The crystal structure and stability of K 2NaAlH 6

• Published in: Journal of alloys and compounds Vol. 415; no. 1; pp. 284 - 287
• Main Authors: Sørby, M.H., Brinks, H.W., Fossdal, A., Thorshaug, K., Hauback, B.C.
• Format: Journal Article
• Language: English
• Published: Elsevier B.V 2006
• Subjects: Alanates; Ball-milling; Crystal structure; Hydrogen storage materials; Pressure-composition isotherms; Hydrogen storage materials; Ball-milling; Alanates; Pressure-composition isotherms; Crystal structure
• ISSN: 0925-8388; 1873-4669
• DOI: 10.1016/j.jallcom.2005.08.010

The mixed alanate K 2NaAlH 6 has been synthesised by ball-milling a mixture of KH and NaAlH 4 and subsequent annealing at 150 °C under a stabilizing hydrogen pressure. It takes a cubic ordered perovskite-type structure (space group F m 3 ¯ m , a = 8.118(1) Å) where the ideal octahedral AlH 6 3− units form a cubic close-packed structure with K + in all tetrahedral sites and Na + in all octahedral sites. K 2NaAlH 6 with 1 mol% TiF 3 decompose quickly and reversibly with a flat plateau. Pressure-composition isotherms have been measured at five temperatures in the range 280–325 °C. The decomposition enthalpy and entropy were determined to 98(2) kJ/mol H 2 and 150(4) J/(K mol H 2) from the van’t Hoff equation.