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Register a new userSuperexchange interactions in AgMF4 (M = Co, Ni, Cu) polymorphs
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Magnetic properties of silver(II) compounds have been of interest in recent years. In covalent compounds, the main mechanism of interaction between paramagnetic sites is the superexchange via the connecting ligand. To date, little is known of magnetic interactions between Ag(II) cations and other paramagnetic centres. It is because only a few compounds bearing Ag(II) cation and other paramagnetic transition metal cation are known experimentally. Recently the high-pressure synthesis of ternary silver(II) fluorides with 3d metal cations AgMF4 (M = Co, Ni, Cu) was predicted to be feasible. Here, we investigate the magnetic properties of these compounds in their diverse polymorphic forms. Using well established computational methods we predict superexchange pathways in AgMF4, evaluate coupling constants and calculate the impact of Ag(II) presence on superexchange between the other cations. The results indicate that the low-pressure form of AgCuF4, the only composed of stacked layers as the parent AgF2, would hold mainly Ag-Ag and Cu-Cu superexchange interactions. Upon compression, or with the nickel(II) cation, the Ag-M interactions in AgMF4 intensify, which is emphasized by an increase of Ag-M superexchange coupling constants and Ag-F-M angles. All the strongest Ag-M superexchange pathways are quasi-linear, leading to the formation of antiferromagnetic chains along the crystallographic directions. The impact of Ag(II) on M-M superexchange turns out to be moderate, due to factors connected to the crystal structure.
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Only several compounds bearing Ag(II) cation and other transition metal cation have been known. Herein, we predict stability and crystal structures of hypothetical ternary silver(II) fluorides with copper, nickel and cobalt in 1:1 stoichiometry at pressure range from 0 GPa up to 20 GPa within the frame of Density Functional Theory. Calculations show that AgCoF4 could be synthesized already at ambient conditions but this compound would host diamagnetic Ag(I) and high-spin Co(III). However, at increased pressure ternary fluorides of Ag(II) featuring Cu and Ni could be synthesized, in the pressure windows of 7-14 and 8-15 GPa, respectively. All title compounds would be semiconducting and magnetically ordered.
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Fe$M_2X_4$ spinels, where $M$ is a transition metal and $X$ is oxygen or sulfur, are candidate materials for spin filters, one of the key devices in spintronics. We present here a computational study of the inversion thermodynamics and the electronic structure of these (thio)spinels for $M=$ Cr, Mn, Co, Ni, using calculations based on the density functional theory with on-site Hubbard corrections (DFT+$U$). The analysis of the configurational free energies shows that different behaviour is expected for the equilibrium cation distributions in these structures: FeCr$_2X_4$ and FeMn$_2$S$_4$ are fully normal, FeNi$_2X_4$ and FeCo$_2$S$_4$ are intermediate, and FeCo$_2$O$_4$ and FeMn$_2$O$_4$ are fully inverted. We have analyzed the role played by the size of the ions and by the crystal field stabilization effects in determining the equilibrium inversion degree. We also discuss how the electronic and magnetic structure of these spinels is modified by the degree of inversion, assuming that this could be varied from the equilibrium value. We have obtained electronic densities of states for the completely normal and completely inverse cation distribution of each compound. FeCr$_2X_4$, FeMn$_2X_4$, FeCo$_2$O$_4$ and FeNi$_2$O$_4$ are half-metals in the ferrimagnetic state when Fe is in tetrahedral positions. When $M$ is filling the tetrahedral positions, the Cr-containing compounds and FeMn$_2$O$_4$ are half-metallic systems, while the Co and Ni spinels are insulators. The Co and Ni sulfide counterparts are metallic for any inversion degree together with the inverse FeMn$_2$S$_4$. Our calculations suggest that the spin filtering properties of the Fe$M_2X_4$ (thio)spinels could be modified via the control of the cation distribution through variations in the synthesis conditions.
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