Showing papers on "Tetragonal crystal system published in 1992"
TL;DR: In this paper, the use of quantum mechanics to make structural predictions and provide insights at the atomic level is reviewed with respect to intermetallics, and the question is addressed of why some tetragonal D022 trialuminides (e.g. TiAl3) can be stabilised with the cubic L12 structure by alloying additions but others cannot, and why if the cubic phase is stabilised, it remains brittle.
Abstract: Recent developments in the use of quantum mechanics to make structural predictions and provide insights at the atomic level are reviewed with respect to intermetallics. In particular, the question is addressed of why some tetragonal D022 trialuminides (e.g. TiAl3) can be stabilised with the cubic L12 structure by alloying additions but others (e.g. NbAl3) cannot, and why, if the cubic phase is stabilised, it remains brittle.MST/1543
1,299 citations
TL;DR: In this article, a review of the science and technology of HfO2 and hafnium-based materials in terms of processing, phase transformation, microstructure, and mechanical properties is presented.
Abstract: Hafnia (HfO2) and hafnium-based materials are traditionally regarded as technologically important materials in the nuclear industry, a consequence of their exceptionally high neutron absorption coefficient. Following the discovery of transformation toughening in the mid 1970s, a considerable research effort has been devoted to zirconia (ZrO2)-toughened ceramics (ZTCs). They are considered to be potentially useful materials for structural applications at low and intermediate temperatures (T 1000 °C) is related to the low temperature of the tetragonal to monoclinic transformation in ZrO2. On the basis that HfO2 exhibits a similar crystal structure and in particular that its tetragonal to monoclinic transformation temperature (∼1700 °C) is approximately 700 °C higher than that for ZrO2, it has been suggested that high-temperature transformation toughening could be possible in HfO2-toughened ceramics (HTCs). Although the concepts behind this suggestion are universally appreciated, only a limited success has been made of the fabrication and the microstructural and mechanical property evaluation of these materials. The fracture toughness values obtained so far in HfO2 toughened ceramics are, in fact, considerably lower than those obtained in their ZrO2 counterparts. A great deal of further research work is therefore required in order to understand fully and to exploit toughened ceramics in the HfO2-based and HfO2-containing systems. This review covers the science and technology of HfO2 and HfO2-toughened ceramics in terms of processing, phase transformation, microstructure, and mechanical properties.
476 citations
TL;DR: In this paper, complex dielectric permittivities of CH3NH3PbX3 (X = Cl, Br, I) were measured at frequencies between 20 Hz and 1 MHz and at temperatures between 20 and 300 K (15 and 350 K for the iodide).
356 citations
TL;DR: In this article, the divalent group IIA intercalant calcium can be intercalated into the f.c. sites of C60 to form a solid solution, and that, near a Ca:C60 ratio of 5:1, a phase transformation occurs to a simple cubic phase.
Abstract: IT has been demonstrated1–6 that solid C60 can be readily intercalated with group IA alkali metals to give metallic or insulating compounds, depending on the dopant concentration. The metal atoms diffuse into tetrahedral and octahedral interstitial sites of the C60 lattice with little disturbance to the face-centred cubic (f.c.c.) packing7. The A3C60f.c.c. phases (A is K, Rb, Cs and mixtures of these) exhibit superconductivity with a transition temperature that increases with lattice constant8. At higher dopant concentration a body-centred tetragonal A4C60 phase9 and an insulating, body-centred cubic A4C60 phase10 are found. Here we report that the divalent group IIA intercalant calcium can be intercalated into the f.c.c. sites of C60 to form a solid solution, and that, near a Ca:C60 ratio of 5:1, a phase transformation occurs to a simple cubic phase. Measurements of microwave loss, magnetic susceptibility and Meissner effect show that the simple cubic phase becomes superconducting below 8.4 K.
249 citations
TL;DR: In this paper, an analysis of the influence of grain size on tetragonal-to-monoclinic transformation temperature in polycrystalline zirconias is presented.
Abstract: In this paper experimental observations of the tetragonal phase transformation behavior in polycrystalline zirconias and the related toughening contribution are presented. An analysis which considers transformation thermodynamics and residual stresses is developed to describe the influence of grain size on tetragonal-to-monoclinic transformation temperature. The model is based on the promotion of the transformation by local internal tensile stress concentrations whose effects scale with grain size. The analysis is supported by observations of the martensite start temperature---grain size behavior in polycrystalline tetragonal zirconia containing 12 mol% ceria (12 CeTZP). Next, the analysis considers the grain-size-dependent behavior of the transformation-toughening contribution, {Delta}K{sup T}, and the transformation zone size, r{sup T}.
226 citations
TL;DR: In this paper, a metastable zinc-blende-structure β-AlN was synthesized by the solid-state reaction between single-crystal Al(001) and TiN(001), grown on MgO(001).
Abstract: Epitaxial metastable zinc‐blende‐structure β‐AlN was synthesized by the solid‐state reaction between single‐crystal Al(001) and TiN(001) layers grown on MgO(001) by ultrahigh vacuum magnetron sputter deposition. At an annealing temperature Ta=600 °C, the interaction proceeded according to the following overall reaction: 4Al+TiN→Al3Ti+AlN, in which β‐AlN was formed pseudomorphically between cubic TiN and tetragonal Al3Ti layers. The lattice constant of β‐AlN was found to be 0.438 nm, which corresponds to a 3.3% lattice mismatch with the underlying TiN layer.
177 citations
170 citations
TL;DR: It is suggested that grain-size reduction after a subducting slab passes through the 670-kilometer discontinuity or after a phase transformation from orthorhombic to tetragonal in perovskite will result in rheological softening in the top portions of the lower mantle.
Abstract: High-temperature creep experiments on polycrystalline perovskite (CaTiO(3)), an analog of (Mg,Fe)SiO(3) perovskite of the lower mantle, suggest that (grain size-sensitive) diffusion creep is important in the lower mantle and show that creep rate is enhanced by the transformation from the orthorhombic to the tetragonal structure. These observations suggest that grain-size reduction after a subducting slab passes through the 670-kilometer discontinuity or after a phase transformation from orthorhombic to tetragonal in perovskite will result in rheological softening in the top portions of the lower mantle.
159 citations
TL;DR: In this paper, U3Si∼5 has been thoroughly investigated with respect to crystal chemistry and magnetic properties; for most compounds, magnetic studies have been carried out at low temperatures for the first time (2 to 300 K, 0 to 5 T).
146 citations
TL;DR: In this article, the authors measured the Ti 2p X-ray absorption spectra for a series of minerals and found that the absorption spectrum is more sensitive to tetragonal distortions than to trigonal distortions.
Abstract: Ti 2p X-ray absorption spectra for a series of minerals have been measured. Crystal field multiplet calculations can explain the spectral shape. The asymmetry of the e
g
, peak is shown to be related to distortions of the Ti
IV
octahedron. It is found, theoretically as well as experimentally, that the absorption spectra are more sensitive to tetragonal distortions than to trigonal distortions. A number of silicate minerals and metamict minerals containing titanium are measured and Ti
III
has not been observed in any of these minerals. A comparison is made to the 1s X-ray absorption, and the potential of both for the study of minerals is discussed.
145 citations
TL;DR: Molecular-dynamics simulation results are reported for systems of strongly interacting dipolar soft spheres and it is found that in addition to the ferroelectric nematic phase, liquid crystals with columnar order can also be obtained.
Abstract: Molecular-dynamics simulation results are reported for systems of strongly interacting dipolar soft spheres. Calculations have been carried out along two isotherms and the structure of the liquid-crystal and solid phases obtained is described in detail. It is found that in addition to the ferroelectric nematic phase we previously reported [Phys. Rev. Lett. 68, 2043 (1992)], liquid crystals with columnar order can also be obtained. The model freezes to form a ferroelectric solid which is shown to have a tetragonal I crystal structure. The influence of different boundary conditions upon the simulation results is also discussed.
TL;DR: A detailed review of the literature dealing with structural studies on the intercalated compounds of C 60 will be given in this article, where the Rietveld profile technique for structure refinement coupled with high resolution synchrotron X-ray powder data has played an important role in providing accurate structural data for many of these compounds.
TL;DR: In this article, the relationship between superconductivity and the crystal structure of La 2− x M x CuO 4 (M Sr, Ba) is studied under high pressures up to 20 kbar.
Abstract: Relations between the superconductivity and the crystal structure of La 2− x M x CuO 4 (M Sr, Ba) are studied under high pressures up to 20 kbar. In the Sr-doped crystal, bulk superconductivity is observed both below and above the critical pressure P c where the high-temperature tetragonal (THT) phase is stabilized down to less than 20 K, the lowest temperature examined, instead of the orthorhombic phase. The superconducting transition temperature T c of the orthorhombic phase below P c is depressed in comparison with that of the THT phase above P c . The depression of T c is caused by a tilt of the CuO 6 octahedra in the orthorhombic phase. It is found for the Ba-doped crystal that the depression of T c for x⋍0.11−0.14 is removed, except for x⋍0.125 when a structure transition at T 1 ∼60 K disappears under high pressure. In the vicinity of x =0.125, there exists another origin for the depression of T c in addition to the structural transition at T c .
TL;DR: In this paper, the coexistence of cubic fluorite and tetragonal phases in rapidly quenched samples was studied iin the Zr02-M01.5 systems for M = Sc, In, Y, and rare earths (R).
Abstract: The coexistence of the cubic fluorite and tetragonal phases in rapidly quenched samples was studied iin the Zr02-M01.5 systems for M = Sc, In, Y, and rare earths (R). Spontaneous transformation from metastable cubic phase was triggered at room temperature by a mechanical force. Isolated tetragonal platelets in the cubic matrix were bounided by (101) habit planes and contained anti-phase boundaries. The tetragonality decreased with stabilizer content and vanished at around 18 mom for M = Y and R, 23 mol% for M = Sc, and 25 mol% for M = In, all at room temperature. With increasing temperature, the tetragonality initially increased because of anisotropic thermal expansion, then decreased rapidly, after reaching a maximum, as the temperature for the tetragonal
TL;DR: Sr 2 CuO 2 (CO 3 ) was prepared at 1273 K and 0.01 MPa CO 2 partial pressure in a flowing gas of O 2 CO 2 using a mixture of SrCO 3 and CuO powders as a starting material as mentioned in this paper.
Abstract: Sr 2 CuO 2 (CO 3 ) was prepared at 1273 K and 0.01 MPa CO 2 partial pressure in a flowing gas of O 2 CO 2 using a mixture of SrCO 3 and CuO powders as a starting material. The compound has a tetragonal structure with lattice constants a = 7.8045(1), and c = 14.993(1) A , and its space group is 14. The formula per unit cell is 8 Sr 2 CuO 2 (CO 3 ), and measured and calculated densities are D m = 4.71 g/cm 3 , and D x = 4.81 g/cm 3 , respectively. The crystal structure was refined by Rietveld analysis on X-ray powder diffraction and neutron powder diffraction data. The final residuals ( R F ) were 4.31 and 4.27% for the X-ray and neutron data, respectively. The structure consists of deformed [CuO 6 ] octahedrons and layers of ordered triangular CO 3 groups. Sr atoms having eight near oxygen neighbors are between the [CuO 6 ] octahedrons and the CO 3 layers.
TL;DR: A structure of CeOCl formed during the synthesis of Pd/CeO 2 catalyst from PdCl 2 and CeO 2 was refined by the Rietveld method from the X-ray powder diffraction data as mentioned in this paper.
TL;DR: The authors have successfully grown single crystals of La 2-x-y Nd y Dr x CuO 4 by the traveling-solvent floating-zone method and enable us to investigate the anisotropic transport properties of the LTT phase.
Abstract: We have successfully grown single crystals of La 2-x-y Nd y Dr x CuO 4 (x=0,0.12,0.20;y∼0.4) by the traveling-solvent floating-zone method. All of these crystals show a tlattice instability to the low-temperature tetragonal (LTT) phase and enable us to investigate the anisotropic transport properties of the LTT phase. For x=0.12, various transport coefficient show a discontinuous jump at the phase transition (evidence for a first-order transition), and superconductivity is suppressed almost completely, while the effects are much reduced for x=0.20, which show a bulk superconductivity with reduced T c
TL;DR: The crystal structures of two ternary, chalcopyrite-structured semiconductors, CuInSe2 and CuInTe2, have been determined from single crystal X-ray diffraction data as mentioned in this paper.
TL;DR: In this article, the dielectric properties of a series of single-crystal and sintered polycrystalline zirconias have been measured at room temperature over the frequency range 1 − 12.5 kHz.
Abstract: The dielectric properties of a series of single-crystal and sintered polycrystalline zirconias have been measured at room temperature over the frequency range 1–12.5 kHz. The additive cations investigated were yttrium, magnesium, calcium and cerium. For monoclinic zirconia, it was found that the permittivity, ɛ′, was 23 at 10 kHz, in agreement with previous work. The addition of stabilizing cations causes a substantial increase in ɛ′ which then lies in the range 32–42 depending on the nature and amount of the stabilizing cation. Analysis of the separate contributions to the overall permittivity arising from the monoclinic, tetragonal or cubic phases has been made for some mixed-phase systems. The results suggest that the permittivity is principally determined by the crystallographic form rather than by the nature or amount of the added cation.
TL;DR: In this article, the structure and properties of the thermodynamically stable δ−phase Mn1−xGax single crystal thin films grown on GaAs were reported and the properties of these films were analyzed.
Abstract: We report the structure and properties of the thermodynamically stable δ‐phase Mn1−xGax single crystal thin films grown on GaAs. X‐ray Θ–2Θ scans and grazing‐incidence scattering measurements confirmed that the unit cell of this phase is tetragonal (a=0.279 nm, c=0.351 nm) and grows with the c‐axis oriented normal to the {001} GaAs substrate surface. X‐ray emission spectroscopy confirmed the composition to be 62±2% Mn. Polar Kerr rotation, SQUID and vibrating sample magnetometer measurements with the field applied along the thin‐film normal showed nearly perfect square hysteresis loops confirming perpendicular anisotropy of the films. The film exhibits a Kerr rotation angle of ∼0.1° at 820 nm, a coercivity of 6.27 kOe and a saturation magnetization of 460 emu/cm3. The optical reflectivity of the film was 65%–70% over a broad range of wavelengths. This unique set of properties make it a very promising material for magneto‐optic recording with the additional potential of integrating semiconductor/magnetic d...
TL;DR: This article reported that polycrystalline monoclinic (m), tetragonal (t), and cubic (c) ZrO[sub 2], sintered at 1500[degrees]C, were annealed in the cubic stability field and rapidly cooled to permit the displacive c [yields] t[sup 1] transformation to occur in compositions containing 0-6 mol% Y [sub 2]O [sub 3]
Abstract: This paper reports that polycrystalline monoclinic (m), tetragonal (t), and cubic (c) ZrO[sub 2], sintered at 1500[degrees]C, were annealed in the cubic stability field and rapidly cooled to permit the displacive c [yields] t[sup 1] transformation to occur in compositions containing 0-6 mol% Y[sub 2]O[sub 3]. The bulk fracture toughness of coarse-grained ([gt] 25[mu]m) m, t[sup 1], and c zirconias were compared with conventionally sintered, fine-grained (typically less than 1 [mu]m) materials. The ferroelastic monoclinic and tetragonal zirconias were more than twice as tough as paraelastic cubic zirconia.
TL;DR: In this paper, the structural and electronic properties of cubic and tetragonal zirconia were studied using a periodic ab initio Hartree-Fock method using only valence electrons explicitly, while effective core potentials were used for describing core electrons.
Abstract: The structural and electronic properties of cubic and tetragonal zirconia (${\mathrm{ZrO}}_{2}$) are studied using a periodic ab initio Hartree-Fock method. Only valence electrons are treated explicitly, while effective core potentials are used for describing core electrons. The equilibrium geometries and elastic properties of the two phases are studied, and the mechanism of phase transition is discussed, with possible effects of applied pressure being taken into consideration. The analysis of the electronic structure shows appreciable departure from a purely ionic type of bond.
TL;DR: In this paper, a phenomenological treatment of domain switching in a piezoelectrically-coupled system is described, which demonstrates the usefulness of micro-Raman spectroscopy for the study of ferroelectric domain structures.
Abstract: Domain switching of 90° ferroelectric domains in tetragonal BaTiO3 and PbTiO3 is induced by the application of stress along specific crystallographic axes. For BaTiO3, single‐domain crystals are obtained from twinned specimens by the application of ∼1.1 MPa of stress parallel to the a axis and twin boundaries are induced by application of ∼0.22 MPa of stress parallel to the c axis. Similar piezoelectrically‐induced domain switching was observed in PbTiO3 at elevated temperature. We observed the rotation of the crystallographic axes associated with domain switching via micro‐Raman spectroscopy. These results were consistent with optical microscope images of the domain switching which demonstrates the usefulness of micro‐Raman spectroscopy for the study of ferroelectric domain structures. A phenomenological treatment of domain switching in a piezoelectrically‐coupled system is described.
TL;DR: The results contradict various presumptions that the disorder in protein crystals can be modeled predominantly by elastic vibrations or rigid body movements.
Abstract: Diffuse scattering data have been collected on two crystal forms of lysozyme, tetragonal and triclinic, using synchrotron radiation. The observed diffraction patterns were simulated using an exact theory for simple model crystals which relates the diffuse scattering intensity distribution to the amplitudes and correlations of atomic movements. Although the mean square displacements in the tetragonal form are twice that in the triclinic crystal, the predominant component of atomic movement in both crystals is accounted for by short-range coupled motions where displacement correlations decay exponentially as a function of atomic separation, with a relaxation distance of approximately 6 A. Lattice coupled movements with a correlation distance approximately 50 A account for only about 5-10% of the total atomic mean square displacements in the protein crystals. The results contradict various presumptions that the disorder in protein crystals can be modeled predominantly by elastic vibrations or rigid body movements.
TL;DR: In this article, crack-free SBN (Sr{sub x}Ba{sub 1{minus}x}Nb{sub 2}O{sub 6}) thin films were obtained by a sol-gel method with metal alkoxides.
Abstract: This paper reports on crack-free SBN (Sr{sub x}Ba{sub 1{minus}x}Nb{sub 2}O{sub 6}) thin films that have been prepared by a sol-gel method with metal alkoxides. A homogeneous and stable precursor solution was obtained from Sr and Ba metal and Nb(OEt){sub 5} in ethanol with a key additive of ethoxyethanol. SBN (where x = 0.5) powder crystallized to orthorhombic phase at 700{degrees}C, and then transformed completely to tetragonal phase at 1200{degrees}C. The formation of tetragonal SBN was observed on sapphire (R) substrates at 700{degrees}C, whereas the tetragonal phase began to appear in the powders at 1000{degrees}C. SBN films with highly preferred orientation were successfully synthesized on MgO (100) substrates at 670{degrees}C.
TL;DR: In this paper, the authors emphasize the need for paying attention to the precursor zirconium salt in preparing the monoclinic or tetragonal phases, and they show that a salt precursor obtained from a supplier at different times may produce different results.
Abstract: Procedures have been developed for preparing zirconia to yield either tetragonal or monoclinic phases following low (400–600 °C) temperature calcination. The data in this note emphasize the need for paying attention to the precursor zirconium salt in preparing the monoclinic or tetragonal phases. A salt precursor obtained from a supplier at different times may produce different results. Generally, it is more difficult to obtain a precursor that will produce the monoclinic phase than the one that will produce the tetragonal form.
TL;DR: In this paper, lattice parameter data of cubic phases and cube roots of unit-cell volumes of tetragonal phases in homogeneous ZrO{sub 2}-containing solid solutions were compiled to examine the validity of Vegard's law.
Abstract: This paper reports that lattice parameter data of cubic phases and cube roots of unit-cell volumes of tetragonal phases in homogeneous ZrO{sub 2}-containing solid solutions were compiled to examine the validity of Vegard's law. Except for ZrO{sub 2}--CeO{sub 2} and ZrO{sub 2}--UO{sub 2} systems, the data for cubic phases were expressed by the equation d = a,X + B, where d, a{sub s}, X, and b denote the lattice parameter, a constant depending on dopant species, the dopant content, and a constant independent of dopant species, respectively. For tetragonal phases, the cube roots of unit-cell volumes could be fitted by a similar equation except for the data in the ZrO{sub 2}--MO{sub 2} systems (M = Ge and U). The constant a{sub s} was calculated using an ion-packing model and was independent of the defect cluster models. The calculated a{sub s} is close to the experimentally observed one, although the former is slightly smaller than the latter in the ZrO{sub 2}--MO{sub u} systems (u = 1 and 1.5). This difference was ascribed to the lack of consideration of the ionic distortions from the ideal sites of the fluorite-type structure.
TL;DR: In this paper, the room-temperature fracture behavior of polycrystalline MoSi2 was characterized using Vickers indentation fracture, aided by the optically active grain structure revealed under polarized light.
Abstract: The room-temperature fracture behavior of polycrystalline MoSi2 was characterized using Vickers indentation fracture. Fracture analysis was aided by the optically active grain structure of MoSi2 revealed under polarized light. Radial crack propagation from indentations was found to be predominantly transgranular. The approximate indentation fracture toughness of MoSi2 was 3 MPa.m1/2, while the measured hardness was 8.7 GPa. Fracture behavior is believed to be controlled by anisotropy and cleavage energy of the tetragonal MoSi2 crystal structure.
TL;DR: In this paper, stoichiometric KNbO3 layers were sputter deposited onto (100) • spinel and (100 • MgO substrates at a substrate temperature of 610 °C. The composition of the layers was determined by Rutherford backscattering (RBS) measurements.
Abstract: Thin, stoichiometric KNbO3 layers were sputter deposited onto (100)‐spinel and (100)‐MgO substrates at a substrate temperature of 610 °C. The composition of the layers was determined by Rutherford backscattering (RBS) measurements. Potassium stoichiometry was maintained by an enrichment of the target with K2CO3. Furthermore, channeling experiments indicated good crystal quality with a χmin of 40%. The x‐ray diffraction spectra showed single‐crystalline tetragonal films. Waveguide modes with losses of only 1.1 dB/cm at λ=632.8 nm could be excited in the sputtered films. The nonlinear optical coefficient d31=5 pm/V was determined by the Maker‐fringe technique using the 1.06 μm Nd:YAG laser fundamental wavelength (compared to d31=15 pm/V measured in orthorhombic single crystals).
TL;DR: The microstrueture development during non-isothermal and isothermal sintering has been studied for tetragonal zireonia ceramics (TZP) containing carious amounts of Y, Ce and Ti.
Abstract: The microstrueture development during non-isothermal and isothermal sintering has been studied for tetragonal zireonia ceramics (TZP) containing carious amounts of Y, Ce and Ti. Smaller grain sizes were obtained when Ce-TZP was doped with yttrium. This could he attributed to segregation of yttrium to the grain boundaries, thus causing an impurity drag. With increasing temperature the grain growth in the Ce-TZP samples became faster, which could be attributed to the absence of a dragging force. The slow grain growth at low temperature in the Ce-TZP samples could he attributed to the slow diffusion kinetics of the diffusing species (trivalent and tetravalent cerium). The critical grain size for retainment of the tetragonal phase at room temperature is larger in the ECe-TZP systems compared to the Y-TZP amt Ce-TZP systems. The chemical stability increased by doping Y-TZP with cerium or titanium.