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Showing papers on "Tetragonal crystal system published in 1991"


Journal ArticleDOI
29 Aug 1991-Nature
TL;DR: In this article, a series of single-phase, isostructural f.c. superconductors with composition A3C60 (where A is K, Rb, Cs or a mixture of these) were presented.
Abstract: THE discovery of conductivity1 in AxC60 (where A represents an alkali metal) and superconductivity2 in KxC60 has been followed by reports of superconductivity in other alkali-metal-doped full-erides with transition temperatures as high as 33 K (ref. 3). Elucidation of phase diagrams and understanding the relationship between structure and superconducting properties is essential to a detailed understanding of superconductivity in these systems. So far, structural data have been reported only for the non-conducting, intercalated body-centred cubic (b.c.c.) structures A6C60 (where A is K or Cs; ref. 4), a body-centred tetragonal structure for A6C60 (where A is K, Rb, Cs; ref. 5) and the superconducting, intercalated face-centred cubic (f.c.c.) material K3C60 (ref. 6). Here we report the preparation of a series of single-phase, isostructural f.c.c. superconductors with composition A3C60 (where A is K, Rb, Cs or a mixture of these), and show that Tcc increases monotonically with the size of the unit cell. Extended Hiickel band-structure calcula-tions also show a monotonic increase in the density of states at the Fermi level, N(EF), with lattice parameter. The primary impli-cation of these results is that all A3C60 superconductors have the same structure and that changes in Tc can be accounted for by changes in N(EF).

463 citations


Journal ArticleDOI
TL;DR: An overall survey of the structural and magnetic features of the La2NiO4+ delta system is presented as a result of neutron diffraction experiments in this article, where a tentative phase diagram is proposed.
Abstract: An overall survey of the structural and magnetic features of the La2NiO4+ delta system is presented as a result of neutron diffraction experiments. The stoichiometric compound ( delta =0) presents two structural phase transitions. At T0 approximately=770 K, La2NiO4 transforms from tetragonal (I4/mmm) to orthorhombic (Bmab); at T1 approximately=80 K, from orthorhombic to a new tetragonal (P42/ncm) phase. Associated with this second phase transition a strong microstrain produces anisotropic broadening of Bragg reflections. La2NiO4 is three-dimensional (3D) antiferromagnetically ordered at room temperature (TN=330 K). A weak ferromagnetic component appears below T1. Oxygen excess suppress the 3D magnetic ordering and the structural phase transformations, giving rise to a non-stoichiometric compound with interstitial oxygens. A tentative phase diagram is proposed.

365 citations


Journal ArticleDOI
Tsutomu Ikeda1, Hiroshi Satoh1
TL;DR: Hard Ti x N and (Ti 1− x Al x )N films were prepared using the cathodic arc ion plating method as discussed by the authors, and they exhibited a cubic structure.

292 citations


Journal ArticleDOI
K.H.J. Buschow1
TL;DR: A review of the occurrence and composition of ternary rare earth compounds RT12−xMx based on the tetragonal ThMn12 structure is given in this paper, where the important role played by the intersublattice coupling on the magnetocrystalline anisotropy of these materials is stressed.

181 citations


Journal ArticleDOI
TL;DR: In this article, the Ba-Sr-Cu-O system was treated at 6 GPa and 1473 K, and it was shown that the system is free from any trivalent or quadrivalent counter cation in contrast with the known superconducting cupric oxides.
Abstract: Superconductivity with T c 's at 90 K and 60 K has been found in the Ba-Sr-Cu-O system treated at 6 GPa and 1473 K. It should be noticed that the system is free from any trivalent or quadrivalent counter cation in contrast with the known superconducting cupric oxides. It is known that use of high pressure for synthesis tends to stabilize a tetragonal structure of the Ca 0.84 Sr 0.16 CuO 2 type for a wide alkaline earth composition range including Sr 1- x Ba x CuO 2 ( x ⪅0.3), while transmission electron microscopic examinations of the present samples revealed three types of modified structures. We suggest that among these, an incommensurate superlattice of a × a × nc with n ≈9 superconducts, though this phase has not been isolated yet.

154 citations


Journal ArticleDOI
TL;DR: In this article, the authors used X-ray diffraction from a synchrotron source to identify the crystal structures in zirconia ceramics produced by the sol-gel method.
Abstract: X-ray diffraction from a synchrotron source was employed in an attempt to identify the crystal structures in zirconia ceramics produced by the sol-gel method. The particles of chemically precipitated zirconia, after calcination below 600 °C, are very fine, and have a diffracting particle size in the range of 7-15 nm. As the tetragonal and cubic structures of zirconia have similar lattice parameters, it is difficult to distinguish between the two. The tetragonal structure can be identified only by the characteristic splittings of the Bragg profiles from the “c” index planes. However, these split Bragg peaks from the tetragonal phase in zirconia overlap with one another due to particle size broadening. In order to distinguish between the tetragonal and cubic structures of zirconia, three samples were studied using synchrotron radiation source. The results indicated that a sample containing 13 mol% yttria-stabilized zirconia possessed the cubic structure with a0 = 0.51420 ± 0.00012 nm. A sample containing 6.5 mol% yttria stabilized zirconia was found to consist of a cubic phase with a0 = 0.51430 ± 0.00008 nm. Finally, a sample which was precipitated from a pH 13.5 solution was observed to have the tetragonal structure with a0 = 0.51441 ± 0.00085 nm and c0 = 0.51902 ± 0.00086.

147 citations


Journal ArticleDOI
TL;DR: In this paper, the authors review the application of Auger and X-ray photoelectron diffraction at high electron kinetic energies to the problem of structure determination in ultrathin epitaxial overlayers.
Abstract: We review recent work in the application of Auger and X-ray photoelectron diffraction at high electron kinetic energies to the problem of structure determination in ultrathin epitaxial overlayers. These closely-related techniques are based on the fact that outgoing Auger and photoelectrons from single-crystal specimens undergo elastic scattering and interference from near-neighbour atoms in the vicinity of the emitter. Such coherent diffraction leads to large intensity modulations as the detected emission direction is varied with respect to the crystal axes of the specimen. The measured modulations are readily interpreted by means of model quantum mechanical scattering calculation in which atomic coordinates in the epitaxial film are systematically varied. Such analyses provide several kinds of useful information, including growth modes accompanying heteroepitaxy, structural details of alloy and compound formation, and quantitative determination of tetragonal distortion at lattice-mismatched hete...

145 citations


Journal ArticleDOI
TL;DR: In this article, a single-phase tetragonal zirconium oxides have been made by the incorporation of 5.4 mol-% of Y3+ or La3+ in ZrO2 to form solid solutions.

142 citations


Journal ArticleDOI
TL;DR: In this article, light scattering measurements as a function of temperature from 10 to 900 K in the tetragonal and cubic phases of a PbTiO3 crystal were investigated and the soft phonon and relaxation mode were both needed to describe the dynamics of this material.
Abstract: Light scattering measurements are investigated as a function of temperature from 10 to 900 K in the tetragonal and cubic phases of a PbTiO3 crystal. Phonon frequencies, ionic effective charges, spontaneous polarization and contribution to the dielectric permittivity are deduced from these data. The results show that the soft phonon and relaxation mode are both needed to describe the dynamics of this material.

135 citations


Journal ArticleDOI
Wenhe Gong1, H. Yun1, Y.B. Ning1, John E. Greedan1, W.R. Datars1, C. V. Stager1 
TL;DR: The grossly nonstoichiometric perovskites SrTiO3−x with x = 0.28, 0.17, and 0.08 were obtained from a reaction of Sr2TiO4, TiO, and TiO2 at 1500°C as mentioned in this paper.

127 citations


Journal ArticleDOI
TL;DR: In this paper, the rare earth-iron intermetallics of the type RTiFe11 can absorb moderate quantities of nitrogen at 500 °C, giving the approximate composition RTIFe11N0.5 at 1 atm.
Abstract: We have discovered that the rare‐earth–iron intermetallics of the type RTiFe11 can absorb moderate quantities of nitrogen at 500 °C, giving the approximate composition RTiFe11N0.5 at 1 atm. X‐ray diffraction showed that the tetragonal structure (I4/mmm) is retained but that the unit cell volume is slightly increased. More significantly, profound change of magnetocrystalline anisotropy have occurred upon the absorption of nitrogen. In this letter, we report the lattice parameters and magnetocrystalline anisotropic properties of RTiFe11Nx compounds.

Journal ArticleDOI
01 Feb 1991
TL;DR: In this paper, a symmetry analysis of the first-order phase transition in cristobalite has been carried out based on the Landau formalism and projection operator methods, and a Landau free-energy expression is given as well as a listing of the three types of domains expected in α-cristobalite from the β→α transition.
Abstract: Cristobalite, a high temperature phase of silica, SiO2, undergoes a (metastable) first-order phase transition from a cubic, \(Fd\bar 3m\), to a tetragonal, P43212 (or P41212), structure at around 220° C. The cubic C9-type structure for β-cristobalite (Wyckoff 1925) is improbable because of two stereochemically unfavorable features: a 180° Si-O-Si angle and an Si-O bond length of 1.54 A, whereas the corresponding values in tetragonal α-cristobalite are ∼146° and 1.609 A respectively. The structure of the β-phase is still controversial. To resolve this problem, a symmetry analysis of the \(Fd\bar 3m \to P4_3 2_1 2\) (or P41212) transition in cristobalite has been carried out based on the Landau formalism and projection operator methods. The starting point is the ideal cubic (\(Fd\bar 3m\)) C9-type structure with the unit cell dimension a (7.432 A) slightly larger than the known a dimension (7.195 A at 205° C) of β-cristobalite, such that the Si-O-Si angle is still 180°, but the Si-O bond length is 1.609 A. The six-component order parameter driving the phase transition transforms according to the X4 representation. The transition mechanism essentially involves a simultaneous translation and rotation of the silicate tetrahedra coupled along 〈110〉. A Landau free-energy expression is given as well as a listing of the three types of domains expected in α-cristobalite from the β→α transition. These domains are: (i) transformation twins from a loss of 3-fold axes, (ii) enantiomorphous twins from a loss of the inversion center, and (iii) antiphase domains from a loss of translation vectors 1/2 〈110〉 (F→P). These domains are macroscopic and static in α-cristobalite, and microscopic and dynamic in β-cristobalite. The order parameter η, couples with the strain components as ɛη2, which initiates the structural fluctuations, thereby causing the domain configurations to dynamically interchange in the β-phase. Hence, the α-β cristobalite transition is a fluctuation-induced first-order transition and the β-phase is a dynamic average of α-type domains.

Journal ArticleDOI
TL;DR: An ellipsoid shell consisting of 30 linked tetragonal VO5 pyramids surrounding a distorted {V4O4}O4 cube is one way to describe the title compound (right:solid circles, V; empty circles, O).
Abstract: An ellipsoid shell consisting of 30 linked tetragonal VO5 pyramids surrounding a distorted {V4O4}O4 cube-this is one way to describe the title compound (right:solid circles, V; empty circles, O). This cluster is not only of topological interest, however. Its magnetic properties make it a molecular model of the monoclinic and the tetragonal VO2 phase. If the cluster structure is roughly regarded as a cubic close packing of oxygen atoms with vanadium atoms occupying the octahedral holes, then its relationship to the solid is clear.

Journal ArticleDOI
TL;DR: Tetragonal BaTiO3 thin films were prepared directly on Ti metal substrates in Ba(OH)2 solutions by a hydrothermal method at temperatures 400° to 800°C for 5 to 240 min this article.
Abstract: Tetragonal BaTiO3 thin films were prepared directly on Ti metal substrates in Ba(OH)2 solutions by a hydrothermal method at temperatures 400° to 800°C for 5 to 240 min. The film thickness estimated from weight gain of Ti plate was in the range from 0.5 to 2.5 μm, and it increased with increasing treatment temperature, treatment time, and Ba(OH)2 concentration. Rectangular crystals having {100} and {001} faces grew idiomorphically with approximate crystal size of 0.3 to 2.0 μm. The tetragonality of the BaTiO3 films became apparent when the average crystal size exceeded about 1 μm. Lattice parameters of the films were a= 3.994 A, c= 4.035 A, and c/a= 1.010. The films formed above 600°C had preferred orientation showing stronger XRD peaks of h00 and 00l than the other peaks.

Journal ArticleDOI
TL;DR: The cubic (c-ZrO2) and tetragonal zirconia phase stability regions in the system ZrO 2 −Y2O3 −Ta2O5 were delineated in this article.
Abstract: The cubic (c-ZrO2) and tetragonal zirconia (t-ZrO2) phase stability regions in the system ZrO2–Y2O3–Ta2O5 were delineated. The c-ZrO2 solid solutions are formed with the fluorite structure. The t-ZrO2 solid solutions having a c/a axial ratio (tetragonality) smaller than 1.0203 display high fracture toughness (5 to 14 MPa · m1/2), and their instability/transformability to monoclinic zirconia (m-ZrO2) increases with increasing tetragonality. On the other hand, the t-ZrO2 solid solutions stabilized at room temperature with tetragonality greater than 1.0203 have low toughness values (2 to 5 MPa · m1/2), and their transformability is not related to the tetragonality.

Journal ArticleDOI
TL;DR: In this article, the elastic and piezoelectric constants at room temperature have been determined on high-quality monodomain tetragonal BaTiO3 single crystals using Brillouin scattering and ultrasound techniques.
Abstract: From velocity of sound measurements, obtained using Brillouin scattering and ultrasound techniques, the elastic and piezoelectric constants at room temperature have been determined on high‐quality monodomain tetragonal BaTiO3 single crystals. The elastic constants are in fair agreement with those measured previously by the low‐frequency equivalent circuit method. However, the electromechanical properties are significantly different. It is suggested that high frequency relaxation may have an influence on the measurements of those parameters.

Journal ArticleDOI
TL;DR: In this paper, the point group of the Ω phase in an Al-4.0 Cu-0.5 Mg- 0.5 Ag (wt%) alloy was determined using CBED and it was shown that the tetragonal structure of the phase is not hexagonal, monoclinic, or orthorhombic as proposed by others.
Abstract: Convergent-beam electron diffraction (CBED) was employed to determine the point group of the Ω phase in an Al-4.0 Cu-0.5 Mg-0.5 Ag (wt%) alloy. The [100]Ω|〈112〉α CBED pattern had 4 mm symmetry and not 2 mm as expected for the orthorhombic Ω structure. The CBED symmetries along the [010]Ω|〈110〉α and [001]Ω|〈111〉α zone axes were 2 mm and the only point group consistent with these symmetries is 4/mmm. Thus, the crystal structure of the Ω phase is not hexagonal, monoclinic, or orthorhombic as proposed by others, but tetragonal. Electron diffraction patterns of the Ω phase can be indexed on the basis of a tetragonal unit-cell with parameters a = b = 0.6066 nm, c = 0.496 nm. In comparison, the equilibrium θ (Al2Cu) phase in AlCu alloys has a tetragonal structure with unit-cell parameters a = b = 0.6066 nm, c = 0.4874 nm. The 1.76% increase in the c-parameter is sufficient for the tetragonal phase (designated θM) to attain nearly perfect atomic matching on the {111}α planes and still maintain 4-fold symmetry along the [001]θM|〈112〉α direction. In addition, the number of variants, hexagonal morphology and preferred growth of the θM phase along 〈112〉α are consistent with symmetry considerations, the proposed θM crystal structure and experiment TEM observations.

Journal ArticleDOI
TL;DR: In this article, mixed perovskites of high quality have been grown from multicomponent melts using the Czochralski method using a mixture of rare-earth aluminium lattices as host lattices.

Journal ArticleDOI
25 Oct 1991-Science
TL;DR: Powder x-ray diffraction at 300 K on equilibrated samples of several nominal compositions χ in RbχC60 is reported and direct evidence for a dilute fcc doped phase, 0 x c ≤ 1, and for a substoichiometric bcc phase, χ ∼ 5, is presented.
Abstract: Powder x-ray diffraction at 300 K on equilibrated samples of several nominal compositions χ in RbχC60 is reported. In addition to the face-centered cubic (fcc) (χ = 3, superconducting), body-centered tetragonal (χ = 4), and body-centered cubic (bcc) (χ = 6) stoichiometric phases, direct evidence for a dilute fcc doped phase, 0 x c ≤ 1, and for a substoichiometric bcc phase, χ ∼ 5, is presented. In contrast, χ = 3 and χ = 4 appear to be line phases with nearly zero solubility of Rb vacancies and interstitials at 300 K. These results are summarized in a provisional binary phase diagram.

Journal ArticleDOI
TL;DR: The crystal structures of four inclusion compounds of 2,6-dimethylideneadamantane-1,3,5,7-tetracarboxylic acid (4) are described in this paper.
Abstract: The crystal structures of four inclusion compounds of 2,6-dimethylideneadamantane-1,3,5,7-tetracarboxylic acid (4) are described, which involve the following guest species: (a) a mixture of 4-methylpent-3-en-2-one, 2,6-dimethylhepta-2,5-dien-4-one, and mesitylene (condensation products of acetone); (b) mesitylene; (c) a mixture of 4-methylpent-3-en-2-one and mesitylene; (d) (tert-butyl)benzene. In all four cases, the host architectures consist of two interpenetrating super-diamond networks built up by the tetra-acid molecules via pairwise H-bonds between the tetrahedrally directed COOH groups. In the first three cases (tetragonal crystal symmetry), the two diamond-like host lattices interpenetrate symmetrically, in the fourth case (monoclinic) asymmetrically. This asymmetry is brought about by the increased steric bulk of the (tert-butyl)benzene guest molecules. Attempts to enforce an inclusion compound of 4 with a single, extremely hollow diamond-like host lattice by offering still bulkier guest molecules have as yet not been met with success. The generally very high propensity of 4 to form inclusion compounds was envisaged and designed beforehand by appropriate evaluation and modulation of the crystal structure of the parent adamantane-1,3,5,7-tetracarboxylic acid, which represents a fivefold diamond-like self-inclusion compound. Crystals of the free, uncomplexed 4 appear to be extremely unstable and could so far not be obtained. On the other hand, from aqueous solution a very stable monohydrate of 4 may be crystallized (4-H2O), which was also subjected to X-ray analysis. The (triclinic) crystal structure of 4·H2O involves an interesting dichotomy inasmuch its pattern of H-bonding may be rationalized either in terms of a double, cross-linked super-zincblende (sphalerite) architecture, or as a system of porous, puckered 4-connected sheets, which interpenetrate each other pairwise and are cross-linked by the H2O molecules. Various structure and (space group) symmetry characteristics of the supramolecular solid-state complexes reported here are highlighted by pointing out analogies with comparable structures retrieved from the literature.

Journal ArticleDOI
TL;DR: In this paper, a detailed analysis of the crystallographic and magnetic structure of YTiFe 11 N 0.5 is presented, and the relationship of the magnetic properties to the crystal structure is discussed.

Journal ArticleDOI
TL;DR: In this paper, a 3 and 4 mol% yttria-doped tetragonal zirconia polycrystalline (TZP) was fabricated by annealing pressureless-sintered samples in air at ∼ 2100°C for 15 min.
Abstract: Polycrystalline, tetragonal (t′) zirconia samples containing 3 and 4 mol% yttria were fabricated by annealing pressureless-sintered samples in air at ∼ 2100°C for 15 min. The grain size of these fully tetragonal samples was on the order of 100 to 200 μm. Domain structure of the samples and of a 3-mol%-yttria-doped tetragonal zirconia single crystal was examined by transmission optical microscopy under polarized light and by transmission electron microscopy. The orientations of the domain/colony boundaries were in accord with the predictions of group theory. As-polished surfaces of polycrystalline t′ materials showed no monoclinic phase even after 1000 h at 275°C in air. By contrast, conventionally yttria-doped tetragonal zirconia polycrystalline (Y-TZP) ceramics of grain size >0.5 μm showed substantial transformation. Surface grinding enhanced the resistance to degradation of Y-TZP but decreased that of t′ materials. Even then, the t′ materials exhibited better resistance to degradation than the Y-TZP ceramics. Excellent resistance of the t′ materials to low-temperature aging despite a very large grain size and the opposite effect of grinding on phase stability are all explained on the basis of ferroelastic domain structure of these materials.

Journal ArticleDOI
TL;DR: In this article, the electrical properties of thin films of η'−Cu3Si phase with a tetragonal crystal structure were investigated and it was found that electrical transport in these films is very sensitive to oxygen exposure.
Abstract: Electrical properties of thin films of η’‐Cu3Si phase with a tetragonal crystal structure are reported on. Electrical transport in these films is found to be very sensitive to oxygen exposure. Cu3Si reacts with oxygen at room temperature to form both Si and Cu oxides, resulting in high‐room‐temperature (∼60 μΩ cm) and even nonmetallic resistivity. This behavior is contrasted with that of low‐resistivity (∼5 μΩ cm at room temperature) Cu3Ge, which is inert in an oxygen environment.

Journal ArticleDOI
TL;DR: In this article, the authors investigated the evolution of the distortion of several oxide perovskites with increasing pressure, using EXAFS in the diamond anvil cell, and interpreted the distortion to the ratio f of the volumes of the AO12 dodecahedron and the BO6 octahedron, and to the compressibilities of the polyhedra.
Abstract: We have investigated the evolution of the distortion of several oxide perovskites with increasing pressure, using EXAFS in the diamond anvil cell. Cubic perovskite BaZrO3 remains cubic up to 52 GPa. Orthorhombic perovskite CaGeO3 becomes less distorted as pressure increases, becomes tetragonal at about 12 GPa and evolves toward cubic structure, still not obtained at 23 GPa. The distortion of orthorhombic perovskite SrZrO3 first increases with pressure up to 8 GPa, then decreases until the perovskite becomes cubic at 25 GPa. The results are interpreted in terms of a systematics, relating the distortion to the ratio f of the volumes of the AO12 dodecahedron and the BO6 octahedron, and to the compressibilities of the polyhedra. For cubic perovskites, f=5, which may correspond to a situation where the compressibilities of octahedra and dodecahedra are equal.

Journal ArticleDOI
TL;DR: In this paper, the precursors for each phase of zirconias were treated with 1N H2SO4 to produce a sulfated material, and the results reveal that sulfation causes the tetragonal phase to be formed for both types of Zirconia contrary to the data before sulfation, and sulfation increases the crystallization exotherm by 150 °C.
Abstract: Zirconia can be prepared to produce either tetragonal phase or predominantly monoclinic phase upon calcination at 500 °C. The precursors for each phase of zirconias was treated with 1N H2SO4 to produce a sulfated material. The results reveal that sulfation causes the tetragonal phase to be formed for both types of zirconia contrary to the data before sulfation, and sulfation increases the crystallization exotherm by 150 °C.

Journal ArticleDOI
TL;DR: In this paper, Monte Carlo simulations of the observed crystal growth of the protein lysozyme are reproduced by Monte Carlo simulation based on a simple model, where the free parameters of the model are the strengths of the three main types of protein-protein contacts occurring in the P43212 lattice of the tetragonal form of the polygonal form of protein and the equilibrium rate of attachment of molecules to lattice sites.

Journal ArticleDOI
TL;DR: In this article, it was shown that two sets of twin variants grow at the expense of the third set upon application of uniaxial compression at 1000° and 1400°C.
Abstract: Ferroelastic domain switching is one of the possible toughening mechanisms in ceramic materials. Microstructural evidence of domain reorientation (switching) in polydomain tetragonal zirconia single crystals is observed upon the application of a unidirectional compressive stress at 1000° and 1400°C. Dark-field imaging of the three {112{ tetragonal twin variants in a [111] zone indicates that two sets of twin variants grow at the expense of the third set upon application of uniaxial compression. The diminishing variant is the one with its c axis parallel to the compression axis. Inducing cracks into polydomain tetragonal zirconia single crystals provides further evidence of domain reorientation near the crack surface. It is noted again that two sets of twin variants grow at the expense of the third set. A construction for the orientation relationship of domains and their twin boundaries is presented, and a relation between shear stress and reorientation is proposed.

Journal ArticleDOI
TL;DR: In this article, Rietveld analysis of neutron and X-ray powder diffraction data has been used to obtain the crystallinity and relative abundances of cubic (22 at% Y), tetragonal (5.8 at % Y), and monoclinic zirconia in synthetic binary and ternary mixtures of the pure phases.
Abstract: Rietveld analysis of neutron and X-ray powder diffraction data has been used to obtain the crystallinity and relative abundances of cubic (22 at% Y), tetragonal (5.8 at% Y), and monoclinic zirconia in synthetic binary and ternary mixtures of the pure phases. The stabilized cubic and tetragonal forms are shown to be 94 (2) and 96 (2)% crystalline, respectively, but no amorphous material was detected in the chemically pure monoclinic phase. In both stabilized polymorphs, the yttrium atoms randomly substitute into the zirconium site, with a charge-compensating proportion of vacancies on the oxygen atom site. In this near worst-case situation of very similar cubic and tetragonal unit cell dimensions, the phase abundances determined by Rietveld analysis of neutron data are very accurate and superior to those determined by integrated-intensity methods of analysis. For X-ray data, the accuracy is diminished by the presence of extinction, and by uncertainty in the definition of the peak shape of the cubic phase due to the absence of intense high-angle reflections and the resultant dependence on strongly overlapping low-angle data.

Journal ArticleDOI
TL;DR: In this paper, the formation of tetragonal-like mullite coincides with that of the Al-Si spinel, which is determined mainly by the processing conditions of the sol-gel precursors.
Abstract: The mullite formation process in both single phase and diphasic sol-gel precursors to mullite was studied using dynamic x-ray diffraction (DXRD). A metastable, tetragonal-like mullite phase was observed in all the single gels at temperatures from 980 °C to 1200 °C, but not in any of the other precursors. The tetragonal to orthorhombic mullite transformation was very slow as the lattice parameters, a and b, split and moved gradually away from each other as a result of a gradual decrease of alumina content in the mullite solid solution with increasing temperature from 1100 °C to 1200 °C. The formation of tetragonal mullite coincides with that of the Al–Si spinel. The occurrence of tetragonal mullite or the spinel (or both) is determined mainly by the processing conditions of the sol-gel precursors.

Journal ArticleDOI
TL;DR: In this paper, microstructural changes associated with the diffusionless cubic-to-tetragonal phase transition (c-t′) in sintered ZrO2-Y2O3 ceramics with 4 to 20 mol% Y2O 3 were investigated.
Abstract: Microstructural changes associated with the diffusionless cubic-to-tetragonal phase transition (c–t′) in sintered ZrO2–Y2O3 ceramics with 4 to 20 mol% Y2O3 are investigated. Ceramics containing 4 to 7 mol% Y2O3 that are quickly cooled from high temperatures experience cubic-to-tetragonal transformation by a diffusionless mechanism. The TEM diffraction pattern of the transformation product, t′-ZrO2, reveals (112) reflections that are suppressed in the c-ZrO2 phase. The microstructural features of the t′-ZrO2 phase include antiphase boundaries and a twinned substructure in the (112) dark-field images. The (112) reflections result from the displacement of oxygen atoms in the lattice during phase transformation. With increasing Y2O3 content, the antiphase domain size, the tetragonality of the lattice, and the intensity of the (112) reflections decrease. When the Y2O3 content reaches 8 mol% or more, the (112) reflections appear, but the dark-field image shows no antiphase boundary, and the tetragonality decreases to 1. In ceramics with 14 to 20 mol% Y2O3, no suppressed reflection appears because the c–t transformation is fully suppressed. A model of the lattice structure is calculated and the intensity of diffraction determined experimentally; on this basis, the mechanism of diffusionless c–t’transformation is discussed.