MnP-type Structures Research Articles

Tuning the Local Coordination of CoP1-xSx between NiAs- and MnP-Type Structures to Catalyze Lithium-Sulfur Batteries.

The slow conversion and rapid shuttling of polysulfides remain major challenges that hinder the practical application of lithium-sulfur (Li-S) batteries. Efficient catalysts are needed to accelerate the conversion and suppress the shuttling. However, the lack of a rational understanding of catalysis poses obstacles to the design of catalysts, thereby limiting the rapid development of Li-S batteries. Herein, we theoretically analyze the modulation of the electronic structure of CoP1-xSx caused by the NiAs-to-MnP-type transition and its influence on catalytic activity. We found that the interacting d-orbitals of the active metal sites play a determining role in adsorption and catalysis, and the optimal dz2-, dxz-, and dyz-orbitals in an appropriately distorted five-coordinate pyramid enable higher catalytic activity compared with their parent structures. Finally, rationally designed catalysts and S were electrospun into carbonized nanofibers to form nanoreactor chains for use as cathodes. The resultant Li-S batteries exhibited superior properties over 1000 cycles with only a decay rate of 0.031% per cycle and demonstrated a high capacity of 887.4 mAh g-1 at a high S loading of 10 mg cm-2. The structural modulation and bonding analyses in this study provide a powerful approach for the rational design of Li-S catalysts.

Pressure-Induced Structural Phase Transition, Anomalous Insulator-to-Metal Transition, and n-p Conduction-Type Switching in Defective, NiAs-Type Cr1-δTe.

Cr1-δTe, as a unique series of defective compounds with a NiAs-type structure and periodic metal vacancies, has attracted intensive research interest because of its diversity in structure and property dependent on the tunable stoichiometric ratio. Another feature of these compounds is their ability to switch between NiAs- and MnP-type structures, in which there often exist composition-, temperature-, or pressure-induced phase transitions accompanied by intriguing physical property switching. Herein, we report the syntheses of a series of Cr1-δTe compounds with similar compositions but distinct crystal structures, their phase transitions, anomalous transport, and photoelectric conduction behaviors under high pressure (HP). For the three Cr1-δTe compounds with δ = 0, 0.25, 0.375, CrTe undergoes pressure-induced NiAs-to-MnP phase transition at around 15 GPa, while Cr3Te4 and Cr5Te8 undergo isostructural phase transitions at around 12 and 11 GPa, respectively. Electrical transport measurements indicate anomalous conduction behaviors: CrTe undergoes a semiconductor-to-metal transition at around 24 GPa, while Cr3Te4 and Cr5Te8 show unexpected metal-semiconductor-metal transitions sequentially upon compression. Besides, CrTe and Cr5Te8 exhibit pressure-induced n-p conduction-type switching at around 9 and 3 GPa, respectively, while Cr3Te4 shows p-type conductivity in the full pressure range. Local structure analyses based on in situ HP Raman spectra are performed to understand the structure and property evolutions of Cr1-δTe under HP. Defective transition-metal chalcogenides with pressure-induced NiAs-to-MnP phase transition and conduction-type conversion provide a potential platform for the rational design of photoelectric conversion devices.

Structure stabilization effect of vacancies and entropy in hexagonal WN.

The structural stability of hexagonal tungsten mononitride (WN) has been studied combining scanning transmission electron microscopy and first-principles calculations. The results show that the WC-type WN with vacancies of 6∼8 at% is more stable than the previously proposed MnP-type and NiAs-type structures. Due to the larger vibrational entropy of the WC-type WN, the vacancy concentration required to stabilize the WC-type structure is lower at high temperatures. The results demonstrate the importance of vacancies and configurational and vibrational entropies in the structural stability of compounds synthesized at high temperatures.

Multiple band crossings and Fermi surface topology: Role of double nonsymmorphic symmetries in MnP-type crystal structures

We use relativistic ab initio methods combined with model Hamiltonian approaches to analyze the normal-phase electronic and structural properties of the recently discovered WP superconductor. Remarkably, the outcomes of such study can be employed to set fundamental connections among WP and the CrAs and MnP superconductors belonging to the same space group. One of the key features of the resulting electronic structure is represented by the occurrence of multiple band crossings along specific high-symmetry lines of the Brillouin zone. In particular, we demonstrate that the eightfold band degeneracy obtained along the $\mathit{SR}$ path at $({k}_{x},{k}_{y})=(\ensuremath{\pi},\ensuremath{\pi})$ is due to inversion time-reversal invariance and a pair of nonsymmorphic symmetries. The presence of multiple degenerate Fermi points along the $\mathit{SR}$ direction constrains the topology of the Fermi surface, which manifests distinctive marks when considering its evolution upon band filling variation. If the Fermi level crosses the bands along the $\mathit{SR}$ line as it happens at the nominal filling of the MnP, these Fermi surfaces are open or closed Fermi pockets. Moving the relative position of the Fermi level away from the eightfold degenerate bands as for the WP and CrAs compounds, the electronic changeover exhibits a simultaneous modification of the Fermi-surface dimensionality and topology. Four two-dimensional (2D) Fermi-surface sheets are centered around the $\mathit{SR}$ line with a corrugated profile along the ${k}_{z}$ direction. Moreover, we show that the spin-orbit interaction determines a selective removal of the band degeneracy and, consequently, a splitting of the quasi-2D Fermi sheets, as it happens in WP. Finally, we comment on the connections between our results and recent experimental and theoretical proposals about the triplet superconductivity in this class of compounds.

Pressure-induced development of bonding in NiAs type compounds and polymorphism of NiP

A reversible, displacive, pressure-induced structural phase transition has been found to occur in nickel monophosphide NiP at approximately 3.5 GPa by means of in situ synchrotron single-crystal X-ray diffraction. The new phase, with Pearson symbol oC56, assumes an orthorhombic structure with Cmc2 1 space group and unit cell parameters a=23.801(2) Å, b=5.9238(6) Å, and c=4.8479(4) Å at 5.79 GPa. The high-pressure phase is a superstructure of the ambient, oP16 phase with multiplicity of 3.5. The phosphorous sublattice gradually converts from the net of isolated P 2 dimers found in the ambient NiP, towards zig-zag polymeric P ∞ chains found in MnP-type structures. The transformation involves development of triatomic phosphorous clusters and interconnected Ni slabs with diamondoid topology. The high-pressure phase, which represents intermediate polymerization step, is a commensurately modulated superstructure of the NiAs aristotype. The phase transformation in NiP bears resemblance to the effect of successive substitution of Si or Ge in place of P found in the series of stoichiometric inhomogeneous linear structures in ternary NiP 1− x Si x and NiP 1− x Ge x systems.

New type of half-metallic antiferromagnet: transition metal pnictides

The electronic structures of transition metal pnictidesABX2, where A and B are the transition metal elements andX = N, P, As, Sb, and Bi, with the total valence d-electron number of the transition metal ionsbeing ten, are investigated in the framework of the first-principles KKR Green’sfunction method. Some possible crystal structures such as NiAs-type, NaCl-type,chalcopyrite, zinc-blende, wurtzite, and MnP-type structures are assumed. Similarly tochalcogenides, a new type of spin-compensated half-metallic ferrimagnet is found forthe case of nitrides. The stability and magnetic transition temperature of thesenitrides indicate that they are good candidates for spintronics materials. For othercases of pnictides such as P, As, Sb, and Bi, the half-metallicity seems not to berealized.

Pressure-induced structural and magnetic phase transition in ferromagnetic CrTe

Self-consistent electronic structure calculations were carried out for ferromagnetic CrTe in NiAs and MnP structure by means of tight-binding linear muffin–tin orbital method. The relative stabilities of the high-pressure phases were studied by means of total energy curves. From the curves it can be clearly seen that NiAs-type CrTe is the stable phase in agreement with the experimental observation. Under pressure, a crystallographic transition from NiAs- to MnP-type occurs around 11.3 GPa. In addition, a magnetic transition from ferromagnetic to non-magnetic state is predicted around 45.3 GPa. The band structure and density of states are plotted for the ferromagnetic (NiAs- and MnP-type structures) and non-magnetic states. The basic ground state properties such as the equilibrium lattice constant, bulk modulus are calculated and compared with the available literature. The possible reason for the structural phase transition is also discussed.

Structures of the ternary iron germanium pnictides FeGe 1− xPn x (Pn=P, As, Sb)

The pseudo-binary FeGe–FePn (Pn=P, As, Sb) systems have been investigated by X-ray diffraction methods. The phosphide and arsenide systems form ternary solid solutions FeGe 1− x P x ( 0.4≤x≤1.0) and FeGe 1− x As x ( 0.3≤x≤1.0) crystallizing in MnP-type structures. The antimonide system is more complex, forming intermetallic compounds that display incipient ordering of Sb 2 pairs as they are stuffed into the FeGe ( x=0) structure while Ge atoms are gradually removed. The structures of an intermediate member FeGe 0.80Sb 0.20 ( x=0.20) and of the limiting member FeGe 0.67Sb 0.33 ( x=0.33) in this progression were determined by single-crystal X-ray diffraction. Both compounds crystallize in hexagonal space groups: P6 3/mmc , a=8.796(2), c=8.004(2) Å, Z=6 for Fe 3Ge 2.4Sb 0.6 (or FeGe 0.80Sb 0.20), and P6/mmm, a=8.9885(5), c=7.9043(6) Å, Z=6 for Fe 3Ge 2Sb (or FeGe 0.67Sb 0.33). Beyond this point, ternary solid solutions FeGe 1− x Sb x ( 0.4≤x≤1.0) are formed, crystallizing in NiAs-type structures.

Spin flip and troilite-MnP structure transition in FeS as studied by Mössbauer spectroscopy

Synthetic FeS (troilite) was analyzed by Mössbauer spectroscopy in the temperature range 283 ⩽ T⩽ 460 K. Spectra were analyzed applying the full Hamiltonian. The α transition between the troilite and MnP-type structures was completed at 413 K on heating. Compared with literature data of a meteoritic troilite, the behavior differed as follows: the transition occurred without hysteresis, with a steeper slope in an MnP structure concentration ([MnP]) vs temperature plot, with a lower [MnP] at room temperature, and with heating and cooling trends coinciding. The spin flip in the MnP structure occurred rapidly and is suggested to be coupled to the ease of the α transition. The absence of trace elements, e.g. Cr, in the synthetic sample is suggested to be of importance to the rapid spin flip and α transition. A reported increase in compressibility of troilite with increased pressure is suggested to be due to a successive weakening of the Fe 3 cluster bond strength. The relatively scarce occurrence of electron transfer enzymes with reaction centres with three Fe atoms is suggested to be due to an evolutionary drawback caused by the possibility of forming energetically stable Fe 3 clusters unsuitable for reaction.

Structural and magnetic properties of Mn 1− tNi tAs

The phase diagram of Mn 1- t Ni t As shows a variety of transitions between states with different magnetic and crystal structure properties. These structures and the corresponding phase transitions are investigated by powder X-ray and neutron diffraction, DSC and magnetic susceptibility measurements as functions of composition ( t), temperature, external pressure and applied magnetic field. At high temperatures complete solid solution of an NiAs type, paramagnetic state exists over the entire composition range. At and below room temperature the crystal structures of the solid solution phases are of the regular NiAs, MnP and NiAs types for the composition intervals 0.00⩽ t⩽0.025±0.005, 0.025±0.005⩽ t⩽0.22±0.02 and 0.65± 0.04⩽ t⩽1.00, respectively. For 0.22±0.02< t<0.65±0.04 below about 650 K incommensurate superstructures, resulting from ordering of the Mn and Ni atoms, are imposed on the basic NiAs and MnP type structures. Helimagnetic ordering of the H a type is found for the MnP type solid solution phase at temperatures below ≈200 K. In the superstructural range, antiferromagnetism prevails for 0.30⩽ t⩽0.50 at low temperatures. The full hysteresis region between NiAs, ferro, MnP, para and MnP, heli ( H a ) type states for compositions close to MnAs is studied in magnetic fields up to 250 kOe and for external pressures up to 10 kbar.

Room- and high-pressure crystal chemistry of CoAs and FeAs

The MnP-type crystal structures of FeAs and CoAs have been studied at both room and high pressure. In the room-pressure study, refinements of both structures were carried out in space groups Pnam and Pna2~ using single-crystal integrated X-ray intensity data collected from a four-circle diffractometer. For CoAs, both space groups refined to a weighted residual, Rw = 0-054, whereas for FeAs, Rw = 0.030 for Pnam and 0-027 for Pna2~. The Hamilton significance test, applied to FeAs, shows a significant improvement in Rw for Pna2,. However, the Hamilton test is not a conclusive test for space-group identity. Comparison of interatomic distances, derived from Pnam and Pna2~ refinements for both CoAs and FeAs, shows no significant change in the structures. High-pressure X-ray data were collected from single crystals enclosed in a Merrill & Bassett-type diamond-anvil cell. Cell parameters and structures were refined at 2.80 (5), 3.70 (5), and 4.90 (5) GPa for CoAs, and at 2.25(5), 4.36 (5), 6-10(5), and 8.3(3) GPa for FeAs. Cell data were used to determine the elastic constants, isothermal bulk modulus and its pressure derivative (Kr and K~r, respectively), by least-squares fitting of the unit-cell volumes to a thirdorder Birch-Murnaghan equation of state. This gave Kr = 123 (6) GPa and K~r = 88(33) for CoAs. For FeAs, KT = 118 (4) GPa and K~r = 72 (1.8). Refinements in Pnam show that the structure remains an MnP-type throughout these pressure ranges. A precession photograph of CoAs at 7.8 GPa revealed previously sharp split and diffuse diffraction spots. Similar effects are known to occur in FeS when it undergoes a change from an MnP-type structure to a high-pressure phase above 6.8 GPa. Thus, these effects in CoAs may indicate the onset of a phase transition. Changes in both interatomic distances and atomic displacements from positions in an NiAs-type structure reveal some interesting aspects of the anisotropic compression these compounds undergo. These changes are correlated with high-pressure phase transitions that may occur in MnP-type structures.

Nouveau phosphure ternaire NiNbP 2 a liaison nickel-nickel et nickel-niobium et compose isotype NiTaP 2

The phosphides NiNbP 2 and NiTaP 2 crystallize in an orthorhombic unit-cell, space group Pnma and Z = 4. The X-ray structure of NiNbP 2 was determined from four-dimensional single crystal counter data and was refined to a final R value of 0.031 for 475 independent reflections. It shows square-pyramidal and 7-prismatic phosphorus coordination for Ni and Nb atoms respectively, and moreover exhibits zigzag and regular Ni-Ni chains bonded together by means of mixed irregular and almost linear Ni-Nb-Ni chains. The structure is derived from MoP 2 type and may be also considered as a mixture of MnP and WC type structures. Magnetic and electrical measurements indicate a metallic behavior of these new phosphides.

On Phase Transitions between the MnP and NiAs Type Structures.

On Phase Transitions between the MnP and NiAs Type Structures.