Showing papers on "Tetragonal crystal system published in 1984"
TL;DR: Two closely related crystal structures of alpha 1-proteinase inhibitor modified at the reactive site peptide bond Met358--Ser359 have been analysed, indicating a major structural rearrangement upon modification of the intact inhibitor.
741 citations
TL;DR: In this paper, structural and metallographic studies were carried out on the Nd-Fe-B alloy system as well as the nd-fe-B tetragonal compound on which record high energy magnets have been developed using a powder metallurgical technique.
Abstract: Structural and metallographic studies were carried out on the Nd-Fe-B alloy system as well as the Nd-Fe-B tetragonal compound on which record high energy magnets have been developed using a powder metallurgical technique. The study on the new magnet has also been extended to other R-Fe-B componds containing various rare earths (R) and to R-Fe-Co-B alloys. The results are as follows; (1) The sintered Nd-Fe-B magnet is composed of mainly three phases, the Nd 2 Fe 14 B matrix phase plus Nd-rich phase and B-rich phase ∼ Nd 2 Fe 7 B 6 ) as minor phases. (2) Nd 2 Fe 14 B has the space group of P4 2 /mnm. The crystal structure of this phase can be described as a layer structure with alternate stacking sequence of a Nd-rich layer and a sheet formed only by Fe atoms. The sheet of Fe atoms has a structure similar to the σ-phase found in Fe-Cr and Fe-Mo systems. (3) The Nd-rich phase containing more than 95 at.% Nd, 3∼5 at.% Fe and a trace of B has fcc structure with a=0.52 nm. This phase is formed around grain boundaries of the matrix phase. Nd 2 Fe 7 B 6 phase has an one-dimentional incommensurate structure with a=a o and c\simeq8 C o , based on a tetragonal structure with a o =0.716 nm and c o =0.391 nm. (4) In the as sintered Nd 15 Fe 77 B 8 alloy periodic strain contrasts are observed along grain boundaries, which disappear after annealing at 870K. This may be related to the enhancement of the intrinsic coercivity of the sintered magnet by post sintering heat treatment. (5) Stable R 2 Fe 14 B phases are formed by various rare earths except La. Of all the R 2 Fe 14 B compounds, Nd 2 Fe 14 B has the maximum saturation magnetization as high as 1.57 T. Dy and Tb form R 2 Fe 14 B phases with the highest anisotropies. Small additions of these elements greatly enhance the coercive force of the Nd 2 Fe 14 B base magnet. (6) Partial replacement of Fe by Co raises the Curie temperature of the Nd 2 Fe 14 B compound, which improves the temperature coefficient of the remanence of the magnet. But the intrinsic coercive force is decreased by the Co addition.
706 citations
TL;DR: In this article, the tetragonal crystallographic structure of the rare earth transition metal Nd 2 Fe 14 B was determined (space group P4 2 /mnm), which is related to the hexagonal CaCu 5 -type structure which is the fundamental basis for the crystal structure of many rare earth-transition metal compounds.
377 citations
TL;DR: The crystal structures of three lithium titanates by neutron diffraction powder profile analysis were determined in this article, where the lithium partially occupies the highly distorted octahedral interstices in the anatase framework in fivefold-coordination with oxygen.
308 citations
TL;DR: In this article, the lattice constants and the X-ray density of ternary tetragonal compounds of the composition R 2 Fe 14 B were determined for R = Y, La, Ce, Pr, Nd, Sm, Gd, Tb, Dy, Ho, Er, Tm, and Lu.
249 citations
TL;DR: The crystal structure and morphology of hydride (deuteride) precipitates were investigated in this paper, where the deuteride was found to have a face-centered tetragonal structure (c/a = 1.09) with an ordered arrangement of hydrogen.
229 citations
TL;DR: In this article, the Raman line from the Si layers of the strained-layer superlattices is indistinguishable from that from single-crystalline Si in both linewidth and frequency.
Abstract: Raman spectroscopy has been used to determine built‐up deformation in GexSi1−x/Si strained‐layer superlattice grown by molecular beam epitaxy. By comparing peak positions in commensurate superlattices and single layers with those from incommensurate thick layers of the same composition we can obtain a quantitative determination of strain. Linewidths are affected by the presence of inhomogeneous strain, dislocations, and disorder. Lines are always narrower in superlattice samples, indicating better crystalline quality. In particular, the Raman line from the Si layers of the strained‐layer superlattices is indistinguishable from that from single‐crystalline Si in both linewidth and frequency. This is consistent with the expectation that the entire lattice mismatch is accommodated as a homogeneous tetragonal strain in the alloy layers only.
215 citations
TL;DR: In this paper, the temperature dependence of the infrared reflectivity spectra in KNbO3 is reported for the cubic, tetragonal and orthorhombic phases in a temperature range extending from 300 to 1200K.
Abstract: The temperature dependence of the infrared reflectivity spectra in KNbO3 is reported for the cubic, tetragonal and orthorhombic phases in a temperature range extending from 300 to 1200K. Spectra have been fitted with a model based on the factorised form of the dielectric function. The temperature dependence of the mode frequencies, the dampings and the oscillator strengths through the successive phase transitions is reported. The experimental data exhibit an unstable mode which decreases continuously in frequency upon cooling across the diverse transitions. The ionic effective charges, the spontaneous polarisation and the static dielectric constant are calculated from these data. All results are discussed in terms of a displacive/order-disorder crossover, which appears especially when the transition from the cubic to the tetragonal phase is approached. A particular phase transition mechanism is proposed, which leads to an understanding of the discrepancy between the experimental and the calculated values of the dielectric constant, the large damping of the soft mode, the incomplete softening of the unstable phonon and the special sequence of phase transitions.
181 citations
TL;DR: In this paper, the analysis of Rayleigh wave propagation in crystals is carried out in the cases for which the Christoffel equations split into two parts providing a rayleigh wave polarized in the sagittal plane, and the boundary conditions simplify under the conditions that some elastic constants vanish.
Abstract: The analysis of Rayleigh wave propagation in crystals is carried out in the cases for which, on the one hand, Christoffel equations split into two parts providing a Rayleigh wave polarized in the sagittal plane, and on the other hand, boundary conditions simplify under the conditions that some elastic constants vanish. It is shown that these requirements are satisfied by 16 configurations in crystals belonging to the orthorhombic, tetragonal, cubic, and hexagonal symmetry systems. The three particular cases solved by Stoneley [R. Stoneley, Proc. R. Soc. London, Ser. A 2 3 2, 447–458 (1955)] are included. The equations giving the velocity and the mechanical displacement are established. The influence of the anisotropy factor on the decay constant is emphasized for crystals belonging to the cubic or tetragonal systems. Curves showing the decrease of the longitudinal and transverse components of the mechanical displacement are given for YAG, Si, GaAs, TiO2, and TeO2. Oscillations and a very slow decrease versus depth of the mechanical displacement components were observed for TeO2. These are ascribed to the strong anisotropy of this crystal.
121 citations
TL;DR: In this paper, the yttria-stabilized zirconias were investigated by spectroscopic methods using a cationic sublattice in which tetravalent and trivalent cations are statistically distributed.
108 citations
TL;DR: In this article, the pH of the solution from which the gel is precipitated determines the calcined zirconia crystal phase, and the tetragonal precursor is obtained for the 3 to 4 or 13 to 14 pH range.
Abstract: The pH of the solution from which the gel is precipitated determines the calcined zirconia crystal phase. Monoclinic zirconia is produced from materials precipitated in the 6.5 to 10.4 pH range; the tetragonal precursor is obtained for the 3 to 4 or 13 to 14 pH range. The calcination atmosphere, temperature history during calcination, and the presence of alkali do not appear to play a dominant role in determining the zirconia phase composition.
TL;DR: In this article, the structural and magnetic behavior of selected metalloid (B,C) containing hexagonal and tetragonal rare earth-transition metal compounds and compound series was investigated.
TL;DR: In this article, a magnetic characterization of the CO 1-x Pt x system with 0.8 anisotropy field has been made, and the solid state transformation diagram at low Pt content has been completed by using the thermomagnetic analysis.
Abstract: A magnetic characterization of the CO 1-x Pt x system with 0\leqx\leq0.8 has been made. The anisotropy field (H A ) of both the hard magnetic phases, i.e. hexagonal at low Pt content and tetragonal around the equiatomic composition, has been measured by the singular point detection technique (SPD). The dependence of H A on composition and temperature is given. The solid state transformation diagram at low Pt content has been completed by using the thermomagnetic analysis (TMA).
TL;DR: In this article, the dielectric, piezoelectric and electromechanical coupling coefficients are significantly large for tetragonal tungsten bronze (T.B.) single crystals, e.g., SBN, BSKNN, KLN, SKN and PBN.
Abstract: Ferroelectric tetragonal tungsten bronze (T.B.) family single crystals, e.g., SBN, BSKNN, KLN, SKN and PBN, have been grown and their ferroelectric properties have been investigated. The results show that the dielectric, piezoelectric, and electromechanical coupling coefficients are significantly large for these crystals; however, they are markedly different when going from smaller to bigger unit cell T.B. crystals. For example, e33 and d33 are substantially larger for the smaller unit cell T.B. crystals, e.g., SBN and SKN, while e11 and d15 are dominant for bigger unit cell bronzes. However, these differences do not have such a significant effect on the respective electro-optic and pyroelectric properties.
TL;DR: In this paper, 27 ternary intermetallic compounds belonging to either the ThCr2Si2-type or the CaBe2Ge2 -type structures were investigated for their superconducting properties.
TL;DR: In this article, a tetragonal and an orthorhombic modification of CH2Cl2-toluene were determined to have a centred crown skeletal geometry.
Abstract: [Au9{P(C6H4OMe-p)3}8](NO3)3crystallizes from CH2Cl2–toluene in two crystalline modifications one tetragonal and the other orthorhombic; single crystal X-ray crystallographic determinations on the two modifications have demonstrated that the tetragonal modification has a skeletal geometry derived from an icosahedron and the orthorhombic modification a centred crown skeletal geometry.
TL;DR: The crystal structure of the [Cu(bipy)2][PF6]2(1) has been determined by X-ray crystallographic methods, using diffractometer data collection; the structure was solved by the heavy-atom method and by successive Fourier syntheses.
Abstract: The crystal structure of the title compound, [Cu(bipy)2][PF6]2(1), has been determined by X-ray crystallographic methods, using diffractometer data collection; the structure was solved by the heavy-atom method and by successive Fourier syntheses. Compound (1) crystallises in the tetragonal space group I41/acd, a= 16.228(3), c= 18.954(3)A, Z= 8.319 unique reflections gave a final R= 0.0487. The structure of (1) involves a unique compressed tetrahedral CuN4 chromophore (dihedral angle 44.6°) with four additional non-bonding PF6– anions at 3.3 A. The e.s.r. spectrum is axial (g∥g⊥ > 2.0) and the dark green crystals have an electronic reflectance spectrum with a band maximum at 15 040 cm–1 with a high-frequency shoulder at 16 950 cm–1. This represents the highest electronic transition yet observed in the [Cu(bipy)2X]Y type complexes and establishes an ‘electronic criterion of stereochemistry’ for the compressed tetrahedral CuN4 chromophore.
TL;DR: Subsolidus phase relations in the low-Y2O3 portion of the system ZrO2-Y 2O3 were studied using DTA with fired samples and X-ray phase identification and lattice parameter techniques with quenched samples.
Abstract: Subsolidus phase relations in the low-Y2O3 portion of the system ZrO2-Y2O3 were studied using DTA with fired samples and X-ray phase identification and lattice parameter techniques with quenched samples. Approximately 1.5% Y2O3 is soluble in monoclinic ZrO2, a two-phase monoclinic solid solution plus cubic solid solution region exists to ∼7.5% Y2O3 below ∼500°C, and a two-phase tetragonal solid solution plus cubic solid solution exists from ∼1.5 to 7.5% Y2O3 from ∼500° to ∼1600°C. At higher Y2O3 compositions, cubic ZrO2 solid solution occurs.
TL;DR: In this paper, the magnetically induced tetragonal lattice distortion observed in an antiferromagnetic metal, γMn, was studied using a total energy local spin density band theory approach.
TL;DR: In this paper, it was shown that the variation of the magnetic moment of Fe in the bct structure can be explained as a superposing effect of both volume expansion and tetragonal expansion.
Abstract: Magnetization and Mossbauer spectra were measured for Fe-C, Fe-N and Fe-Ni-C systems. The mass ratio of the tetragonal martensite in the mixed phases was determined by means of Mossbauer spectroscopy, leading to the average magnetic moment of Fe atoms in the body centered tetragonal (bct) structure. The magnetic moment of Fe atoms increases from 2.2 µ B to 2.6 µ B as the axial ratio and the volume of unit cell increase. In other words, the volume expansion and/or the tetragonal elongation seem to cause an increase in the magnetic moment of Fe in bct alloys. It is concluded that the variation of the magnetic moment of Fe in the bct structure can be explained as a superposing effect of both volume expansion and tetragonal expansion. The mechanism of these effects is discussed in terms of energy band splitting of d electrons and magnetovolume effects.
TL;DR: An exact relation between the thermal expansion coefficient and the bulk modulus of statistically isotropic polycrystalline aggregates composed of crystals of hexagonal, tetragonal or trigonal symmetry was developed in this article.
Abstract: An exact relation is developed between the thermal expansion coefficient and the bulk modulus of statistically isotropic polycrystalline aggregates composed of crystals of hexagonal, tetragonal or trigonal symmetry. This relation is exploited to derive simple close bounds for the thermal expansion coefficient in terms of single crystal properties. Comparison of bounds to experimentally obtained expansion coefficients shows fair to very good agreement.
TL;DR: In this paper, the authors derived X-ray structure amplitudes [F~o[ for urea at 123 K for 318 reflections, IF[ > 3tr(Fo), sin 0/h < 1.15,~-~.
Abstract: X-ray structure amplitudes [F~o[ for urea at 123 K have been derived experimentally for 318 reflections, IF[ > 3tr(Fo), sin 0/h < 1.15 ,~-~. Corresponding amplitudes [F'ol have also been calculated for a simulated crystal structure with the charge density derived from a 6-31G** wavefunction for the isolated urea molecule. The nuclear positional and anisotropic thermal parameters at 123 K were those obtained by neutron diffraction. The [F~[ were placed on an 'absolute' scale with the [F'ol. Simple pseudoatom models for the charge density have been fitted by least squares using the set of ]FX[ and IF'o] as observations in separate refinements. Maps of the resulting static deformation density distribution are similar to each other and to the map derived directly from the wavefunction. Thus the pseudoatom models are efficient, although there are limitations which are attributed primarily to the restricted nature of the radial functions. The most significant differences between the experimental and theoretical model maps are close to the H atoms and may be related to H-bonding effects in the crystal. [Crystal data: a5-578(1), c = 4.686(1)~, tetragonal, P42~m with two molecules per unit cell lying in special positions having point symmetry 2mm.]
TL;DR: The detailed structure of deuterated Cu(II) Tutton's salt has been determined at 5, 50, 123, 150, 203, 250, 295, and 295 K by neutron powder diffraction as mentioned in this paper.
TL;DR: In this paper, the unitcell parameters of the sodium tungstate perovskites, NaxWO3(x=0.55, 0.62, and 0.70), have been determined at a number of pressures to 5.3 GPa with single-crystal x-ray diffraction techniques.
Abstract: Unit‐cell parameters of the sodium tungstate perovskites, NaxWO3(x=0.55, 0.62, and 0.70), have been determined at a number of pressures to 5.3 GPa with single‐crystal x‐ray diffraction techniques. Pseudocubic Na0.55WO3 and Na0.62WO3 (probably tetragonal, C4/mmb, c>a at room pressure) have compressibilities of 0.0095(1) and 0.0084(1) GPa−1, respectively. The Na0.55WO3 perovskite compresses uniformly with pressure to 4.2 GPa, but Na0.62WO3 transforms reversibly at 1.8(1) GPa to a second single phase (presumably tetragonal, I4/mmm) with c
TL;DR: In this paper, an unpaired electron is trapped as a Ti3+ ion in a crystal field that is predominantly octahedral, but with a strong tetragonal component.
TL;DR: In this article, fine-grained, homogeneously dispersed alumina-zirconia powders were prepared by evaporative decomposition of solutions, and the pure metastable tetragonal zirconium powder transformed to the monoclinic form when it was heated to 1150°C.
Abstract: Fine-grained, homogeneously dispersed alumina-zirconia and zirconia powders were prepared by evaporative decomposition of solutions. The pure metastable tetragonal zirconia powder transformed to the monoclinic form when it was heated to 1150°C. The zirconia in the alumina-zirconia powder, which was also in the tetragonal form, did not transform when the powder was heated to 1150° C. This result is explained in terms of inhibition of coarsening of the zirconia grains by the alumina particles.
TL;DR: In this article, a method to grow single ammonium vanadate (IV, V) (NH 4 ) 2 V 3 O 8 has been devised and the crystal structure is tetragonal P4bm; residual factor is R = 0.030.