Crystal Structures

In my previous article, I wrote about an introduction to material science. I believe that you understood my previous article clearly. But I could not touch on Crystal Structures in that article. Now I’m going to try explaining Crystal Structures in Material Science.

            First, I want to define what a crystal means. Let’s study..

            If a short define is necessary, we can say it is a repeating sequence which is appearing between solid’s atoms. In my opinion, it is a true but not enough defining. I want to tell my model about Crystal Structures.

            Imagine a group which is consisting of 1000 students. Probably, there are little groups in that big group that consisted by close friends.  For an example, a class which consisted by 20 students, it means there are 50 classes in our big group. As you know, almost in every class there are a lot of little groups which consisted of 3, 4 or 5 students are close friends. At the outset, we have a 1000 people group, now it is current still but we have a little problem that is disorder due to existence of a lot of little groups. We have to capacitate a coach who would organize them. By the way, that coach has a whistle. The first and the only condition is students must obey that whistle. When he blows his whistle, students must line up in a repeating sequence what their coach wants.

Did you hear it? Whistle is blown. Students lined up as a square after whistle was blown. What a perfect arrangement. After coach’s gone, they disrupt their perfect sequence and they lined up again but this time only our little close-friends groups among of their own. So it means that, perfect crystal has been disrupted. A new sequence existed now which is not long-term, only in short sequences in little close-friends groups.

When an observation made from above of those for each situation, two different shapes observe. First of them, a sequenced perfect arrangement square when the coach is on there, second of them, that group is in disarray, situation is distorted, when the coach releases them free.

At the outset that big square shaped students which we called it “crystallized” but after coach’s gone, there was a disorder. Now, they had a short-distance sequence according to the first. And we called that disarray “amorphous” in Material Science. It must be remembered, this situation is not an issue. This is just a different orientation due to atom’s natures. For an example, while metals are in crystalline forms but glass is an amorphous solid.

Solids are like that, too.  If you can control pressure and temperature correctly, you can obtain a crystal structure if that material’s nature permits. Some materials can never crystallized by themselves. You have to control environmental conditions such as pressure, temperature, humidity etc.. If you can’t control those variables otherwise solid materials can’t be crystallized.

            Crystalline materials have a sequence between their atoms or molecules. They can be metals, ceramics and some polymers but except glasses. Because of glassy materials cannot be crystallized by themselves due to they do not have long-distance sequence according to other crystalline materials.

            Types of Crystal Structures

            Discovery of Crystallite is based with mine and minerals sciences. In 1832, metals and their hydrate complexes have been discovered by an English mineralogist who was W.H. Miller has found those crystalline materials have a sequence and a selective orientation. While Miller was investigating them, he realized that they have repeating units in a sequence and he called those smallest repeating units were “unit cell”. He put forth that, if those unit cells repeats their selves in a 3-dimensional sequence, they form a solid structure. Also those unit cells have difference characteristics each one which could be separated from the others. According to Material Scientists, that is the most important method while identifying a solid material as a specific fingerprint of that material.

            In 1850, Auguste Bravais who has been inspired by Miller has published his new article which was about “7 Different Crystal Structures of Solid Materials” he defined 7 different specific lattices and combinations of their orientations at the total 14 different lattices. Also he put forth those crystal structures as a fingerprint of minerals and solid materials which they are different from materials to materials.

Those different crystal structures are below.

(Source: Wikipedia)

I can hear you’re asking “instead of might be 243 unit cells which are combinations of seven, why only existing 14 Bravais Lattices?

I’m going to explain this situation now why it can not be like that. In nature, only 14 Bravais Lattices exist as we know. Because face-centered, body-centered and base-centered Bravais Lattices can not be existed in hexagonal, rhombohedra and triclinic micro structures. If it can be like that, those structures repeat the other base microstructures. Due to we accept the smallest lattice as a unit cell, there are no more lattices in the nature except of 14 different Bravais lattices. 

With today’s technology, their characterization is too easy with using XRD. X-Ray Diffractometer can determine those different crystal structures clearly. So we can separate them which is different from the others.

           There are amorphous solids in nature except of crystals. Glass and glassy materials do not have any selective orientated grains. Their grains and atoms align randomize. Those amorphous materials best example is silica which is base materials of glasses. This is the point why glasses are transparent. Due to they have amorphous micro-structure, their grains reflect the light randomize so they can be seen transparent. I give their structural properties below.

Amorphous Silica- (SiO2)

            Also some special lattices are existing in nature. For an example, diamond, zinc blend, rutile etc.. I give those crystal structures at below.

I want to touch on a new material group which are called “piezoelectric materials”and used in sensor systems. They can generate electricity when they are below stress or vibration. How can they do that?

Their crystal structure can yield by elastically a little at that exact moment, a little voltage is induced and we can collect that little voltage. Their special micro-structure is “Perovskite”.

Diamond Shape

Zinc Blend Structure

Rutile (TiO2)

Perovskite (BaTiO3)

Stress-Voltage Relationship of
           Piezoelectric Materials

I tried to put on words that, there are two type solid materials. One of thing is “Crystallines” and the other one is “Amorphous”. I’m thinking that my modal which I touched on “group”, it may be benefit in terms of understanding crystalline materials. All my studies and models belong to me. I’ll write about “Crystal defects” in my next article. Keep in follow me please.