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Perovskite Structure (perovskite + structure)
Kinds of Perovskite Structure Selected AbstractsChemInform Abstract: Copper-Rich Framework Sulfides: A4Cu8Ge3S12 (A: K, Rb) with Cubic Perovskite Structure.CHEMINFORM, Issue 38 2010Ren-Chun Zhang Abstract The new title compounds (IV) are isotypic and crystallize in the cubic space group Fmc with Z = 2. [source] Water Splitting into H2 and O2 over Ba5Nb4O15 Photocatalysts with Layered Perovskite Structure Prepared by Polymerizable Complex Method.CHEMINFORM, Issue 52 2006Yugo Miseki Abstract ChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 200 leading journals. To access a ChemInform Abstract, please click on HTML or PDF. [source] Nd2Ba2CaZn2Ti3O14.4: A New High Dielectric Constant Oxide Having a Disordered (Cubic) Perovskite Structure.CHEMINFORM, Issue 36 2003Pika Jha Abstract For Abstract see ChemInform Abstract in Full Text. [source] Role of oxygen vacancies in the coloration of 0.65Pb(Mg1/3Nb2/3)O3 -0.35PbTiO3 single crystalsCRYSTAL RESEARCH AND TECHNOLOGY, Issue 5 2007Zhiguo Xia Abstract The coloration and oxygen vacancies in 0.65Pb(Mg1/3Nb2/3)O3 -0.35PbTiO3 (PMN-PT(65/35)) (starting composition) single crystals grown by a so-called modified Bridgman technique were investigated in this paper. Light yellow and dark brown colored crystals were generally observed for the typical as-grown PMN-PT(65/35) single crystals. X-ray diffraction results demonstrated that they were both of pure perovskite structure, but good electric properties were only obtained for the light yellow crystal. X-ray photoelectron spectroscopy (XPS) was used to investigate the electronic structure of its components. The O 1s photoelectron spectra of the dark brown colored crystals located at the higher binding energy side, which meant the existence of the more oxygen vacancies. It accordingly led to the formation of the low valence cations associated with the coloration of the crystals, which is also testified by the obtained X-ray photoelectron spectra of Ti and Nb. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Crystal growth and structural refinement of NaMn7O12CRYSTAL RESEARCH AND TECHNOLOGY, Issue 10-11 2005E. Gilioli Abstract We report the crystal growth and the structural refinement of NaMn7O12, a manganite having a double perovskite structure. As in many similar compounds, there is coexistence of Mn3+ and Mn4+ but in this material they orderly occupy different sites for crystallographic reasons. Therefore, this peculiar structure can be considered as a model system for studying complex mechanisms such as charge, orbital and spin ordering. High purity bulk samples and "large" single crystals are needed to study tiny modifications in the crystallographic and magnetic structures associated to the ordering phenomena. Almost single phase (more than 96% pure) and single crystals (up to about 150 µm) of NaMn7O12 were synthesized by solid state reaction under pressure in a multi-anvil apparatus. Single crystal x-ray diffraction and SEM analysis have been used to characterize the crystals. The structure refinement indicates that NaMn7O12 crystallizes in the cubic Im3 space group, with a = 7.312 Å and Z = 2. Further studies are in progress to optimize the synthesis conditions, in order to grow larger crystals. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Sc3AlN , A New PerovskiteEUROPEAN JOURNAL OF INORGANIC CHEMISTRY, Issue 8 2008Carina Höglund Abstract Sc3AlN with perovskite structure has been synthesized as the first ternary phase in the Sc,Al,N system. Magnetron sputter epitaxy at 650 °C was used to grow single-crystal, stoichiometric Sc3AlN(111) thin films onto MgO(111) substrates with ScN(111) seed layers as shown by elastic recoil detection analysis, X-ray diffraction, and transmission electron microscopy. The Sc3AlN phase has a lattice parameter of 4.40 Å, which is in good agreement with the theoretically predicted 4.42 Å. Comparisons of total formation energies show that Sc3AlN is thermodynamically stable with respect to all known binary compounds. Sc3AlN(111) films of 1.75 µm thickness exhibit a nanoindentation hardness of 14.2 GPa, an elastic modulus of 249 GPa, and a room-temperature electrical resistivity of 41.2 µ, cm. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2008) [source] Ordered Ferroelectric Lead Titanate Nanocellular Structure by Conversion of Anodic TiO2 NanotubesADVANCED MATERIALS, Issue 30 2009Jan M. Macak The novel synthesis of ferroelectric perovskite PbTiO3 layers is reported. For that, anodic self-organized TiO2 nanotubes are used as a template for deposition of Pb inside the nanotubes. Upon thermal annealing, the filled template is converted to desired perovskite structure with nanocellular architecture. This approach could be advantageously used for synthesis of other piezoelectric or composite materials. [source] Phase Relations in the Na0.5Bi0.5TiO3,Li3xLa(2/3),x,(1/3),2xTiO3 SystemINTERNATIONAL JOURNAL OF APPLIED CERAMIC TECHNOLOGY, Issue 2 2009Matja, Spreitzer The phase relations and the mechanism of solid-state synthesis for the Na0.5Bi0.5TiO3,Li3xLa(2/3),x,(1/3),2xTiO3 system were investigated using X-ray powder diffraction, scanning electron microscopy, and thermal analysis. The study revealed that the extent of the homogeneity range,which is related to the A-site substitution between (Na0.5Bi0.5)2+ and (Li3xLa(2/3),x,(1/3),2x)2+ pseudo cations of a perovskite structure,depends strongly on the ordering of the (Li3xLa(2/3),x,(1/3),2x)2+ species. The solid-state reaction of the compounds in the homogeneity range is completed only after multiple high-temperature firings. However, the system is also subjected to a slow thermal decomposition; this is particularly so for the compounds with a high × value and an increased Li3xLa(2/3),x,(1/3),2xTiO3 concentration. [source] Multiferroic Properties of Nanocrystalline PbTiO3 CeramicsJOURNAL OF THE AMERICAN CERAMIC SOCIETY, Issue 8 2010Min Wang Nanocrystalline PbTiO3 (PTO) powders in a perovskite structure have been synthesized by the sol,gel process using lead acetate, glycerin, and titanium (IV) isopropoxide as precursors. PTO ceramics were obtained by sintering the powders at temperatures ranging from 600° to 1000°C. The structure and morphology of the ceramics have been determined by X-ray diffraction, transmission electron microscopy, and field emission scanning electron microscopy. The PTO powder calcined at 450°C shows weak ferromagnetism at room temperature. The PTO ceramics sintered at various temperatures exhibit coexistence of ferroelectricity and weak ferromagnetism at room temperature. Enhancement in the magnetic moment and ferroelectricity with a reduction in the grain size of PTO ceramics was observed. This result facilitates the possibilities of new perovskite electromagnetic devices at the nanoscale level. [source] Electrode Properties of the Ruddlesden,Popper Series, Lan+1NinO3n+1 (n=1, 2, and 3), as Intermediate-Temperature Solid Oxide Fuel CellsJOURNAL OF THE AMERICAN CERAMIC SOCIETY, Issue 8 2010Suguru Takahashi The Ruddlesden,Popper phases, Lan+1NinO3n+1 (n=1, 2, and 3), were synthesized by a solid-state reaction for use as cathodes in an intermediate-temperature (500°,700°C) solid oxide fuel cell. The samples crystallized into an orthorhombic layered perovskite structure. The overall electrical conductivity increased with the increase of n in the intermediate temperature range. Single test-cells, which consisted of samarium-oxide-doped ceria (SDC; Sm0.2Ce0.8Ox) as an electrolyte, Ni,SDC cermet (Ni,SDC) as an anode, and Lan+1NinO3n+1 as a cathode, were fabricated for measurements of cell performance at 500°,700°C. Current interruption measurements revealed that both the ohmic and overpotential losses at 700°C decreased with the increase of n. La4Ni3O10 was found to exhibit the best cathode characteristics in the Lan+1NinO3n+1 series. Maximum test-cell power densities with La4Ni3O10 (n=3) were 10.2, 36.5, and 88.2 mW/cm2 at 500°, 600°, and 700°C, respectively. [source] Orientation-Control Synthesis of KTa0.25Nb0.75O3 NanorodsJOURNAL OF THE AMERICAN CERAMIC SOCIETY, Issue 3 2010Yong-Ming Hu Single-crystalline KTa0.25Nb0.75O3 (KTN) nanorods with an orthorhombic perovskite structure were synthesized via a polymer-assisted hydrothermal method. The preferred crystallographic orientation of the nanorods were found to be controllable by using different types of polymers in the process; that is, [001]-oriented KTN were obtained upon the use of polyvinyl alcohol (PVA) and [110]-oriented ones were formed with the use of a combination of polyacrylic acid and PVA. The two types of nanorods showed different properties. The [110]-oriented nanorods exhibited a better photocatalytic decolorization efficiency but very likely a lower thermal conductivity (as indicated by the Raman spectrum measurements) than the [001] nanorods. [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] Photoluminescence of High-Aspect-Ratio PbTiO3 Nanotube ArraysJOURNAL OF THE AMERICAN CERAMIC SOCIETY, Issue 11 2008Yang Yang High-aspect-ratio PbTiO3 nanotube arrays have been synthesized by the hydrothermal method. X-ray diffraction result shows that PbTiO3 nanotube arrays have a tetragonal perovskite structure without any other impurity. The photoluminescence property of PbTiO3 nanotube arrays studied at room temperature reveals a strong green emission band peaking at 550 nm (2.25 eV). Both Raman spectroscopy and X-ray photoelectron spectroscopy demonstrate the existence of local defects in PbTiO3 nanotube arrays, which act as emission source and result in the photoluminescence behavior of PbTiO3 nanotube arrays. [source] Structural and Electrical Properties of Er2O3 -Doped Na1/2Bi1/2TiO3 Lead-Free PiezoceramicsJOURNAL OF THE AMERICAN CERAMIC SOCIETY, Issue 11 2007Mengjia Wu Sodium bismuth titanates Na1/2Bi1/2TiO3 (NBT) doped with 0,3 wt% Er2O3 were prepared by the conventional solid-state reaction method. The X-ray diffraction results revealed that the sintered Er-doped NBT ceramics exhibited a pure perovskite structure with Er3+ concentrations ranging from 0 to 1 wt%. At a low Er2O3 concentration, the Er-doped NBT ceramics showed enhanced electrical properties with dielectric constant ,33T/,0=636, a low dielectric dissipation factor (tan ,=3.3%), a low coercive field (Ec=4.56 kV/mm), and a high piezoelectric constant (d33=75 pC/N). The relationship between the composition and properties of Er-doped NBT ceramics has been discussed. [source] Structure and Dielectric Properties of Pb(Sc2/3W1/3)O3,Pb(Zr/Ti)O3 RelaxorsJOURNAL OF THE AMERICAN CERAMIC SOCIETY, Issue 11 2004Pavol Juhás The structure and dielectric properties of (1,x)Pb(Sc2/3W1/3)O3,(x)Pb(Zr/Ti)O3 ceramics have been investigated over a full substitution range. All compositions with x < 0.5 adopt a cubic perovskite structure; however, for x, 0.25 a doubled cell results from a 1:1 ordered distribution of the B-site cations. The structural order in Pb(Sc2/3W1/3)O3 (PSW) can be described by a random-site model with one cation site occupied by Sc3+ and the other by a random distribution of (Sc1/33+W2/36+). The ordering is destabilized in solid solutions of PSW with PbZrO3 (PSW,PZ), but stabilized by PbTiO3 in the (1,x)PSW,(x)PT system. The changes in order are accompanied by alterations in the dielectric response of the two systems. For PSW,PZ the temperature of the permittivity maximum (T,,max) increases linearly with x; however, for PSW,PT T,,max decreases in the ordered region (up to x= 0.25) and then increases rapidly as the order is lost. Similar effects were produced by modifying the degree of order of (0.75)PSW,(0.25)PT; when the order parameter was reduced from ,1.0 to ,0.65, T,,max increased by more than 60°C. [source] A-Site and B-Site Order in (Na1/2La1/2)(Mg1/3Nb2/3)O3 PerovskiteJOURNAL OF THE AMERICAN CERAMIC SOCIETY, Issue 5 2004Rachel Tarvin (Na1/2La1/2)(Mg1/3Nb2/3)O3 undergoes a series of phase transitions that involve cation order on the A- and B-sites of the parent perovskite structure. At high temperatures both sites contain a random distribution of cations; below 1275°C a ,111, layering of Mg and Nb leads to the formation of a 1:2 ordered structure with a monoclinic supercell. A second transition was observed at 925°C, where the Na and La cations order onto alternate A-site positions along the ,001, direction of the parent subcell. By quenching samples from above 1275°C to preserve the disorder on the B-site, a fourth variant of this compound was obtained by inducing A-site order through a subsequent anneal at 900°C. Although the changes in structure do not produce significant alterations in the relative permittivity (,r, 35), they do have a significant effect on the value of the temperature coefficient of the capacitance. [source] Phase Composition and Vaporization Study of LaGa1,xAlxO3, 0 ,x, 1, and La0.9Sr0.1Ga0.8,xAlxMg0.2O2.85, x= 0.1, 0.2, 0.3JOURNAL OF THE AMERICAN CERAMIC SOCIETY, Issue 11 2003Aleksandra Matraszek The vaporization of the LaGa1,xAlxO3 solid solution, 0 ,x, 1, of the perovskite structure, was investigated using Knudsen effusion mass spectrometry in the temperature range of 1623,1928 K. The partial pressures of the gaseous species O2, Ga, GaO, Ga2O, and LaO were determined for the samples investigated. The equilibrium partial pressures were used for the computation of thermodynamic activities of Ga2O3 and La2O3 at 1800 K. Thermodynamic activities of Al2O3 were obtained using Gibbs,Duhem integration. Gibbs energies of formation of the solid-solution LaGa1,xAlxO3 resulted from the thermodynamic activities of the oxide components. Samples of the chemical composition La0.9Sr0.1Ga0.8,xAlxMg0.2O2.85, x= 0.1, 0.2, 0.3, were of practical importance in solid oxide fuel cell (SOFC) technology and also were investigated. The influence of the aluminum concentration in the perovskite phase on the thermodynamic activity and volatility of Ga2O3 is presented and implications for the potential use of this material in SOFC technology are discussed. [source] Barium Holmium Zirconate, A New Complex Perovskite Oxide: I, Synthesis, Characterization, and Potential Use as a Substrate for High-Critical-Temperature SuperconductorsJOURNAL OF THE AMERICAN CERAMIC SOCIETY, Issue 10 2002Rajan Jose Barium holmium zirconate, a new complex perovskite ceramic oxide, has been synthesized through liquid-phase sintering for the first time. The conventional solid-state reaction method using constituent oxides and carbonates was found to be inadequate for the synthesis of Ba2HoZrO5.5 material. During high-temperature annealing, the development of stable BaZrO3 and BaHoO2.5 phases prevented the formation of Ba2HoZrO5.5 as a single-phase material, even at 1650°C. However, an addition of a small amount of CuO (1 wt%) in the reaction mixture has resulted in the formation of an ordered complex perovskite Ba2HoZrO5.5 phase during the heating process. The structure of Ba2HoZrO5.5 was studied by X-ray diffraction and found to have a cubic perovskite structure with a lattice constant of a= 8.482 Å. Dielectric constant and loss factor values of Ba2HoZrO5.5 are also in the range suitable for use as a substrate for microwave applications. The X-ray diffraction and resistivity measurements have shown that there is no detectable chemical reaction in YBa2Cu3O7,,,Ba2HoZrO5.5 and Bi(2212),Ba2HoZrO5.5 composites, even under extreme processing conditions. Dip-coated and melt-textured YBa2Cu3O7,, and Bi(2212) thick films developed on polycrystalline Ba2HoZrO5.5 gave zero-resistivity transition temperatures of Tc(0) = 92 and 85 K, respectively. [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] Relationship between the Bond Valence and the Temperature Coefficient of the Resonant Frequency in the Complex Perovskite (Pb1,xCax)[Fe0.5(Nb1,yTay)0.5]O3JOURNAL OF THE AMERICAN CERAMIC SOCIETY, Issue 1 2001Heung Soo Park The temperature coefficient of the resonant frequency (TCF) of complex perovskite (Pb1,xCax)[Fe0.5(Nb1,yTay)0.5]O3 ceramics (x= 0.5, 0.55; 0.0 ,y, 1.0) was investigated, relative to the bond valence of the A- and B-site ions in the ABO3 perovskite structure (such as the barium-, strontium-, and calcium-based complex perovskites). The TCF of these complex perovskite compounds varied with the bond valence of the A- and B-sites and the tolerance factor (t) in the perovskite structure. In the tilted region (t < 1.0), the tilting of the oxygen octahedra increased and the TCF decreased, because of the increased bond valence of the B-site. Also, the dependence of TCF on the bond valence of the A-site was similar to its dependence on t. [source] Microstructure, dielectric, and piezoelectric properties of 0.38Bi(Gax Sc1,x)O3,0.62PbTiO3 high temperature piezoelectric ceramicsPHYSICA STATUS SOLIDI - RAPID RESEARCH LETTERS, Issue 1 2008Yihang Jiang Abstract 0.38Bi(Gax Sc1,x)O3,0.62PbTiO3 (BGSPTx) ceramics have been prepared by using the conventional mixed oxide method. X-ray diffraction analysis revealed that BGSPTx has a pure perovskite structure, and the crystal symmetry of BGSPTx changed from rhombohedral to tetragonal with increasing Ga content (x). The Curie temperature (TC) of BGSPTx ceramics is in the range of 448,467 °C for different x. The ferroelectric phase transition of BGSPTx was found to be of the first order type according to the Curie,Weiss law. For x = 0.125, BGSPTx ceramics show enhanced piezoelectric properties: piezoelectric constant d33 = 420 pC/N and d31 = ,142 pC/N, planar and thickness electromechanical coupling factors kp = 56.27% and kt = 56.00%, respectively. The high- TC of BGSPTx coupled with its excellent piezoelectric properties suggests those future high-temperature applications. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Microstructure and electrical properties of (1,x)(K0.5Na0.5)NbO3,x BiFeO3 piezoelectric ceramicsPHYSICA STATUS SOLIDI (A) APPLICATIONS AND MATERIALS SCIENCE, Issue 5 2008Xiang Li Abstract Lead,free ceramics (1,x)(K0.5Na0.5)NbO3,x BiFeO3 doped with 1 mol% Fe2O3 (KNNBF/x) have been synthesized by pressureless sintering. With the Fe2O3 doping, the KNNBF/x ceramics can be well sintered at 1085,1100 °C and exhibit a pure perovskite structure with x < 0.013. It was found that the crystal structure of the KNNBF/x ceramics changed from orthorhombic to tetragonal and then to pseudocubic phase with the increase of BiFeO3 content. The composition KNNBF/0.013 near the tetragonal symmetry that separates the orthorhombic and pseudocubic phases exhibits improved electrical properties: d33 = 173 pC/N, kp = 0.40, ,r = 905, tan , = 4%, Pr = 26 ,C/cm2, and Ec = 11.2 kV/cm, with a high Curie temperature (TC) of about 388 °C. Our results suggest that KNNBF/x are promising lead-free high temperature piezoelectric ceramics. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Preparation and optical properties of amorphous and crystalline BLT thin films grown on SiO2/Si(100) substrates by a CSD processPHYSICA STATUS SOLIDI (A) APPLICATIONS AND MATERIALS SCIENCE, Issue 5 2004Ziping Cao Abstract Amorphous and crystalline Bi3.25La0.75Ti3O12 thin films deposited on SiO2/Si(100) substrates have been prepared by a chemical solution deposition (CSD) process. X-ray diffraction shows that the crystal structure is a bismuth-layered perovskite structure with some preferred (117) orientation. The refractive indices and extinction coefficients of the amorphous and crystalline films were obtained by spectroscopic ellipsometry as a function of photo energy in the range from 1.8 to 4.0 eV. The dispersion of refractive indices was fitted by the Wemple,Didomenico single electronic oscillator dispersion mode. An interesting exponential absorption was found in the amorphous and crystalline BLT films, which may be ascribed to the substitution of lanthanum for bismuth. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Radiation amorphization of orthoferrite YFeO3PHYSICA STATUS SOLIDI (A) APPLICATIONS AND MATERIALS SCIENCE, Issue 2 2003Yu. G. Chukalkin Abstract Polycrystalline orthoferrite YFeO3 samples irradiated by fast neutron fluence 3 × 1024 m,2 were investigated by the X-ray and magnetic methods. It has been shown experimentally that the irradiation of the perovskite-structure oxide leads to its full amorphization and, as a consequence, to the magnetic transformation antiferromagnetic , spin glass. The authors believe that the amorphization is the consequence of the instability of the perovskite structure to radiation disordering. (© 2003 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Structural analysis of lithium lanthanum titanate with perovskite structurePHYSICA STATUS SOLIDI (C) - CURRENT TOPICS IN SOLID STATE PHYSICS, Issue 5 2009Koji Ohara Abstract Neutron and high-energy X-ray diffraction analysis of polycrystalline La4/3-xLi3xTi2O6 have been performed to clarify the extent of disorder of the distribution of La and Li ions and to understand the relation of these distributions to ionic conduction. The distributions of the La and Li ions in a 10 × 10 × 20 cubic box (i.e., 10 × 10 × 10 unit cell) super-structure, in which Ti and O atoms are fixed onto their regular sites, were obtained by the reverse Monte Carlo (RMC) structural modelling of both diffraction data sets. When the occupancy of La ions in the planes perpendicular to the c-axis is analysed, one can find a La-rich and La-poor layers alternating, which is consistent with the results of earlier Rietveld analysis (Stramare et al., Chem. Mater. 15, 3974 (2003) [1]). Of particular interest, the Li ions are found mainly on the interstitial sites between the O-3 triangle plane of the TiO6 octahedron and a La ion, which is different from the earlier work (Yashima et al., J. Am. Chem. Soc. 127, 3491 (2005) [2]). (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Investigation of the structure and phase transitions in the novel A-site substituted distorted perovskite compound Na0.5Bi0.5TiO3ACTA CRYSTALLOGRAPHICA SECTION B, Issue 2 2002G. O. Jones Rietveld neutron powder profile analysis of the compound Na0.5Bi0.5TiO3 (NBT) is reported over the temperature range 5,873,K. The sequence of phase transitions from the high-temperature prototypic cubic structure (above 813,K), to one of tetragonal (673,773,K) and then rhombohedral structures (5,528,K) has been established. Coexisting tetragonal/cubic (773,813,K) and rhombohedral/tetragonal (with an upper temperature limit of 145,K between 528 and 673,K) phases have also been observed. Refinements have revealed that the rhombohedral phase, space group R3c, with aH = 5.4887,(2), cH = 13.5048,(8),Å, V = 352.33,(3),Å3, Z = 6 and Dx = 5.99,Mg,m,3, exhibits an antiphase, a,a,a, oxygen tilt system, , = 8.24,(4)°, with parallel cation displacements at room temperature. The tetragonal phase, space group P4bm, with aT = 5.5179,(2), cT = 3.9073,(2),Å, V = 118.96,(1),Å3, Z = 2 and Dx = 5.91,Mg,m,3, possesses an unusual combination of in-phase, a0a0c+ oxygen octahedra tilts, , = 3.06,(2)°, and antiparallel cation displacements along the polar axis. General trends of cation displacements and the various deviations of the octahedral network from the prototypic cubic perovskite structure have been established and their systematic behaviour with temperature is reported. An investigation of phase transition behaviour using second harmonic generation (SHG) to establish the centrosymmetric or non-centrosymmetric nature of the various phases is also reported. [source] Phosphate tungsten bronze series: crystallographic and structural properties of low-dimensional conductorsACTA CRYSTALLOGRAPHICA SECTION B, Issue 5 2001P. Roussel Phosphate tungsten bronzes have been shown to be conductors of low dimensionality. A review of the crystallographic and structural properties of this huge series of compounds is given here, corresponding to the present knowledge of the different X-ray studies and electron microscopy investigations. Three main families are described, monophosphate tungsten bronzes, Ax(PO2)4(WO3)2m, either with pentagonal tunnels (MPTBp) or with hexagonal tunnels (MPTBh), and diphosphate tungsten bronzes, Ax(P2O4)2(WO3)2m, mainly with hexagonal tunnels (DPTBh). The general aspect of these crystal structures may be described as a building of polyhedra sharing oxygen corners made of regular stacking of WO3 -type slabs with a thickness function of m, joined by slices of tetrahedral PO4 phosphate or P2O7 diphosphate groups. The relations of the different slabs with respect to the basic perovskite structure are mentioned. The structural description is focused on the tilt phenomenon of the WO6 octahedra inside a slab of WO3 -type. In this respect, a comparison with the different phases of the WO3 crystal structures is established. The various modes of tilting and the different possible connections between two adjacent WO3 -type slabs involve a great variety of structures with different symmetries, as well as the existence of numerous twins in MPTBp's. Several phase transitions, with the appearance of diffuse scattering and modulation phenomena, were analysed by X-ray scattering measurements and through the temperature dependence of various physical properties for the MPTBp's. The role of the W displacements within the WO3 -type slabs, in two modulated structures (m = 4 and m = 10), already solved, is discussed. Finally, the complexity of the structural aspects of DPTBh's is explained on the basis of the average structures which are the only ones solved. [source] Effects of Barium Substitution on Perovskite Formation, Dielectric Properties, and Diffuseness Characteristics of Lead Zinc Niobate CeramicsJOURNAL OF THE AMERICAN CERAMIC SOCIETY, Issue 7 2000Byung-Yong Ahn Perovskite-phase developments in partially/fully substituted (Ba,Pb)(Zn1/3Nb2/3)O3, synthesized via B-site precursor routes, were investigated. The pyrochlore structure, present at no barium content, was immediately replaced by perovskite with a small amount (,8 at.%) of barium substitution. Variations in the lattice parameters of the pyrochlore and perovskite structures were studied. The weak-field radio-frequency dielectric constants and losses of the system ceramics were examined. A highest maximum dielectric constant of 9000 (76°C, 1 MHz) was observed at 8 at.% barium substitution. The dielectric-constant spectra were analyzed further in terms of diffuseness characteristics (e.g., diffuseness exponent and degree of diffuseness). The dielectric hysteresis loops of the ceramics were measured. The microstructures of the sintered ceramics also were observed. [source] Symmetry rules and strain/order-parameter relationships for coupling between octahedral tilting and cooperative Jahn,Teller transitions in ABX3 perovskites.ACTA CRYSTALLOGRAPHICA SECTION B, Issue 2 2009Space groups, order-parameter and strain/order-parameter coupling relationships in ABX3 perovskite structures which combine cooperative Jahn,Teller distortions and octahedral tilting have been investigated from the perspective of group theory using the computer program ISOTROPY. Two common Jahn,Teller ordering schemes are associated with the irreducible representations and of the space group . A third, less-common ordering scheme is associated with . These combine with tilting instabilities associated with and to generate a predicted suite of Jahn,Teller structure types that includes many of the known structures of manganites, vanadates, Cu and Cr halides. Order-parameter coupling and possible phase transitions are described using Landau free-energy expansions, and general expressions for the relationships between symmetry-adapted spontaneous strains and particular order-parameter components are presented. These provide a general formal framework for determining structural evolution across multi-component order-parameter space and for characterizing the influence of tilting instabilities on Jahn,Teller instabilities or of Jahn,Teller ordering on octahedral tilting. [source] Metal,Organic Perovskites: Synthesis, Structures, and Magnetic Properties of [C(NH2)3][MII(HCOO)3] (M=Mn, Fe, Co, Ni, Cu, and Zn; C(NH2)3= Guanidinium)CHEMISTRY - A EUROPEAN JOURNAL, Issue 44 2009Ke-Li Hu Abstract We report the synthesis, crystal structures, and spectral, thermal, and magnetic properties of a family of metal,organic perovskite ABX3, [C(NH2)3][MII(HCOO)3], in which A=C(NH2)3 is guanidinium, B=M is a divalent metal ion (Mn, Fe, Co, Ni, Cu, or Zn), and X is the formate HCOO,. The compounds could be synthesized by either diffusion or hydrothermal methods from water or water-rich solutions depending on the metal. The five members (Mn, Fe, Co, Ni, and Zn) are isostructural and crystallize in the orthorhombic space group Pnna, while the Cu member in Pna21. In the perovskite structures, the octahedrally coordinated metal ions are connected by the anti,anti formate bridges, thus forming the anionic NaCl-type [M(HCOO)3], frameworks, with the guanidinium in the nearly cubic cavities of the frameworks. The Jahn,Teller effect of Cu2+ results in a distorted anionic Cu,formate framework that can be regarded as Cu,formate chains through short basal CuO bonds linked by the long axial CuO bonds. These materials show higher thermal stability than other metal,organic perovskite series of [AmineH][M(HCOO)3] templated by the organic monoammonium cations (AmineH+) as a result of the stronger hydrogen bonding between guanidinium and the formate of the framework. A magnetic study revealed that the five magnetic members (except Zn) display spin-canted antiferromagnetism, with a Néel temperature of 8.8 (Mn), 10.0 (Fe), 14.2 (Co), 34.2 (Ni), and 4.6,K (Cu). In addition to the general spin-canted antiferromagnetism, the Fe compound shows two isothermal transformations (a spin-flop and a spin-flip to the paramagnetic phase) within 50,kOe. The Co member possesses quite a large canting angle. The Cu member is a magnetic system with low dimensional character and shows slow magnetic relaxation that probably results from the domain dynamics. [source] | ||||||||||||||||