X-ray single crystal refinements on some RT(2)Ge(2) compounds (R=Ca,Y,La,Nd,U; T=Mn-Cu,Ru-Pd): Evolution of the chemical bonds

We report on single crystal refinement of 22 ThCr2Si2-type structure germanides of composition RT(2)Ge(2) (R = Ca, Y, La, Nd, U; T = Mn, Fe, Co, Ni, Cu, Ru, Rh, Pd). The interatomic distances and their variations with R size, R valency and the position of the transition metal in the Periodic Table are analysed from 35 refined structures. The T-Ge bond is always strong except in the Mn compounds. The Ge-Ge bond is stronger in the right-hand side transition metal compounds. This effect is discussed in term of a previous theoretical work. Among the other contacts T-T, R-T and R-R, the T-T contacts seem to play an important role in the Ru and Rh compounds, whereas it is believed that the R-T contacts control the cell dimensions of Fe and Co compounds. This paper examines the relationships between the interatomic distances and the magnetic behaviour of manganese ThCr2Si2-type structure compounds.