J.B. Taylor

Rare-earth bismuthides

Abstract The binary systems of the rare-earth elements (except Eu and Lu) with bismuth, have been examined using X-ray and metallographic techniques. The phases identified were RBi2 (LaBi2-type), RBi (NaCl-type), R4Bi3 (antiTh3P4-type), R5Bi3 (Mn5Si3-type), R5+xBi3 (Y5Bi3-type) and R2Bi (La2Sbtype). The LaBi2-type structure is new and single-crystal studies are reported. Refined lattice parameters are given for all the phases. Studies of the system ytterbium-bismuth are incomplete but reveal that it differs markedly from the other binary systems.

Hydrides and deuterides of CaNi5

Abstract CaNi5 is unique among the known AB5 hydrogen storage compounds in that three distinct hydride phases can be formed at room temperature and modest H2 pressures. These hydrides have average compositions of approximately CaNi5H, CaNi5H5 and CaNi5H6, designated herein as β, γ and δ hydride respectively. Deuteride phases of identical composition can be formed but the isotopic pressure effects are different for each of the three phases. Thermodynamics of the β and γ hydrides are presented along with comments on the tendency for CaNi5 to disproportionate when hydrided.

Technical and economic assessment of methods for the storage of large quantities of hydrogen

Abstract The storage of large quantities of hydrogen is analysed on the basis of five conceptual scenarios. These scenarios are representative of possible practical situations. They involve storage above ground as a cryogenic liquid as well as high-pressure storage both above and below ground. The latter includes natural and specially created cavities. No comparison is made between these various methods since in practice one would rarely be free to make a choice among various alternatives. Each scenario is defined and then costed in detail. The incremental cost of storage is calculated and the sensitivity determined for variations of throughput, capital cost and electricity cost. Also included in the data are the costs of electrolytic hydrogen and of hydrogen liquefaction. The cost of storage can add between 30 and 300% to the cost of hydrogen so that large offsetting benefits have to exist in order to give economic viability. There is a considerable interplay between the magnitude of the capital charges and the system throughput or utilization. In the case of the electrolyser alone, non-continuous operation results in a considerable increase in the unit hydrogen costs. For storage systems where the unit capital charges are high then the operating costs are very sensitive to changes in both throughput and capital costs. A hidden capital charge occurs in some cases of underground storage where a large inventory of cushion gas is inaccessible to the user, but is necessary to define the minimum size of the reservoir. The results clearly show the need to consider many factors in order to optimize a storage system. In practice, this has to be done for each specific situation and general consideration can be very misleading.

Rare-earth arsenides

Abstract X-ray powder diffraction techniques have been used to examine the binary systems formed between arsenic and the elements Y, La, Ce, Pr, Nd, Sm, Gd, Tb, Ho, Er, and Yb, in the temperature range 500 °–900 °C. All the binary systems have a phase at the equiatomic composition which crystallises in the NaCl-type structure. In six of the systems studied (Y, Sm, Gd, Tb, Ho, Er/As) this is the only intermediate phase observed. The next most frequently occuring structure type is the NdAs2 type which is found for the diarsenides of La, Ce, Pr, and Nd. Single crystal measurements indicated that the symmetry is monoclinic, space group solP21 c . • LaAs2 (below 750 °C) a = 4.212 A , b = 6.935 A , c = 10.647 A , β = 106.60 ° • CeAs2 a = 4.165 A , b = 6.871 A , c = 10.561 A , β = 106.72 ° • PrAs2 a = 4.139 A , b = 6.844 A , c = 10.509 A , β = 106.69 ° • NdAs2 a = 4.109 A , b = 6.819 A , c = 10.449 A , β = 106.68 ° The LaAs2-type structure is formed above 750 °C only by LaAs2. Single crystal measurements indicated that the symmetry is monoclinic, space group solB2 b , a = 12.891 A , b = 9.140 A , c = 14.450 A , γ = 135.16 °. Three phases were found crystallising in the anti-Th3P4 structure, Ce4As3, Pr4As3 and Yb4As3 with a = 9.053, 8.994 and 8.791 A, respectively. A rhombohedral distortion (space group R3) of the Th3P4 structure was also observed in some samples of Yb4As3, a = 8.784 A , α = 90.80 °. The only phase obtained with the stoichiometry 5:3 was Yb5As3. It has the Mn5Si3 structure, a = 8.480 A , c = 6.671 A , solc a = 0.787 . A small range of composition extends in the direction of metal deficiency.

The thermodynamics of the LaNi5-H2 system by differential heat flow calorimetry I: Techniques; the α + β two-phase region

Abstract The use of a heat flow calorimeter with the differential twin cell configuration to study intermetallic compound-hydrogen reactions is described. The techniques of calibration and operation of both the gas titration system and the calorimeter, which are required to obtain high precision enthalpies, are described and discussed. Enthalpies for the reaction in the two-phase region of the LaNi5-H2 system are independent of the overall composition but exhibit a small hysteresis with ΔHabs = −32.30 ± 0.07 kJ (mol H2)−1ΔHdes = 31.83 ± 0.09 kJ (mol H2)−1 where ΔHabs is the enthalpy of absorption, ΔHdes is the enthalpy of desorption and the errors are the standard deviations. The value of ΔHdes is the best value for the enthalpy of reaction between equilibrium phases with a total probable error of ±0.5 kJ (mol H2)−1.

The rare-earth arsenides: Non-stoichiometry in the rocksalt phases

Abstract The rare-earth arsenides with the rocksalt structure are non-stoichiometric compounds. Both anion and cation lattice sites are incompletely occupied. A range of arsenic solubility is observed in all cases, and in samarium arsenide at 700 °C, it extends from Sm 0.98 As 0.81 to Sm 0.98 As 0.98 . Over part of the solubility range at the arsenic-rich end, variations in arsenic content can occur with no detectable change in lattice parameter. Further decrease in arsenic content can occur at 700 °C in all but Y, Ho, Er and Yb arsenides, and is accompanied by a reduction in the lattice parameter. A range of metal solubility exists at high temperatures for all phases and results in considerable reduction of lattice parameter at the lower metal concentrations. There is no evidence for vacancy ordering.

The thermodynamics of the LaNi5-H2 system by differential heat flow calorimetry II: The α and β single-phase regions

Abstract Simultaneous pressure-composition isotherms and partial relative molar enthalpies of hydrogen for the single-phase portions of the system LaNi5Hx with 0 ⩽ x ⩽ 6.4 were obtained. In absorption the system exhibits chemisorption for 0 ⩽ x ⩽ 0.05 with ¦Δ H ∞ o ¦ = 80 kJ ( mol H 2 ) −1 , followed by trapping for 0.05 ⩽ x ⩽ 0.23 with ¦ Δ H o ¦ = 59 kJ ( mol H 2 ) −1 at x = 0.05 and then contains an α solid solution to the phase limit at x = 0.42. In desorption this phase limit is at x = 0.34. Both the α−(α + β) and the (α + β)−β phase boundaries exhibit discontinuous changes in Δ H with single-phase ¦ΔH¦ values of 26.5 kJ (mol H2)−1 and 38.5 kJ (mol H2)−1 respectively. The partil relative enthalpy for the β single phase for x > 6.05 decreases slowly with increasingx.

Rare earth arsenides: The metal-rich Europium arsenides

Abstract The binary system europium-arsenic has been investigated in the range 50 – 100 at.% europium. Numerous experimental difficulties were encountered because of the high liquidus temperatures and the extreme reactivity of the system towards container materials. Seven phases were identified. Single crystals of six phases were examined by X-ray precession methods and the powder patterns were indexed as follows: • Eu11As10 (distorted Ho11Ge10-type), orthorhombic, a = 11.255(2), b = 11.715(3), c = 17.396(4) A ; • Eu5As4, Ccmb, a = 8.0222(5), b = 15.806(1), c = 8.0586(5) A ; • Eu3As2, (Ba3P2-type, defect anti-Th3P4), I43d, a = 9.2895(4) A ; • Eu3+xAs2, I4 1 acd ,a = 16.464(5), c = 22.246(7) A ; • Eu5As3, (Ca5Pb3-type), P63mc, a = 15.245(2), c = 7.2501(7) A ; • Eu5As3, (Mn5Si3-type), P6 3 mcm , a = 8.8526(3), c = 7.0376(4) A . A ternary was also found, Eu4As2O (structure derived from La2Sb), I4 mmm , a = 4.7939(7), c = 16.1936(24). This and other new data for rare earth pnictides are reviewed. Some correlations are presented and the crystal chemistry of some of the phases is discussed.

Phase relationships and thermodynamics of refractory metal pnictides: The metal-rich tantalum arsenides

The metal-rich tantalum-arsenic system has been reinvestigated. X-ray powder diffraction techniques were used in conjunction with controlled thermal decomposition experiments in an effusion cell connected to a vacuum electrobalance. Monophase polycrystalline samples of Ta3As, Ta2As, Ta5As4 and TaAs have been prepared. Single crystals of each of these phases have been obtained by halogen vapour transport. Single-crystal precession camera studies indicated that Ta3As was of a new structure type with a monoclinic lattice and space group B2b, a = 14.683(2), b = 14.558(1), c = 5.098(1) A, γ = 90.562° (3). The structure of TaAs was refined by using single-crystal diffractometer data. The results confirm the structure assigned earlier on the basis of powder data. In addition, a modification of the TaAs phase has been identified but not, as yet, fully characterized. Powder and single-crystal data indicate a primitive tetragonal cell with structure and lattice parameters very similar to the body-centered form. n nRefined lattice parameters have been determined for all the phases. The difficulties of constructing a phase diagram for the system are discussed.

Differential heat flow calorimetry of the hydrides of intermetallic compounds

Abstract The thermodynamic properties of selected hydrides of intermetallic compounds are measured using a gas-solid equilibration apparatus (Sieverts) coupled to a Tian-Calvet differential calorimeter. This combination allows simultaneous measurement of enthalpy changes and Gibbs energy; the differential configuration eliminates the pΔV correction. The experimental technique and apparatus are briefly described and the limits to accurate measurements are discussed. Preliminary small-sample measurements of the partial molar enthalpies of hydrogen in the two-phase coexistence region of the systems LaNi5 + H2 and CaNi5 + H2 have been performed.