Showing papers on "Tetragonal crystal system published in 2001"

Selective Preparation of Monoclinic and Tetragonal BiVO4 with Scheelite Structure and Their Photocatalytic Properties

Abstract: BiVO4 powder with scheelite structure was obtained by hydrolyzing a nitric acid solution of Bi(NO3)3 and Na3VO4 with bases (Na2CO3 and NaHCO3) at room temperature. Tetragonal BiVO4 of a high-temperature form was obtained after 4.5 h of preparation time while monoclinic BiVO4 was done after 46 h. Although the structure and the band gap of tetragonal BiVO4 with scheelite structure were similar to those of monoclinic BiVO4, the photocatalytic activity of the tetragonal BiVO4 for O2 evolution from an aqueous AgNO3 solution under visible light irradiation was negligible. In contrast, the monoclinic BiVO4 showed high photocatalytic activity. Distortion of a Bi−O polyhedron by a 6s2 lone pair of Bi3+ plays an important role for high photocatalytic activity of the monoclinic BiVO4 under visible light irradiation.

New High Temperature Morphotropic Phase Boundary Piezoelectrics Based on Bi(Me)O3?PbTiO3 Ceramics

Abstract: New morphotropic phase boundary (MPB) piezoelectrics, with ferroelectric phase transition (Tc) exceeding that of PbZrO3–PbTiO3 (PZT), were investigated. Based on a perovskite tolerance factor-Tc relationship, new high Tc MPB systems were projected in the Bi(Me)O3–PbTiO3 system, where Me is a relatively large B+3-site cation. For the (1-x)BiScO3–(x)PbTiO3 solid solution, a MPB was found at x-0.64 separating the rhombohedral and tetragonal phases, with correspondingly enhanced dielectric and piezoelectric properties. A transition temperature Tc of ~ 450°C was determined with evidence of Tc's on the order of ≥ 600°C in the BiInO3 and BiYbO3 analogues, though issues of perovskite stability remain for the smaller tolerance end-member systems.

Polarization Rotation via a Monoclinic Phase in the Piezoelectric 92%PbZn1/3Nb2/3O3-8%PbTiO3

Abstract: The origin of ultrahigh piezoelectricity in the relaxor ferroelectric PbZn(1/3)Nb(2/3)O3-PbTiO3 was studied with an electric field applied along the [001] direction. The zero-field rhombohedral R phase starts to follow the direct polarization path to tetragonal symmetry via an intermediate monoclinic M phase, but then jumps irreversibly to an alternate path involving a different type of monoclinic distortion. Details of the structure and domain configuration of this novel phase are described. This result suggests that there is a nearby R-M phase boundary as found in the Pb(Ti,Zr)O3 system.

Investigation of CO and CO2 Adsorption on Tetragonal and Monoclinic Zirconia

Abstract: The adsorption of CO and CO2 on tetragonal and monoclinic ZrO2 was investigated using infrared spectroscopy and temperature-programmed desorption spectroscopy. For this study, samples were prepared of tetragonal ZrO2 (t-ZrO2) with surface areas of 20 and 187 m2/g and of monoclinic ZrO2 (m-ZrO2) with surface areas of 19 and 110 m2/g. The CO2 adsorption capacity of m-ZrO2 is more than an order of magnitude higher than that of t-ZrO2. The principal species observed on m-ZrO2 are HCO3- and m- and b-CO32-, whereas the principal species observed on t-ZrO2 are p- and b-CO3-. The higher CO2 adsorption capacity of m-ZrO2 is attributed to the higher concentration and basicity of the hydroxyl groups on this polymorph, as well as the stronger Lewis acidity of Zr4+ cations and the stronger Lewis basicity of O2- anions. Depending on the adsorption temperature, the CO adsorption capacity of m-ZrO2 is 5- to 10-fold higher than that of t-ZrO2. Below 450 K, CO is adsorbed as HCO3- and CO32- species on both polymorphs of Zr...

Micro-Raman scattering and dielectric investigations of phase transition behavior in the BaTiO3–BaZrO3 system

Abstract: In this study, the phase transition behavior of the BaTiO3–BaZrO3 system was studied using micro-Raman scattering and dielectric measurement techniques. BaZrxTi1−xO3 ceramics were prepared for x=0.00, 0.05, 0.08, 0.15, 0.20, and 1.00 compositions using a solid-state reaction technique. A single-phase perovskite structure of the ceramics was identified by the x-ray diffraction technique. The basic phase transition temperatures in these compositions were studied in the temperature range of 70–575 K. The tetragonal to cubic transition temperature was found to decrease with increasing Zr content. The orthorhombic to tetragonal transition temperature that increases with an initial increase in Zr content merges with the tetragonal–cubic transition for x⩾0.15 compositions. Raman spectra of rhombohedral and orthorhombic phases could not be distinguished. Excellent agreement between the crystallographic transition temperatures obtained by both techniques suggested that Zr substituted octahedra were uniformly distr...

Vibrational and electronic spectroscopic properties of zirconia powders

Abstract: Ten zirconia powders have been characterized by XRD, FT-IR/FT-FIR and Raman skeletal spectroscopies and DR-UV electronic spectroscopy. The vibrational features of a yttria-stabilized cubic zirconia and of tetragonal, monoclinic and tetragonal + monoclinic pure zirconia mixtures have been discussed. Assignments for the vibrational features of cubic and tetragonal samples are proposed. The origin of very broad vibrational peaks in tetragonal zirconia is briefly addressed. The vibrational spectra provide evidence for the presence of trigonal oxide ions in the monoclinic phase only. The UV spectra show that monoclinic zirconia (where the coordination of zirconium is sevenfold) absorbs at lower energy than the cubic and tetragonal phases (where the coordination of zirconium is eightfold). However, it absorbs at higher energy than the perovskite SrZrO3 where Zr adopts octahedral coordination. The origin of the shifts of the corresponding absorption edges is discussed.

Phase Transformation in the Surface Region of Zirconia Detected by UV Raman Spectroscopy

Abstract: The phase transformation of zirconia from tetragonal to monoclinic is characterized by UV Raman spectroscopy, visible Raman spectroscopy, and XRD. Electronic absorption of ZrO2 in the UV region makes UV Raman spectroscopy more sensitive at the surface region than XRD or visible Raman spectroscopy. Zirconia changes from the tetragonal phase to the monoclinic phase with calcination temperatures elevated and monoclinic phase is always detected first by UV Raman spectroscopy for the samples calcined at lower temperatures than that by XRD and visible Raman spectroscopy. When the phase of zirconia changes from tetragonal to monoclinic, the slight changes of the phase at very beginning can be detected by UV Raman spectroscopy. UV Raman spectra clearly indicate that the phase transition takes place initially at the surface regions. It is found that the phase change from tetragonal to monoclinic is significantly retarded when amorphous Zr(OH)4 was agglomerated to bigger particles and the particle agglomeration of ...

Crystallographic symmetry and magnetic structure of CoO

Abstract: The crystallographic symmetry of antiferromagnetic CoO was studied using high-resolution synchrotron powder diffraction in the temperature range 10--300 K. The high-quality powder patterns unambiguously revealed a monoclinic symmetry (space group $C2/m)$ and allowed the extraction of accurate values for the lattice constants. The temperature dependence of the monoclinic deformation scales with the much stronger tetragonal distortion as determined from laboratory x-ray diffraction. Magnetic ordering is associated with a cubic-to-monoclinic transition that is, thus, of first order. Neutron powder-diffraction data are compatible with a collinear magnetic structure with the moments ordered in the monoclinic ac plane.

Miniband formation in a quantum dot crystal

Abstract: We analyze the carrier energy band structure in a three-dimensional regimented array of semiconductor quantum dots using an envelope function approximation. The coupling among quantum dots leads to a splitting of the quantized carrier energy levels of single dots and formation of three-dimensional minibands. By changing the size of quantum dots, interdot distances, barrier height, and regimentation, one can control the electronic band structure of this artificial quantum dot crystal. Results of simulations carried out for simple cubic and tetragonal quantum dot crystal show that the carrier density of states, effective mass tensor and other properties are different from those of bulk and quantum well superlattices. It has also been established that the properties of artificial crystal are more sensitive to the dot regimentation rather then to the dot shape. The proposed engineering of three-dimensional mini bands in quantum dot crystals allows one to fine-tune electronic and optical properties of such nanostructures.

Tetragonal Y-doped zirconia: Structure and ion conductivity.

Abstract: Zirconia is of great industrial importance as support for catalysts and as ion conductor. Especially when mechanical stability is needed in environments with varying temperature the tetragonal modification is superior, because of its high resistivity against physical shocks and thermal stresses. In this study density functional theory is used to investigate the interaction between Y impurities and oxygen vacancies and to develop in this way a structural model for Y-doped zirconia. For every two Y defects an O vacancy is formed at a distance of about $4.11 \AA{}$ to each dopant. An activation energy for ion conductivity of 0.87 eV is calculated in very good agreement with experimental findings.

Stoichiometry and microstructure effects on tungsten oxide chemiresistive films

Abstract: Highly oriented tungsten trioxide thin films have been grown on (0 1 2) r-cut sapphire substrates using reactive rf magnetron sputtering of a tungsten metal target in various oxygen/argon mixtures Parameters for growth of stoichiometric tungsten trioxide (WO 3 ) films were determined by X-ray photoelectron spectroscopy (XPS) In situ four-point Van der Pauw conductivity measurements were performed on as-grown films and after post-deposition annealing to study changes in the oxygen vacancy population Reflection high-energy electron diffraction (RHEED) and X-ray diffraction (XRD) indicate that stoichiometric WO 3 films deposited at 450°C are dominated by the tetragonal phase with (0 0 1) orientation along the growth direction, and films deposited at 650°C possess coexisting (0 0 2), (0 2 0), and (2 0 0) in-plane orientations of the monoclinic phase Tetragonal and monoclinic-phase WO 3 films exhibit a change in conductivity of 01 Ω −1 cm −1 to 20 ppm H 2 S at 250°C, but display different response kinetics

Characterisation of ceria–zirconia solid solutions after hydrothermal ageing

Abstract: Ce1−xZrxO2 (x=0–0.84) solid solutions prepared by co-precipitation were characterised after calcination at 700 or 900°C, and after hydrothermal ageing at 1000 or 1200°C. The solid solutions were formed at 700°C, and crystallise as cubic or tetragonal phases depending on their compositions. Despite the rather high surface areas obtained after calcination at 700°C, the sintering is important at 900°C, and tremendous after hydrothermal ageing at 1000°C. For all compositions between 16 and 83 mol% ceria, complete de-mixing of the solid solutions into two phases was observed after ageing at 1200°C: one Zr-rich, tetragonal phase, and one Ce-rich, cubic phase. XPS and ISS measurements show that the phase separation takes place with surface enrichment in Zr, the Zr-rich phase being formed at the periphery of the particles, whereas the core is composed of the Ce-rich phase.

Highly selective guest uptake in a silver sulfonate network imparted by a tetragonal to triclinic shift in the solid state.

Abstract: The silver sulfonate network presented herein, silver 3-pyridinesulfonate, reversibly and selectively absorbs MeCN while undergoing a major structural rearrangement. The origin of this structural flexibility is a coupling of the weak coordinating ability of the SO3 group with the geometrically pliant silver(I) center. Single crystal and powder X-ray structures of both the desolvated and solvated forms are presented in addition to the mechanism of their reversible interconversion. A heterogeneous gas chromatographic study showing selective extraction of the MeCN is also presented. Extended solid frameworks which reorder to any extent are not common but the structure presented herein transforms from a tetragonal to a triclinic crystal system. The results indicate that cooperative interactions in systems based on supposedly weaker interactions can yield softer yet functional networks with behavior unlike that observed in more rigid inorganic frameworks.

Magnetic and Thermal Properties of CeIrIn5 and CeRhIn5

Abstract: We studied the magnetic properties in a non-magnetic heavy-fermion compound CeIrIn 5 and an antiferromagnetic compound CeRhIn 5 with the tetragonal structure. High-field magnetization of CeIrIn 5 s...

Nucleation and crystal growth in commercial LAS compositions

Abstract: Thermal cycles for crystal nucleation of two commercial compositions of the Li2O·Al2O3·SiO2 system, Ceran® and Robax®, were determined by DTA. Both glasses show two exothermic peaks, the first peak corresponding to crystallisation from the glassy phase and the second one to a phase transformation. Crystalline phases formed on heating up to the first maximum exothermic peak temperature are a lithium alumino-silicate (LixAlxSi1–xO2) and an unidentified phase, which then transform to β-spodumene, tetragonal silica and an unidentified phase at the second maximum exothermic peak temperature. Kinetic parameters for crystallisation were determined. For both glasses, the Avrami parameter (n) calculated from the Ozawa equation is between 1 and 3, indicating that surface and volume crystallisation occur simultaneously. The activation energies calculated from the Kissinger model yield 195.8 kJ/mol for Ceran® and 113.2 kJ/mol for Robax®. For each glass, the thermal cycle for nucleation was optimised by studying the influence of time and temperature on the position of the maximum crystallisation peak temperature. It was observed that sintering completion requires temperatures higher than the crystallisation onsets, hindering powders' full densification by pressureless sintering.

CuMn2O4: properties and the high-pressure induced Jahn-Teller phase transition

Abstract: Single crystal x-ray diffraction, x-ray photoelectron spectroscopy and magnetic susceptibility measurements at normal pressure have shown that, in spite of two Jahn-Teller active ions in CuMn2O4, the crystal is cubic with partly inverse spinel structure, the inversion parameter being \mbox{$x = 0.8$}. The cation configuration at normal pressure was determined as Cu0.2+Mn2+0.8[Cu2+0.8Mn3+0.2Mn4+1.0]O4. The high-pressure behaviour of the crystal was investigated up to 30 GPa using the energy dispersive x-ray diffraction technique and synchrotron radiation. A first-order phase transition connected with a tetragonal distortion takes place at Pc = 12.5 GPa, the c/a ratio being 0.94 at P = 30 GPa. The high-pressure phase has been described in terms of ligand field theory and explained by the changes to the valence and electronic configuration of the metal ions, leading to the formula Cu2+0.2Mn3+0.8[Cu2+0.8Mn3+1.2]O4. The electron configuration of the tetrahedrally coordinated Cu2+ and Mn3+ is (e4)t5 and e2t2, respectively. On the other hand, the electron configuration of Cu2+ located at octahedral sites is (t62g)e3g. While six electrons with antiparallely aligned spins occupy the triplet (t62g), three electrons on the orbital eg can be distributed in two ways (double degeneracy): (dx2-y2)1(dz2)2 and (dx2-y2)2(dz2)1. The first alternative leads to an axially elongated octahedron; the second one causes flattening of the octahedron. The contraction of the c axis indicates, that in the high-pressure phase the second configuration with unpaired electron on the dz2 orbital occurs. A similar effect of the octahedral contraction brings the orbital degeneracy of Mn3+ with the t32ge1g distribution. It follows that at high pressure the ligand field forces the two metals to take the valences that they show in the parent oxides CuO and Mn2O3.

Hydrothermal precipitation and characterization of nanocrystalline BaTiO3 particles

Abstract: Crystalline BaTiO3 powders were precipitated by reacting fine TiO2 particles with a strongly alkaline solution of Ba(OH)2 under hydrothermal conditions at 80°C to 240°C. The characteristics of the powders were investigated by X-ray diffraction, transmission electron microscopy, thermal analysis and atomic emission spectroscopy. For a fixed reaction time of 24 hours, the average particle size of BaTiO3 increased from ∼50 nm at 90°C to ∼100 nm at 240°C. At synthesis temperatures below ∼150°C, the BaTiO3 particles had a narrow size distribution and were predominantly cubic in structure. Higher synthesis temperatures produced a mixture of the cubic and tetragonal phases in which the concentration of the tetragonal phase increased with increasing temperature. A bimodal distribution of sizes developed for long reaction times (96 h) at the highest synthesis temperature (240°C). Thermal analysis revealed little weight loss on heating the powders to temperatures up to 700°C. The influence of particle size and processing-related hydroxyl defects on the crystal structure of the BaTiO3 powder is discussed.

Evidence for another low-temperature phase transition in tetragonal Pb(Zr x Ti 1-x )O 3 (x=0.515,0.520)

Abstract: Results of dielectric and resonance frequency (f r ) measurements below room temperature are presented for Pb(Zr x Ti 1-x )O 3 , x=0.515 and 0.520. It is shown that the temperature coefficient of f r changes sign from negative to positive around 210 and 265 K for x=0.520 and 200 and 260 K for x=0.515. Anomalies in the real part of the dielectric constant (e') are observed around the same temperatures at which the temperature coefficient of f r changes sign because of the electrostrictive coupling between the elastic and dielectric responses. Low-temperature powder x-ray-diffraction (XRD) data, however, reveal only one transition from the tetragonal to monoclinic phase similar to that reported by Noheda et al. [Phys. Rev. B, 61, 8687 (2000)]. Electron-diffraction data, on the other hand, reveal yet another structural transition at lower temperatures corresponding to the second anomaly in the e' vs T and f r vs T curves. This second transition is shown to be a cell-doubling transition not observed by Noheda et al. in their XRD studies. The observation of superlattice reflections raises doubts about the correctness of the Cm space group proposed by Noheda et al. for the monoclinic phase of Pb(Zr x Ti 1-x )O 3 below the second transition temperature.

Structure of tetragonal hen egg-white lysozyme at 0.94 A from crystals grown by the counter-diffusion method.

Abstract: Very high quality crystals of tetragonal hen egg-white lysozyme were grown in the Advanced Protein Crystallization Facility (APCF) on board the Space Shuttle using a modified free-interface diffusion (FID) reactor designed ad hoc to have a longer diffusion path. This design allows the performance of true counter-diffusion experiments. Crystals were obtained under the classical chemical conditions defined 50 y ago with NaCl as a crystallizing agent and acetate pH 4.5 as a buffer. Counter-diffusion crystallization allows a `physical' instead of chemical optimization of growth conditions: indeed, this method screens for the best supersaturation conditions in a single trial and yields crystals of very high quality. A complete diffraction data set was collected at atomic resolution from one of these crystals using synchrotron radiation at the DESY–EMBL beamlines. The overall Rmerge on intensities in the resolution range 31–0.94 A was 5.2% and the data were 98.9% complete. Refinement was carried out with the programs CNS and SHELX97 to a final crystallographic R factor of 12.26% for 72 390 reflections. A mean standard uncertainty in the atomic positions of 0.024 A was estimated from inversion of blocked least-squares matrices. 22 side chains show alternate conformations and the loop 59–75 adopts in the same crystal packing two conformations that were observed for either triclinic or tetragonal lysozyme in previous high-resolution studies. In addition to 255 water molecules, the crystallizing agent (one hexacoordinated sodium ion and five chloride anions) participates in the ordered lysozyme hydration shell.

Structure and the location of the morphotropic phase boundary region in (1-x)[Pb(Mg1/3Nb2/3)O3]-xPbTiO3

Abstract: The structure of (1-x)[Pb(Mg1/3Nb2/3)O3]-xPbTiO3 is tetragonal and rhombohedral for x≥0.35 and x≤0.30, respectively. The intrinsic width of the morphotropic phase boundary region (0.30