Michael G. Down

Solubilities, and solution and solvation enthalpies, for nitrogen and hydrogen in liquid lithium

The solubilities of nitrogen and of hydrogen in liquid lithium have been determined up to 2.77 mol% N and 5.68 mol% H by electrical resistance methods, and they can be represented, in part, by the equations log10xN = 1.168 −2036T 473 < T < 708 Klog10xH = 1.523 −2308T 523 < T < 775 K where x is the solute mole fraction. The results show that the nitrogen and hydrogen content of lithium can be reduced to 0.08 and 0.03 mol%, respectively, by filtration at 200 °C. Solubilities provide values of the partial molar enthalpies, H(soln), and entropies, S(soln) (with respect to the precipitating phase), of solution of 39.23 kJ mol−1 and 22.35 J K−1 mol−1 (for Li3N), and 44.18 kJ mol−1 and 29.15 J K−1 mol−1 (for LiH). The values of H(soln) are used to derive solvation enthalpies of −3473 and −427 kJ mol−1 for nitride and hydride ions, respectively, in the metal. The hydrogen solubilities augment the liquidus of the Li-LiH phase diagram.

Hydrogen in liquid alkali metals

Abstract The chemistry of liquid alkali metal-hydrogen solutions has been surveyed. Solubility data for hydrogen ( Investigations of interactions between hydrogen and non-metals in liquid alkali-metal solutions have shown that, whereas hydrogen and nitrogen act independently in lithium at 420 °C, hydrogen and oxygen interact in sodium at 400 °C according to the equilibrium: O2− + H−⇌ OH− + 2e−. Hydrogen-oxygen interactions in the other alkali metals are also considered and are rationalised in terms of the enthalpy changes of the corresponding solid-state reaction. Furthermore, yttrium has been shown to react, rapidly, with hydrogen dissolved in lithium at a relatively low temperature (400 °C) to form a mixture of a solid solution of hydrogen in yttrium and yttrium dihydride according to the reaction: Li(H) + Y → Li + Y(H) + YH2−x.

Low temperature method for the growth of lithium nitride single crystals

Abstract A novel, low temperature method for the growth of single crystals of lithium nitride is reported. Elemental lithium and nitrogen are reacted in a liquid sodium solvent which is subsequently removed by vacuum distillation.

Solutions of lithium salts in liquid lithium: preparation and X-ray crystal structure of the dilithium salt of carbodi-imide (cyanamide)

The salt Li2[NCN] has been prepared by the solid-state reaction of Li2C2 with Li3N at 600 °C. When the reaction is carried out in molten lithium, colourless single crystals of the salt form on evaporation of the metal. This reaction is pertinent to the purification of liquid lithium for fusion reactors and to corrosion and dissolution of containment metals. An X-ray diffraction study has shown that the structure of Li2[NCN] is tetragonal, space group I4/mmm, with unit-cell dimensions a=b= 3.687(3), c= 8.668(5)A, for Z= 2. Full matrix least-squares refinement with anisotropic temperature factors for each atom converged to give an R value of 0.036. The lattice is composed of Li+ and centrosymmetric [NCN]2– ions [r(C–N) 1.230(3)A]. Each Li+ ion is at the centre of a squashed tetrahe-dron of N atoms [r(Li–N) 2.068(l)A, N–Li–N 101.85(5) and 126.1 1(13)°]. The i.r. spectrum of the solid consists of bands at 2000 (ν3, Σu+) and 690 cm–1(ν2, πu,) for the [NCN]2– ion. Comparisons are made with the crystal structures of the analogous compounds MNCN (M = Ca, Sr, or Pb) and H2NCN. The i.r. spectrum is compared with those of the compounds M2NCN (M = Na, K, Ag, or Tl) and MNCN (M = Zn or Pb).

Electrical resistivity of liquid and solid lithium

The electrical resistivity of lithium contained in a stainless steel capillary has been determined from 15 to 460 °C by means of a Kelvin–Wheatstone bridge. The resistivity of the liquid between 180 and 460 ° obeys equation (i)ρ1/Ω m = 16·476 × 10–8+ 4·303 × 10–10θc– 2·297 × 10–13θc2(i) where θc is in °C. The corresponding equation for the solid metal is (ii) from 15 to 180 °C. Lithium which has ρs/Ω m = 8·685 × 10–8+ 3·261 × 10–10θc+ 1·821 × 10–13θc2(ii) been purified by filtration followed by gettering with titanium and yttrium at 480 °C has the lowest resistivity. Less-elaborate methods of purification produce metal with a higher resistivity. For the liquid, dρ/dθc, though positive, decreases with increasing temperature, whereas for the solid, it increases with increasing temperature.

Solutions of lithium salts in liquid lithium. The electrical resistivity of solutions of nitride, hydride and deuteride

The electrical resistivities, ρ, of solutions of lithium nitride, lithium hydride and lithium deuteride in liquid lithium have been determined for concentrations, x, up to 2.77, 5.68 and 2.22 mol % non-metal over the temperature ranges 200–450, 257–551 and 276–500°C, respectively. For each solute, resistivity increases linearly with increasing concentration, except for very dilute solutions, and the coefficient, dρ/dx increases with increasing temperature. Nitride causes the greatest increase in resistivity [dρ/dx= 7.0 × 10–8Ωm (mol % N)–1 at 400°C], and hydride and deuteride show no detectable isotope effect [dρ/dx= 4.9 × 10–8Ωm (mol % H or D)–1 at 400°C]. The resistivities of mixtures of nitride and hydride in lithium are additive, thereby showing lack of association between these solutes. Ammonia vapour reacts with the metal to form hydride and nitride which dissolve to increase the resistivity by their characteristic amounts.

Synthesis of the dilithium salt of cyanamide in liquid lithium; X-ray crystal structure of Li2NCN

Dilithium acetylide reacts with a solution of lithium nitride in liquid lithium at 530 °C to form the dilithium salt of cyanamide, Li2NCN; subsequent evaporation of the liquid metal solvent leaves colourless single crystals of the salt which X-ray diffraction shows is composed of centrosymmetric NCN2– ions and Li+ ions surrounded by a distorted tetrahedron of nitrogen atoms.

Electrical resistivity of liquid sodium + lithium mixtures. Evidence for incipient immiscibility?

The electrical resistivity of liquid sodium + lithium solutions has been accurately determined by a capillary method from 100 to 450°C. The resistivity obeys a parabolic relationship over almost the full concentration range, but slight deviation occurs near the sodium axis. The excess resistivity of the solutions over that of the linear interpolation between the pure metals is relatively small. A comparison with the other alkali metals shows that the excess resistivity increases in the order Na + Li < Na + K < Na + Rb < Na + Cs. The temperature coefficient of resistivity, dρ/dT, shows a peak near 63 mole % Li but only at temperatures just above 305°C. This is attributed to incipient immiscibility. At higher temperatures the coefficient changes smoothly from Na to Li.

Sodium–lithium phase diagram: redetermination of the liquid immiscibility system by resistance measurement

The sodium–lithium phase diagram has been determined over the entire composition range by a combination of resistance and thermal methods. Each method is especially effective for specific parts of the phase diagram. Two liquid phases separate below 305 ± 1 °C; this temperature is variously reported between 303 and 442 °C in the literature. The consolute composition is 63 mol % Li. The immiscibility boundary extends from 10.1 to 97.0 mol % Li at the monotectic temperature 170.75 ± 0.05 °C. A eutectic occurs at 3.0.mol % Li and 92.10 ± 0.05 °C. Positive deviation from ideality has been observed for both sodium- and lithium-rich solutions.