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Accumulated evidence indicates that phase separation occurs in potassium intercalated iron selenides, a superconducting phase coexisting with the antiferromagnetic phase K2Fe4Se5, the so-called '245 phase'. Here, we report a comparative study of substitution effects by Co and Mn for Fe sites in K0.8Fe2-ySe2 within the phase separation scenario. Our results demonstrate that Co and Mn dopants have distinct differences in occupancy and hence in the suppression mechanism of superconductivity upon doping of Fe sites. In K0.8Fe2-xCoxSe2, Co prefers to occupy the lattice of the superconducting phase and suppresses superconductivity very quickly, obeying the magnetic pair-breaking mechanism or the collapse of the Fermi surface nesting mechanism. In contrast, in K0.8Fe1.7-xMnxSe2, Mn shows no preferential occupancy in the superconducting phase or the 245 phase. The suppression of superconductivity can be attributed to restraining of the superconducting phase and meanwhile inducing another non-superconducting phase by Mn doping.

The fabrication of heterojunctions is an intriguing approach to boost the reactivity of catalysts. However, facile preparation of desirable heterojunction materials remains a challenge. Here, novel CoPS3/CoS2 heterojunctions are created by an intuitive phosphatization process on the basis of structurally flexible cobalt sulfide precursors. Significant stress effect exists at these heterogeneous interfaces, resulting in lattice distortions and the exposure of more active sites. The interface interaction also modifies the catalyst's electronic structure to improve its electrical conductivity and hydrogen adsorption capabilities. Notably, the overpotential of the CoPS3/CoS2 heterojunction for electrocatalytic hydrogen evolution reaction is just 36.3 mV, which is far superior to those of single‐component catalysts and competitive with comparable catalysts reported. This work not only offers an innovative electrocatalyst but also a strategy that can be used to create heterojunctions based on related transition metal thiophosphates. [ABSTRACT FROM AUTHOR]

Molybdenum diselenide and cobalt diselenide have been commonly implemented in electrocatalytic hydrogen evolution reaction (HER). However, there have been few research on the creation of their three‐phase heterojunctions and the associated HER process. Herein, we constructed a three‐phase heterostructure sample consisting of orthorhombic CoSe2, cubic CoSe2 and MoSe2 and we investigated its HER performance. The sample shows microsphere morphology composed of nanosheets with interfacial interactions between the components. It possesses an overpotential of −136 mV at −10 mA cm−2 in acid medium, which is superior to that of single component and most two‐phase heterostructures. Especially, the overpotential at −200 mA cm−2 is smaller than that of Pt/C. The excellent performance can be attributed to the d‐orbital upshift of the Co active sites due to charge redistribution between the three‐phase heterojunction and the optimization of the hydrogen free energy. This work provides inspiration for exploring the application of other multi‐component heterojunctions in electrocatalytic hydrogen evolution. [ABSTRACT FROM AUTHOR]

Optimizing the hydrogen adsorption Gibbs free energy (ΔGH) of active sites is essential to improve the overpotential of the electrocatalytic hydrogen evolution reaction (HER). We doped graphene‐like Co0.85Se with sulfur and found that the active sites are reversed (from cationic Co sites to anionic S sites), which contributed to an enhancement in electrocatalytic HER performance. The optimal S‐doped Co0.85Se composite has an overpotential of 108 mV (at 10 mA cm−2) and a Tafel slope of 59 mV dec−1, which exceeds other reported Co0.85Se‐based electrocatalysts. The doped S sites have much higher activity than the Co sites, with a hydrogen adsorption Gibbs free energy (ΔGH) close to zero (0.067 eV), which reduces the reaction barrier for hydrogen production. This work provides inspiration for optimizing the intrinsic HER activity of other related transition metal chalcogenides. [ABSTRACT FROM AUTHOR]

Theorigin of transition metal vacancies and their effects on theproperties of ThCr2Si2-type compounds have beenless studied and poorly understood. Here we carefully investigatethe structure, physical properties, and electronic structure for aseries of lanthanum cobalt arsenides with nominal composition of LaCoxAs2(1.6 ≤ x≤ 2.1). It is revealed that the occupancy of Co can be tunedbetween 1.98(1) and 1.61(1). The structural analyses based on X-rayand neutron diffractions show the existence of Co vacancies resultsfrom charge balance due to the formation of bond between As–As.These Co vacancies affect the magnetic and electrical properties greatly,adjusting the Curie temperature from 205 to 47 K and increasing theresistivity by more than 100%. First-principles calculations indicatethat the Co vacancies weaken the spin polarization and reduce thedensity of states at the Fermi level, resulting in decreased Curietemperature and increased resistivity, respectively. Our results addressthe importance of transition metal vacancies in ThCr2Si2-type materials and offer a reliable route to tune the magnetismof ThCr2Si2-type structure. [ABSTRACT FROM AUTHOR]

The new title compounds are synthesized by solid state reaction of stoichiometric mixtures of BaO, Co or Mn, Ag, and Se (alumina crucibles, 400 °C for 15 h and 850 °C for 24 h). [ABSTRACT FROM AUTHOR]