# How do you test if a crystal is anisotropic?

Discussion in 'Physics' started by physicsstudent1, Apr 12, 2011.

1. ### physicsstudent1 Thread Starter New Member

Apr 11, 2011
7
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Hi

So I have been searching the Internet for a long time trying to find any experiments or information about testing to determine whether a particular crystal is anisotropic. This is one of a few physics assignments I have which I am having difficulty writing up. I would be gratified if anyone could help.

Some info on the assignment:
The physical properties of some crystals are different when measured in different directions in the crystal. Such a crystal is said to be anisotropic. In the case where resistivity is the physical property under investigation, how does one design an experiment to test whether a particular crystal is anisotropic (assuming that the small crystal is in the shape of a rectangular block, approximately 2mm in length)?

I must include a list of the measurements and how they would be made and an explanation of how these measurements would be used in any calculations that would be performed (along with a procedure to be followed).

Finally, in the case where the resistivity of a particular crystal is large (and thus difficult to measure), what is one design feature of the experiment that would enable one to overcome this problem. Just to be clear, I don't have to actually conduct this experiment - since the experiment is hypothetical - so I just need to know how one would carry out this experiment.

Thank you for any help!!!

Darien

Jul 7, 2009
1,585
141
You might search the web for "resistivity measurements". Semiconductor fabs do it all the time. Hint: think "Kelvin". Also go to the basic physical definition of resistivity and see if that doesn't cause the creative juices to flow.

3. ### mjhilger Member

Feb 28, 2011
119
16
If you can mentally visualize some of the different crystal geometries and think about how they stack to form the bonds for the material, you might see something reveal itself. With that picture, and the information Someonesdad provided, you should begin to see the path(s) of conduction of various orientations. From there you should be able to notice some measurements that might be handy to differentiate the paths.

4. ### physicsstudent1 Thread Starter New Member

Apr 11, 2011
7
0
thank you so much.

Last edited: Apr 13, 2011
5. ### mcike New Member

Apr 17, 2011
12
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Measuring electrical resistivity on single crystaline samples works exactly the same way as for isotropic (fe polycrystaline) samples. In principle you would have to contact your sample with standard 4 point technique (this helps to prevent errors concerning contact resistance and voltage drops in wires). the 2 outer wires are used for impressing a current ((AC or DC), the 2 inner wires are used for measuring the voltage drop between those contacts. Ohm's law tells you the resistance of your sample. Taking into account the geometry of your sample, you can calculate the resistivity.

for measuring anisotropy, you would have to cut several samples (with different orientation) out of your crystal and conduct resistivity measurements for each one.

If your sample has very high resistance, leakage currents in the voltmeter cause errors. Maybe a bridge circuit could be useful tool...

A few weeks ago, i saw a nice e-seminar about this issue. I think i found it on keithley homepage. Maybe you can find it again.
http://www.keithley.com/re/cel

here's one example, how results of such measurements could look like:
http://hal.archives-ouvertes.fr/docs/00/24/66/18/PDF/ajp-jp1v2p1257.pdf

greets
matthias