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Showing papers on "Tetragonal crystal system published in 1990"


Journal ArticleDOI
29 Mar 1990-Nature
TL;DR: Superplasticity is defined phenomenologically as the ability of a material to exhibit exceptionally large elongations during tensile deformation, and is well established for metals and alloys.
Abstract: SUPERPLASTICITY is defined phenomenologically as the ability of a material to exhibit exceptionally large elongations during tensile deformation1. It is a property of some poly crystalline solids, and is well established for metals and alloys2. Superplasticity has also been observed in some ionic crystals, such as Y2O3-stabilized tetragonal ZrO2 polycrystals3,4, but has not been found previously for covalent crystals. Here we report superplastic elongation (by more than 150%) of a covalent crystal composite, Si3N4/SiC. The superplasticity is probably related to the presence of an inter-granular liquid phase. Combined with its hardness, this property suggests several useful applications for the novel material: for example, to form engine components—superplasticity will make it readily mouldable at high temperatures.

323 citations


Journal ArticleDOI
TL;DR: In this paper, the Mossbauer ligand-field spectra and magnetic behaviour of both the hydrated and non-hydrated compounds are discussed, and the spin transition has been followed by EPR measurements with the aid of traces of Cu2+ ions which could be substituted for Fe2+ in the tetragonal structure.

175 citations


Journal ArticleDOI
TL;DR: In this article, it has been found that Mn or Cr can substitute for Al in Al 3 Ti and change the crystal structure from tetragonal DO 22 to cubic L 1 2.
Abstract: It has been found that Mn or Cr can substitute for Al in Al 3 Ti and change the crystal structure from tetragonal DO 22 to cubic L1 2 . The new phases have been produced with negligible porosity and second phases, and have compositions near Al 67 Mn 8 Ti 25 and Al 67 Cr 8 Ti 25 . As with other cubic forms of Al 3 Ti, the ternary addition can range from bout 5 to 15 at. pct. while still maintaining the cubic phase. These new cubic phases have significantly reduced hardnesses and greatly improved crack resistance compared to binary Al 3 Ti, as well as other cubic forms based on alloying with Fe, Ni, or Cu. In addition to appreciable ductility in compression, the new phases show some tensile ductility

165 citations


Journal ArticleDOI
TL;DR: In this paper, the effect of elevated temperature on the structural stability and performance of Mo-Si multilayer mirrors is investigated, characterized by an increase in the width of the amorphous interlayer regions, as well as the nucleation of microcrystallites of silicide.
Abstract: The effect of elevated temperature on the structural stability and performance of Mo‐Si multilayer mirrors is investigated. Mo‐Si multilayers deposited by magnetron sputtering are annealed at temperatures ranging from 200 to 800 °C. A detailed and consistent picture of the thermally induced changes in the microstructure is obtained using an array of complementary measurement techniques including small‐ and large‐angle x‐ray scattering and high‐resolution electron microscopy. The first significant structural changes are observed at 400 °C, characterized by an increase in the width of the amorphous interlayer regions, as well as the nucleation of microcrystallites of silicide in these regions. At higher temperature the Mo layers transform completely into polycrystalline mixtures of Mo5 Si3 and MoSi2 in both the hexagonal and tetragonal phase. The layers of silicide remain intact but exhibit a structural instability, resulting in severely warped layers surrounded by pockets of amorphous Si and voids. By 800 °C the layered structure is completely destroyed and the composition is predominately tetragonal MoSi2 . The performance of the multilayers as normal‐incidence x‐ray mirrors is measured and correlated with the observed structural modifications. Finally, our results are compared and contrasted with other annealing studies of the Mo‐Si system.

151 citations


Journal ArticleDOI
TL;DR: In this paper, the phase relationship over a wide range of temperature and compositions in the ZrO2-CeO2 system has been reinvestigated and the extent of the different single and two-phase fields were determined with precise lattice parameter measurements on quenched samples.
Abstract: The phase relationships over a wide range of temperature and compositions in the ZrO2-CeO2 system have been reinvestigated. From DTA results, thermal expansion measurements andKIC determinations it was established that additions of CeO2 to ZrO2 decreases the monoclinic to tetragonal ZrO2 transition temperature, from 990 ° C to 150 ∓ 50 ° C, and an invariant eutectoid point at approximately 15 mol% CeO2 exists. The extent of the different single- and two-phase fields were determined with precise lattice parameter measurements on quenched samples. Evidence for the existence of a binary compound Ce2Zr3O10 (o-phase) was obtained by X-ray diffraction. The o-phase was stable below approximately 800 ° C, above which it decomposes into tetragonal zirconia + fluorite ceria solid solutions. Taking into account the polymorphic tetragonal-cubic transition and the narrowness of the two-phase tetragonal zirconia + fluorite ceria field above 2000 ° C, the existence of a new invariant eutectoid point was assumed, in which the metastable fluorite zirconia solid solution decomposes into tetragonal zirconia + fluorite ceria solid solutions. From the results obtained, the phase diagram also incorporates a eutectic point located at approximately 2300 ° C and 24 mol % CeO2.

140 citations



Journal ArticleDOI
TL;DR: In this paper, the phase relationships in the CuO-rich region of these ternary diagrams progressively change from the La system through the Nd, Sm, Eu, Gd, Y, Ho systems to the Er system.

129 citations


Journal ArticleDOI
TL;DR: In this article, the isothermal nucleation and crystallization kinetics of hydrothermally prepared monoclinic and tetragonal ZrO2 have been determined at various pH conditions.
Abstract: The isothermal nucleation and crystallization kinetics of hydrothermally prepared monoclinic and tetragonal ZrO2 have been determined at various pH conditions. It is shown that monoclinic ZrO2 precipitates at low pH whereas at high pH tetragonal ZrO2 crystallizes from an amorphous zirconium (hydrous) oxide, Zr(OH)xOy, precursor. At intermediate pH conditions mixtures of the polymorphs are formed suggestive of kinetically competing particle formation mechanisms. The data are explained by the proposed existence of three controlling regimes for the formation of crystalline ZrO2: dissolution/precipitation at low pH, a solubility controlled regime at intermediate pH values, and a gel structure controlled regime at high pH. Apparent activation energies for the nucleation and crystallization of monoclinic and tetragonal ZrO2 formed under hydrothermal conditions are presented.

124 citations


Journal ArticleDOI
TL;DR: In this article, a tetragonal "1-2-3" type was synthesized as a superconductor with an onset temperature of 60 K, where a and c are 3.7949 and 11.4102 A, respectively.
Abstract: At a high oxygen pressure of 7 GPa and high temperature of 1380°C, YSr2Cu3Oy was synthesized as a single phase. The crystal structure is a tetragonal "1-2-3" type, where a and c are 3.7949 A and 11.4102 A, respectively. This material is a superconductor with an onset temperature of 60 K.

123 citations


Journal ArticleDOI
TL;DR: In this article, a small addition of CuO to 2-mol-Yz03-stabilized tetragonal zirconia polycrystals significantly enhances superplasticity by forming an amor hous grain-boundary phase containing primarily Cu', Y", Zr", and 0'-.
Abstract: .. . A small addition of CuO to 2-mol%-Yz03-stabilized tetragonal zirconia polycrystals significantly enhances superplasticity by forming an amor hous grain-boundary phase containing primarily Cu', Y", Zr", and 0'-. This phase apparently melts at around 1130°C, but it already provides a fast diffusion path even below the melting temperature. There are abrupt changes in stress exponent, activation energy, and grain size exponent across the melting temperature. Superplasticity is diffusion-controlled below the melting temperature and is interfaced-controlled above that. [Key words: yttria-stabilized tetragonal zirconia polycrystals, plasticity, liquid phase, creep, grain boundaries.]

116 citations


Journal ArticleDOI
01 Jun 1990
TL;DR: The following binary metal oxides have yielded high-resolution 17O nuclear magnetic resonance (NMR) spectra, obtained by magic-angle spinning (MAS) without isotopic enrichment: α-Al2O3, TiO2 (anatase and rutile), Ti2O 3, monoclinic VO2, Cu2O, ZnO, tetragonal and monocallinic ZrO2, Ag2O 2, tetragonagonal SnO, hexagonal La 2O 3, hexagonal Na β-Al 2
Abstract: The following binary metal oxides have yielded high-resolution 17O nuclear magnetic resonance (NMR) spectra, obtained by magic-angle spinning (MAS) without isotopic enrichment: α-Al2O3, TiO2 (anatase and rutile), Ti2O3, monoclinic VO2, Cu2O, ZnO, tetragonal and monoclinic ZrO2, Ag2O, tetragonal SnO, hexagonal La2O3, CeO2, HfO2, α-PbO; also Na β-Al2O3 and cubic MgO-ZrO2. Detection sensitivity limits observation to cases where the nuclear quadrupole coupling is less than 4 MHz. Different ceramic phases can readily be distinguished by their chemical shift. As empirical chemical shift correlation is suggested between 17O in the oxide M2Om, and 19F in the corresponding fluoride MFm.

Journal ArticleDOI
TL;DR: ZrO{sub 2} powder was prepared by a sol-emulsion-gel method at temperatures below 140{degrees}C from ZrO(NO{sub 3} ){center dot} nH{ sub 2}O. The asprepared powder was amorphous, but crystallized into the tetragonal structure by 600
Abstract: ZrO{sub 2} powder was prepared by a sol-emulsion-gel method at temperatures below 140{degrees}C from ZrO(NO{sub 3}){sub 2} {center dot} nH{sub 2}O. The asprepared powder was amorphous, but crystallized into the tetragonal structure by 600{degrees}C. The metastable tetragonal powder (600{degrees}C) was comprised of ultrafine 4- to 6-nm size particles. On heat treatment, the tetragonal form completely transformed into the monoclinic state at 1100{degrees}C. Preliminary studies indicate good sinterability with densities greater than 94% at 1100{degrees}C and with a grain size of 0.25 {mu}m.

Journal ArticleDOI
TL;DR: In this article, rare earth (Nd, Sm, Er, Yb, Sc)-doped zirconia was melted using an arc-imaging furnace, and rapidly quenched with a hammer and anvil apparatus (cooling rate >105 K sec−1).
Abstract: Rare-earth (Nd, Sm, Er, Yb, Sc)-doped zirconia was melted using an arc-imaging furnace, and rapidly quenched with a hammer and anvil apparatus (cooling rate >105 K sec−1). These ZrO2-RO1.5 samples were investigated by X-ray diffraction and transmission electron microscopy. The existing region of metastable tetragonal zirconia is from 2 to 14 mol% of RO1.5 regardless of the species of dopants, RO1.5. In the lattice parameters of the tetragonal phases, the unit cell volume corresponds to the ionic radii of dopants, whereas the tetragonality (c/a) is independent of the species of dopants, but dependent on the content of dopants.

Journal ArticleDOI
TL;DR: In this paper, the compounds SrVO3 and Sr2VO4 have been synthesized by decomposition of the nitrates in air followed by high purity hydrogen reduction and they have been studied by X-ray and neutron diffraction measurements.

Journal ArticleDOI
27 Apr 1990-Science
TL;DR: Observations suggest that under the experimental synthesis conditions, and perhaps in the earth's lower mantle, the stable phase of MgSiO3 might have the cubic perovskite structure.
Abstract: Crystals of MgSiO3 perovskite synthesized at high pressures and temperatures have orthorhombic symmetry under ambient conditions. Examination by transmission electron microscopy shows that the microstructure of crystals synthesized at 26 gigapascals and 1600°C is dominated by a large number of twin domains that are related by reflection operations with respect to {112} and {110} planes. These twins may be associated with the transformations of MgSiO3 perovskite from the cubic to tetragonal and tetragonal to orthorhombic phases, respectively, upon decreasing pressure and temperature. These observations suggest that under the experimental synthesis conditions, and perhaps in the earth's lower mantle, the stable phase of MgSiO3 might have the cubic perovskite structure.

Journal ArticleDOI
TL;DR: In this article, the powder X-ray structure analysis of VO2(A) has been performed by the use of EXAFS and the Rietveld method; possible structure models based on the ex-AFS results were examined by the Rieveld analysis.

Journal ArticleDOI
TL;DR: The phase diagram of the BaO-CuO binary system has been investigated in air and in a mixed gas of Ar + 0.21 atm of O2 as mentioned in this paper.
Abstract: The phase diagram of the BaO-CuO binary system has been investigated in air and in a mixed gas of Ar + 0.21 atm of O2. The existence of two compounds, BaCuO2 and Ba2CuO3, was confirmed. The phase transition of Ba2CuO3 from an orthorhombic to a tetragonal phase was observed to occur at 1083 K. The lattice constants of the tetragonal Ba2CuO3 phase were determined to be a= 1.2975 nm and b= 0.3992 nm. BaCuO2 was shown to melt incongruently by a synthetic reaction at 1289 K. Furthermore, the existence of two eutectic reactions and a peritectic reaction in the present system was confirmed.

Journal ArticleDOI
TL;DR: In this paper, a tetragonal structure with a distribution of the cationic vacancies on octahedral sites of the spinel lattice is proposed for γ and δ aluminas obtained by dehydration of boehmite.

Journal ArticleDOI
TL;DR: In this paper, large-grained (100-to 200-μm) yttria-doped, polycrystalline t'-zirconia ceramics were fabricated by heat-treating presintered samples at temperatures 2100°C.
Abstract: Large-grained (100- to 200-μm), yttria-doped, polycrystalline t'-zirconia ceramics were fabricated by heat-treating presintered samples at temperatures 2100°C. Polarized light microscopy revealed the ferroelastic domain structure in the t' samples. XRD showed that no monoclinic phase was detected on as-polished, ground and fracture surfaces, or on surfaces while under a tensile stress as high as 400 MPa. By contrast, relative changes occurred in the tetragonal peak intensities, which were attributed to ferroelasatic domain switching. The higher toughness of 3-mol%-Y2O3-doped t' samples (7.7 MPa · m1/2) compared to that of 8 mol% Y2O3 cubic samples (2.4 MPa · m1/2) was explained in part by ferroelastic domain switching.

Journal ArticleDOI
TL;DR: In this article, a new stable phase has been identified in the Nd-Fe-Si system using X-ray diffraction examination of single crystals, and the crystal structure is an ordered version of Nd 6 Ga 3 Fe 11.
Abstract: A new stable phase has been identified in the Nd-Fe-Si system. X-ray diffraction examination of single crystals was used to establish the crystal structure of Nd 6 Fe 13 Si. The tetragonal space group is I 4/ mcm and the lattice parameters are a = b = 0.8034(3)nm, c = 2.278(1)nm. The crystal structure is an ordered version of Nd 6 Ga 3 Fe 11 .Magnetic measurements show an antiferromagnetic behaviour.

Journal ArticleDOI
TL;DR: P1 nuclease, a zinc-dependent single-strand specific endonuclease from Penicillium citrinum, has been crystallized in three different space groups using either ammonium sulphate or polyethylene glycol 4000 as the precipitating agent.

Journal ArticleDOI
TL;DR: A classification of p-type superconductors is proposed in this article, where a tetragonal symmetry (a = 0.388 nm, c = 2.914 nm) and three blocks are a (Pb/Cu)-O single layer, a Cu-O 5 pyramid and a fluorite-type Pr 2 -O 2 layer.
Abstract: A classification of p-type superconductors is proposed. According to this classification, PbPr 2.4 Sr 1.6 Cu 2.6 O y ceramic was designed and synthesized as a ceramic having a new crystal structure. This ceramic has a tetragonal symmetry ( a =0.388 nm, c =2.914 nm) and consists of three blocks which are a (Pb/Cu)-O single layer, a Cu-O 5 pyramid and a fluorite-type Pr 2 -O 2 layer. It is expected to be a mother material of superconductors.

Journal ArticleDOI
TL;DR: In this paper, it was shown that the tetragonal (t) → monoclinic (m) martensitic transformation in ZrO2 can occur thermo-elastically in certain ZRO2-containing ceramics, and that microcracking accompanying the transformation is more common than had previously been recognized.
Abstract: Recent evidence is summarized showing that the tetragonal (t) → monoclinic (m) martensitic transformation in ZrO2 can occur thermoelastically in certain ZrO2-containing ceramics, and that microcracking accompanying the transformation is more common than had previously been recognized. The implications of these new data for the conditions under which the stress-induced transformation is irreversible, and for the particle size dependence of the transformation start (Ms), temperature, are discussed.

Journal ArticleDOI
TL;DR: In this article, relative-enthalpy measurements have been made on the hexagonal, tetragonal, glass and liquid phases of GeO2 and they have been used to derive a consistent set of thermodynamic properties.
Abstract: Relative-enthalpy measurements have been made on the hexagonal, tetragonal, glass and liquid phases of GeO2. The glass transition is very sensitive to the impurity content, with a T g ranging from 980 K for a pure product to 780 K for a Li-doped sample with 0.06 mol % Li. The relative C p change at T g of about 5% increases with the impurity content as a result of lower glass transition temperatures. Above 298 K the derived heat capacities are similar for all forms, with slightly higher values for the amorphous phases and two C p cross-overs at 400 and 1000 K between the hexagonal and tetragonal modifications. For both GeO2 and SiO2 the coordination state markedly affects C p and the entropy below 300 K, where the properties are much lower for the tetragonal than for the hexagonal modifications, i.e., S 298 = 39.7 vs 55.3 J/mole K and 27.8 vs 41.4 J/ mole K for GeO2 and SiO2, respectively. The high-temperature C p's of coesite and stishovite are likely similar to those of the low-pressure SiO2 forms. Finally, these results, low-temperature C p data and enthalpy-of-solution measurements have been used to derive a consistent set of thermodynamic properties for the GeO2 modifications.

Journal ArticleDOI
01 Feb 1990-EPL
TL;DR: In this paper, the structural and magnetic properties of stoichiometric Nd2NiO4 have been investigated in the presence of a ferromagnetic component along c-axis in the Ni2+ spin structure.
Abstract: Neutron diffraction on polycrystalline samples of stoichiometric Nd2NiO4 shows a complex structural and magnetic behaviour as a function of temperature. The room temperature (RT) phase is orthorhombic (Bmab) and Ni2+ ions are 3D antiferromagnetically ordered (TN ≈ 320 K), with a propagation vector k = [100] and spins oriented parallel to the propagation vector, i.e. along the a-axis. The magnetic structure can be described as a gx mode (Shubnikov group BPmab'). The magnetic moment for Ni2+ is 1.57μB at 160 K. The system undergoes a structural phase transition from orthorhombic to tetragonal (P42/ncm) at T1 ≈ 130 K. The tetragonal phase allows the existence of a ferromagnetic component along c-axis in the Ni2+ spin structure, the magnetic structure can be described either as a gxcyfz mode (Shubnikov group (Pc' c' n) or as a gx + cyfz mode (Shubnikov group P42/nc' m'). At a temperature as high as 70 K the polarization of the Nd3+ ions becomes noticeable. At low temperature (TN ≈ 8 K) Nd3+ ions cooperative order takes place with a magnetic moment of 3.2μB at 1.5 K and a magnetic structure belonging to the same Shubnikov group (either Pc' c' n or P42/nc' m'). Fully oxidized samples of Nd2NiO4+δ as obtained in air atmosphere, seems to be also orthorhombic at RT and do not show static magnetic order or structural phase transitions between 1.5 K and 300 K. Partially oxidized samples (δ = 0.04) are also orthorhombic at RT, and show a similar magnetic behaviour as stoichiometric ones, but no structural phase transition at low temperatures is observed.

Journal ArticleDOI
01 Mar 1990-EPL
TL;DR: In this article, a single-domain PbTiO3 crystal was used for light scattering measurements extended down to 2 cm−1 of the laser line and the appearance of a central peak below the soft-phonon line when the cubic-tetragonal phase transition is approached with decreasing and increasing temperature.
Abstract: Light scattering measurements extended down to 2 cm−1 of the laser line were performed on a single-domain PbTiO3 crystal. Spectra reveal the appearance of a central peak below the soft-phonon line when the cubic-tetragonal phase transition is approached with decreasing and increasing temperature. The relaxation process associated with this central peak presents a characteristic frequency smaller than 1 GHz and is shown to possess the main contribution to the dielectric permittivity near the transition temperature. This feature constitutes the first direct evidence of some order-disorder mechanism in the ferroelectric transition of PbTiO3 which was up to now considered as a typical example of a displacive mechanism.

Journal ArticleDOI
TL;DR: In this paper, the XRD pattern and thermal behavior of barium titanyl oxalate tetrahydrate (BTOT) were investigated and it was shown that BTOT crystallizes in the monoclinic system, 2/m, with unit cell dimensions of a= 14954 A, b= 19332 A, c= 13947 A, and β= 10643° Unit cell content (Z) is 12 and the Bravais lattice is P Theoretical density is 231 g/cm3 At a relatively low temperature (∼60°C
Abstract: The XRD pattern and the thermal behavior of barium titanyl oxalate tetrahydrate (BTOT) were investigated BTOT crystallizes in the monoclinic system, 2/m, with unit cell dimensions of a= 14954 A, b= 19332 A, c= 13947 A, and β= 10643° Unit cell content (Z) is 12 and the Bravais lattice is P Theoretical density is 231 g/cm3 At a relatively low temperature (∼60°C), BTOT starts to dehydrate, resulting in a less-than-calculated weight loss of ignition Barium titanate powder obtained by calcining the oxalate exists as the cubic perovskite phase, instead of the tetragonal phase at low temperatures This happens when the particle size of the crystals is smaller than ∼ 30 nm As the crystal coarsens, barium titanate powder transforms to the tetragonal state

Journal ArticleDOI
TL;DR: The superconducting oxide in the Sr-Ca-Y-Pb-Cu-O system was identified and prepared as a single phase as mentioned in this paper, and the compound examined has a tetragonal cell with a=3.815 A and c=11.85 A.
Abstract: The superconducting oxide in the Sr-Ca-Y-Pb-Cu-O system was identified and prepared as a single phase. The compound examined has a tetragonal cell with a=3.815 A and c=11.85 A. The Tc was 17 K for the sample quenched from 860°C to 77 K, and below 4.2 K for the sample annealed in air at 400°C.

Journal ArticleDOI
TL;DR: In this paper, a polycrystalline tetragonal superconductors Tl 2 Ba 2 CuO 6± δ with critical temperatures ranging from 12 K to 92 K were synthesized by combining two techniques, synthesis in evacuated ampoules and annealing in an H 2 /Ar flow at 290°C.
Abstract: Polycrystalline tetragonal superconductors Tl 2 Ba 2 CuO 6± δ with critical temperatures ranging from 12 K to 92 K were synthesized by combining two techniques, synthesis in evacuated ampoules and annealing in an H 2 /Ar flow at 290°C. It is demonstrated that several tetragonal forms exhibit bulk superconductivity with diamagnetic volume fractions ranging from 25% to 65%, whereas other tetragonal forms do not superconduct. However, results obtained for the orthorhombic samples suggest that this material does not superconduct. These results show that two factors play an important role in the appearance of superconductivity in the thallium cuprates: the oxygen nonstoichiometry, which governs the hole carrier density, and the homogeneous distribution of the oxygen and vacancies, which should improve superconductivity. Relationships between structure, chemical bonding and superconductivity in Tl 2 Ba 2 CuO 6± δ are also discussed.

Journal ArticleDOI
TL;DR: In this paper, the polarization mechanisms of the morphotropic phase boundary compositions of tungsten bronze relaxor ferroelectrics, Pb1−xBaxNb2O6 (PBN[( 1−x) %]), were investigated by means of electrical, thermal, optical, and electron microscopic methods.
Abstract: Investigations of the polarization mechanisms in tungsten bronze relaxor ferroelectrics, Pb1–xBaxNb2O6 (PBN[(1−x) %]), in particular, of the morphotropic phase boundary compositions have been carried out for both ceramics and bulk crystals by means of electrical, thermal, optical, and electron‐microscopic methods. Relations of phase‐transition behavior with compositional and polarizational fluctuations have been carefully studied. Instead of a sharp boundary composition, it is found that a rather wide composition range, 1−x≊0.63±0.03 in Pb1−xBaxNb2O6, corresponds to the orthorhombic and tetragonal morphotropic phase boundary. It is demonstrated with the help of optical conoscopic interference figures that for the near‐morphotropic phase boundary single crystal PBN of tetragonal symmetry, the polarization vector can be switched orthogonally to an orthorhombic phase simply by the application of an electric field, which authenticates optically the 90° polarization rotation. A transmission electron microscope study in the temperature range from −180 °C to room temperature revealed the presence of incommensurate ferroelastic domains in the morphotropic phase boundary composition PBN crystals. In light of the above observed experimental data, the polarization mechanisms of the morphotropic phase‐boundary compositions are discussed in terms of qualitative thermodynamics.