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Register a new userTheoretical study of ternary silver fluorides AgMF4 (M = Co, Ni, Cu) formation at pressures up to 20 GPa
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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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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.
The silver-fluorine phase diagram has been scrutinized as a function of external pressure using theoretical methods. Our results indicate that two novel stoichiometries containing Ag+ and Ag2+ cations (Ag3F4 and Ag2F3) are thermodynamically stable at ambient and low pressure. Both are computed to be magnetic semiconductors at ambient pressure conditions. For Ag2F5, containing both Ag2+ and Ag3+, we find that strong 1D antiferromagnetic coupling is retained throughout the pressure-induced phase transition sequence up to 65 GPa. Our calculations show that throughout the entire pressure range of their stability the mixed valence fluorides preserve a finite band gap at the Fermi level. We also confirm the possibility of synthesizing AgF4 as a paramagnetic compound at high pressure. Our results indicate that this compound is metallic in its thermodynamic stability region. Finally, we present general considerations on the thermodynamic stability of mixed valence compounds of silver at high pressure.
We report the properties of two new isostructural compounds, U3Bi4Ni3 and U3Bi4Rh3. The first of these compounds is non-metallic, and the second is a nearly ferromagnetic metal, both as anticipated from their electron count relative to other U-based members of the larger 3-4-3 family. For U3Bi4Rh3, a logarithmic increase of C/T below 3 K, a resistivity proportional to T^4/3, and the recovery of Fermi-liquid behavior in both properties with applied fields greater than 3T, suggest that U3Bi4Rh3 may be a new example of a material displaying ferromagnetic quantum criticality.
We report ab initio calculations of the melting curve and Hugoniot of molybdenum for the pressure range 0-400 GPa, using density functional theory (DFT) in the projector augmented wave (PAW) implementation. We use the ``reference coexistence technique to overcome uncertainties inherent in earlier DFT calculations of the melting curve of Mo. Our calculated melting curve agrees well with experiment at ambient pressure and is consistent with shock data at high pressure, but does not agree with the high pressure melting curve from static compression experiments. Our calculated P(V) and T(P) Hugoniot relations agree well with shock measurements. We use calculations of phonon dispersion relations as a function of pressure to eliminate some possible interpretations of the solid-solid phase transition observed in shock experiments on Mo.
Optical conductivity [$sigma(omega)$] of YbCu$_2$Ge$_2$ has been measured at external pressures ($P$) to 20 GPa, to study the $P$ evolution of $f$ electron hybridized states. At $P$=0, $sigma(omega)$ shows a marked mid-infrared (mIR) peak at 0.37 eV, which is due to optical excitations from $f^{14}$ (Yb$^{2+}$) state located below the Fermi level. With increasing $P$, the mIR peak shows significant shifts to lower energy, reaching 0.18 eV at $P$=20 GPa. This result indicates that the $f^{14}$ energy level increases toward the Fermi level with $P$. Such a shift of the $f$ electron level with $P$ has been expected from theoretical considerations, but had never been demonstrated by spectroscopic experiment under high $P$. The obtained results are also analyzed in terms of the $P$ evolution of the conduction-$f$ electron hybridization.
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