced minima in conductivity isotherms (the mixed-alkali effect in glass) is attributed to disappearance of mobile interstitial pairs, e.g. (Li₂*)²⁺ or (K₂*)²⁺, and the stabilization (by polarization interactions) of apparently immobile mixed-alkali pairs, (LiK*)²⁺. The phenomenon of coionic conduction in certain β-alumina crystals is an interesting departure from this general pattern. The orientation dependence of the electrical modulus spectrum of monocrys-talline β-alumina highlights the presence of a bimodal distribution of relaxation times, in which the low-frequency component ( v ₀=10¹¹ Hz) may arise from the rearrangement of interstitial pairs and the high-frequency component ( v ₀=2×10¹² Hz) may arise from less hindered ionic motions. It is suggested that the motions of interstitial pairs and surrounding cations are mutually catalytic and that some form of combined motion is responsible for both the electrical and mechanical relaxations in β-alumina and glass.     

Solubilities of Magnesia, Titania, and Magnesium Titanate in Aluminum Oxide

On heat treatment in air the solubility of MgO or TiO₂, in Al₂₃ is too small to detect by lattice parameter shifts. The solubility of MgTiO₃ in Al₂O₃ in air increased to the measured values, expressed as atomic fractions Mg:A1or Ti:A1of0.82 × lo-², 1.43 × 10^-2, and 1.75 × 10^-2 at 1250°, 1650°, and 1850°C, respectively. In 1 atm hydrogen the TiO₂ solubility expressed as the atomic fraction Ti:A1 is 0.55 × lo^-2, 0.75 × 10W², 1.15 × 10^-2, and 1.50 × 10^p2 at 1400°, 1500°, 1600°, and 1700°C, respectively. The increased solubility in H₂ was attributed to reduction of the titanium ion. The solubility of MgO in A1₂O₃ in vacuum (0.3μ) expressed as the atomic fraction Mg:A1 was measured as 1.10 × loW⁴, 3.00 × 10"⁴, 6.80 × 10–⁴, and 1.40 × 10^-3 at 1530°, 1630°, 1730°, and 183O°C, respectively. These contents did not cause an observable change in lattice parameter, but a slight change was observed when MgO was dissolved in A1₂O₃ in a hydrogen atmosphere.     

Anisotropic Thermal Expansion of β-Ca2SiO4 Monoclinic Crystal

Crystals of β-Ca₂SiO₄ (space group P 12₁/ n 1) were examined by high-temperature powder X-ray diffractometry to determine the change in unit-cell dimensions with temperature up to 645°C. The temperature dependence of the principal expansion coefficients (α_i) found from the matrix algebra analysis was as follows: α₁= 20.492 × 10⁻⁶+ 16.490 × 10⁻⁹ ( T - 25)°C⁻¹, α₂= 7.494 × 10⁻⁶+ 5.168 × 10⁻⁹( T - 25)°C⁻¹, α₃=−0.842 × 10⁻⁶− 1.497 × 10⁻⁹( T - 25)°C⁻¹. The expansion coefficient α₁, nearly along [302] was approximately 3 times α₂ along the b -axis. Very small contraction (α₃) occurred nearly along [     01]. The volume changes upon martensitic transformations of β↔α_L' were very small, and the strain accommodation would be almost complete. This is consistent with the thermoelasticity.     

Correlation between Radiative Transition Probabilities of Nd3+ and Composition in Silicate, Borate, and Phosphate Glasses

Compositional dependence of spontaneous emission probabilities between initial ⁴ F _3/2 and terminal ⁴ I _J J = 9/2, 11/2, 13/2, 15/2) levels of Nd³⁺ were studied for about 90 samples of silicate, borate, and phosphate glasses using the Judd–Ofelt theory. The effect of the covalency of the Nd–O bond on the magnitude of intensity parameters was estimated from the variation of spectral profiles of the ⁴ I _9/2→⁴ G _5/2, ² G _7/2 and ⁴ F _7/2, ⁴ S _3/2 transitions. Intensity parameters Ω₄ and Ω₆ and the spontaneous emission probabilities were strongly affected by the ionic packing ratio of the glass host. The results were discussed in terms of the site selectivity of Nd³⁺ ions in glasses.