Self-Catalyzed Hydrogenated Carbon Nano-Onions Facilitates Mild Synthesis of Transparent Nano-Polycrystalline Diamond.

Transparent nano-polycrystalline diamond (t-NPD) possesses superior mechanical properties compared to single and traditional polycrystalline diamonds. However, the harsh synthetic conditions significantly limit its synthesis and applications. In this study, a synthesis routine is presented for t-NPD under low pressure and low temperature conditions, 10 GPa, 1600 °C and 15 GPa, 1350 °C similar with the synthesis condition of organic precursor. Self-catalyzed hydrogenated carbon nano-onions (HCNOs) from the combustion of naphthalene enable synthesis under nearly industrial conditions, which are like organic precursor and much lower than that of graphite and other carbon allotropes. This is made possible thanks to the significant impact of hydrogen on the thermodynamics, as it chemically facilitates phase transition. Ubiquitous nanotwinned structures are observed throughout t-NPD due to the high concentration of puckered layers and stacking faults of HCNOs, which impart a Vickers hardness about 140 GPa. This high hardness and optical transparency can be attributed to the nanocrystalline grain size, thin intergranular films, absence of secondary phase and pore-free features. The facile and industrial-scale synthesis of the HCNOs precursor, and mild synthesis conditions make t-NPD suitable for a wide range of potential applications.

Structural Phase Transition and Decomposition of XeF2 under High Pressure and Its Formation of Xe-Xe Covalent Bonds.

Noble gases with inert chemical properties have rich bonding modes under high pressure. Interestingly, Xe and Xe form covalent bonds, originating from the theoretical simulation of the pressure-induced decomposition of XeF2, which has yet to be experimentally confirmed. Moreover, the structural phase transition and metallization of XeF2 under high pressure have always been controversial. Therefore, we conducted extensive experiments using a laser-heated diamond anvil cell technique to investigate the above issues of XeF2. We propose that XeF2 undergoes a structural phase transition and decomposition above 84.1 GPa after laser heating, and the decomposed product Xe2F contains Xe-Xe covalent bonds. Neither the pressure nor temperature alone could bring about these changes in XeF2. With our UV-vis absorption experiment, I4/mmm-XeF2 was metalized at 159 GPa. This work confirms the existence of Xe-Xe covalent bonds and provides insights into the controversy surrounding XeF2, enriching the research on noble gas chemistry under high pressure.

First-principles study of multifunctional Mn2B3 materials with high hardness and ferromagnetism.

Transition metal boride TM2B3 is widely studied in the field of physics and materials science. However, Mn2B3 has not been found in Mn-B systems so far. Mn2B3 undergoes phase transitions from Cmcm (0-28 GPa) to C2/m (28-80 GPa) and finally to C2/c (80-200 GPa) under pressure. Among these stable phases, Cmcm- and C2/m-Mn2B3s comprise six-membered boron rings and C2/c-Mn2B3 has wavy boron chains. They all have good mechanical properties and can become potential multifunctional materials. The strong B-B covalent bonding is mainly responsible for the structural stability and hardness. Comparison of the hardness of the five TM2B3s with different bonding strengths of TM-B and B-B bonds reveals a nonlinear change in the hardness. According to the Stoner model, these structures possess ferromagnetism, and the corresponding magnetic moments are almost the same as those of GGA and GGA + U (U = 3.9 eV, J = 1 eV).

Synthesis, Characterization, and First-Principles Analysis of the MAB-Like Ternary Transition-Metal Boride Fe(MoB)2.

Novel transition-metal borides have attracted considerable attention because they exhibit high stability under extreme conditions. Compared with binary borides, ternary transition-metal borides (TTMBs) exhibit novel boron substructures and diverse properties, which result in excellent designability. In this study, we synthesized the MAB-like (where M = iron, A = molybdenum, and B = boron) phase Fe(MoB)2 using a high-pressure and high-temperature method. Fe(MoB)2 exhibited ferromagnetic metastable characteristics with a saturation magnetization of 8.35 emu/g at room temperature. Microhardness measurement revealed an indentation hardness of 10.72 GPa, which was higher than those of conventional magnetic materials. First-principles calculations revealed excellent mechanical properties, which mainly originated from the strong covalent short B2 chains. Furthermore, magnetism was attributed to the Fe 3d electrons. Numerous d-d hybridizations existed between the Fe 3d eg and Mo 4d orbitals, and the antibonding/nonbonding state difference for up/down-spin electrons in the hybridization orbitals led to the local magnetic moment of Fe(MoB)2. The magnetic anisotropy energy analyses reveal that Fe(MoB)2 prefers the easy magnetization axis along the z direction, and Mo atom acts as a medium to realize the exchange action between two Fe atoms. The B-B and Fe-B bonds were considerably stronger than the Fe-Mo and Mo-B bonds, and Fe(MoB)2 exhibited a class of atomically laminate composed of FeB2 and Mo layers. These results may provide guidance for the design of novel multifunctional TTMBs by adjusting the interactions between binary metal components.

Correction: Synthesis and characterization of a strong ferromagnetic and high hardness intermetallic compound Fe2B.

Correction for 'Synthesis and characterization of a strong ferromagnetic and high hardness intermetallic compound Fe2B' by Xingbin Zhao et al., Phys. Chem. Chem. Phys., 2020, 22, 27425-27432, DOI: 10.1039/D0CP03380D.

Synthesis and characterization of a strong ferromagnetic and high hardness intermetallic compound Fe2B.

Magnetic materials attract great attention due to their fundamental importance and practical application. However, the relatively inferior mechanical properties of traditional magnetic materials limit their application in a harsh environment. In this work, we report an outstanding magnetic material that exhibits both fantastic mechanical and excellent magnetic properties, CuAl2-type Fe2B, synthesized by the high pressure and high temperature method. The magnetic saturation of Fe2B is 156.9 emu g-1 at room temperature and its Vickers hardness is 12.4 GPa which outclasses those of traditional magnetic materials. It exhibits good conductivity with a resistivity of 5.6 × 10-7 Ω m. Fe2B is a promising strong ferromagnetic material with high hardness, which makes it a good candidate for multifunction applications in a harsh environment. The high hardness of Fe2B originates from the Fe-B bond framework, and the strong ferromagnetism is mainly attributed to the large number of unpaired Fe 3d electrons. The competition of Fe 3d electrons to fall into Fe-B bonds or Fe-Fe bonds is the main factor for its magnetism and hardness. This work bridges the chasm between strong ferromagnetism and high hardness communities.

Revealing the Unusual Rigid Boron Chain Substructure in Hard and Superconductive Tantalum Monoboride.

Poor electrical conductivity severely limits the diverse applications of high hardness materials in situations where electrical conductivities are highly desired. A "covalent metal" TaB with metallic electrical conductivity and high hardness has been fabricated by a high pressure and high temperature method. The bulk modulus, 302.0(4.9) GPa, and Vickers hardness, 21.3 GPa, approaches and even exceeds that of traditional insulating hard materials. Meanwhile, temperature-dependent electrical resistivity measurements show that TaB possesses metallic conductivity that rivals some widely-used conductors, and it will transform into a superconductor at Tc =7.8 K. Contrary to common understanding, the hardness of TaB is higher than that of TaB2 , which indicates that low boron concentration borides could be mechanically better than the higher boron concentration counterparts. Compression behavior and first principles calculations denote that the high hardness is associated with the ultra-rigid covalent boron chain substructure. The hardness of TaB with different topologies of boron substructure shows that besides incorporating higher boron content, manipulating light element backbone configurations is also critical for higher hardness amongst transition metal borides with identical boron content.

Role of TM-TM Connection Induced by Opposite d-Electron States on the Hardness of Transition-Metal (TM = Cr, W) Mononitrides.

Recent reports exposed an astonishing factor of high hardness that the connection between transition-metal (TM) atoms could enhance hardness, which is in contrast to the usual understanding that TM-TM will weaken hardness as the source of metallicity. It is surprising that there are two opposite mechanical characteristics in the one TM-TM bond. To uncover the intrinsic reason, we studied two appropriate mononitrides, CrN and WN, with the same light-element (LE) content and valence electron concentration. The two high-quality compounds were synthesized by a new metathesis under high pressure, and the Vickers hardness is 13.0 GPa for CrN and 20.0 GPa for WN. Combined with theoretical calculations, we found that the strong correlation of d electrons in TM-TM could seriously affect hardness. Thus, we make the complementary suggestions of the previous hardness factors that the antibonding d-electron state in TM-TM near the Fermi level should be avoided and a strong d covalent coupling in TM-TM is very beneficial for high hardness. Our results are very important for the further design of high-hardness and multifunctional TM and LE compounds.

Double-zigzag boron chain-enhanced Vickers hardness and manganese bilayers-induced high d-electron mobility in Mn3B4.

The D7b-type structure Mn3B4 was fabricated by high-temperature and high-pressure (HPHT) methods. Hardness examination yielded an asymptotic Vickers hardness of 16.3 GPa, which is much higher than that of Mn2B and MnB2. First principle calculations and XPS results demonstrated that double zigzag boron chains form a strong covalent skeletons, which enhances this structure's integrity with high hardness. Considering that the hardensses of MnB and Mn3B4 are higher than those of Mn2B and MnB2, zigzag and double zigzag boron backbones are superior to isolated boron and graphite-like boron layer backbones for achieving higher hardness. This situation also states that a higher boron content is not the sole factor for the higher hardness in the low boron content transition metal borides. Futhermore, the co-presence of metallic manganese bilayers contribute to the high d-electron mobility and generate electrical conductivity and antiferromagnetism in Mn3B4 which provide us with a new structure prototype to design general-purpose high hardness materials.

Crossover from metal to insulator in dense lithium-rich compound CLi4.

At room environment, all materials can be classified as insulators or metals or in-between semiconductors, by judging whether they are capable of conducting the flow of electrons. One can expect an insulator to convert into a metal and to remain in this state upon further compression, i.e., pressure-induced metallization. Some exceptions were reported recently in elementary metals such as all of the alkali metals and heavy alkaline earth metals (Ca, Sr, and Ba). Here we show that a compound of CLi4 becomes progressively less conductive and eventually insulating upon compression based on ab initio density-functional theory calculations. An unusual path with pressure is found for the phase transition from metal to semimetal, to semiconductor, and eventually to insulator. The Fermi surface filling parameter is used to describe such an antimetallization process.

Insights into Antibonding Induced Energy Density Enhancement and Exotic Electronic Properties for Germanium Nitrides at Modest Pressures.

Here, the electronic and bonding features in ground-state structures of germanium nitrides under different components that not accessible at ambient conditions have been systematically studied. The forming essence of weak covalent bonds between the Ge and N atom in high-pressure ionic crystal Fd-3 m-Ge3N4 is induced by the binding effect of electronic clouds originated from the Ge_ p orbitals. Hence, it helps us to understand the essence of covalent bond under high pressure, profoundly. As an excellent reducing agent, germanium transfer electrons to the antibonding state of the N2 dimer in Pa-3-GeN2 phase at 20 GPa, abnormally, weakening the bonding strength considerably than nitrogen gap (N≡N) at ambient pressure. Furthermore, the common cognition that the atomic distance will be shortened under the high pressures has been broken. Amazingly, with a lower range of synthetic pressure (∼15 GPa) and nitrogen contents (28%), its energy density is up to 2.32 kJ·g-1, with a similar order of magnitude than polymeric LiN5 (nonmolecular compound, 2.72 kJ·g-1). It breaks the universal recognition once again that nitrides just containing polymeric nitrogen were regarded as high energy density materials. Hence, antibonding induced energy density enhancement mechanism for low nitrogen content and pressure has been exposed in view of electrons. Both the highest occupied molecular orbitals (HOMO) and the lowest unoccupied molecular orbitals (LUMO) are usually the separated orbitals of N_π* and N_σ*, which are the key to stabilization. Besides, the sp2 hybridizations that exist in N4 units are responsible for the stability of the R-3 c-GeN4 structure and restrict the delocalization of electrons, exhibiting nonmetallic properties.

Pressure-Stabilized Superconductive Ionic Tantalum Hydrides.

High-pressure structures of tantalum hydrides were investigated over a wide pressure range of 0-300 GPa by utilizing evolutionary structure searches. TaH and TaH2 were found to be thermodynamically stable over this entire pressure range, whereas TaH3, TaH4, and TaH6 become thermodynamically stable at pressures greater than 50 GPa. The dense Pnma (TaH2), R3̅m (TaH4), and Fdd2 (TaH6) compounds possess metallic character with a strong ionic feature. For the highly hydrogen-rich phase of Fdd2 (TaH6), a calculation of electron-phonon coupling reveals the potential high-Tc superconductivity with an estimated value of 124.2-135.8 K.

Bonding Properties of Aluminum Nitride at High Pressure.

Exploring the bonding properties and polymerization mechanism of stable polymeric nitrogen phases is the main goal of our high-pressure study. The pressure versus composition phase diagram of the Al-N system is established. In addition to the known Fm3̅m phase of AlN, a notable monoclinic phase with N66- anion polymeric nitrogen chains for AlN3 in the pressure range from 43 to 85 GPa is predicted. Its energy density is up to 2.75 kJ·g-1, and the weight ratio of nitrogen is nearly 61%, which make it potentially interesting for the industrial applications as a high energy density material. The high-pressure studies of atomic and electronic structures in this predicted phase reveal that the formation of N66- anion is driven by the sp2 hybridization of nitrogen atoms. The resonance effect between alternating π-bonds and σ-bonds in polymeric nitrogen chains are all responsible for the structural stability. Because of the electrons transfer from aluminums to polymeric nitrogen chains, there is a pseudogap in the electronic structures of AlN3. The N_p electrons form π-type chemical bonds with the neighboring atoms, resulting in the delocalization of π electrons and charge transfer in polymeric nitrogen chains. Furthermore, disparities of charge density distribution between nitrogen atoms in polymeric nitrogen chains are the principal reason for the metallicity.

Investigation of superconductivity in compressed vanadium hydrides.

Aiming at finding new superconducting materials, we performed systematical simulations on phase diagrams, crystal structures, and electronic properties of vanadium hydrides under high pressures. The VH, VH2, VH3, and VH5 species were found to be stable under high pressures; among these, VH2 had previously been investigated. Moreover, all three novel stoichiometries showed a strong ionic character as a result of the charge transfer from V to H. The electron-phonon coupling calculations revealed the potentially superconductive nature of these vanadium hydrides, with estimated superconducting critical temperature (Tc) values of 6.5-10.7 K for R3[combining overline]m (VH), 8.0-1.6 K for Fm3[combining overline]m (VH3), and 30.6-22.2 K for P6/mmm (VH5) within the pressure range from 150 GPa to 250 GPa.

Investigating Robust Honeycomb Borophenes Sandwiching Manganese Layers in Manganese Diboride.

We report a robust honeycomb boron layers sandwiching manganese layers compound, MnB2, synthesized by high pressure and high temperature. First-principle calculation combined with X-ray photoelectron spectrum unravel that the honeycomb boron structure was stabilized by filling the empty π-band via grabbing electrons from manganese layers. Honeycomb boron layers sandwiching manganese layers is an extraordinary prototype of this type of sandwiched structure exhibiting electronic conductivity and ferromagnetism. Hydrostatic compression of the crystal structure, thermal expansion, and the hardness testing reveal that the crystal structure is of strong anisotropy. The strong anisotropy and first-principle calculation suggests that B-B bonds in the honeycomb boron structure are a strong directional covalent feature, while the Mn-B bonds are soft ionic nature. Sandwiching honeycomb boron layers with manganese layers that combine p-block elements with magnetic transition metal elements could endow its novel physical and chemical properties.

Pressure-induced structural transformation of CaC2.

The high pressure structural changes of calcium carbide CaC2 have been investigated with Raman spectroscopy and synchrotron X-ray diffraction (XRD) techniques in a diamond anvil cell at room temperature. At ambient conditions, two forms of CaC2 co-exist. Above 4.9 GPa, monoclinic CaC2-ii diminished indicating the structural phase transition from CaC2-ii to CaC2-i. At about 7.0 GPa, both XRD patterns and Raman spectra confirmed that CaC2-i transforms into a metallic Cmcm structure which contains polymeric carbon chains. Along with the phase transition, the isolated C2 dumbbells are polymerized into zigzag chains resulting in a large volume collapse with 22.4%. Above 30.0 GPa, the XRD patterns of CaC2 become featureless and remain featureless upon decompression, suggesting an irreversible amorphization of CaC2.

Phase diagram, mechanical properties, and electronic structure of Nb-N compounds under pressure.

Niobium-nitrogen compounds, which are potential candidates for superhard multifunctional materials, may possess multiple stoichiometries and structures under pressure. Based on ab initio evolutionary structural searches, we predict three ground states (oP6-Nb2N, CW-NbN, and hP22-Nb5N6) and six stable high pressure phases (ε-NbN, AsNi-NbN, U2S3-Nb2N3, oC24-NbN2, mP8-NbN3, and mP20-NbN4) for Nb-N compounds at pressures up to 100 GPa. Among them, the oP6-Nb2N, oC24-NbN2, mP8-NbN3, and mP20-NbN4 have never been reported, and N-rich oC24-NbN2, mP8-NbN3, and mP20-NbN4 high pressure phases are recoverable to ambient pressure. We find that the structure of N-rich Nb-N compounds consists of NbNx polyhedral stacking configurations and connected with Nn (n = 2, 3, 4, and n) polymerizations, which can remarkably improve the elastic modulus. It is found that CW-NbN and mP20-NbN4 are two potential ultra-incompressible and hard materials with the hardness calculated to be 24.56 and 19.86 GPa, respectively, while other N-rich phases such as U2S3-Nb2N3, oC24-NbN2, and mP8-NbN3 are soft materials. Detailed electronic structure and chemical bonding analysis proved that the high hardness of CW-NbN and mP20-NbN4 stems from the strong covalent bonding and the fullfilled Nb-N bonding and antibonding states.

The low coordination number of nitrogen in hard tungsten nitrides: a first-principles study.

Tungsten-nitrogen (W-N) compounds are studied via a combination of first-principles calculations and variable-composition evolutionary structure searches. New candidate ground states and high-pressure phases at 3 : 2, 1 : 1, and 5 : 6 compositions are uncovered and established for possible synthesis. We found that the structures in 4/5-fold N coordination (i.e., NbO-WN and W5N6) are more favoured for the W-N system at low-pressures compared with the conventional 6-fold phases (rs-WN and δ-WN). We attribute the low N coordination feature of W-N ground states to the enhanced W 5d-N 2p orbital hybridization and strong covalent W-N bonding, which involves the full-filling of W-N bonding and antibonding states and can remarkably improve the mechanical strength and hardness. These findings not only clarify the phase diagram of the W-N system, but also shed light on the correlations of hardness with microscopic crystal and electronic structures.

Investigation of stable germane structures under high-pressure.

The evolutionary structure-searching method discovers that the energetically preferred compounds of germane can be synthesized at a pressure of 190 GPa. New structures with the space groups Ama2 and C2/c proposed here contain semimolecular H2 and V-type H3 units, respectively. Electronic structure analysis shows the metallic character and charge transfer from Ge to H. The conductivity of the two structures originates from the electrons around the hydrogen atoms. Further electron-phonon coupling calculations predict that the two phases are superconductors with a high Tc of 47-57 K for Ama2 at 250 GPa and 70-84 K for C2/c at 500 GPa from quasi-harmonic approximation calculations, which may be higher than under actual conditions.

Pressure-induced structural changes in NH4Br.

We report angle dispersive X-ray diffraction (XRD) measurements and Raman spectroscopy on NH4Br up to 70.0 GPa at room temperature. Three thermodynamically stable phases (phases II, IV, and V) are confirmed and a new possible phase (phase VI) of P21/m symmetry is proposed whose structure was established from Rietveld refinement of synchrotron XRD data for the first time. The phase sequence observed in NH4Br is in accordance with phase II → IV → V → VI. Phase V transforms into phase VI at about 57.8 GPa with a huge volume reduction of 30%. Still, the intramolecular distances are analyzed to better understand the nature of structures. The H-H interactions become markedly more important as the N-Br distances are compacted, which is probably the reason of the kink of symmetric stretching band (ν1) at the transition pressure.