Alkali halide clusters

T. P. Martin, Alkali-Halide Clusters and Micro-Crystals. Phys. Rev. 1983, 95, 167-199. 

The different homo- and hetero-complexes observed and their thermochemical data are summarized in various review articles by McPhail et al. [424], Schafer [425, 426], 0ye and Gruen [427], Hastie [428, 429], Novikov and Gavryuchenkov [430], Biichler and Berkowitz-Mattuk [268], as well as Bauer and Porter [59]. Gaseous metal halide species is the only subject of these articles. Rules were developed to predict many properties of complexes (see Refs. 426, 428, 430). Brooker and Papatheodorou [431] as well as Papatheodorou [432] give an account of the vibrational properties and spectroscopic studies of the complexes. Theoretical and experimental investigations of alkali halide clusters are reviewed by Martin [433]. 

The first indications that certain systems might violate the phase rule came from computer simulations of small clusters of atoms. A number of studies revealed clearly defined solid-like and liquid-like forms [5-14]. These embraced both molecular dynamics and Monte Carlo simulations, and explored a variety of clusters. These included several based on atomic models with interparticle Lennard-Jones forces, which mimic rare gas clusters rather well. There were also models of alkali halide clusters. Hence, the existence of solid and liquid forms for such small systems seemed not only plausible but general, not restricted to any one kind of system. Shortly after these studies appeared, another, of a 55-atom cluster with Lennard-Jones interparticle forces, showed not only solid and liquid forms but also a form in which the surface of the cluster (with icosahedral structure) is liquid... 

U. Landman, D. Scharf, and J. Jortner, Isomerization and Melting in Alkali Halide Clusters (unpublished). 

Ochsenfeld C, Gauss J, Ahlrichs R. An ab initio treatment of the electronic absorption spectra of excess-electron alkali halide clusters Na , , CI up to Na18Cl17. J Chem Phys 1995 103 7401-7407. 

It is possible to infer these structures, that of lowest energy and others of higher energy as well, from simple models for some kinds of clusters. This is particularly so for atomic clusters whose binding forces are well represented by pairwise interactions such as the Lennard-Jones potentials that approximate van der Waals interactions, or the Born-Mayer interactions that describe the forces between the ions in alkali halide clusters. Many molecular clusters also behave much like their simple models, models... 

CN clusters and the alkali halide clusters, which formed compact cubic or planar structures. 

Bloomfield LA, Conover CWS, Yang YA, Twu YJ, Phillips NG. Experimental and theoretical studies of the structure of alkali halide clusters. Nucl Instrum Methods Phys Res B. 

Pittner, /. Chem. Phys., 121, 9898 (2004). Different Approaches for the Calculation of Electronic Excited States of Nonstoichiometric Alkali Halide Clusters The Example of NasF. 

Nonstoichiometric alkali halide clusters X Y with X = Na, Li, K and Y = Cl, F containing single and multiple excess electrons have been extensively studied experimentally " and theoretically " as prototypes of possible metal-insulator transitions and segregation into metallic and ionic parts in finite systems. Hydrogenation of lithium clusters has also been investigated but considerably less than alkali halides (cf. Ref. 36 and references therein). Of course the ground state properties such as ionization potentials (IPs) were first available for both halides and hydrides. In the case of the optical probes the visible region was until recently experimentally more easily accessible than the infrared and therefore the data were incomplete. 

For alkali halide clusters it was first proposed to classify species corresponding to (i) cuboid or deformed sections of the rock salt lattice with the localized excess electrons in the vacancy (surface F-centers), for which relatively high IPs were measured and a strong absorption in the visible-infrared region, in analogy with the bulk color centers, is expected (ii) filled cuboids with excess electrons highly delocalized... 

Excess electrons in clusters also exhibit the above structural characteristics as well as Other quantum mechanical effects that arise when the electron wavelength is comparable to the cluster diameter. Path integral studies of an excess electrons in alkali halide clusters have established the existence of internal cavity structures and of extended surface states. Dissociative attachment, leading to the formation of an isolated alkali atom, and structural isomerization have also been observed. Similar calculations have demonstrated the prevalence of surface states in water clusters. " Representative structures are shown in Figure 5. 

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