Contrary to interstitial metal hydrides, where the thermodynamic stability of the metal hydride can be tuned by substitutions on the metal lattice, few examples of such substitutions have been reported for complex hydrides. The present paper shows a new and alternative approach to affect the stability of complex hydrides by substitution on the anion lattice. A mixed hydride-flouride has been prepared from NaF, Al, and H 2 . The resulting complex hydride Na 3 AlH 6-x F x is less stable than Na 3 AlH 6 . Pressure composition isotherms indicate a plateau pressure of about 25 bar for dehydrogenation at 120 °C. The crystal structure has been determined by combined neutron diffraction and synchrotron X-ray diffraction to be isostructural with Na 3 AlF 6 and Na 3 AlD 6 . The space group is P2 1 /n, and x was determined to be approximately 4. No ordering of D and F was observed.

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