The widely recognized MAX-phase materials consist of an early transition metal (M), an A-group element, and carbon or nitrogen (X) in a hexagonal layered crystal structure. Recently, materials known as MAB phase materials have been developed by substituting boron (B) for the carbon or nitrogen. We have studied an MAB phase alloy system, Hf2(S,Te)B${\rm Hf}_{2}{\rm (S,Te)B}$, by mixing the elements on the A site in the prototypical Cr2AlC-type${\rm Cr}_{2}{\rm AlC\text{-}type}$ MAX-phase crystal structure instead of the more common M site. We have considered thermodynamic, mechanical and electronic properties of the resulting Hf2S1-xTexB${\rm Hf}_{2}{\rm S}_{1-x}{\rm Te}_{x}{\rm B}$ alloy system in the entire composition range, 0 <= x <= 1$0 \le x \le 1$. With increasing Te content, the modulus of elasticity and hardness show a decreasing trend, while the material retains its electrical conductivity. Further analysis of the optical properties shows that the studied solid solutions are good candidates for effective absorbing materials in the UV region. Our study indicates that strategic alloying within the A site of MAB phases can selectively tailor certain material properties while preserving their favorable electrical and mechanical performance.

Layered van der Waals (vdW) materials, characterized by their interlayer vdW gaps, offer exceptional tunability of magnetic properties via intercalation chemistry. A wide range of magnetic behaviors have been observed in nonmagnetic transition-metal dichalcogenides intercalated with magnetic atoms. Beyond the incorporation of magnetic ions, we propose the controlled alkali-ion intercalation of intrinsic vdW magnets as a strategy to probe and manipulate spin populations and exchange interactions within individual magnetic layers. Unlike conventional solid-state methods typically used for atomic intercalation, this approach depends on postsynthetic, solution-based reactions, which remain relatively underdeveloped and present unique synthetic challenges. Hence, in this work, we demonstrate precise potassium intercalation of VOCl, a layered antiferromagnet with square-like motifs, using stoichiometric organic reductants, potassium naphthalene and potassium pyrene. Our synthetic approach addresses thermodynamic and kinetic challenges via redox-matching reductants and electrolyte-assisted homogenization. Magnetic measurements reveal a continuous evolution from antiferromagnetism (x = 0) to a spin-glass state (0 < x < 1) with magnetic memory and ultimately to ferrimagnetism (x = 1) in KxVOCl (0 <= x <= 1). Ab initio calculations support the existence of a spin-glass state, stabilized by mixed valence and competing magnetic interactions. Taken all together, this work establishes a programmable intercalation methodology to access metastable phases and tailor magnetic properties, offering new insights into magnetism in layered compounds with complex spin interactions.

Transition metal dichalcogenides are interesting candidates as photocatalysts for hydrogen evolution reaction. The MnPSe3/WS2 heterostructure is hence studied here with first principles calculations by exploring its electronic properties under the application of an electric field. It is discovered that the band gap will decrease from the WS2 monolayer to the MnPSe3/WS2 heterostructure with Perdew-Burke-Ernzerhof functional, while increase slightly when electron correlation is involved. The conduction band minimum of the heterostructure is determined by the MnPSe3 layer, while the valence band maximum is contributed by the WS2 layer. The band edges and band gap suggest that the heterostructure will have good photocatalytic properties for water splitting. Moreover, comparing to monolayer WS2, the light absorption in both the ultraviolet and visible regions will be enhanced. When an electric field is present, a linear relation is observed between the electric field and the band gap within specific range, which can thus modulate the photocatalytic performance of this heterostructure.

过渡金属硫属化合物是光催化分解水制氢的可能催化剂。在本工作中我们通过第一性原理计算 研究了一个锰磷硒单层薄膜和二硫化钨单层薄膜所构成的异质结的电子结构性质, 及其对外加电场的 响应。我们发现构建异质结之后, 体系的带隙比单层二硫化钨薄膜有所减小, 其价带和导带的带隙边 缘分别源自锰磷硒单层薄膜和二硫化钨单层薄膜。体系带隙的大小以及带隙边缘的位置表面该异质结 可能是光催化分解水的良好催化材料。与单层二硫化钨薄膜相比较, 所构建的异质结在紫外光和可见 光区域对光的吸收都有所增强。在具有外加电场的情况下, 在特定电场强度范围内电场强度和带隙存 在线性关系, 所以, 施加外加电场可以有效地调节该异质结的带隙和带隙边缘位置, 进而提升其催化 性能。 

Magnetic properties of NiO have been studied in the multimegabar pressure range by nuclear forward scattering of synchrotron radiation using the 67.4 keV Mossbauer transition of Ni-61. The observed magnetic hyperfine splitting confirms the antiferromagnetic state of NiO up to 280 GPa, the highest pressure where magnetism has been observed so far, in any material. Remarkably, the hyperfine field increases from 8.47 T at ambient pressure to similar to 24 T at the highest pressure, ruling out the possibility of a magnetic collapse. A joint x-ray diffraction and extended x-ray-absorption fine structure investigation reveals that NiO remains in a distorted sodium chloride structure in the entire studied pressure range. Ab initio calculations support the experimental observations, and further indicate a complete absence of Mott transition in NiO up to at least 280 GPa.

A pressure-induced interaction between core electrons, the core-level crossing (CLC) transition, has been observed in hcp Os at P approximate to 400 GPa [L. Dubrovinsky et al., Nature (London) 525, 226 (2015)]. By carrying out a systematic theoretical study for all metals of the 5d series (Hf, Ta, W, Re, Os, Ir, Pt, Au) we have found that the CLC transition is a general effect for this series of metals. While in Pt it occurs at approximate to 1500 GPa, at a pressure substantially higher than in Os, in Ir it occurs already at 80 GPa. Moreover, we predict that in Re the CLC transition may take place already at ambient pressure. We explain the effect of the CLC and analyze the shift of the transition pressure across the series within the Thomas-Fermi model. In particular, we show that the effect has many common features with the atomic collapse in rare-earth elements.

Metallic osmium (Os) is one of the most exceptional elemental materials, having, at ambient pressure, the highest known density and one of the highest cohesive energies and melting temperatures(1). It is also very incompressible(2-4), but its high-pressure behaviour is not well understood because it has been studied(2-6) so far only at pressures below 75 gigapascals. Here we report powder X-ray diffraction measurements on Os at multi-megabar pressures using both conventional and double-stage diamond anvil cells(7), with accurate pressure determination ensured by first obtaining self-consistent equations of state of gold, platinum, and tungsten in static experiments up to 500 gigapascals. These measurements allow us to show that Os retains its hexagonal close-packed structure upon compression to over 770 gigapascals. But although its molar volume monotonically decreases with pressure, the unit cell parameter ratio of Os exhibits anomalies at approximately 150 gigapascals and 440 gigapascals. Dynamical mean-field theory calculations suggest that the former anomaly is a signature of the topological change of the Fermi surface for valence electrons. However, the anomaly at 440 gigapascals might be related to an electronic transition associated with pressure-induced interactions between core electrons. The ability to affect the core electrons under static high-pressure experimental conditions, even for incompressible metals such as Os, opens up opportunities to search for new states of matter under extreme compression.

The elastic properties of fcc Fe-Mn-X (X = Cr, Co, Ni, Cu) alloys with additions of up to 8 at.% X were studied by combinatorial thin film growth and characterization and by ab initio calculations using the disordered local moments (DLM) approach. The lattice parameter and Youngs modulus values change only marginally with X. The calculations and experiments are in good agreement. We demonstrate that the elastic properties of transition metal alloyed Fe-Mn can be predicted by the DLM model.

The influence of the valence electron concentration of X in fcc Fe-Mn-X (X=Cr, Co, Ni, Cu) alloys on the elastic and magnetic properties has been studied by means of ab initio calculations for alloy element concentrations of up to 8 at. % X. We observe that Cu increases the bulk-to-shear modulus (B/G) ratio by 19.2%. Simultaneously magnetic moments of Fe and Mn increase strongly. The other alloying elements induce less significant changes in B/G. The trends in B/G may be understood by considering the changes in G induced by the variation in valence electron concentration (VEC). As the VEC is increased, more pronounced metallic bonds are formed, giving rise to smaller shear modulus values. The changes in magnetic moments may be explained by the magnetovolume effect. Alloys with smaller VEC as Fe-Mn exhibit a decrease in local magnetic moments and equilibrium lattice parameters, while alloys with larger VEC as Fe-Mn demonstrate an increase in local magnetic moments and equilibrium lattice parameters. These data provide the basis for the design of Mn-rich steels with enhanced elastic properties.

Magnetic and elastic properties of Ni metal have been studied up to 260 GPa by nuclear forward scattering of synchrotron radiation with the 67.4 keV Mossbauer transition of Ni-61. The observed magnetic hyperfine splitting confirms the ferromagnetic state of Ni up to 260 GPa, the highest pressure where magnetism in any material has been observed so far. Ab initio calculations reveal that the pressure evolution of the hyperfine field, which features a maximum in the range of 100 to 225 GPa, is a relativistic effect. The Debye energy obtained from the Lamb-Mossbauer factor increases from 33 me V at ambient pressure to 60 me V at 100 GPa. The change of this energy over volume compression is well described by a Gruneisen parameter of 2.09.

We discover that hcp phases of Fe and Fe0.9Ni0.1 undergo an electronic topological transition at pressures of about 40 GPa. This topological change of the Fermi surface manifests itself through anomalous behavior of the Debye sound velocity, c/a lattice parameter ratio, and Mossbauer center shift observed in our experiments. First-principles simulations within the dynamic mean field approach demonstrate that the transition is induced by many-electron effects. It is absent in one-electron calculations and represents a clear signature of correlation effects in hcp Fe. DOI: 10.1103/PhysRevLett.110.117206