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Monoclinic Phase (monoclinic + phase)
Selected AbstractsAtmospheric Pressure Synthesis of Heavy Rare Earth Sesquioxides Nanoparticles of the Uncommon Monoclinic PhaseJOURNAL OF THE AMERICAN CERAMIC SOCIETY, Issue 11 2007Bing Guo We report, for the first time, the atmospheric pressure synthesis of nonagglomerated nanoparticles (20,60 nm in diameter) of the uncommon monoclinic phase of some heavy rare earth sesquioxides RE2O3 (RE=Dy, Ho, Er, Tm, and Yb). The RE2O3 nanoparticles, prepared by a flame synthesis process, were characterized by X-ray diffraction, transmission electron microscopy, and electron energy loss spectroscopy. Monoclinic nanoparticles were formed when the flame temperature was sufficiently high; lower temperatures led to the formation of the normal cubic (C-type) phase. We explain the formation of the uncommon monoclinic phase on the basis of pressure,temperature phase equilibria, and the extra internal pressure induced by surface curvature of the nanoparticles. [source] ChemInform Abstract: Ternary Al,Pd,Co Monoclinic Phases.CHEMINFORM, Issue 24 2002Shaobo Mi Abstract ChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a "Full Text" option. The original article is trackable via the "References" option. [source] Heteroleptic Guanidinate- and Amidinate-Based Complexes of Hafnium as New Precursors for MOCVD of HfO2EUROPEAN JOURNAL OF INORGANIC CHEMISTRY, Issue 11 2010Ke Xu Abstract The synthesis and characterization of four new heteroleptic complexes [Hf{,2 -(iPrN)2CNMe2}2Cl2] (1), [Hf{,2 -(iPrN)2CNMe2}2Me2] (2), [Hf{,2 -(iPrN)2CMe}2Cl2] (3), and [Hf{,2 -(iPrN)2CMe}2Me2] (4) are reported. All the complexes were characterized by spectroscopic methods, while compounds 1,3 were further examined by single-crystal X-ray diffraction, revealing that the complexes are monomers with the hafnium center in a distorted octahedral geometry. The thermal properties of the chlorine-free complexes (2, 4) were examined to determine their suitability for metalorganic chemical vapor deposition (MOCVD) applications, and compound 2 showed good volatility and thermal stability. On the basis of these results, compound 2 was selected for MOCVD of HfO2 with oxygen as oxidant. Depositions were carried out on Si(100) substrates in the temperature range 300,700 °C. The as-deposited HfO2 films crystallized in the monoclinic phase at temperatures above 500 °C, and the composition analysis determined by Rutherford back-scattering (RBS) and X-ray photoelectron spectroscopy (XPS) revealed that the films were stoichiometric and free of carbon. Thus, alkylguanidinatohafnium complex 2 is a promising precursor for growing HfO2 films in a wide temperature range with the desired stoichiometry, because of its adequate volatility, sufficient temperature window between vaporization and decomposition, as well as its ability to decompose cleanly in the presence of oxygen. [source] Martensitic Phase Transformation of Isolated HfO2, ZrO2, and HfxZr1,,,xO2 (0,<,x,<,1) Nanocrystals,ADVANCED FUNCTIONAL MATERIALS, Issue 10 2005J. Tang Abstract We previously reported that, during the reactions to make nanocrystals of HfO2 and Hf-rich HfxZr1,,,xO2, a tetragonal-to-monoclinic phase transformation occurs that is accompanied by a shape change of the particles (faceted spherical to nanorods) when the temperature at which the reaction is conducted is changed from 340,to 400,°C. We now conclude that this concomitant phase and shape change is a result of the martensitic transformation of isolated nanocrystals in a hot liquid, where twinning plays a crucial role in accommodating the shape-change-induced strain. That such change was not observed during the reactions forming ZrO2 and Zr-rich HfxZr1,,,xO2 nanocrystals is attributed to the higher driving force needed in those instances compared to that needed for producing HfO2 and Hf-rich HfxZr1,,,xO2 nanocrystals. We also report here the post-synthesis, heat-induced phase transformation of HfxZr1,,,xO2 (0,<,x,<,1) nanocrystals. As temperature increases, all the tetragonal nanocrystals transform to the monoclinic phase accompanied by an increase in particle size (as evidenced by X-ray diffraction and transmission electron microscopy), which confirms that there is a critical size for the phase transformation to occur. When the monoclinic nanorods are heated above a certain temperature the grains grow considerably; under certain conditions a small amount of tetragonal phase appears. [source] Preparation and microstructure characterization of ball-milled ZrO2 powder by the Rietveld method: monoclinic to cubic phase transformation without any additiveJOURNAL OF APPLIED CRYSTALLOGRAPHY, Issue 5 2002S. Bid The phase transformation kinetics of high-energy ball-milled monoclinic ZrO2 have been studied in detail by Rietveld powder structure refinement analysis. In the present study, no stabilizing compound was required to obtain the cubic phase. The fine-grain powder was milled in a planetary ball mill for up to several hours at different BPMRs (ball to powder mass ratios): 10:1, 20:1, 35:1 and 40:1. During the process of ball milling, the monoclinic phase is gradually transformed to the cubic phase. The relative phase abundances of the respective phases, the particle sizes, the r.m.s. strains, the lattice parameter changes, etc., have been estimated from Rietveld analysis of X-ray powder diffraction data. It has been found that a higher BPMR exerts more influence on rapid phase transformation. In the m - to c -ZrO2 phase transformation, no formation of an intermediate tetragonal ZrO2 phase has been found. The small change in the lattice volume of m -ZrO2, which is very close to the lattice volume of c -ZrO2, caused by ball milling may be attributed to this phase change. The formation of the c phase is noticed, in general, after just 1,h of ball milling, and the particle size of the m phase is reduced to a large extent at the first stage of milling and remains almost unchanged with increasing milling time. However, the particle size of the c phase increases with increasing milling time for the samples milled with higher BPMRs (35:1 and 40:1), suggesting that quenching caused by a high impact energy followed by an annealing effect may play a vital role, which is further manifested in the agglomeration of small particles. [source] Accelerating aging of zirconia femoral head implants: Change of surface structure and mechanical propertiesJOURNAL OF BIOMEDICAL MATERIALS RESEARCH, Issue 2 2007S. Chowdhury Abstract Recently, alternations of zirconia ceramic femoral heads of total hip prostheses during in vivo conditions have caused concern in the medical disciplines regarding phase transformation of zirconia prosthetic components. In this paper, we have investigated the mechanical and structural properties of different laboratory aged zirconia femoral heads and correlated changes in mechanical properties with the phase compositions of the sample. From laser microscope observation, cross-sectional Scanning electron microscopy imaging, and X-ray diffraction analysis on the surface of the zirconia femoral heads, we found monoclinic to tetragonal phase transformation in zirconia prostheses over time during the aging process in the laboratory. Mechanical properties, mainly hardness (H) and Young's modulus (E) values, were measured by nanoindentation technique on the surface of these implants. The results showed that both H and E values decreased with increased monoclinic phase in zirconia, thus confirming a phase transformation over time during aging. © 2006 Wiley Periodicals, Inc. J Biomed Mater Res Part B: Appl Biomater, 2006 [source] Two new paracetamol/dioxane solvates,a system exhibiting a reversible solid-state phase transformationJOURNAL OF PHARMACEUTICAL SCIENCES, Issue 10 2003Ranko M. Vrcelj Abstract This work reports on the crystal structures of two dioxane solvates of paracetamol that are true polymorphs. The high temperature phase is an orthorhombic form, space group Pbca, Z,=,8, a,=,12.6078(3) Å, b,=,12.1129(2) Å, c,=,13.4138(3) Å, V,=,2048.52(7) Å3, (at 295 K) and the low temperature form is monoclinic, space group P21/c, Z,=,4, a,=,12.325(6) Å, b,=,11.965(4) Å, c,=,13.384(6) Å, ,,=,92.01°, V,=,1972.6(14)Å3 (at 123 K). The structures of these polymorphs are described as is the interrelationship between the two structures. In addition to the structural interrelationship, it is shown that the two forms undergo a reversible phase transformation. Desolvation of either form generates the stable monoclinic phase of paracetamol. © 2003 Wiley-Liss, Inc. and the American Pharmacists Association J Pharm Sci 92:2069,2073, 2003 [source] UV Raman spectroscopic study on the phase transformation of ZrO2, Y2O3,ZrO2 and SO42,/ZrO2JOURNAL OF RAMAN SPECTROSCOPY, Issue 5 2002Can Li The phase evolution of zirconia (ZrO2), sulfated zirconia (SO42,/ZrO2) and yttrium oxide incorporated zirconia (Y2O3,ZrO2) from the tetrahedral phase to the monoclinic phase was studied using UV Raman spectroscopy, visible Raman spectroscopy and x-ray diffraction (XRD). It is clearly observed that there are discrepancies between the results from the UV Raman spectra, visible Raman spectra and XRD patterns. The phase change from tetragonal to monoclinic is always earlier or at lower calcination temperatures as observed by UV Raman spectroscopy than by visible Raman spectroscopy and XRD. UV Raman spectroscopy is found to be more sensitive at the surface region while visible Raman spectroscopy and XRD supply the information mainly from the bulk. The inconsistency in the results from the three techniques suggests that the phase transformation of zirconia starts from its surface region and then gradually develops into its bulk. For SO42,/ZrO2 and Y2O3,ZrO2, the transformation from the tetragonal to the monoclinic phase is significantly retarded owing to the presence of the sulfated groups and the yttrium oxide. Particularly, the tetragonal phase of Y2O3,ZrO2 can be maintained up to 800 °C although its phase at the surface region changed into monoclinic at 500 °C. Copyright © 2002 John Wiley & Sons, Ltd. [source] Reproducible Solvent,Thermal Synthesis, Controlled Microstructure, and Photoluminescence of REPO4:Eu3+, Tb3+ (RE=Y, La, and Gd) NanophosphorsJOURNAL OF THE AMERICAN CERAMIC SOCIETY, Issue 8 2010Xiuzhen Xiao By altering the temperature and solvents, we have synthesized hydrated and dehydrated Eu3+/Tb3+ -doped REPO4 (RE=Y, La, and Gd) nanophosphors via a solvo-thermal technology. X-ray powder diffraction and scanning electronic microscopy reveal that they have different structures and different morphologies. REPO4 prepared under subsequent heating at 80°, 120°, and 160°C for 1 day, respectively, present larger particle size than that formed by heating at 160°C for 3 days. Moreover, at the same temperature of 160°C and pH 3, three different solvents (ethanol, N,N -dimethylformamide (DMF), and water) were used, whose influence on the microstructure of LaPO4 has been examined. As a result, LaPO4 samples from anhydrous ethanol solvent show a pure hexagonal phase and nanowire morphology, just like that prepared from the water solvent. On the other hand, the microstructure of LaPO4 samples from DMF,H2O-mixed solvents have been changed: with the increasing volume ratio of DMF to H2O, the crystal phase of LaPO4 has been changed from hexagonal phase to monoclinic phase and the morphology from nanowires to nanoparticles. Finally, the photoluminescence properties of these Eu3+ (Tb3+)-activated rare earth phosphates have been investigated, indicating that the photoluminescent behavior are related to their crystal phases and microstructures. [source] Low-Temperature Aging Behavior of Alumina-Toughened ZirconiaJOURNAL OF THE AMERICAN CERAMIC SOCIETY, Issue 11 2008Jens Schneider The corrosion of alumina-toughened zirconia (ATZ) as a consequence of hydrothermal treatment was investigated, especially the transformation kinetics from tetragonal zirconia to the monoclinic phase. For this purpose, polished ATZ specimens (Ra<5 nm) were aged in water vapor at different temperatures ranging from 70° to 134°C. The fraction of the monoclinic phase was determined using X-ray diffraction and Rietveld refinement. The isothermal transformation curves obtained were fitted to the Mehl,Johnson,Avrami equation by least squares. An Arrhenius plot of the fitted transformation rates was used to determine the activation energy and the pre-exponential factor. Following this procedure, the kinetic parameters of the phase transformation were extrapolated down to body temperature and the formation of the monoclinic phase was simulated. In addition, optical interferometry on well-polished specimens (Ra<2 nm) was alternatively used to calculate the monoclinic fraction from the histogram dataset. The results agree very well with those of the X-ray measurements. Additionally, the development of surface roughness with increasing aging time is discussed. [source] Atmospheric Pressure Synthesis of Heavy Rare Earth Sesquioxides Nanoparticles of the Uncommon Monoclinic PhaseJOURNAL OF THE AMERICAN CERAMIC SOCIETY, Issue 11 2007Bing Guo We report, for the first time, the atmospheric pressure synthesis of nonagglomerated nanoparticles (20,60 nm in diameter) of the uncommon monoclinic phase of some heavy rare earth sesquioxides RE2O3 (RE=Dy, Ho, Er, Tm, and Yb). The RE2O3 nanoparticles, prepared by a flame synthesis process, were characterized by X-ray diffraction, transmission electron microscopy, and electron energy loss spectroscopy. Monoclinic nanoparticles were formed when the flame temperature was sufficiently high; lower temperatures led to the formation of the normal cubic (C-type) phase. We explain the formation of the uncommon monoclinic phase on the basis of pressure,temperature phase equilibria, and the extra internal pressure induced by surface curvature of the nanoparticles. [source] Solid-Solution Effects of a Small Amount of Nickel Oxide Addition on Phase Stability and Mechanical Properties of Yttria-Stabilized Tetragonal Zirconia PolycrystalsJOURNAL OF THE AMERICAN CERAMIC SOCIETY, Issue 3 2003Hiroki Kondo Stability and mechanical properties of the tetragonal phase were investigated for NiO-doped yttria-stabilized tetragonal zirconia polycrystal (Y-TZP) systems. Only 0.3 mol% of NiO in solid solution could be added to the Y-TZP while maintaining the tetragonal phase. Fracture toughness improved remarkably on addition of a small amount of NiO. Raman spectroscopy analysis around cracks introduced by Vickers indentation revealed that the amount of monoclinic phase transformed from tetragonal phase was increased. It was confirmed that fracture toughness improvement was due not only to increased grain size, but also to Y-TZP destabilization by solid solution of NiO. [source] Crystallite and Grain-Size-Dependent Phase Transformations in Yttria-Doped ZirconiaJOURNAL OF THE AMERICAN CERAMIC SOCIETY, Issue 2 2003Arun Suresh In pure zirconia, ultrafine powders are often observed to take on the high-temperature tetragonal phase instead of the "equilibrium" monoclinic phase. The present experiments and analysis show that this observation is one manifestation of a much more general phenomenon in which phase transformation temperatures shift with crystallite/grain size. In the present study, the effect of crystallite (for powders) and grain (for solids) size on the tetragonal , monoclinic phase transformation is examined more broadly across the yttria,zirconia system. Using dilatometry and high-temperature differential scanning calorimetry on zirconia samples with varying crystallite/grain sizes and yttria content, we are able to show that the tetragonal , monoclinic phase transformation temperature varies linearly with inverse crystallite/grain size. This experimental behavior is consistent with thermodynamic predictions that incorporate a surface energy difference term in the calculation of free-energy equilibrium between two phases. [source] Zirconia-Based Metastable Solid Solutions through Self-Propagating High-Temperature Synthesis: Synthesis, Characterization, and Mechanistic InvestigationsJOURNAL OF THE AMERICAN CERAMIC SOCIETY, Issue 8 2000Filippo Maglia Cubic Zr1,xMexOy(Me = Fe, Co, Ni, Cu) metastable solid solutions with metal content significantly higher than equilibrium levels have been synthesized by the self-propagating high-temperature synthesis method based on a thermite reaction between metallic zirconium and the transition-metal oxides CoO, Fe2O3, CuO, and NiO. Through in situ XRD analysis, it was determined that when heated to 1100°C, the cubic solid solution transformed to the tetragonal phase with the concomitant formation of iron oxide. When cooled to lower temperatures, the tetragonal phase transformed to the monoclinic phase at or below 500°C. Results of auxiliary experiments strongly suggest that the formation of the solid solution takes place behind the combustion front by a reaction between zirconia and the metal. [source] Raman tensor analysis of baddeleyite single-crystal and its application to define crystallographic domains in polycrystalline zirconiaPHYSICA STATUS SOLIDI (B) BASIC SOLID STATE PHYSICS, Issue 2 2010Kyoju Fukatsu Abstract The angular dependence of polarized Raman intensity for the Ag and Bg modes was investigated and the full set of Raman tensor elements defined for a baddeleyite single-crystal, namely the monoclinic polymorph of zirconia (ZrO2). Based on the quantitative knowledge of the tensor elements, a method has been proposed for the determination of unknown crystallographic textures in monoclinic zirconia. An application of this method is also shown, which consists of a Raman analysis of crystal orientation on the microscopic scale in polycrystalline ZrO2 after its tetragonal-to-monoclinic (t,m) polymorphic transformation (i.e., occurred under an externally applied stress field). This working example not only confirms the well-known phenomenon of stress-induced phase transformation in polycrystalline zirconia, but also proves the existence of textured domain patterns in the monoclinic phase on a scale larger than that of individual grains. This finding might suggest that the structural and functional properties of polycrystalline zirconia after partial phase transformation should be reinterpreted with taking into account a crystallographic reorientation effect. [source] Analysis of phase transition and expansion behaviour of Al2(WO4)3 by temperature-regulated X-ray diffractionPHYSICA STATUS SOLIDI (B) BASIC SOLID STATE PHYSICS, Issue 11 2008Takuya Hashimoto Abstract X-ray diffraction measurements of Al2(WO4)3 were carried out under various temperatures from 123 K to 373 K. Diffraction patterns above 273 K and below 253 K could be successfully explained as orthorhombic Pbcn (No. 60) and monoclinic P21/n (No. 14) with superstructure, respectively, which were in agreement with the results of convergent-beam electron diffraction. Thermal expansion of low-temperature monoclinic phase and slight shrinkage of high-temperature orthorhombic phase were observed from the temperature dependence of lattice constants and molar volume, showing agreement with the result of dilatometry. It is revealed that abrupt expansion of Al2(WO4)3 at the structural phase transition is attributed to an abrupt increase of the c -axis, which is parallel to the 21 screw axis and perpendicular to the n-glide plane. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Phase transitions and transport phenomena in Li0.25Cu1.75Se superionic compoundPHYSICA STATUS SOLIDI (B) BASIC SOLID STATE PHYSICS, Issue 15 2004M. Kh. Abstract Phase transformation points in Li0.25Cu1.75Se mixed electronic,ionic conductor have been determined by calorimetric, conductometric and thermoelectric measurements. The phase transformation (PT) from triclinic to monoclinic occurs at 403,413 K. At 503,515 K the monoclinic phase is followed by a rhombohedral modification. Both of these PTs are accompanied by drops on the calorimetric curve. At about 653 K observed anomalies in the temperature dependencies of the ionic conductivity, of the chemical diffusion coefficient and the jump of the ionic Seebeck coefficient have been induced by the PT to hexagonal phase. Neutron diffraction studies reveal the cubic structure of Li0.25Cu1.75Se compound (with space group Fm3m) at 773 K. The corresponding PT causes anomalies in the electrical and diffusion properties at 703,713 K. Cu ions are statistically distributed over tetrahedral and trigonal voids in an Fm3m cage; lithium ions randomly occupy 32(f) positions. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Pressure-induced phase transitions in l -alanine, revisitedACTA CRYSTALLOGRAPHICA SECTION B, Issue 4 2010N. A. Tumanov The effect of pressure on l -alanine has been studied by X-ray powder diffraction (up to 12.3,GPa), single-crystal X-ray diffraction, Raman spectroscopy and optical microscopy (up to ,,6,GPa). No structural phase transitions have been observed. At ,,2,GPa the cell parameters a and b become accidentally equal to each other, but without a change in space-group symmetry. Neither of two transitions reported by others (to a tetragonal phase at ,,2,GPa and to a monoclinic phase at ,,9,GPa) was observed. The changes in cell parameters were continuous up to the highest measured pressures and the cells remained orthorhombic. Some important changes in the intermolecular interactions occur, which also manifest themselves in the Raman spectra. Two new orthorhombic phases could be crystallized from a MeOH/EtOH/H2O pressure-transmitting mixture in the pressure range 0.8,4.7,GPa, but only if the sample was kept at these pressures for at least 1,2,d. The new phases converted back to l -alanine on decompression. Judging from the Raman spectra and cell parameters, the new phases are most probably not l -alanine but its solvates. [source] Structural study of KxNa1,,,xNbO3 (KNN) for compositions in the range x = 0.24,0.36ACTA CRYSTALLOGRAPHICA SECTION B, Issue 1 2009D. W. Baker The structure of the A -site substituted perovskite KxNa1,,,xNbO3, x = 0.24,0.36, where a phase boundary was previously reported, has been determined by high-resolution X-ray powder and neutron powder diffraction studies. The structure of the x = 0.3 compound was refined in the monoclinic space group Pm at 293,K and in P4mm at 523,K. The Glazer tilt system of the room-temperature monoclinic phase is a0b+c0, which has implications for the nature of the next symmetry change with composition towards pure potassium niobate. A phase-coexistence region at the transition between monoclinic and tetragonal phases was also identified, consistent with a first-order phase boundary. There is also evidence for an intermediate oxygen-octahedra tilted tetragonal phase. [source] Incommensurate structure of InAl1,,,xTixO3,+,x/2 [x = 0.701,(1)]: comparison between modulated and composite modelsACTA CRYSTALLOGRAPHICA SECTION B, Issue 4 2008P. J. Bereciartua The structure of the monoclinic phase of the compound InAl1,,,xTixO3,+,x/2 with x = 0.701,(1) has been analyzed within the (3,+,1)-dimensional superspace formalism. Two different models were refined describing the structure as an incommensurate modulated layer and modulated composite, respectively. Both models include the same composition,structure relation. In the composite approach it is derived from the mismatching between the two subsystems. In the incommensurate modulated system, it is derived from a closeness condition between O atomic domains. The distribution and coordination of the cations is discussed and compared with previously proposed models for similar compounds. [source] X-ray structure determination of the monoclinic (121,K) and orthorhombic (85,K) phases of langbeinite-type dithallium dicadmium sulfateACTA CRYSTALLOGRAPHICA SECTION B, Issue 6 2000A. Guelylah The structures of the monoclinic and the orthorhombic phases of type I langbeinite Tl2Cd2(SO4)3 have been determined at 121 and 85,K, respectively, by X-ray diffraction. A precise analysis of these structures shows the existence of some differences compared to langbeinites of type II. The monoclinic structure differs very little from the high-temperature cubic structure and the distortion relating the monoclinic structure to the cubic one is very small. SO4 tetrahedra seem to rotate under orthorhombic symmetry in the monoclinic phase. A symmetry distortion analysis of the ferroelectric monoclinic distortion discloses the importance of the secondary modes with orthorhombic symmetry, especially for the O atoms of the SO4 groups. [source] Redetermination of hydronium perchlorate at 193 and 293,KACTA CRYSTALLOGRAPHICA SECTION C, Issue 9 2003Azhar A. Rahman A sample of hydronium perchlorate, H3O+·ClO4,, crystallized from ethanol at ambient temperature, was found to be orthorhombic (space group Pnma) at both 193 and 293,K, with no phase transition observed in this temperature range. This contrasts with the earlier observation [Nordman (1962). Acta Cryst. 15, 18,23] of a monoclinic phase (space group P21/n) at 193,K for crystals grown at that temperature from perchloric acid. The hydronium and perchlorate ions lie across a mirror plane but it is not possible to define at either temperature a simple description of the H-atom positions due to the three-dimensional tumbling of the hydronium cation. [source] Deposition of HfO2, Gd2O3 and PrOx by Liquid Injection ALD Techniques,CHEMICAL VAPOR DEPOSITION, Issue 3 2005J. Potter Abstract Thin films of hafnium oxide (HfO2), gadolinium oxide (Gd2O3), and praseodymium oxide (PrOx) have been deposited by liquid injection atomic layer deposition (ALD) and for comparison, have also been deposited by "thermal" metal-organic (MO) CVD using the same reactor. The ALD-grown films were deposited on Si(100) over a range of substrate temperatures (150,450,°C) using alternate pulses of [Hf(mmp)4], [Gd(mmp)3], or [Pr(mmp)3] (mmp = OCMe2CH2OMe) and water vapor. X-ray diffraction (XRD) analysis showed that as-grown films of HfO2 were amorphous, but these crystallized into the monoclinic phase after annealing in air at 800,°C. XRD analysis showed that as-grown Gd2O3 and PrOx films had some degree of crystallinity. Residual carbon (0.8,3.3 at.-%) was detected in the HfO2 and PrOx films by Auger electron spectroscopy (AES), but not in the Gd2O3 films. The self-limiting behavior of the precursors was investigated at 225,°C by varying the volume of precursor injected during each ALD cycle and, in each case, oxide growth was not fully self-limiting. We propose a mechanism for this involving ,-hydride elimination of the mmp group, and also propose some general mechanistic principles which may influence the growth of oxides by ALD using other precursors. [source] On the tetragonality of the room-temperature ferroelectric phase of barium titanate, BaTiO3JOURNAL OF APPLIED CRYSTALLOGRAPHY, Issue 3 2009Dean S. Keeble The room-temperature phase of the important ferroelectric material barium titanate, BaTiO3, was re-investigated by single-crystal X-ray diffraction on a sample grown by the top-seeded solution growth method, with the intention of demonstrating once again that the structure has tetragonal symmetry consistent with the space-group assignment P4mm and thus resolving recent controversy in the scientific community and literature [Yoshimura, Kojima, Tokunaga, Tozaki & Koganezawa (2006). Phys. Lett. A, 353, 250,254; Yoshimura, Morioka, Kojima, Tokunaga, Koganezawa & Tozaki (2007). Phys. Lett. A, 367, 394,401]. To this end, the X-ray diffraction pattern of a small (341,µm3) sample of top-seeded solution-grown BaTiO3 was measured using an Oxford Diffraction Gemini CCD diffractometer employing Mo,K, radiation and an extended 120,mm sample-to-detector distance. More than 104 individual diffraction maxima observed out to a maximum resolution of 0.4,Å were indexed on two tetragonal lattices. These were identical to within the standard deviations on the lattice parameters and were related to each other by a single rotation of 119.7° about the [11] direction of the first tetragonal lattice (the major twin component), although the actual twinning operation that explains the observed diffraction pattern both qualitatively and quantitatively is shown to be conventional 90° twinning by the m[101] operation. Importantly, it is not necessary to invoke either monoclinic symmetry or a coexistence of tetragonal and monoclinic phases to explain the observed diffraction data. [source] Phase stability in ferroelectric bismuth titanate: a first-principles studyACTA CRYSTALLOGRAPHICA SECTION A, Issue 3 2008Anurag Shrinagar Experimental data on the structure of ferroelectric oxide bismuth titanate suggest two different kinds of structures, i.e. orthorhombic and monoclinic. Density-functional-theory-based (DFT) first-principles calculations have been performed to determine the most stable structure of bismuth titanate among experimentally observed structures. Orthorhombic and monoclinic phases are optimized to zero pressure and lattice parameters were determined as a = 5.4370, b = 5.4260, c = 32.6833,Å and Z = 4 for the structure with space group B2cb, and a = 5.4289, b = 5.4077, c = 32.8762,Å, , = 90.08° and Z = 4 for the structure with space group B1a1. Static and relaxation calculations show that the monoclinic structure with space group B1a1 is the most stable structure. [source] Structures of K0.05Na0.95NbO3 (50,300,K) and K0.30Na0.70NbO3 (100,200,K)ACTA CRYSTALLOGRAPHICA SECTION B, Issue 3 2009N. Zhang Rietveld refinement using neutron powder diffraction data is reported for the potential lead-free piezoelectric material KxNa1,,,xNbO3 (x = 0.05, x = 0.3) at low temperatures. The structures were determined to be of rhombohedral symmetry, space group R3c, with the tilt system a,a,a, for both compositions. It was found that some of the structural parameters differ significantly in the two structures, and particularly the NbO6 octahedral strains as a function of temperature. The 300,K profile for K0.05Na0.95NbO3 shows the coexistence of rhombohedral and monoclinic phases, which indicates that the phase boundary is close to room temperature; the phase boundary for K0.30Na0.70NbO3 is found to be at approximately 180,K. [source] | |||||||||||||