Centimeter-sized diamond composites with high electrical conductivity and hardness.

Achieving high-performance materials with superior mechanical properties and electrical conductivity, especially in large-sized bulk forms, has always been the goal. However, it remains a grand challenge due to the inherent trade-off between these properties. Herein, by employing nanodiamonds as precursors, centimeter-sized diamond/graphene composites were synthesized under moderate pressure and temperature conditions (12 GPa and 1,300 to 1,500 °C), and the composites consisted of ultrafine diamond grains and few-layer graphene domains interconnected through covalently bonded interfaces. The composites exhibit a remarkable electrical conductivity of 2.0 × 104 S m-1 at room temperature, a Vickers hardness of up to ~55.8 GPa, and a toughness of 10.8 to 19.8 MPa m1/2. Theoretical calculations indicate that the transformation energy barrier for the graphitization of diamond surface is lower than that for diamond growth directly from conventional sp2 carbon materials, allowing the synthesis of such diamond composites under mild conditions. The above results pave the way for realizing large-sized diamond-based materials with ultrahigh electrical conductivity and superior mechanical properties simultaneously under moderate synthesis conditions, which will facilitate their large-scale applications in a variety of fields.

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.

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).

Modulating Charge-Density Wave Order and Superconductivity from Two Alternative Stacked Monolayers in a Bulk 4Hb-TaSe2 Heterostructure via Pressure.

Two-dimensional (2D) van der Waals heterostructures (VDWHs) containing a charge-density wave (CDW) and superconductivity (SC) have revealed rich tunability in their properties, which provide a new route for optimizing their novel exotic states. The interaction between SC and CDW is critical to its properties; however, understanding this interaction within VDWHs is very limited. A comprehensive in situ study and theoretical calculation on bulk 4Hb-TaSe2 VDWHs consisting of alternately stacking 1T-TaSe2 and 1H-TaSe2 monolayers are investigated under high pressure. Surprisingly, the superconductivity competes with the intralayer and adjacent-layer CDW order in 4Hb-TaSe2, which results in substantially and continually boosted superconductivity under compression. Upon total suppression of the CDW, the superconductivity in the individual layers responds differently to the charge transfer. Our results provide an excellent method to efficiently tune the interplay between SC and CDW in VDWHs and a new avenue for designing materials with tailored properties.

Extreme conditions research using the large-volume press at the P61B endstation, PETRA III.

Penetrating, high-energy synchrotron X-rays are in strong demand, particularly for high-pressure research in physics, chemistry and geosciences, and for materials engineering research under less extreme conditions. A new high-energy wiggler beamline P61 has been constructed to meet this need at PETRA III in Hamburg, Germany. The first part of the paper offers an overview of the beamline front-end components and beam characteristics. The second part describes the performance of the instrumentation and the latest developments at the P61B endstation. Particular attention is given to the unprecedented high-energy photon flux delivered by the ten wigglers of the PETRA III storage ring and the challenges faced in harnessing this amount of flux and heat load in the beam. Furthermore, the distinctiveness of the world's first six-ram Hall-type large-volume press, Aster-15, at a synchrotron facility is described for research with synchrotron X-rays. Additionally, detection schemes, experimental strategies and preliminary data acquired using energy-dispersive X-ray diffraction and radiography techniques are presented.

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.

Pressure-Dependent Structural and Band Gap Tuning of Semiconductor Copper(I) Thiocyanate (CuSCN).

Copper(I) thiocyanate (CuSCN) is a p-type semiconductor with exceptional properties for optoelectronic devices such as solar cells, thin-film transistors , organic light-emitting diodes, etc. Understanding the structure-optical property relationships in CuSCN is critical for its optoelectronic applications. Herein, high-pressure techniques combined with theoretical calculations are used to thoroughly investigate the structural and optical changes of CuSCN upon compression. Under high pressure, CuSCN exhibits a progressive decrease of the band gap with different rates, which is relevant to the ß to α phase transition in CuSCN and the subsequent amorphization through polymerization. UV-vis spectra measurements reveal a reduction in band gap from 3.4 to 1.3 eV upon decompression to ambient conditions. Such transitions could be attributed to the pressure-induced rotation of CuNS3 tetrahedron and bond length shrinkage. The severe distortion of the polyhedral units prompts breakdown of the structure and thus the amorphization, which is quenchable to ambient conditions. Our study demonstrates that high pressure can be utilized to adjust the structure and optical characteristics of CuSCN compound, potentially extending the material's uses in optoelectronic devices.

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.

Tricolor Ho

The photoluminescence (PL) characterization spectrum has been widely used to study the electronic energy levels. Ho^{3+} is one of the commonly used doping elements to provide the PL with concentration limited to 1% atomic ratio. Here, we present a tricolor PL achieved in pyrochlore Ho_{2}Sn_{2}O_{7} through pressure treatment at room temperature, which makes a non-PL material to a strong multiband PL material with Ho^{3+} at the regular lattice site with 18.2% concentration. Under a high pressure compression-decompression treatment up to 78.0 GPa, the Ho_{2}Sn_{2}O_{7} undergoes pyrochlore (Fd 3m), to cotunnite (Pnma), then amorphous phase transition with different Ho^{3+} coordinations and site symmetries. The PL emerged from 31.2 GPa when the pyrochlore to cotunnite phase transition took place with the breakdown of site symmetry and enhanced hybridization of Ho^{3+} 4f and 5d orbitals. Upon decompression, the materials became an amorphous state with a partial retaining of the defected cotunnite phase, accompanied with a large enhancement of red-dominant tricolor PL from the ion pair cross-relaxation effect in the low-symmetry (C_{1}) site, in which two distinct Ho^{3+} emission centers (S center and L center) are present.

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.

Emergent property of high hardness for C-rich ruthenium carbides: partial covalent Ru-Ru bonds.

Hard materials are being investigated all the time by combining transition metals with light elements. Combining a structure search with first-principles functional calculations, we first discovered three stable stoichiometric C-rich ruthenium carbides in view of three synthesis routes, namely, the ambient phases of Ru2C3 and RuC, and two high pressure phases of RuC4. There is a phase transition of RuC4 from the P3[combining macron]m1 structure to the R3[combining macron]m structure above 98 GPa. The calculations of elastic constants and phonon dispersions show their mechanical and dynamical stability. The large elastic modulus, high Debye temperature and the estimated hardness values suggest that these hard ruthenium carbides have good mechanical properties. The analyses of electronic structure and chemical bonding indicate that chemical bonding, not carbon content, is the key factor for the hardness in these metallic C-rich ruthenium carbides. The partial covalent Ru-C bonds and strong covalent C-C bonds are responsible for the high hardness. Moreover, the emergence of partial covalent Ru-Ru bonds can enhance the hardness of RuC, while the ionic Ru-Ru bonds can weaken the hardness of Ru2C3.

WB2: not a superhard material for strong polarization character of interlayer W-B bonding.

In this work, the structure of WB2 synthesized at high pressure and high temperature (HPHT) was accurately determined by X-ray diffraction and Rietveld refinement. Its asymptotic Vickers hardness (Hv) value is 25.5 GPa which is much lower than the previous theoretical results (36-40 GPa). It is worth noting that the chemical bonds between the W layers and two different kinds of B layers show obvious polarization character based on the results obtained from X-ray photoelectron spectroscopy (XPS) and electron localization functions (ELFs), density of states (DOS), topological analysis of the static electron density and Mulliken population. This result can well clarify that WB2 is only a hard but not superhard material. Thus, a 3D network structure can not be formed between the W layers and the B layers which is previously predicted by theoretical calculations. Our results are helpful to understand the hardness mechanism and design superhard materials in TMBs.

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.

Acoustic emission detection of micro-cracks under high pressure and high temperature in a deformation large-volume apparatus at the endstation P61B, PETRA III.

We successfully developed an in situ acoustic emission (AE) detection setup that allows recording of AE waveforms (triggered and streaming) and simultaneous x-ray diffraction and imaging on samples deformed at high pressure and high temperature (HPHT) conditions in the Aster-15 Large Volume Press at the synchrotron beamline station P61B. This high pressure AE detection system is a powerful tool to investigate AE phenomena from the HPHT chamber. Six commercial acoustic sensors, protected by a tungsten carbide support ring on each anvil of the same material, have excellent survivability throughout each successive experiment. By pulsing each sensor in succession, the average wave velocity through the anvils and cell assembly can be determined at any press load. The distance between the sensors is obtained by x-ray radiography and by logging the positions of each hydraulic ram. This provides a basis for accurately locating AE events in the sample. The feasibility of this AE detection setup was confirmed by compression and deformation test runs using several different self-designed AE sources in specialized assemblies. The present setup proves to be extremely efficient and accurate in measuring brittle processes in samples under HPHT. It is now available for applications for beam time and experiments without x rays at P61B. Combined with synchrotron x rays, in situ pressure, temperature, strain rate and stress, and phase changes can be monitored while recording AE activity. We provide a powerful tool to investigate the origin of earthquakes, for example, causing AE emissions due to brittle dehydration reactions or phase transformations in the Earth.

Pressure-induced bandgap engineering and photoresponse enhancement of wurtzite CuInS2 nanocrystals.

Wurtzite CuInS2 exhibits great potential for optoelectronic applications because of its excellent optical properties and good stability. However, exploring effective strategies to simultaneously optimize its optical and photoelectrical properties remains a challenge. In this study, the bandgap of wurtzite CuInS2 nanocrystals is successfully extended and the photocurrent is enhanced synchronously using external pressure. The bandgap of wurtzite CuInS2 increases with pressure and reaches an optimal value (1.5 eV) for photovoltaic solar energy conversion at about 5.9 GPa. Surprisingly, the photocurrent simultaneously increases nearly 3-fold and reaches the maximum value at this critical pressure. Theoretical calculation indicates that the pressure-induced bandgap extention in wurtzite CuInS2 may be attributed to an increased charge density and ionic polarization between the In-S atoms. The photocurrent preserves a relatively high photoresponse even at 8.8 GPa, but almost disappears above 10.3 GPa. The structural evolution demonstrates that CuInS2 undergoes a phase transformation from the wurtzite phase (P63mc) to the rock salt phase (Fm3̄m) at about 10.3 GPa, which resulted in a direct to indirect bandgap transition and fianlly caused a dramatic reduction in photocurrent. These results not only map a new route toward further increase in the photoelectrical performance of wurtzite CuInS2, but also advance the current research of AI-BIII-CVI2 materials.

The Synthesis and Characterisation of the High-Hardness Magnetic Material Mn2N0.86.

High-quality P6322 Mn2N0.86 samples were synthesised using a high-pressure metathesis reaction, and the properties of the material were investigated. The measurements revealed that the Vickers hardness was 7.47 GPa, which is less than that predicted by commonly used theoretical models. At low air pressure, Mn2N0.86 and MnO coexist at 500 to 600 °C, and by excluding air, we succeeded in producing Mn4N by heating Mn2N0.86 in nitrogen atmosphere; we carefully studied this process with thermogravimetry and differential scanning calorimetry (TG-DSC). This gives a hint that to control temperature, air pressure and gas concentration might be an effective way to prepare fine Mn-N-O catalysis. Magnetic measurements indicated that ferromagnetism and antiferromagnetism coexist within Mn2N0.86 at room temperature and that these magnetic properties are induced by nitrogen vacancies. Ab intio simulation was used to probe the nature of the magnetism in greater detail. The research contributes to the available data and the understanding of Mn2N0.86 and suggests ways to control the formation of materials based on Mn2N0.86.

Revealing the Unusual Boron-Pinned Layered Substructure in Superconducting Hard Molybdenum Semiboride.

Improving the poor electrical conductivity of hard materials is important, as it will benefit their application. High-hardness metallic Mo2B was synthesized by high-pressure and high-temperature methods. Temperature-dependent resistivity measurements suggested that Mo2B has excellent metallic conductivity properties and is a weakly coupled superconductor with a T c of 6.0 K. The Vickers hardness of the metal-rich molybdenum semiboride reaches 16.5 GPa, exceeding the hardness of MoB and MoB2. The results showed that a proper boron concentration can improve the mechanical properties, not necessarily a high boron concentration. First-principles calculations revealed that the pinning effect of light elements is related to hardness. The high hardness of boron-pinned layered Mo2B demonstrated that the design of high-hardness conductive materials should be based on the structure formed by light elements rather than high-concentration light elements.