Time‐Resolved High and Low Temperature Phase Transitions of the Nanocrystalline Cubic Phase in the Y2O3‐ZrO2 and Fe2O3‐ZrO2 System

The nanocrystalline cubic Phase of zirconia was found to be thermally stabilized by the addition of 2.56 to 17.65 mol % Y₂O₃ (5.0 to 30.0 mol % Y, 95.0 to 70.0 mol % Zr cation content). The cubic phase of yttria stabilized zirconia was prepared by thermal decomposition of the hydroxides at 400°C for 1 hr. 2.56 mol % Y₂O₃‐ZrO₂ was stable up to 800°C in an argon atmosphere. The samples with 4.17 to 17.65 mol % Y₂O₃ were stable to 1200°C and higher. All samples at temperatures between 1450°C to 1700°C were cubic except the sample with 2.56 mol % Y₂O₃ which was tetragonal. The crystallite sizes observed for the cubic phase ranged from 50 to 150 Å at temperatures below 900°C and varied from 600 to 800 nm between 1450°C and 1700°C. Control of furnace atmosphere is the main factor for obtaining the cubic phase of Y‐SZ at higher temperature. Nanocrystalline cubic Fe‐SZ (Iron Stabilized Zirconia) with crystallite sizes from 70 to 137 Å was also prepared at 400°C. It transformed isothermally at temperatures above 800°C to the tetragonal Fe‐SZ and ultimately to the monoclinic phase at 900°C. The addition of up to 30 mol % Fe(III) thermally stabilized the cubic phase above 800°C in argon. Higher mol % resulted in a separation of Fe₂O₃. The nanocrystalline cubic Fe‐SZ containing a minimum 20 mol % Fe (III) was found to have the greatest thermal stability. The particle size was a primary factor in determining cubic or tetragonal formation. The oxidation state of Fe in zirconia remained Fe³⁺. Fe‐SZ lattice parameters and rate of particle growth were observed to decrease with higher iron content. The thermal stability of Fe‐SZ is comparable with that of Ca‐SZ, Mg‐SZ and Mn‐SZ prepared by this method.     

Transformation of three-dimensional three-connected silicon nets in SrSi2

Dimorphic SrSi₂ is the first compound for which the two simplest three-dimensional three-connected nets are found in its polymorphs. The cubic net of three-connected silicon atoms (SrSi₂ type of structure) can be transformed into the tetragonal one (α-ThSi₂ type of structure) by a high-pressure-high-temperature treatment. The tetragonal phase is quenchable. Heating of this phase to 600–700°C at ambient pressure results in transformation into the cubic one. At a heating rate of 20°C/min complete transformation can be achieved within 5 min in a DTA apparatus. The energy of transformation has been obtained from the peak areas of the DTA curves to ?1.6 ± 0.3 kcal/mole. Although the transformation between the three-dimensional three-connected sets in SrSi₂ must be formally classified as a reconstructive one, a relatively small entropy change (ΔS = 1 ·₁ cal/deg · mole) has been calculated from the change in molar volume and p-T equilibrium conditions. Therefore, structural relations between the cubic and the tetragonal nets are discussed.     

Oxydes de plomb: V. Etude de la texture des phases quadratique et orthorhombique de Pb3O4: Influence des defauts sur la transition de phase

Several samples of Pb₃O₄ have been prepared by oxidizing PbO in air at various temperatures in the range 705–815°K. A correlation is established between the nonstochiometry of the samples and their X-ray diffraction line profiles at 295°K which are characteristic of an orthorhombic distortion of the tetragonal lattice. In the high-temperature phase (T > 170°K), orthorhombic microdomains exist in the tetragonal matrix. The mean distortion increases with the nonstochiometry of the compound. Below 170°K Pb₃O₄ exhibits an orthorhombic phase with orthorhombic domains according to two orientation states, and para crystalline distortion. A model of texture is proposed and compared with the high-temperature one. The pretransitional effect which is observed between 250 and 170°K is correlated with the presence of orthorhombic microdomains in the high-temperature phase (tetragonal).     

Phase equilibria,thermal analysis,and reactivity of tin tungsten bronzes and related phases

Crystal chemistry and phase relations of the bronze forming region of the SnWO system have been investigated. Above 780°C the tin bronzes Sn_xWO₃ are shown to be thermally unstable and an equilibrium diagram is established at 700°C which shows that the composition limits of the tetragonal phase are 0.21 ? x ? 0.29. Below x = 0.21 a series of single and two phase regions containing orthorhombic bronzes exists for which the composition limits have been established. In the range 0.29 ? x ? 0.76 the system comprises the tetragonal bronze, Sn₂W₃O₈ and SnWO₄, while above 0.76 there is no bronze, only Sn₂W₃O₈, SnWO₄ and free Sn. The phase Sn₂W₃O₈ has been isolated and shown to have a hexagonal unit cell, a = 7.696 Å, c = 18.654 Å. The evidence of differential thermal analysis and X-ray studies suggests that this hexagonal phase arises from the decomposition of the tungsten bronze phase and is itself decomposed to cubic SnWO₄ above 700°C. Small thermal effects observed in the DTA scans of tin-containing tetragonal bronzes are interpreted in terms of an order-disorder phenomenon arising from asymmetric tunnel occupancy by Sn²⁺ ions caused by the presence of the lone pair of electrons. Hydrogen reduction of Sn_xWO₃ has been shown to result in complete removal of oxygen, producing Sn + α-W in the range 600–850°C. Some activation energy data are given for the reduction process.     

Thallium alloys of the rare earth: Sm?TI Phase diagram and molar volumes of the rare earth thallides

The Sm? Tl system has been studied by differential thermal, metallographic and X-ray analyses. The following intermediate phases were observed: Sm₂Tl (decomposes at 1030 ± 10°C); Sm₅Tl₃ (decomposes at 1060 ± 10°C); SmTl (melting point, 1220 ± 20°C); Sm₃Tl₅ (decomposes at 940 ± 10°C); SmTl₃ (melting point, 870 ± 5°C). Three eutectics occur: β-Sm—Sm₂Tl (840 ± 10°C, 18.0 ± 0.5 at % Tl); Sm₃Tl₅—SmTl₃ (860 ± 5°C, 72.0 ± 0.5 at % Tl); SmTl₃—β-Tl (～303°C, greater than 99.5 at % Tl); there is an eutectoidal reaction at 760 ± 10°C and 10 ± 1 at % Tl (decomposition of β-Sm phase). Following crystal structures have been determined or confirmed: Sm₂Tl hexagonal hP6—Ni₂In-type, Sm₅Tl₃ tetragonal tI32—W₅Si₃-type, SmTl cubic cI2—W or cP2—CsCl-type, SmTl tetragonal tP4—AuCu I-type, Sm₃Tl₅ orthorhombic oC32—Pu₃Pd₅ like-type, SmTl₃ cubic cP4—AuCu₃-type. The characteristics of the phase diagram and the molar volumes of the Sm? Tl compounds are compared with those of other RE? Tl alloys and briefly discussed.     

Electrical properties of the α, β, γ, and δ phases of bismuth sesquioxide

Conductivity measurements have been performed on compressed powder specimens of Bi₂O₃ in the temperature region 300–800°C. The conductivity in the β, γ, and δ phases is predominantly ionic. Oxide ions are the mobile charge carriers. The disorder in these phases is intrinsic even when the samples are contaminated with Au or Pt. The conductivity in the α phase is predominantly p type. The disorder in the α phase is extrinsic up to the α → δ transition at 729°C. From 650 to 729°C a rapidly increasing contribution of oxygen vacancies to the conductivity is apparent.     

Time‐Resolved Phase Transitions of the Nanocrystalline Cubic to Submicron Monoclinic Phase in Mn2O3‐ZrO2

The nanocrystalline cubic phase of zirconia was found to be thermally stabilized by the addition of 3 to 40 mol % manganese. The nanocrystalline cubic, tetragonal and monoclinic phases of zirconia stabilized with manganese (III)oxide (Mn‐Stabilized Zirconia) were prepared by thermal decomposition of carbonate and hydroxide precursors. Both the crystallization and isothermal phase transitions associated with Mn‐SZ were studied using high temperature x‐ray diffraction and x‐ray diffraction of quenched samples. Cubic Mn‐SZ initially crystallized and progressively transformed to tetragonal, and monoclinic structures above 700°C. The nanocrystalline cubic Mn‐SZ containing 25 mol % Mn was found to have the greatest thermal stability, retaining its cubic form at temperatures as high as 800°C for periods up to 25 hours. Higher than 40 mol %, cubic Mn₂O₃ was found to coexist with cubic Mn‐SZ. The crystallite sizes observed for the cubic, tetragonal and monoclinic Mn‐SZ phases ranged from 50 to 137, 130 to 220, and 195 to 450 Å respectively, indicating, for ZrO₂, that particle size was a primary factor in determining its polymorphs. The classical Avrami equation for nucleation and growth was applied to the observed phase transformations.     

Die Untersuchung des Phasenübergangs von FeZrF6 mit der Mössbauer-Spektroskopie

FeZrF₆ exhibits a phase transition from the trigonal LiSbF₆ structure type to the cubic high-temperature modification with an ordered ReO₃ lattice at 212.3°K. From the temperature dependence of the quadrupole splitting in the temperature range 212°K ? T ? 4.1°K an axial splitting parameter |Δ₁| = 68 cm^?1 could be deduced, which is valid for T < 50°K and decreases linearly to 27 cm^?1 (210°K) with increasing temperature. There is structural evidence supporting this interpretation and identifying the axial ligand field component as being of trigonal symmetry. Ligand field and EPR spectroscopic results, however, prove the existence of an additional dynamic Jahn-Teller coupling of tetragonal symmetry, which is obviously not seen by Mössbauer spectroscopy.     

High pressure synthesis,crystal growth and magnetic properties of TiOF

Polycrystalline samples of TiOF have been prepared at 1300 °C and 8 GPa, with small single crystals grown at the same conditions. The crystal structure remains tetragonal rutile-type down to at least 90 K (space group P4₂/mnm, a = 4.6533 (2) Å and c = 3.0143 (2) Å at 90 K) and the Ti(O,F)₆ octahedra are slightly compressed, consistent with Jahn-Teller distortion of 3d¹ Ti³⁺. Diffuse scattering reveals disordered structural correlations that may arise from local cis-order of oxide anions driven by covalency. TiOF is paramagnetic down to 5 K and observation of a small paramagnetic moment and a substantial Pauli term indicates that the d-electrons are partially delocalised.     

Processing and Characterization of Sol-Gel Derived Modified PbTiO3 for Ferroelectric Composite Applications

Powders of (Pb_0.8Ca_0.2)(Ti_0.99Mn_0.01)O₃ have been prepared by sol-gel processing. A tetragonal phase is formed after heat treatment at as low as 800°C. The tetragonality was found to be 1.053±0.005 and Curie temperature 315°C. Composite films with 0–3 connectivity were prepared from 800°C heat treated powders and P(VDF-TrFE) by the solvent casting technique. Composites poled at 20 MV/m, exhibited a pyroelectric coefficient of 17.4 μC/m²K and a pyroelectric figure of merit (FOM_p=p/ε) of 0.51 μC/m²K.     

A novel route to perovskite lead zirconate titanate from glycolate precursors via the sol–gel process

A perovskite lead zirconate titanate was synthesized by the sol‐gel process, using lead glycolate, sodium tris(glycozirconate) and titanium glycolate as the starting precursors. For the mole ratio Pb:Zr:Ti of 1:0.5:0.5 [Pb(Zr_0.5Ti_0.5)O₃], TGA‐DSC thermal analysis indicated that the percentage of ceramic yield was 55.8, close to the calculated chemical composition value of 49.5. The exothermic peak occurred at 268 °C below the theoretical Curie temperature of 400 °C. The pyrolysis of Pb(Zr_0.5Ti_0.5)O₃ of the perovskite phase was investigated in terms of calcination temperature and time. The structure obtained was of the tetragonal form when calcined at temperatures below 400 °C; it transformed to the tetragonal and the cubic forms of the perovskite phase on calcination above the Curie temperature, as verified by X‐ray data. The lead zirconate titanate synthesized and calcined at 400 °C for 1 h had the highest dielectric constant, the highest electrical conductivity and the dielectric loss tangent of 10 190, 0.803 × 10^?3 (Ω.m)^?1 and 1.513 at 1000 Hz, respectively. The lead zirconate titanate powder synthesized has potential applications as an electronic material. Copyright © 2008 John Wiley & Sons, Ltd.     

Existiert eine Wurtzit‐Modifikation von Lithiumbromid? — Untersuchungen im System LiBr/LiI —

Is there a Wurtzite‐Modification of Lithium Bromide? — Studies on the System LiBr/LiI — Deposition of mixtures of LiBr/LiI (ratio: LiBr/LiI = 3:1, 2:1, 1:1, 1:2, 1:3, 1:4) and of pure LiI and LiBr from the gas phase onto a sapphire substrate at ‐196 °C in a high vacuum chamber were investigated by means of temperature‐dependent X‐ray diffraction. Below 0 °C LiI crystallizes in the hexagonal Wurtzite‐modification (β‐LiI) with a = 451.4(1) und c = 731.1(2) pm, which transforms into the cubic rock salt modification (α‐LiI, a = 602.57(3) pm) by heating up to room temperature. Co‐depositions of LiBr/LiI formed solid LiBr_1‐xI_x solutions that also crystallize in the Wurtzite‐modification, below room temperature. Compared to β‐LiI, these solid solutions are more stable and transform into the cubic phase at the significantly higher temperature of 80 °C. The lattice constants of LiBr_1‐xI_x with x ≈ 0.7 are a = 445.48(7), c = 719.1(1) pm and with x ≈ 0.4 are a = 431.50(5), c = 691.7(1) pm. The hexagonal phase LiBr_1‐xI_x is observed for the complete series of mixed crystals with 0.25 ≤ x ≤ 0.8. Both cubic phases, α‐LiI and LiBr, show solubilities of up to ca. 10 % of the respective other compound. In case of pure LiBr only the cubic modification (a = 551.54(2) pm, 25 °C) was observed in the complete temperature range (‐196 °C to 25 °C).     

Time‐Resolved Phase Transitions of the Nanocrystalline Cubic to Submicron Monoclinic Phase in Zirconia

The nanocrystalline cubic, tetragonal, and submicron monoclinic phases of pure zirconia were prepared by thermal decomposition of carbonate and hydroxide precursors. The crystallization and isothermal phase transformations of the oxide were studied using high temperature X‐ray diffraction, X‐ray diffraction and Raman spectra of quenched samples. Cubic zirconia formed first, and then progressively transformed to tetragonal and monoclinic phases at temperatures as low as 320°C. The cubic, tetragonal, and monoclinic phases for ZrO₂ were found to be distinct functions of crystallite size, indicating the nanocrystalline nature of these phases. They were found to exist within critical size ranges of 50 to 140 Å, 100 to 220 Å and 190 to 420 Å (±5 Å), respectively. Thus, as the crystallites grow during annealing, they first transform from cubic to tetragonal and then from tetragonal to monoclinic at critical sizes. The classical Avrami equation for nucleation and growth was applied to the tetragonal to monoclinic phase transition.     

Microstructure and microanalysis of bismuth molybdates

The microstructures of commercially important bismuth molybdate catalysts in relation to olefin oxidation reactions are examined by electron microscopy (EM) techniques. The microstructural characterization has been carried out using dynamic (in situ) EM, high resolution EM, and microanalysis. The coprecipitated catalyst system Bi₂MoO₆ or γ, together with the γ phase, contains small amounts of tetragonal Bi₂MoO₆ phase, Bi₂Mo₃O₁₂ (α phase), Bi₂O₃, and MoO₃. In reduction with propylene, at catalyst operating temperatures of 400–500°C, in the dynamic experiments conducted on α- and γ-phase crystallites under reaction conditions no evidence for extended defects such as crystallographic shear planes has been obtained, instead an ordered intermediate phase similar to (101) Bi₂Mo₂O₉ (β phase) is observed which is found to be unstable. Observations by electron microscopy have been confirmed with parallel measurements made in a reactor connected to a gas chromatograph and mass spectrometer system. The possible influence of the microstructural changes on the catalytic behavior of the system is examined.     

Synthesis and characterization of FePt nanoparticles and FePt nanoparticle/SiO2-matrix composite films

Superparamagnetic face-centered cubic (fcc) FePt nanoparticles were synthesized using a polyol process. The effect of reaction temperature and molar ratio of Fe(CO)₅ to Pt(acac)₂ on the structure, composition and morphology of nanoparticles has been investigated. The optimum processing condition has been obtained for producing well-monodisperse fcc-phase FePt nanoparticles with the 2:1?molar ratio of Fe-Pt at 220?°C. In order to circumvent the problem of FePt particle coalescence during high temperature annealing for the L1₀ ordering, FePt nanoparticle/SiO₂-matrix composite films have been fabricated by sol?Cgel method. The experimental results confirm that the amorphous SiO₂ matrix effectively inhibits the grain growth and particle aggregation during 700?°C annealing for 1?h. Well-monodisperse face-centered tetragonal (fct) FePt particles embedded in the SiO₂ matrix can be obtained with the long-range chemical order parameter S of ~0.74, indicating partially ordered L1₀ phase transition in FePt/SiO₂ composite films. The FePt/SiO₂ system exhibits a hysteretic behavior with smaller coercive field of 1,450 Oe. The incomplete phase transition from cubic deredat height maxsium (A ₁-disordered phase to tetragonal L1₀-ordered phase) might be responsible for it.     

Dependence of the properties of Ce-Zr-Y-La-M-O systems on synthetic conditions and on the nature of the transition metal M (Mn,Fe, Co)

The effects of synthetic conditions, component ratios, and the nature of the transition metal on the physicochemical and catalytic properties of Ce-Zr-Y-La-M-O (M = Mn, Fe, Co) systems are studied. The Ce-Zr-Y-La-M-O samples precipitated at ～23°C and calcined at 600°C are single-phase and are solid solutions with a fluorite structure, which persists upon calcination at 1150°C. The Ce-Zr-Y-La-Fe(Co)-O samples precipitated at 70°C and calcined at 1150°C consist of two solid solutions, one cubic, and the other tetragonal. The specific surface area (S _sp) of the samples precipitated at ～23°C and calcined at 600°C increases in the order Ce-Zr-Y-La-O < Ce-Zr-Y-La-Mn-O < Ce-Zr-Y-La-Co-O ≈ Ce-Zr-Y-La-Fe-O. The specific surface area of the samples precipitated at 70°C is independent of M and is ～110 m²/g. Calcination at 1150°C reduces S _sp approximately by two orders of magnitude. The TPR of the unpromoted systems in H₂ proceeds in two steps at 600–650 and 750–840°C. The introduction of M decreases the reduction temperatures and gives rise to a lower temperature peak between 150 and 300°C. The most effective promoter is cobalt. The fluorite-type catalysts containing no noble metal are active in NO reduction (X _NO ≈ 100%) at T _react = 400–450°C. The cobalt-containing catalysts are the most active in the oxidation of CO (X _max = 28%) and hydrocarbons (X _max = 4.3%).     

Thermal stability of Bi<Subscript>2</Subscript>O<Subscript>3</Subscript>

Differential thermal analysis (DTA) and thermogravimetric analysis (TGA) of an α-Bi₂O₃ sample revealed staged phase transitions in the range 720–800°C (at 720, 780, and 800°C) and the elimination of oxygen to the composition Bi₂O_2.967 during heating to 895°C in air at 16 K/min. In dynamic vacuum (p = 1.33 Pa) at 780–800°C, Bi₂O₃ consecutively transforms to a phase with the cubic γ-Bi₂O₃ structure and tetragonal Bi₂O_2.3?2.4. In the latter, electron diffraction in a transmission electron microscope (ED/TEM) shows a superstructure with the superstructure vector q ₁₁₀ ≈ 1/9, which indicates an ordered arrangement of oxygen vacancies.     

A novel route to perovskite lead titanate from lead and titanium glycolates via the sol–gel process

Pure perovskite lead titanate powder (PbTiO₃) is successfully produced via the sol–gel process using lead and titanium glycolates as starting precursors and has been synthesized by the oxide one spot synthesis process. The obtained lead titanate is of the tetragonal form of the perovskite phase, with high purity and nearly zero moisture content. From high‐resolution mass spectra, the XRD technique, Raman‐FTIR and TGA‐DTA analysis, the lead–titanium glycolates undergo sol–gel transition through the formation of Pb? O? Ti bonds. From the SEM micrographs, the PbTiO₃ particle shape transforms from an agglomerate sphere to a needle and fiber‐like shapes as the calcination temperature is varied above T_c. The corresponding molecular structural transformation, from the tetragonal form to the cubic form, occurs at 430 °C. The lead titanate powder calcined at 300 °C for 3 h has the highest dielectric constant and electrical conductivity values, namely 17470 and 1.83 × 10^?3, respectively. Copyright © 2006 John Wiley & Sons, Ltd.     

Temperature effect on hydrothermal synthesis of wairakite and hsianghualite

The zeolite minerals wairakite (Ca₈(Al₁₆Si₃₂O₉₆)x16H₂O) and hsianghualite (Li₁₆Ca₂₄(Be₂₄Si₂₄O₉₆)xF₁₆) were synthesized in a temperature range between 150°C and 500°C by hydrothermal treatment of artificial glasses of the respective same composition at one kbar H₂O pressure. The crystal symmetry of wairakite varied systematically with temperature under the given experimental conditions starting from orthorhombic symmetry at low synthesis temperatures, tetragonal at medium, leading to cubic symmetry at highest zeolite formation temperatures. In contrast, the crystal symmetry of hsianghualite remained cubic in the whole temperature range of synthesis. The crystal sizes varied between 500 nm and 100 μm. The investigations showed the direct correlation between chemical composition of the starting materials and the formed zeolite phase under the given pressure-temperature conditions.     

Synthesis and Characterization of Fluorite-Like Ce-Zr-Y-La-O Systems

Ce-Zr-O and Ce-Zr-Y-La-O materials obtained under various conditions and at varying component ratios are characterized. At Ce/Zr ≈ 1, a tetragonal phase that can hardly be distinguished from a cubic phase by X-ray diffraction forms in the ternary system. Raising the precipitation temperature favors the formation of two-phase systems. Promoting the Ce/Zr = 0.26–0.62 materials with both yttrium and lanthanum favors the formation of a single-phase specimen, namely, a (Ce, Zr, Y, La)O₂ fluorite-like solid solution at 600°C. This structure persists up to 1150°C. The specific surface area of the (Ce, Zr, Y, La)O₂ materials is primarily determined by their calcination temperature: S_sp = 50–80 m²/g at 600°C and 0.6–0.8 m²/g at 1150° C. The specimens calcined at 600°C are mesoporous, with uniformly sized pores of mean diameter 32 ± 2 Å, and have no micropores. According to TPR data, the specimens calcined at 600°C are reduced with hydrogen in two steps that can apparently be interpreted as surface and bulk reduction. The Ce/Zr = 0.26 and 0.40 specimens calcined at 1150°C are reduced in a single step, giving rise to TPR peaks at 707 and 686°C, respectively, and their degree of reduction increases with decreasing Ce/Zr.