CLOSE PACKING IN CRYSTALS (2D AND 3D):

The structure of crystalline solids is determined by packing of their constituents .In order to

understand the packing of the constituent particles in a crystal; it is assumed that these particles are 

hard spheres of identical size (eg those of metal). The packing of these spheres takes place in such a 

way that they occupy the maximum available space and hence the crystal has maximum density. This 

type of packing is called close packing.

TWO DIMENSIONAL PACKING (2D): 

When the rows are combined touching each other, the crystal plane is obtained. The rows can be combined in two different ways

(A) Square close packing:

(1)  The particles when placed in the adjacent rows show a horizontal as well as vertical (head to head) alignment and form squares. This type of packing is called square close packing.

   

(2) Packing efficiency calculation:

(1) One sphere will be in constant contact with 4 other spheres hence coordination number is 4

(2) Side of Square is (a) and a=2r where r is the radius of atom (sphere).

(3) Area of square = a2 = 4r² 

(4) Area of atoms in the square is

(5) Packing fraction (PE) , fraction of area occupied by spheres .

(B) Hexagonal close packing:

(1)  The particles in every next row are placed in the depressions between the particles of the first row. The particles in the third row will be vertically aligned with those in the first row. This type of packing gives a hexagonal pattern and is called hexagonal close packing

(2) Packing efficiency calculation:

(1) One sphere will be in constant contact with 6 other spheres hence coordination number is 6

(2) Side of hexagon is a and a=2r where r is the radius of atom (sphere) .

(3) Area of hexagon unit cell is 6*area of six equilateral triangles 

(4) Area of atoms in the hexagon unit 

(5)  Packing fraction (PE) , fraction of area occupied by spheres 

THREE DIMENSIONAL PACKING (3D):

(1) 3D SIMPLE CUBIC CELL AND BODY CENTRED CELL (SCC AND BBC):

 (A) Simple (primitive) Cubic Cell (SCC) structure:

(1) 2D square close packing sheets are involved to generate simple cubic cell as well as body centred cell. In which each corner atom is touching potion with its adjacent corner atom.

(2) Take two 2D square close packing sheet and Placing a second square packing layer (sheet) directly over a first square packing layer forms a "simple cubic" structure.

(3) The simple “cube” appearance of the resulting unit cell is the basis for the name of this three dimensional structure.

(4) This packing arrangement is often symbolized as "AA...", the letters refer to the repeating order of the layers, starting with the bottom layer.

(5) The coordination number of each lattice point is six. This becomes apparent when inspecting part of an adjacent unit cell.

(6) The unit cell contain eight corner spheres, however, the total number of spheres within the unit cell is 1 (only 1/8th of each sphere is actually inside the unit cell). The remaining 7/8ths of each corner sphere resides in 7 adjacent unit cells.
(7)  PACKING EFFICIENCY AND COORDINATION NUMBER:

(2) Body Centered Cubic (bcc) Structure:

(1) A more efficiently packed cubic structure is the "body-centered cubic" (bcc).

(2) The first layer of a square array is expanded slightly in all directions. Then, the second layer is shifted so its spheres nestle in the spaces of the first layer.

(3) This repeating order of the layers is often symbolized as "ABA...". Like Figure 3b, the considerable space shown between the spheres in Figure 5b is misleading: spheres are closely packed in bcc solids and touch along the body diagonal.

(4) The packing efficiency of the bcc structure is about 68%. The coordination number for an atom in the bcc structure is eight.

(5) PACKING EFFICIENCY AND COORDINATION NUMBER:

THREE DIMENSIONAL CLOSED PACKING (3D): 
(2) CUBIC CLOSE PACKING AND HEXAGONAL PACKING (CCP AND HCP)

Step-(1) In order to develop three dimensional close packing take a 2D hexagonal close packing sheet as  first layer (A- layer).

Step-(2) Another 2D hexagonal close sheet (B-layer) is taken and it is just over the depression (Pit) of the first layer (A) .When the second layer is placed in such a way that its spheres find place in the ‘b’ voids of the first layer, the ‘c’ voids will be left unoccupied. Since under this arrangement no sphere can be placed in them, (c voids), i.e. only half (50%) the triangular voids in the first layer are occupied by spheres of the second layer (i.e. either b or c)

Step-(3) There are two alternative ways in which spheres in the third layer can be arranged over the second layer

(1)  When a third layer is placed over the second layer in such a way that the spheres cover the tetrahedral or ‘a’ voids; a three dimensional closest packing is obtained where the spheres in every third or alternate layers are vertically aligned (i.e. the third layer is directly above the first, the fourth above the second layer and so on) calling the first layer A and second layer as layer B, the arrangement is called ABAB …………. pattern or hexagonal close packing (HCP) as it has hexagonal symmetry.   

(2)  When a third layer is placed over the second layer in such a way that spheres cover the octahedral or ‘c’ voids, a layer different from layers A and B is produced. Let it be layer ‘C’. Continuing further a packing is obtained where the spheres in every fourth layer will be vertically aligned to the spheres present in the first layer.  This pattern of stacking spheres is called ABCABC ……….. pattern or cubic close packing (CCP). It is similar to face centred cubic (fcc) packing as it has cubic symmetry

COORDINATION NUMBERS:

In both HCP and CCP methods of stacking, a sphere is in contact with 6 other spheres in its own

layer. It directly touches 3 spheres in the layer above and three spheres in the layer below. Thus

sphere has 12 close neighbours. The number of nearest neighbours in a packing is called coordination

number. In close packing arrangement (HCP & CCP) each sphere has a coordination number of 12.

(3) FACE CENTRED UNIT CELL (FCC/CCP):

CCP or FCC has two lattice point corner as well as face centred:

Suppose ‘r’ be the radius of sphere and ‘a’ be the edge length of the cube As there are 4 sphere in FCC unit cell

(1) Relation between radius (r) and side (a)

In FCC, the corner spheres are in touch with the face centred sphere. Therefore, face diagonal AD is equal to four times the radius of sphere AC = 4r

But from the right angled triangle ABC:

(2)  Effective no. of atoms per unit cell (Z):

(3) Volume of four spheres (atoms):

(4) Volume of unit cube:

(5) Packing efficiency (PE): Percentage of space occupied by sphere

(6) Percentage Voids: 100- PE= 26 %

(7) Density of FCC(CCP):

(8) Coordination Numbers:

(9) LOCATION OF VOIDS: FCC/CCP UNIT CELL:

(A) Tetrahedral voids: The FCC/CCP unit cell has eight tetrahedral voids per unit cell. Just below every corner of the unit cell, there is one tetrahedral void. As there are eight corners, there are eight tetrahedral voids.

(B) Octahedral voids: In an FCC/CCP unit cell, there are four octahedral voids. They are present at all the edge centres and at the body centre. The contribution of the edge centre is 1/4

Hence, total number of octahedral voids:

In CCP/FCC:

 Rank (Z) = 4,

 Number of tetrahedral voids = 8 and

 Number of tetrahedral voids = 2 × Z

Number of tetrahedral voids in close packing = 2 × eff. no. of spheres.

Hence, there are two Tetrahedral Voids per sphere in closed packing arrangements.

In CCP/FCC:

 Z = 4

Number of octahedral voids = 4 

Number of octahedral voids = Z

There is exactly one OV per sphere in close packing.

(4) HEXAGONAL CLOSE PACKING (HCP):

Step-(1) In order to develop three dimensional close packing take a 2D hexagonal close packing sheet as  first layer (A- layer).

Step-(2) Another 2D hexagonal close sheet (B-layer) is taken and it is just over the depression (Pit) of the first layer (A) .When the second layer is placed in such a way that its spheres find place in the ‘b’ voids of the first layer, the ‘c’ voids will be left unoccupied. Since under this arrangement no sphere can be placed in them, (c voids), i.e. only half (50%) the triangular voids in the first layer are occupied by spheres of the second layer (i.e. either b or c)

Step-(3) There are two alternative ways in which spheres in the third layer can be arranged over the second layer

(1)  When a third layer is placed over the second layer in such a way that the spheres cover the tetrahedral or ‘a’ voids; a three dimensional closest packing is obtained where the spheres in every third or alternate layers are vertically aligned (i.e. the third layer is directly above the first, the fourth above the second layer and so on) calling the first layer A and second layer as layer B, the arrangement is called ABAB …………. pattern or hexagonal close packing (HCP) as it has hexagonal symmetry.   

ANALYSIS OF HCP UNIT CELL:

(1) Number of effective atoms in HCP unit cell (Z):

Lattice point:  corner- total 12 carbon contribute 1/6 to the unit cell

Lattice point:   face- total face 2 contribute ½

Lattice point: body centre- total atom 3 (100% contribution)

(2) Radius of atom in HCP unit cell:

Let the edge of hexagonal base =( a) And the height of hexagon =( h) And radius of sphere =( r)

(3) Area of hexagon:

 Area of hexagonal can be divided into six equilateral triangles with side 2r

(4) Height of HCP unit cell:

(5)  Volume of hexagon = area of base x height

(6)  Volume of spheres:

(7) Packing efficiency: Percentage of space occupied by sphere.   

(8) Voids %: 100 - PE= 26 %

(9) Coordination Numbers:  12 (each spheres touches 6 spheres in its layer,3 above and 3 below).

SPINEL STRUCTURE :

(1) Structure of FeO, Fe₂O₃, and Fe₃O₄  :

(1) FeO: This oxide is non-stoichiometric and has a composition FexO (Generally ‘x’ varying from0.92 to 0.97). The oxide ions form a cubic close packing. The octahedral voids are occupied by Fe2+ but a small number of Fe²⁺ is replaced by Fe³⁺ ions. Three Fe²⁺will be replaced by two Fe³⁺ to maintain electrical neutrality but then what we obtain is an iron-deficient crystal.

(2) Fe₂O₃: If all Fe2+ are replaced by Fe3+, the ratio between Fe:O will be 2 : 3 (since 3 Fe²⁺ are replaced by 2Fe³⁺⁾ and not 1 : 1. As such, we obtain Fe₂O₃.

(3) Fe₃O₄: This is obtained by replacing exactly two thirds of Fe²⁺ by Fe³⁺ (in FeO structure).The remaining Fe²⁺ ions and 50% of Fe³⁺ ions occupy the octahedral voids. The remaining Fe³⁺ ions occupy tetrahedral voids. If in the structure of Fe₃O₄, the Fe²⁺ ions are replaced by divalent cations such as Mg^2+, Zn^2+, etc., the compounds obtained are called ferrites. In ferrites, divalent cations occupy tetrahedral voids and trivalent cations occupy octahedral voids. This structure is called spinel structure.

(2) NORMAL SPINEL (AB₂O₄ ) STRUCTURE:

Example of Spinel is a MgAl₂O₄.( mineral) In it oxide ions (O^-2) are arranged in ccp with Mg⁺² ions occupying tetrahedral voids and Al⁺³ ions in a set of octahedral voids.

Many ferrites (such as ZnFe₂O₄) also possess spinel structure. These are very important magnetic materials and are used in telephone and memory loops in computers.

(3) INVERSE SPINEL STRUCTURE (Fe₃O₄-Magnetite):

In Fe₃O₄, Fe⁺² and Fe⁺³ ions are present in the ratio 2:1. it may be considered as having composition FeO.Fe₂O₃. In Fe₃O₄ Oxide arranged in ccp. Fe⁺² ions occupy octahedral voids while Fe⁺³ ions are equally distributed between octahedral and tetrahedral voids

MgFe₂O₄ also has structure similar to magnetite. In this Mg⁺² ions are present in place of Fe⁺² ion in Fe₃O₄. Magnetite has inverse spinet structure.

ANTIFLUORITE STRUCTURE (REVERE OF FLUORITE) Na2O:

The compound having A₂B formula are compounds having anti fluorite structure :

Anti fluorite structure is having arrangement of cations and anions opposite to the fluorite structure Li₂O has an anti fluorite structure.

(1)  In the crystal structure of Li₂O, the O^-2 ions constitute a cubic close packed lattice (fcc structure) and the Li⁺ ions occupy all the tetrahedral voids

(2)  Each oxide ion, O^-2 ion is in contact with 8 Li⁺ ions and each Li⁺ ions having contact with 4 oxide ion. Therefore, Li₂O has 4:8 coordination
(3) Stoichiometric ratio  of Na₂O is 2:1
(4)  radius ratio , Packing efficiency, density and void %:

 Other Examples: – Na₂O, K₂O, K₂S, Na₂S, Rb₂O, Rb₂S

Note: Metals like Al, Ag, Au, Cu, Ni, and Pt have ccp structure and Be, Mg, Co and Zn have a hcp structure. And Noble gases (except He has a hcp structure) have ccp structure.

CALCIUM FLORIDE (FLORITE) STRUCTURE:

The salient features of fluorite structure are:

(1)  The Ca⁺² ions are arranged in ccp arrangement, i.e. these ions occupy all the corners and the centres of each face of the cube

(2)  The F^– ions occupy all the tetrahedral holes.

(3)  Since there are two tetrahedral holes for each Ca⁺² ion and F^- ions occupy all the tetrahedral holes, there will be two F^- ions for each Ca⁺² ions, thus the stoichiometry of the compound is 1:2.

(4) Each Ca⁺² ion is surrounded by 8F^- ions and each F^- ions is surrounded by 4Ca⁺² ions. The Coordination number of Ca⁺² ion is eight and that of F^- ion is four, this is called 8:4 Coordination.

 (5) Each unit cell has 4 calcium ions and 8 fluoride ions so formula of unit cell is Ca₄F₈ which is explained as  below

      No. of Ca⁺² ions = 8(at corners)´1/8 + 6 (at face centres)´1/2

      No. of F ions = 8 (within the body)´1 = 8

      Thus the number of CaF₂ units per unit cell is 4.

(6) Radius Ratio, packing efficiency ,void % and density:

Other examples: of structure are SrF₂, BaCl₂, BaF₂, PbF₂, CdF₂, HgF₂, CuF₂, SrCl₂, etc.