Abrupt symmetry decrease in the ThT₂Al₂₀ alloys (T = 3D transition metal)

Th-T-Al system, where T-3d transition metals, was studied at ThT₂Al₂₀ stoichiometry to establish the influence of T on the structural stability of ternary aluminide formed. Different alloys were prepared, varying T in the row from Ti to Fe. Using electron microscopy and X-ray diffraction methods it was found that ThT₂Al₂₀ phase adopts CeCr₂Al₂₀ structure type when T = Ti, V, and Cr. Starting from Mn, the symmetry of the stable Al-rich phase, which forms in the alloys with the same composition, decreases from cubic to orthorhombic. The results of Density Functional Theory (DFT) calculations coincide with experiments. Concepts of the Theory of Coordination Compounds and Jahn-Teller effect were used to explain the observed abrupt change of the symmetry. These considerations were supported by DFT calculations.