1. H. Poulet and J.-P. Mathieu, Vibration Spectra and Crystal’s Symmetry (Gordon and Breach, Paris, 1970).
2. A. E. Chmel’, A. N. Smirnov, and V. S. Shashkin, Fiz. Tved. Tela (St. Petersburg) 43(5), 804 (2001) [Phys. Solid State 43 (5), 836 (2001)].
3. H. E. Jackson and C. T. Walker, Phys. Rev. B: Solid State 3, 1428 (1971).
4. C. C. Ackerman, B. Bertman, H. A. Fairbank, and R. A. Guer, Phys. Rev. Lett. 16, 789 (1966).
5. Y. R. Shen, The Principles of Nonlinear Optics (Willey, New York, 1984).