Aluminium nitride with the wurtzite structure, w-A1N, has attracted special attention as an alternative ceramic substrate of high power applications in microcircuits.1 The conspicuous advantages are high thermal conductivity greater than 100 W/mK2> and with a low coefficient of thermal expansion which, in the temperature range of interest (20-200°C), closely matches that of silicon. It also has a high electrical resistivity, a moderate dielectric loss, and good resistance to thermal shock and thermal decomposition. On the analogy of other III-V compounds with the wurtzite structure, A1N is expected to assume the cubic structure at high pressures. Van Vechten3~ predicted that, at 90 GPa and room temperature, A1N would transform to a metallic phase with the a-tin structure. Kieffer et a1.41 reported a new phase in the system AlN-ALO:, which they claimed to be cubic A1N. The new phase, however, was proved later to be AION, occurring in hot pressing experiments in the system. ~~ On the basis of the shock wave experiments on A1N, Kondo et al.°~ suggested a transformation to a denser phase at about 21 GPa. Despite of more general searches,7~-9 no definite evidence has been obtained for the existence of the new dense phase of A1N. It is the purpose of the present paper to report the successful synthesis of the new cubic phase of A1N at high pressures and temperatures. The fine powder of w-A1N (grain size, <1.0 µm; purity, >99%) kindly provided by Tokuyama Soda Co. Ltd. was used as the starting material. High pressure and high temperature experiments were carried out using a uniaxial split sphere apparatus driven by 1000 ton press (USSA-1000) at the Institute for Study of the Earth's Interior. The assembly of tungsten carbide cubes with 5 mm truncation was used with semisintered magnesia octahedron of 10 mm edge length as pressure medium. The A1N starting material was charged into a cylindrical tantalum heater embedded in the octahedron. The detailed sample assembly was similar to those described elsewhere.1o1 The sample temperature was monitored by the Pt/Pt13 % Rh thermocouple. The pressure values were calibrated at room temperature by the conventional fixed points of Bi III-V (7.7 GPa), ZnS metallic transition (15.6±0.6 GPa) and GaAs metallic transition (18.2±0.7 GPa). The experimental conditions were varied in the ranges of 11.5-16.5 GPa and

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Thermodynamic considerations suggest the Curie temperature can be represented by means of differential thermal analysis. For this purpose, the heat capacity is used as parameter for the Curie-point range; the basic theoretical connections for this are reported.In passing, an account is given of some experimental results as well as of tests made to use magnetic substances with certain characteristic features instead of the usual inert ones.

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