Thermally Reentrant Crystalline Phase Change in Perovskite-Derivative Nickelate Enabling Reversible Switching of Room-Temperature Electrical Resistivity.

Reversible switching of room-temperature electrical resistivity due to crystal-amorphous transition is demonstrated in various chalcogenides for development of non-volatile phase change memory. However, such reversible thermal switching of room-temperature electrical resistivity has not reported in transition metal oxides so far, despite their enormous studies on the electrical conduction like metal-insulator transition and colossal magnetoresistance effect. In this study, a thermally reversible switching of room-temperature electrical resistivity is reported with gigantic variation in a layered nickelate Sr2.5 Bi0.5 NiO5 (1201-SBNO) composed of (Sr1.5 Bi0.5 )O2 rock-salt and SrNiO3 perovskite layers via unique crystalline phase changes between the conducting 1201-SBNO with ordered (O-1201), disordered Sr/Bi arrangements in the (Sr1.5 Bi0.5 )O2 layer (D-1201), and insulating oxygen-deficient double perovskite Sr2 BiNiO4.5 (d-perovskite). The O-1201 is reentrant by high-temperature annealing of ≈1000 °C through crystalline phase change into the D-1201 and d-perovskite, resulting in the thermally reversible switching of room-temperature electrical resistivity with 102 - and 109 -fold variation, respectively. The 1201-SBNO is the first oxide to show the thermally reversible switching of room-temperature electrical resistivity via the crystalline phase changes, providing a new perspective on the electrical conduction for transition metal oxides.

Slow magnetic relaxation in a ferromagnetic CuII chain complex, induced by a phonon bottleneck effect.

The first slow magnetic relaxation in a ferromagnetic Cu(II) chain compound, Cu(dipic)(OH2)2 (dipicH2 = pyridine-2,6-dicarboxylic acid), induced by a phonon bottleneck effect under a magnetic field of 0.6 T, with a relaxation time of 2.2 s at 2.8 K, was observed.

Superconductivity and improved electrical conduction in anti-ThCr2Si2-type RE 2O2Sb and RE 2O2Bi with pnictogen square net.

Layered compounds have shown rich physical properties such as high-temperature superconductivity. Recently, monatomic honeycomb lattice systems, such as graphene, have been studied extensively, whereas monatomic pnictogen square nets in layered compounds also exhibit interesting electronic properties owing to their unusual negative valence states. Among them, anti-ThCr2Si2-type RE 2O2 Pn (RE = rare earth, Pn = Sb, Bi) with monoatomic Pn square nets were recently found to exhibit interesting electronic properties such as superconductivity and high carrier mobility. In this article, we review recent studies on crystal structures, electronic properties, and thin-film growth of RE 2O2 Pn.

Versatile control of the superconducting transition temperature in anti-ThCr2Si2-type Y2O2Bi via H, Li, or F doping.

In Y2O2Bi with Bi square net, H substitution and Li intercalation led to higher superconducting transition temperature (Tc), while F substitution led to lower Tc, where Tc is universally scaled by unit cell tetragonality c/a. Li intercalated Y2O2Bi showed a higher Tc than previously reported Y2O2Bi even for the similar c/a.

Heterospin frustration in a metal-fullerene-bonded semiconductive antiferromagnet.

Lithium-ion-encapsulated fullerenes (Li+@C60) are 3D superatoms with rich oxidative states. Here we show a conductive and magnetically frustrated metal-fullerene-bonded framework {[Cu4(Li@C60)(L)(py)4](NTf2)(hexane)}n (1) (L = 1,2,4,5-tetrakis(methanesulfonamido)benzene, py = pyridine, NTf2- = bis(trifluoromethane)sulfonamide anion) prepared from redox-active dinuclear metal complex Cu2(L)(py)4 and lithium-ion-encapsulated fullerene salt (Li+@C60)(NTf2-). Electron donor Cu2(L)(py)2 bonds to acceptor Li+@C60 via eight Cu‒C bonds. Cu-C bond formation stems from spontaneous charge transfer (CT) between Cu2(L)(py)4 and (Li+@C60)(NTf2-) by removing the two-terminal py molecules, yielding triplet ground state [Cu2(L)(py)2]+(Li+@C60•-), evidenced by absorption and electron paramagnetic resonance (EPR) spectra, magnetic properties and quantum chemical calculations. Moreover, Li+@C60•- radicals (S = ½) and Cu2+ ions (S = ½) interact antiferromagnetically in triangular spin lattices in the absence of long-range magnetic ordering to 1.8 K. The low-temperature heat capacity indicated that compound 1 is a potential candidate for an S = ½ quantum spin liquid (QSL).

Low-temperature topotactic oxidation using the solid-state oxidant Zr-doped CeO2.

A new topotactic oxidation was developed using the solid-state oxidant Zr-doped CeO2. For the anti-ThCr2Si2 type Y2O2Bi, which became superconducting via oxygen intercalation during solid-state oxidation at 1000 °C, the topotactic oxidation enabled not only the oxygen intercalation at a much lower temperature of 200 °C, hampering the segregation of impurity phases, but also the highest superconducting transition temperature for Y2O2Bi.

Increased hole mobility in anti-ThCr2Si2-type La2O2Bi co-sintered with alkaline earth metal oxides for oxygen intercalation and hole carrier doping.

Metallic anti-ThCr2Si2-type RE2O2Bi (RE = rare earth) with Bi square nets show superconductivity while insulating La2O2Bi shows high hole mobility, by expanding the c-axis length through oxygen intercalation. In this study, alkaline earth metal oxides (CaO, SrO and BaO) were co-sintered with La2O2Bi. CaO and BaO served as oxygen intercalants without the incorporation of Ca and Ba in La2O2Bi. On the other hand, SrO served not only as an oxygen intercalant but also as a hole dopant via Sr substitution for La in La2O2Bi. The oxygen intercalation and hole doping resulted in the expansion of the c-axis length, contributing to the improved electrical conduction. In addition, the hole mobility was enhanced up to 150 cm2 V-1 s-1 in La2O2Bi, which is almost double the mobility in a previous study.

Clean Solid-Electrolyte/Electrode Interfaces Double the Capacity of Solid-State Lithium Batteries.

Solid-state lithium (Li) batteries using spinel-oxide electrode materials such as LiNi0.5Mn1.5O4 are promising power supplies for mobile devices and electric vehicles. Here, we demonstrate stable battery cycling between the Li0Ni0.5Mn1.5O4 and Li2Ni0.5Mn1.5O4 phases with working voltages of approximately 2.9 and 4.7 V versus Li/Li+ in solid-state Li batteries with contamination-free clean Li3PO4/LiNi0.5Mn1.5O4 interfaces. This clean interface has the effect of doubling the capacity of conventional battery cycling between the Li0Ni0.5Mn1.5O4 and Li1Ni0.5Mn1.5O4 phases. We also investigated the structural changes between the Li0Ni0.5Mn1.5O4 and Li2Ni0.5Mn1.5O4 phases during battery cycling. Furthermore, we found an inhomogeneous distribution of the Li2Ni0.5Mn1.5O4 phase in the LiNi0.5Mn1.5O4 electrode, induced by spontaneous Li migration after the formation of the Li3PO4/LiNi0.5Mn1.5O4 interface. These results indicate that the formation of a contamination-free clean Li3PO4/LiNi0.5Mn1.5O4 interface is key to increase the battery capacity.

Tetragonality induced superconductivity in anti-ThCr2Si2-type RE2O2Bi (RE = rare earth) with Bi square nets.

We report a series of layered superconductors, anti-ThCr2Si2-type RE2O2Bi (RE = rare earth), composed of electrically conductive Bi square nets and magnetic insulating RE2O2 layers. Superconductivity was induced by separating the Bi square nets as a result of excess oxygen incorporation, irrespective of the presence of magnetic ordering in RE2O2 layers. Intriguingly, the transition temperature of all RE2O2Bi including nonmagnetic Y2O2Bi was approximately scaled by unit cell tetragonality (c/a), implying a key role in the relative separation of the Bi square nets to induce superconductivity.

Atomically Well-Ordered Structure at Solid Electrolyte and Electrode Interface Reduces the Interfacial Resistance.

Using synchrotron surface X-ray diffraction, we investigated the atomic structures of the interfaces of a solid electrolyte (Li3PO4) and electrode (LiCoO2). We prepared two types of interfaces with high and low interface resistances; the low-resistance interface exhibited a flat and well-ordered atomic arrangement at the electrode surface, whereas the high-resistance interface showed a disordered interface. These results indicate that the crystallinity of LiCoO2 at the interface has a significant impact on interface resistance. Furthermore, we reveal that the migration of Li ions along the interface and into grain boundaries and antiphase domain boundaries is a critical factor reducing interface resistance.

Superconductivity in Anti-ThCr2Si2-type Er2O2Bi Induced by Incorporation of Excess Oxygen with CaO Oxidant.

Recently, superconductivity was induced by expanding interlayer distance between Bi square nets in anti-ThCr2Si2-type Y2O2Bi through incorporation of excess oxygen with increased nominal amount of oxygen. However, such oxygen incorporation was applicable to only Y2O2Bi among R2O2Bi ( R = rare earth metal), probably due to a larger amount of oxygen incorporation for Y2O2Bi. In this study, the interlayer distance in Er2O2Bi was increased by cosintering with CaO, which served as an oxidant, indicating that excess oxygen was incorporated in Er2O2Bi. As a result, superconductivity was induced in Er2O2Bi at 2.2 K.

Extremely Low Resistance of Li3PO4 Electrolyte/Li(Ni0.5Mn1.5)O4 Electrode Interfaces.

Solid-state Li batteries containing Li(Ni0.5Mn1.5)O4 as a 5 V-class positive electrode are expected to revolutionize mobile devices and electric vehicles. However, practical applications of such batteries are hampered by the high resistance at their solid electrolyte/electrode interfaces. Here, we achieved an extremely low electrolyte/electrode interface resistance of 7.6 Ω cm2 in solid-state Li batteries with Li(Ni0.5Mn1.5)O4. Furthermore, we observed spontaneous migration of Li ions from the solid electrolyte to the positive electrode after the formation of the electrolyte/electrode interface. Finally, we demonstrated stable fast charging and discharging of the solid-state Li batteries at a current density of 14 mA/cm2. These results provide a solid foundation to understand and fabricate low-resistance electrolyte/electrode interfaces.