By means of X-ray scattering and ultrasonic experiments, the structure of different melts of the Cd-Ga system was investigated. The structure factors S (q) obtained for the pure elements agree well with those reported by other authors. The concentration dependence of the structure factors shows no special features. From the structure factors, the total pair-distribution functions were calculated by a Fourier transformation. The nearest-neighbour distances, obtained from the total pair-distribution functions, amount to 3.04 Å for pure Cd and 2.83 Å for pure Ga and are nearly constant for all alloys. The concentration dependence of the experimental nearest-neigh-bour distances and coordination numbers was determined and is compared with statistical distribution and macroscopic segregation models. These comparisons tend to indicate a preference for like-atom neighbours. From the long-wavelength limit S(0) of the structure factors of Cd and Ga, the isothermal compressibilities were calculated. These show a satisfactory agreement with those calculated from adiabatic sound velocity data. The anomalous ultrasonic absorption of a single melt with nearly critical concentration indicates the existence of concentration fluctuations. This result is in agreement with those obtained by X-ray smallangle scattering experiments which are described in Part 2 of this work [1].

X-ray small-angle scattering experiments were performed on nine melts of the Cd-Ga system at different temperatures up to 440°C. Evaluation of the data follows the Ornstein-Zernike theory of critical scattering, thus yielding correlation lengths £ of concentration fluctuations and the long-wavelength limit Sec (0) of the Bhatia-Thornton structure factor. Studies of the concentration and temperature dependence of £ and Sec (0) indicate that the critical point occurs at cc = 50.0 ± 1-0 at % Ga and Tc -295.2 ± 0-1° C. For a melt with the critical concentration, Sec (0) increases up to 3500 times the ideal $cc (0)=CACB-This indicates a strong segregation tendency. In the vicinity of the critical point of the Cd-Ga system, experimental correlation lengths | > 100 A were obtained. The critical-point exponents v and y were determined. It follows that the behaviour of a critical Cd-Ga melt satisfies the prediction of the classical mean-field theory for higher temperatures, whereas, within experimental accuracy, the lattice-gas predictions are satisfied upon approaching the critical temperature.