THE P,T-STATE DIAGRAM AND SOLID-PHASE SYNTHESIS OR ALUMINUM-HYDRIDE

The P,T-diagram for aluminium hydride obtained at pressures 0.5-6.5 GP a and temperatures 100-700 degrees C indicates that the decomposition of AlH3 is reversed at high pressures and the compound can be prepared from the elements at pressures above 2.5 Gpa. However, synthesis of A lH3 via the solid-state exchange reactions of LiAlH4 or LiH with AlCl3 under quasi-hydrostatic conditions is more advantageous. The syntheti c P,T-conditions are limited by two straight lines that correspond to the decomposition of AlH3 into elements and the phase transition alpha -LiAlH4 --> beta-LiAlH4. Similar reaction of alpha-LiAlH4 with alumini um chloride does not occur at all, while reactions with alpha- and bet a-Li3AlH6 lead to complete decomposition of the starting compounds. Th e reaction of AlCl3 with LiH to give AlH3 proceeds only at P > 5.6 GPa and T > 550 degrees C. Only the most compact cr (hexagonal) modificat ion of AlH3 is formed under the P,T-conditions studied.