MgAl₂Ge₂ – a new representative of the structure type CaAl₂Si₂

Chem. Met. Alloys 5 (2012) 59-65

https://doi.org/10.30970/cma5.0220

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 r_M/r_R (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