MgAl2Ge2 a new representative of the structure type CaAl2Si2

 

Chem. Met. Alloys 5 (2012) 59-65

 

Svitlana PUKAS, Liliya PYLYPCHAK, Oksana MATSELKO, Pavlo DEMCHENKO1, Roman GLADYSHEVSKII

 

The magnesium alumogermanide MgAl2Ge2 was synthesized in a high-frequency furnace and its crystal structure was studied by X-ray powder diffraction. It crystallizes with a CaAl2Si2-type structure: Pearson symbol hP5, space group P-3m1, a = 4.11693(5), c = 6.7873(1) Å. For the isotypic compounds RAl2M2 (R = Mg, Ca, Sr, M = Si, Ge) on increasing the ratio of the covalent radii rM/rR (from 0.700 for SrAl2Si2 to 0.982 for MgAl2Ge2) the R-centered elongated trigonal antiprisms M6 become compressed, whereas the initially compressed trigonal pyramids M4 centered by Al become elongated. The trigonal structure type CaAl2Si2 can be derived from the hexagonal structure type AlB2 via the structure types ZrBeSi and γ-EuPtP.

 

 

Deformation of R-centered M6 octahedra and Al-centered M4 tetrahedra deviation of the c/a ratio and z-coordinate of the M-element atoms from the ideal values for the compounds RAl2M2.

 

Keywords

Magnesium / Aluminum / Germanium / X-ray powder diffraction / Crystal structure