Superexchange interactions in AgMF4 (M = Co, Ni, Cu) polymorphs

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 a Ag(II) cation and other paramagnetic transition metal cation are known from experimental work. Recently the high-pressure synthesis of ternary silver(II) fluoridometallates 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 compounds, evaluate coupling constants and calculate the impact of the Ag(II) presence on superexchange between the other cations. The results indicate that the low-pressure form of AgCuF4, the only one composed of stacked layers like the parent AgF2, would show mainly Ag–Ag and Cu–Cu superexchange interactions. Upon compression, or with the nickel(II) cation, the Ag–M interactions in AgMF4 compounds are intensified, 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.

Energy Transfer Between Yb3+- Mn2+ in Zn(Po3 ) 2 Glasses Giving Green, Red and Infrared Light

The Yb3+-Mn2+codoped zinc metaphosphate glass gives rise to several processes such as upconversion, downshifting and double ion absorptions producing light from the visible to the IR wavelengths. These processes are possible since the Mn2+ and the Yb3+ ions replaces the Zn2+ ion in nanoparticles of the α phase of the compound Zn(PO3)2. An important result is that the α structure presence allows the formation of Yb3+-Mn2+ dimers, that gives rise to a superexchange coupling that allows an upconversion process from the IR to the red region of the electromagnetic spectrum. The experimental results also show that these dimers can couple to produce Yb3+ ion pairs that led to a cooperative emission in the green, around 500nm, coupling also the Mn2+ ions that in turn allows to produce a double absorption of this ions in the red region of the electromagnetic spectrum and as well a downshifting process conducting to the Yb3+ emission in the infrared. If the manganese ion concentration is higher than 10% most of these effects are masked. All these results make the material an effective option for different applications, due to the large wavelength variety that can be selected for excitation or emission

Energy Transfer Between Yb3+- Mn2+ in Zn(Po3 ) 2 Glasses Giving Green, Red and Infrared Light

The Yb3+-Mn2+codoped zinc metaphosphate glass gives rise to several processes such as upconversion, downshifting and double ion absorptions producing light from the visible to the IR wavelengths. These processes are possible since the Mn2+ and the Yb3+ ions replaces the Zn2+ ion in nanoparticles of the α phase of the compound Zn(PO3)2. An important result is that the α structure presence allows the formation of Yb3+-Mn2+ dimers, that gives rise to a superexchange coupling that allows an upconversion process from the IR to the red region of the electromagnetic spectrum. The experimental results also show that these dimers can couple to produce Yb3+ ion pairs that led to a cooperative emission in the green, around 500nm, coupling also the Mn2+ ions that in turn allows to produce a double absorption of this ions in the red region of the electromagnetic spectrum and as well a downshifting process conducting to the Yb3+ emission in the infrared. If the manganese ion concentration is higher than 10% most of these effects are masked. All these results make the material an effective option for different applications, due to the large wavelength variety that can be selected for excitation or emission.

Novel Three-Dimensional Copper(II) 1,2- Ethylenediphosphonate Framework with Channel-like Voids

Turquoise monoclinic single crystals of the novel three-dimensional Cu2[μ8-O3P(CH2)2PO3)].3.2H2O coordination polymer have been prepared using the silica gelmethod. Space group C2/m (no. 12) with a = 1483.6(2), b = 668.44(8), c = 436.30(6) pm, beta =93.28(2)°. The Cu2+ cation is coordinated by four oxygen atoms stemming from the 1,2-ethylenediphosphonate dianions in a square planar manner and two water molecules in theaxial positions. The connection between the Cu2+ cations and the [PO3C] units from the 1,2-ethylenediphosphonate dianions leads to layers parallel to (100), which are linked by theethylene groups to a three-dimensional framework with channel-like voids. The voidsaccommodate water molecules not bound to Cu2+ and extend parallel along [001] with anopening of about 550 260 pm. Magnetic measurements reveal an antiferromagneticbehaviour due to a superexchange coupling between Cu2+ ions through an oxygen bridge. TheUV-Vis spectrum reveals three dd transition bands at 694, 774, and 918 nm. The compoundcan be fully dehydrated by thermal treatment and rehydrated by storage in ambient air.

Successive Spin-Correlated Local Processes Underlying the Magnetism in Diluted Magnetic Semiconductors and Related Magnetic Materials

Recent works have suggested that the defect induced magnetism in Diluted Magnetic Semiconductors (DMSs), Transition Metal Oxides (TMOs) and related materials is facilitated and enhanced by codoping and the synergistic action between the codopants. In the present work we demonstrate that the proposed defect synergy is the result of the interplay among correlated spin-polarization processes which take place in a successive way in neighborhoods centered at the codopants and include their first nearest neighbors. These processes result in a reduction in the superexchange coupling which in turn causes an enhancement in the ferromagnetic coupling (FMC) among the magnetic dopants. The proposed FMC is demonstrated using ab initio calculations of the electronic properties of codoped ZnO, GaN and TiO2.