Showing papers on "Tetragonal crystal system published in 1986"
TL;DR: Crystallographic data are presented in phase I (cubic, diamond), II (tetragonal, ..beta..-Sn), V (simple hexagonal), VII (hexagonal close-packed), and the metastable phase III (body-centered-cubIC (BC8)) and on the coexistence of the phases.
Abstract: X-ray-diffraction data have been obtained on Si in a diamond anvil cell to pressures of \ensuremath{\sim}50 GPa. Crystallographic data are presented in phase I (cubic, diamond), II (tetragonal, \ensuremath{\beta}-Sn), V (simple hexagonal), VII (hexagonal close-packed), and the metastable phase III [body-centered-cubic (BC8)] and on the coexistence of the phases. Comparison is made between these data and the predictions of ab initio calculations for these structures and their equations of state.
412 citations
TL;DR: In this article, the 57Fe Mossbauer effect was applied to the SrFeOx system with 2.5 ≤ x ≤ 3.0 and the results indicated the ideal series of SrFeO 3− 1 n, where n = ∞, 8, 4, and 2 give x = 3, 2.75, 2, and 1.50, correspondingly.
400 citations
TL;DR: In this article, the detrimental aging phenomenon observed in ZrO2-Y2O3 materials was investigated with respect to the gaseous aging environment and the Y 2O3 and SiO2 content of the material and it was shown that the aging phenomenon is caused by water vapor and that intergranular silicate glassy phases play no significant role.
Abstract: The detrimental aging phenomenon observed in ZrO2-Y2O3 materials, which causes tetragonal ZrO2 to transform to its monoclinic structure at temperatures between 150 and 400°C, was investigated with respect to the gaseous aging environment and the Y2O3 and SiO2 content of the material. It is shown that the aging phenomenon is caused by water vapor and that inter-granular silicate glassy phases play no significant role. Transmission electron microscopy of thin foils, before and after aging, showed that the water vapor reacted with yttrium in the ZrO2 to produce clusters of small (20 to 50 nm) crystallites of α-Y(OH)3. It is hypothesized that this reaction produces a monoclinic nucleus (depleted of Y2O3) on the surface of an exposed tetragonal grain. Monoclinic nuclei greater than a critical size grow spontaneously to transform the tetragonal grain. If the transformed grain is greater than a critical size, it produces a microcrack which exposes subsurface tetragonal grains to the aging phenomenon and results in catastrophic degradation. Degradation can be avoided if the grain size is less than the critical size required for microcracking.
360 citations
TL;DR: In this article, it was argued that the tetragonal to monoclinic transformation in zirconia is exothermic, which guarantees that the surface free energy of the former is less than that of the latter structure.
Abstract: It is argued that because the tetragonal (t) to monoclinic (m) transformation in zirconia is exothermic, this guarantees that the surface free energy of the former is less than that of the latter structure. It is argued further that for pristine, unconstrained, single crystals,A
s ∼M
s ∼ 1447 K. It follows then, from a thermodynamic analysis that the reciprocal crystallite size is a linear function of the transformation temperature. Quantitative agreement was obtained between calculated and experimental crystallite size-temperature data which ranged over three orders of magnitude.
287 citations
TL;DR: The protein concentration dependence of the growth rate of lysozyme crystals (tetragonal form) was investigated at two NaCl concentrations (3.5%, 5% w/v) as discussed by the authors.
223 citations
TL;DR: In this paper, electron diffraction measurements on (100) GaAs1−xSbx layers with x≊0.5 grown by organometallic vapor phase epitaxy indicate that ordered phases are formed during growth.
Abstract: Electron diffraction measurements on (100) GaAs1−xSbx layers with x≊0.5 grown by organometallic vapor phase epitaxy indicate that ordered phases are formed during growth. Two ordered phases are observed. The simple, tetragonal AuCu‐I type phase consists of alternating {100} oriented GaAs and GaSb layers. Only the two variants with the tetragonal c axes perpendicular to the growth direction are observed. At least two variants are observed for the chalcopyrite E11 structure with alternating {210} oriented GaAs and GaSb layers.
181 citations
TL;DR: In this article, the average grain size of ZrO2(+Y, o) materials sintered at 1400°C was observed to depend significantly on the Y2O3 content.
Abstract: The average grain size of ZrO2(+Y, o,) materials sintered at 1400°C was observed to depend significantly on the Y2O3 content. The average grain size decreased by a factor of 4 to 5 for Y2O3 contents between 0.8 and 1.4 mol% and increased at Y2O3 contents of 6.6 mol%. Grain growth control by a second phase is the concept used to interpret these data; compositions with a small grain size lie within the two-phase tetragonal + cubic phase field, and the size of the tetragonal grains is believed to be controlled by the cubic grains. This interpretation suggests that the Y2O3-rich boundary of the two-phase field lies between 0.8 and 1.4 mol% Y2O3. Transformation toughened materials fabricated in this binary system must have a composition that lies within the two-phase field to obtain the small grain size required, in part, to retain the tetragonal toughening agent.
165 citations
TL;DR: In this article, the authors investigated the role of free energy on subsequent transformation nucleation during degradation of tetragonal polycrystalline t-ZrO2 polycrystals with 2 and 3 mol% Y2O3.
Abstract: Various aspects of the tetragonal (t) to monoclinic (m) transformation during degradation have been studied experimentally and theoretically in yttria-stabilized tetragonal zirconia polycrystals (Y-TZP), i.e., polycrystalline t-ZrO2 containing Y2O3 in solution. Transmission electron microscopy (TEM) revealed that protruding grains at the surface of Y-TZP specimens do not transform under corrosive conditions (250°C, humid atmosphere) even after an annealing time of 168 h. Eigenstresses due to anistropic thermal expansion In and around protruding and bulk grains have been calculated for Y-TZP containing 2 and 3 mol% Y2O3. The prominent role of these stresses on subsequent transformation nucleation during degradation is shown to agree qualitatively with an established free energy concept. The lack of complete transformation of m-ZrO2 is attributed to characteristics of the nucleation- and growth-controlled transformation process.
152 citations
TL;DR: The unit cell of HCa 2 Nb 3 O 10, as determined from powder X-ray diffraction, is tetragonal with a = 3.851 A and c = 14.38 A and contains one formula unit as mentioned in this paper.
Abstract: The alkali cations in the layered perovskites MCa 2 Nb 3 O 10 (M ≡ K, Rb, Cs) are exchanged by protons in aqueous acid in a topochemical reaction to give HCa 2 Nb 3 O 10 . The unit cell of HCa 2 Nb 3 O 10 , as determined from powder X-ray diffraction, is tetragonal with a = 3.851 A and c = 14.38 A and contains one formula unit. The anhydrous hydrogen compound hydrates to form a composition HCa 2 Nb 3 O 10 ·1.5H 2 O which is also tetragonal with a = 3.854 A and c = 16.23 A . The hydration-dehydration behavior at 24 °C as a function of the partial water pressure shows significant hysteresis as a result of the expansion and contraction of the unit cell along the c axis.
136 citations
TL;DR: Rietveld analyses of room-temperature neutron diffraction data for seven Nd2(Co/x/Fe/1-x/)14B alloys (x = 0,0.1, 0.3, 0.,5, 0,7, 0.9, 1) are reported in this paper.
Abstract: Rietveld analyses of room-temperature neutron diffraction data for seven Nd2(Co/x/Fe/1-x/)14B alloys (x = 0,0.1, 0.3, 0.5, 0.7, 0.9, 1) are reported. Throughout the entire composition range the Nd2Fe14B-type tetragonal crystal structure is maintained, with the lattice constants decreasing significantly as the Co concentration x increases. It is found that the J2-type transition-metal sites are preferentially occupied by Fe ions in the pseudoternary systems, a result which is analogous to the preferential Fe occupation of c sites previously observed in hexagonal Nd2(Co/x/Fe/1-x/)17 alloys.
TL;DR: In this article, it is shown that lattice plane bending near the specimen surfaces can cause tetragonal distortion observed in bulk, cubic strained layer superlattices, and spinodally decomposed alloys.
Abstract: Composition modulations in defect‐free crystalline materials are generally accompanied by a lattice distortion where unit cell dimensions are not characteristic of the local composition. The distortion originates from the constraint that the interatomic spacings perpendicular to the modulation direction be commensurate from one layer to the next. Thus there is an internal stress field set up in the material which attempts to suppress unit cell expansions and contractions in these perpendicular directions. A well‐known example is the tetragonal distortion observed in bulk, cubic strained layer superlattices, and spinodally decomposed alloys. However, it is frequently overlooked that these stresses can be significantly relaxed near surfaces of the bulk material, particularly those surfaces lying perpendicular to the modulation direction. This relaxation can be a dominating effect in materials which have been thinned in cross section for study by TEM. Lattice plane bending near the specimen surfaces can caus...
TL;DR: The crystal size of the orthorhombic form grown at 40°C was less sensitive to pH and P than the tetragonal form, and many differences between the two crystal forms are ascribed to different interactions to promote crystallization.
Abstract: If protein single crystals larger than those suitable for x-ray analysis are obtained, various spectroscopic, thermal, mechanical, and electrical measurements become possible. To understand the factors governing the crystal size, tetragonal lysozyme crystals were grown in batches at 15°C from solutions of different protein and salt concentrations between pH 4–7. The number and size of the crystals, and the protein concentration remaining in the supernatant, varied markedly with the initial salt amount, pH, and cation species, but large crystals always grew when the initial protein concentration (P) was in a narrow range of 2.5–3 times the crystal solubility (S). It was also shown (1) that the period before the first crystals appeared (D) varied as D ∝ (P/S)−n, where n ≃ 5, and (2) that many previous experiments used more supersaturated solutions than the optimal ones thus determined. The reason why large crystals grow only from moderately supersaturated solutions is discussed. The crystal size of the orthorhombic form grown at 40°C was less sensitive to pH and P than the tetragonal form. An effort to measure D and the solubility at 40°C revealed many differences between the two crystal forms, which we ascribe to different interactions to promote crystallization.
TL;DR: In this paper, a method for immobilizing protein crystals in small volumes to determine growth rates on various faces was applied to study the growth kinetics of the (100) face of tetragonal hen-egg white lysozyme crystals at different degrees of bulk saturation.
TL;DR: In this paper, the coarsening rates of both cubic and tetragonal Al3Zr precipitates in Al were measured and the effect of partial substitution of V for Zr reduced the precipitate-matrix mismatch for both phases.
Abstract: The coarsening rates of both cubic and tetragonal Al3Zr precipitates in Al were measured The tetragonal Al3Zr coarsened 16 times faster than the cubic modification in keeping with the fact that the latter is coherent and coplanar with the matrix while the former forms a semicoherent interface with the matrix giving a larger interfacial energy Partial substitution of V for Zr reduced the precipitate-matrix mismatch for both phases and slowed both coarsening rates as well as retarded the cubic to tetragonal transformation Reducing strain and interfacial energy no doubt is the origin of this effect Since the cubic particles are spherical, their volume fraction is small, and the coherency strains are small, this would appear to be an ideal system for testing the Lifshitz-Slyozov-Wagner theory of diffusion controlled Ostwald ripening While the theory seems to hold, the calculated diffusivity of Zr in Al is much higher than the value reported in the literature Because of the low coarsening rates of the dispersed particles, the Al-Zr-V system shows promise as the basis for a high temperature Al alloy
TL;DR: In this article, it was shown that SnO2 films at temperatures above 400°C are polycrystalline with the tetragonal rutile structure and grow with a (301) preferred orientation.
TL;DR: In this article, the Raman spectra of bismuth sodium titanate in its rhombohedral, tetragonal, and cubic phases were measured, with special attention paid to the phase transitions at 584K and 837K (heating).
Abstract: We have measured the Raman spectra of bismuth sodium titanate in its rhombohedral, tetragonal, and cubic phases, with special attention paid to the phase transitions at 584K and 837K (heating). Both transitions appear to be order-disorder and strongly first order, with large thermal hysteresis. The phonon spectra at temperature slightly below the tetragonal phase are remarkably similar to BaTiO3 with A1(To) modes at 130, 269, and 541 cm−1 (compared with 170, 270, 520 cm−1 in BaTiO3) and an E(TO) at 52 cm−1.
TL;DR: The anomaly in the thermal expansion near 25 K suggests the presence of crystal-field effects, in agreement with recent calculations for the magnetic susceptibility at elevated temperatures.
Abstract: Thermal-expansion and specific-heat measurements have been performed on a monocrystalline sample of U${\mathrm{Ru}}_{2}$${\mathrm{Si}}_{2}$ in the temperature region between 14 and 100 K The thermal expansion of this tetragonal compound is strongly anisotropic and exhibits several anomalies in the investigated temperature range Broad anomalies are centered around 5 and 25 K, whereas sharp peaks in the thermal-expansion coefficients are observed at the transition to the antiferromagnetic state at 175 K The $\frac{c}{a}$ ratio exhibits a similar temperature dependence as the susceptibility along the tetragonal axis The anomaly in the thermal expansion near 25 K suggests the presence of crystal-field effects, in agreement with recent calculations for the magnetic susceptibility at elevated temperatures
TL;DR: In this paper, anisotropic elasticity was used to explain the differences in lattice parameters of precipitate and cubic (c) ZrO2 matrix in the three systems.
Abstract: Tetragonal (t) ZrO2 precipitates in Mg-, Ca-, and Y-partially-stabilized ZrO2 (Mg-PSZ, Ca-PSZ, Y-PSZ) have different habit planes and different morphologies. These differences arise because of differences in lattice parameters of precipitate and cubic (c) ZrO2 matrix in the three systems. The approximate {001} habit plane and oblate spheroid precipitate morphology observed in Mg-PSZ are explained in terms of anisotropic elasticity using the theory of Khachaturyan. The aspect ratio of ∼5 of these particles is used to calculate a c/t interfacial energy of ∼0.15 J·m–2. The aligned equiaxed precipitates observed in Ca-PSZ and the twinned colonies observed in Y-PSZ can also be explained using this theory and arise from interactions between strain fields during coarsening; the aligned particles in Ca-PSZ may actually represent an intermediate state before the formation of colonies in this system.
TL;DR: The spontaneous transformation of the metastable (Form II) tetragonal crystals into the stable twinned hexagonal (Form I) crystals in polybutene-1 was studied using several techniques as mentioned in this paper.
Abstract: The spontaneous transformation of the metastable (Form II) tetragonal crystals into the stable twinned hexagonal (Form I) crystals in polybutene-1 was studied using several techniques. The mechanical properties of the heat moulded material undergo significant changes in the process and the crystalline melting point increases from 112 to 128‡ C. Results from an Avrami analysis suggest the nucleation of the stable crystalline phase occurs shortly after crystallization from the melt and the subsequent growth of the nuclei follows a rod-like geometry. Transmission electron microscopy of melt-grown thin films shows that nucleation occurs at random positions within a spherulite and growth propagates along the radially oriented fibrillar crystals. As a result of multiple nucleation, each tetragonal crystal generates several twinned hexagonal crystallites with different crystallographic orientations. Besides exhibiting multiple nucleation within each individual tetragonal crystal, solution-grown single crystals also reveal twisting of the crystal lattice about thec-axis. The information obtained shows that residual stresses present in the material do not appear essential for the nucleation of the stable phase. Current concepts of the transformation mechanism are examined.
TL;DR: In this paper, the Nowotny phases with compositions MnSi 2−x are studied by means of electron diffraction and high-resolution electron microscopy and it is concluded that the compounds exhibit a particular type of disorder.
TL;DR: The fracture toughness of these glass-ceramics increased with increasing crystallite size of the tetragonal ZrO/sub 2/, reaching approx. = 5.0 Mn/M/sup 32/ at a size of approximately 40 Nm as discussed by the authors.
Abstract: Glasses of composition 3ZrO/sub 2/.2SiO/sub 2/ were prepared by the sol-gel process from metal alkoxides. Tetragonal ZrO/sub 2/ was precipitated by appropriate heat treatment at 1000/sup 0/ to 1200/sup 0/C. The fracture toughness of these glass-ceramics increased with increasing crystallite size of the tetragonal ZrO/sub 2/, reaching approx. = 5.0 Mn/M/sup 32/ at a size of approx. = 40 Nm. The higher fracture toughness was attributed to tetragonal ..-->.. monoclinic ZrO/sub 2/ transformation toughening.
TL;DR: Il est montre qu'un champ electrique applique parallelement a la direction d'un monocristal cubique centre du BP-I transforme l'axe d'ordre 2 en axe d’ordre 4 d' un cristal quadratique a faces centrees a un champ critique
Abstract: Convergent beam diffraction patterns from a bcc blue-phase (I) monocrystal in an electric field applied in the [110] direction show this twofold axis transforming through an orthorhombic distortion to become, at a critical field, the fourfold axis of a tetragonal crystal. The implication on structures of blue phase I is important. Although the transition is weakly first order because the two order parameters describing it are coupled, the diffraction pattern changes continuously through the transition.
TL;DR: In this article, the authors examined the structure of monoclinic ZrO2 particles dispersed in MgO partially stabilized zirconia and found that the product of the martensitic tetragonal-to-monoclic transformation is noted to comprise parallel variants of the monocliic structure in both athermally transformed and stress-transformed particles.
Abstract: TEM and electron microdiffraction are presently used to systematically examine the structure of monoclinic ZrO2 particles dispersed in MgO partially stabilized zirconia. The product of the martensitic tetragonal-to-monoclinic transformation is noted to comprise parallel variants of the monoclinic structure in both athermally transformed and stress-transformed particles. The microcracking associated with transformed particles appears to be closely related to the substructure adopted by the particles. Attention is given to the boundary between variants and the orientation relationship between tetragonal and monoclinic lattices. 32 references.
TL;DR: In this article, anisotropic thermal expansion coefficients based on lattice parameter determinations at room temperature and 600° and 800°C are presented for tetragonal zirconia containing 2 and 3 mol% Y2O3.
Abstract: The transformability of the grains in tetragonal zirconia polycrystals is determined by thermal stresses (eigenstresses) which develop because of anisotropic expansion behavior of the tetragonal grains on cooling. Anisotropic thermal expansion coefficients based on lattice parameter determinations at room temperature and 600° and 800°C are presented for tetragonal zirconia containing 2 and 3 mol% Y2O3. The sources of errors in these data are discussed.
TL;DR: In this article, a metastable tetragonal form is obtained for either low (less than about 5) or high (greater than about 13) p-italicH.
Abstract: The p-italicH of the solution that zirconia is precipitated from defines the crystal phase formed after calcination of the material at 400 /sup 0/ to 600 /sup 0/C. A metastable tetragonal form is obtained for either low (less than about 5) or high (greater than about 13) p-italicH. The tetragonal phase formed at high p-italicH is much more stable at the calcination temperature than the material obtained at a low p-italicH is. For a material obtained by precipitation at a low p-italicH, monoclinic crystallites, determined by x-ray line broadening, were obtained that were smaller than the tetragonal crystals that produced them. A crystallite size effect, based on x-ray line broadening, is not responsible for the formation or stabilization of the tetragonal phase.
TL;DR: In this article, the relative stability of monoclinic and tetragonal ZrO2 in these ultrafine particles was studied by X-ray diffractometry, and it was concluded that the monOClinic phase was stabilized by the presence of water vapor, which probably decreases the surface energy.
Abstract: Mixtures of ultrafine monoclinic zirconia and aluminum hydroxide were prepared by adding NH4OH to hydrolyzed zirconia sols containing varied amounts of aluminum sulfate. The mixtures were heat-treated at 500° to 1300°C. The relative stability of monoclinic and tetragonal ZrO2 in these ultrafine particles was studied by X-ray diffractometry. Growth of ZrO2 crystallites at elevated temperatures was strongly inhibited by Al2O3 derived from aluminum hydroxide. The monoclinic-to-tetragonal phase transformation temperature was lowered to ∼500°C in the mixture containing 10 vol% Al2O3, and the tetragonal phase was retained on cooling to room temperature. This behavior may be explained on the basis of Garvie's hypothesis that the surface free energy of tetragonal ZrO2 is lower than that of the monoclinic form. With increasing A12O3 content, however, the transformation temperature gradually increased, although the growth of ZrO2 particles was inhibited; this was found to be affected by water vapor formed from aluminum hydroxide on heating. The presence of atmospheric water vapor elevates the transformation temperature for ultrafine ZrO2. The reverse tetragonal-to-monoclinic transformation is promoted by water vapor at lower temperatures. Accordingly, it was concluded that the monoclinic phase in fine ZrO2 particles was stabilized by the presence of water vapor, which probably decreases the surface energy.
TL;DR: In this article, a series of tetragonal melilite-like Ca2−xNdxGa2+xSi1−xO7 single crystals with desordered structure were grown using Czochralski technique and their precies X-ray structural analysis was carried out.
Abstract: Full concentration series of tetragonal melilite-like Ca2−xNdxGa2+xSi1−xO7 single crystals with desordered structure are grown using Czochralski technique and their precies X-ray structural analysis is carried out. Intensity, absorption, and luminescence characteristics including orientational ones of Nd2+ ions in Ca2Ga2SiO7 are investigated. Using conventional lamp-pumping technique pulse stimulated emission at the wavelengths of two channels (4F3/2 4I11/m, 13/2) is excied and investigated both, at low and room temperatures. Raman spectra of Ca2Ga2SiO7 are also measured.
[Russian Text Ignored.]
IBM1
TL;DR: The three-dimensional reconstruction of the connector of bacteriophage phi 29 has been obtained from tilt series of negatively stained tetragonal ordered aggregates under low-dose conditions and up to a resolution of (1/1.8) nm-1.
TL;DR: The crystal structure of hollandite from Gowari Wadhona, India has been refined and the crystal structure is monoclinic, I2m, and cell constants are a=10.17° as discussed by the authors.
Abstract: The crystal structure of hollandite from Gowari Wadhona, India has been refined. Chemical formula determined by EPMA and moisture counter is (Ba0.75K0.38)1.13 (Mn7.85Fe0.15)8O16·0.41H2O. The structure is monoclinic, I2⁄m, and cell constants are a=10.006(3)A, b=2.866(1)A, c=9.746(2)A, β=91.17(3)°. Each atom exists on a mirror plane.(y=0, or 0.50). Manganese atom is in octahedral coordination. MnO6 octahedra are distorted and manganese atoms shift to unshared oxygens. The tunnel site is occupied by Ba2+, K+ and H2O. These cations and molecules do not show continuous distribution along the tunnel but only one single peak is observed. This mineral gives an X-ray powder pattern different from that of tetragonal hollandite. The relationship between the site occupancies of tunnel cations and the powder diffraction intensities is discussed. As the occupancy of tunnel site increase, the intensity ratio of I(200)+I(002) to I(301)+I(-103) decreases.