High Pressure Temperature Conditions Research Articles

In Situ Measurement Techniques Using Diamond Anvil Cell at High Pressure–Temperature Conditions: A Review

Diamond anvil cells have garnered significant attention in high‐pressure studies as a valuable tool for investigating material preparation, phase transition dynamics, and ultra‐high‐pressure physical chemistry. Its potential applications span fields such as materials science, condensed matter physics, chemistry, and geology. This study conducts a comprehensive review of the utilization of laser‐heated diamond anvil cell (LH‐DAC) devices in conjunction with in situ optical characterization techniques, such as X‐Ray and Raman scattering. Further, diverse in situ performance measurement methods encompassing electrical, thermal, magnetic, and acoustic analyses are examined. The development and role of the prevailing in situ measurement techniques have been described, along with the current progress in applied research for each technique. This study aims to facilitate the discovery of new structures and properties of materials under high pressure–temperature conditions.

Annealing process and temperature effects on silicon-vacancy and germanium-vacancy centers in CVD grown polycrystalline diamond

The annealing treatment plays a crucial role in tailoring the properties of synthetic diamond materials, especially those doped with various elements in order to form specific color centers like nitrogen-vacancy (NV), silicon-vacancy (Si-V), germanium-vacancy (Ge-V), etc. This study delves into the annealing of 175 μm-thick Ge-doped polycrystalline diamond (PCD) films grown by microwave plasma-assisted chemical vapor deposition (MPCVD). Large-area PCD plate was cut into smaller equivalent 5 × 5 mm2 pieces, which were separately subjected to annealing in microwave plasma in H2 atmosphere, to annealing in vacuum or to annealing under high-pressure high-temperature conditions (HPHT, 5.9 GPa, 2000 °C). The structure, phase composition and photoluminescence (PL) of samples before and after various annealing processes were investigated. All applied types of annealing enhance both the Si-V and Ge-V lines in PL at room temperature. Increasing annealing temperature leads to gradual decrease of full widths at half-maxima (FWHM) of diamond Raman peak (1332.5 cm−1), as well as Si-V (738 nm) and Ge-V (602 nm) PL peaks. In addition, the limitations for each type of annealing are established. The obtained results are crucial for the design of CVD-grown Ge-doped and Si-doped PCD materials that can be used for applications in photonics such as single photon sources, biomarkers, as well as for the fabrication of optical diamond thermometers.

High-pressure high-temperature synthesis of NdRe2.

In this study, we report the synthesis of a new cubic neodymium-rhenium metallic alloy NdRe2 through the utilization of high pressure and laser heating in a diamond anvil cell. NdRe2 crystallizes in the space group with a lattice parameter equal to 7.486 (2) Å and Z = 8at 24 (1) GPa and 2,200 (100) K. It was studied using high-pressure single-crystal X-ray diffraction. The compound crystallizes in the cubic MgCu2 structure type. Its successful synthesis further proves that high-pressure high-temperature conditions can be used to obtain alloys holding a Laves phase structure. Ab initio calculations were done to predict the mechanical properties of the material. We also discuss the usage of extreme conditions to synthesize and study materials present in the nuclear waste.

Evaluating the Effect of Claytone-EM on the Performance of Oil-Based Drilling Fluids.

This study provides a detailed characterization and evaluation of Claytone-EM as a rheological additive to enhance the performance of oil-based drilling fluids (OBDFs) under high-pressure, high-temperature (HPHT) conditions. It also offers a comparative evaluation of the effectiveness of Claytone-EM with an existing organoclay, analyzing their mineral and chemical compositions, morphologies, and particle sizes. A series of experiments are performed to evaluate Claytone-EM's influence on crucial drilling mud properties, such as mud density, electrical stability, sagging tendency, rheology, viscoelastic properties, and filtration properties, to formulate a stable and high-performing OBDF. Results indicated that Claytone-EM had no significant impact on mud density but remarkably enhanced emulsion stability. Claytone-EM effectively mitigated sagging issues under both static and dynamic conditions, leading to improvements in the plastic viscosity (PV), yield point (YP), apparent viscosity (AV), and YP/PV ratio. The PV, YP, AV, and YP/PV ratios were improved by 11, 85, 28, and 66% increments, respectively, compared with those of the drilling fluid formulated with MC-TONE. The addition of Claytone-EM resulted in enhancing gel strength and improving the filtration properties of the drilling fluid. The filtration volume was reduced by 2% from 5.0 to 4.9 cm3, and the filter cake thickness had a 13% reduction from 2.60 to 2.26 mm. These findings highlight Claytone-EM as a valuable additive for enhancing OBDF performance, particularly under challenging HPHT conditions. Its ability to provide emulsion stability, reduce static and dynamic sag, and control filtration holds the potential to enhance drilling operations, minimize downtime, and bolster wellbore stability. This study acknowledges certain limitations, including its temperature range, which could benefit from exploration at extreme temperatures. Additionally, the absence of flow experiments limits a comprehensive understanding of sag effects, and further research and field-scale evaluations are recommended to validate and optimize the application of Claytone-EM in OBDFs.

Cr5.7Si2.3P8N24-A Chromium(+IV) Nitridosilicate Phosphate with Amphibole-Type Structure.

The first nitridic analog of an amphibole mineral, the quaternary nitridosilicate phosphate Cr5.7Si2.3P8N24 was synthesized under high-pressure high-temperature conditions at 1400 °C and 12 GPa from the binary nitrides Cr2N, Si3N4 and P3N5, using NH4N3 and NH4F as additional nitrogen source and mineralizing agent, respectively. The crystal structure was elucidated by single-crystal X-ray diffraction with microfocused synchrotron radiation (C2/m, a=9.6002(19), b=17.107(3), c=4.8530(10) Å, β=109.65(3)°). The elemental composition was analyzed by energy dispersive X-ray spectroscopy. The structure consists of vertex-sharing PN4-tetrahedra forming zweier double chains and edge-sharing (Si,Cr)-centered octahedra forming separated ribbons. Atomic resolution scanning transmission electron microscopy shows ordered Si and Cr sites next to a disordered Si/Cr site. Optical spectroscopy indicates a band gap of 2.1 eV. Susceptibility measurements show paramagnetic behavior and support the oxidation state Cr+IV, which is confirmed by EPR. The comprehensive analysis expands the field of Cr-N chemistry and provides access to a nitride analog of one of the most prevalent silicate structures.

Microfluidic devices, materials, and recent progress for petroleum applications: A review

AbstractMicrofluidic devices are miniaturized systems that manipulate fluids on a small scale, typically at microlitre or nanolitre volumes. They have received significant attention in various industries, including petroleum applications. The recent advancements in microfluidic devices for petroleum applications have the potential to improve oil recovery efficiency, reduce costs, and provide valuable insights into fluid behaviour and reservoir characterization. This paper aims to provide a comprehensive overview of microfluidic devices, their materials, and their applications in the petroleum industry. The first section of the paper focuses on describing the materials used in microfluidic devices specifically tailored for energy sector applications. In the second section, the paper highlights relevant applications and discoveries in petroleum research, showcasing the innovative techniques and special features enabled by microfluidic devices. These applications include but are not limited to asphaltenes characterization, enhanced oil recovery (EOR), and water treatment. The versatility and customization of microfluidic devices have allowed researchers to accurately represent fractures, ensure chemical conformance, simulate high pressure–high temperature conditions and reservoir heterogeneity, and study geochemical interaction. Additionally, molecular tagging and machine learning techniques have been employed for image analysis, further enhancing the capabilities of microfluidic devices. Throughout the paper, the advantages and disadvantages of implementing microfluidic devices and utilizing specific materials are thoroughly discussed. This analysis provides valuable insights into the challenges and potential limitations associated with this technology. To conclude, the paper offers suggestions, ideas, and highlights for future research paths, pointing toward promising directions for further exploration in this field.

Carbon Isotopic Behavior During Hydrocarbon Expulsion in Semiclosed Hydrous Pyrolysis of Type I and Type II Saline Lacustrine Source Rocks in the Jianghan Basin, Central China

Abstract Organic carbon isotopic analysis is a significant approach for oil-source correlation, yet organic carbon isotopic behavior during oil expulsion from saline lacustrine source rocks is not well constrained, and this hinders its wide application for fingerprinting oils generated by saline lacustrine source rock. To resolve this puzzle, semiclosed hydrous pyrolysis was conducted on typical saline lacustrine source rocks from the Qianjiang Formation (type I kerogen) and Xingouzui Formation (type II kerogen) sampled in the Jianghan Basin, China, under high-temperature high-pressure conditions (T = 275℃–400℃; P = 65–125 MPa). Experimental results show that there is minor carbon isotopic fractionation (<3‰) between pyrolyzed and nonpyrolyzed retained oil fractions during the main oil generation/expulsion stage of both type I and II source rocks. Carbon isotopic fractionations between expelled and retained oil fractions are also minor (<2‰) during this stage. The δ13C values of retained and expelled oil fractions generated by the type I saline lacustrine source rock correlate positively with the degree of oil expulsion, whereas the influence of oil expulsion on the δ13C values of oil fractions generated by the type II source rock was not consistent. In addition, carbon isotopic analysis also unravels the mixing of oil-associated gases with different maturity levels and/or generated via different processes. Outcomes of this study demonstrate that oil expulsion from type I and II saline lacustrine source rocks cannot be able to result in large-degree carbon isotopic fractionation, indicating that carbon isotopic analysis is a feasible approach for conducting oil-source correlation works in saline lacustrine petroleum systems.

Formation of grain boundaries in nanocrystalline SiC ceramics examined by powder diffraction supported by MD simulations

Atomic structure of nanocrystalline SiC powder sintered under high-pressure high-temperature conditions was examined with use of powder diffraction technique supported by molecular dynamics simulation. Shape and atomic structure of grain surfaces was identified based on real space PDF analysis of experimental diffraction data and theoretical calculations performed for atomic models of nanograins relaxed by MD simulation. It is shown that in presence of about 5 % of excess carbon in the initial 11 nm size SiC powder HPHT sintering leads to formation of (100) and (110) grain boundaries formed by single C planes and (111) boundaries formed by double C planes. Under pressure 3 GPa and 1600°C sphere-like grains tend to transform to dodecahedrons terminated by single C atomic planes. Under 8 GPa the grains adopt the shape of cube-octahedrons terminated by (100) single C planes and (111) planes terminated by single and double C planes.

Preparation of nanodiamonds based on phase transformation of vertical sheet under atmospheric pressure

A basic and important way to prepare diamond is to make graphite experience the phase transformation under the high-pressure high-temperature (HPHT) condition. However, this method needs stringent equipment and high investment cost. Recently, we proposed a method to prepare the diamond by phase transformation of graphite at atmospheric pressure with monodispersed Ta atoms. It is found that a phase transformation happens to H atoms under atmospheric pressure, but the role of O atoms has not been investigated. Here, we use tantalum wires as Ta source and heat the filaments to prepare vertical graphene containing Ta atoms in hot filament chemical vapor deposition (HFCVD) system. And then the vertical graphene layers are annealed in oxygen-containing environment, and nanodiamonds are obtained by phase transformation from the vertical graphene under atmospheric pressure. The results show that the sample morphologies are the same as the untreated vertical graphene’s, when the annealed ambient air pressure is at 10 Pa and 50 Pa with oxygen atom content of 1.96% and 2.04%, respectively; TEM tests reveal TaC and graphite but no diamond in these samples . Nanodiamond grains with the size range of 2–4 nm are observed in the amorphous carbon region of samples annealed at 100 Pa and 500 Pa air pressure with oxygen atom content increasing to 2.77% and 3.11%, respectively, indicating that oxidation facilitates the phase change from Ta-containing vertical graphene to diamond at atmospheric pressure. When the air pressure of the annealing environment rises to 1000 Pa with the oxygen atom content of 3.54%, the sample is extensively oxidized and the graphite structure is severely damaged,which means that a large number of oxygen atoms tend to disrupt the graphite structure rather than promote the phase change into diamond. These results supply a way to prepare nanodiamond and show the effect of O atoms in the graphite phase transition at atmospheric pressure.

Stabilization Of The CN35- Anion In Recoverable High-pressure Ln3O2(CN3) (Ln = La, Eu, Gd, Tb, Ho, Yb) Oxoguanidinates.

A series of isostructural Ln3O2(CN3) (Ln = La, Eu, Gd, Tb, Ho, Yb) oxoguanidinates was synthesized under high-pressure(25-54 GPa) high-temperature (2000-3000 K) conditions in laser-heated diamond anvil cells. The crystal structure of this novel class of compounds was determined via synchrotron single-crystal X-ray diffraction (SCXRD)as well as corroborated by X-ray absorption near edge structure (XANES) measurements and density functional theory (DFT) calculations. The Ln3O2(CN3) solids are composed of the hitherto unknown CN35- guanidinate anion - deprotonated guanidine. Changes in unit cell volumes and compressibility of Ln3O2(CN3) (Ln = La, Eu, Gd, Tb, Ho, Yb) compounds are found to be dictated by the lanthanide contraction phenomenon. Decompression experiments show that Ln3O2(CN3) compounds are recoverable to ambient conditions. The stabilization of the CN35- guanidinate anion at ambient conditions provides new opportunities in inorganic and organic synthetic chemistry.

Modified locally derived graphene nanoplatelets for enhanced rheological, filtration and lubricity characteristics of water-based drilling fluids

Drilling fluid rheology, filtration, and lubricity are fundamental parameters that influence the efficiency and ease of drilling operations. The drilling industry is constantly exploring different research areas for stabilizing and improving these parameters at high temperatures. The use of drilling fluid additives, especially synthetic polymers, poses a threat to the environment. Herein, rice husk char (RHC) was used to synthesize graphene Nanoplatelets (GNP) whose surface is modified with triton-X100 nonionic surfactant. Field emission scanning electron microscopy (FESEM), Fourier transform infrared (FTIR), Particle size distribution (PSD), Energy Dispersive X-Ray Analysis (EDX), and Zeta potential analysis are performed on the synthesized nanomaterials to verify their morphology, functional groups, particle size, elemental composition, and material stability. Synthesized GNP from RHC (GNP-RHC) along with surface-modified GNP-RHC (GNP-TXT) were examined critically and their properties compared using a water-based mud (WBM) formulation. The rheological and lubricity characteristics of all the mud samples were determined before and after hot rolling (AHR), and the filtration properties were determined under API and high pressure high temperature (HPHT) conditions. The TGA results indicate that GNP-TXT nanomaterial resists thermal degradation with 10% weight loss at 300 °C, while the zeta potential analysis indicates the materials are stable. Since the GNP-TXT particles are so well dispersed, the GNP-TXT-based muds worked better than the GNP-RHC-based muds and was more stable at controlling filtration and improving rheological properties. For example, at 1.5 g AHR, the plastic viscosity of GNP-TXT-based mud decreased by 22.2% from 9 to 7 cP, while that of GNP-RHC-based mud decreased from 17 to 7 cP by 59%. Also, the API and HPHT FL of WBM of 11 and 24.8 mL, respectively, were reduced by GNP-TXT-based muds to the range of 8.6 and 9.8 mL for API and 18.6 and 22.3 mL for HPHT. However, GNP-RHC-based drilling muds showed a lower fluid loss control of the WBM between 9.6 and 10.2 mL for API and between 19.6 and 23.6 mL for HPHT. Furthermore, the coefficient of friction of the WBM of 0.45 was decreased with GNP-RHC concentrations between 0.47 and 0.32, while GNP-TXT demonstrated a higher decrease between 0.26 and 0.20.

A coupling prediction model of annular pressure build-up for deepwater oil and gas wells during transient-state testing

To accurately predict annular pressure build-up of deepwater high pressure-high temperature oil and gas wells during the transient-state testing, a coupling prediction model of trapped & untrapped annular pressure build-up is proposed. For the short-term process of gas well transient testing, a wellbore transient heat transfer model has been established. Then the improved coupling model of trapped & untrapped annular pressure build-up is established, which is not only novelly considering fluid thermodynamic properties (isobaric expansion coefficient and isothermal compression coefficient) under high pressure-high temperature conditions and annulus fluid mass changes of untrapped annulus, but also considering annulus volume changes. For precise coupling calculating the annular pressure build-up, the annulus is divided into infinitesimal grid cells along well depth. The impact of different considering factors, testing times, testing production rates, formation permeability and cement sheath sealing positions on the annular pressure build-up in the model is investigated. The results show that conventional models (high pressure-high temperature properties and mass changes of the fluid are not considered) will greatly overestimate the annular pressure build-up; as the testing time increases, the impact of untrapped annulus fluid mass changes on the annular pressure build-up will gradually take the dominant role; reasonably optimizing the setting of untrapped annulus open hole section in high permeability formations and increasing the distance between cement sheath and the upper casing shoe can effectively relieve the increasing annular pressure build-up. These findings can achieve accurate prediction of trapped and untrapped annular pressure build-up, and play an important role in guaranteeing the wellbore integrity and safety of deepwater oil and gas wells during transient-state testing.

Optimizing Sour Gas Qualification Testing—Modeling the Effects of Temperature and Total Pressure on H2S Fugacity, Activity, and Solubility Coefficients up to 138 MPa and 204°C

Traditionally, the sour severities of high-pressure high-temperature (HPHT) oil and gas production wells were assessed by H2S partial pressure (PH2S): The mole fraction of H2S in the gas (yH2S) multiplied by the total pressure (PT). However, PH2S usually over-predicts the actual sour severity of HPHT systems, leading to suboptimal material selection choices. To reflect recent advances in thermodynamic modeling and to avoid over-conservatism, after careful deliberation, ANSI/NACE MR0175-2021/ISO 15156-2:2022 recently expanded the number of sour severity metrics to four: PH2S, fugacity (fH2S), chemical activity (aH2S), and dissolved concentration (CH2S) of the aqueous phase. The new metrics are often computationally derived and account for thermodynamic nonidealities, which are significant at HPHT conditions. Regardless of the preferred metric, quantifying the sensitivity of each metric to a wide range of temperatures and total pressures is critical when conducting H2S service assessments. In this article, the effect of increasing temperature and total pressure on the thermodynamically derived apparent H2S solubility (KH2S = CH2S/PH2S) was investigated. KH2S is a critical parameter for quantifying changes in H2S phase behavior/sour severity of HPHT systems. Apparent KH2S values were calculated by two different thermodynamic models and benchmarked to two publicly available H2S/H2O datasets up to 120°C and 10.3 MPa equilibrated in a brine containing 165,000 mg/L Cl−. The model that provided the best match to the experimental data was later used in a much broader thermodynamic sensitivity study of the H2S/CH4/H2O/NaCl “oilfield” system. For this sensitivity analysis, changes in fH2S, aH2S, CH2S, and KH2S were individually modeled between 4°C and 204°C, at total pressures up to 138 MPa, and in brines containing up to 25 wt% NaCl (180,000 mg/L Cl−). Lastly, a comparison of the predicted sour severity by pseudo-PH2S, fH2S, and CH2S metrics, over the same temperature and total pressure parameter space, is presented.

The homogeneous mixing of MgO and H2O at extreme conditions

AbstractInvestigating water worlds presents a unique opportunity to understand the fundamental processes of planetary formation and evolution. One key aspect is characterizing the interactions between water and rock under the pressures and temperatures present within these worlds. Investigating the conditions for the homogeneous mixing of these materials is imperative to characterizing bulk properties and evolution of water‐rich exoplanets. Here we use density functional molecular dynamics simulations to study MgO‐H2O mixtures at high pressure–temperature conditions where H2O occurs in solid, superionic, or liquid form. MgO, the representative rocky material, can be either solid or liquid. We start from 500 K at 120 GPa, increasing the temperature step by step up to 8000 K. By inspection, we determine the temperature at which MgO‐HO homogeneously mix in our simulations. At 6000 K and 174 GPa is when we find the system to homogeneously mix. This heat‐until‐it‐mixes approach provides us with an upper bound on the temperature for the mixing of MgO and H2O. We find that homogeneous mixing occurs at sufficiently low temperatures to be relevant for the collisional growth of a water‐rich planet.

In situ and operando study of catalysts during high-temperature high-pressure catalysis in a fixed-bed plug flow reactor with x-ray absorption spectroscopy.

Numerous important catalytic reactions, such as Fischer-Tropsch synthesis (FTS), are performed under harsh conditions in terms of high temperature of a catalyst in a mixture of reactants at a high pressure. There has been a lack of an intrinsic correlation between a catalytic performance and its corresponding catalyst structure due to the unavailable information on the authentic structure of the catalyst during catalysis under a high-temperature high-pressure (HTHP) condition. Here, we report in situ/operando studies of Co catalysts during catalysis under HTHP conditions using x-ray absorption spectroscopy (XAS). A high-temperature high-pressure catalysis-XAS (HTHP Catalysis-XAS) system using a thin, small quartz or beryllium tube as the reactor was built for in situ/operando characterization of high-energy absorption edges of 4d transition metals or low-energy absorption edges of 3d/4d transition metals under high-temperature high-pressure conditions, respectively. This reactor can be used for HTHP catalysis performed at a temperature of up to 550 °C and a gas pressure of up to 60 bars for uncovering the chemical states and coordination environments of metal atoms of these catalysts during HTHP catalysis. The capability of collecting XAS data during HTHP catalysis was confirmed through tests at 400oC in the mixture of 20 bar mixture of reactants at beamline endstation. The operando studies of Ru catalyst particles under Fischer-Tropsch catalytic conditions with extended x-ray absorption fine structure spectroscopy revealed a restructuring of the Ru catalyst at 250 °C in the mixture of 6 bars CO and 12 bars H2 during FTS (30 ml/min), which was not observed at 300 °C in 1bar H2 (20 ml/min). This observation suggests new chemistry for metal catalysts under HTHP condition inaccessible due to a lack of applicable characterizations. These tests confirmed the function of this HTHP Catalysis-XAS system for in situ/operando characterizations of catalysts during HTHP catalysis.

Evidence of near-ambient superconductivity in a N-doped lutetium hydride.

The absence of electrical resistance exhibited by superconducting materials would have enormous potential for applications if it existed at ambient temperature and pressure conditions. Despite decades of intense research efforts, such a state has yet to be realized1,2. At ambient pressures, cuprates are the material class exhibiting superconductivity to the highest critical superconducting transition temperatures (Tc), up to about 133 K (refs. 3-5). Over the past decade, high-pressure 'chemical precompression'6,7 of hydrogen-dominant alloys has led the search for high-temperature superconductivity, with demonstrated Tc approaching the freezing point of water in binary hydrides at megabar pressures8-13. Ternary hydrogen-rich compounds, such as carbonaceous sulfur hydride, offer an even larger chemical space to potentially improve the properties of superconducting hydrides14-21. Here we report evidence of superconductivity on a nitrogen-doped lutetium hydride with a maximum Tc of 294 K at 10 kbar, that is, superconductivity at room temperature and near-ambient pressures. The compound was synthesized under high-pressure high-temperature conditions and then-after full recoverability-its material and superconducting properties were examined along compression pathways. These include temperature-dependent resistance with and without an applied magnetic field, the magnetization(M) versus magnetic field (H) curve, a.c. and d.c. magnetic susceptibility, as well as heat-capacity measurements. X-ray diffraction (XRD), energy-dispersive X-ray (EDX) and theoretical simulations provide some insight into the stoichiometry of the synthesized material. Nevertheless, further experiments and simulations are needed to determine the exact stoichiometry of hydrogen and nitrogen, and their respective atomistic positions, in a greater effort to further understand the superconducting state of the material.

Solid-State Transformations of Mayenite and Core-Shell Structures of C12A7@C Type at High Pressure, High Temperature Conditions

Calcium aluminate of a mayenite structure, 12CaO∙7Al2O3 (C12A7), is widely applicable in many fields of modern science and technology. Therefore, its behavior under various experimental conditions is of special interest. The present research aimed to estimate the possible impact of the carbon shell in core-shell materials of C12A7@C type on the proceeding of solid-state reactions of mayenite with graphite and magnesium oxide under High Pressure, High Temperature (HPHT) conditions. The phase composition of the solid-state products formed at a pressure of 4 GPa and temperature of 1450 °C was studied. As is found, the interaction of mayenite with graphite under such conditions is accompanied by the formation of an aluminum-rich phase of the CaO∙6Al2O3 composition, while in the case of core-shell structure (C12A7@C), the same interaction does not lead to the formation of such a single phase. For this system, a number of hardly identified calcium aluminate phases along with the carbide-like phrases have appeared. The main product of the interaction of mayenite and C12A7@C with MgO under HPHT conditions is the spinel phase Al2MgO4. This indicates that, in the case of the C12A7@C structure, the carbon shell is not able to prevent the interaction of the oxide mayenite core with magnesium oxide located outside the carbon shell. Nevertheless, the other solid-state products accompanying the spinel formation are significantly different for the cases of pure C12A7 and C12A7@C core-shell structure. The obtained results clearly illustrate that the HPHT conditions used in these experiments lead to the complete destruction of the mayenite structure and the formation of new phases, which compositions differ noticeably depending on the precursor used-pure mayenite or C12A7@C core-shell structure.

HPHT-Treated Impact Diamonds from the Popigai Crater (Siberian Craton): XRD and Raman Spectroscopy Evidence

Phase change and graphitization of diamonds from the Popigai impact crater (Krasnoyarsk Territory, Siberian platform, Russia) exposed to high-pressure high-temperature (HPHT) conditions of 5.5 GPa and 2000–2200 °C are studied by Raman spectroscopy and X-ray diffractometry (XRD). Light-color diamonds of type 1, free from inclusions, with 0 to 10 % lonsdaleite, are more resistant to HPHT effects than dark diamonds of type 2 rich in lonsdaleite and graphite. The lonsdaleite/diamond ratios in lonsdaleite-bearing impact diamonds become smaller upon annealing, possibly because lonsdaleite transforms to cubic diamond simultaneously with graphitization. Therefore, lonsdaleite is more likely a structure defect in diamond than a separate hexagonal phase.

Structural, optical and transport properties of layered europium disulfide synthesized under high pressure

A novel layered compound — europium disulfide — was synthesized under high-temperature–high-pressure conditions and characterized by complementary methods.

Electronic properties of single-crystalline Fe4O5.

We synthesized single and polycrystals of iron oxide with an unconventional Fe4O5 stoichiometry under high-pressure high-temperature (HP-HT) conditions. The crystals of Fe4O5 had a CaFe3O5-type structure composed of linear chains of iron with octahedral and trigonal-prismatic oxygen coordinations. We investigated the electronic properties of this mixed-valence oxide using several experimental techniques, including measurements of electrical resistivity, the Hall effect, magnetoresistance, and thermoelectric power (Seebeck coefficient), X-ray absorption near edge spectroscopy (XANES), reflectance and absorption spectroscopy, and single-crystal X-ray diffraction. Under ambient conditions, the single crystals of Fe4O5 demonstrated a semimetal electrical conductivity with nearly equal partial contributions of electrons and holes (σn ≈ σp), in line with the nominal average oxidation state of iron as Fe2.5+. This finding suggests that both the octahedral and trigonal-prismatic iron cations contribute to the electrical conductivity of Fe4O5via an Fe2+/Fe3+ polaron hopping mechanism. A moderate deterioration of crystal quality shifted the dominant electrical conductivity to n-type and considerably worsened the conductivity. Thus, alike magnetite, Fe4O5 with equal numbers of Fe2+ and Fe3+ ions can serve as a prospective model for other mixed-valence transition-metal oxides. In particular, it could help in the understanding of the electronic properties of other recently discovered mixed-valence iron oxides with unconventional stoichiometries, many of which are not recoverable to ambient conditions; it can also help in designing novel more complex mixed-valence iron oxides.