NaNiO 2 is a layered material consisting of alternating layers of NaO 6 and Jahn–Teller-active NiO 6 edge-sharing octahedra. At ambient pressure, it undergoes a broad phase transition from a monoclinic to rhombohedral structure between 465 and 495 K, associated with the loss of long-range orbital ordering. In this work, we present the results of a neutron powder diffraction study on powdered NaNiO 2 as a function of pressure and temperature from ambient pressure to ∼5 GPa between 290 and 490 K. The 290 and 460 K isothermal compressions remained in the monoclinic phase up to the maximum pressures studied, whereas the 490 K isotherm was mixed-phase throughout. The unit-cell volume was fitted to a second-order Birch–Murnaghan equation of state, where B = 119.6(5) GPa at 290 K. We observe at 490 K that the fraction of the Jahn–Teller-distorted phase increases with pressure, from 67.8(6)% at 0.71(2) GPa to 80.2(9)% at 4.20(6) GPa. Using this observation, in conjunction with neutron diffraction measurements at 490 K on removing pressure from 5.46(9)  to 0.342(13) GPa, we show that the Jahn–Teller transition temperature increases with pressure. Our results are used to present a structural pressure–temperature phase diagram for NaNiO 2 . To the best of our knowledge, this is the first diffraction study of the effect of pressure on the Jahn–Teller transition temperature in materials with edge-sharing Jahn–Teller-distorted octahedra and the first variable-pressure study focusing on the Jahn–Teller distortion in a nickelate.

Electrochemical energy storage (EES) technologies are playing a leading role in the global effort to address the energy challenges. Current EES systems are limited by their energy density, capacity, and cycling stability. Some of those limitations arise from nanoscale phenomena, which are not fully understood or accounted for. Electrochemical activation (ECA), an often-overlooked process, creates more active sites on the electrode material and boosts the activity of the system to achieve a higher storage capacity. Herein, the ECA of bimetallic Ni–Co oxyphosphides is investigated via a plethora of spectroscopic techniques, including transmission electron microscopy enhanced by multivariate statistical analysis as a tool to better analyze the obtained spectra. Interestingly, ECA induces an in situ reconstruction of the pre-electrode via phosphorus leaching, together with accelerated surface segregation of the reconstructed Ni and Co species. The electrodes with reconstructed composition showed 110% higher supercapacitive performance than their pre-electrode counterparts. Thanks to the electrochemical optimization approach, a hybrid device has been assembled with a superb performance. The device exhibits energy density values comparable with batteries: 89 W h kg–1 at a power density of 848 W kg–1 with an excellent stability over 10,000 galvanic charge–discharge cycles as manifested by the steady capacitive retention (94.2–100.9%) even during the last 1000 cycles.

Frustrated lanthanide oxides are promising candidates for cryogen-free magnetic refrigeration due to their suppressed ordering temperatures and high magnetic moments. While much attention has been paid to the garnet and pyrochlore lattices, the magnetocaloric effect in frustrated face-centered cubic (fcc) lattices remains relatively unexplored. We previously showed that the frustrated fcc double perovskite Ba 2 GdSbO 6 is a top-performing magnetocaloric material (per mol Gd) because of its small nearestneighbor interaction between spins. Here we investigate different tuning parameters to maximize the magnetocaloric effect in the family of fcc lanthanide oxides, A

NaNiO 2 is a layered material consisting of alternating layers of NaO 6 and Jahn-Teller-active NiO 6 edge-sharing octahedra. At ambient pressure it undergoes a broad phase transition from a monoclinic to rhombohedral structure between ∼465 K and ∼495 K, associated with the loss of long-range orbital ordering. In this work, we present the results of a neutron powder diffraction study on powdered NaNiO 2 as a function of pressure and temperature from ambient pressure to ∼5 GPa between 290 K and 490 K. The 290 K and 460 K isothermal compressions remained in the monoclinic phase up to the maximum pressures studied, whereas the 490 K isotherm was mixedphase throughout. The unit-cell volume was fitted to a 2nd-order Birch-Murnaghan equation of state, with B = 113(1) GPa. We observe at 490 K that the fraction of Jahn-Teller-distorted phase increases with increasing pressure, from 67.8(6)% at 0.71(2) GPa