High-pressure Synthesis Research Articles

Thermoelectric performance of p-type CaxFe1.3Co2.7Sb12 skutterudites from high pressure synthesis

P-type CaxFe1.3Co2.7Sb12 skutterudites were successfully synthesized with a high pressure synthesis method followed by spark plasma sintering. The structure, composition and thermoelectric properties were investigated. With fixed Fe substitution level, the hole concentration was tuned with Ca filling. Compared with the unfilled Co3.25Fe0.73Sb12, Ca-filled samples possessed a relatively large effective mass, which is beneficial for enhancements in Seebeck coefficient and power factor. Moreover, the joint impurity and rattling mode scattering further suppressed the lattice thermal conductivity. Consequently, the Ca0.6Fe1.3Co2.7Sb12 sample showed the optimal ZT of 0.70 at 820 K. Composition analysis and simple electron counting suggested 0.77 hole per molecular formula for this sample, close to the value (about 0.7) we proposed for optimal filling and substitution contents in Co-rich p-type skutterudites.

High-pressure synthesis of a novel compound in Mg–Y system

This study involved the investigation of a novel compound in the Mg–Y system using a high-pressure synthesis technique. The crystal structure, thermal stability, and hydrogenation properties of the newly found compound were comprehensively evaluated. Mg3Y exhibiting a face-centered cubic structure was prepared at a pressure in the order of GPa. From X-ray diffraction analysis, the lattice parameter was determined to be a = 7.29939(4) Å. Following an endothermic reaction at approximately 588 K under an Ar atmosphere, Mg3Y was found to decompose into Mg24Y5 and Mg2Y phases. Hydrogenation of Mg3Y at 473 K under 8 MPa of H2 for 72 h resulted in decomposition into ambient-pressure and high-pressure phases of both MgH2 and YH3. The hydrogen content of hydrogenated Mg3Y was established to be 3.52 mass%. Hydrogenation reactions at high pressure (in the order of GPa) were also conducted for Mg3Y and Mg3La, which display analogous crystal structures. A Mg3La hydride was obtained as a tetragonal structure at a pressure of more than 2 GPa, whereas formation of a Mg3Y hydride was not observed.

High-pressure synthesis of SmGe3

AbstractThe new samarium germanide SmGe3is obtained by high-pressure high-temperature synthesis of pre-reacted mixtures of samarium and germanium at a pressure of 9.5 GPa and temperatures between 1073 and 1273 K. SmGe3decomposes at 470(5) K into SmGe2,α-Sm3Ge5and a hitherto unknown phase. SmGe3exhibits a superstructure of the cubic Cu3Au-type. Transmission electron microscopy measurements of crystalline particles and prepared lamellae indicate a high density of defects on the nanoscale. Selected area electron diffraction and elaborate X-ray powder diffraction measurements consistently indicate a 2a0 × 2a0 × 2a0superstructure adopting space groupFm3¯m$Fm\overline{3}m$witha = 8.6719(2) Å.

Two-step high-pressure high-temperature synthesis of nanodiamonds from naphthalene**Project supported by the National Key R&D Program of China (Grant No. 2018YFB0406500), the National Natural Science Foundation of China (Grant Nos. U1804155, U1604263, and 11804307), and the China Postdoctoral Science Foundation (Grant Nos. 2018M630830 and 2019T120631).

Nanodiamonds have outstanding mechanical properties, chemical inertness, and biocompatibility, which give them potential in various applications. Current methods for preparing nanodiamonds often lead to products with impurities and uneven morphologies. We report a two-step high-pressure high-temperature (HPHT) method to synthesize nanodiamonds using naphthalene as the precursor without metal catalysts. The grain size of the diamonds decreases with increasing carbonization time (at constant pressure and temperature of 11.5 GPa and 700 °C, respectively). This is discussed in terms of the different crystallinities of the carbon intermediates. The probability of secondary anvil cracking during the HPHT process is also reduced. These results indicate that the two-step method is efficient for synthesizing nanodiamonds, and that it is applicable to other organic precursors.

Metastable (CuAu-type) CuInS2 Phase: High-Pressure Synthesis and Structure Determination.

We report on the high-pressure solid-state synthesis and the detailed structural characterization of the metastable, CuAu-type CuInS2 (CA-CIS) phase. Although often present in CIS thin films as unwanted phase, it has been never synthesized in pure form, and its effect on the performance of CIS-based solar cells has been long debated. In this work, pure CA-CIS phase is synthesized in bulk polycrystalline form through a high-pressure-high-temperature solid-state reaction. Single-crystal X-rays diffraction reveals the formation of tetragonal CA-CIS (a = 3.9324(5), c = 5.4980(7) Å) either in cation-ordered and disordered phase, pointing out the role of the pressure/temperature increase on the Cu/In ordering. The resistivity measurements performed on CA-CIS show low resistivity and a flat trend vs temperature and, in the case of the ordered phase, highlight a bad-metallic behavior, probably due to a high level of doping. These findings clearly rule out the possibility of a beneficial effect of this phase on the CIS-based thin film solar cells.

Charge disproportionation and interchange transitions in twelve-layer BaFeO3

A fully oxygenated hexagonal 12-layer perovskite $\mathrm{BaFe}{\mathrm{O}}_{3}$ with nominal $\mathrm{F}{\mathrm{e}}^{4+}$ was obtained by high-pressure synthesis. The structure consists of face-sharing $\mathrm{F}{\mathrm{e}}_{3}{\mathrm{O}}_{9}$ octahedral trimers and corner-sharing octahedra. A remarkable series of electronic transitions that relieve the electronic instability of $\mathrm{F}{\mathrm{e}}^{4+}$ are discovered on cooling; $2\mathrm{F}{\mathrm{e}}^{4+}\ensuremath{\rightarrow}\mathrm{F}{\mathrm{e}}^{3+}+\mathrm{F}{\mathrm{e}}^{5+}$ charge disproportionation and ordering over two sites accompanied by a structural distortion at 500 K; charge disproportionation of the remaining $\mathrm{F}{\mathrm{e}}^{4+}$ with magnetic order at 280 K; and an unprecedented charge interchange where $\mathrm{F}{\mathrm{e}}^{3+}$ and $\mathrm{F}{\mathrm{e}}^{5+}$ states are exchanged between two sites with spin orientation change at 50 K. These properties demonstrate the presence of multiple competing ground states in this complex, highly condensed network of corner- and face-sharing octahedra.

Ab initio study of band gap properties in metastable BC8/ST12 SixGe1−x alloys

The cubic Ia3¯ (BC8) and tetragonal P43212 (ST12) high pressure modifications of Si and Ge are attractive candidates for application in optoelectronic, thermoelectric, or plasmonic devices. SixGe1−x alloys in BC8/ST12 modifications could help overcome the indirect and narrow bandgaps of the pure phases and enable tailoring for specific use-cases. Such alloys have experimentally been found to be stable at ambient conditions after release from high pressure synthesis; however, their fundamental properties are not known. In this work, we employ ab initio calculations based on density functional theory (DFT) to investigate the electronic properties of these compounds as a function of composition x. We obtain the effective band structures of intermediate alloys by constructing special quasi-random structures (SQSs) and unfolding their band structure to the corresponding primitive cell. Furthermore, we show that the indirect bandgap of the ST12 Ge end-member can be tuned to become direct at xSi≈0.16. Finally, our investigations also demonstrate that the BC8 modification, on the other hand, is insensitive to compositional changes and is a narrow direct bandgap semiconductor only for the case of pure Si.

Fourier Transform Limited Linewidth of Optical Transitions in Single SiV Centers in “Adamantane” Nanodiamonds

Silicon-vacancy (SiV) color centers in diamond offer a promising system for quantum information applications owing to their intense narrowband emission and optically detectable spin states. Here, the fluorescent properties of ensembles and single SiV centers in diamonds obtained by high-pressure high-temperature synthesis from adamantane (below referred to as adamantane diamonds) are investigated at liquid-helium temperatures. Ensembles of SiV centers (~103) are studied in large diamond crystals (1–2 µm in size). Despite the large number of excited centers, the fine structure of the zero-phonon line, corresponding to four allowed optical transitions between the ground- and excited-state doublets of the SiV center, can be observed in the fluorescence spectra of these crystals. The width of individual lines is in the range of 60–80 GHz, which is explained by their inhomogeneous broadening. Single SiV centers are studied in diamond crystallites with sizes of about 200 nm. The width of the narrowest transition lines observed in the fluorescence spectra of single SiV centers under resonant excitation is almost three orders of magnitude smaller than linewidths in the spectra of SiV ensembles and is as small as 94 MHz; i.e., these lines are so narrow that their width is determined by the lifetime of the upper state of the transition. Therefore, adamantane nanodiamond features the narrowest emission line width of a single SiV center at cryogenic temperatures among other types of SiV-con-taining nanodiamonds of similar sizes obtained by the high-pressure high-temperature and chemical vapor deposition techniques.

Phase Transitions in the Metastable Perovskite Multiferroics BiCrO3 and BiCr0.9Sc0.1O3: A Comparative Study

The temperature behavior of the crystal structure as well as dielectric and magnetic properties of the perovskite bismuth chromate ceramics with the 10 mol % Cr3+-to-Sc3+ substitution were studied and compared with those of the unmodified compound. Using a high-pressure synthesis, BiCrO3 and BiCr0.9Sc0.1O3 were obtained as metastable perovskite phases which are monoclinic C2/c with the √6ap × √2ap × √6ap superstructure (where ap is the primitive perovskite unit-cell parameter) under ambient conditions. At room temperature, the unit cell volume of BiCr0.9Sc0.1O3 is ∼1.3% larger than that of BiCrO3. Both perovskites undergo a reversible structural transition into a nonpolar GdFeO3-type phase (orthorhombic Pnma, √2ap × 2ap × √2ap) in the temperature ranges of 410-420 K (BiCrO3) and 470-520 K (BiCr0.9Sc0.1O3) with a relative jump of the primitive perovskite unit cell volume of about -1.6 and -2.0%, respectively. Temperature dependences of the complex dielectric permittivity demonstrate anomalies in the phase transition ranges. The Pnma-to-C2/c crossover in BiCrO3 is accompanied by a decrease in the direct current (dc) conductivity, while in BiCr0.9Sc0.1O3 the conductivity increases. The onset of an antiferromagnetic order in BiCr0.9Sc0.1O3 is observed at the Néel temperature (TN) of about 85 K as compared with TN = 110 K in BiCrO3. In contrast to BiCrO3, which exhibits a spin reorientation at Tsr ∼ 72 K, no such a transition occurs in BiCr0.9Sc0.1O3.

High-pressure synthesis, crystal structure, and magnetic properties of the Shastry-Sutherland-lattice oxides BaLn2ZnO5 (Ln = Pr, Sm, Eu)

BaPr2ZnO5, BaSm2ZnO5, and BaEu2ZnO5 – a series of 4f magnetic insulators comprising the Shastry-Sutherland lattice – were synthesized via a solid-state reaction under high-pressure and high-temperature conditions. The magnetic behaviors are well characterized by the localized 4f electrons of each Ln3+ (= Pr, Sm, Eu) under the influence of the crystal-electric field of the surrounding ions. The magnetic susceptibility and heat capacity measurements of BaPr2ZnO5 (Pr3+; 4f23H4) indicate the splitting of the ground state J-multiplet due to the crystal-electric field. BaSm2ZnO5 (Sm3+; 4f56H5/2) is a strong Van Vleck paramagnet, while BaEu2ZnO5 is non-magnetic due to the 7F0 ground state (Eu3+; 4f67F0). The series of new oxides enriches a group of the Shastry-Sutherland lattice materials and helps to develop a range of two-dimensional quantum materials.

High-pressure synthesis, crystal structure and physical properties of a new Cr-based arsenide La3CrAs5

In La-Cr-As system, the first ternary compound La3CrAs5 has been successfully synthesized under high- pressure and high-temperature conditions. La3CrAs5 crystallizes into a hexagonal Hf5Sn3Cu-anti type structure with a space group of P 63/ mcm (No. 193) and lattice parameters of a = b = 8.9845 A and c = 5.8897 A. The structure contains face-sharing octahedral CrAs6 chains along the c -axis, which are arranged triangularly in the ab -plane and separated by a significantly large distance of 8.9845 A. The magnetic properties, resistivity and specific heat measurements were performed. La3CrAs5 exhibits a metallic state with Fermi liquid behavior at low temperatures and undergoes a ferromagnetic transition at Curie temperature T C ~50 K. First-principles theoretical studies were conducted to calculate its band structure and density of states (DOS), which indicated that the non-negligible contribution of La to the DOS near the Fermi level caused La3CrAs5 to be a three-dimensional (3D) metal. The crystal orbital Hamilton population (−COHP) was also calculated to explain the global stability and bonding characteristics in the structure of La3CrAs5.

High-pressure synthesis of heavily hole-doped cuprates Mg1−xLixCu2O3 with quasi-one-dimensional structure

We investigate electronic properties of hole-doped cuprates ${\mathrm{Mg}}_{1\ensuremath{-}x}{\mathrm{Li}}_{x}{\mathrm{Cu}}_{2}{\mathrm{O}}_{3}$ with the quasi-one-dimensional two-leg-ladder structure. We succeeded in extending the solubility limit of Li in ${\mathrm{Mg}}_{1\ensuremath{-}x}{\mathrm{Li}}_{x}{\mathrm{Cu}}_{2}{\mathrm{O}}_{3}$ from $x=0.12$ to $x=0.60$ by using the high-pressure synthesis technique. The antiferromagnetic transition temperature rapidly decreases with increasing Li content from 94 K at $x=0$ to 7.5 K at $x=0.30$, and takes an almost constant value (3--6 K) at $x\ensuremath{\ge}0.35$. The antiferromagnetic order still exists even at $x=0.60$, where the formal valence of Cu is as large as $+2.30$. The temperature dependence of the specific heat suggests the finite contribution of the electronic specific heat at $x=0.20--0.60$, which is consistent with high valence of Cu. Nevertheless, the temperature dependence of resistivity shows a variable range hopping behavior in the whole $x$ ranges, and the insulating behavior survives under the pressure up to 2.9 GPa. This peculiar behavior is owing to the disorder originating from the intersite atom exchanges due to the similar ionic radius of cations in ${\mathrm{Mg}}_{1\ensuremath{-}x}{\mathrm{Li}}_{x}{\mathrm{Cu}}_{2}{\mathrm{O}}_{3}$. Nonmagnetic ions of ${\mathrm{Mg}}^{2+}$ and ${\mathrm{Li}}^{+}$ are introduced into the ${\mathrm{Cu}}_{2}{\mathrm{O}}_{3}$ planes of ${\mathrm{Mg}}_{1\ensuremath{-}x}{\mathrm{Li}}_{x}{\mathrm{Cu}}_{2}{\mathrm{O}}_{3}$, resulting in the localization of doped hole carriers.

High-Pressure Synthesis and Thermal Transport Properties of Polycrystalline BAsx**Supported by the National Key R&D Program of China (Grant Nos. 2018YFE0103200 and 2018YFA0305700), the National Natural Science Foundation of China (Grant Nos. 51772324, 11934017, 11921004, and 11574378), and the Chinese Academy of Sciences (Grant Nos. QYZDB–SSW–SLH013 and GJHZ1773).

Polycrystalline BAsx (x = 0.80–1.10) compounds with different boron-to-arsenic elemental molar ratios were synthesized by a high-pressure and high-temperature sintering method. Compared with other ambient-pressure synthesis methods, high pressure can significantly promote the reaction speed as well as the reaction yield. As the content of arsenic increases from x = 0.91 to 1.10, the thermal conductivity of BAsx gradually increases from 53 to 65 W⋅m−1⋅K−1. Furthermore, the temperature dependence of thermal conductivities of these samples reveals an Umklapp scattering due to the increasing phonon population. This work provides a highly efficient method for polycrystalline BAs synthesis.

High-pressure Synthesis of Ba2CoO2Ag2Te2 with Extended CoO2 Planes

Using a high-pressure synthesis method, we prepared the layered oxychalcogenide Ba2CoO2Ag2Te2 (space group: I4/mmm) with alternating stacks of CoO2 and Ag2Te2 layers, separated by Ba atoms. The CoO2 plane is greatly extended (Co-O = 2.19 Å on average) due to tensile strain from adjacent Ag2Te2 layers, causing displacement of oxide anions. Layered cobaltates with trans-CoO4X2 (X = chalcogen, halogen) coordination feature large spin-orbit coupling, which is linearly scaled by the tetrahedral factor of dCo-X/dCo-O. However, applying this relation to Ba2CoO2Ag2Te2 yields a magnetic moment of ∼4 μB, which is nearly twice the experimentally observed value of 1.87(17) μB. This result, along with a reduced Néel temperature (TN = 60 K), originates from the off-centered position of otherwise under-bonded oxide anions, which changes the crystal field splitting of Co d orbitals.

Perovskite-Type CuNbO3 Exhibiting Unusual Noncollinear Ferrielectric to Collinear Ferroelectric Dipole Order Transition

We report a perovskite oxide with a new type of polar structure, CuNbO3, thanks to the use of high-pressure and high-temperature synthesis. The perovskite-type CuNbO3 was previously obtained under ...

Congruent melting of tungsten phosphide at 5 GPa and 3200 °C for growing its large single crystals**Project supported by the National Key Research and Development Program of China (Grant Nos. 2016YFA0401503 and 2018YFA0305700), the National Natural Science Foundation of China (Grant No. 11575288), the Youth Innovation Promotion Association of Chinese Academy of Sciences (Grant No. 2016006). the Key Research Platforms

As one of important members of refractory materials, tungsten phosphide (WP) holds great potential for fundamental study and industrial applications in many fields of science and technology, due to its excellent properties such as superconductivity and as-predicted topological band structure. However, synthesis of high-quality WP crystals is still a challenge by using tradition synthetic methods, because the synthesis temperature for growing its large crystals is very stringently required to be as high as 3000 °C, which is far beyond the temperature capability of most laboratory-based devices for crystal growth. In addition, high temperature often induces the decomposition of metal phosphides, leading to off-stoichiometric samples based on which the materials’ intrinsic properties cannot be explored. In this work, we report a high-pressure synthesis of single-crystal WP through a direct crystallization from cooling the congruent W–P melts at 5 GPa and ∼ 3200 °C. In combination of x-ray diffraction, electron microscope, and thermal analysis, the crystal structure, morphology, and stability of recovered sample are well investigated. The final product is phase-pure and nearly stoichiometric WP in a single-crystal form with a large grain size, in excess of one millimeter, thus making it feasible to implement most experimental measurements, especially, for the case where a large crystal is required. Success in synthesis of high-quality WP crystals at high pressure can offer great opportunities for determining their intrinsic properties and also making more efforts to study the family of transition-metal phosphides.

High-pressure synthesis of a new compound of the Mg–Co system

High-pressure synthesis is an effective technique to obtain novel compounds. Under high pressure of the order of GPa, the atomic radius of Mg undergoes large shrinkage compared with that of transition metals (TM) and novel compounds with unusual atomic sizes of Mg and TMs can be synthesized. This study describes the high-pressure synthesis of a novel Mg–Co alloy of formula Mg44Co7 at above 5 GPa and 973 K for 8 h. Investigation of the crystal structure of Mg44Co7 using X-ray powder diffraction reveals its Mg44Rh7-type structure (space group F-43 m) with lattice parameter a = 20.127 (1) Å. Mg44Co7 decomposes into Mg and MgCo phases at 663 K in an Ar atmosphere via an exothermic reaction. Hydrogenation of Mg44Co7 at 573 K–623 K under 8 MPa of H2 results in its decomposition into MgH2, Mg2CoH5 and Mg6Co2H11 with a total hydrogen content of 3.5–5.0 mass%.

High-Pressure Synthesis of Metal-Inorganic Frameworks Hf4 N20 ⋅N2 , WN8 ⋅N2 , and Os5 N28 ⋅3 N2 with Polymeric Nitrogen Linkers.

Polynitrides are intrinsically thermodynamically unstable at ambient conditions and require peculiar synthetic approaches. Now, a one‐step synthesis of metal–inorganic frameworks Hf4N20⋅N2, WN8⋅N2, and Os5N28⋅3 N2 via direct reactions between elements in a diamond anvil cell at pressures exceeding 100 GPa is reported. The porous frameworks (Hf4N20, WN8, and Os5N28) are built from transition‐metal atoms linked either by polymeric polydiazenediyl (polyacetylene‐like) nitrogen chains or through dinitrogen units. Triply bound dinitrogen molecules occupy channels of these frameworks. Owing to conjugated polydiazenediyl chains, these compounds exhibit metallic properties. The high‐pressure reaction between Hf and N2 also leads to a non‐centrosymmetric polynitride Hf2N11 that features double‐helix catena‐poly[tetraz‐1‐ene‐1,4‐diyl] nitrogen chains [−N−N−N=N−]∞.

Synthesis and performance of TiB2–B4C composites by high pressure of TiC–B mixture

AbstractTiB2–B4C composites were in situ synthesized and consolidated by high pressure synthesis method from a mixture of TiC and B powders at the pressure and temperature of 5.0 GPa and 1500℃‐1900℃. The phase composition, microstructure, density, hardness, thermal conductivity, and electrical resistivity of TiB2–B4C composites were analyzed. As the increase in the synthesis temperature, the products were TiB2 and B4C phases and that crystallinity improved. TiB2–B4C composites were dense without obvious pores. TiB2–B4C composites synthesized at 1800℃ obtained the optimized performance, including the relative density of 98.2%, the Vickers hardness of 31.7 ± 1.2 GPa with the load of 9.8 N, the thermal conductivity of 30.3 ± 0.7 W/(m K), and the electrical resistivity of 3.3 × 10−3 Ω cm, respectively. The grain size of the TiB2–B4C composites changed with the increase in synthesis temperature, leading to the changes in hardness, thermal conductivity, and electrical resistivity.

Effect of synthesis parameters on the structure and properties of carbon particles formed from amorphous fullerites

The effect of high-pressure synthesis temperature on the structure and indentation characteristics of the superelastic hard carbon formed from amorphous fullerites and on the tribological properties of the Co-based composite materials (CM) reinforced by the particles of such carbon has been studied by Raman spectroscopy, high-resolution transmission electron microscopy (HRTEM), indentation measurements, and tribological tests. It is shown that ball milling (for 48 h) of C60 fullerite crystals results in the amorphization of the product of fullerite transformation upon their high-pressure treatment at temperatures above the stability limit of fullerene molecule (∼800°C). An increase in synthesis temperature at 8 GPa from 800°C to 1200°C leads to a gradual graphitization of the structure of amorphous fullerite derived carbon. This decreases its hardness and indentation modulus from 32 to 18 GPa and from 256 to 95 GPa, respectively, and increases the elastic recovery (from 80% to 86%). The best tribological characteristics of the CM are attained at the maximum particle hardness, which is realized in the CM synthesized at 800°C. When the synthesis temperature is elevated to 1200°C, the friction coefficient and wear rate of the CM increase, but they remain substantially lower than those of the matrix cobalt.