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Metastable Phase (metastable + phase)
Selected AbstractsEffect of Oxygen Partial Pressure on the Formation of Metastable Phases from an Undercooled YbFeO3 Melt Using an Aerodynamic LevitatorJOURNAL OF THE AMERICAN CERAMIC SOCIETY, Issue 4 2009Malahalli S. Vijaya Kumar 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 P63cm and P63/mmc, were found through rapid solidification of an undercooled YbFeO3 melt in an atmosphere with reduced Po2. The crystal structure of the as-solidified samples changed from orthorhombic Pbnm to hexagonal P63cm and P63/mmc with decreasing Po2. 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. [source] Solid,Solid Phase Transitions: Interface Controlled Reactivity and Formation of Intermediate StructuresCHEMISTRY - A EUROPEAN JOURNAL, Issue 36 2007Stefano Leoni Dr. Abstract Finding new pathways to novel materials is an open challenge in modern solid-state chemistry. Among the reasons that still prevent a rational planning of synthetic routes is the lack of an atomistic understanding at the moment of phase formation. Metastable phases are, in this respect, powerful points of access to new materials. For the synthetic efforts to fully take advantage of such peculiar intermediates, a precise atomistic understanding of critical processes in the solid state in its many facets, that is, nucleation patterns, formation and propagation of interfaces, intermediate structures, and phase growth, is mandatory. Recently we have started a systematic theoretical study of phase transitions, especially of processes with first-order thermodynamics, to reach a firm understanding of the atomistic mechanisms governing polymorphism in the solid state. A clear picture is emerging of the interplay between nucleation patterns, the evolution of domain interfaces and final material morphology. Therein intermediate metastable structural motifs with distinct atomic patterns are identified, which become exciting targets for chemical synthesis. Accordingly, a new way of implementing simulation strategies as a powerful support to the chemical intuition is emerging. Simulations of real materials under conditions corresponding to the experiments are shedding light onto yet elusive aspects of solid,solid transformations. Particularly, sharp insights into local nucleation and growth events allow the formulation of new concepts for rationalizing interfaces formed during phase nucleation and growth. Structurally different and confined in space, metastable interfaces occurring during polymorph transformations bring about distinct diffusion behavior of the chemical species involved. More generally, stable structures emerge as a result of the concurrence of the transformation mechanism and of chemical reactions within the phase-growth fronts. [source] Formation of Metastable Na2CrO4 -Type LiNiPO4 from a Phosphate,Formate PrecursorEUROPEAN JOURNAL OF INORGANIC CHEMISTRY, Issue 1 2010Violeta Koleva Abstract High-pressure modification of LiNiPO4 with a Na2CrO4 -type structure was obtained at ambient pressure and low temperature from a mixed LiNi,phosphate,formate precursor, LiNiPO4Hx(HCOO)x·yH2O (where x , 1.2 and y , 2.5). The structural and thermal characterization of the precursor and the LiNiPO4 compositions were carried out by powder XRD analysis, IR spectroscopy, and DSC analysis. Thermal treatment of LiNiPO4Hx(HCOO)x·yH2O precursors between 450 and 650 °C yields a mixture of the two structural modifications of LiNiPO4: the Na2CrO4 type and the olivine type. It was established that the obtained Na2CrO4 -type LiNiPO4 is a metastable phase, which completely transforms at 700 °C into the olivine-type phase. The enthalpy of the phase transition is ,H = ,43.40 kJ,mol,1. The mechanism of formation of the two forms of LiNiPO4 from the LiNi,phosphate,formate precursor is discussed. [source] Abinitio structure determination of m -toluidine by powder X-ray diffractionJOURNAL OF APPLIED CRYSTALLOGRAPHY, Issue 3 2004Mwaffak Rukiah The powder X-ray diffraction pattern of the crystalline phase of m -toluidine has been recorded with a sensitive curved detector (CPS120) at 150,K. The structure has been solved by real-space methods (simulated annealing) followed by Rietveld refinements with phenyl rings as rigid bodies and with soft constraints on bond lengths for peripheral atoms. The cell is monoclinic with space group P21/c and Z = 8. Equivalent molecules form chains along c. The crystalline cohesion is achieved by N,H,N hydrogen bonds between neighbouring chains of non-equivalent molecules and by van der Waals interactions of neighbouring chains of equivalent molecules. The hydrogen-bonding network has been confirmed by lattice-energy minimization. Anisotropic strain effects of the cell have been calculated. The directions of the minimal strains correspond to the directions of the hydrogen bonds. An explanation of the difficulty to crystallize the metastable phase is given. [source] Kinetics of decompressional reactions in eclogitic rocks , formation of plagioclase coronas around kyaniteJOURNAL OF METAMORPHIC GEOLOGY, Issue 3 2002D. Nakamura Abstract This paper describes a kinetic study on reaction textures in eclogitic rocks from the Sulu region, eastern China. Some of the eclogitic rocks display a decompressional reaction texture, whereby kyanite grains are surrounded by plagioclase coronas and are never in contact with quartz. The change in mineral parageneses with progress of the reaction was predicted by constructing chemical potential diagrams in a model system. The chemical potential diagrams indicated that the chemical potential of 2Na2O + CaO (2µNa2O + µCaO) in intergranular regions between kyanite and quartz should decrease with decreasing pressure, whereas 2µNa2O + µCaO in intergranular regions between garnet and omphacite should increase with decreasing pressure. Thus, upon decompression, an inequality in chemical potential arises in the rock. To reduce this inequality, garnet and omphacite react to produce amphibole and plagioclase and release Na2O and CaO. Then, the released Na2O and CaO components diffuse into the regions between kyanite and quartz grains and react to produce plagioclase between them. This model also indicates that the chemical potential of SiO2 should decrease around kyanite grains during the progress of the decompressional reaction, and Si-undersaturated conditions should have formed around kyanite grains in spite of the presence of quartz in these eclogitic rocks. Thus, spinel or corundum that are not stable in the system with excess quartz can form as a metastable phase, as observed in eclogitic rocks from the study areas. Phase diagrams in the system with excess quartz should be carefully applied for analysis of such reaction textures. [source] Metastable Phase Formation from an Undercooled Rare-Earth Orthoferrite MeltJOURNAL OF THE AMERICAN CERAMIC SOCIETY, Issue 10 2002Kosuke Nagashio High-speed digital imaging was conducted during the containerless solidification of rare-earth orthoferrites (RE = La, Sm, Dy, Y, Yb, and Lu) with the perovskite structure to determine the metastable phase and elucidate its growth behavior. Observation using a high-speed video camera revealed that the formation of the metastable phase became pronounced, and double recalescence from the metastable phase to a stable phase occurred, as the ionic radius of the rare-earth element decreased. In the present paper, the formation of the metastable phase is discussed systematically in view of the stability of the perovskite structure and the activation energy of nucleation. [source] Structures of stable and metastable Ge2Bi2Te5, an intermetallic compound in a GeTe,Bi2Te3 pseudobinary systemACTA CRYSTALLOGRAPHICA SECTION B, Issue 3 2007Toshiyuki Matsunaga Ge2Bi2Te5 in the GeTe,Bi2Te3 pseudobinary system has two single-crystalline phases: a metastable phase with an NaCl-type structure and a stable phase with a nine-layer trigonal structure. In the metastable phase, the structure consists, in the hexagonal notation, of infinitely alternating stacks of Te and Ge/Bi layers at equal intervals along the c axis. On the other hand, in the stable phase those two layers are stacked alternately nine times to form an NaCl block. The blocks are then piled to construct a nine-layered trigonal structure with cubic close-packed stacking. Both ends of each block are covered with Te layers, contrary to the infinite alternation of Ge/Bi and Te layers in the structure of the metastable phase. The Ge/Bi layers in the metastable phase contain as much as 20,at.,% vacancies; on the other hand, those in the stable phase are filled with atoms. These two crystalline phases in Ge2Bi2Te5 have identical atomic configurations to the two corresponding phases found in Ge2Sb2Te5. [source] Nano-Scale Design of TiAl Alloys Based on ,-Phase Decomposition,ADVANCED ENGINEERING MATERIALS, Issue 5 2006F. Appel Abstract Phase decomposition and ordering reactions in ,/B2-phase containing TiAl alloys were utilized to establish a novel, previously unreported, type of laminate microstructure. The characteristic constituent of this microstructure are laths with a nanometer-scale substructure that are comprised of several stable and metastable phases. Microstructural control can be achieved by conventional thermomechanical processing and leads to a structurally and chemically very homogeneous material with excellent mechanical properties. The physical metallurgy of this novel type of alloy has been assessed by transmission electron microscope investigations and mechanical testing. [source] Effect of Oxygen Partial Pressure on the Formation of Metastable Phases from an Undercooled YbFeO3 Melt Using an Aerodynamic LevitatorJOURNAL OF THE AMERICAN CERAMIC SOCIETY, Issue 4 2009Malahalli S. Vijaya Kumar 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 P63cm and P63/mmc, were found through rapid solidification of an undercooled YbFeO3 melt in an atmosphere with reduced Po2. The crystal structure of the as-solidified samples changed from orthorhombic Pbnm to hexagonal P63cm and P63/mmc with decreasing Po2. 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. [source] Integral modeling approach to study the phase behavior of complex solids: application to phase transitions in MgSiO3 pyroxenesACTA CRYSTALLOGRAPHICA SECTION A, Issue 5 2010Sandro Jahn A combination of electronic structure calculations, classical molecular dynamics simulations and metadynamics is proposed to study the phase behavior of complex crystals. While the former provide accurate energetics for thermodynamic properties, molecular dynamics and metadynamics simulations may reveal new metastable phases and provide insight into mechanisms and kinetics of the respective structural transformations. Here, different simulation methods are used to investigate the polymorphism of MgSiO3 pyroxenes (enstatites) up to high pressures and temperatures. A number of displacive phase transitions are observed within the three basic structure types clino-, ortho- and protoenstatite using classical molecular dynamics simulations. Transitions between these types require a change of stacking order, which is modeled using a combination of molecular dynamics and metadynamics. [source] Towards a generalized vision of oxides: disclosing the role of cations and anions in determining unit-cell dimensionsACTA CRYSTALLOGRAPHICA SECTION B, Issue 3 2010Ángel Vegas Theoretical calculations of the electron-localization function show that, at the volumes of the two CaO phases (rocksalt and CsCl type), the parent Ca structures (fcc: face-centred cubic and sc: simple cubic, respectively) exhibit charge concentration zones which coincide with the positions occupied by the O atoms in their oxides. Similar features, also observed for the pairs Ca/CaF2 and BaSn/BaSnO3, are supported by recent high-pressure experiments as well as electron-localization function (ELF) calculations, carried out on elemental K. At very high pressures, the elemental K adopts the hP4 structure, topologically identical to that of the K atoms in high-pressure K2S and high-temperature ,-K2SO4. Moreover, the ELF for the hP4 structure shows charge concentration (,,2 electrons) at the sites occupied by the S atoms in the high-pressure K2S phase. All these features confirm the oxidation/high-pressure equivalence as well as the prediction of how cation arrays should be metastable phases of the parent metals. For the first time to our knowledge, the structure type, dimension and topology of several oxides and fluorides (CaO, CaF2 and BaSnO3) are explained in univocal physical terms. [source] | |||||||||||||