Showing papers in "Journal of The Less Common Metals in 1978"

Bond lengths in oxygen and halogen compounds of d and f elements

Abstract: The variation of the bond length D with the bond strength s in oxygen and halogen compounds of 3d, 4d, 5d-4f and 6d-5f elements is discussed The functional form D(s) = D(1) (1—Alns) = D(1) —Bins is adopted and the parameters D(1) and A (or B) are given for a great many bonds

Phase diagrams of thE CaF2- (Y, Ln) F3 svstems I. Experimental

Abstract: The phase diagrams of the CaF 2 - (Y, Ln) F 3 systems, where Ln is any one of the lanthanides except Pm and Eu, were studied by thermal and X-ray phase analysis methods. The investigations were made from 800 (600)°C up to the melting temperature. The oxygen concentration of the specimens was checked after thermal treatment. For the systems with Ln = Ce, Pr, Tb, Ho, Tm and Lu the fusibility data have been obtained for the first time. Phases with the following structure types are present in the systems investigated: (a) solid solutions Ca 1− x (Y, Ln) x F 2+ x with the fluorite structure type, which undergo ordering in the systems with Ln = HoLu, Y; (b) non-stoichiometric phases (Y, Ln) 1− y Ca y F 3− y with the tysonite (LaF 3 ) structure type, which undergo ordering in the systems with Ln = TbLu, Y; (c) solid solutions with the α-UO 3 structure type, based on α modifications (Y, Ln) F 3 (Ln = ErLu); and (d) modifications (Y, Ln) F 3 (Ln = GdLu) of the β-YF 3 type. Successive transitions, from solid solutions of the components of the system to the berthollide phases and the stoichiometric compound, were found for phases with the tysonite structure type. The maxima on fusion curves of non-stoichiometric phases were noticed in Sm, Gd, Tb, Ho and Er for Ca 1− x - (Y, Ln) x F 2− x and for Ln = PrLu in (Y, Ln) 1− y Ca y F 3− y . The coordinates of non-variant points for equilibria of the liquid-solid type and also the equations describing the dependence of the lattice parameters of nonstoichiometric phases on the composition are given for the systems investigated.

The molybdenum-nitrogen phase diagram

Abstract: In high and low pressure experiments in the Mo-N system the solidus line (α + L) and the composition and temperature of the eutectic (L = α-Mo + γ-Mo 2 N) have been determined. Mo dissolves 1.08 at.% N at the eutectic temperature of 1860°C and at the equilibrium pressure of 670 atm (6.7 × 10 7 Pa). The eutectic composition is 19 at.% N and the corresponding N content of γ-Mo 2 N is 27 at.% N. The solubility of N 2 in Mo(1) and the liquidus line (α + L) L have been calculated on the basis of existing data. For γ-Mo 2 N a melting temperature of 2000°C has been estimated. An Mo-N phase diagram is presented and the phases are discussed in detail. Equations for the solubility of N 2 in solid and liquid Mo, the solid solubility limit and the dissociation and plateau pressures are given together with the Gibbs free energy of the corresponding reactions. The special behaviour of the metal-gas system Mo-N is additionally treated in a p - c diagram.

Über einige phasen der Mischungen ZnSbN und CdSbN

Abstract: Zusammenfassung Die Phasendiagramme der Mischungen ZnSbN und CdSbN wurden in wesentlichen Punkten bestatigt. Im Phasendiagramm CdSbNTeN′ wurden keine stabilen nichtmarginalen Phasen gefunden. Die Kristallstruktur von Zn4Sb3.r ist von einem R12.10 Typ. Es werden energetische Grunde fur das Auftreten und die Struktur der Phasen Zn4Sb3.r, ZnSb und CdSb, fur das positive Bildungsvolumen der Phasen und fur einige Metastabilitaten angegeben.

Phase relations in the ternary Ti-Ni-Cusystem at 800 and 870 degrees C

Abstract: We have investigated the isothermal cross sections through the ternary phase diagram Tiz.sbnd;Niz.sbnd;Cu at 800 and 870 °C by means of the diffusion couple technique. The results have been corroborated on essential points by the investigation of equilibrated alloys. Use has been made of optical, microprobe a nd X-ray analyses. The results differ from those mentioned in the literature. For two hitherto undescribed ternary intermetallic compounds X-ray diffraction data are given and crystallographic cell parameters are proposed.

Hydride properties of AB2 laves phase compounds

Abstract: The hydrogen sorption and physical properties of Laves phase AB2 intermetallic compounds are reviewed. The work contains a list of AB2 compounds and their hydrogen capacities. The preferred location of hydrogen in different tetrahedral interstitial sites is calculated assuming the formation of “binary hydrides” between the hydrogen and the four atoms that form the tetrahedral site. A phenomenological model is described to explain the variation of the hydrogen capacity in pseudobinary compounds A(B1−xB′x)2. The model assumes a local hydrogen capacity that is a function of the number of B′ atoms that surround the Zr atom; it is described by a double-step function. The influence of hydrogen on the magnetic properties of the compounds is discussed.

Lattice parameters of some ThCu2Si2-type phases in ternary alloys of rare earths with cobalt (or iron) and silicon (or germanium)

Abstract: The structural characteristics of 19 compounds with the composition RMe 2 X 2 have been studied for R = Y or a rare earth, Me = Fe or Co and X = Si or Ge. These compounds add to other similar rare earth alloys that are already known, resulting in the tI 10 ThCu 2 Si 2 -type structure. The data obtained are briefly discussed.

CoCr binary system: experimental re-determination of the phase diagram and comparison with the diagram calculated from the thermodynamic data

Abstract: The CoCr binary system was experimentally re-investigated by differential thermal analysis, isothermal annealing and high temperature X-ray diffraction. In the Cr-rich region, the transformation of the a phase into the b.c.c. α phase occurred at about 1280 °C, but no δ phase was found to exist at high temperature. The other parts of the diagram agree with the data selected by Hansen. The thermodynamic values were described by a polynomial representation which took into account all the published experimental thermodynamic data; the phase equilibrium was then calculated and compared with the experimental results.

A comparative study of hydrogen permeabilities and solubilities in some palladium solid solution alloys

Abstract: Reproducible permeability data have been obtained for the approximately isoelectronic alloys Pd-25 at.% Ag, Pd-5.75 at.% Ce and Pd-8 at.% Y; the previously observed enhanced performance of the Pd-Y alloy has been confirmed. Hydrogen solubility measurements have also been made for the three alloys over the same range of pressures and temperatures as those employed in the permeability studies. Using these data the detailed shapes of the permeability versus temperature plots have been successfully analysed and it has been shown that the hydrogen diffusion coefficients in all three alloys are very similar. The present results show clearly that the enhanced performance of the Pd-Y diffusion membrane is due to the substantially greater hydrogen solubility gradient in this alloy membrane compared with that in Pd-25% Ag.

M5X3 intermediate phases formed by Ca, Sr and Ba

Abstract: The crystal structures of some M 5 X 3 compounds (M = Ca, Sr, Ba; f,X = element of Group II B, III B, IV B or V B) were determined by single crystal and powder techniques. The phases Ca 5 Zn 3 , Sr 5 Cd 3 , Ca 5 Ga 3 , Sr 5 In 3 , Sr 5 Tl 3 , Sr 5 Sn 3 , Ba 5 Sn 3 and Sr 5 Pb 3 crystallize with the tetragonal Cr 5 B 5 structure type; Ca 5 As 3 , Sr 5 As 3 , Ca 5 Sb 3 and Ba 5 Bi 3 belong to the hexagonal Mn 5 Si 3 type; Sr 5 Bi 3 is isostructural with orthorhombic β-Yb 5 Sb 3 . A survey of the known M 5 X 3 phases formed by the alkaline earth elements is given and the influence of the size factor on the occurrence of the various structural types is discussed.

The formation of metastable hydrides Ti0.75Al0.25Hx with x < 1.5

Abstract: Ternary hydrides Ti 0.75 Al 0.25 H x were formed at temperatures below 200 °C by absorption from hydrogen gas. The hydriding was studied by means of volumetric analysis and X-ray diffraction. Phases were identified as α(h.c.p.) for x x x 2 formation. The pressure of the β + γ plateau is given as JRTlnP H 2 = ΔH − TΔS with ΔH = −11.3 kcal and AS = −30.4 cal K −1 . The β + γ plateau pressure was 1 atm at 100 °C which approaches a useful range for hydrogen storage applications. The alloy has an attractive hydrogen-to-metal ratio of about 1.5, but the γ phase formed irreversibly. The attainment of reversibility in the γ phase formation is the dominant problem in developing useful Ti-Al-H alloys for hydrogen storage.

The phase diagram of the niobium-germanium system

Abstract: The NbGe equilibrium phase diagram has been investigated by means of X-ray diffractometry, metallography, hardness measurements, microprobe analysis and differential thermal analysis up to 2200 °C. For temperatures higher than 1870 °C, the solidus lines were determined by thermal analysis on levitating samples. Particular attention was devoted to the homogeneity range of the superconducting A15 phase. We have found that the Ge-rich limit of this phase varies with temperature from 20 ± 1 at.% Ge at 1600 °C to 23 ± 1 at.% Ge at 1865 °C, the eutectic temperature. It is thus shown that the stoichiometric composition Nb 3 Ge is not reached in this system.

On the superconductivity of PbMo6S8 and the series MxPbMo6S8 and MxPb1−xMo6S8

Abstract: We have studied the correlation between the lattice parameters and the superconducting critical temperature T c in PbMo 6 S 8 and the series M x -PbMo 6 S 8 and M x Pb 1− x Mo 6 S 8 ( M = Cu , Ag, Ba, Sn, Eu, La, Gd, Lu, Y, Th, U). A net correlation was found between the volume V H and T c when M is trivalent or tetravalent. We also found that the high T c modification of PbMo 6 S 8 has a maximal occupation of the lattice site centred about the origin of the rhombohedral lattice cell and that this can be obtained by insertion of trivalent and tetravalent elements.

Tungsten carbide powders produced by the vapor phase reaction of the WCl6-CH4-H2 system

Abstract: Black powders of tungsten carbides (WC, W2C), including tungsten metal, were produced by the vapor phase reaction of the WCl6-CH4-H2 system at 1000–1400 °C. The C/W ratio of the product increased sharply with increasing reaction temperature and increasing methane concentration. At 1400 °C the product consisted of almost pure WC. The particle size of the carbide powders, which was significantly influenced by the mixing temperature of WCl6 and H2, was 0.04–0.05 μm for low mixing temperatures and 0.08–0.11 μm for high mixing temperatures. The formation of tungsten carbide powders in this reaction system was found to occur in two steps, namely the initial formation of W particles and their subsequent carburization. This was confirmed by separate examinations of the hydrogen reduction of WCl6 and the carburization of W powders with methane.

The concept of frustration in spin glasses

Abstract: This paper gives a precise definition of those spin systems which have the property of “frustration” which is thought to be essential to the canonical spin-glass behavior. The definition is that the coupling energy across a generalized plane boundary between large blocks of spins grows only as the square root of the area of that boundary.

Hydrogen sorption properties of the La1−xCaxNi5 and La(Ni1−xCux)5 systems

Abstract: The hydrogen sorption properties of the Lai 1− x Ca x Ni 5 and La(Ni 1− x Cu x 5 systems were investigated at various temperatures and at pressures up to 20 atm. It was found that initial substitution of La by Ca in LaNi 5 caused an increase in the hydrogen dissociation pressure, up to Ca 0.3 La 0.7 Ni 5 . In the Ca 0.3 La 0.7 Ni 5 −CaNi 5 range, the dissociation pressure decreased. The absorption capacity of CaNi 5 was dependent on the purity of the sample and increased significantly at low temperatures. The stability of La(Ni 1− x Cu x ) 5 hydrides increased linearly from LaNi 5 to LaCu 5 . These features are discussed in the light of existing models of ternary and pseudoternary hydride stability. Finally, the role of the measured change in entropy ΔS in determining the occupied interstitial sites in the hydride is outlined and discussed in relation to these systems.

Neutron and X-ray diffraction investigation of deuterium storage in La7Ni3

Abstract: Quantitative phase analysis by neutron powder diffractometry yields conclusive evidence for the irreversible decomposition of La7Ni3 into LaD3 and LaNi5 during deuteration at room temperature. Refinement of the LaD3 structure confirms that the D atoms are distributed over the tetrahedral and octahedral interstices of the f.c.c. metal atom arrangement. The D atoms occupying the octahedral interstices are not exactly at the centre but appear to be shifted away from it along different directions. It is suggested that this delocalization results from large anharmonic thermal displacements of the D atoms mainly along 〈111〉 directions. The crystal structure of La7Ni3 was refined both by single-crystal X-ray analysis and by neutron powder diffractometry. The results are compared and discussed in relation to other Th7Fe3-type structures.

Lattice parameters oF RMn2Si2 and RMn2Ge2 compounds

Abstract: The ternary compounds of the rare earths with Mn and Si in the stoichiometric ratio 1:2:2 have been prepared and studied by X-ray analysis. They were found to be of the tI 10 Al 4 Ba-ThCu 2 Si 2 structure type. A number of the previously known isostructural RMn 2 Ge 2 compounds have also been prepared. The molar volumes of these phases are compared with those of other similar groups of compounds and are briefly discussed.

On the occurrence of M2X intermetallic compounds (M = Ca, Sr, Ba; X = Si, Ge, Sn, Pb)

Abstract: The M2X phases formed by the alkaline earth metals with the elements of Group IVB of the periodic system have been prepared and studied. Their crystal structure has been determined by the X-ray powder and single crystal methods. They crystallize in the orthorhombic anti-PbCl2 type (space group D162h). The relation between this structure and the Co2Si or anti-CaF2 structures, which occur for other homologous compounds formed by transition elements or magnesium, is discussed. The stability of these three types of structure as a function of atomic sizes, electronegativities and the electronic configuration of the valence shell of the component atoms is examined.

Die struktur des BaPtSn3, BaNiSn3 und SrNiSn3 und ihre verwandtschaft zum ThCr2Si2-strukturtyp

Abstract: Zusammenfassung Es wurden die neuen Verbindungen BaPtSn3, BaNiSn3 und SrNiSn3 dargestellt und strukturell untersucht. Sie kristallisieren tetragonal (I4mm) mit zwei Formeleinheiten in der Elementarzelle. Die Gitterkonstanten sind: BaPtSn3, a = 485 ± 1 pm, c = 1107 ± 2 pm, c a = 2.28 ; BaNiSn3, a = 482 ±1 pm, c = 1093 ± 2 pm, c a = 2.27 ; SrNiSn3, a = 474 ± 1 pm, c = 1088 ±2 pm, c a = 2.30 . Die Struktur kann als Splitvariante des ThCr2Si2-Typs beschrieben werden.

Si-Stabilised C14 laves phases in the transition metal systems

Abstract: A systematic exploration of the ABSi systems, where A and B are two transition elements of large and small atomic radii, respectively, has been made in order to detect Si-stabilised Laves phases. In all, 27 Si-stabilised Laves phases have been found. The stabilised Laves phases are of the hexagonal C14 type, irrespective of whether the A-B binary system had no Laves phase or had a C15-type cubic Laves phase.

Zusammenhang zwischen strukturellen und thermodynamischen eigenschaften bei phasen der Cu-Familie in den T10-B3-systemen

Abstract: Zusammenfassung Die Gitterparameter der Phasen der Cu-Familie in den T10-B3-Systemen wurden in ihrem ganzen Homogenitatsbereich ermittelt. Es zeigte sich, dass Palladium und Platin auch mit Thallium marginale Mischkristalle PdTlm und PtTlm bilden, obwohl der Atomradienunterschied etwa 25% betragt. Die Abhangigkeit der integralen Atomvolumina von der chemischen Zusammensetzung wurde fur alle mischbaren T10-B3-Systeme im ganzen Konzentrationsbereich bestimmt und im Bereich der Phasen der Cu-Familie analysiert. Es wurde festgestellt, dass die partiellen Atomvolumina der B3-Metalle fur grosse Verdunnung von den als Losungsmittel benutzten T10-Metallen unabhangig sind. Die aus den Gitterparametern ermittelten effektiven Atomradien der B3-Metalle konnten den Pauligschen Atomradien fur metallische und kovalente Bindung zugeordnet werden. Zwischen der Differenz der partiellen molaren Atomvolumina der B3- und T10-Metalle ( V m B 3 − V T 10 ) und der partiellen molaren Mischungsenthalpie der B3-Metalle ΔH m B 3 fur grosse Verdunnungen wurde eine lineare Korrelation gefunden.

Interaction between superconductivity and magnetism in the pseudoternary system (Lu1−xHox)Rh4B4

Abstract: The boundaries between the normal paramagnetic, superconducting and normal magnetically ordered phases in the tetragonal pseudoternary rare earth system (Lu 1− x Ho x )Rh 4 B 4 have been established by means of a.c. magnetic susceptibility measurements to temperatures as low as 0.07 K. The heat capacities of the ternary compounds LuRh 4 B 4 and HoRh 4 B 4 and the pseudoternary compound (Lu 0.5 Ho 0.5 Rh 4 B 4 have been measured from 0.5 K to 36 K. The static magnetic susceptibility and a.c. electrical resistance have been determined between about 1 K and room temperature for the ferromagnetic ternary compound HoRh 4 B 4 .

Ternary alkali molybdenum sulfides AxMoS2 and AxMo3S4 from alkali halide melts

Abstract: Reduction of MoS 2 with alkali metals in alkali halide melts at temperatures of 700–900°C results in the formation of ternary phases A x Mo 3 S 4 and A x MoS 2 . The latter are built up of distorted layers of the MoS 2 type with the alkali metal between the chalcogenide sheets. They are able to undergo topotactic hydration and cation exchange in aqueous electrolytes. At pH x MoS 2 which behaves as a Brnsted acid and reacts with Lewis bases L to give layered intercalates L y H x MoS 2 . The A x Mo 3 S 4 phases are structurally closely related to the ternary molybdenum chalcogenides MMo 3 X 4 with three-dimensional Mo 3 X 4 networks.

Note on the Hf-Co phase diagram

Abstract: The Co-rich part of the Hf-Co phase diagram has been re-investigated by X-ray diffraction, thermal analysis and metallography. The compound HfCo6 reported in the literature proved to be non-existent. Three new compounds were found: Hf2Co7, Hf6Co23 and HfCo7. The latter two compounds give rise to a eutectoid decomposition close to 1000 °C.

Studies on the formation of FeSi2 from the FeSiFe2Si5 eutectic

Abstract: The unidirectional solidification of the eutectic between α-Fe2Si5 and ϵ-FeSi brings about an aligned composite with a lamellar structure. Using this well-defined composite, the formation of β-FeSi2 was studied as a function of annealing temperature and time. Examinations were carried out by metallographic, X-ray and electron probe microanalysis (EPM) techniques. The existence of two different reactions, which are dominant in the temperature ranges above and below 860 °C, respectively, was revealed. Above 865 °C β-FeSi2 is formed around ϵ-FeSi by the reaction α + ϵ → β, while below 860 °C it is formed mainly by the disproportionation reaction α → β + Si, followed by the subsidiary reaction Si + ϵ → β. The rate of the reaction in the higher temperature range decreases with increasing temperature, while that in the lower temperature range increases.

Kinetics of reaction of Zirconium alloy with hydrogen

Abstract: The kinetics of hydriding of Zircaloy-2 tube were studied at hydrogen partial pressures from 0.2 to 1 atm, in the temperature range 300 – 500 °C, by means of thermogravimetric techniques. As a result of the analysis of the reaction curves, it has been suggested that the rate-controlling step is a phase boundary controlled reaction, the activation energy of which is 16.3 kcal mol −1 . The t in ∞ Δl relationship was studied on the oxidized specimens, where t in is the initiation time of hydride precipitation and Δl is the oxide film thickness. This relationship suggests that hydrogen diffusion in ZrO 2−x on a Zircaloy surface is a rate-controlling step until the film is broken. Marked hydriding in H 2 -H 2 O gas mixtures occurred when the ratio of hydrogen to water vapor pressure reached more than about 10 5 and 10 2 at 300 and 400 °C, respectively.

Heterodiffusion model for the activated sintering of molybdenum

Abstract: Sintering studies were conducted on a 2.2 μm Mo powder in the temperature range 1000–1350°C. The influence of thirteen transition metal additions on the sintering kinetics was determined by shrinkage measurements. On the basis of a grain boundary heterodiffusion model it is shown that Ni and Pd are the best sintering activators when present as coatings approximately 10 monolayers thick on the powder surface. Calculated activation energies are in reasonable agreement with the experimental values. In addition, the sintering of pure Mo was found to be controlled by a volume diffusion process with an apparent activation energy of 405 ± 20 kJ mol −1 .

Hydrides of LaNi and CeNi intermetallic compounds

Abstract: The hydriding characteristics of La7Ni3, LaNi and Ce7Ni3 have been investigated. These compounds form the stable hydrides La7Ni3H19.3, LaNiH3.85 and Ce7Ni3H19.2. In structural and magnetic investigations La7Ni3is found to decompose into LaH3 and LaNi5 on rapid hydrogenation. On desorption at elevated temperatures the original La7Ni3 structure is reformed. An analogous decomposition is found in Ce7Ni3. The enthalpies of formation ΔH have been measured calorimetrically for the hydrides of LaNi compounds. ΔH varies linearly with the LaNi composition, independently of the various compound structures and of structural transformations on hydrogenation. The experimental values of ΔH are described by a phenomenological model. Despite their high hydrogen content, these hydrides are not suitable for use as hydrogen storage materials because of their high stability.