Scanning tunneling spectroscopy permits real-space observation of one-dimensional electronic states on a Fe(100) surface alloyed with Si. These states are localized along chains of Fe atoms in domain boundaries of the Fe(100) c͑2 3 2͒Si surface alloy, as confirmed by first-principles spinpolarized calculations. The calculated charge densities illustrate the d-like orbital character of the one-dimensional state and show its relationship to a two-dimensional state existing on the pure Fe(100) surface. [S0031-9007(96)00335-3]

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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.

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The systematic variations of the crystal structure, phase stability, electronic structure and chemical bonding properties of equiatomic alkali-tin alloys as functions of the size of the alkali atom have been studied for the example of equiatomic alkali-tin alloys using ab initio local density calculations. It is demonstrated that the formation of the polyanionic phases of KSn and NaSn with tetrahedral Sn4 clusters may be interpreted within the Zintl principle: the large electronegativity difference leads to an at least formally complete electron transfer from the alkali to the tin atoms and to the formation of strong covalent bonds stabilizing the Sn44- `Zintl ions' which are isoelectronic and isostructural to the P4 molecule. Charge transfer is also the dominant mechanism in LiSn; however, due to the smaller size of its alkali ion, the remaining intercluster interactions are too strong, so the Sn ions form an extended network (in the form of corrugated planes) rather than isolated polyanions. The LiSn structure is also discussed from the point of view of a simple ionic model such as is realized in the CsCl structure. It is shown that the simple ionic model is destabilized by direct Sn-Sn interactions. Local density functional theory is shown to provide an accurate description of the complex crystal structures of these alloys and a rationale for the observed structural trends in alkali-group-IV alloys.