Structure and stability of possible new alanates

Three new stable bialkalimetallic alanates are predicted by accurate density functional calculations: $\chem{K_2LiAlH_6}$, $\chem{K_2NaAlH_6}$, and $\chem{KNa_2AlH_6}$. Their detailed crystal structure has been determined by a systematic search through a large part of the probable space of crystal structures. They are thermodynamically stable at 0$\un{K}$ compared to their monoalkali constituents by 9 to 49$\un{kJ/mol}$ formula units. The crystal structure of the already known alanates $\chem{Li_3AlH_6}$, $\chem{Na_3AlH_6}$, $\chem{K_3AlH_6}$, and $\chem{LiNa_2AlH_6}$ were also determined, and found to be in excellent agreement with experimental data where available. The two last bialkali alanates studied, $\chem{Li_2NaAlH_6}$ and $\chem{KLi_2AlH_6}$, were found to be unstable.