Melting and polymorphism of Zn 3 As 2 and Cd 3 As 2 at high pressures

The melting and solid-solid transitions in Zn 3 As 2 and Cd 3 As 2 have been investigated at pressures up to about 40 kb by means of differential thermal analysis. For Zn 3 As 2 , the melting point decreases from about 1015°C at zero pressure to 945°C near 23 kb; at higher pressures, the melting temperature remains nearly constant. For Cd 3 As 2 , the melting point decreases from 697 ± 3°C at zero pressure to 640–650°C near 17 kb, above which it increases only very slightly with pressure. In both cases, the break in the melting slope presumably indicates a solid-solid-liquid triple point. The temperature of the solid-solid transition, occurring near ~656°C in Zn 3 As 2 and near ~578°C in Cd3As2 at zero pressure, decreases to 646°C near 32 kb and to ~520°C near 18 kb, respectively. Thermal arrests could not be observed at higher pressures; these points (646°C near 32 kb for Zn 3 As 2 ; ~520°C near 18 kb for Cd 3 As 2 ) are interpreted as solid-solid-solid triple points. A reversible transition, with a small decrease in resistance upon compression, was observed in Cd 3 As 2 near 24 kb at room temperature. The postulated high pressure phases could not be retained on release of pressure at room temperature. In an effort to anticipate the structures of the unidentified Zn 3 As 2 and Cd 3 As 2 polymorphs, attempts are made to correlate data for some of the A3B2 compounds.