Showing papers on "Tetragonal crystal system published in 1983"

Formation of Ultrafine Tetragonal ZrO2 Powder Under Hydrothermal Conditions

Abstract: Ultrafine tetragonal ZrO2 powder was prepared by hydrothermal treatment at 100 MPa of amorphous hydrous zirconia with distilled water and LiCl and KBr solutions. The resulting powder consisted of well-crystallized particles; at 200°C, the particle size was 16 nm and at 500°C, 30 nm. Under hydrothermal conditions tetragonal ZrO2 appears to crystallize topotactically on nuclei in the amorphous hydrous zirconia.

Revised Phase Diagram of the System ZrO2‐CeO2 Below 1400°C

Abstract: The phase diagram of the system ZrO2-CeO2 was reinvestigated using hydrothermal techniques. Cubic, tetragonal, and monoclinic solid solutions are present in this system. The tetragonal solid solution decomposes to monoclinic and cubic solid solutions by a eutectoid reaction at 1050°±50°C. The solubility limits of the tetragonal and cubic solid solutions are about 18 and 70 mol% CeO2, respectively, at 1400°C, and about 16 and 80 mol% CeO2, respectively, at 1200°C. Solubility limits of the monoclinic and cubic solid solutions are about 1.5 and 88 mol% CeO2 at 1000°C, and 1.5 and 98 mol% CeO2 at 800°C, respectively. The compound Ce2Zr3O10 is not found in this system.

Role of Water Vapor in Crystallite Growth and Tetragonal‐Monoclinic Phase Transformation of ZrO2

Abstract: The role of water vapor in crystallite growth and the tetragonal-to-monoclinic phase transformation of ZrO2 was studied using three specially prepared samples: an ultrafine powder of monoclinic ZrO2 obtained by hydrolysis of ZrOCI2, an aggregated powder of tetragonal ZrO2 obtained by thermal decomposition of Zr(OH)4 under reduced pressure, and an ultrafine powder of tetragonal ZrO2 obtained by thermal decomposition of zirconyl acetate dispersed in caramel. The samples were heat-treated up to 1000°C in dry and wet atmospheres saturated with water vapor at 90°C. It was found that water vapor markedly accelerated crystallite growth for both monoclinic and tetragonal ZrO2 and facilitated the tetragonal-to-monoclinic phase transformation. Water vapor increases surface diffusion and thus enhances crystallite growth and decreases surface energy, which leads to stabilization of the tetragonal phase.

Magnetic properties and structure of cobalt-platinum thin films

Abstract: The magnetic properties of RF sputtered Co-Pt alloy thin films were studied as a function of Pt content from 0 to 80 at%. At room temperature, ferromagnetic films were obtained in the range 0-32 and 40-80 at% Pt. For Pt contents between 32 and 40 at%, discontinuities in the magnetization, magnetostriction, and coercivity versus Pt content were observed; however no discontinuity was observed in the resistivity. The structure of films containing about 25 at% Pt is a mixture of hexagonal and face-centered cubic (FCC) phases. At this composition the magnetostriction is small, but coercivities are large-700 to 2000 Oe-and dependent upon film thickness. The coercivities of these films do not change with heat treatment up to temperatures of 600°C but decrease markedly at 700°C. The properties of equiatomic Co-Pt film s are similar to those of bulk alloys. In particular the large coercivity observed in films after heal treatment at 500° to 700°C is due to the formation of an ordered tetragonal phase within the face-centered cubic matrix. The structure of films of about 75 at% Pt is initially a disordered face-centered cubic phase and with heat treatment beginning at 500°C an ordered face-centered cubic phase forms. The coercivity of these films (∼200 Oe) does not change with annealing at 500°C. It decreases slightly upon further annealing at 600°C to 700°C. Electron microscope observations were used to correlate the magnetic properties with film structure.

Microstructural development of sub-eutectoid aged MgO-ZrO2 alloys

Abstract: The microstructural development in fully and partially stabilized MgO-ZrO2 alloys during sub-eutectoid heat treatment at 1100° C, has been studied by optical and electron microscopy and powder X-ray diffraction in order to correlate the observed microstructures with the mechanical properties. The materials used were 14 mol % MgO-ZrO2 (Mg-CSZ) of eutectoid composition and 9 mol % MgO-ZrO2 (Mg-PSZ). In Mg-CSZ the decomposition reaction proceeds almost to completion before the maximum thermal up-shock properties are attained. For the Mg-PSZ material the most significant event leading to the enhanced mechanical properties appears to be the nucleation and growth of a long-range ordered anion vacancy phase within the cubic zirconia matrix. This, in turn, leads to the destabilization of the tetragonal ZrO2 precipitates also present in Mg-PSZ, causing some of them to acquire monoclinic intergrowths whilst others transform to the monoclinic form on cooling. Beyond about 4 h ageing at 1100° C the decomposition product in the grain boundaries began to influence the mechanical properties.

Fracture of metastable tetragonal zirconia crystals

Abstract: Crystals of single-phased metastable tetragonal zirconia (MTZ) were prepared from skullmelting at the composition ZrO2-3 mol % M2O3 (M = Y, Yb or Gd). The fracture of the tetragonal crystals occurred differently than for the cubic stabilized zirconia ones. The toughness was much higher for the tetragonal specimens and the cleavage planes were not the same as for the fluorite crystals. The results were interpreted by considering the domain microstructure induced by the cubic → tetragonal phase transformation undergone by the crystals.

The structure of metallic complexes of polyacetylene with alkali metals

Abstract: The crystal structures of sodium, potassium, rubidium, and cesium doped polyacetylene have been determined using crystal packing and x‐ray diffraction analyses. Each of these metallic complexes is tetragonal, with the polyacetylene chains forming a host lattice in which the alkali metal ions are present in channels. Lithium appears to be too small to stabilize the channel structure and an amorphous structure is observed. Predicted unit cell parameters and x‐ray diffraction intensities are in agreement with observed values. Similarities with the alkali metal doped graphite suggest that hybridization between carbon pz orbitals and metal s orbitals occurs. Such hybridization is expected to result in a high conductivity component normal to the chain direction. On the other hand, direct overlap between polymer chains appears small, since alkali metal columns separate polymer chains. Compositions calculated for the channel structures (from meridional diffraction spacings, the intensity of equatorial diffraction lines, measured volume expansion, and distances in model complexes) all range from y=0.12 to 0.18 for (CHMy)x, where M is sodium, potassium, rubidium, or cesium.

Barium Titanate Perovskite Sintered with Lithium Fluoride

Abstract: The sintering of barium titanate with, respectively, 1 and 2 wt% LiF for two stoichiometries, Ti/Ba=1 and 0.975, was studied using two calcined powders. One was pure barium titanate; the other contained BaTi03 plus BaC03 and TiO2 that did not react when burning. The sintering chronology—intermediate phases, appearance, and disappearance of a liquid phase that has been pointed out for the first time—is directly dependent on the used calcined powder, on the LiF amount, and on the firing schedule. In the same way, the obtained perovskite symmetry varies during sintering from tetragonal to cubic and then to a second tetragonal form, whereas most of the Li and F disappear from the ceramic with two different kinetics.

Dielectric, Electromechanical, Optical, and Mechanical Properties of Lanthanum-Modified Lead Titanate Ceramics

Abstract: Dielectric, electromechanical, optical, and mechanical properties of hot-pressed lanthanum-modified PbTiO/sub 3/ ceramics were measured as a function of crystal tetragonality, c/a, in a range of tetragonal-to-cubic phases. The results obtained in this study are summarized as follows: The c/a changes linearly as a function of lanthanum content. Similarly, Curie point changed linearly and remanent polarization, coercive field, and coupling factors decreased with decreasing c/a. The sample with c/a = 1,010 showed the maximum transmittance of 61% wavelengths above 600 nm. The internal stress measured using the microindentation technique decreased with decreasing c/a in the tetragonal region and was zero in the cubic region.

Molecular dynamics and structures of amine boranes of the type R3N.BH3. I. X-ray investigation of H3N.BH3 at 295K and 110K

Abstract: The results of an X-ray diffraction investigation of powdered H3N.BH3 are reported. A first-order rotational order-disorder phase transition has been observed at 225K. In the high-temperature tetragonal phase the space group permits twelvefold reorientations of GH3 and NH3 groups. The cell parameters are linear functions of temperature in this phase. The low-temperature structure is orthorhombic. The length of the c axis is independent of temperature while the a axis decreases non-linearly in length as the temperature is increased. The length of the b axis increases non-linearly with increasing temperature. With the exception of the c axis which changes very little in length at the phase transition, all cell dimensions as well as the volume of the unit cell show drastic changes at the phase transition. The temperature dependences of the cell parameters are explained qualitatively in terms of the reorientations and relative orientations of the BH3 and NH3 groups.

Phase distributions in plasma-sprayed zirconia-yttria

Abstract: The distribution of phases in plasma-sprayed zirconia-yttria has been determined over a range of yttria levels from 0 to 26.1 molpct YO(1.5) using room temperature X-ray diffractometry. Pure, plasma-sprayed zirconia is composed almost entirely of the monoclinic phase. At levels of yttria between 4 and 10 percent, a quenched-in tetragonal phase predominates, and at higher levels the cubic phase predominates. The phase distributions are compared with previously reported test lives of thermal barrier coatings formed from these materials. Regions of optimal lives were found to correlate with regions having high amounts of the tetragonal phase, small but nonzero amounts of the monoclinic phase, and little or none of the cubic phase. Possible relationships between phase composition and coating performance are discussed.

Stability of Ultrafine Tetragonal ZrO2 Coprecipitated with Al2O3 by the Spray‐ICP Technique

Abstract: The transformation of ultrafine powders (particle size, 0.01 to 0.04 μm) of the system ZrO2–Al2O3, prepared by spraying their corresponding nitrate solutions into an inductively coupled plasma (ICP) of ultrahigh temperature, was investigated. The powders were composed of metastable tetragonal ZrO2 (mt-ZrO2) and γ-Al2O3. On heating, the mt-ZrO2 (or tetragonal ZrO2, t-ZrO2) was retained up to 1200°C. At 1380°C the transformation to monoclinic ZrO2 (m-ZrO2) occurred and the amount of the m-ZrO2 decreased with the increase in Al2O3 content, thus indicating the stabilization of the t-ZrO2 by the Al2O3, which seems to be explained in terms of the retardation of grain growth.

Reduced forms of LaNiO3 perovskite. Part 2.—X-ray structure of LaNiO2 and extended X-ray absorption fine structure study: local environment of monovalent nickel

Abstract: One of the products of the reduction of LaNiO3 is LaNiO2, containing monovalent nickel. This valence state of nickel has scarcely been observed in mineral compounds. X-ray data from powder samples and extended X-ray absorption fine structure studies (EXAFS) allow one to define a tetragonal structure with lanthanum at the centre of a prism composed of oxygen atoms having a square base and nickel in the centre of the square. The Ni—O distance (nearest neighbour) was found to be 1.983 A for this monovalent nickel.

Charge-density waves in the quasi-one-dimensional compounds NbTe4 and TaTe4

Abstract: Superlattice spots observed by X-ray and electron diffraction in the tetragonal, metallic, quasi-one-dimensional compounds NbTe4 and TaTe4 are interpreted in terms of charge-density waves. It is shown that three slightly different types of columns occur in the crystals. In NbTe4 at room temperature two types of chains have incommensurate distortions with wavevectors q1=(0, 0, 0.311c*) and q2=(0.5a*, 0.5b*, 0.344c*): the third type has a very weak distortion with q3=(0.5a*, 0, 1/3c*). The wavevectors for TaTe4 are as above except that the component along the metal chain is exactly 1/3c* in each case. The vectors q1 and q2 are independent of temperature but can be varied by substitution of Ta for Nb. These are structurally the simplest inorganic crystal structures in which CDW have been observed and are the only reported crystals in which three CDW coexist.

Crystal structure of bis(2,2':6',2''-terpyridyl) cobalt (II) iodide dihydrate at 295 K and at 120 K

Abstract: The crystal structure of the title compound, [CO(C15H11N3)2] 12.2H2O, has been determined by single crystal X-ray diffraction methods at 295 K and at 120 K, being refined by least squares to residuals of 0.051 and 0.035 respectively for 982 and 1024 'observed' reflections at these temperatures. Both structures are based on a P42/n tetragonal cell, a c. 8.9, c c. 19.4 A, Z 2, in which only a quarter of the cation is independent, being located about a site of 4 symmetry. At room temperature (μoff c. 3.2 BM) Co-N (central, distal) are 1.942(7), 2.104(5) A diminishing to 1.912(5), 2.083(4) A at 120 K, with μoff c. 2.2 BM, corresponding to a fully populated low-spin ground state.

Structural studies of manganese stabilised lead-zirconate-titanate

Abstract: Piezoelectric compositions Pb(MnuNbvNiwTixZry)O3 where u+v+w+x+y = 1.0 were studied by means of X-ray powder diffractometry, scanning electron microscopy and thermal analytical techniques. These studies showed that Mn is incorporated into the tetragonal lattice as trivalent ions at the Ti, Zr octahedral (B) sites.

Ion channeling studies of InGaAs/GaAs strained-layer superlattices

Abstract: The first ion channeling studies of the InxGa1−xAs/GaAs strained‐layer superlattice (SLS) system are reported. The strong orientation dependence of the dechanneling for both axial and planar directions relative to the [100] growth axis indicates that the ≊1% lattice mismatch is accommodated by lattice strain. Tetragonal distortions along the growth direction give rise to alternating tilts for inclined crystal directions which result in significant [110] and (110) dechanneling. Measurements for 40‐layer structures with different In concentrations demonstrate that the channeling technique is a depth‐sensitive probe of the degree of strain in SLS systems.

Investigations of the magnetic properties of CeRh2Si2 using lattice parameter, LIII edge and extended X-ray absorption fine structure measurements

Abstract: The lattice parameters of tetragonal CeRh2Si2 are anomalous compared with those of the other members of the series CeM2Si2 (M ≡ Ru, Rh, Pd and Ag). Measurements of the susceptibility of CeRh2Si2 as a function of temperature in the range 15–300 K reveal interesting features. At high temperatures the susceptibility obeys a Curie-Weiss law with an effective moment per cerium ion that is 20% higher than that of the free Ce3+ ion. There is a cusp-like peak at about 37 K and a relatively smaller peak at 5.2 K. The lattice parameters a and c show a slight decrease between 300 and 40 K and a rather small increase between 40 and 12 K. The LIII absorption edge and the extended X-ray absorption fine structure spectra are unchanged between 20 and 300 K. This clearly indicates that the valence of the cerium ion does not change and that the first-nearest-neighbour environment (eight rhodium atoms and eight silicon atoms) of the cerium ion is not affected by the transition at 37 K.