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Composition

Structure

Properties

Performance

Design and construction

Solid Materials Package of Properties

Properties f(composition)

Properties of Materials

Choice of composition + Construction

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• Solid properties = f(Composition)

• Solid properties = f(Atomic arrangement)

Ex: diamond vs. graphite

Composition alone can’t give the properties of a material, they are dependent on the atomic arrangement.

3 pictures from wikipedia

Solid State Chemistry

• Electronic structure of the elements holds the key to the understanding of the long range atomic order in solids;

• Electronic structure of the atomic constituents and symmetry arguments are the criteria for the material selection process

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1. What atoms are involved and their electronic configuration?

2. What types of chemical bonds are formed?

3. How are the atoms arranged in the crystal structure?

4. What is the symmetry of the crystal?

5. Do these arrangements promote certain mechanisms for

electronic or atomic motions?

6. How do these mechanisms give rise to the observed properties?

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Identifying solid materials by use of

Atomic radii

Chemical bond strengths

Anisotropic atomic groupings

Symmetry arguments

Electronic band structure

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Classification of solids based on bonding type

1. Ionic solids (e- transfer)

2. Covalent solids (e- sharing)

3. Metals (delocalized e-)

4. Molecular (Hydrogen bonds or Van der Waals forces)

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1. Ionic Solids Electrostatic attractions between oppositely charged ions which is the same in all directions i.e. NaCl

Ionic interactions are omni-directional and non-saturated

The resulting low energy configuration: An ordered 3-dimensional network, a "crystalline" solid.

Na + Cl [Na]+ + [Cl]-

1e-

1. Ionic solids

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because of the repulsive forces in the cleavage plane ionic crystals are hard and brittle

Have high degree of stability due to achievement of valence shell octets through bonding Non directional forces in such solids will result in highly ordered macroscopic bodies

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(1) Non–directionality of the electrostatic forces leads to an ordered solid (crystalline).

(2) Octet stabilization makes the solid non–conducting (electrical insulator). (3) Stability will also most likely make the material transparent (high Eg) in the visible. (4) The magnitude of the attractive forces makes the solid melt at elevated temperature only. (NaCl-801ᵒC, MgO-2800ᵒC, CaF2-1418ᵒC)

Basic properties of ionic solids

2. Covalent Solids The electrons are shared between two interacting atoms in a molecular

orbital (electron pairs);

Electron-pair bonds are positioned to maximize the orbital overlap;

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sp3carbon in diamond sp2 carbon in graphite

Bond directionality because the atomic and hybrid orbitals are quite definite and point in fixed directions

Ex:

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The directionality of the bond makes the properties to vary widely depending on the arrangement of the bonds

sp3 hybridization of carbon Cubic structure The electrons are very tightly bound very strong bonds •Hard ( used as cutting tool) •Insulator (Eg=5.4eV) •Transparent to visible light •High refractive index

sp2 hybridization of carbon Hexagonal structure Strong covalent bonds in plane weak delocalized bonds normal to the planes the planes can slide over one another •Lubricity ( used as lubricant) •Decent electronic conduction •Absorbs the photons and appears black

diamond graphite

Properties of Covalent solids

Diamond is the metastable form of carbon. The stable form is graphite!

Memorial diamonds from carbonized human remains by companies such as GemLife (US), Algordanza (CH), HeartIn diamond (Ru)

Diamond can be synthesized from graphite at high pressure

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Diamond vs. Silicon

Graphite - conductor Boron Nitride – insulator In BN – two e- are only on N; the N atoms are separated by an B atom in the plane poor orbital overlap and therefore insulator In graphite – one unpaired e- is on each C atom good orbital overlap and therefore good conductor

Graphite vs. Boron Nitride

Diamond – insulator Silicon – semiconductor (Band gap, Eg = 1.1eV) In Silicon, the Si atom is larger and the bond is longer (dSi-Si=2.35Å vs. dC-C=1.54Å; weaker bonds the e- are more easily liberated) smaller band gap and therefore semiconductor

dB-N=1.45Å dC-C=1.42Å 14

3. Metals

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ion cores

gas of free electrons

In metallic bond, the atoms loose their outer electrons to a common electron band that runs throughout the solid (e- delocalization).

We may therefore visualize metals as a lattice of ion cores being held together by a gas of free electrons.

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• Displacement of neighboring planes does not lead to charge effects and therefore malleability and ductility

•High conductivity because the valence electrons never remain near any particular atom very long

Properties of Metals

•The atoms pack together like spheres and the bonding is non-directional

Planes slip is a common phenomenon in metals

4. Molecular solids

solid He(1-1.5K and 2.5MPa)

+ - + -

Atomic or molecular dipoles

1. Van der Waals or secondary bonding results from the coulombic attraction between induced dipoles

Van der Waals solids have extremely low melting points

2. Hydrogen bonding – a special type of secondary bonding exists between some molecules that have hydrogen as one of the constituents

ice

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Directional vs. non-directional bonds

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In solids with directional bonds, the atoms cannot pack together in a dense manner, yielding to a low mass density [ (g/cm3)]

density of covalently solids < than metallic or ionic ones;

•Ionic solids: NaCl (2.6 ) •Metals*: K (0.86); Ca (1.55); Sc (2.98); Ti (4.5) •Covalent solids: Diamond (3.53); graphite (2.23); SiC (3.21 )

Density of solids - (g/cm3):

* The fewer electrons lost to get full outer shells, the lower density

Classification of solids based on atomic arrangements:

Classes Ordered Disordered

Atomic arrangement Regular Random*

Ordering type Long range Short range

Name Crystalline “crystal”

Amorphous “glass”

*Not totally random

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•Short-range order (SRO) - The regular and predictable arrangement of the atoms over a short distance - usually one or two atom spacings.

LRO vs. SRO

•Long-range order (LRO) - A regular repetitive arrangement of atoms in a solid which extends over a very large distance.

Eordered < Eirregular

crystalline form is favored

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Transparent crystal vs. Opaque glass

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Obsidian (volcanic glass: 70–75% SiO2, plus MgO, Fe3O4)

pictures form Wikipedia

Diamond (Eg=5.4eV)

Transparency: Eg>>3eV; Transparency is not a property linked to the long range atomic ordering of a solid; it is a function of the band gap of the solid;

Solids classification summary

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A. Based on bond type

1. Ionic solids 2. Covalent solids

3. Metals 4. Molecular solids

B. Based on atomic arrangements I. Ordered solids = Crystalline II. Disordered solids = Amorphous

Crystalline solids

• have sharp melting points

• are anisotropic by nature

Particles vibrate about their equilibrium positions

• have characteristic geometric shapes

diamond NaCl

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Crystalline forms

•Polycrystalline = the regularity or periodicity is interrupted at the grain boundaries*

•Single crystal = the regularity of the pattern extends throughout a certain piece of solid

When cut or hammered gently it shows a clean fracture along a smooth surface.

Silicon crystal

*grain boundary = where different aligned grains meet with each other

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• Single Crystals are anisotropic = properties vary with direction

• Polycrystals are isotropic = properties may/may not vary with direction

200 mm

Single vs. Polycrystals

the degree of anisotropy increases with decreasing structural symmetry

E (diagonal) = 273 GPa

E (edge) = 125 GPa

Epoly Fe = 210 GPa

the measured property represents some average of the directional values

Fe

25 From Callister 5e

Polycrystals

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• Each "grain” is a single crystal. • crystals are randomly oriented, overall component properties are not directional. • Crystal sizes varies (from 1 nm to 2 cm)

Most engineering materials are polycrystals.

Different color stands for different grain orientation

Crystals forms

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• Isomorphism = crystals of different composition with the same form

• Polymorphism = different crystal forms with the same chemical composition; allotropes

halite (NaCl)

fluorite (CaF2) sylvite (KCl)

Ex:

Ex:

)()()(

16631183..BCCFCCBCC FeKFeKFetr

)()(..

cubicdiamanthexagonalgraphite CpressurehighCtr

Next

• Introduction to Crystallography

• Bravais lattices

• Unit cell

• Crystal structure

• Characteristic of cubic systems

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