Cmcm Structure Research Articles

Stability and properties of six new Ru3Al structures: A first-principles study of mechanical, electronic, and superconducting properties

Ruthenium–aluminium (Ru–Al) alloys demonstrate excellent properties; however, the Ru3Al structure has a higher formation energy than those of other Ru–Al structures. Herein, to identify more stable Ru3Al structures, we employed the CALYPSO code and discovered six stable structures: one hexagonal structure, two monoclinic structures and three orthorhombic structures. First-principles calculations and the density functional theory were used to assess their mechanical, electrical and superconducting properties. The formation energies of these newly discovered Ru3Al structures were lower than those of previously proposed structures, confirming their thermodynamic stability. Elastic constant calculations indicated that all the structures were mechanically stable and ductile. Moreover, electronic structure analyses revealed that the structures were metallic, primarily owing to the contribution of Ru d orbitals. Phonon dispersion and density of states analyses also confirmed the dynamic stability of the structures, with low-frequency phonons contributing considerably to electron–phonon coupling. The superconducting transition temperatures (Tc) of the structures were calculated using the Allen–Dynes-modified McMillan equation. The Cmcm structure showed the highest Tc of 3.99 K, and the P2/c structure showed the lowest Tc of 1.56 K. Overall, this study identified six novel and stable Ru3Al structures with good mechanical and superconducting properties, potentially advancing the development of Ru–Al-based superconductors.

Pressure-induced phase transitions of intermetallic TMBe2 (TM = Zr and Hf) from first-principles calculations

High-pressure stable structures of intermetallic TMBe2 (TM = Zr and Hf) have been fully explored up to 40 GPa using particle swarm structure searches combined with first-principles calculations. The resulting findings show that both intermetallics undergo a structure transition from the ambient-pressure layered AlB2-type structure to a uniform orthorhombic Cmcm structure at 4.5 GPa and 13.8 GPa, respectively. The significant volume collapses under high pressure indicate that the structural transitions of two intermetallics are typical first-order. During the pressure-induced phase transition process, the formation of newly folded Be hexagonal rings in the high-pressure Cmcm structure with unusual change of coordination numbers of TM atoms was attributed to the alternating distortions of the layer-like Be hexagonal rings in ambient AlB2-type structure. The contributions of atomic orbitals near the Fermi level changes correspondingly before and after the structural phase transition, demonstrated by an analyses of electronic band structures and density of states. An obvious degeneracy of TM-d orbitals in the new high-pressure Cmcm structure was further revealed. Additionally, a systematic comparative analysis of mechanical parameters and anisotropic distributions of TMBe2 intermetallics with two structures was performed.

Exploration of novel structures and related physical properties of Fe2Pt ordered alloys

The Fe–Pt phase diagram suggests the possible existence of ordered Fe2Pt structures, however, the ordered structures of Fe2Pt and related physical properties are unclear until now. In this work, four new low-energy ferromagnetic Fe2Pt ordered structures are predicted based on the particle swarm optimization algorithm. Their structural stability is verified by the formation enthalpy, mechanical stability and lattice dynamic stability. The order of the thermodynamic stability is Cmcm > Immm > I4/mmm > P3¯m1. All four structures are dynamically (except tetragonal I4/mmm structure) and mechanically stable. The fundamental physical properties including magnetic, elastic, phonon and thermal properties are investigated in detail. Their total magnetic moments, as well as the bulk modulus are almost the same. The shear modulus and Young's modulus of Cmcm structure are close to the P3¯m1, and larger than that of other two structures. The order of the elastic anisotropy is Immm > P3¯m1 > I4/mmm > Cmcm. The orthorhombic Immm structure can be regarded as a distorted tetragonal I4/mmm structure, and the distortion effectively improves the physical properties of the I4/mmm structure. The thermal properties at finite temperatures show that the trigonal P3¯m1 structure is more suitable for high-temperature applications compared to other structures. The results of linear thermal expansion coefficient indicate that anisotropy order of thermal expansion is Immm > I4/mmm > Cmcm > P3¯m1. More interesting is the discovery of the Invar effect along the y-axis direction in orthorhombic Immm structure.

Flux Growth and Characterization of Bulk InVO4 Crystals

The flux growth of InVO4 bulk single crystals has been explored for the first time. The reported eutectic composition at a ratio of V2O5:InVO4 = 1:1 could not be used as a self-flux since no sign of melting was observed up to 1100 °C. Crystals of InVO4 of typical size 0.5 × 1 × 7 mm3 were obtained using copper pyrovanadate (Cu2V2O7) as a flux, using Pt crucibles. X-ray powder diffraction confirmed the orthorhombic Cmcm structure. Rests of the flux material were observed on the sample surface, with occasional traces of Pt indicating some level of reaction with the crucible. X-ray absorption spectroscopy showed that oxidation states of indium and vanadium ions are +3 and +5, respectively. The size and high quality of the obtained InVO4 crystals makes them excellent candidates for further study of their physical properties.

The Structure of Terbium in the Ferromagnetic State.

Metals typically crystallize in highly symmetric structures due to their nondirectional and nonsaturated metallic bonds. Here, we report that terbium metal in its ferromagnetic state adopts an unusual low-symmetry orthorhombic structure with a Cmcm space group. A similar structure has been previously observed only in a few actinide metals with bonding 5f electrons at ambient pressure, such as uranium, neptunium, and plutonium, but with different nearest coordination numbers and bond-length variations. The Tb atom occupies the 4c site (0, ∼0.1661, 1/4), building up -[Tb-Tb]- layers stacking along the b-axis. Our first-principles many-body calculations of the crystal field splitting in the correlated Tb 4f-shell demonstrate that the Cmcm structure for ferromagnetic terbium is stabilized by magneto-elastic forces due to a secondary order of quadrupolar moments in the ferromagnetic state. These findings are significant for further understanding of the nature of terbium, including its electron structure, energy bands, phonons, and magnetism.

Structural phase stability, elastic properties, and electronic structure of pressure-induced CaC2: A first-principles study

This work investigates the structural phase transition of CaC2 material under 0–150 GPa pressure by using first-principles calculation. The enthalpy difference results suggest that the C2/c structure transforms into the Cmcm structure at 0.08 GPa, while the C2/m structure transforms into the Cmcm structure at 0.11 GPa. The Cmcm structure transforms into the Immm structure at 13.70 GPa and the P6/mmm structure at 110.65 GPa. Moreover, the enthalpy values of C2/c, C2/m, I4/mmm, and Cmcm are very close between 0 and 0.11 GPa, indicating that these four structures may coexist in this pressure range. The change of volume with pressure shows that the volume decreases with the increased pressure, especially it decreases sharply at the pressure transition point. In addition, the results show that the elastic modulus of different structures increases linearly with the increase of pressure in their respective pressure stable ranges, but it will increase sharply at the pressure transition point, indicating that CaC2 exhibits better stiffness under high pressure than under low pressure. Finally, the change of the electronic structure of CaC2 with pressure is also analyzed, the results show that C2/c and C2/m are indirect band gap semiconductor structures between 0–0.11 GPa, but the band gap becomes 0 ⅇV at the first pressure transition point 0.11 GPa, and it is always 0 ⅇV in the pressure range of 0.11–150 GPa, which means that it becomes a metal phase. This work will provide a valuable reference for the experiment and application of CaC2 under high pressure.

Structural, mechanical, and electronic properties of Ta2B by First-principle calculations

The structure, phase stability, and elastic properties of Ta2B have been carefully investigated by density functional theory. We propose a new orthorhombic Cmcm structure that is more stable than the previous I4/mcm. Combined with a study of the relationship between enthalpy pressure and convex hull, the Cmcm structure is found to be stable at atmospheric pressure and, at zero pressure, the stable phases are Ta2B, TaB, Ta3B4, Ta5B6, and TaB2, in agreement with experimental results. Above a pressure of 60 GPa, Ta2B becomes unstable.

Crystal Structures and Physicochemical Properties of Be2N and Mg2N as Electride Materials

Electrides are a special family of materials with exotic physical properties and various crystal structures in different dimensions. Among them, alkaline-earth metal nitrides (${\mathrm{AE}}_{2}\mathrm{N}$) have attracted enormous attention, since the synthesis of ${\mathrm{Ca}}_{2}\mathrm{N}$ as a two-dimensional (2D) electride. However, apart from ${\mathrm{Ca}}_{2}\mathrm{N}$, the physicochemical properties of other ${\mathrm{AE}}_{2}\mathrm{N}$ are not well understood, and in some cases, their crystal structures remain unknown. To overcome these issues, here, we conduct systematic investigations into the crystal structures, electronic structures, and optical properties of two typical ${\mathrm{AE}}_{2}\mathrm{N}$, i.e., ${\mathrm{Mg}}_{2}\mathrm{N}$ and ${\mathrm{Be}}_{2}\mathrm{N}$, based on first-principles calculations. By symmetry analysis, we derive a crystal structure with $R\overline{3}m$ symmetry for ${\mathrm{Be}}_{2}\mathrm{N}$, which is energetically more stable than all the other structures proposed previously. Our calculations show that $R\overline{3}m$ ${\mathrm{Be}}_{2}\mathrm{N}$ is a 2D electride with the anionic electrons located in the interstitial space, forming a 2D Kagome lattice. Moreover, distinct from traditional 2D electrides, it exhibits anomalous physicochemical properties, such as having a small interstitial space, a high work function, and a large cleavage energy, mainly due to the relatively strong bonding effect between anionic electrons and cationic framework. For ${\mathrm{Mg}}_{2}\mathrm{N}$, on the other hand, we find that the most stable structure has R3m symmetry, which shows a total energy of 0.005 eV per formula unit lower than the previously predicted Cmcm structure. Moreover, different from $R\overline{3}m$ ${\mathrm{Be}}_{2}\mathrm{N}$, R3m ${\mathrm{Mg}}_{2}\mathrm{N}$ is identified as a zero-dimensional (0D) electride with a semiconducting band structure, since its anionic electrons are confined in separated 0D cavities and cannot interact with each other. These studies thus provide a deeper understanding of the crystal structures and physicochemical properties of ${\mathrm{Be}}_{2}\mathrm{N}$ and ${\mathrm{Mg}}_{2}\mathrm{N}$ as potential electride materials.

Structure Effect on Pseudocapacitive Properties of A2LaMn2O7 (A = Ca, Sr)

Herein, the effect of structure on pseudocapacitive properties in alkaline conditions is demonstrated through the investigation of isoelectronic oxides Ca2LaMn2O7 and Sr2LaMn2O7, where the difference in ionic radii of Ca2+ and Sr2+ leads to a change in structure and lattice symmetry, resulting in an orthorhombic Cmcm structure for the former and a tetragonal I4/mmm structure for the latter. While calcium and strontium do not make a direct contribution to the near‐surface faradaic processes that are essential to the pseudocapacitive properties, their effect on the structure leads to a change in the oxygen intercalation process and the associated pseudocapacitive energy storage. It is shown that Sr2LaMn2O7 has a significantly greater specific capacitance than Ca2LaMn2O7. In addition, the former shows a considerably higher‐energy density compared to the latter. Furthermore, these materials show highly stable energy‐storage properties, and retain their specific capacitance over 10 000 cycles of charge–discharge in a symmetric pseudocapacitive cell. Importantly, these findings show the structure–property relationships, where a change in the structure and lattice symmetry can result in a significant change in pseudocapacitive charge–discharge properties in isoelectronic systems.

Pressure dependent structural, dynamical, mechanical and electronic properties of magnesium dicarbide

ABSTRACT Magnesium dicarbide (MgC2) at high pressure has been systematically investigated using the first-principle methods. Ten possible structures were studied and three of them (Cmcm, C2/m, P-3m1) were found to be dynamically and mechanically stable under the given pressure. The calculated formation enthalpies indicated that the Cmcm structure was thermodynamically unstable while the C2/m and P-3m1 structures were thermodynamically stable. The interesting thing was that the P-3m1 structure of MgC2 could also be as potential hard materials owing to their Vickers hardness closing to 40 GPa with increasing pressure. The Cmcm and C2/m structures presented ductile behaviour while P-3m1 structure displayed brittle character. The elastic anisotropy of Young’s modulus E and shear modulus G became smaller with the increasing pressure, and the P-3m1 structure possessed a lower anisotropy in E and G than Cmcm and C2/m structures. The Cmcm and C2/m structures of MgC2 had metallic characteristic; however, the P-3m1 structure of MgC2 was semiconductor with an indirect band gap.

Feiite: Synthesis, stability, and implications for its formation conditions in nature

Abstract Feiite (Fe3TiO5) is a high-pressure Fe-Ti oxide mineral recently discovered in martian meteorite Shergotty. Feiite is isostructural with Fe4O5, a high-pressure iron oxide stable at pressures >10 GPa. The stability of feiite has yet to be studied, as it has not previously been synthesized in the laboratory. To determine the minimum pressure at which feiite can be synthesized, we have conducted multi-anvil experiments at 1200 °C and at pressures ranging from 7 to 12 GPa. Major element compositions and XRD patterns indicate that we successfully synthesized feiite with an orthorhombic unit cell (Cmcm structure) in experiments conducted at pressures 8 GPa or greater. Relative to A2B2O5 phases with similar structure, feiite can be synthesized at lower pressures. The coexistence of feiite and liuite (FeTiO3-perovskite) in Shergotty indicates that the upper pressure limit of feiite stability is above 15 GPa. To investigate the effect of oxygen fugacity on the composition and stability of feiite, we conducted an additional series of experiments at 1200 °C and 10 GPa pressure in which we varied the Fe3+/Fetotal ratio of the experimental starting materials. In doing so, we identified a minimum Fe3+ content necessary to stabilize the feiite structure (Fe3+/Fetotal = 0.26 at 10 GPa and 1200 °C). The importance of Fe3+ for feiite stability suggests this phase would not form in lunar or HED meteorites, where iron-titanium oxides contain little to no ferric iron. Though our experimental results can only place a lower limit on the shock pressures experienced in Shergotty, the determined pressure stability indicates feiite could also be present in diamond-bearing terrestrial rocks sourced from the upper mantle or transition zone. Additionally, the presence of feiite would be an indicator of source Fe3+/Fetotal.

Structural, magnetic and magnetocaloric properties in the rare earth ruthenate RE3RuO7 (RE = Pr, Nd, Gd and Tb) oxides with fluorite related structure

The magnetic refrigeration (MR) based on magnetocaloric effect (MCE) is a key technique, since it possesses ideally higher performance than the traditional gas compression technology. However, the lack of MR materials with excellent MCE performance is a bottle-neck for practical MR applications. Herein, we systematically report the structural properties using powder X-ray diffraction and scanning electron microscope as well as the magnetic and magnetocaloric properties through the magnetization measurements of four rare-earth (RE) ruthenate RE3RuO7 (RE = Pr, Nd, Gd and Tb) compounds. Light RE-based Pr3RuO7 and Nd3RuO7 compounds form an orthorhombic Cmcm structure, while heavy RE based Gd3RuO7 and Tb3RuO7 compounds crystallize in an orthorhombic P21nb structure. The magnetic phase transition of all the present compounds are confirmed to be of the first-order type with TN of 52, 15, 11 and 12 K for Pr3RuO7, Nd3RuO7, Gd3RuO7 and Tb3RuO7 compounds, respectively. The related MCE parameters including the maximum isothermal magnetic entropy change, temperature-averaged entropy change (5 K lift), and refrigerant capacity with the magnetic field change of 0–5 T were computed to be 1.35 J/kgK, 1.31 J/kgK, and 27.4 J/kg for Pr3RuO7, to be 2.18 J/kgK, 2.04 J/kgK, and 25.6 J/kg for Nd3RuO7, to be 8.12 J/kgK, 7.90 J/kgK, and 124.6 J/kg for Gd3RuO7, and to be 7.78 J/kgK, 7.49 J/kgK, and 128.8 J/kg for Tb3RuO7, respectively.

Stability, elastic and electronic properties of the CsSrI3 halid perovskite

Due to the importance of halide perovskite in many applications, namely optical and electronic ones, it is a challenging task to determine the precise stable studied structure for a given overall use. In view of this point, we present in this work, for the first time, theoretical calculations carried out by means of the full-potential linearized augmented plane wave (FP−LAPW) method on the CsSrI3compound. Deep analysis of energetic and dynamical investigations shows that this compound adopts an orthorhombic Cmcm structure. Meanwhile, both bonding and mechanical properties reveal a strong ionic/anisotropic behavior of the studied materials. This trend drastically influences electronic properties, giving rise to a wide band gap energy of 4 eV. The analysis of band structure and effective mass predicts favorable properties for UV applications.

Stabilizing superconductivity of ternary metal pentahydride hbox {CaCH}_{{5}} via electronic topological transitions under high pressure from first principles evolutionary algorithm

We explored the phase stability of ternary pentahydride hbox {CaCH}_{{5}} based on the first principles evolutionary algorithm. Here, we successfully search for a candidate structure up to 500 GPa. As a consequence, the possible stable structure of hbox {CaCH}_{{5}} is found be to a monoclinic structure with space group Pm at a pressure of 50 GPa. Moreover, the orthorhombic structure with a space group of Cmcm is found to be thermodynamically stable above 316 GPa. With this, the Kohn-Sham equation plays a crucial role in determining the structural stability and the electronic structure. Therefore, its structural stability is discussed in term of electronic band structure, Fermi surface topology, and dynamic stability. With these results, we propose that the superconducting transition temperature (hbox {T}_{{c}}) of Cmcm structure is estimated to be 50 K at 450 GPa. This could be implied that the proposed Cmcm structure may be emerging as a new class of superconductive ternary metal pentahydride. Our findings pave the way for further studies on an experimental observation that can be synthesized at high pressure.

Li5Sn, the Most Lithium-Rich Binary Stannide: A Combined Experimental and Computational Study.

From reaction of excess lithium with tin, we isolate well-crystallized Li5Sn and solve the crystal structure from single-crystal X-ray diffraction data. The orthorhombic structure (space group Cmcm) features the same coordination polyhedra around tin and lithium as previously predicted by electronic structure calculations for this composition, however differently arranged. An extensive ab initio analysis, including thermodynamic integration using Langevin dynamics in combination with a machine-learning potential (moment tensor potential), is conducted to understand the thermodynamic stability of this Cmcm Li5Sn structure observed in our experiments. Among the 108 Li5Sn structures systematically derived using the structure enumeration algorithm, including the experimental Cmcm structure and those obtained in previous ab initio studies, another new structure with the space group Immm is found to be energetically most stable at 0 K. This computationally discovered Immm structure is also found to be thermodynamically more stable than the Cmcm structure at finite temperatures, indicating that the Cmcm Li5Sn structure observed in our experiments is favored likely due to kinetic reasons rather than thermodynamics.

Observation of a Novel Lattice Instability in Ultrafast Photoexcited SnSe

There is growing interest in using ultrafast light pulses to drive functional materials into nonequilibrium states with novel properties. The conventional wisdom is that above-gap photoexcitation behaves similarly to raising the electronic temperature and lacks the desired selectivity in the final state. Here, we report a novel nonthermal lattice instability induced by ultrafast above-gap excitation in SnSe, a representative of the IV−VI class of semiconductors that provides a rich platform for tuning material functionality with ultrafast pulses due to their multiple lattice instabilities. The new lattice instability is accompanied by a drastic softening of the lowest-frequency Ag phonon. This mode has previously been identified as the soft mode in the thermally driven phase transition to a Cmcm structure. However, by a quantitative reconstruction of the atomic displacements from time-resolved x-ray diffraction for multiple Bragg peaks and excitation densities, we show that ultrafast photoexcitation with near-infrared (1.55 eV) light induces a distortion toward a different structure with Immm symmetry. The Immm structure of SnSe is an orthorhombic distortion of the rocksalt structure and does not occur in equilibrium. Density functional theory calculations reveal that the photoinduced Immm lattice instability arises from electron excitation from the Se 4p- and Sn 5s-derived bands deep below the Fermi level that cannot be excited thermally. The results have implications for optical control of the thermoelectric, ferroelectric, and topological properties of the monochalcogenides and related materials. More generally, the results emphasize the need for ultrafast structural probes to reveal distinct atomic-scale dynamics that are otherwise too subtle or invisible in conventional spectroscopies.8 MoreReceived 15 June 2021Revised 10 November 2021Accepted 20 December 2021DOI:https://doi.org/10.1103/PhysRevX.12.011029Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI.Published by the American Physical SocietyPhysics Subject Headings (PhySH)Research AreasElectron-phonon couplingOptical phononsTechniquesUltrafast pump-probe spectroscopyX-ray diffractionCondensed Matter, Materials & Applied Physics

SnSe:Kx intermetallic thermoelectric polycrystals prepared by arc-melting

Neutron powder diffraction and thermoelectric characterization of SnSe:Kx intermetallic alloys are presented. Nanostructured ingots were prepared by arc-melting elemental tin and selenium along with potassium hydride. Up to x = 0.1 of K can be incorporated into SnSe. Rietveld refinement of the diffractograms locates potassium on the Sn site in the high-temperature Cmcm structure. However, in the low-temperature Pnma structure, K cannot be localized by difference Fourier maps, indicating the incorporation of K in a disordered form in the interlayer space. STEM-EELS indicates the incorporation of K into the SnSe grains. The resistivity upon K-doping at intermediate temperatures decreases by 1–2 orders of magnitude, but at high temperature is higher than the undoped SnSe. The Seebeck coefficient of K-doped SnSe remains p-type and almost temperature independent (400 μV/K for x = 0.1). The ultralow thermal conductivity of undoped SnSe decreases further upon K-doping to below 0.3 W/m K.

Structure, stability, and mechanical properties of Nb2B under high pressure

A new Cmcm structure of Nb2B is predicted by first-principles density functional theory calculations, which is energetically much superior to the previously proposed I4/m and I4/mcm structures. The theoretical elastic properties and phonon spectroscopy confirm that the Cmcm-Nb2B is stable at ambient pressure. The obtained density of states shows that Cmcm-Nb2B is metallic. According to the calculated Vicker hardness based on a semi-empirical method, we find that the Cmcm-Nb2B has a hardness value of 15.2 GPa. The 3D network B–B covalent bonds are critical to the mechanical properties of the structure.

Prediction of stable Cu structure and phase transition mechanism at ultra-high pressure: A comprehensive properties characterization by DFT calculation

Evolutionary algorithms and the particle swarm optimization method have been used to predict stable and metastable Cu structures between 0 and 2000 GPa that have not been discussed in previous studies, Cmcm phase is identified to be the most energetically competitive candidate. Density functional theory calculations illustrate that face-centered Cu atoms in (0 1 0), (0 1(−) 0) and (1 1 0) move along the <0 0 1 > under the ultrahigh-pressure during the phase transformation from FCC (ambient pressure) to Cmcm (2000 GPa) structure. Phonon calculation further confirms the dynamical stability of this predicted structure, and the temperature dependence of electrical resistivity for Cmcm Cu will get reduced, predicted by phonon hardening with increasing pressure. The high-pressure structure still exhibits metallic behavior due to the finite electron DOS at the Fermi level, and the calculated elastic properties display superior bulk and shear modulus of Cmcm Cu with dramatic increase at 2000 GPa.

The unconventionally stoichiometric compounds in the Na–K system at high pressures

Recent discoveries of alkali metal structures with novel properties at high pressures have stimulated considerable interest in binary mixtures of these metals under compression. This work reports a broad investigation of the physical and chemical behaviors of different Na–K stoichiometries by first-principles calculations and swarm-intelligence structure searching methodology. In addition to five structures previously proposed, seven new structures with stoichiometries of NaK, NaK2, and NaK3 were uncovered. New NaK2 compounds with Pm and Pnma structures are potential superconductors with Tc of 12.17 K at 250 GPa and 6.26 K at 400 GPa, respectively. The NaK and NaK3 compounds show intriguing decomposition–recombination behavior. The newly found Cmcm structure of NaK, Pm structure of NaK2, and Pmmm and Cmcm structures of NaK3 are electrides with strong interstitial electron localization. What's more, interesting reversible charge transfer between Na and K occurred as the pressure increased. Our discoveries prove that pressure is a powerful tool for creating new exotic materials in the binary alkali compounds.