The crystal structure and stability of K2NaAlH6

The mixed alanate K2NaAlH6 has been synthesised by ball-milling a mixture of KH and NaAlH4 and subsequent annealing at 150 deg C under a stabilizing hydrogen pressure. It takes a cubic ordered perovskite-type structure (space group Fm3m, a = 8.118(1) A) where the ideal octahedral AlH6(3-) units form a cubic close-packed structure with K(+) in all tetrahedral sites and Na(+) in all octahedral sites. K2NaAlH6 with 1 mol% TiF3 decompose quickly and reversibly with a flat plateau. Pressure-composition isotherms have been measured at five temperatures in the range 280-325 deg C. The decomposition enthalpy and entropy were determined to 98(2) kJ/mol H2 and 150(4) J/(K mol H2) from the van't Hoff equation.