3.18 Crystals with Several Atoms per Lattice Point

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Del curso dictado por Georgia Institute of Technology

Material Behavior

282 calificaciones

Georgia Institute of Technology

Material Behavior

282 calificaciones

Have you ever wondered why ceramics are hard and brittle while metals tend to be ductile? Why some materials conduct heat or electricity while others are insulators? Why adding just a small amount of carbon to iron results in an alloy that is so much stronger than the base metal? In this course, you will learn how a material’s properties are determined by the microstructure of the material, which is in turn determined by composition and the processing that the material has undergone. This is the first of three Coursera courses that mirror the Introduction to Materials Science class that is taken by most engineering undergrads at Georgia Tech. The aim of the course is to help students better understand the engineering materials that are used in the world around them. This first section covers the fundamentals of materials science including atomic structure and bonding, crystal structure, atomic and microscopic defects, and noncrystalline materials such as glasses, rubbers, and polymers.

De la lección

Crystalline Structure [Level of Difficulty: Medium || Student Effort: 2hrs 30mins]

This module covers how atoms are arranged in crystalline materials. Many of the materials that we deal with on a daily basis are crystalline, meaning that they are made up of a regularly repeating array of atoms. The "building block" of a crystal, which is called the Bravais lattice, dtermines some of the physical properties of a material. An understanding of these crystallographic principles will be vital to discussions of defects and diffusion, which are covered in the next module.

3.16 Crystals with 2 Atoms per Lattice Point6:24

3.17 Crystals with 2 Ions or 2 Different Atoms per Lattice Point7:02

3.18 Crystals with Several Atoms per Lattice Point9:42

3.19 Polycrystalline Materials and Liquid Crystals10:20

3.20 X-Ray Diffraction and Crystal Structure14:26

Conoce a los instructores

Thomas H. Sanders, Jr.
Regents' Professor
School of Materials Science and Engineering

In this lesson, we're going to be talking about more complicated structures where

rather than just having one or two, we now have several atoms per lattice point.

So we can begin with a structure that is of the type Mx2.

And when we look at a structure like this Mx2 what I have done is to illustrate one

of the ions as being blue and the other two as being black filled in circles.

And I've coupled them with dotted lines to indicate how those structures

are coupled together or how those ions are coupled together in the structure.

So when we look at this, what we're seeing here is again a face centered cubic

lattice and this time it has a total of three atoms per lattice point so

that gives us then a total of 12 ions in the unit cell.

And because of the stoichiometry,

what we have to have is two of the X type ions and

one of the M type so that the charge is balanced out in the unit cell.

We can look at the structure of a material which is based upon the Silica

tetrahedron, where we have silicons that sits in the center of the tetrahedron and

is surrounded by a total of four oxygen ions.

And one of the structures that can develop as a consequence of this tetrahedral

arrangement of silicon and oxygen is one of the forms of silica,

which is referred to a crystobalite.

There's several different crystal structures that are associated with SiO2

but this happens to be one of them and this one in particular

is a variation again on the face centered cubic structure.

So let's examine this in a bit more detail.

In terms of the FCC structure,

we're going to see that they're a total of six atoms per lattice point.

As we go through this, because we're dealing with the face centered cubic

structure, and we have six of the atoms associated with each lattice point,

we therefore must have a total of 24 of these atoms in the unit cell.

So let's take a look at this.

So here is our structure, and what I'm going to do is to identify the number

of the various types of species that are inside of the structure.

So the first thing we're looking at are all those indicated by

the number 1 and that 1 indicates the total number of silicon

atoms that are at those eight corner points.

Remember each one of those corner points has a total of eight unit cells around it,

so since we have eight corners, we there for

have one silicon atom that is associated with a corner.

Now what we're going to do is we're going to take a look at

the silicon atoms that occupy the face centering positions.

So, when we look at those,

those are designated as 3s, meaning we have six faces.

Each one of the faces shares with an adjacent above or below,

in front or back and since we have six faces in every share we

have then a total of three silicon atoms that are associated with those faces.

Now we still have some silicons that are not labelled.

And we're going to go through and label those, and those there indicated as four.

And in this particular case when we look at the four, we see that there are a total

of four of those silicons that lie wholly inside of the unit cell.

And therefore, we must have a total of four silicon atoms that are inside and

associated with those particular atoms in space.

Now we're going to sum up all of the atoms that appear in the unit cell

in terms of the structure of silicon dioxide.

Remember that we had a total of 8 silicon atoms and because the structure is

one to two, we therefore must have a total of 16 oxygen atoms per unit cell.

And as a consequence, we have a total of 24 atoms that

are in this unit cell of crystobalite, SiO2.

Let's look at another structure.

This happens to be once again a simple cubic structure.

And when you look at the structure, and

what I'll do is indicate what those various points mean

by recognizing that I have assigned the center position

as titanium, and I've put calciums on the corners.

And then what I've done, is to put the oxygens on the faces of the unit cell.

Now it's clear that the corner positions are different than the face positions,

which are different than the center position

which means this structure must be simple cubic.

And it turns out, it's referred to as calcium titanate,

and it has the stoichiometry of one titanium, one calcium and three oxygens.

And that is consistent with the picture that is up on the screen and

this particular structure, because of the importance and

the number of different compounds that crystallize in this form,

it's referred to as the Perovskite Structure.

What we can do is we can look at an alternative Perovskite structure of

titanium, calcium, oxygen material.

And this time what we're going to do is reposition the atoms.

So the atoms now are located at the titanium at the corners.

And when we start looking at the calcium, it lies in the center.

And those two, then, are just simply interchanged and consequently,

we maintain the one to one relationship between titanium and calcium.

All right, now let's take a look at the oxygens.

Because of our new origin, the oxygens are not located on the faces,

but now they're located on the edges.

So let's do a little bit of counting here.

It turns out that we're dealing with a queue,

which means that we're going to have a total of 12 edges.

When we look at each one of those edges wherein oxygen is occupied.

Then what we find is,

each one of those oxygens is contributing a total of one quarter.

So hence, when we sum up all the edges and the one quarter contribution,

we'll get a total then of 12 divided by three or

four which is ultimately going to give us three.

So now we go back and we see the structure of calcium titanate.

Again, one titanium, one calcium and three oxygen.

So everything is consistent here.

If we were to go down the column of the periodic chart and replace calcium with

barium, we would come up with a structure barium titanate.

When we look at the structure of barium titanate,

it's similar to what we saw with calcium titanate with the exception that

the titanium is now displaced in the center of the unit cell.

As a result of that displacement, what we see is on the diagram to the right,

we see that the titanium has been moved up relative to the oxygen.

And then what we find is as a consequence of that charge displacement,

we develop a dipole that's associated with the structure now.

So we have a dipole.

And what does that mean?

Well, what happens in this particular case.

If we were to take this unit so and compress it along the C direction or

the Z direction, what we could do is to move the titanium back into

the center of the cell and the dipole then winds up the superion.

Well it turns out that there is a very fascinating material that can be

developed as a consequence of utilizing this structure, and that is we can produce

a material which can take electrical energy and mechanical energy,

or mechanical and electrical, and go back and forth between those two.

And this then becomes the basis for

something that we refer to as a transducer.

The last structure that I want to introduce in this lesson

is that associated with a crystalline polyethylene.

And remember we can begin to think about these polymer chains as spaghetti.

And what we've done in this particular case is to align each one of those

change in such a way that we produce this unit cell that happens to be orthogonal.

So the a, b and c have different dimensions but

again all of those interaxial angles are 90 degrees.

So you can begin to see as we move away from some of these simple structures and

we come in to these more complicated structures that we can still begin to

analyze the individual crystal structures of the various materials.

It just takes a lot more detail in order to do that.

Thank you.

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