Survival of polyanions in expanded liquid alloys

We report the first neutron diffraction experiments of the equiatomic liquid alloys NaSn and KPb over a temperature range from 873 to 1873K and pressures up to 60 bars. In the crystalline state, these are semiconducting compounds containing tetrahedrally coordinated polyanions, Sn⁴⁻₄ and Pb⁴⁻₄. The diffraction results indicate the survival of polyanions in the expanded liquid alloys even up to the highest temperatures measured, as indicated by the presence of the first peak in the radial distribution function and by the persistence of the peak at Q≈1Å^-1, which is indicative of intermediate-range order. The position of this peak shifts towards lower Q-values, its height decreases, and its width drastically increases with increasing temperature and correspondingly decreasing density. One can conclude that in these alloys, which are representative of a wider class of compounds containing Zintl ions in their crystalline state, the polyanions survive at these high temperatures and at densities which are about 25% lower than those at the melting point.