Frameworks of clusters in alkali metal-gallium phases: Structure, bonding and properties

The occurrence of polyanions in a group of liquid ionic alloys, viz. alloys of the alkali metals with 13, 14, 15 and 16 elements (post-transition-metal groups 3, 4, 5 and 6), is discussed. It is shown that there are strong parallels with the corrresponding crystalline phases, in which polyanions such as .Pb4/4 , .Sb /1 and .Te2/2 exist, known for a long time as 'Zintl ions'. Little evidence exists for remnants of the diamond-lattice-type Zintl ion in liquid alkali-group-13 alloys, but in the alloys of Tl with K, Rb and Cs very long-range superstructures have been found, which are related to the occurrence of large, compact, polyhedral clusters in many crystalline phases. A stability rule, first proposed by Geertsma, relating the stability of the polyanions to the size of the cations, is universally observed. Furthermore it is shown that the range of the superstructure in the liquid is with remarkable precision proportional to the intercluster distance in the corresponding solids. The interpretation of the experimental data in terms of polyanion formation is discussed critically. There remains a paradox. Some sharp experimental features (superstructure peaks and maxima in the resistivities and stability functions) seem to indicate that the Zintl ions in the liquid are near perfect whereas specific heat results and ab initio computer simulations clearly show that the structure of the liquid is seriously blurred.

https://iopscience.iop.org/article/10.1088/0953-8984/8/34/003/pdf

Polyanions in liquid ionic alloys: a decade of research

A modified “hole” theory for solute impurity diffusion in liquid metals is presented. The theory calculates an “effective” valence for the solute impurities by assuming that the bulk valences of the solvent and solute metals are those given by the Engel-Brewer theory (one for bcc structures, two for cph structures, and three for fcc structures). The effects of balancing the Fermi energy level of the solute with that of the solvent and differences in zero point energy are then considered in calculating the effective valence. The effective valence is then used to calculate a value of diffusional activation energy from a modified Lazarus-LeClaire approach. It is shown that the proposed theory works very well for solute impurity diffusion of a number of solutes in Al, Cu, Ga, Ag, Pb, and Sn, but the paucity of diffusion data is a hindrance to development of theoretical models.

A modified “Hole” theory for solute impurity diffusion in liquid metals

The authors have measured the electrical resistivity of liquid K-Bi and Cs-Bi alloys as a function of composition and temperature. In both alloy systems strong indications for chemical compound formation were found: in a large composition range the resistivity attains values that are characteristic for Mott's 'regime of diffusive motion'. In K-Bi the octet compound K3Bi dominates. In Cs-Bi at 550 degrees C the resistivity reaches a maximum of more than 4000 mu Omega cm at a composition of 40 at.% Bi (T=550 degrees C). This is explained by assuming that the Bi ions form poly-anions (Zintl ions), quite possibly chain fragments Bin(n+2)- of variable length. The equi-atomic liquid alloy CsBi displays the unusual feature of a high resistivity (2000 mu Omega cm) combined with an almost negligible temperature dependence.

Resistivity of liquid K-Bi and Cs-Bi alloys

We have studied the structural, dynamical, and electronic properties of liquid Li12Si7 by means of first-principles molecular-dynamics simulations. We find that the Si atoms give rise to a covalently bonded network that can be described as originating from interconnected short chains and stars. The elements of the crystal structure that survive in the liquid state are the preferential twofold Si coordination and the angle between Si bonds. The structure factor is in overall agreement with neutron-scattering data, except for a slight nonuniform shift of peak positions. Electronic-structure calculations show that the Fermi energy is in a minimum of the electronic density of states. The resulting dc conductivity is in good agreement with the resistivity measurements by Meyer et al. Our results appear consistent with the extended Zintl principle recently proposed by Hafner and Jank.

First-principles molecular-dynamics simulation of liquid Li12Si7

The electrical resistivity rho and the 7Li and 71Ga Knight shifts of liquid Li-Ga alloys were measured. The maximum value of rho on the isotherm at 775 K is 161.1 mu Omega cm at 32.6 at.% Ga. The temperature coefficient delta rho / delta T exhibits a negative minimum at this concentration. This value of rho for liquid Li-Ga alloys does not exceed values appropriate to the nearly free electron (NFE) model; resistivity values calculated by the Faber-Ziman formula are in fair agreement with experiment, though the peak position of the calculated isotherm of rho is shifted slightly to the Li-rich side. However, the experimental Knight shifts provide clear evidence of charge transfer from Li to Ga. The Knight shift KLi of 7Li decreases rapidly with the addition of Ga up to 30 at.% Ga where rho is maximum. Beyond this Ga concentration KLi becomes constant. On the other hand, the Knight shift of 71Ga, KGa, decreases almost linearly with the addition of Li up to 53 at.% Ga. This is attributed to increasing p character of the Ga conduction electrons if 7Li is added. For a more quantitative interpretation it will be necessary to incorporate charge transfer in the NFE model.

Resistivity and knight-shift of liquid li-ga alloys

The phase diagram and the electrical resistivity of liquid Na-Ga alloys were studied experimentally. Compound formation in the solid state was observed at 20 at.% and at 37 at.% Na. The latter composition closely corresponds to the stoichiometric compound Na22Ga39, reported recently by Ling and Belin. The present DSC and electrical resistivity measurements do not show the existence of a region of immiscibility around 70 at.% Na. The electrical resistivity ϱ shows a maximum value of 203.9 μΩ cm in the isothermal curve at 560° for an alloy containing 57 at.% Na. In the neighbourhood of this composition, the temperature derivative dϱ dT becomes slightly negative. The values of ϱ and dϱ dT remain within the metallic regime; yet, a description by the diffraction model is complicated by a lack of information on both liquid structure and ion-electron potentials.

The phase-diagram and the electrical-resistivity of liquid na-ga alloys

DSC measurements were performed for Li-Ga alloys of 54 different compositions. The solid compound at the maximum liquidus temperature (739 °C) is observed not at 50 at.% Li (as reported in previous studies) but at 47.1 at.% Li. A eutectic point was detected at 97 at.% Li, in excellent agreement with the result of Thummel and Klemm and in disagreement with that of Yatsenko et al., Solid compounds were found at 22.1, 33.3, 47.1, 60 and 66.7 at.% Li, of which those at 47.1 and 66.7 at.% Li seem to be accompanied by a non-stoichiometric region. The compound at 22.1 at.% Li corresponds to LiGa3.42, the existence and structure of which have recently been clarified by Belin.

T. Itami

Papers

Resistivity and knight-shift of liquid li-ga alloys

The phase-diagram and the electrical-resistivity of liquid na-ga alloys

Study of the Li-Ga system by differential scanning calorimetry

The Phase Diagram and the Electrical Resistivity of Liquid Na-Ga Alloys

The resistivities of liquid K-Ga alloys