The material class of rare earth nickelates with high Ni³⁺ oxidation state is generating continued interest due to the occurrence of a metal-insulator transition with charge order and the appearance of non-collinear magnetic phases within this insulating regime. The recent theoretical prediction for superconductivity in LaNiO₃ thin films has also triggered intensive research efforts. LaNiO₃ seems to be the only rare earth nickelate that stays metallic and paramagnetic down to lowest temperatures. So far, centimeter-sized impurity-free single crystal growth has not been reported for the rare earth nickelates material class since elevated oxygen pressures are required for their synthesis. Here, we report on the successful growth of centimeter-sized LaNiO₃ single crystals by the floating zone technique at oxygen pressures of up to 150 bar. Our crystals are essentially free from Ni²⁺ impurities and exhibit metallic properties together with an unexpected but clear antiferromagnetic transition.

Designing efficient catalyst for the oxygen evolution reaction (OER) is of importance for energy conversion devices. The anionic redox allows formation of O-O bonds and offers higher OER activity than the conventional metal sites. Here, we successfully prepare LiNiO₂with a dominant 3d⁸Lconfiguration (Lis a hole at O 2p) under high oxygen pressure, and achieve a double ligand holes 3d⁸L²under OER since one electron removal occurs at O 2porbitals for Ni^IIIoxides. LiNiO₂exhibits super-efficient OER activity among LiMO₂,RMO₃(M = transition metal,R = rare earth) and other unary 3d catalysts. Multiple in situ/operando spectroscopies reveal Ni^III→Ni^IVtransition together with Li-removal during OER. Our theory indicates that Ni^IV(3d⁸L²) leads to direct O-O coupling between lattice oxygen and *O intermediates accelerating the OER activity. These findings highlight a new way to design the lattice oxygen redox with enough ligand holes created in OER process.

${\mathrm{SmFeO}}_3$ has attracted considerable attention very recently due to its reported multiferroic properties above room temperature. We have performed powder and single crystal neutron diffraction as well as complementary polarization dependent soft X-ray absorption spectroscopy measurements on floating-zone grown ${\mathrm{SmFeO}}_3$ single crystals in order to determine its magnetic structure. We found a $k=0$ $G$-type collinear antiferromagnetic structure that is not compatible with inverse Dzyaloshinskii-Moriya interaction driven ferroelectricity. While the structural data reveal a clear sign for magneto-elastic coupling at the Néel-temperature of $\sim 675\mathrm{K}$, the dielectric measurements remain silent as far as ferroelectricity is concerned.

We report on the spin structure of the pyrochlore iridate ${\mathrm{Nd}}_2{\mathrm{Ir}}_2{\mathrm{O}}_7$ that could be directly determined by means of powder neutron diffraction. Our magnetic structure refinement unravels a so-called all-in/all-out magnetic structure that appears in both the Nd and the Ir sublattice. The ordered magnetic moments at 1.8 K amount to $0.34\left(1\right){\mu }_{\mathrm{B}}/{\mathrm{Ir}}^{4+}$ and $1.27\left(1\right){\mu }_{\mathrm{B}}/{\mathrm{Nd}}^{3+}$. The ${\mathrm{Nd}}^{3+}$ moment size at 1.8 K is smaller than that expected for the ${\mathrm{Nd}}^{3+}$ ground state doublet. On the other hand, the size of the ordered moments of the ${\mathrm{Ir}}^{4+}$ ions at 1.8 K agrees very well with the value expected for a $J_{\mathrm{eff}}=1/2$ state based on the presence of strong spin-orbit coupling in this system. Finally, our measurements reveal a parallel alignment of the ${\mathrm{Nd}}^{3+}$ moments with the net moment of its six nearest neighboring ${\mathrm{Ir}}^{4+}$ ions.

Magnetoelectric multiferroics have attracted enormous attention in the past years because of their high potential for applications in electronic devices, which arises from the intrinsic coupling between magnetic and ferroelectric ordering parameters. The initial finding in TbMnO₃ has triggered the search for other multiferroics with higher ordering temperatures and strong magnetoelectric coupling for applications. To date, spin-driven multiferroicity is found mainly in oxides, as well as in a few halogenides. We report multiferroic properties for synthetic melanothallite Cu₂OCl₂, which is the first discovery of multiferroicity in a transition metal oxyhalide. Measurements of pyrocurrent and the dielectric constant in Cu₂OCl₂ reveal ferroelectricity below the Néel temperature of ~70 K. Thus, melanothallite belongs to a new class of multiferroic materials with an exceptionally high critical temperature. Powder neutron diffraction measurements reveal an incommensurate magnetic structure below T_N, and all magnetic reflections can be indexed with a propagation vector [0.827(7), 0, 0], thus discarding the claimed pyrochlore-like “all-in–all-out” spin structure for Cu₂OCl₂, and indicating that this transition metal oxyhalide is, indeed, a spin-induced multiferroic material.

An hour-glass-shaped magnetic excitation spectrum appears to be a universal characteristic of the high-temperature superconducting cuprates. Fluctuating charge stripes or alternative band structure approaches are able to explain the origin of these spectra. Recently, an hour-glass spectrum has been observed in an insulating cobaltate, thus favouring the charge stripe scenario. Here we show that neither charge stripes nor band structure effects are responsible for the hour-glass dispersion in a cobaltate within the checkerboard charge-ordered regime of La_2−xSr_xCoO₄. The search for charge stripe ordering reflections yields no evidence for charge stripes in La_1.6Sr_0.4CoO₄, which is supported by our phonon studies. With the observation of an hour-glass-shaped excitation spectrum in this stripeless insulating cobaltate, we provide experimental evidence that the hour-glass spectrum is neither necessarily connected to charge stripes nor to band structure effects, but instead, probably intimately coupled to frustration and arising chiral or non-collinear magnetic correlations.