Binary crystal structures

Binary compounds have two elements present. And there are many ways two atoms can fit together. Knowing the binary structure is often enough to describe the properties of the binary compounds that are talked about later in this chapter. 

There are four main factors that influence what structure a binary crystal forms. 

The size of the ionic radii The sharing of electrons is required to stabilize the total charge of a binary compound. However, if the atoms can t get close enough to one another, they can t share electrons effectively. The ionic radii is a measure of how close a cation-anion pair of ions can get to one another. To form a stable solid, the cation-anion pair needs to be as close to one another as possible. 

1 The ratio of the radii The ratio of the radii between the cation and anion determines the structure because it affects the coordination number of the constituents. If each radii is similar, then you can expect high coordination numbers for both. The ratio is defined as where r is the 

The polarizahility of the ion An external electric field creates a dipole within the atom. This affects larger ions more than smaller ones. In a crystal, this happens when other ions are close by. The electric field from nearby ion s electron cloud can polarize an anion. This stabilizes the net crystal structure, because it lowers the energy of ions in the crystal. Sometimes Van der Waals forces between ions (see Chapter 6) come into play and influence the structures that are made (this is especially the case when large ions are present). 

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Binary Crystal Structures in Terms of Close Packing... 

Binary crystal structures Binary crystal structures are made of two type of atoms in the crystal, such as NaCl (table salt), for example. 

In this and the following chapter, we will describe the most important simple (binary) crystal structures found in ceramic materials. You need to know the structures we have chosen because many other important materials have the same structures and because much of our discussion of point defects, interfaces, and processing will use these materials as illustrations. Some, namely FeSi, TiOi, CuO, and CU2O, are themselves less important materials and you would not be the only ceramist not to know their structure. We include these oxides in this discussion because each one illustrates a special feature that we find in oxides. These structures are just the tip of the topic known as crystal chemistry (or solid-state chemistry) the mineralogist would have to learn these, those in Chapter 7, and many more by heart. In most examples we will mention some applications of the chosen material. 

The catalytic subunit of cAPK contains two domains connected by a peptide linker. ATP binds in a deep cleft between the two domains. Presently, crystal structures showed cAPK in three different conformations, (1) in a closed conformation in the ternary complex with ATP or other tight-binding ligands and a peptide inhibitor PKI(5-24), (2) in an intermediate conformation in the binary complex with adenosine, and (3) in an open conformation in the binary complex of mammalian cAPK with PKI(5-24). Fig.l shows a superposition of the three protein kinase configurations to visualize the type of conformational movement. 

As a template for an intermediate conformation of protein kinase, the crystal structure of the binary complex of cAPK with adenosine (Ibkx.pdb in the Protein Data Bank) was used. As templates for open conformations... 

Fig. 2. Conformational free energy of closed, intermediate and open protein kinase conformations. cAPK indicates the unbound form of cAMP-dependent protein kinase, cAPKiATP the binary complex of cAPK with ATP, cAPKiPKP the binary complex of cAPK with the peptide inhibitor PKI(5-24), and cAPK PKI ATP the ternary complex of cAPK with ATP and PKI(5-24). Shown are averaged values for the three crystal structures lATP.pdb, ICDKA.pdb, and ICDKB.pdb. All values have been normalized with respect to the free energy of the closed conformations.

The catalytic subunit then catalyzes the direct transfer of the 7-phosphate of ATP (visible as small beads at the end of ATP) to its peptide substrate. Catalysis takes place in the cleft between the two domains. Mutual orientation and position of these two lobes can be classified as either closed or open, for a review of the structures and function see e.g. [36]. The presented structure shows a closed conformation. Both the apoenzyme and the binary complex of the porcine C-subunit with di-iodinated inhibitor peptide represent the crystal structure in an open conformation [37] resulting from an overall rotation of the small lobe relative to the large lobe. 

Table 8. Properties and Crystal Structure Data for Important Actinide Binary Compounds...

TFIIA and TFIIB are two basal transcription factors that are involved in the nucleation stages of the preinitiation complex by binding to the TBP-TATA box complex. Crystal structures of the ternary complex TFIIA-TBP-TATA box have been determined by the groups of Paul Sigler, Yale University, and Timothy Richmond, ETH, Zurich, and that of the TFIIB-TBP-TATA box by Stephen Burley and collaborators. The TBP-DNA interactions and the distortions of the DNA structure are essentially the same in these ternary complexes as in the binary TBP-TATA complex. 

Colloidal crystals . At the end of Section 2.1.4, there is a brief account of regular, crystal-like structures formed spontaneously by two differently sized populations of hard (polymeric) spheres, typically near 0.5 nm in diameter, depositing out of a colloidal solution. Binary superlattices of composition AB2 and ABn are found. Experiment has allowed phase diagrams to be constructed, showing the crystal structures formed for a fixed radius ratio of the two populations but for variable volume fractions in solution of the two populations, and a computer simulation (Eldridge et al. 1995) has been used to examine how nearly theory and experiment match up. The agreement is not bad, but there are some unexpected differences from which lessons were learned. 

The binary oxides and hydroxides of Ga, In and T1 have been much less extensively studied. The Ga system is somewhat similar to the Al system and a diagram summarizing the transformations in the systems is in Fig. 7.13. In general the a- and y-series have the same structure as their Al counterparts. )3-Ga203 is the most stable crystalline modification (mp 1740°) it has a unique crystal structure with the oxide ions in distorted ccp and Ga " in distorted tetrahedral and octahedral sites. The structure appears to owe its stability to these distortions and, because of the lower coordination of half the Ga ", the density is 10% less than for the a-(corundum-type) form. This preference of Ga "... 

C.19 Aluminum oxide, alumina, exists in a variety of crystal structures, some of which are beautiful and rare. Write the formula for aluminum oxide, which is a binary compound of aluminum and oxygen. The mass of a rectangular slab of aluminum oxide of dimensions 2.5 cm X 3.0 cm X 4.0 cm is 102 g. What is the density of aluminum oxide ... 

The crystal structures of the borides of the rare earth metals (M g) are describedand phase equilibria in ternary and higher order systems containing rare earths and B, including information on structures, magnetic and electrical properties as well as low-T phase equilibria, are available. Phase equilibria and crystal structure in binary and ternary systems containing an actinide metal and B are... 

When two metals A and B are melted together and the liquid mixture is then slowly cooled, different equilibrium phases appear as a function of composition and temperature. These equilibrium phases are summarized in a condensed phase diagram. The solid region of a binary phase diagram usually contains one or more intermediate phases, in addition to terminal solid solutions. In solid solutions, the solute atoms may occupy random substitution positions in the host lattice, preserving the crystal structure of the host. Interstitial soHd solutions also exist wherein the significantly smaller atoms occupy interstitial sites... 

The transition-metal monopnictides MPn with the MnP-type structure discussed above contain strong M-M and weak Pn-Pn bonds. Compounds richer in Pn can also be examined by XPS, such as the binary skutterudites MPn , (M = Co, Rh, Ir Pn = P, As, Sb), which contain strong Pn-Pn bonds but no M-M bonds [79,80], The cubic crystal structure consists of a network of comer-sharing M-centred octa-hedra, which are tilted to form nearly square Pnn rings creating large dodecahedral voids [81]. These voids can be filled with rare-earth atoms to form ternary variants REM Pnn (RE = rare earth M = Fe, Ru, Os Pn = P, As, Sb) (Fig. 26) [81,82], the antimonides being of interest as thermoelectric materials [83]. 

It should not be inferred that the crystal structures described so far apply to only binary compounds. Either the cation or anion may be a polyatomic species. For example, many ammonium compounds have crystal structures that are identical to those of the corresponding rubidium or potassium compounds because the radius NH4+ ion (148 pm) is similar to that of K+ (133 pm) or Rb+ (148 pm). Both NO j and CO, have ionic radii (189 and 185 pm, respectively) that are very close to that of Cl- (181 pm), so many nitrates and carbonates have structures identical to the corresponding chloride compounds. Keep in mind that the structures shown so far are general types that are not necessarily restricted to binary compounds or the compounds from which they are named. 

Four simple crystal structural types encompass the majority of elemental or binary semiconductors. The high symmetry of the structures has important consequences for the NMR spectra in several respects ... 

The ZB and WZ crystal structures are the most common types for binary octet semiconductors [23], and for III-V semiconductors they constitute practically the only ones known to occur. The energy differences between the two forms have been... 

The fourth and final crystal structure type common in binary semiconductors is the rock salt structure, named after NaCl but occurring in many divalent metal oxides, sulfides, selenides, and tellurides. It consists of two atom types forming separate face-centered cubic lattices. The trend from WZ or ZB structures to the rock salt structure takes place as covalent bonds become increasingly ionic [24]. 

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