Interlayer bonding in compounds with the ThCr_2Si_2-type structure: Insight on the ferromagnetism of SrCo_2(Ge_(1-x)P_x)_2 from electronic structure calculations

摘要

The strong composition dependence of the cell parameters in (more than 700) compounds with the ThCr_2Si_2-type structure has been the subject of discussion for several decades. Recent findings of ferromagnetism in SrCo_2[Ge_(1-x)P_x]_2 with a quantum critical point at x ≈ 0.325 have revived interest in the composition-property relationship in such systems. In this article, we present a theoretical insight on these phenomena, supported by electronic structure calculations. We discuss the differences between SrCo_2[Ge_(1-x)P_x]_2 and related compounds, and we conclude that previous analyses based on the filling of "relevant" molecular orbitals are case specific and do not describe the general mechanism behind the changes in the cell parameters. We propose a more physical reasoning, based on electronic kinetic energy and electron-core interactions, which gives a unified picture of all cases. With respect to the magnetism in SrCo_2[Ge_(1-x)P_x]_2, we find that the density of states at the Fermi level and the Curie temperature display very similar dependences on the composition. Our calculations support the classification of the magnetic dome as a weak itinerant ferromagnetism, and they are consistent with the Stoner-enhanced paramagnetism observed in samples with x ＞ 0.75. We argue that the interlayer bonding is metallic (instead of covalent) from x = 0 up to the value at which the magnetic remanence disappears.