W. van der Lugt

Polyanions in liquid ionic alloys: a decade of research

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.

ELECTRICAL RESISTIVITIES OF LIQUID ALKALI LEAD AND ALKALI INDIUM ALLOYS

Measurements of the resistivities of liquid K-Pb, K-In, Li-Pb and Na-In alloys are presented. The resistivities of the lead alloys exhibit a sharp peak at cPb=21 at.% for Li-Pb and at cPb=49 at.% for K-Pb. This is in agreement with the transition from simple ionic behaviour in Li-Pb to poly-anion formation in K-Pb. The maximum resistivity of K-Pb distinctly exceeds the range of metallic conduction. In the indium alloys the resistivity maximum is less pronounced and the theoretical description is more complicated.

Electronic structure and charge-transfer-induced cluster formation in alkali-group-IV alloys

In this paper the authors apply model tight-binding calculations to the electronic structure of systems in which there are essentially two covalent interactions: the intracluster interaction U and the intercluster interaction V. The atomic structure is simulated by a modified Bethe lattice which incorporates closed loops as a possibility. The ratio alpha =V/U is an important parameter to be varied. At a distinct value, alpha crit, a metal-non-metal transition takes place. The results are applied to the electronic structure and chemical stability of anion cluster in liquid and solid alkali-group-IV alloys. The physical parameters which govern the process of clustering are discussed.

First‐principles molecular‐dynamics simulation of liquid CsPb

Many alkali–post‐transition group IV alloy systems exhibit clearly defined equiatomic compounds together with a pronounced intermediate range ordering, indicated by a first sharp diffraction peak at ≊0.9 A−1. These phenomena have been explained assuming that tetrahedral group IV anions, ‘‘Zintl’’ ions, survive in the liquid state. As a prototype system we considered liquid CsPb, for which several experimental results are available, and studied it by means of first‐principles molecular‐dynamics. Agreement with experiment is satisfactory, provided the 5s and 5p electrons of cesium are explicitly taken into account in the computation of the electronic valence charge density. In particular, our calculations reproduce the structure factor prepeak reasonably well. The local liquid structure however is quite complex. This can be described as a disordered network, which still has many features in common with the ‘‘Zintl’’ ion model. For instance, the average Pb‐Pb coordination is close to 3, the value for perfect...

Resistivity of liquid Rb-Pb and Cs-Pb alloys

Measurements of the resistivity rho of liquid Rb-Pb and Cs-Pb alloys as a function of composition and temperature are presented. Around the equiatomic composition, the resistivity reaches non-metallic values in both systems; equiatomic Cs-Pb ( rho =7000 mu Omega cm at T=620 degrees C) may be classified as a liquid semiconductor. This is explained in terms of the liquid structure of these alloys, in which the Pb ions presumably join in covalently bound clusters. With rising temperatures, these clusters disintegrate. As a consequence, the resistivity decreases rapidly. For the Cs-Pb system, a DSC (differential scanning calorimetry) survey of the liquidus temperature as a function of composition has been performed. A tentative Cs-Pb phase diagram is presented.

Thermodynamic calculations for liquid alloys with an application to sodium-caesium

A semi-empirical model is presented for the analysis in liquid alloys of the long wavelength limit of the structure factor, the free energy of mixing and the electrochemical potential. The formalism is partly based on statistical mechanics assuming a concentration-dependent hard-sphere behaviour of liquid alloys. Although being conceptually quite different, the description incorporates the Flory-Huggins model as a first approximation.

The splitting of the nuclear magnetic resonance lines in vivianite

Abstract The splitting of the nuclear magnetic resonance lines of vivianite (Fe 3 (PO 4 ) 2 ·8H 2 O) was studied at temperatures of liquid nitrogen, liquid hydrogen and liquid helium. At a temperature between 4°K and 12°K the crystal becomes antiferromagnetic. Resonance diagrams with H 0 rotating in the ac -plane and in a plane perpendicular to the c -axis were determined at temperatures above as well as below the Neel temperature. In the antiferromagnetic state the internal magnetic fields at the position of the nuclei were determined from measurements at constant resonance frequency and from zero external field measurements. Conditions for the antiferromagnetic spin arrangement of Fe ++ spins have been derived from the symmetry properties of the internal field vectors. Two types of temperature dependences of the magnetization have been found in the paramagnetic as well as in the antiferromagnetic state. They correspond to two systems of Fe ++ ions occupying different types of lattice sites. The resonance lines of the P nuclei exhibit a shift due to electron transfer from the Fe ++ ions to the phosphate groups.

STRUCTURE OF LIQUID LI-SN ALLOYS

Neutron diffraction measurements of liquid Li-Sn alloys provide evidence for appreciable ordering in the liquid, probably accompanied by charge transfer from Li to Sn. The effects are strongest in Li4Sn, which happens to be close to the zero-alloy composition SCC(q) and rho CC(r) have been determined at the latter composition. For two other compositions, combination with available X-ray results has led to reasonable estimates of SNN(q) and SCC(q). The structural properties of the Li-Sn alloy system closely resemble those of the Li-Pb system. Some essential features of the diffraction pattern can be described by the mean spherical approximation (MSA).

Nuclear magnetic resonance and relaxation in zero field and near the threshold field

Synopsis As a result of previous measurements, proton magnetic resonance in antiferromagnetic copper chloride was expected to occur also in zero external field. Due to the interaction between the two protons in one crystalline water two resonance lines were observed. The proton-proton distance in a crystalline water was calculated from the frequency distance between these two resonance lines. The calculated distance is in good agreement with the results of the measurements of Levy and Peterson using neutron diffraction. It can be concluded that the magnetic moments of the Cu ions cannot be considered as mathematical dipoles located in the centre of the Cu ion (or Cl ion). The magnetization of a sublattice was measured as a function of temperature. The protonspin-lattice relaxation time t1 was measured in zero field as a function of temperature. The temperature dependence of t1 is even more pronounced than in a finite external field. Measurements of t1 as a function of the external field strength were carried out up to the threshold field.

Magnetic phase transitions of cobaltchloride

Abstract In single crystals of CoCl2·6H2O the Neel temperature was determined as a function of the external magnetic field. The external magnetic field was varied from 0–10000 Oe and was directed along the preferred axis and along a direction perpendicular to it. The threshold field was determined as a function of the temperature in the temperature range from 1.0°K up to the Neel point. The spontaneous magnetization was measured as a function of temperature in zero external field from 1°K up to 0.1°K below the Neel temperature.