Thermal expansion of NZP-family alkali-metal (Na, K) zirconium phosphates

The thermal expansion of the A _x Zr_2.25-0.25x(PO₄)₃ phosphates with A = Na(x = 0.5,1.0,2.0,3.0,4.0, 5.0) and K(x = 1.0, 3.0, 5.0), crystallizing in structures of the NaZr₂(PO₄)₃ type (sp. gr.R3c or C2/c), was studied by high-temperature x-ray powder diffraction in the range 20–700‡C. The lattice parametersa and c and thea- andc-axis thermal expansion coefficients (α_a and α_c) were determined. The thermal expansion of the phosphates was found to be highly anisotropic (α_a < 0, α_c > 0). The strongest anisotropy was found in NaZr₂(PO₄)₃ (α_a = -4.89 x 10^-6 K^-1, α_c = 22.02 x 10^-6 K^-1), KZr₂(PO₄)₃ (α_a =-5.30 x 10^-6 K^-1, α_c = 5.41 x 10^-6 K^-1), and Na₅Zr(PO₄)₃ (α_a = -5.82 x 10^-6 K^-1, α_c = 20.73 x 10^-6 K^-1). K₅Zr(PO₄)₃ exhibited the smallest thermal expansion and weak anisotropy (α_a = -2.14 x 10^-6 K^-1, α_c = 2.65 x 10^-6 K^-1). The effects of M(l) and M(2) site occupancies on α_a, α_c,a, and c were assessed. The relative magnitudes of crystal-chemical and thermal expansion in the Na and K compounds were analyzed.     

s -Wave Symmetry Along the c -Axis and s + d In-plane Superconductivity in Bulk YBa2Cu4O8

To clarify the order parameter symmetry of cuprates, the magnetic penetration depth λ was measured along the crystallographic directions a, b, and c in single crystals of YBa₂Cu₄O₈ via muon spin rotation. This method is direct, bulk sensitive, and unambiguous. The temperature dependences of λ _a ⁻² and λ _b ⁻² exhibit an inflection point at low temperatures as is typical for two-gap superconductivity (TGS) with s+d-wave character in the planes. Perpendicular to the planes a pure s-wave gap is observed thereby highlighting the important role of c-axis effects. We conclude that these are generic and universal features in the bulk of cuprates.     

Structural,magnetic and electrical study of MgCoMnO4

A new compound MgCoMnO₄ has been synthesised by the oxide method. It crystallizes in a tetragonal spinel structure witha=8.30 A andc=8.46 A. The observed crystal symmetry is associated with the existence of Mn⁺³ on theB sites. The compound isn type semi-conductor with activation energy ΔE=0.33 eV. The electrical properties show that it can be regarded as a properly substituted MgMn₂O₄ by Co⁺³ ions. It is paramagnetic withC _M=5.75 and ϑ _a =−160°K. All these results show the ionic configuration of the compound as Mg⁺² [Co⁺³ Mn⁺³] O₄.     

Crystal Structure of Tl2[(UO2)2(MoO4)3] and Crystal Chemistry of the Compounds M2[(UO2)2(MoO4)3] (M = Tl, Rb, Cs)

A new compound, Tl₂[(UO₂)₂(MoO₄)₃], was prepared by a solid-phase reaction. The compound crystallizes in a rhombic system, space group Pna2₁, a = 20.1296(9), b = 8.2811(4), c = 9.7045(4), V = 1617.69(13) ų, Z = 4. The crystal structrue was solved by the direct method and refined to R ₁ = 0. 04 for 4884 unique reflections. The structural motif is a framework consisting of UO₇ pentagonal bipyramids and MoO₄ tetrahedra. The Tl coordination polyhedra are irregular, with seven and eight vertices. Large channels of the size 6 × 10.8 Å, occupied by Tl⁺ cations, are arranged parallel to the [001] direction. The compound is isostructural to the previously described α-Cs₂(UO₂)₂(MoO₄)₃ and Rb₂(UO₂)₂ (MoO₄)₃. __________ Translated from Radiokhimiya, Vol. 47, No. 5, 2005, pp. 408–411. Original Russian Text Copyright ? 2005 by Nazarchuk, Krivovichev, Burns.     

Electronic Energy Levels in High-Temperature Superconductors

Parent materials of high-temperature superconductors (HTSC) need to be doped to become superconducting. The optimum doping for maximum critical transition temperature T _c has been analyzed for more than 20 materials. Assuming a uniform doping distribution the distance x between doped unit cells—projected into the CuO₂ plane for cuprates—shows a strong linear correlation to the inverse of T _c in the form (2x)²=m ₁1/T _c with a slope of m ₁=(2.786±0.029)×10⁻¹⁵ m² K. The mercury cuprate homologous series HgBa₂Ca _n−1Cu _n O_2n+2+δ with n=1,2,3 has been used to demonstrate the procedure deriving the doping distance x from the optimum doping value δ.     

Mixed-Cation Thallium Phosphates Containing Di-, Tri-, and Pentavalent Cations

The mixed phosphates TlMn²⁺P₃O₉(a= 9.422 Å, b= 7.3199 Å, c= 12.227 Å sp. gr. Pnma), TlGaHP₃O₁₀(a= 12.01 Å, b= 8.471 Å, c= 9.098 Å, = 111.98° sp. gr. C2/c), and TlTa(PO₄)₂(a= 5.0308 Å, c= 8.497 Å sp. gr. P321) were obtained by precipitation from polyphosphoric-acid melts containing di-, tri-, and pentavalent cations in combination with Tl⁺. The crystal structures of TlMn²⁺P₃O₉and TlTa(PO₄)₂were determined by single-crystal x-ray diffraction.     

Hydrothermal synthesis and characterization of CsNiP and 2(LiZnHP2O7) crystals

CsNiP and 2(LiZnHP₂O₇) crystals were synthesized by hydrothermal technique at moderate P-T conditions. Solubility results of both the compound shown positive thermal coefficient and single crystal X-ray studies revealed, CsNiP crystallized in hexagonal system with cell parameters; a = 7.173(2), c = 5.944(9) Å, V = 264.87(7) ų and space group P63/mmc and 2(LiZnHP₂O₇) crystallized in orthorhombic system with cell parameters; a = 12.3636 Å, b = 27.5330 Å, c = 6.8647 Å and space group, Pca21 exhibiting ring type of cavities with open aperture in the structure. CsNiP is a frequency dependent paramagnetic and 2(LiZnHP₂O₇) is a diamagnetic.     

Structural model for thermal expansion in MZr2P3O12 (M=Li,Na, K,Rb, Cs)

A structural model is proposed to describe the highly anisotropic thermal expansion in the sodium zirconium phosphate NaZr₂P₃O₁₂ structure as a result of the thermal motion of the polyhedra in the structure. In the proposed model the rotations of the phosphate tetrahedra are coupled to the rotation of the zirconium octahedra. Of the two versions considered, the first one allows angular distortions to occur only in the ZrO₆ octahedra; the second one permits all polyhedra to be distorted.     

Structural and noise characterization of VO2 films on SiO2/Si substrates

Multi-technique structural and electrophysical investigations of VO₂ films on SiO₂/Si substrates are carried out to study the microscopic nature of fluctuator defects — sources of lowfrequency flicker noise. It is established that the noise intensity is determined by the magnitude of the microstress fluctuations 〈ε 〉={〈(δc/c)²〉}, where c is the lattice parameter along the c-axis parallel to [011] direction in the blocks of which the film is formed. The dimensions of the blocks were determined in the direction of the c-axis (t _c∼1000 Å). The suggestion is put forward that the samples contain two types of fluctuator defects: 1) V atoms jumping between the two nearest interstitial sites and 2) V atoms jumping between these interstitial sites near lattice defects. Pis’ma Zh. Tekh. Fiz. 23, 58–65 (July 12, 1997)     

Behavior of Anhydrous Uranyl Acetate at Heating in CH3CN. Crystal Structures of New Uranyl Acetates

Finely crystalline anhydrous uranyl acetate UO₂(OOCCH₃)₂ (I) was prepared by recrystallization from acetonitrile at 140-145°C. Its X-ray diffraction pattern was indexed in the monoclinic system: a = 7.4311(5), b = 12.6622(9), c = 4.1985(2) Å, = 92.01(1)°, V = 394.8(2) ų, Z = 2, _{c a l c} = 3.265 g cm^{- 3}; probable space group C2, Cm, or C2/m. Presumably, in structure I, the coordination polyhedra of U atoms (hexagonal bipyramids), sharing common equatorial edges, are linked to form infinite chains via bridging oxygen atoms of acetate ions. Under the same conditions, the presence of water in acetonitrile results in formation of crystalline [UO₂(OOCCH₃)₂·HOOCCH₃] (II) and (NH₄)₂[(UO₂)₅(₃-O)₂(OOCCH₃)₈] (III), whose composition and structure were determined by single crystal X-ray analysis. In the structure of II, one acetate ion is bidentate chelate and the other, bidentate bridging; the coordination number (CN) of the U atom is 7. In the structure of III, there are three crystallographically independent U atoms with CN 7 and 8. The coordination polyhedra of the U atoms, sharing common edges and vertices, are linked via bridging O^{2 -} ions and oxygen atoms of acetate ions.     

Quantum molecular tunneling of NH4 + ions in mixed ammonium-alkali halide systems

The rotational excitations of NH₄ ⁺ ions in dilute solution in metal alkali halide lattices have been studied at 4K by inelastic neutron scattering technique. It is found that the ions exist in matrix isolation at very low ammonium ion concentration,c (c → 0), and it transforms to orientational glass phase on increase ofc. It is shown that the onset of the orientational glass phase is moderated through the strain field generated by the substituted impurity in the lattice. The variation of the intensity profiles with increase ofc have been successfully explained by a phenomenological model based on gaussian distributed heights.     

Van der Waals chain-molecule fluid in self-consistent field approximation: Some thermal properties

The van der Waals equation for a monomer is used to derive the equation of state for a fluid consisting of chain molecules of equal length. The evolution of a part of the diagram of state pertaining to liquid-vapor equilibrium is treated for the case of an increase in the number of links in a molecule. The dependences on the number of links n are found for the following properties of polymer fluid: the critical temperatureT _c , the critical pressurep _c , the critical density p_c, the critical compressibilityz _c , the temperature of normal boiling, the Riedel parameter of similarity a, the acentric factor Ω, and the enthalpy of vapor formation δH_V, A comparison with the experimental data forn-alkanes and 1-alkanols reveals that the obtained dependences reflect qualitatively correctly the variation of the above-identified properties with an increase of the number of links in a molecule. For long chains(n ≪ 1), the scaling dependences are determined for the properties of a chain-molecule fluid:T _c ∽n ^−1/2,p _c ∽n ^−3/2ρ_c∽n ^−1/2,z _c∽n ⁻¹,α∽n, ω∽n ^2/3,ΔH _ν∽n.     

Crystal Structure of (H3O)2[(UO2)2(SeO4)3(H2O)2](H2O)3.5

Crystals of (H₃O)₂[(UO₂)₂(SeO₄)₃(H₂O)₂](H₂O)_3.5 were prepared from aqueous solutions by evaporation. The crystal structure [monoclinic system, space group P2₁/m, a = 11.9402(11), b = 13.6452(14), c = 13.7271(12) Å, β = 109.436(7)°, V = 2109.1(3) ų] was solved by the direct method and refined to R ₁ = 0. 048 (wR ₂ = 0. 082) for 3677 reflections with |F _hkl |F _hkl |. The structure consists of [(UO₂)₂(SeO₄)₃(H₂O)₂]₂₋ layers arranged parallel to the (010) plane. The layers are formed by uranium and selenium coordination polyhedra sharing common vertices and are linked with each other by hydrogen bonds through the H₂O and H₃O⁺ groups arranged between the layers. __________ Translated from Radiokhimiya, Vol. 47, No. 5, 2005, pp. 412–414. Original Russian Text Copyright ? 2005 by Krivovichev, Kahlenberg.     

Fatigue limit and crack arrest in alkali-containing silicate glasses

Crack growth, including fatigue limit and crack arrest, have been investigated for glasses of the systemsxNa₂O-11Al₂O₃-(89-x)SiO₂,xNa₂O-(100-x)SiO₂ andxNa₂O-7CaO-(93−x)SiO₂ in water as well as in acid and alkaline solutions. From studies of the dependence of the crack arrest on the alkali content of the glass, the kind of alkali (K⁺, Na⁺, Li⁺), the pH of the corrosive medium, the ageing time and ageing loading in conjunction with measuring the alkali leaching behaviour, the basic mechanism of crack arrest and fatigue limit can be concluded. Owing to load- and medium-dependent diffusion processes, a crack-growth retarding leached layer at the crack is generated with modified strength and crack growth properties compared to the bulk properties. In high alkali-containing glasses the process is additionally stimulated by stresses produced in the leached layer at the crack tip and at the crack surfaces.     

Electron microscopic observations of Bi and Tl bearing cuprate high temperature superconductors

The second generation of high temperature superconductors typified by Bi₂Sr₂Ca _n−1Cu _n O_2n+4 and Tl₂Ba₂Ca _n−1Cu _n O_2n+4 exhibits curious structural properties which have direct relevance to the superconducting behaviour particularly transition temperature (T _c ). The present paper reports on investigations of structural properties at microlevel in Bi-bearing HTCS. We have found curious structural characteristics which manifests itself in the form of transformation froma _p ×a _p ×c to (2)^1/2 a _p ×(2)^1/2 a _p ×c through the loss of calcium atoms and the formation of five-fold modulated phase alongb through the loss of Ca and Sr atoms. We have also found the evidence of high periodicities (n=4) Bi₂Sr₂Ca₃Cu₄O₁₂. The microstructural characteristics of HTCS showing the higherT _c (R=0) ∼ 120 K exhibits unusual characteristics.     

Electrical and photoelectric properties of GeS layered crystals grown by different techniques

Thermally stimulated currents and anisotropic electrical conductivity are studied in GeS layered crystals prepared by the Bridgman-Stockbarger, Pizzarello, and sublimation techniques. All the crystals arep-type, regardless of the growth technique, owing to the presence of Ge vacancies. The conductivity anisotropy in the melt-grown crystals is high compared to the vapor-grown GeS. The anisotropy rises exponentially with temperature. The concentrations and ionization energies of traps in GeS crystals are determined from thermally stimulated current curves. The spectral response of the photocurrent through the crystals prepared by sublimation, whose structural perfection is higher than that of the melt-grown crystals, is governed by the spectral dependence of the absorption coefficient forad ≪ 1 (near-edge region) and by the spectral dependence of reflectivity for αd > 1 (high-α region). Regardless of the growth technique, the 293-K photocurrent spectra of GeS crystals show strongly polarized peaks at 1.65 (E ∥a) and 1.78 eV (E ∥b), which are due to the Λ ₁ ^v → Λ ₁ ^c and Δ ₂ ^v → Δ ₂ ^c optical transitions. The low-temperature photoresponse athv < 1.7 eV is due to absorption by Si impurity.     

Analysis of the Normal-State Magnetotransport in CeIrIn5

We present an analysis of the normal-state magnetotransport in the heavy-fermion superconductor CeIrIn₅. The Hall effect and the transverse magnetoresistance in this material do not appear to be uniquely correlated, as inferred from the field dependence of the current ratio (R _σ =σ _xy /σ _xx ² H). The Hall coefficient is seen to satisfy a scaling equation of the form R _H=f[H/(a+bT ^c )]. These results are compared to those observed earlier in CeCoIn₅, and are discussed in terms of the contrasting phase diagram which the CeIrIn₅ system exhibits in relation to its Co counterpart.     

Phase diagram study of the formation and crystallization of Ge-metal amorphous alloy films

The Ge-Au and Ge-Ag alloy films were deposited in vacuum at room temperature and then systematically observed in the TEM. The maximum metallic concentrations in the alloy films,C _max, which form the stable amorphous alloy phases of germanium with gold and silver, were obtained. The annealed crystallization temperatureT _c, which falls with increasing metallic content in these films was also found. The structures of these films and their annealed specimens were also studied. There are various factors which influence the formation of amorphous alloy films deposited in vacuum for Ge-metal systems. A new formula forC _max has been derived. The annealed crystallization character has been explained by means of the variation of the free energy and the activation energy of crystallization. The activation energy of crystallization,E _a, can be obtained from the data values ofT _c. For Ge-Au films,E _a (Au)=E _a ^o /(–18.66C _Au ² +16.83C _Au+1)±3.3 (kcal mol^–1); for Ge-Ag films,E _a (Ag)=E _a ^o /(–2.754C _Ag ² +3.815C _Ag+ 1)±2.6 (kcal mol^–1). In order to explain all these results, two kinds of phase diagram for the alloy films have been introduced. One is the three-dimensional relationship diagrams of phase formation in semiconductor-metallic alloy films; it was introduced to explain the influencing factors. The other is the three-dimensional phase diagrams of annealed semiconductor-metallic films systems. From this diagram all the phase transitions can be found.     

Truncated Gröbner Bases for Integer Programming

 The toric ideal I _A of a matrix A=(a ₁, . . . ,  a _n )∈ℤd ^× n is the kernel of the monoid algebra map π^ _A  : k[x ₁, . . . ,  x _n ]→k[t ^±1 ₁, . . . ,  t ^±1 _d ], defined as x _j ?t ^aj . It was shown in [4] that the reduced Gr?bner basis of I _A , with respect to the weight vector c, can be used to solve all integer programs minimize {cx : Ax=b, x∈ℕ ⁿ }, denoted IP _A,b,c,= , as b varies. In this paper we describe the construction of a truncated Gr?bner basis of I _A with respect to c, that solves IP _A,b,c,= for a fixed b. This is achieved by establishing the homogeneity of I _A with respect to a multivariate grading induced by A. Depending on b, the truncated Gr?bner basis may be considerably smaller than the entire Gr?bner basis of I _A with respect to c. For programs of the form maximize{cx : Ax≦b, x≦u, x∈ℕ  ⁿ } in which all data are non-negative, this algebraic method gives rise to a combinatorial algorithm presented in [17]. Received: April 10, 1995; revised version: February 6, 1996     

Preparation of SiB4±x and SiB6 plates by chemical vapour deposition of SiCl4 + B2H6 system

SiB_4±x and SiB₆ plates were prepared by chemical vapour deposition (CVD) using SiCl₄, B₂H₆ and H₂ gases under the conditions of deposition temperatures (T _dep) from 1323–1773 K, total gas pressures (P _tot) from 4–40 kPa and B/Si source gas ratio (m _B/Si=2B₂H₆/SiCl₄) from 0.2–2.8. The effects of CVD conditions on the morphology, structure and composition of the deposits were examined. High-purity and high-density SiB_4±x and SiB₆ plates about 1 mm thick were obtained at the deposition rates of 71 and 47 nm s⁻¹, respectively. The lattice parameter, composition and density of CVD SiB_4±x plates were dependent on their non-stoichiometry. The lattice parameter,a, was 0.6325 nm, butc ranged from 1.262–1.271 nm.The B/Si atomic ratio ranged from 3.1–5.0, and the density ranged from 2.39–2.45×10³ kg m⁻³. The CVD SiB₆ plates showed constant values of lattice parameters (a=1.444 nm,b=1.828 nm,c=0.9915 nm), composition (B/Si=6.0) and density (2.42×10³ kg m⁻³), independent of CVD conditions.