By the use of the electron beam plasma technique, it has been found that the solid solutions of Ga1−xInxN can be synthesized over the entire composition region. From the optical measurements, the direct energy gap at 78 °K was determined to be 3.46 eV for GaN and 2.11 eV for InN. Also its composition dependence was found to deviate downward from linearity. From the infrared reflectivity measurement and the resultant K-K dispersion analysis, the transverse optical frequencies for long-wavelength phonons of GaN and InN were 563 and 478 cm−1, respectively. The optical phonons in this quasibinary system were concluded to exhibit a one-mode–type behavior. The Brout sum rule was discussed for a large number of the diatomic crystals of ANB8−N type and its relation with respect to the reduced mass was derived as Σiω2i(k=0) =A μ−1.5. According to this relation, the longitudinal optical frequency of InN was deduced to be 694 cm−1. On the other hand, from the result of the annealing treatment for the solid-solution alloys and the theoretical consideration, it was pointed out that this quasibinary system has a solid-phase misibility gap in chemical equilibrium.

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Relationships between displacive phase transitions and soft modes are discussed and illustrated using experimental examples. Ferroelectric and ferroelastic transitions are dealed with together with the "band-Jahn-Teller" and antiferromagnetic transitions which are closely connected with acoustic soft modes. Since an acoustic mode softening has been observed in the vicinity of the martensitic transformation temperatures in ß-phase alloys, it is of importance to discuss the close relation between the nucleation mechanism of martensite and the crystal anharmonicity near the Ms temperature

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