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Chapter 3

Chapter 3. Atomic Scale Structure ： Atomic Packing and Coordination Numbers. I. Introduction. Atomic packing ： arrangement of the atoms surrounding a centered atom or ion ： coordination number  type of bonding , relative size of atoms or ions

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Chapter 3

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  1. Chapter 3 Atomic Scale Structure：Atomic Packing and Coordination Numbers

  2. I. Introduction • Atomic packing： arrangement of the atoms surrounding a centered atom or ion ： coordination number  type of bonding, relative size of atoms or ions • Atomic packing (Atomic Scale Structure) : a few A  short range order.(SRO) Crystalline structure : > a few nm)  long range order.(LRO) F22 27 28

  3. Several important macroscopic properties of materials can be estimated on the basis of only a knowledge of the atoms present, the type of bonding between atoms, and the shape of the bond-energy curve. Other properties such as density, largely depend on the arrangement of the atoms in the solid. • Atomic packing (arrangement of atoms) within a solid can be principally characterized by the number of nearest neighbors, or coordination number, CN, which is influenced primarily by the type of bonding present and by the relative sizes of the atoms or ions .

  4. F 3.1 F 3.2 T 3.1 F 12.3 F 3.2 T 12.3 II. Radius of Atoms and Ions • Atomic radius • Ionic radiie.g., Consider the ionic compound CsC1.r(Cl -) + r(Cs +) = X0 (CsCI) • Using this procedure, a self-consistent set of ionic radii can be calculated.

  5. III. Determination of Coordination Number (Atomic packing← Coordination Number) • Coordination Number (CN) ● type of bonding i.e., metallic, covalent, ionic ● relative size of atoms or ions • Type of bonding ● Metals：pure metallic element, i.e., composed of one single element e.g., Fe, Cu, Al,…...  metallic bonding

  6. ● ceramics：mostly are compounds composed of metallic nonmetallic elements, i.e., composed of at least two different elements, usually metallic elements form cations, and nonmetallic elements form anions e.g., Al2O3, SiO2, TiO2, AlN, BN, ……(exception : diamond graphite, ……) (Most ceramics have mixedionic and covalent bonding, (exception： a few with pure covalent bonding, e.g., diamond)

  7. A. Metallic Bonding • Type of bonding ● Metals：pure metallic element, i.e., composed of one single element , e.g., Fe, Cu, Al,…... ＊all atoms：spherical , same size. • CN is primarily determined by geometric arrangement (consideration) ＊usually (better) described by crystalline structure. ＊CN：8-12. F3.1 F3.2 T3.1

  8. T 12.2 T 12.3 F 12.1 F 2.6-1 B. Ionically Bonded Materials • CN can be determined using the following constraints: (1) cations “touch” anions, (2) the number of longer ions(e.g.,) anions , surrounding the smaller ions(e.g.,) cation will be as high as geometrically possible, (3) the ions cannot overlap. • We usually consider the smaller ions (with radius r )at the center and see how the larger ions (with radius R)are packed surrounding the smaller ions. • r/R must always be less than or equal to one, for most ironically bonded compounds, r(cation) < R(anion). r(cation) /R(aion) is used to estimate the CN of cation. If, however, r(anion) < R(cation) then the r(aion)/R(cation) ratio should be used to estimate the CN of the anion. Once the CN of thesmaller ion is known, the CN of the larger ion can be determined based on the cation : anion ratio, or the stoichiometry of the compound.

  9. F 12.10 2-36 F 12.12 C. Covalently Bonded materials

  10. IV. Effects of Coordination Number on Material Properties : density • ●Density • atomic mass • CN • CN density • ● Since the CN of most ceramic materials are smaller that • that of metallic materials, ceramics are usually lighter • (smaller density ) than metals. 27 CN metallic bonding：8-12 covalent bonding：mostly 4 ionic bonding：4-8

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