Showing papers on "Tetragonal crystal system published in 1994"

Effect of Dopants on Zirconia Stabilization—An X‐ray Absorption Study: I, Trivalent Dopants

Abstract: Local atomic structures of Zr and dopant cations in zirconia solid solutions with Fe2O3, Ga2O3, Y2O3, and Gd2O3 have been determined. The Zr ions in both partially stabilized tetragonal and fully stabilized cubic zirconia have their own characteristic structures which are dopant-independent. The dopant cations substitute for Zr ions despite severe local distortions necessitated by the large difference in dopant–O distance ana Zr─O distance. Dopant ionic size determines the preferred locations of oxygen vacancies. Vacancies introduced by oversized dopants (Y and Gd) are located as nearest neighbors to Zr atoms, leaving 8-fold oxygen coordination to dopant cations. Undersized dopants (Fe and Ga) compete with Zr ions for the oxygen vacancies in zirconia, resulting in 6-fold oxygen coordination and a large disturbance to the surrounding next nearest neighbors. Since oxygen vacancies associated with Zr can provide stability for tetragonal and cubic zirconia, these results suggest an explanation for the observation that oversized trivalent dopants are more effective than undersized trivalent dopants in stabilizing cubic and tetragonal phases.

Domain configurations due to multiple misfit relaxation mechanisms in epitaxial ferroelectric thin films. i: theory

Abstract: Possible mechanisms for strain relaxation in ferroelectric thin films are developed. The models are applicable to tetragonal thin film ferroelectrics grown epitaxially on (001) cubic single crystal substrates. We assume growth at temperatures in excess of the Curie temperature (Tc). The extent of strain accommodation by misfit dislocations is considered at the growth temperature (Tg). On cooling to Tc, further misfit dislocation generation is possible due to differences in thermal expansion behavior of the film and substrate. During the paraelectric to ferroelectric transition (PE→FE) additional strains develop in the film. The total strain for the FE phase may be relieved either by further misfit generation or by domain formation. We have developed temperature dependent stability maps that predict the stable domain structure that forms during the PE→FE transition. The stability maps incorporate the role of the following parameters: (i) substrate lattice parameter, (ii) differential thermal expansion characteristics between the film and substrate, (iii) cooling rate, and (iv) depolarizing fields and electrode geometry. Further, the role of dislocation stabilization of domain patterns is discussed.

Raman Scattering Study of Cubic–Tetragonal Phase Transition in Zr1−xCexO2 Solid Solution

Abstract: A structural phase transition between the cubic (space group, Fm3m) and tetragonal (space group, P42/nmc) phases in a zirconia–ceria solid solution (Zr1−xCexO2) has been observed by Raman spectroscopy. The cubic–tetragonal (c–t″) phase boundary in compositionally homogeneous samples exists at a composition X0 (0.8 < X0 < 0.9) at room temperature, where t″ is defined as a tetragonal phase whose axial ratio c/a equals unity. The axial ratio c/a decreases with an increase of ceria concentration and becomes 1 at a composition X′0 (0.65 < X′0 < 0.7) at room temperature. The sample with a composition between X0 and X′0 is t″ ZrO2. By Raman scattering measurements at high temperatures, the tetragonal (t″) → cubic and cubic → tetragonal phase transitions occur above 400°C in Zr0.2 Ce0.8O2 solid solution.

The equilibrium shape of a misfitting precipitate

Abstract: We examine the equilibrium morphologies of precipitates with either a tetragonal or purely dilatational misfit in an elastically anisotropic medium with cubic symmetry under conditions of plane strain. We find that particles with a dilatational misfit are nearly spherical at small sizes, take on four-fold symmetric shapes at intermediate sizes and then undergo a supercritical symmetry-breaking bifurcation to two-fold symmetric shapes aligned along the elastically soft directions of the crystal. A tetragonal misfit breaks this four-fold to two-fold supercritical bifurcation when the direction of the tetragonality is coincident with one of the elastically soft directions of the crystal. Such a tetragonal misfit can lead to two-fold equilibrium particle shapes which are local energy minima, or metastable, and in some cases have large negative interfacial curvatures. When the tetragonality is not in an elastically soft direction, the supercritical bifurcation is not broken and the particles can take on unusual diamond-like or S-shaped morphologies.

Effect of Particle Size on the Room‐Temperature Crystal Structure of Barium Titanate

Abstract: The room-temperature tetragonal-to-cubic transformation in BaTiO3 powders with decreasing particle size has been carefully studied, using materials prepared mainly by hydrothermal methods. Hydrothermal BaTiO3 powders exhibited a more uniform particle size distribution than oxalate-route powders, with X-ray diffraction and electron microscopy indicating that powders 0.19 μm in size were fully cubic while powders 0.27 μ were completely tetragonal (within a 5% detection limit for cubic material) at room temperature. The tetragonal-to-cubic transformation temperature was also found to lie in the range of 121°± 3°C for BaTiO3 powders with room-temperature (c/a) values > 1.008. No transformation could be detected using differential scanning calorimetry for BaTiO3 particles with a (c/a) > 1.008 at room temperature. BaTiO3 powder with a particle size just too small (0.19 μm) to be tetragonal at room temperature remained cubic down to 80 K. Different models for the cubic-to-tetragonal room-temperature transformation are discussed. Hydroxyl ions do not appear to greatly affect the cubic-to-tetragonal transformation, which appears to be essentially dependent on particle size. It is concluded that a model based on surface free energy, as previously discussed for the monoclinic-to-tetragonal transformation at room temperature of fine ZrO2 particles, is consistent with the experimental data.

Oxygen-induced structural change of the tetragonal phase around the tetragonal–cubic phase boundary in ZrO2–YO1.5 solid solutions

Abstract: In the ZrO 2 -YO 1.5 solid solutions, the phase changes among the cubic phase (Fm3m, Z=4), the t″ form and the t' form were investigated by neutron and X-ray powder diffraction, where the t″ and t' forms are defined as tetragonal phases (P4 2 /nmc, Z=2) with axial ratios of c/a f =1 and c/a f >1, respectively, which were prepared by a diffusionless transition from the high-temperature cubic phase during quenching. a f is the lattice parameter of the pseudo-fluorite cell

The spontaneous relaxor to normal ferroelectric transformation in La-modified lead zirconate titanate

Abstract: Tetragonal La-modified lead zirconate titanate (PLZT) ceramics (Zr/Ti=40/60) with La-contents of 8, 12, and 15at% have been studied using dielectric spectroscopy, X-ray diffraction, and hot-stage transmission electron microscopy (TEM) Increasing La-content drives the lead zirconate titanate from near conventional ferroelectric to typical relaxor-type behaviours The PLZT composition with 12at% La possessed a unique transformation Dielectric constant measurements revealed a spontaneous switching between relaxor and normal ferroelectric behaviours at a temperature significantly below that of the dielectric maximum Pronounced X-ray line broadening was observed above the switching temperature indicating the presence of local cubic-tetragonal structural transformations, while line splitting was observed below the switching temperature indicating the formation of a macroscopic tetragonal structure Neither the higher or lower La-content compositions exhibited an analogous spontaneous switching In

Hydrothermal Synthesis and Dielectric Properties of Tetragonal BaTiO3

Abstract: Hydrothermal synthesis of barium titanate (BaTiO{sub 3}) at 240{degrees}C using different barium salts as starting reactants is examined. It is found that halide salts (I{sup {minus}}, Br{sup {minus}}, Cl{sup {minus}}) result in the highest tetragonal content, followed by acetate, nitrate, and hydroxide salts. The tetragonal content was quantified from differential scanning calorimetry by following the endothermic Curie transition around 130{degrees}C and reached as high as 37-39% for the materials made with the barium halides. The particle sizes ranged from 0.15 to 0.37 {mu}m, as estimated by scanning electron microscopy. Sintering and dielectric properties of the barium titanates synthesized from different barium sources were examined. BaTiO{sub 3} made from the chloride salt sintered with high densities at 1250{degrees}C with grain sizes of 1-5 {mu}m and a room-temperature dielectric constant (1 kHz) of 2960. 35 refs., 10 figs., 1 tab.

Effect of composition and temperature on electric fatigue of La-doped lead zirconate titanate ceramics

Abstract: Composition and temperature of ferroelectric La‐doped lead zirconate titanate ceramics influence its electric fatigue behavior, defined as the degradation of the electrical properties under the action of an ac field applied for a long time. Compositions of rhombohedral symmetry exhibit little or no fatigue compared with those of tetragonal and orthorhombic symmetry. At temperatures higher than the dielectric maximum, no fatigue effect was detected. Compositions close to phase boundaries (FE‐AFE, FE‐FE, or FE‐PE) display significant fatigue behavior. Electric fatigue arises from the pinning of domains by space charges or injected carriers or from microcracking. The former (which are charge related) is accompanied by smaller strains and is recoverable by thermal and electrical treatment, while the latter (arising from microcracking) arises from large incompatible stresses between grains and is a permanent damage. The understanding of the mechanism of electric fatigue gained in the present study provides gui...

Solid-State and Surface Spectroscopic Characterization of BaTiO3 Fine Powders

Abstract: The bulk and surface structure of two BaTiO 3 powders has been investigated by FT-IR and FT-Raman spectroscopies. A high-area sample (39 m 2 /g), produced by a hydrothermal method, cubic to XRD, appears to have lower symmetry to Raman spectroscopy, with unit-cell distortions similar to those of tetragonal ferroelectric BaTiO 3 . It contains BaCO 3 impurities, leading to a bulk Ba/Ti ratio lower than 1, as also evidenced by Raman spectra. Bulk hydroxy groups are evident, possibly needed to balance such a charge defect. Calcination at 1223 K causes the disappearance of these impurities and converts this material in the usual tetragonal phase. The surface properties of both powders, investigated through water and pyridine adsorption, are similar and appear to be dominated by Ba 2+ ions, so leading to a solid characterized by extremely weak Lewis acidity. An interpretaion for the solid-state behavior of the starting powder is proposed

Structural and magnetic studies of metal-insulator transition in thiospinel CuIr2S4

Abstract: Crystal structure and magnetic susceptibility were investigated for a thiospinel compound CuIr 2 S 4 , which exhibits a transition from metallic to insulating on cooling at the transition temperature T c of 230 K. It was found that the transition is accompanied by a first-order structural transition. As the compound is cooled through T c , the structure transforms from cubic to tetragonal with a volume contraction of 0.7%. The tetragonal phase has an axial ratio of about 1.03. Magnetic susceptibility, which is paramagnetic in the metallic region, decreases sharply and becomes diamagnetic at T c on cooling. The susceptibility is mostly temperature independent in each region above and below T c . Magnetic measurements also revealed that high pressure stabilizes the insulating phase, in contrast with most of the compounds that exhibit metal-insulator transitions. Possible mechanisms of the metal-insulator transition are discussed.

Investigation of the structure of barium titanate thin films by Raman spectroscopy

Abstract: Raman spectroscopy was used to examine the structure of barium titanium oxide thin films grown by metal‐organic chemical vapor deposition (MOCVD) and laser‐assisted deposition The spectra were compared with the spectra of a ceramic specimen and a single crystal Raman peaks specific to the tetragonal ferroelectric phase of BaTiO3 were seen in the spectra of several films Other Raman peaks were ascribed to impurity (non‐BaTiO3) phases in the films or to the substrates (fused quartz, MgO) Some of the Raman peaks showed a strong polarization dependence The MOCVD films were also characterized by x‐ray diffraction, energy‐dispersive x‐ray spectroscopy, and transmission electron microscopy The film‐to‐film variation of the strength of BaTiO3 features in the Raman spectrum, relative to impurity‐phase features, was qualitatively consistent with the x‐ray diffraction and electron microscopy results Spatially resolved Raman measurements showed that the structure of the laser‐deposited film varies significantly over the deposited area The temperature dependencies of the Raman spectra of two MOCVD films were examined in the 25–175 °C range Raman peaks due to the tetragonal phase of BaTiO3 were observed at temperatures well above the Curie temperature of bulk single‐crystal BaTiO3 (132 °C) This observation suggests that the tetragonal ferroelectric phase is stabilized by an anisotropic film‐substrate interaction that gives rise to a two‐dimensional stress in the plane of the film

New High-Tc Superconductor Family of Cu-Based Cu1-xBa2Can-1CunO2n+4-δ with Tc>116 K

Abstract: A new high-temperature and nontoxic superconductor family of Cu1-xBa2Can-1CunO2n+4-δ with critical temperature (Tc)>116 K was discovered. These materials were prepared by the high-pressure technique. A superconductor sample with the highest T c contains mainly a Cu1-x Ba2Ca3Cu4O12-δ phase. The T c value of the Cu1-x Ba2Ca3Cu4O12-δ phase is 116.4 K. The phase has two kinds of tetragonal structures: body-centered tetragonal (bct) and simple tetragonal. The lattice parameters of bct are a=7.7114 A and c=35.986 A. The disordering or full occupation of Cu-vacancy sites leads to the simple tetragonal phase with the space group of P4/mmm and the half values of the lattice constants (a=3.8557 A and c=17.993 A) of the bct phase. The relationship between the lattice constant c and the number of CuO2 layers n is c/2=8.4+3.2(n-1) ( A). The small CuO2-block spacing (8.4 A) of this family suggests a lower anisotropy and higher J c than in Hg- and Tl-based superconductor families.

Ultrafine-Grained Dense Monoclinic and Tetragonal Zirconia

Abstract: Nanoparticles of ZrO[sub 2] with diameters ranging from 4 to 8 nm were synthesized by gas condensation. As-prepared n-ZrO[sub 2] particles have a monoclinic and a high-pressure tetragonal structure depending on size. Pure ZrO[sub 2] was sintered to full density under vacuum at 0.4T[sub m] within the monoclinic phase field. Final grain sizes in theoretically dense pellets are below 60 nm. By sintering below the monoclinic-tetragonal transition temperature, microcracking was completely avoided. Tetragonal ZrO[sub 2] stabilized with 3 mol% Y[sub 2]O[sub 3] was prepared by interdiffusion of nanoparticles and sintered to near-theoretical density.

117 K superconductivity in the BaCaCuO system

Abstract: Superconductivity was observed at 117 K in the BaCaCuO samples synthesized at 950°C under 5.0 GPa for 1 h. The samples were multiphase. Electrical resistivity and DC magnetic susceptibility measurements revealed bulk superconductivity at Tconset = 117 K, Tczero = 107.6 K and Tcmag = 116.0 K. Crystals having primitive tetragonal unit cells with the lattice parameters of a=3.88 A , and c=15.0 A , 18.3 A and 21.6 A found by electron diffraction. The dominant peaks of the X-ray powder diffraction pattern were tentatively assigned to the hypothetical Cu-1234 ( a=3.85 A and c=18.30 A ) and Cu-1223 ( a=3.88 A and c=14.94 A ). structures. It is likely that these are new high-Tc superconducting phases.

Single Uranium-Site Properties of the Dilute Heavy Electron System UxTh1-xRu2Si2 (x≤0.07)

Abstract: Specific heat C , electrical resistivity ρ , magnetization M and magnetic susceptibility χ have been measured for the dilute tetragonal heavy-electron compounds U x Th 1- x Ru 2 Si 2 ( x ≤0.07), in the temperature range 100 mK≤ T ≤360 K and in the magnetic fields H up to 55 kOe. Non-Fermi-liquid behavior with unusual lnT dependence of C , ρ and χ has been observed at the low temperatures: as the temperature is lowered below about 10 K, C / T and χ increase logarithmically, and ρ decreases logarithmically. Systematical variations of these quantities with uranium concentration indicate that the observed non-Fermi-liquid behavior is ascribed to the properties of a single U ion embedded in the Th site of the ThRu 2 Si 2 metal. The magnetization curves M vs H / T indicate a strong uniaxial magnetic anisotropy along the tetragonal c -axis as well as a strong reduction of the 5 f moment of U ions. A possible interpretation based on the recent developments of the S =1/2 two-channel Kondo model is given.

Quantitative determination of the cubic, tetragonal and monoclinic phases in partially stabilized zirconias by Raman spectroscopy

Abstract: Vibrational Raman spectroscopy was employed successfully for the simultaneous quantitative determination of the cubic, tetragonal and monoclinic phases of partially stabilized zirconia. Pressed pellets with known composition were prepared and used as standard mixtures. From the standard pellets, a calibration curve was obtained which was found to be a straight line if (I 640−0.47I 260)/I 177 is plotted against the inverse monoclinic molar fraction.I 260,I 177 are the Raman intensities for the tetragonal and monoclinic peaks at 260 and 177 cm−1, respectively, and were calculated by subtracting the “background”. In contrast,I 640 represents the intensity for the 640 cm−1 band, which is attributed to all three phases, and was measured including the “background”.

Low-Temperature Phase Equilibria by the Flux Method and the Metastable–Stable Phase Diagram in the ZrO2–CeO2 System

Abstract: Low-temperature phase equilibria ranging from 1,000 to 1,200 C in the ZrO[sub 2]-CeO[sub 2] system were investigated by annealing compositionally homogeneous ZrO[sub 2]-CeO[sub 2] solid solutions in a Na[sub 2]B[sub 2]O[sub 7][center dot]NaF flux. The 5 mol% CeO[sub 2] samples decomposed into monoclinic (m) and tetragonal (t) phases during annealing at 1,100 and 1,120 C, and the t-phase transformed diffusionlessly into monoclinic (m[prime]) symmetry during quenching. A eutectoid reaction, t [yields] (m + c), was confirmed to occur at 1,055 [+-] 10 C, where the equilibrium compositions of the t-, m-, and c-phases were 11.2 [+-] 2.8, 0.9 [+-] 0.9, and 84 [+-] 1 mol% CeO[sub 2], respectively. The equilibrium phase boundaries were almost independent of the annealing time and/or the flux:sample ratio, which indicates that the flux accelerates the reaction rate without affecting the equilibration. The previous data are discussed using metastable-stable phase diagrams. The discrepancies of the low-temperature phase diagram in the literature are attributable to either regarding the metastable phase boundaries as stable ones of ignoring the sluggish kinetics.

Structures of three crystal forms of the sweet protein thaumatin.

Abstract: Three crystal forms of the sweet-tasting protein thaumatin from the African berry Thaumatococcus daniellii have been grown. These include two naturally occurring isoforms, A and B, that differ by a single amino acid, and a recombinant form of isoform B expressed in yeast. The crystals are of space groups C2 with a = 117.7, b = 44.9, c = 38.0 A, and beta = 94.0 degrees, P2(1)2(1)2(1) with a = 44.3, b = 63.7 and c = 72.7 A, and a tetragonal form P4(1)2(1)2 with a = b = 58.6 and c = 151.8 A. The structures of all three crystals have been solved by molecular replacement and subsequently refined to R factors of 0.184 for the monoclinic at 2.6 A, 0.165 for the orthorhombic at 1.75 A, and 0.181 for the tetragonal, also at 1.75 A resolution. No solvent was included in the monoclinic crystal while 123 and 105 water molecules were included in the higher resolution orthorhombic and tetragonal structures, respectively. A bound tartrate molecule was also clearly visible in the tetragonal structure. The r.m.s. deviations between molecular structures in the three crystals range from 0.6 to 0.7 A for Calpha atoms, and 1.1 to 1.3 A for all atoms. This is comparable to the r.m.s. deviation between the three structures and the starting model. Nevertheless, several peptide loops show particularly large variations from the initial model.

Raman scattering of ferroelectric Sr1-xCaxTiO3, x=0.007

Abstract: First-order Raman scattering of A1g and Eg modes originating from the antiferrodistortive F2u soft mode confirms the cubic-tetragonal (Oh-D4h) phase transition in Sr0.993Ca0.007TiO3, at T0=125 K. Within the cubic and the tetragonal phase first-order Raman scattering of the soft ferroelectric F1u mode (TO1) and of the hard TO2 and TO4 optic modes evidences the existence of Ca2+-centred polar microregions. Their size increases rapidly on cooling towards the ferroelectric ordering temperature, Tc=18 K. Splitting and incomplete softening of the F1u components is consistent with an order-disorder transition into an orthorhombic low-temperature phase with C2v symmetry.

Thickness of Cubic Surface Phase on Barium Titanate Single‐Crystalline Grains

Abstract: The crystallographic structure of BaTiO3 particles prepared by conventional solid-state reaction has been investigated by X-ray diffractometry and dielectric measurements. The systematic analysis of the diffraction pattern has clearly shown the coexistence of tetragonal and cubic phases at room temperature. The relative amount of cubic phase increases and the dielectric constant decreases with decreasing particle size. These experimental results are consistent with the model that the cubic phase is located at the surface of BaTiO3 grains. The thickness of the surface layer is estimated to be approximately 5 nm, irrespective of the grain size.

Synthesis of Metastable Tetragonal (t′) Zirconia–Ceria Solid Solutions by the Polymerized Complex Method

Abstract: Homogeneous metastable tetragonal (t′) solid solutions of ZrO2—x mol% CeO2 (x= 20 and 50) were successfully synthesized by the organic polymerized complex method. The citric acid-ethylene glycol solution containing Zr and Ce ions was polymerized at about 140°C and then heat-treated at about 350°C to obtain a precursor. The black precursor was heated at 450°C and then fired up to 1300° or 1590°C, resulting in the homogeneous solid solutions.

Isotropic and anisotropic magnetoelastic interactions in heavy and light RCo2 Laves phase compounds

Abstract: The thermal expansion and anisotropic magnetostriction of the RCo2 Laves phases were studied in the temperature range 4-500 K using the X-ray powder diffraction method. In the heavy RCo2 the magnetic moment of the itinerant d electron subsystem derived from the magnetovolume effect was found to fit well with the magnetization curve of YCo2. A pronounced paraprocess above the metamagnetic transition has been observed when increasing the f-d exchange field. The type and temperature variation of the distortion of the cubic unit cell of the three compounds PrCo2, NdCo2 and SmCo2 have been studied in detail. The corresponding magnetostriction constants lambda 111 or lambda 100 were calculated. At 4 K the following values have been obtained: PrCo2, tetragonal distortion, easy axes (100), lambda 100=-3.4*10-3: SmCo2, rhombohedral distortion, easy axes (111), lambda 111=-4.6*10-3; NdCo2, tetragonal distortion and easy axes (100) for 42 K