Diamond, crystal structure melting point

Tin exists in three different forms (allotropes). Grey tin has a diamond structure, a density of 5.75 gem-3 and is stable below 286 K. White tin exists as tetragonal crystals, has a density of 7.31 gem-3 and is stable between 286 and 434 K. Between 434 K and the melting point of tin, 505 K, tin has a rhombic structure, hence the name rhombic tin , and a density of 6.56gem-3. 

Silicon and germanium crystallize in the diamond structure. However, they have somewhat weaker covalent bonds than carbon as a consequence of less efficient orbital overlap. These weaker bonds result in lower melting points for silicon (1420°C for Si and 945°C for Ge, compared with 4100°C for diamond) and greater chemical reactivity. Both sihcon and germanium are semiconductors, described in Chapter 7. 

Material Crystal structure Lattice constant (A) Difference from diamond (%) Melting point (°C) Thermal expansion coefficient (10- /K)... 

Essentially the characteristic temperature is a measure of the temperature at which the atomic heat capacity is changing from zero to 6 cal deg for silver (0 = 215 K) this occurs around 100 K, but for diamond (0 = 1860 K) with a much more rigid structure, the atomic heat capacity does not reach 5 cal deg i until 900 K. Those elements that resist compression and that have high melting points have high characteristic temperatures. Equations have been derived relating y/ u ) to the characteristic temperature 0. At room temperature diamond, with a characteristic temperature of 1860 K, has a root-mean-square amplitude of vibration, / u ) of 0.02 A, while copper and lead, with characteristic temperatures of 320 and 88 K, respectively, have values of 0.14 and 0.28 A for (u ). - Similar types of values are obtained for crystals with mixed atom (or ion) types. For example, average values of / u ) for Na+ and Cl in sodium chloride (0 = 281 K) are 0.14 A at 86 K and 0.23 A at 290 K. ° ... 

Each constituent atom of a covalent crystal is linked to its neighbours through directed covalent bonds. The crystal structure is determined by the spatial dispositions of these bonds. Because primary valence forces are involved, such solids are hard and have high melting points, e.g. diamond, silicon carbide, etc. Relatively few entirely covalent solids have been studied at elevated temperatures and it is, therefore, premature to comment on their decomposition characteristics. 

In Group 14, only carbon and tin exist as allotropes under normal conditions. For most of recorded history, the only known allotropes of carbon were diamond and graphite. Both are polymeric solids. Diamond forms hard, clear, colorless crystals, and was the first element to have its structure determined by x-ray diffraction. It has the highest melting point and is the hardest of the naturally occurring solids. Graphite, the most thermodynamically stable form of carbon, is a dark gray, waxy solid, used extensively as a lubricant. It also comprises the lead in pencils. 

On the other hand, diamond, a form of solid carbon, is one of the hardest substances known and has an extremely high melting point (about 3500 °C). The incredible hardness of diamond arises from the very strong covalent carbon-carbon bonds in the crystal, which lead to a giant molecule. In fact, the entire crystal can be viewed as one huge molecule. A small part of the diamond structure is represented below. 

There are some solids, often called covalent network solids, that are composed only of atoms interconnected by a network of covalent bonds. Quartz and diamond are two common examples of network solids. In contrast to molecular solids, network solids are typically brittle, nonconductors of heat or electricity, and extremely hard. Analyzing the structure of a diamond explains some of its properties. In a diamond, each carbon atom is bonded to four other carbon atoms. This tetrahedral arrangement, which is shown in Figure 8.25, forms a strongly bonded crystal system that is extremely hard and has a very high melting point. 

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