Formation Of Tetrahedra Research Articles

Structure of Na3YSi6O15 – a unique silicate based on discrete Si6O15 units, and a possible fast-ion conductor

Hydrothermal investigations in the high silica region of the Na₂O-Y₂0₃-Si₆O₁₅ system, carried out in a search for novel fast-ion conductors (FIC's), yielded the new compound trisodium yttrium hexasilicate, Na₃YSi₆O₁₅. Single-crystal X-ray methods revealed that Na₃YSi₆O₁₅ crystallizes in space group Ibmm, has lattice constants a=10.468 (2), b=15.2467 (13) and c=8.3855 (6) A, Z=4, and 11 atoms in the asymmetric unit. Refinement was carried out to a weighted residual of 3.53% using anisotropic temperature factors for all atoms. The structure is unique in that the silica tetrahedra form isolated SiSi₆O₁₅⁶⁻ double dreier-rings, rather than layers as might be expected from the Si to O ratio of 0.4. No isomorphs to Na₃YSi₆O₁₅ have been reported.

Experimental study of silicon isotope dynamic fractionation and its application in geology

Silicon shows no variation in its chemical valence in nature and exists mainly in the form of silicon-oxygen tetrahedra, so very small silicon isotope thermodynamic fractionation occurs and the resultant silicon isotope variation is limited. Dynamic fractionation of Si isotopes during precipitation of SiO2 from a solution is a main factor leading to substantial variations in silicon isotopes in nature. In this experimental study, we determined the dynamic fractionation factorα for silicon isotopes during precipitation of SiO2 from the solution. And in combination of α, a theoretical explanation is presented of the considerably low δ30Si values of black smokers on modern seafloor, Archean banded magnetite-quartzite and clay minerals of weathering origin, and of clearly high δ30Si values of siliceous rocks in shallow-sea carbonate platforms.

Self‐Assembly of Molecular‐Sized Boxes

A simple recipe for tailored cavities for the recognition of neutral organic guests: just mix difunctional chelate ligands with metal ions having a preferred coordination geometry. Boxes, cylinders, or tetrahedra form in high yields by self‐assembly. Of course it's not quite this easy, but the studies discussed here indicate that this strategy for the construction of macrocyclic hosts has a bright future.

Microstructural evolution in low-dose neutron-irradiated Fe-16Ni-15Cr alloy

Reactor irradiation of the base alloy of modified austenitic stainless steel, a candidate alloy for fusion reactor application, was performed with improved temperature control. All three types of defects — vacancy clusters in the form of stacking-fault tetrahedra, interstitial-type dislocation loops and voids — are more pronounced in samples irradiated with conventional temperature control than those with improved control. They are attributed to the nucleation of those defects during the start-up of the reactor. From the irradiation of thin foils, the role of freely migrating interstitials was extracted. For bulk irradiation, the variations of the three types of defects mentioned above with irradiation temperature are thoroughly investigated. Reaction of the less freely migrating point defects in SUS than in pure metals is pointed out.

Stacking Fault Tetrahedra Formation During Growth of Si1-xGex Strained Layers on 〈111〉 Oriented Si Substrates: Tem Observations and Defect Modeling

AbstractWe report the observation of stacking fault tetrahedra (SFT) in strained Si1-xGex layers grown via rapid thermal CVD on (111) Si substrates. It is shown that these defects provide a mechanism for strain relief in films strained in compression due the presence of bounding edge-type stair rod partials whose Burgers vectors lie parallel to the strained layer interface. Cross section and plan view TEM were used to characterize this defect structure in epilayers (30 to 650nm thick) of Si1-xGex (0 < xGe < 0.27) grown on 〈111〉 oriented Si wafers. Stacking fault tetrahedra were observed only in alloys in the compositional range xGe ≥ 0.13 and only when growth proceeded on the 〈111〉 surface. A critical strain energy model that identifies conditions for the stable growth of stacking fault tetrahedra in a strained layer is presented. The model was based on conventional strain energy considerations where the energy of the stacking fault area plus the bounding dislocation network (including dislocation interactions but neglecting the free surface) was balanced against the strain energy released by the introduction of the defect. In addition, a formation mechanism consistent with these observations is described that involves the dissociation of Frank partial dislocation loops bounding stacking faults lying in the growth plane.

Darstellung und Kristallstruktur von K49Tl108 / Preparation and Crystal Structure of K49T1108

K49T1108 was prepared from the elements (molar ratio K: Tl = 1:2) at 700 K. The crystal structure was determined by single crystal X-ray diffraction methods (a = 1728.7(1) pm, Pm 3, Z = 1, R = 0.086). It contains T112 icosahedra and Tl12 truncated tetrahedra (Kasper polyhedra). 20 truncated tetrahedra form Tl84-polyhedra (Samson polyhedra) by sharing hexagonal faces. These polyhedra are connected by common edges. Folded Tl13-chains connect the icosahedra and the Samson polyhedra to a framework. The truncated tetrahedra are occupied by potassium atoms.

Neutron Powder Diffraction Study of the Crystal Structure of YSr2AlCu2O7

The structure of YSr2AlCu2O7 has been analyzed by neutron powder diffraction techniques. This compound crystallizes with space group P4/mmm symmetry and with lattice parameters a = 3.8646(1), c = 11.1139(3)Å. The general structural features of YSr2AlCu2O7 are similar to those found for the 123 superconductor YBa2Cu3O6+x, and in particular the sequence of layers is the same in the two materials, with SrO substituting for BaO. The Al ions substitute for the copper ions of the chain sites of the 123 parent structure and have tetrahedral coordination. The tetrahedra form chains propagating along the a and b axes on the basal plane of the unit cell. Thus the structure is made of short-range domains within which the local symmetry is orthorhombic. All the oxygen atoms in this compound have a disordered configuration around the specialized positions of the 123 structure. This arrangement results in tilting of the CuO5 pyramids and in significant distortions of the coordination polyhedra of the other cations.

Formation of stacking-fault tetrahedra in low defect density oxygen-implanted silicon-on-insulator material

Fabrication of integrated circuits on SOI (Silicon-On-Insulator) material is very attractive because it offers high component density, immunity to latch-up and radiation hardness. Among various SOI techniques SIMOX Separation by IMplantation of OXygen) provides the best material, with carrier mobilities and defect densities approaching bulk silicon values, Early SIMOX wafers were implanted at temperatures below 600°C and annealed at high temperature (>1300°C), which gave a high defect density (109cm−2), including threading dislocations and narrow stacking faults (SFs), as shown in Figure 1. Higher temperature (>600°C) implantation of SIMOX reduced defect densities to 106cm-2 with pairs of narrow SFs in the top silicon layer, as shown in Figure 2. This paper describes a further reduction of defect density in SIMOX material through various annealing conditions, which has resulted in a defect density less than 105cm−2. A new formation mechanism for stacking fault tetrahedra is also discussed.Silicon (100) wafers were sequentially implanted (620°C) and annealed (at 1320°C for 5 hours) to doses of 0.5, 0.5, and 0.8×l018cm-2.

The structure of glasses along the Na 2OGeO 2 join

Raman spectra of glasses with compositions along the Na 2OGeO 2 join and corresponding crystalline phases have been investigated using micro-Raman techniques. The main vibrational band in the Raman spectrum of GeO 2, which is associated with symmetric stretch of the GeOGe bonds, is observed to contain fine structure. These weak vibrational features may be indicative of several distinct 4-membered ring populations within the network. Changes in the intensity of the fine structure with addition of Na 2O indicate that the ring statistics change systematically with composition. No evidence is observed along the join for the formation of 6-fold coordinated germanium atoms. The ‘germanate anomaly’ exhibited by these glasses appears to result from the formation of 3-membered rings of GeO 4 tetrahedra. The maximum (or minimum) in the anomaly occurs when the network becomes saturated in 3-membered GeO 4 rings. Beyond this point, the continued formation of Q 3 tetrahedra (tetrahedra containing 1 NBO) is responsible for the decline (increase) in some properties. The glass structure begins to resemble the crystalline digermanate structure at ≥ 30–35 mol% added Na 2O. Crystallization occurs at ∼ 40 mol% Na 2O compositions at which large numbers of Q 2 tetrahedra form and there is total breakdown of the glass network.

Crystal structure of α-Li 2.5Al 0.5SiO 4

The crystal structure of α-Li 2.5Al 0.5SiO 4, refined from neutron HRPD data, is derived from that of γ-LiAlO 2. Oxygens form a distorted tetragonal packed array with cations in fully occupied tetrahedral sites; one such site contains Li and Al at random. The tetrahedra form edge-sharing pairs which link up at their corners to form infinite columns and the columns then link up to form a 3D framework. The high temperature phase, α-Li 2.5Ga 0.5SiO 4, appears to be isostructural. These structures are compared with others that have tetragonal packed anions.

Diffusion of sodium ions in borosilicate glasses by molecular dynamics method

Molecular dynamics simulations of borosilicate glasses with different sodium oxide content has been carried out. The generated structure indicates that the major number of oxygen atoms introduced as Na2O consumes in the borate network to convert the BO3 triangles to BO4 tetrahedra. The formation of nonbridging oxygen ions in the silica site occurs during the formation of BO4 tetrahedra with low rate. At higher concentrations of Na2O, the NBO's increases abruptly. The self diffusion of sodium ions depends essentially on the temperature, composition (number of Na+ ions) and the rate of cooling. The phase separation mechanism in borosilicate glasses can be detected by MD simulation since the environment of the B−O, Si−O and B−O−Si can be easily reached.

The crystal structure of monoclinic Ag5Cu3(PO4)2[(PO4)H(PO4)] and its relationship to the triclinic sodium analogue

Abstract The crystal structure of Ag5Cu3(PO4)2[(PO4)H(PO4)] was determined from 2611 single crystal X-ray data [a = 17.725(6) Å, b = 5.162(2) Å, c = 18.584(6) Å, β = 125.66(2)°, space group C2/c, Z = 4, Rw = 0.032, sinθ/λ ≤ 0.80 Å−1]. (One part of the PO4 tetrahedra form dimers via a symmetrically restricted, very short O – H – O bond of 2.428(6) Å). Simultaneously this hydrogen bond is a common edge between two coordination polyhedra of Ag atoms. The Cu(1) atom is in a square planar [4] environment. The Cu(2) atom is tetragonal pyramidal [5] coordinated with an uncommon distribution of Cu – O bond lengths [1.933(2), 1.934(2), 1.976(2), 2.105(2) and 2.180(2) Å]. Monoclinic Ag5Cu3(PO4)2[(PO4)H(PO4)] has topologically the same [Cu3(PO4)4]2− layers as triclinic Na5Cu3(XO4)2[(XO4)H(XO4)] (X = P, As), but a different stacking sequence. In both cases these layers are connected by hydrogen bonds and by Na and Ag atoms, respectively, to a three-dimensional network structure.

Some studies of the optical properties of molybdenum-phosphate glasses

A series of binary MoO3-P2O5 and ternary MoO3-In2O3-P2O5 glasses was prepared and their optical properties were investigated. The optical absorption edge of the glasses was measured for specimens in the form of thin blown films. It was found that the fundamental absorption edge of these glasses usually occurs in the ultraviolet region. The linear variation of (αħω)1/2 with ħω where α is the absorption coefficient and ħω is the incident photon energy, is taken as evidence of non-direct interband transitions. The linear dependence of optical gap with molybdenum content indicates that the molybdenum content controls the absorption phenomena in this glassy system. The infrared spectra of all the glasses appeared to be almost the same, indicating that the infrared bands arise primarily from the vibrations of the phosphate and molybdate groups. The structure of molybdenum phosphate glasses is discussed in terms of the formation of mixed phosphate and MoO4 tetrahedra.

Combined experimental and theoretical investigation of the stress field of tensioned plates with local thinnings

The article presents the results of a combined theoretical and experimental investigation of the state of stress of a tensioned plate with pits in the form of spherical or ellipsoidal segments. The experiments were carried out by the method of bulk photoelasticity with the use of freezing. For the numerical solution of the problem the finite element method in volumetric statement with elements in the form of tetrahedra was used. An algorithm was worked out for the automatic optimal division of the region into elements ensuring high accuracy with economy of computer time. The results of the calculations are in good agreement with the experimental data. On the basis of the numerical results and with the aid of methods of planning experiments we devised interpolation dependences of the stress concentration factors on the parameters of thinning. It is shown that when, in eliminating defects on the surface of parts, the pits are given a certain shape, it is sometimes possible to reduced considerably the stress concentration factors.

Na10{Li2[MnO4]4}, a Compound in Which LiO4 and MnO4 Tetrahedra Form Chains Consisting of Six‐ and Eight‐Membered Rings

Annealing thoroughly mixed amounts of the oxides Na2O2 and LiMnO4 (Na:Mn = 2.8:1; Ag bomb, 580°C, 30d) results in formation of Na10 {Li2[MnvO4]4} as blue‐green, orthorhombic crystals. Structure analysis revealed that the MnO4 and LiO4 tetrahedra are linked in a way previously unknown for metal oxides: chain‐shaped anions made up of six‐ and eight‐membered rings are linked through sodium ions.

The morphology of precipitates in an AlGe alloy—II. Analysis using symmetry

The range of morphologies observed for Ge precipitates in quenched and aged AlGe alloys (4.0 wt% Ge) has been analysed in terms of the symmetry of precipitate and matrix phases and the orientation relationship between them. For precipitates in the form of (i) laths parallel to 〈100〉 Al, (ii) rectangular plates on {100} Al, (iii) hexagonal plates on {111} Al, and (iv) rods parallel to 〈110〉 Al, each precipitate shape is found to be consistent with the symmetry of the intersection point group defined by point symmetry elements common to precipitate and matrix structures in the observed orientation relationship. For those precipitates which exhibit an identity or “cube-cube” relationship with the matrix phase, there is a correspondence between translational elements of symmetry in the two structures and a complete analysis requires the determination of the full space group defined by the intersection of the individual space groups of the precipitate (Ge) and matrix (Al) structures. Those precipitates in the form of regular tetrahedra with {111} Al facets are found to be consistent with the intersection space group ( F 43m )_describing the lattice obtained by superposition of the Ge and Al lattices in an identity relationship, with a Ge atom site somewhere coincident with an Al atom site. However, it remains impossible to account for those Ge precipitates occurring as triangular plates on {111} Al in terms of this analysis. It seems likely that such precipitates nucleate heterogeneously and that it will be necessary to incorporate the symmetry of the nucleating defect in a complete symmetry analysis. One possible analysis is outlined briefly.

Chemical short-range order in dense random-packed models

We used a dense random-packed model of an amorphous alloy to calculate the partial correlation functions, which in turn were used to derive the radial distribution functions, the Bathia-Thorton number-concentration fluctuations and the partial interference functions. We applied the model to amorphous Ni 35Zr 65 using atomic radii of 1.10 and 1.58 Å for nickel and zirconium respectively. We included chemical short-range order (CSRO) in the model by permuting nickel-zirconium nearest-neighbor atom pairs in response to a decrease in the enthalpy of the alloy. The permutations decreased the Warren-Cowley order parameter from zero to −0.038, a value in good agreement with the measured value of −0.040. The increase in CSRO is accompanied by the appearance of a peak in the partial interference function I Ni- Ni ( K) at K = 1.9 A ̊ −1 . The peak is similar in form to the socalled prepeak commonly observed in rapidly quenched binary amorphous alloys containing nickel and is located at the same value of K. We tentatively atribute the increase in CSRO and the appearance of the prepeak in I Ni- Ni ( K) to the formation of double tetrahedra with three zirconium atoms at the base and two nickel atoms at the apices.

Exafs study of borate glasses containing copper

The EXAFS of Cu atoms in Na2O-CuO-B2O3 glass systems have been measured in a laboratory EXAFS facility. In order to achieve good EXAFS data, gas proportional counter for I0 and scintillation detector for I are used simultaneously to cancel out emission lines and the pulse height discriminator to out off harmornic. The local structure parameters around Cu atom have been determined by using of a common analysis process for EXAFS data. The monoolinic crystalline CuO is used as a standard specimen. The results show that the coordination number of Cu atom and the length of Cu-O bond in Na2O-CuO-B2O3 glass systems is respectively four and 1.92A as near as those in crystalline CuO. It can be concluded that Cu2+ ions still exist in the glassy network with the form of (CuO4) tetrahedron. The coordination number of Cu atom and the length of Cu-O bond don't change with increased CuO content in the glass.

In-Situ Observations of Point-Defect Precipitation at Dislocations in Electron-Irradiated Silver

In-situ weak-beam observations of the development of electron irradiation damage at dislocations in silver are described. Dislocations constrict and promote in their vicinity the formation of stacking-fault tetrahedra. The possibility that these are of interstitial nature is discussed.

Ferroelectric properties of oxygen deficient PbTiO 3-related phases

New oxygen deficient perovskites derived from PbTiO 3 have been prepared in the PbTiO 3- 1 2 Pb 2 Fe 2 O 5- CaTiO 3- 1 2 Ca 2 Fe 2 O 5 system. A short range ordering of the vacancies leading to formation of (FeO 4) tetrahedra has been shown by Mössbauer spectroscopy. The symmetry at 300K is tetragonal. A ferroelectric, ferroelastic-paraelectric, paraelastic transition has been detected for each composition. The spontaneous strain and the Curie temperature decrease both with the vacancy concentration. Ferroelectric and ferroelastic properties are coupled. The decreasing rate of the Curie temperature has been discussed in terms of cationic substitutions and of formation of (FeO 4) tetrahedra via oxygen vacancy ordering.