Theoretical and 27 Al NMR Spectroscopic Investigations of Binary Intermetallic Alkaline-Earth Aluminides.

The binary alkaline-earth aluminides AE Al 2 ( AE = Ca and Sr) and AE Al 4 ( AE = Ca-Ba) have been synthesized from the elements and investigated via powder X-ray diffraction experiments. CaAl 2 adopts the cubic MgCu 2 -type structure ( Fd 3̅ m ), while SrAl 2 crystallizes in the orthorhombic KHg 2 -type ( Imma ). LT-CaAl 4 crystallizes with the monoclinic CaGa 4 -type ( C 2/ m ), while HT-CaAl 4 , SrAl 4 , and BaAl 4 adopt the tetragonal BaAl 4 -type structure ( I 4/ mmm ). The close structural relation of the two CaAl 4 polymorphs was established using a group-subgroup relation in the Bärnighausen formalism. In addition to the room-temperature and normal pressure phase of SrAl 2 , a high-pressure/high-temperature phase has been prepared using multianvil techniques, and its structural and spectroscopic parameters were determined. Elemental analysis by inductively coupled plasma mass spectrometry showed that no significant impurities with other elements besides the weighed ones are present and the chemical compositions match the synthesized ones. The title compounds have been furthermore investigated by 27 Al solid-state magic angle spinning NMR experiments to validate the crystal structure and to gain information about the influence of the composition on the electron transfer and the NMR characteristics. This has also been investigated from a quantum chemical point of view using Bader charges, while the stabilities of the binary compounds in the three phase diagrams (Ca-Al, Sr-Al and Ba-Al) have been studied by calculations of formation energies per atom.