Z. Akdeniz

Structure and diffusion in aluminium and gallium trihalide melts from simulations based on intramolecular force laws

Diffraction studies of the liquid structure of AlCl3, AlBr3, GaBr3 and GaI3 close to their respective freezing points have revealed fourfold coordination of the trivalent metal ions, consistent with dimeric M2X6 molecules being the dominant species. We evaluate the species-resolved pair distribution functions and liquid structure factors in all these melts by carrying out classical molecular-dynamics simulations, based on polarizable-halogen force laws that were determined on isolated molecular monomers and dimers in the gaseous phase. We also report results for mean-square displacements and diffusion coefficients of the two species in each melt. The model reproduces the main features of the total neutron-diffraction structure factors, showing peaks due to intermediate-range order and to charge and density short-range order, and accounts for the experimental data at a good quantitative level. Direct simulation of the pair distribution functions yields agreement with the diffraction data on metal–halogen and halogen–halogen bond lengths in the melt and on the stability of the first-neighbour shell of the metal ions. We examine the temperature dependence of the liquid structure in our models for GaBr3 and AlCl3 and emphasize the structural role of van der Waals interactions between the halogens.

Tests of the deformation-dipole model for metal–halide molecular clusters

An ionic model that was originally built for chloroaluminate clusters by combining truncated expansions in classical multipoles and in quantal overlaps has been extensively applied to describe cohesion, structure, and vibrational spectra of a wide variety of molecular clusters in polyvalent metal halides [for a review see M.P. Tosi, Phys. Chem. Liq., 43, 409 (2005)]. In this work we test on simple ionic molecules two crucial aspects of the model, namely (i) the transferability of the overlap parameters for the halogen ions across families of halide compounds, and (ii) the anharmonicity in the halogen – metal ion interaction potential over a very broad range of interionic distances. Transferability is tested by means of a parallel discussion of alkali and alkaline-earth halide monomers near equilibrium. With regard to anharmonicity, the full potential energy curve yielded by the model for the sodium chloride monomer is compared with the results of quantum mechanical calculations for the ionic state of the molecule within a configuration-interaction approach.

Stability analysis for complexes in calcium-alkali bromide solutions

Abstract We discuss the dependence of the stability of tetrahedral complexes in molten halide mixtures on the halogen species. This is done by calculating the equilibrium concentration of (CaBr4)2− complexes in calcium-alkali bromide solutions as a function of composition, in comparison with earlier calculations on the calcium-alkali chloride systems. The comparison supports a possible trend of increasing stability from chlorides to bromides, provided that halogen polarizability or chemical bonding contribute appreciably to the binding of a complex. Supporting evidence is noted and further experiments are suggested.

Molecular clusters in gaseous and liquid AlCl3

Motivated by results on molecular clusters formed from octahedral connectivity in NbF5 and by simulation and neutron diffraction studies of liquid AlCl3 and related materials, we discuss the gaseous n-mers of AlCl3 built from corner-sharing or edge-sharing tetrahedra. We use an interionic force-law model to evaluate the energetics of these clusters and examine their relevance to liquid structure near freezing and at higher temperatures as determined by means of classical molecular-dynamics simulation.

From molecular clusters to liquid structure in AlCl3 and FeCl3

Melting of aluminum and iron trichloride is accompanied by a structural transition from sixfold to fourfold coordination of the trivalent metal ions, and a widely accepted interpretation of the structure of their melts near freezing is that they mainly consist of strongly correlated dimers formed from two edge-sharing tetrahedra. We carry out classical molecular dynamics simulations to examine how a polarizable-ion force law, determined on isolated molecular monomers and dimers in the gaseous phase of these compounds, fares in accounting for the pair structure of their liquid phase and for mean square displacements and diffusion coefficients of the two species in each melt. The model reproduces the main features of the neutron diffraction structure factor, showing peaks due to intermediate range order and to charge and density short-range order, and accounts for the experimental data at a good semi-quantitative level. We find agreement with the neutron and X-ray diffraction data on metal–halogen and Cl–Cl bond lengths in the melt, and demonstrate the high sensitivity of the results for the width of the first-neighbor shell to truncation in obtaining it by Fourier transform of the neutron-weighted structure factor in momentum space. We also report comparisons with a recent first-principles study of the structure of the AlCl3 melt by the Car–Parrinello method. Finally, we demonstrate break-up of dimers into monomers upon raising the liquid temperature in the case of AlCl3.

Microstructure of mixed-nitrate melts and glasses

We evaluate the stability of various structures for a Ca2(NO3)7 core unit compensated by K or Rb ions, as a basic constituent of the glass-forming melts of CKN and CRbN compounds [3ANO3 · 2Ca(NO3)2 with A = K or Rb]. We find that three alternative structures are stable for the core unit and lie at approximately the same energy, provided the Ca-NO3 overlap repulsion is stiff enough as suggested by the planar structure of the NO3 group, but independent of the amount of electronic charge transfer to these groups. In these structures, each Ca ion can lie in a fourfold or a fivefold coordination by NO3 groups, so that the two Ca ions can be bridged by one, two, or three NO3 groups. This picture is compatible with the accepted view of CKN as a prototype fragile glass-former and with the stability of mixed-nitrate glasses over a broad range of composition.

Models for structural transitions in nitrates

A number of alkali and alkaline-earth nitrates crystallize in ionic structures related to the NaCl and fluorite crystal structures, respectively. This suggests that their cohesive properties may be usefully described by means of a phenomenological ionic model (Z. Akdeniz, M.P. Tosi, Zs. Naturforsch., 59a, 957 (2004)). In the present work we discuss from this viewpoint the structural transitions that take place in these materials. We first present a Bragg–Williams type approach to the orientational disordering of the NO3 groups and to positional melting in sodium nitrate. We then discuss the stability of the superionic CKN glass in terms of strongly bound Ca2(NO3)7 units surrounded by highly mobile potassium ions.

Structure of rare-earth/group-IIIa chloride complexes

We evaluate the structures taken by vapour complexes of chloride compounds with the chemical formula MnRCl3(n+1) where R is a selected rare-earth element, M a group-IIIA element, and n = 1, 2, or 3. The main predictions that emerge for the most stable structures from our model calculations are as follows: (i) in MRCl6 a fivefold coordination of the rare-earth element (for R = La, Nd, Er, or Lu) is very stable relative to a fourfold one, with the excess binding energy decreasing slightly from La to Lu and being almost the same when M = Al or Ga; (ii) a sixfold coordination of Nd becomes very stable in Ga2NdCl9; and (iii) sevenfold and eightfold coordinations of Nd can arise in Ga3NdCl12, with the latter being more stable. All these structures are obtained from the RCl3 monomer by substituting n chlorines with n MCl4 distorted tetrahedra, which complete the coordination shell of the rare-earth ion via edge or face-sharing. This criterion combines high coordination of the rare-earth ion with shielding of its Coulomb field by bonding chlorines in double or triple sets. The possible appearance of the unusual fivefold and sevenfold coordination states in the vapour complexes should provide further motivation for experimental structural studies and for refined quantum-chemical calculations

Interionic Force Model for Pentahalide Molecules and Higher Niobium-Based Halide Clusters

Molecular bound states tend to become progressively more stable in the melts of polyvalent metal halides as the nominal valence of the metal increases. We examine in this work the case of pentavalent metal halides. First we propose a simple ionic model for the binding in several pentahalide clusters: the chlorides of Nb, Ta, Sb, and Mo and the bromides of Nb and Ta. The molecular monomers of these compounds have a D3h trigonal-bipyramidal structure in the ground state, and we make use of data on equatorial bond lengths and breathing mode frequencies in the vapour to determine the main force-law parameters of the metal ion. We also find that the C4v square-pyramidal structure is mechanically unstable against transformation into the D3h shape.We then consider higher molecular clusters, i. e. the dimers of Nb pentahalides and the bound states formed by NbCl5 with the chlorides of Cs, Al, Ga, and Sb. We propose structural models for all these stable clusters and compare their calculated vibrational frequencies with the available data from vibrational spectroscopy of mixed melts.

On the Ionic Equilibrium between Complexes in Molten Fluoroaluminates

We discuss theoretically (i) the effect of the alkali cation species on the ionic equilibrium between (AIF 6 ) 3- and (AIF 4 ) - complexes in molten alkali fluoroaluminates, and (i) the possible presence of (AIF 5 ) 2- complexes in molten cryolite, in relation to very recent Raman scattering experiments by Gilbert and Materne