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[8] (a) Consider a cubic crystal with lattice constant a = 3 A. A phonon with wave- length 7 À travelling along [101] collides with a phonon of the same wave- length travelling along (100). Draw a reciprocal space diagram to show the magnitude and direction of the resultant phonon within reduced zone scheme, ensuring you clearly state the relevant numerical values. (b) Consider now a tetragonal crystal with lattice constants a = b = 2 A and C = 3 A. A phonon with wavelength 7 À travelling along [101] collides with a phonon of the same wavelength travelling along (100). Draw a recipro- cal space diagram to show the magnitude and direction of the resultant phonon within reduced zone scheme, ensuring you clearly state the rele- vant numerical values. (c) Explain how the scattering events above may or may not affect the thermal conductivity of a solid. [7] [2]