Why Is Longitudinal Lattice Ignored When Proving Debye Frequency in Crystals?

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in a crystal, when proving the debye frequency, why is the existence of longitudinal lattice ignored? why is there longitudinal lattice in the first place?
 
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Can you provide a reference? I don't recall "proving the debye freq", actually I'm not sure what that statement means.
 
i think it's in "physical metallogy principles" by reed
 
What is a longitudinal lattice ?

Are you saying that Debye's calculation ignores longitudinal modes ? That is not true.

i think it's in "physical metallogy principles" by reed
That's not enough. You must provide the exact quote from Reed-Hill.
 
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