Investigations of perovskite oxides triggered by the discovery of high-temperature and unconventional superconductors have had crucial roles in stimulating and guiding the development of modern condensed-matter physics. Antiperovskite oxides are charge-inverted counterpart materials to perovskite oxides, with unusual negative ionic states of a constituent metal. No superconductivity was reported among the antiperovskite oxides so far. Here we present the first superconducting antiperovskite oxide Sr3−xSnO with the transition temperature of around 5 K. Sr3SnO possesses Dirac points in its electronic structure, and we propose from theoretical analysis a possibility of a topological odd-parity superconductivity analogous to the superfluid 3He-B in moderately hole-doped Sr3−xSnO. We envision that this discovery of a new class of oxide superconductors will lead to a rapid progress in physics and chemistry of antiperovskite oxides consisting of unusual metallic anions.

The field of high entropy oxides (HEOs) flips traditional materials science paradigms on their head by seeking to understand what properties arise in the presence of profound configurational disorder. This disorder, which originates from multiple elements sharing a single lattice site, can take on a kaleidoscopic character due to the vast numbers of possible elemental combinations. High configurational disorder appears to imbue some HEOs with functional properties that far surpass their nondisordered analogs. While experimental discoveries abound, efforts to characterize the true magnitude of the configurational entropy and understand its role in stabilizing new phases and generating superior functional properties have lagged behind. Understanding the role of configurational disorder in existing HEOs is the crucial link to unlocking the rational design of new HEOs with targeted properties. In this Perspective, we attempt to establish a framework for articulating and beginning to address these questions in pursuit of a deeper understanding of the true role of entropy in HEOs.

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Spinel oxides are an ideal setting to explore the interplay between configurational entropy, site selectivity, and magnetism in high-entropy oxides (HEOs). In this work, we characterize the magnetic properties of the spinel (Cr, Mn, Fe, Co, Ni)3O4 and study the evolution of its magnetism as a function of nonmagnetic gallium substitution. Across the range of compositions studied here, from 0 to 40% Ga, magnetic susceptibility and powder neutron diffraction measurements show that ferrimagnetic order is robust in the spinel HEO. However, we also find that the ferrimagnetic order is highly tunable, with the ordering temperature, saturated and sublattice moments, and magnetic hardness all varying significantly as a function of Ga concentration. Through X-ray absorption and magnetic circular dichroism, we are able to correlate this magnetic tunability with strong site selectivity between the various cations and the tetrahedral and octahedral sites in the spinel structure. In particular, we find that while Ni and Cr are largely unaffected by the substitution with Ga, the occupancies of Mn, Co, and Fe are each significantly redistributed. Ga substitution also requires an overall reduction in the transition metal valence, and this is entirely accommodated by Mn. Finally, we show that while site selectivity has an overall suppressing effect on the configurational entropy, over a certain range of compositions, Ga substitution yields a striking increase in the configurational entropy and may confer additional stabilization. Spinel oxides can be tuned seamlessly from the low-entropy to the high-entropy regime, making this an ideal platform for entropy engineering.

Bulk superconductivity was recently reported in the antiperovskite oxide Sr3−xSnO, with a possibility of hosting topological superconductivity. We investigated the evolution of superconducting properties such as the transition temperature Tc and the size of the diamagnetic signal, as well as normal-state electronic and crystalline properties, with varying the nominal Sr deficiency x0. Polycrystalline Sr3−xSnO was obtained up to x0 = 0:6 with a small amount of SrO impurities. The amount of impurities increases for x0 > 0.6, suggesting phase instability for high deficiency. Mössbauer spectroscopy reveals an unusual Sn4− ionic state in both stoichiometric and deficient samples. By objectively analyzing superconducting diamagnetism data obtained from a large number of samples, we conclude that the optimal x0 lies in the range 0.5 < x0 < 0.6. In all superconducting samples, two superconducting phases appear concurrently that originate from Sr3−xSnO but with varying intensities. These results clarify the Sr deficiency dependence of the normal and superconducting properties of the antiperovskite oxide Sr3−xSnO will ignite future work on this class of materials.

Abstract(Ba,K)BiO3 constitute an interesting class of superconductors, where the remarkably high superconducting transition temperature Tc of 30 K arises in proximity to charge density wave order. However, the precise mechanism behind these phases remains unclear. Here, enabled by high-pressure synthesis, we report superconductivity in (Ba,K)SbO3 with a positive oxygen–metal charge transfer energy in contrast to (Ba,K)BiO3. The parent compound BaSbO3−δ shows a larger charge density wave gap compared to BaBiO3. As the charge density wave order is suppressed via potassium substitution up to 65%, superconductivity emerges, rising up to Tc = 15 K. This value is lower than the maximum Tc of (Ba,K)BiO3, but higher by more than a factor of two at comparable potassium concentrations. The discovery of an enhanced charge density wave gap and superconductivity in (Ba,K)SbO3 indicates that strong oxygen–metal covalency may be more essential than the sign of the charge transfer energy in the main-group perovskite superconductors.

These quaternary chalcogenides are isostructural, crystallizing in a unique structure type comprising localized Cu clusters and Te(2)(2-) dumbbells. With less than six Cu atoms per formula unit, these materials are p-type narrow-gap semiconductors, according to the balanced formula Ba(2+)(Cu(+))6Q(2-)(Te(2)(2-))3 with Q = S, Se. Encouraged by the outstanding thermoelectric performance of Cu(2-x)Se and the low thermal conductivity of cold-pressed BaCu(5.7)Se(0.6)Te(6.4), we determined the thermoelectric properties of hot-pressed pellets of BaCu(5.9)STe(6) and BaCu(5.9)SeTe(6). Both materials exhibit a high Seebeck coefficient and a low electrical conductivity, combined with very low thermal conductivity below 1 W m(-1) K(-1). Compared to the sulfide-telluride, the selenide-telluride exhibits higher electrical and thermal conductivity and comparable Seebeck coefficient, resulting in superior figure-of-merit values zT, exceeding 0.8 at relatively low temperatures, namely, around 600 K.

A large variety of perovskite oxide superconductors are known, including some of the most prominent high-temperature and unconventional superconductors. However, superconductivity among the oxidation state inverted material class, the antiperovskite oxides, was reported just recently for the first time.In this superconductor, Sr3-xSnO, the unconventional ionic state Sn 4is realized and possible unconventional superconductivity due to a band inversion has been discussed. Here, we discuss an improved facile synthesis method, making it possible to control the strontium deficiency in Sr3-xSnO. Additionally, a synthesis method above the melting point of Sr3SnO is presented. We show temperature dependence of magnetization and electrical resistivity for superconducting strontium deficient Sr3-xSnO (Tc ~ 5 K) and for Sr3SnO without a superconducting transition down to 0.15 K. Further, we reveal a significant effect of strontium raw material purity on the superconductivity and achieve 40% increased superconducting volume fraction (~100%) compared to the highest value reported so far. More detailed characterisation utilising powder X-ray diffraction and energy-dispersive X-ray spectroscopy show that a minor cubic phase, previously suggested to be a Sr3-xSnO, is SrO. The improved characterization and controlled synthesis reported herein enable detailed investigations on the superconducting nature and its dependency on the strontium deficiency in Sr3-xSnO.