Showing papers in "Journal of The Less Common Metals in 1990"

Some factors affecting the cycle lives of LaNi5-based alloy electrodes of hydrogen batteries

Abstract: The electrode characteristics, the hydrogen equilibrium properties, the crystallographic and mechanical properties of simple ternary alloys of the type LaNi5−xMx(M ≡ Ni, Mn, Cu, Cr, AlandCo) were examined. The effect of substituting elements to improve the cycle life increased in the order: M ≡ Mn, Ni, Cu, Cr, AlandCo. The lower the capacity, the smaller the volume expansion ratio, the slower the pulverizing rate and the lower the Vickers hardness were, the longer the cycle life became. The alloy LaNi2.5Co2.5, which showed the highest durability as a negative electrode, satisfied all these requirements.

Crystal-amorphous phase transition induced by ball-milling in silicon

Abstract: We present an experimental study of the first reported crystalamorphous phase transition induced by ball-milling in a pure element: a silicon powder. Scanning electron microscope observations show some particular spherical particles with smooth shapes. Only minor contamination with iron and chromium (less than or equal to 0.2 at.%) has been determined by chemical EDX microanalyses. The analyses of the X-ray patterns reveal the coexistence of silicon microcrystallites and an amorphous phase. Transmission electron microscope investigations (selected area diffraction patterns) show that such mixing is on the micrometer scale. The various enthalpic evolutions which have been studied by differential scanning calorimetry experiments reveal three major contributions which may be successively interpreted starting from room temperature up to 725°C as a release of the strain energy (exothermic event), followed by an equilibration between the in situ differential scanning calorimetry annealed silicon crystallites and the amorphous phase (endothermic effect), and subsequently by the crystallization of the amorphous phase (exothermic contribution). The influence of the ball-milling conditions on the dynamic equilibrium between the crystalline and the amorphous phases has been studied.

Hydrogenation, disproportionation and desorption (HDD): An effective processing route for NdFeB-type magnets

Abstract: Polymer-bonded magnets have been made from powdered material of the alloys Nd16Fe76B8 and Nd12.3Fe81.9B5.8 (Nd2.1Fe14B) by a hydrogenation-disproportionation-desorption (HDD) process. The HDD process resulted in the production of fine-grained homogeneous materials (from the coarse-grained, non-homogeneous cast alloy) which exhibited appreciable coercivities and remanences characteristic of isotropic materials. Optimum annealing conditions were established for both alloys and superior coercivities were observed for the alloy richer in neodymium.

The influence of small amounts of added elements on various anode performance characteristics for LaNi2.5Co2.5-based alloys

Abstract: The addition of small amounts of elements such as silicon, aluminium and titanium to LaNi2.5Co2.5 greatly influenced anode performance characteristics such as usable temperature range, capacity and its decay rate during repeated cycles, rate capability, low temperature dischargeability and self-discharge rate. The capacity decay was suppressed by the addition of silicon, but the rate capability decreased and the self-discharge rate increased. The alloy containing titanium exhibited a much longer cycle life, but much lower storage capacity and worse low temperature dischargeability. The addition of aluminium was very useful for improving the usable temperature range, cycle life and charge retention, and it did not cause too great a decrease in capacity and/or increase in overpotentials.

Nouveaux isotypes de CeNiSi2 dans les systemes R-M-X (R ≡ La-Lu, M ≡ metaux des groupes 7 A 11 ET X ≡ Ge, Sn). I Compositions et parametres cristallins

Abstract: Resume De nombreux germaniures et stannures isotypes de CeNiSi 2 ont ete caracterises par diffraction X de poudres et analyse a la microsonde electronique. La plupart de ces composes sont non-stoechiometriques, avec de larges domaines d'homogeneite; leurs formules sont donc RM x X 2 avec 0 x min x x max 2 de type ZrSi 2 . De simples considerations geometriques ne permettent pas d'expliquer les fortes variations des indices x min et x max , observees en fonction de la nature du constituant M. Deux nouveaux germaniures binaires sont aussi decrits: LuGe 2 et TmGe 2 , isotypes de ZrSi 2 .

Neutron diffraction study of the structural and magnetic properties of the R2Fe17Hx(Dx) ternary compounds (R Ce, Nd and Ho)

Abstract: A structural and magnetic characterization of R2Fe17 alloys and their corresponding hydrides has been carried out by means of powder neutron diffraction. The hydrides have been found to retain the host metal symmetry. Hydrogen is accommodated in rare-earth-rich octahedral and tetrahedral sites. The effects of hydrogenation on the magnetic properties are discussed.

Hydrogen uptake by Pd-coated Mg: absorption-decomposition isotherms and uptake kinetics

Abstract: The phase diagram of the hydrogen-magnesium system, and the associated uptake-decomposition kinetics, have been measured in the pressure range 0.01–0.6 Torr, i.e. at 105 times lower pressure than previously reported, and in the temperature range 290–370 K. The measurements were performed using thin, ultrahigh vacuum evaporated samples of magnesium covered with a thin palladium coating. Hydrogen uptake was measured by a piezoelectric quartz crystal microbalance on which the samples were deposited. Sample purity was controlled by Auger electron spectroscopy and X-ray photoelectron spectroscopy. In a limited pressure-temperature regime, equilibrium can be established between H2 gas and magnesium at isothermal conditions. The experimental data in this region reveal several features not previously reported for the H-Mg system. In the α phase, fast kinetics and p 1 2 dependence of the uptake at low hydrogen concentrations are followed by a stronger p dependence and accompanying slower kinetics at concentrations of about [H] [Mg] ≈ 0.01 . The latter observation may indicate the onset of attractive H-H interaction at concentrations below the solvus. In the α + β phase, the absorption and decomposition isotherms (plateau pressures) are widely separated in pressure, i.e. large hysteresis effects are observed. The enthalpies of hydride formation and decomposition derived from the measured plateau pressures are 60.7 ± 6.3 kJ mol − 1 H2 and 71 ± 4.2 kJ mol − 1 H2 respectively. The measured decomposition plateau pressures agree well with extrapolations from reported high pressure measurements. The results obtained in this work are compared, whenever possible, with the available high temperature data for the H2-Mg system, and with other metal-hydrogen systems.

Disproportionation in Nd16Fe76B8-type hydrides

Abstract: This work has shown that heating Nd 16 Fe 76 B 8 -type materials in a hydrogen atmosphere, with subsequent vacuum annealing at greater than 750°C produces coercive powders which are suitable for the production of polymer-bonded magnets. A study of the material at different stages of the process has confirmed that the Nd-Fe-B alloy disproportionates into iron, Fe 2 B and neodymium hydride before recombining during the vacuum annealing stage to produce the fine-grained microstructure necessary for highly coercive material. The grain size of the final microstructure has been found to be very dependent on the temperature at which the hydrogenation-disproportionation-desorption cycle is carried out with optimum properties for the Nd 16 Fe 76 B 8 alloy being obtained for a desorption temperature of 785°C for 1 h.

Ammonothermalsynthese von kupfernitrid, Cu3N

Abstract: Zusammenfassung Tetraamminkupfer(II)-nitrat komproportioniert bei Raumtemperatur in flussigem Ammoniak mit Kupfer zu Tri- bzw. Diamminkupfer(I)-nitrat (f(T) und p(NH3)). [Cu(NH3)x]NO3 mit 2 ⩽ x ⩽ 3 geht ab 350 °C bis 580 °C und p(NH3) = 6 kbar im Temperaturgefalle von Autoklaven in Cu3N, N2 und H2O uber; das Nitrid kristallisiert in der heiβen Zone. Rontgenographische Einkristallstrukturbestimmungen bei Raumtemperatur und −143 °C bestatigen den ReO3-typ: a = 3,819(1) A , Z = 1 , Pm3m, R R w = 0,010/0,014 bei Z(F0)2)⩾3σ(F0)2) = 108. Die kovalente NCuNBindung ist mit 173 ° bei 21 °C und 176 ° bei − 143 °C leicht gewinkelt. Modelle dynamischer und statischer Verkippung von NCu6-Oktaedern werden diskutiert. Cu3N sieht dunkel glanzend aus mit einem Grunschimmer. Es kann ohne Zersetzung an der Luft gehandhabt werden. Die Verbindung ist ein diamagnetischer Nichtleiter. Sie ist bei Raumtemperatur metastabil und zerfallt bei 470°C mit einer Enthalpie von 83,7 kJ mol−1 zu Cu und N2.

Solidification and solid state transformations in FeNd: A revised phase diagram

Abstract: A new stable ferromagnetic phase is found to exist in the binary FeNd system, with composition Fe-22.8at.%Nd and Tc = 230°C. A revised phase diagram is presented. A metastable ferromagnetic phase with Tc = 245°C is found to occur in arc-melted and differential thermal analysis (DTA) samples, with different eutectic morphologies. The transformation of this phase in binary FeNd was investigated. The double peak thermal events occurring upon the solidification of the DTA samples are discussed.

RMX compounds formed by alkaline earths, europium and ytterbium—I. Ternary phases with M ≡ Cu, Ag, Au; X ≡ Sb, Bi

Abstract: 19 intermetallic compounds with the general formula RMX (R ≡ Ca, Sr, Ba, Eu, Yb; M ≡ Cu, Ag, Au; X ≡ Sb, Bi) were synthesized and structurally studied by X-ray methods on powders and single crystals. The ternary phases crystallize in four ordered structural types: ZrBeSi, LiGaGe, TiNiSi and MgAgAs. Some comments are given on the parameters controlling the structure of these phases, and the volume effects are calculated.

Thermal stability and characterization of γ-Al2O3 modified with rare earths

Abstract: Rare earth modification was effective for improving the thermal stability of γ-Al2O3. It inhibited the grain growth of transition Al2O3 and the formation of α-Al2O3. The surface area was 12 m2 g−1 for pure Al2O3, 40 m2 g−1 for ceriummodified Al2O3 and 51–59 m2 g−1 for Al2O3 modified with lanthanum and others, heated at 1200°C for 5 h. The activation energy of α transformation, evaluated by differential thermal analysis, was 581–583 kJ mol−1 for pure and cerium-modified Al2O3 and 635–655 kJ mol−1 for Al2O3 modified with lanthanum, samarium and ytterbium. Lanthanum inserted into the crystal lattice of Al2O3 with spinel structure, whereas cerium existed on the Al2O3 surface as a form of CeO2. The difference in states between La3+ and Ce4+ can explain that the modification of lanthanum is more effective than that of cerium for improving the thermal resistance of γ-Al2O3.

Comparison of uranium and zirconium cobalt for tritium storage

Abstract: The utility of ZrCo for tritium storage has been studied and a comparison with uranium has been made. Loading, unloading, and delivery operations typically required in tritium handling loops were conducted using two identical beds: one containing 25.0 g of ZrCo and the other 25.8 g of uranium powder. Hydrogen was the working gas. The two beds have similar performance characteristics although the bed containing uranium has faster loading kinetics and can attain a lower vacuum at room temperature. Particulates are transported into the loop by both beds and appear to be more difficult to contain in the ZrCo case. The temperature ramp rate to the unloading temperature for ZrCo must be controlled if operating at high H/M ratios. A transformation of the ZrCo alloy into a phase which has inferior loading rate characteristics may be induced by rapid temperature ramping.

Rod milling for solid-state formation of Al30Ta70 amorphous alloy powder

Abstract: A rod milling technique has been used to produce amorphous Al30Ta70 alloy powders. The rod-milled powders, starting with elemental aluminium and tantalum powders, have been characterized by X-ray diffraction, differential thermal analysis, scanning electron microscopy and chemical analysis. The results obtained by rod milling are compared with those from a conventional ball milling technique. The rod-milled amorphous Al30Ta70 alloy powders have high thermal stability and a low concentration of contamination.

A new phase in Nd2Fe14B magnets. Crystal structure and magnetic properties of Nd6Fe13Si

Abstract: A new stable phase has been identified in the Nd-Fe-Si system. X-ray diffraction examination of single crystals was used to establish the crystal structure of Nd 6 Fe 13 Si. The tetragonal space group is I 4/ mcm and the lattice parameters are a = b = 0.8034(3)nm, c = 2.278(1)nm. The crystal structure is an ordered version of Nd 6 Ga 3 Fe 11 .Magnetic measurements show an antiferromagnetic behaviour.

On the evolution of microstructure and diffusion paths in the titanium-steel explosion weld interface during heat treatment

Abstract: The microstructure and composition of the bond interface of titanium-steel explosion welds were investigated by means of optical microscopy and electron probe microanalysis after heat treatments between 923 and 1273 K. Heating at 923 K does not produce any intermetallic compounds, whereas heating at 1023 and 1273 K results in the formation of various intermetallic compounds and the solid solution phases. The diffusion paths at 1023 and 1273 K were interpreted by means of the FeTiC phase diagram assuming that local equilibrium is established at the phase boundaries.

Li3FeN2, a ternary nitride with ∞1[FeN4/23-]-chains: Crystal structure and magnetic properties

Abstract: Li3FeN2 was prepared from the melt under nitrogen and the crystal structure was determined by single-crystal diffraction (Ibam; a = 4872(1) pm, b = 9641(2) pm, c = 4792(1) pm, Z = 4) The crystal structure can be described in terms of a fluorite superstructure which contains infinite chains 1 ∞ [FeN 4 2 3−] of edge-sharing FeN4 tetrahedra Measurements of the magnetic susceptibilities vs temperature reveal Curie-Weiss behaviour Magnetic ordering occurs below 10 K The calculated magnetic moment of 17(1) μB indicates a low spin state of the Fe3+ ion

Hydriding characteristics of Mg-xwt.%LaNi5 sintered alloys

Abstract: The initial and subsequent hydriding process and hydrogen storage characteristics of Mg-x wt.%LaNi5 (x = 10–60) sintered alloys were investigated in comparison with those of pure magnesium, Mg2Ni and LaMg12. LaNi5 addition significantly lowered the temperature at which the hydrogen absorption of magnesium was initiated during the initial hydriding process. The subsequent hydriding rate was also accelerated by the addition of LaNi5 up to 30%, beyond which the hydriding rate decreased steeply. Microstructural observations revealed that Mg-xwt.%LaNi5 sintered alloys (x ⩽ 25.6) consisted of primary magnesium, a eutectic mixture of Mg-Mg2Ni and LaMg12, while the alloys (25.6 ⩽ x ⩽ 44.7) consisted of primary Mg2Ni, LaMg12 and a eutectic mixture of Mg-Mg2Ni. The alloys with x ⩾ 44.7 consisted of Mg2Ni, LaMg12 and LaNi5. It was found that the initial and subsequent hydriding rates were closely related to the microstructure and the types of phases present in the alloys. For the initial hydriding process, the presence of LaNi5 together with the Mg2Ni phase in the alloy play a crucial role in the acceleration of the initial hydriding process. The subsequent hydriding rates are probably dependent on the amounts of Mg-Mg2Ni eutectic mixture present in the alloy, whose complicated structure may be effective for hydriding. The pressure-composition isotherms of Mg-xwt.%-LaNi 5 sintered alloys consisted of two plateaux. The pressures of the lower plateau were consistent with those of magnesium and LaMg12, while the pressures of the higher plateau were consistent with that of Mg2Ni. It may be concluded that Mg-30wt.%LaNi5 sintered alloy would be the most suitable material for hydrogen storage of the materials investigated in this study. That is, the initial and subsequent hydriding rates were faster and the hydrogen storage capacity of about 5 wt.% was larger than those of Mg2Ni.

A new phase in the NdFe system: Crystal structure of Nd5Fe17

Abstract: A new stable phase in the NdFe system has been identified. X-ray diffraction examination of single crystals was used to establish the crystal structure of Nd5Fe17. The hexagonal space group is P6 3 mcm and the lattice parameters are a = 2.0214(8) nm, c = 1.2329(8) nm, Z = 12. The Nd5Fe17 structure is built up of layers which are a tessellation of triangles, pentagons, hexagons and heptagons. The stacking of layers forms antiprisms. Clusters of neodymium atoms only are made of trigonal antiprisms or regular octahedra stacked upon each other to form infinite columns. Each neodymium cluster is surrounded by a first ring of nine pentagonal antiprisms centred by iron atoms and a second ring of 12 antiprisms with hexagonal and heptagonal faces centred by neodymium atoms.

Variation of the superconducting and crystallographic properties and their relation to oxygen stoichiometry of highly homogeneous YBa2Cu3Ox

Abstract: A great number of YBa2Cu3Ox (if6.2 ⩽ x ⩽ 7.0) samples were investigated by a.c. susceptibility and field cooling magnetization measurements, X-ray and microprobe analyses, microstructural examinations in a light microscope and a scanning electron microscope. The oxygen content x was determined by the reduction of the samples in a hydrogen flow. To reach the (x-T-p) equilibrium, well-oxidized YBCO samples were annealed in pure oxygen flow (10−1 atm ⩽ P(O2) ⩽ 1 atm) at temperatures between 200 and 1000 °C and subsequently quenched into liquid gallium. Other samples were prepared under 80 atm O2 (200 °C ⩽ T⩽ 1000 °C). We were able to produce macroscopically very homogeneous oxygen deficient material with sharp χa.c. transitions in the entire range of critical temperatures. The evolution of the Meissner fraction is non-monotonic with a relative maximum near x = 6.6, a concentration where the orthorhombicity 2 (b − a) (a + b) also deviates from monotonic behaviour.

Structure determination of Y0.95Ni2 by X-ray powder diffraction

Abstract: Cette etude permet d'expliquer la structure observee dans le diagramme de diffraction de Debye-Scherrer de YNi 2 , en termes de lacunes d'yttrium

Thermodynamic characterization of the ZrNiH system by reaction calorimetry and p-c-t measurements

Abstract: In this research we have characterized the thermodynamics of hydrogen absorption in the range H ZrNi = n = 0 to n ≈ 1.0; this region has been previously neglected because the equilibrium hydrogen pressures are outside the ranges of the gauges which are usually employed for metal-hydrogen research, e.g. about 10−5 Pa at 323 K. With the aid of reaction calorimetry and X-ray diffraction there is shown to be a two-solid phase plateau region from n ≈ 0.2 to n = 0.65 (323 K). A technique consisting of heat treating the hydrogen-containing sample in order to attain internal equilibrium was employed between each calorimetric determination in this region of low hydrogen content. For the hydride formation reaction corresponding to this low pressure plateau ΔHf = −47.5 ± 1 kJ (mol H)−1. For larger hydrogen contents, i.e. the two solid phase plateau region, which extends from n ≈ 1.0 to n = 2.3, the enthalpy for hydride formation for samples which had been hydrided and then evacuated and annealed at 623 K prior to the run was determined calorimetrically to be −35.0 kJ (mol H)−1. Enthalpies for this plateau for both hydride formation and decomposition have also been determined from van't Hoff plots of pf and pd. The pressures for this plateau exhibit a relatively large hysteresis and the enthalpy values determined from the van't Hoff plots reflect this.

57Fe and 169Tm Mössbauer effect and magnetic properties of Tm2Fe15M2(M=Al, Ga, Si)

Abstract: The Curie temperatures Tc and spin reorientation temperatures TSR were found to increase strongly when aluminium, gallium or silicon was substituted for iron in Tm2Fe17. By means of 57Fe Mossbauer spectroscopy we were able to show that there is no preferential substitution of these elements into the 4f or dumbbell site. The 169Tm Mossbauer effect was studied in Tm2Fe15Si2. We found that silicon substitution has led to a marked shift of the second-order crystal field parameter A02 to more negative values, the corresponding increase in K1 explaining the shift of TSR towards higher temperatures.

The effect of thermal annealing on the hydriding kinetics of uranium

Abstract: The effects of vacuum heat pretreatments on the hydriding kinetics of uranium (pure and chromium alloyed) were studied. Two pretreatment temperature regimes were correlated with changes displayed in the two stages of the hydriding reaction. The lower pretreatment regime (below about 200 °C) has a drastic effect on the initial stage of the reaction (the “induction period”. However, it does not alter the rate of the subsequent massive reaction stage. This low temperature region is associated with surface modification processes, the most prominent process being gas desorption. However, the higher pretreatment regime (above about 450 °C) affects the rate of the massive reaction stage (i.e. the velocity of the hydride reaction front) and is attributed to heat-induced changes in the microstructure of the metal (grain growth). The increased rate induced by chromium addition is attributed to the precipitation of chromium at grain boundaries.

A NON POLLUTING SINGLE CRYSTAL GROWTH PROCESS FOR YBaCuO AND PHASE DIAGRAM STUDIES

Abstract: A new method of growing single crystals of YBa2Cu3O6 + x is presented which uses an Y2O3 crucible containing a composition close to the binary eutectic BaO-CuO. Large crystals are obtained which may be homogeneised by a subsequent oxygen treatment (superconducting transition at 88 K, 2 K wide). The phase diagram of the subsystem Y2BaCuO5-BaCuO2-CuO centered on the 123 compound was investigated. The composition of two peritectic points corresponding to the reactions YBa2Cu3O6 + x → Y2BaCuO5 + L and YBa2Cu3O6 + x + CuO → Y2BaCuO5 + L were determined and the boundaries of the primary crystallization field of the YBa2Cu3O6 + x phase was precised. These results allowed to optimize the crystal growth parameters.

Raman spectra of some actinide dioxides and of EuF2

Abstract: Raman spectra have been obtained for the first time from polycrystalline samples of NpO2, PuO2 and EuF2. Raman spectra were also obtained from polycrystalline samples of ThO2, UO2 and CeO2 for comparison with published results. Each of these compounds exhibits the CaF2 (fluorite)-type cubic crystal structure. The observed Raman active phonon (T2g) frequencies were used in conjunction with reported optically active IR phonon (T1u) frequencies to calculate the force constants for both the cation-anion stretching (K) and anion-anion repulsion (F).

Phase transformation of AnX compounds under high pressure (An ≡ Np, Pu; X ≡ Sb, Te)

Abstract: High pressure X-ray diffraction studies were performed on AnX compounds (An ≡ Np, Pu; X ≡ Sb, Te) in the pressure range up to about 57 GPa, at room temperature, using a diamond anvil cell in an energy-dispersive X-ray diffraction facility. These AnX compounds have an f.c.c., NaCl-type structure at ambient pressure and undergo first-order phase transitions under pressure. Whereas NpTe and PuTe transform to a primitive cubic cell of the CsCl type at 13 GPa and 15 GPa respectively, a tetragonal structure, space group P4/mmm, is observed for NpSb above 10 GPa. This tetragonal structure can be regarded as a “distorted” CsCl-type structure, the atomic positions being the same as in the cubic CsCl cell. In PuSb two transitions are detected: the CsCl-type structure is observed above 17 GPa; then PuSb transforms to the tetragonal P4/mmm structure at about 42 GPa. In addition, the compressibility of the low pressure phase was determined from the V(p) data for each compound.

Glass formation and extended solubilities in mechanically alloyed cobalt-transition metal alloys

Abstract: Binary Co-TM (TM ≡ Mn, Cr, V) alloys have been prepared by mechanical alloying of elemental powders in a planetary ball mill and investigated by X-ray diffraction, differential scanning calorimetry and saturation magnetization measurements. For Co-V, an amorphous phase appears in the central composition range, whereas for Co-Cr and Co-Mn an extended solid solution can be obtained during mechanical alloying. The extent of the glass-forming range is discussed in terms of a thermodynamic approach. The larger the driving force for the interdiffusion reaction the wider the glass-forming range.

Comparison of YBCO-films prepared by laser ablation and sputtering

Abstract: Two different preparation techniques, i.e. laser ablation and dc-sputtering have been applied for the production of thin films from YBa2Cu3O7 − x. Both techniques do not require postannealing steps, as the condensation of the superconducting film is done onto a hot substrate (600–780 °C) at high oxygen pressures (1–6 mbar). Besides the superconducting properties (Tc(R = 0) ≥ 90 K, jc(77 K) > 4 × 106 A/cm2) also the structural properties are very similar (single crystalline, c-axis oriented) as measured by ion channeling (χmin ≈ 5.5 %), x-ray scattering and TEM. The preparation methods require quite different process times, < 10 min for laser ablation compared to about one hour for sputtering. They yield different SEM-appearances of the surface: the sputtered surface is clean and smooth, but the laser oblated film shows in addition balls of 200 nm dia. The properties of films made by both techniques are compared and discussed in detail. In addition results on various buffer layers between YBCO and Si are presented. The interdiffusion of the various components have been analysed by RBS. Promising results have been obtained for buffer layers from ZrO2, where only little interdiffusion could be observed, resulting in good superconducting YBCO-overlayers.

The high Tc superconducting phases of the Y2Ba4Cu6 + nO14 + n family

Abstract: A brief overview is presented of some thermodynamic, structural and physical properties of the HTc compounds YBa2Cu3O7 (single chains; nonstoichiometric; Tc = 92 − 55 K), YBa2Cu4O8 (double chains; stoichiometric; Tc = 80 K) and Y2Ba4Cu7O15 (alternating single and double chains; nonstoichiometric; Tc = 70 − 14 K). This family of compounds is an excellent model system for the study of the influence of the chains in superconductivity. A great advantage of YBa2Cu4O8 is the high oxygen stability (up to 850 °C) and the narrow nonstoichiometric range, both due to the stability of the double chains. The oxygen stability keeps orthorhombicity and hinders a structural phase transition like that in YBa2Cu3O7. Therefore, single crystals of YBa2Cu4O8 (and Y2Ba4Cu7O15) do not have a high concentration of twins. Crystals without twins are easily grown and allow for the first time the investigation of the a-b anisotropy which is shown to be appreciable. Although we have put great efforts in the investigation of the thermodynamic phase diagram of the Y-Ba-Cu-O system, the slow kinetics do not allow yet an exact determination of the phase boundaries. The up to now existing data are discussed and the dependence of the Cu Ba ratio on the oxygen pressure is presented.