# Does amorphous solids support phonons or nomal modes of vibration.?

1. ### ns_phonon

34
Does amorphous solids support phonons or nomal modes of vibration.??

Can we apply the concept of phonons and Debye Model in case of amorphous and poly-crystalline solids??

http://en.wikipedia.org/wiki/Phonon

http://en.wikipedia.org/wiki/Debye_model

The links I have attached above discusses only about quantization of normal vibrational modes in case of monocrystals. But, in real world most objects which we see around seems to be appearing polycrystalline(i.e. with grains and grain boundaries) or amorphous.

So,how can we apply highly simplified concepts of phonons and debye model in case of such non-crystalline objects...???

"Does an amorphous solid also has normal modes of vibrations of atoms/molecules just like crystals do have...???"

Please answer me as simple as possible as I am not an expert in this field.This question is not for homework,the question was born just out of curiosity...

I have already searched about this matter on google but I did not find appropriate material,rather the material which I explored contained too high level mathematics which is out of my mind's regime.

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From your 1st reference: a "phonon is a collective excitation in a periodic, elastic arrangement of atoms or molecules in condensed matter".
Amorphous and poly-crystalline solids are not periodic.
The mathematical model does not apply.

3. ### ns_phonon

34
So, how can we describe the vibrational motion of atoms/molecules in case of amorphous solids like glass???? Also, we often say glass has resonances in ultraviolet and infrared regions of electromagnetic system.This implies that amorphous solids like glass also has quantized normal vibrational modes.

So, there must be some other model for calculating heat capacity or phonon contribution to heat capacity for amorphous solids. Am I right...????

So, according to my understanding, amorphous solids do possess quantised vibrational modes,otherwise we can't say solids like glass has resonances in infrared. The word resonance itself describes the existence of normal modes in amorphous solids like glass....

Only the debye model we described for crystalline solids must be modified for amorphous and poly-crystalline solids to find resonance modes in such materials.

Right....?????

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5. ### DrDu

4,247
Of course amorphous systems have vibrational modes like any other many particle system. The only difference is that they can't be calculated as easily as in periodical solids using a Bloch ansatz. The Debye model is certainly applicable to calculate the heat capacity as heat capacity depends mainly on accustical modes with wavelengths much larger than the atomic structure of the solid which will therefore not depend on the microscopic lattice being periodic or not but can be calculated in terms of elastic moduli and density.