I'm reading the review on phonon and Raman scattering in 2D transition metal dichalcogenides (2D TMDCs). At the beginning of Section 2.1 it is said that, since there are two X-M-X units in the crystal's unit cell with a total six atoms, then there are 18 phonon modes. What is the logic behind the counting how many phonons there are?

Seems it is not specific for these particular materials, but some general ideology behind.

  • $\begingroup$ Number of atoms times number of dimensions $\endgroup$ – LLlAMnYP May 28 '16 at 5:27
  • $\begingroup$ Although one should take out the rigid translations of the solid. $\endgroup$ – Jon Custer May 28 '16 at 14:34

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