Structure of Liquid Sb-Zn and As-Zn Alloys

The existence of a chemical short range order in many binary alloys has been confirmed by different experiments. At our institute we always had a special interest in liquid binary alloys of the group IIB-VA elements in the periodic table. Thermodynamic investigations [emf (electromotive force) and calorimetric measurements], determination of the electrical conductivity and magnetic investigations were carried out at such systems. The binary Sb-Zn system was extensively studied and we found strong temperature dependent anomalies of the thermodynamic properties and also of the electrical conductivity. From these results the existence of Sb3Zn4 clusters in the melt were assumed. The As-Zn system should even show a stronger indication of the cluster formation in the melt. The proposed neutron diffraction experiments should not only provr the existence of clusters but also should give some indication how the coordination number of liquid arsenic. Which is in the solid and liquid state 3, will change with the addition of zinc. Besides the experimental work we will also carry out some calculations. We will do it in collaboration with the group of Prof. Hafner in Vienna. It will be abinitio density functional molecular dynamic studies and the Vienna Abinitio Simulation Program (VASP) will be used.

Alloys have in the solid state an exact defined crystal structure. A-atoms are surrounded by a given number of B- atoms. These structures can be determined by x-ray analysis. In the liquid state a complete random distribution is assumed. A-atoms can be surrounded by A-atoms or by B-atoms or by A and B-atoms. There is a complete statistical distribution of the atoms in the melt. Has an alloy an exact melting point, say at 500C the alloy is ordered with a defined crystal structure but if we heat this material above the melting point the atoms are not ordered anymore. But some measurements with different methods indicate that in some alloys there is still some kind of order in the melt. However this type of order decreases sharply with increasing temperature. At approximately 100C above the melting point the ordering has disappeared. This type of ordering is called: "Chemical Short Range Order CSRO" From some investigations by colleagues and of our group it is known that in the systems Sb-Zn and As-Zn at the compositions of Sb3 Zn4 and As3 Zn2 such ordering phenomenon exist in the melt. In order to determine the size of such clusters neutron diffraction experiments were carried out at the Institute Laue-Langevin in Grenoble, France. Since the samples are not transparent and have a high density only neutron beams can be used for such experiments. It took us 8 hours of beam time to get a useful spectrum. The results of our experiment showed us that there is an interaction of unlike atomic pairs near the liquidus. With increasing temperature the clusters will dissociate and we found a statistical distribution of the atoms in the melt. However it was not possible to determine the exact size of the clusters. The use of this investigations is to understand the liquid state of alloys better. Model calculations of the system Al- Sn-Zn show that there is a segregation of tin to the surface during melting. Since this system is used as a lead-free solder the accumulation of tin at the surface is of great importance especially since the soldering bumps are getting smaller, down to 25 -50 (1 = 10-6 m). At this size the surface compared to the bulk is getting larger and more important.