On the order and disorder of the transition metal (T) and silicon atoms in ternary thorium transition metal silicides of the compositions Th2TSi3 and ThTSi

During phase investigations of the ternary systems thorium-transition metal-silicon, 12 ternary thorium transition metal silicides Th2TSi3 (T = Mn, Fe, Co, Ni, Cu, Ru, Rh, Pd, Os, Ir, Pt, Au), nine of them for the first time, were prepared by arc melting of the elemental components and subsequent annealing. The silicides with Co, Ni and Cu crystallize with a hexagonal AlB2-type structure, while all other compositions adopt the tetragonal structure of α-ThSi2. Although it can be assumed that the transition metal and silicon atoms occupy the boron and silicon positions of AlB2 and α-ThSi2 with at least some short-range order, no evidence for an ordered occupancy of these positions was found. In contrast, in the new equiatomic compound ThAuSi the gold and silicon atoms occupy the boron positions of AlB2 in an ordered manner. There is a principal difference for compounds with order-disorder transitions depending on whether the ordering results in a translationengleiche or a klassengleiche subgroup. For structures with a translationengleiche subgroup the order changes the intensities of the reflections already present in the disordered structure, while for ordered structures with klassengleiche subgroups the order can be detected only through the observation of superstructure reflections.