二氢化镁（$\mgh$）储存7.7重量%的氢，但其具有高的热力学稳定性和缓慢的氢化（脱氢化）动力学。Mg与轻过渡金属（TM = Sc、Ti、V、Cr）合金化旨在改善热力学和动力学性能。采用第一性原理计算，在密度泛函理论水平上研究了Mg $_x$ TM $_{1-x}$ H $_2$化合物[$x=[0$ -1]的结构和稳定性.我们发现，实验观察到的急剧下降的氢化速率为$x\gtrsim0.8$与相转变的镁$_x$ TM $_{1-x}$ H $_2$从萤石相的金红石相。这些化合物的稳定性沿着Sc、Ti、V、Cr系列降低。改变过渡金属（TM）和组成$x$，Mg $_x$ TM $_{1-x}$ H $_2$的形成焓可以在0-2 eV/f. u的基本范围内调节。然而，假设合金Mg $_x$ TM $_{1-x}$在脱氢时不分解，则与具有高镁含量的化合物（$x=0.75$）的可逆氢化相关的焓接近于纯Mg的焓。
英文摘要：
Magnesium dihydride ($\mgh$) stores 7.7 weight % hydrogen, but it suffers from a high thermodynamic stability and slow (de)hydrogenation kinetics. Alloying Mg with lightweight transition metals (TM = Sc, Ti, V, Cr) aims at improving the thermodynamic and kinetic properties. We study the structure and stability of Mg$_x$TM$_{1-x}$H$_2$ compounds, $x=[0$-1], by first-principles calculations at the level of density functional theory. We find that the experimentally observed sharp decrease in hydrogenation rates for $x\gtrsim0.8$ correlates with a phase transition of Mg$_x$TM$_{1-x}$H$_2$ from a fluorite to a rutile phase. The stability of these compounds decreases along the series Sc, Ti, V, Cr. Varying the transition metal (TM) and the composition $x$, the formation enthalpy of Mg$_x$TM$_{1-x}$H$_2$ can be tuned over the substantial range 0-2 eV/f.u. Assuming however that the alloy Mg$_x$TM$_{1-x}$ does not decompose upon dehydrogenation, the enthalpy associated with reversible hydrogenation of compounds with a high magnesium content ($x=0.75$) is close to that of pure Mg.
The family of binary compounds composed of magnesium and silicon is rather rich. In addition to the well-known magnesium silicide Mg$_2$Si, other compounds, including MgSi$_2$, Mg$_4$Si$_7$, Mg$_5$Si$_6$, MgSi, and Mg$_9$Si$_5$, have also been identified and/or proposed in precipitated Al-Mg-Si solid solutions. Nevertheless, computational studies show that only Mg$_2$Si is thermodynamically stable at ambient conditions while certain non-zero hydrostatic pressure can stabilize Mg$_9$Si$_5$ so that it can co-exist with Mg$_2$Si. We conduct a comprehensive search for viable binary compounds of Mg…
Being a lithophile element at ambient pressure, magnesium is long believed to be immiscible with iron. A recent study by Gao et al. [1] showed that pressure turns magnesium into a siderophile element and can produce unconventional Fe-Mg compounds. Here, we extend the investigation to exoplanetary pressure conditions using an adaptive genetic algorithm-based variable-composition structural prediction approach. We identify several Fe-Mg phases up to 3 TPa. Our cluster alignment analysis reveals that most of the predicted Fe-Mg compounds prefer a BCC packing motif at terapascal pressures. This st…
We have synthesized magnesium cage compounds $R$Ni$_{2}$Mg$_{20}$ ($R$ = Pr and Nd). We report the measurements of electrical resistivity $ρ$, magnetic susceptibility $χ$, isothermal magnetization $M$, and specific heat $C$. Polycrystalline samples with $R$ = Pr and Nd were obtained by annealing, while the counterparts for $R$ = La, Ce, and Y were found to be absent. The $ρ(T)$ data for $R$ = Pr and Nd monotonically decrease on cooling from 300 K to 40 K and exhibit shoulders at around 13 K and 15 K, respectively. The Curie-Weiss behaviors of the $χ(T)$ data indicate the trivalent states of bo…
Dimethylammonium magnesium formate, [(CH$_3$)$_2$NH$_2$][Mg(HCOO)$_3$] or DMAMgF, is a model to study high temperature hybrid perovskite-like dielectrics. This compound displays a phase transition from para to ferroelectric at about 260~K. Using multifrequency electron spin resonance in continuous wave and pulsed modes, we herein present the quantum dynamic of Mn$^{2+}$ ion probe in DMAMgF. In the high temperature paraelectric phase, we observe a large distribution of the zero field splitting that is attributed to high local disorder and further supported by DFT computations. In the low temper…
Based on first-principles calculations, the anomalous Hall (AHC) and anomalous Nernst conductivities (ANC) of the XMnP (X=Ti, Zr, Hf) compounds were evaluated, and the possibility to tailor such properties by changing the magnetization directions was also investigated. We observed that nodal lines and small gap areas consist the dominant contributions to AHC and ANC, where the significant variations in AHC and ANC when altering the magnetization direction originate from the whole Brillouin zone. Our studies gave a promising clue on engineering magnetic intermetallic compounds for tunable trans…
Calcium and magnesium carbonates are believed to be the host compounds for most of the oxidized carbon in the Earth's mantle. Here, using evolutionary crystal structure prediction method USPEX, we systematically explore the MgO-CO2 and CaO-CO2 systems at pressures ranging from 0 to 160 GPa to search for thermodynamically stable magnesium and calcium carbonates. While MgCO3 is the only stable magnesium carbonate, three calcium carbonates are stable under pressure: well-known CaCO3, and newly predicted Ca3CO5 and CaC2O5. Ca3CO5 polymorphs are found to contain isolated orthocarbonate (CO4)4- tetr…
New allotropes of two-dimensional (2D) GaP and InP binary compounds were predicted from the structural optimization and dynamical stability analysis in the framework of the density functional calculations. These stabilized GaP and InP monolayers possess unique high buckled orthorhombic lattices symmetry with their cohesive energies of ~0.14 and ~0.17 eV/pair lower than those of 2D low buckled honeycomb GaP and InP binary compounds, demonstrating that these newly predicted 2D GaP and InP binary compounds are energetically preferential. More interesting, their energy band gaps are even ~ 0.52 eV…
Topological materials have been discovered so far largely by searching for existing compounds in crystallographic databases, but there are potentially new topological materials with desirable features that have not been synthesized. One of the desirable features is high tunability resulted from the band inversion with a very small direct band gap, which can be tuned by changes in pressure or strain to induce a topological phase transition. Using density functional theory (DFT) calculations, we have studied the septuple layered AB2X4 series compounds, where A=(Ge, Sn and Pb), B=(As, Sb and Bi) …
The symmetry of local moments plays a defining role in the nature of exotic grounds states stabilized in frustrated magnetic materials. We present inelastic neutron scattering (INS) measurements of the crystal electric field (CEF) excitations in the family of compounds MgRE$_2$Se$_4$ (RE $\in$ $\{$Ho, Tm, Er and Yb$\}$). These compounds form in the spinel structure, with the rare earth ions comprising a highly frustrated pyrochlore sublattice. Within the symmetry constraints of this lattice, we fit both the energies and intensities of observed modes in the INS spectra to determine the most lik…
Heusler compounds are a large family of binary, ternary and quaternary compounds that exhibit a wide range of properties of both fundamental and potential technological interest. The extensive tunability of the Heusler compounds through chemical substitutions and structural motifs makes the family especially interesting. In this article we highlight recent major developments in the field of Heusler compounds and put these in the historical context. The evolution of the Heusler compounds can be described by four major periods of research. In the latest period, Heusler 4.0 has led to the observa…
Doping tunable charge density waves in misfit layer compounds
Institut des Nanosciences de Paris（巴黎纳米科学研究所）；Universität Würzburg（维尔茨堡大学）；Institut des Matériaux de Nantes Jean Rouxel（南特让·鲁克塞尔材料研究所）；University of Trento（特伦托大学）；Pavol Jozef Šafárik University（帕沃尔·约瑟夫·沙法里克大学）
Building on discoveries in graphene and two-dimensional (2D) transition metal dichalcogenides, van der Waals (VdW) layered heterostructures - stacks of such 2D materials - are being extensively explored with resulting new discoveries of novel electronic and magnetic properties in the ultrathin limit. Here we review a class of naturally occurring heterostructures - so called misfits - that combine disparate VdW layers with complex stacking. Exhibiting remarkable structural complexity and diversity of phenomena, misfits provide a platform on which to systematically explore the energetics and loc…
Recent interest in topological semimetals has lead to the proposal of many new topological phases that can be realized in real materials. Next to Dirac and Weyl systems, these include more exotic phases based on manifold band degeneracies in the bulk electronic structure. The exotic states in topological semimetals are usually protected by some sort of crystal symmetry and the introduction of magnetic order can influence these states by breaking time reversal symmetry. Here we show that we can realize a rich variety of different topological semimetal states in a single material, $\rm CeSbTe$. …
We report the computational investigation of a series of ternary X$_4$Y$_2$Z and X$_5$Y$_2$Z$_2$ compounds with X={Mg, Ca, Sr, Ba}, Y={P, As, Sb, Bi}, and Z={S, Se, Te}. The compositions for these materials were predicted through a search guided by machine learning, while the structures were resolved using the minima hopping crystal structure prediction method. Based on $\textit{ab initio}$ calculations, we predict that many of these compounds are thermodynamically stable. In particular, 21 of the X$_4$Y$_2$Z compounds crystallize in a tetragonal structure with $\textit{I-42d}$ symmetry, and e…
The quaternary arsenide compounds XCuYAs (X=Zr, Hf; Y= Si, Ge) belong to the vast family of the 1111-type quaternary compounds, which possess outstanding physical properties ranging from $p$-type transparent semiconductors to high-temperature Fe-based superconductors. In this paper, we study the electronic structure topology, spin Hall effect (SHE) and spin Nernst effect (SNE) in these compounds based on density functional theory calculations. First we find that the four considered compounds are Dirac semimetals with the nonsymmorphic symmetry-protected Dirac line nodes along the Brillouin zon…
AB5 compounds (A = rare earth, B = transition metal) have been widely studied as anodes for Ni-MH applications. However, they have reached their technical limitations and the search for new promising materials with high capacity is foreseen. ABy compounds (2 < y < 5) are good candidates. They are made by stacking [AB5] and [A2B4] units along the c crystallographic axis. The latter unit allows a large increase in capacity, while the [AB5] unit provides good cycling stability. Consequently, the AB3.8 composition (i.e. A5B19 with three [AB5] for one [A2B4]) is expected to exhibit better cycling s…
Field-tunable partial antiferromagnetism, glassy spin dynamics, and magnetodielectric coupling in the quasi-one-dimensional spin-chain compound Ca3CoIrO6
Compound-tunable embedding potential (CTEP) method is proposed. A fragment of some chemical compound, "main cluster" in the present paper, is limited by boundary anions such that the nearest environmental atoms are cations. The CTEP method is based on constructing the embedding potential as linear combination of short-range "electron-free" spherical "tunable" pseudopotentials for cations from nearest environment of the main cluster, whereas the long-range CTEP part consists of Coulomb potentials from optimized fractional point charges centered on both environmental cations and anions. A pilot …
Herein, we demonstrate that first-principles calculations can be used for mapping electronic properties of two-dimensional (2d) materials with respect to non-uniform strain. By investigating four representative single-layer 2d compounds with different symmetries and bonding characters, namely 2d-${MoS_2}$, phosphorene, $α$-Te, and $β$-Te, we reveal that such a mapping can be an effective guidance for advanced strain engineering and development of strain-tunable nanoelectronics devices, including transistors, sensors, and photodetectors. Thus, we show that $α$-Te and $β$-Te are considerably mor…
Magnesium hydride owns the largest share of publications on solid materials for hydrogen storage. The Magnesium group of international experts contributing to IEA Task 32 Hydrogen Based Energy Storage recently published two review papers presenting the activities of the group focused on magnesium hydride based materials and on Mg based compounds for hydrogen and energy storage. This review article not only overviews the latest activities on both fundamental aspects of Mg-based hydrides and their applications, but also presents a historic overview on the topic and outlines projected future deve…
The hydrogenation and deuteration of graphite with potassium intercalation compounds (GICs) as starting materials was investigated. Characterization of the reactions products (hydrogenated and deuterated graphene) was carried out by thermogravimetric analysis coupled with mass spectrometry (TG-MS) and Raman spectroscopy including statistical Raman spectroscopy (SRS) and -microscopy (SRM). The results reveal that the choice of the hydrogen/deuterium source, the nature of the graphite (used as starting material), the potassium concentration in the intercalation compound as well as the choice of …
Tunable Magnetic Frustration in the Cu-Ru-based Double Perovskite La$_{2-x}$Sm$_x$CuRuO$_6$ (x = 0, 1, 2) Oxides
S. N. Bose National Centre for Basic Sciences（S.N. 玻色基础科学国家中心）；M. V. Lomonosov Moscow State University（莫斯科国立罗蒙诺索夫大学）；National University of Science and Technology MISiS（俄罗斯科学与技术国立大学 MISiS）；CSIR-National Physical Laboratory（CSIR-国家物理实验室）；Academy of Scientific and Innovative Research (AcSIR)（科学与创新研究学院 (AcSIR)）；UGC-DAE Consortium for Scientific Research（UGC-DAE 科学研究联盟）
PdCoO2 , belonging to a family of triangular oxides called delafossite, is one of the most conducting oxides. Its in-plane conductivity is comparable to those of the best metals, and exhibits hydrodynamic electronic transport with extremely long mean free path at cryogenic temperatures. Nonetheless, it is nonmagnetic despite the presence of the cobalt ion. Here, we show that a mild hydrogenation process reduces PdCoO2 thin films to an atomically-mixed alloy of PdCo with strong out-of-plane ferromagnetism and sign-tunable anomalous Hall effect. Considering that many other compounds remain littl…
Electronic structure study of YNbTiO$_6$ vs. CaNb$_2$O$_6$ with U, Pu and minor actinide substitutions using compound-tunable embedding potential method
The ZrSiS-type compounds are Dirac semimetals and have been attracting considerable interest in recent years due to their topological electronic properties and possible applications. In particular, gapped Dirac nodes can possess large spin Berry curvatures and thus give rise to large spin Hall effect (SHE) and spin Nernst effect (SNE), which may be used to generate pure spin current for spintronics and spin caloritronics without applied magnetic field or magnetic material. In this paper we study both SHE and SNE in ZrXY (X = Si, Ge; Y = S, Se, Te) based on \textit{ab initio} relativistic band …
Despite many years of efforts, attempts to reach the quantum regime of topological surface states (TSS) on an electrically tunable topological insulator (TI) platform have so far failed on binary TI compounds such as Bi2Se3 due to high density of interfacial defects. Here, utilizing an optimal buffer layer on a gatable substrate, we demonstrate the first electrically tunable quantum Hall effects (QHE) on TSS of Bi2Se3. On the n-side, well-defined QHE shows up, but it diminishes near the charge neutrality point (CNP) and completely disappears on the p-side. Furthermore, around the CNP the syste…
We report the first detailed investigation of K$_2$MnS$_2$ and K$_2$MnSe$_2$ from the K$_2$MnS$_2$ structure type and their magnetic solid solution K$_2$MnS$_{2-x}$Se$_x$ and find that compounds of this structure type consist of strongly-coupled pseudo-one-dimensional antiferromagnetic chains that collectively represent a frustrated two-dimensional triangular antiferromagnet. Bulk samples of K$_2$MnS$_{2-x}$Se$_x$ with $0 \leq x \leq 2$ are characterized using X-ray diffraction, neutron diffraction, magnetization and heat capacity measurements. An incommensurate cycloid magnetic structure with…
A Kagome-Derived Mosaic Lattice Family A3V9Te13 (A = Cs, Rb) with Tunable Strong Electronic Correlations
Tsung-Dao Lee Institute, Shanghai Jiao Tong University（上海交大李政道研究所）；School of Physics and Astronomy, Shanghai Jiao Tong University（上海交通大学物理与天文学院）；Shenzhen Institute for Quantum Science and Engineering, Southern University of Science and Technology（南方科技大学量子科学工程学院）；International Quantum Academy（国际量子科学院）
Compound-tunable embedding potential method to model local electronic excitations on $f$-element ions in solids: Pilot relativistic coupled cluster study of Ce and Th impurities in yttrium orthophosphate, YPO$_4$
High pressure is a proven effective tool for modulating inter-layer interactions in semiconducting transition metal dichalcogenides, which leads to significant band structure changes. Here, we present an extended infrared study of the pressure-induced semiconductor-to-metal transition in 2H-MoTe2, which reveals that the metallization process at 13-15 GPa is not associated with the indirect band gap closure, occuring at 24 GPa. A coherent picture is drawn where n-type doping levels just below the conduction band minimum play a crucial role in the early metallization transition. Doping levels ar…
We report a high-throughput ab-initio study of the thermodynamic and superconducting proper- ties of the recently synthesized XB$_3$C$_3$ clathrates. These compounds, in which boron and carbon form a sponge-like network of interconnected cages each enclosing a central X atom, are attractive candidates to achieve high-Tc conventional superconductivity at ambient pressure, due to the simultaneous presence of a stiff B-C covalent network and a tunable charge reservoir, provided by the guest atom. Ternary compounds like CaB$_3$C$_3$, SrB$_3$C$_3$ and BaB$_3$C$_3$ are predicted to exhibit T$_c$ $\l…
We report on the synthesis and superconductivity of high-entropy-alloy-type (HEA-type) compounds TrZr2 (Tr = Fe, Co, Ni, Rh, Ir), in which the Tr site satisfies the criterion of HEA. Polycrystalline samples of HEA-type TrZr2 with four different compositions at the Tr site were synthesized by arc melting method. The phase purity and crystal structure were examined by Rietveld refinement of X-ray diffraction profile. It has been confirmed that the obtained samples have a CuAl2-type tetragonal structure. From analyses of elemental composition and mixing entropy at the Tr site, the HEA state for t…
