Group Zintl phases with

The increasing charge on the group-13 and group-15 cluster ions must be counterbalanced by a larger number of counter ions. A large value of n requires more and thus smaller counter ions. For this reason, polyhedral Zintl ions of group-13 elements occur preferentially in Zintl phases with alkali- or alkaline-earth metal... 

Table 15.1. Compilation of the Ca Zintl phases with layered polyanions and of typical sheet polymers based on an sp hybridized backbone for the different group IV atoms. The arrows indicate the route for obtaining the polymers.

Knowledge of the topochemistry of the Zintl phases of Ca with Si and Ge known is still rather limited. New approaches could include the use of different sources of chlorine or of protic solvents in the topochemical transformation of the layered Zintl phases. A first attempt in this direction has involved the use of aqueous solutions of C0CI2 for the removal of A different approach to broaden the topochemistry is the transformation of Zintl phases with different structures of the group IV polyanions, leading to Si or Ge compounds with other backbone structures.An example of this is the reaction of CaSi with HCl, which leads to the formation of linear polysilane chain polymers. However, the Zintl phases of Si and Ge have an even richer structural chemistry, containing polyanions in the form of tetrahedra or planar Si rings, which could lead to Si compounds of great interest from both a chemical and a physical point of view. ° ... 

Calcium disilicide CaSi2 is a Zintl phase with an extreme formulation of Ca " (Si 2 (Schafer, Eisenmann, and Muller 1973). Si is isoelectronic, with group V elements, and forms anion partial lattices of corrugated layers like As. The Ca " " planer layers are sandwiched between the anion layers (Bohm and Hassel 1927). There are two kinds of layered modifications, three- and six-layer types, in which the CaSi2 stacked layers are repeated after three and six... 

The performance of VASP for alloys and compounds has been illustrated at three examples The calculation of the properties of cobalt dislicide demonstrates that even for a transition-metal compound perfect agreement with all-electron calculations may be achieved at much lower computational effort, and that elastic and dynamic properties may be predicted accurately even for metallic systems with rather long-range interactions. Applications to surface-problems have been described at the example of the. 3C-SiC(100) surface. Surface physics and catalysis will be a. particularly important field for the application of VASP, recent work extends to processes as complex as the adsorption of thiopene molecules on the surface of transition-metal sulfides[55]. Finally, the efficiciency of VASP for studying complex melts has been illustrate for crystalline and molten Zintl-phases of alkali-group V alloys. 

The components of polar intermetallics generally include an active metal from the group 1 or 2 or the rare-earth series plus, sometimes, a late-transition metal, and a metal from the p-block. Because of the presence of an electron-poorer late transition metal, polar intermetallics generally have lower e/a values (about 2.0-4.0) than classic Zintl phases (>4.0) [45], Note these values are traditionally calculated over only electronegative atoms [45], in contrast to those of Hume-Rothery phases (<2.0) [45] and QC/ACs (2.0 0.3) [25], for which electron counts are considered to be distributed over all atoms. The former two higher values are decreased to about 1.5-2.5 and >2.5, respectively, when counted over all atoms (but with omission of any dw shells). For comparison purposes, Fig. 3 sketches the distribution of all these intermetallic phases according to e/a counted over all atoms, as we will use hereafter. 

The polyhedral boranes and carboranes discussed above may be regarded as boron clusters in which the single external orbital of each vertex atom helps to bind an external hydrogen or other monovalent atom or group. Post-transition main group elements are known to form clusters without external ligands bound to the vertex atoms. Such species are called bare metal clusters for convenience. Anionic bare metal clusters were first observed by Zintl and co-workers in the 1930s [2-5], The first evidence for anionic clusters of post-transition metals such as tin, lead, antimony, and bismuth was obtained by potentiometric titrations with alkali metals in liquid ammonia. Consequently, such anionic post-transition metal clusters are often called Zintl phases. 

A review about the Zintl phases has been published by Sevov (2002) from the introduction of this publication we quote a few remarks. It was preliminary observed that the number of Zintl phases has increased many-fold since Zintl s time and that the definition of a Zintl phase has never been very exact often compounds that include non-metals have been considered in this family. The paper by Sevov is mainly dedicated to clearly intermetallic Zintl phases (that is phases containing main group metals, semi-metals or semiconductors only). Attention has therefore been dedicated to compounds of alkali metals with the elements of the 13th, 14th and 15th groups (without B, Al, C, N and P). To this end the following definitions and statements have been considered. 

Another contribution is represented by an investigation of a cubic thallium cluster phase of the Bergmann type Na13(TlA.Cdi A.)27 (0.24 < x <0.33) (Li and Corbett 2004). For this phase too the body centred cubic structure (space group Im 3, a = 1587-1599 pm) may be described in terms of multiple endo-hedral concentric shells of atoms around the cell positions 0, 0, 0, and 14,14,14. The subsequent shells in every unit are an icosahedron (formed by mixed Cd-Tl atoms), a pentagonal dodecahedron (20 Na atoms), a larger icosahedron (12 Cd atoms) these are surrounded by a truncated icosahedron (60 mixed Cd-Tl atoms) and then by a 24 vertices Na polyhedron. Every atom in the last two shells is shared with those of like shells in adjacent units. A view of the unit cell is shown in Fig. 4.38. According to Li and Corbett (2004), it may be described as an electron-poor Zintl phase. A systematic description of condensed metal clusters was reported by Simon (1981). 

The Structural principles of the elements can also be found in a number of ternary Zintl phases. For example, KSnSb contains (SnSb ) layers as in a-arsenic. In other ternary ZiNTL phases the anionic part of the structures resembles halo or oxo anions or molecular halides. For example, in Ba4SiAs4 there are tetrahedral SiAs particles that are isostructural to SiBr4 molecules. In Ba3AlSb3 dimeric groups Al2Sbg2 are present, with a structure as in A Clg molecules (Fig. 13.4). Ca3AlAs3 contains poly-... 

The term Zintl phase is applied to solids formed between either an alkali- or alkaline-earth metal and a main group p-block element from group 14, 15, or 16 in the periodic table. These phases are characterized by a network of homonuclear or heteronuclear polyatomic clusters (the Zintl ions), which carry a net negative charge, and that are neutralized by cations. Broader definitions of the Zintl phase are sometimes used. Group 13 elements have been included with the Zintl anions and an electropositive rare-earth element or transition element with a filled d shell (e.g. Cu) or empty d shell (e.g. Ti) has replaced the alkali- or alkaline-earth element in some reports. Although the bonding between the Zintl ions and the cations in the Zintl phases is markedly polar, by our earlier definition those compounds formed between the alkali- or alkaline-earth metals with the heavier anions (i.e. Sn, Pb, Bi) can be considered intermetallic phases. 

The anion connectivity of many Zintl phases can be rationalized in terms of Hume-Rothery s (8 V) mle. For example, in BaSi2 (with Si clusters), the Si anion is isoelectronic with the nitrogen group elements, that is, it has five valence electrons. The (8 N) rule correctly predicts that each silicon atom will be bonded to three other sUicon atoms. Similarly, in Ca2Si, Si is isoelectronic with the noble gas elements. Again, the 8 A mle correctly predicts that silicon will occur as an isolated ion. Indeed, this compound has the anti-PbCl2 stmcffire, in which the sUicon is surrounded by nine calcium ions at the comers of a tricapped trigonal prism. 

A typical Zintl phase is a compound like NaSi [12]. It is described by the formulation Na Si . The anion Si" has to be triply bonded, which is expressed by the symbol (3b) [13]. Si" is isoelectronic with phosphorus and arsenic (diagonal relationship) and it is, therefore, according to Klemm, a pseudo element of group 15. This means it should accomodate a structure which is typical for group 15 elements. In NaSi (3b)Sr anions form [SiJ " tetrahedra, like phosphorus and arsenic do in white phosphorus and yellow arsenic, respectively. In CaSi2, a comparable situation oecurs. According to the formulation Ca (Si )2, anions (3b)Si" are expected as well. 

Further examples of formally subvalent main group compounds that contain element-element bonds but not necessarily clusters are the Zintl phases. The bonding in these has been described as the octet rule for all atoms . The archetypal Zintl compound is NaTl, in which charges are assigned as Na+ and Tl, representing a formal transfer of electrons from the more to the less electropositive element. The Tl ion can be considered to be a group 14 pseudoelement, and in fact exists in NaTl as a three-dimensional polyanionic diamond framework (TN) stuffed with Na+ cations. The Zintl concept is extended more broadly to other binary and ternary solid-state compounds, whose structures show the formation of element-element bonds in one, two, or three dimensions. ... 

The structures adopted by the binary compounds of the Group I or II metals with elements from Groups III—VI, the so-called Zintl-phases, have been reviewed, and the bonding in such species has been discussed in terms of the transition between metallic and ionic bonding.347... 

---