3.091_Notes_4 - LN4 3.091 Introduction to Solid State...

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LN–4 1 3.091 – Introduction to Solid State Chemistry Lecture Notes No. 4 THE NATURE OF CRYSTALLINE SOLIDS In an assembly of atoms or molecules a solid phase is formed whenever the interatomic (intermolecular) attractive forces significantly exceed the disruptive thermal forces and thus restrict the mobility of atoms, forcing them into more-or-less fixed positions. From energy considerations it is evident (as discussed in LN-2) that in such solids the atoms or molecules will always attempt to assume highly ordered structures which are characterized by symmetry . Depending on the nature of the acting interatomic forces, all solids may be subdivided into: (a) Ionic solids (NaCl) (b) Covalent solids (Diamond) (c) Metallic solids (Fe, Ni, etc.) (d) Van der Waals solids (ice, solid He) Solids as we encounter them in nature may or may not reflect the internally ordered arrangement in their appearance. We find, for example, well-formed quartz crystals, garnets, diamonds and snowflakes which are all characterized by flat bounding planes which intersect at characteristic angles. On the other hand, we also observe rounded stones and man-made cast solid objects with no external evidence of internal order (fig. 1). water ice snow flake melt crystals single crystal c 1 c 2 c 3 c 4 c 5 poly crystalline solid consisting of intergrown randomly oriented single crystals q (heat) q (heat) Figure 1 Crystals, internal order and external appearance
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LN–4 2 To understand the external appearance of the solid state it is necessary to consider the formation of solids from different phases . Solids, for example, are formed upon cooling of liquids (melts) - by freezing or solidification; this solidification process normally proceeds in total confinement and the resulting “cast” structure will have an external appearance which reflects in detail the confining geometry (and not the internal order). Moreover, depending on solidification conditions, the solid body may be either a single crystal or polycrystalline . Polycrystalline solids (in excess of 95% of the solid state encountered) may be thought of as an assembly of microscopic single crystals with random orientation held together like a maze structure by the interwoven irregular shapes of the individual crystals. A typical example of an “unconfined” phase transformation is the formation of snowflakes where the external boundaries of the solid have assumed crystalline appearance, reflecting in detail the internally ordered molecular (H 2 O) arrangement. Another unconfined formation of a solid is precipitation from solution (sugar crystals, CuSO 4 and the like). Similarly, the formation of crystals from the vapor phase leads to bodies which externally reflect elements of internal order. 1. CRYSTAL STRUCTURE From the earlier discussion it should be apparent that, when strong interatomic forces exist, atoms tend to pack closely together - the closeness of packing being particularly pronounced in the solid state. In this case atoms can be regarded as hard spheres and
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This note was uploaded on 11/02/2009 for the course CHEMISTRY 3.091 taught by Professor Donsadoway during the Fall '04 term at MIT.

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3.091_Notes_4 - LN4 3.091 Introduction to Solid State...

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