I find the expression: "In general, one can expand the atomic displacements in terms of all the lattice wave modes (resembles a Fourier series expansion)" at: https://courses.cit.cornell.edu/ece407/Lectures/handout17.pdf But I have not found the expression in any other literature. (In fact, some literature, like P.Y. Yu and M. Cardona: Fundamentals of Semiconductors, a long wavelength limitation seems to have been imposed.) Is the above-cited expression correct (in 3D lattice)? Thank for your assistance.