First-principles studies of the stability of Zintl ions in alkali-tin alloys: II. Liquid alloys

Abstract

Following our preceding work on the structure and electronic properties of crystalline Zintl phases in alkali-tin systems, we have studied the structural, dynamic and electronic properties of these alloys in the molten state using ab initio local density functional molecular dynamics. Extended simulations (up to 124 atoms per cell, simulation times of up to 60 ps) have been performed for K-Sn, in which, according to the results for the crystalline compounds, the tetrahedral Sn4 polyanions can be expected to be most stable. Simulations at the experimental density and starting from configurations with no polyanions at all, but allowing for sufficient time for equilibration, lead to excellent agreement with the structure factor and radial distribution function measured by means of neutron diffraction. The analysis of the instantaneous multi-atom configurations leads to the conclusion that at least a certain fraction of the Sn atoms form polyanionic clusters. The existence of such clusters is also supported by the calculation of the electronic and vibrational spectra; the electronic structure in particular shows the signatures of tetrahedral polyanions. The precise percentage of Sn atoms forming polyanions depends, however, on density and on the ability of the system to simultaneously accommodate all An atoms in electronically saturated clusters. At a slightly higher density we find that a 64-atom ensemble condenses to a `plastic' or `rotor' phase in which almost all Sn atoms are arranged in tetrahedral polyanions performing only librational motions, whereas the K atoms continue to show liquid-like behaviour. Simulations for liquid Na-Sn and Li-Sn show a less complex behaviour: in the melt the Sn atoms form an entangled three-dimensional network instead of isolated polyanions. Again the calculated structure factors are in good agreement with diffraction measurements. The analysis of the vibrational and electronic spectra confirms the breakdown of polyanionic ordering. The trend in the series K, Na, Li-Sn is attributed to the decreasing size of the alkali atoms which leads to a less efficient screening of the polyanions by a surrounding shell of alkali atoms. The Zintl picture for the chemical bonding is supported insofar as we find that the electronic and chemical bonding properties are dominated by the strong attractive potential of the polyvalent Sn atoms resulting in the formation of at least partially covalent Sn-Sn bonds.

Structure of liquid Hg-Rb alloys: An x-ray absorption study

“…Indeed, the first peak minimum is markedly deep, even approaching zero for the systems alloyed with large alkali atoms. 12,14 We have found a number of arguments favoring the formation of stable Hg n polyanions, the strongest one being the lack of dependence on composition of the g Hg-Hg (r) low r region. In other systems, e.g., K-Pb alloys, it has been suggested 14 that Zintl structures are progressively stabilized when approaching central compositions.…”

“…Both group-IV-alkali and Hg-alkali [9][10][11] crystalline phases exhibit polyanion units. It has been shown that these units are preserved in liquid group-IV-alkali alloys, 12 for which transport properties are closely related to those of Hg-alkali alloys. 3,13 It has been proposed 12-14 that the formation of polyanion, Pb 4 4Ϫ is responsible for this maximum of resistivity at 50 at.…”

First-principle molecular dynamics study of the structural and electronic properties of liquid and amorphous Ni–Al alloys

“…The VASP code makes use of ultrasoft pseudopotentials [18], and an expansion of the electronic wave functions in plane waves. The approach has been applied widely in AIMD simulations of liquid metals and alloys [19][20][21][22][23][24][25][26][27]. Our simulations were performed using a cubic supercell containing 128 particles with periodic boundary conditions and C-points sampling.…”

"Non-simple liquids" - a challenge for neutron Brillouin scattering?

“…The main problem with simulations of such complicated liquids like those of the Zintl-type is the huge computational demand. First ab initio MD simulations of the structure and vibrational properties of liquid K x Sb 12x [15] or Na x Sn 12x [16] used relatively small cells and were made over short simulation times. To get a converged static structure factor or even the S ij (Q, v)s at small Q one would need several orders of magnitude more computer power.…”