Bulk Chromium Research Articles

Mineralogical anatomy of the Cr-rich quenched angrite Northwest Africa 12774: Implication to mantle heterogeneity

Angrites are a small group of basaltic meteorites and their origin is currently disputed. Among these, Northwest Africa (NWA) 12774 is a quenched angrite that was reported having an anomalously high bulk Cr2O3 content (∼0.45 wt%). However, the reason behind this anomaly, which is critical for understanding the evolution of the angrite parent body, remains unknown. Here, we performed a detailed petrographic, mineralogical, and bulk oxygen and chromium isotopic composition study on this meteorite. NWA 12774 consists of porous olivine macrocrysts, phenocrysts of olivine and Al-Ti-rich augite, and spinel micro-phenocrysts with fine-grained groundmass. The olivine macrocrysts and the magnesian cores of olivine phenocrysts show compositional correlations distinctly different from typical olivine phenocrysts. The olivine macrocrysts contain small chromite/chrome-spinel inclusions which have the highest Cr2O3 content (53.2 wt%) for chromite/spinel in angrites to date. Based on these textural and chemical features, the olivine macrocrysts and the magnesian cores of olivine phenocrysts are identified as xenocrysts. Some pyroxene phenocrysts contain regions with complexly zoned microtextures, which have much larger chemical variations compared with those with simply zoned microtextures. The regions with complexly zoned microtextures are likely to be of xenocrystic origin. The bulk Cr2O3 content in NWA 12774 was estimated through two approaches, both of which show the bulk Cr2O3 content to be around 0.3 wt% or possibly up to ∼ 0.45 wt%, which is consistent with previously measured values. The high Cr2O3 content in NWA 12774 could be attributed to both the high abundance of spinel micro-phenocrysts and their high Cr2O3 contents, rather than the presence of Cr-rich xenocrysts. The calculated melt rare earth element concentration in NWA 12774 equilibrated with the most Mg-rich augite is essentially identical to those in LEW 87051 and Asuka 881371, which however have Cr2O3 contents much lower than NWA 12774. We suggest that the mantle source of NWA 12774 may not be as depleted in Cr and probably other volatile elements as other angrite sources. The various Cr2O3 contents in different quenched angrites probably reflect that their mantle sources have not been homogenized.

The Tribological Mechanism of Chromium at Extreme Stress-temperature Conditions Simulating the Interior Ballistic Environment in Artillery Barrels

The tribological mechanism of bulk chromium materials has been investigated at high-temperature and high-stress conditions simulating the interior ballistic environment in gun barrels. The results show a higher level of weight change after testing at 1, 000 °C as compared to that at 600 °C. The topographical difference is more evident in the friction surface after testing at 1, 000 °C than that at 600 °C. The detailed analysis of surface morphology and chemistry indicates a tribological mechanism combining oxidative wear, abrasive wear, and adhesive wear at a testing temperature of 600 °C, whilst the tribological mechanism is combined with adhesive wear and oxidative wear at a testing temperature of 1, 000 °C.

Impact of surface oxide on hydrogen permeability of chromium membranes

Hydrogen permeation through pure and oxidised bulk chromium membranes was measured by the classical gas technique to get insight into oxide as a hydrogen permeation barrier (HPB). An additional palladium-coated reference chromium membrane was tested to avoid the influence of native Cr oxide. Key parameters for Cr permeability: P0 = 3.23 × 10−7 mol H2/s/m/Pa0.5 and Ea = 0.68 eV and Cr diffusivity D0 = 9.0 × 10−5 m2/s and Ea = 0.59 eV. In the sample preparation stage, a thin ∼2 nm thick oxide was formed. Additional oxidation in pure oxygen at 400 °C increased the thickness from 20 to 50 nm. At this temperature, its efficiency as HPB was evaluated by comparing permeation rates to the reference chromium membrane. The highest permeation reduction factor of ∼3900 corresponded to only a ∼28 nm thick Cr oxide layer. Surface morphology and oxide thickness were investigated by SEM, while the thickness and type of chromium oxide by XPS.

Shidian meteorite, a new fall analog of near‐Earth asteroid (101955) Bennu

AbstractShidian is a recent meteorite which fell in Yunnan province, China, on November 27, 2017, and has been classified as a CM2 chondrite. Petrography, mineralogy, oxygen and chromium isotopic composition, reflectance spectrum, and density studies of Shidian are reported in this study. Clasts with different aqueous alteration degree, two type 1 clasts with nontypical CM petrography, and one metamorphic clast are observed in Shidian. Mineralogically, Shidian main body consists of phyllosilicates (∼70 vol%), forsterite (∼13 vol%), fayalitic olivine, carbonates, sulfide, high‐Ca pyroxene, magnetite framboids, and Fe‐Ni metal. The average electron microprobe analysis (EMPA) analytical totals of phyllosilicates are 84.07 ± 1.75 wt%, with average FeO/SiO2of tochilinite–cronstedtite intergrowths (TCIs) in different clasts ranging from 1.18 to 3.29. The bulk geochemical composition is characterized by flat rare earth element pattern, and by depletion of highly volatile elements. The whole rock oxygen isotopic composition is −0.51 ± 0.73‰, 5.44 ± 1.01‰, and −3.38 ± 0.20‰ for δ17O, δ18O, and Δ17O, respectively, with bulk chromium isotopic composition as ε54Cr = 1.00 ± 0.11. The grain density, bulk density, and porosity are 2.758 ± 0.008 g cm−3, 2.500 ± 0.004 g cm−3, and 9.37 ± 0.59%, respectively. The reflectance spectrum shows “blue” (negative) continuum slope across the visible and near‐infrared range, with characteristic absorption features (such as 0.765, 0.923, and 1.160 μm for phyllosilicates). These characteristics indicate that Shidian is an unheated, brecciated CM chondrite and may be an analog of asteroid Bennu.

Real- and momentum-space description of the excitons in bulk and monolayer chromium tri-halides

Excitons with large binding energies ~2–3 eV in CrX3 have been characterized as being localized (Frenkel) excitons that emerge from the atomic d − d transitions between the Cr-3d-t2g and eg orbitals. The argument has gathered strength in recent years as the excitons in recently made monolayers are found at almost the same energies as the bulk. The Laporte rule, which restricts such parity forbidden atomic transitions, can relax if a symmetry-breaking mechanism is present. While what can be classified as a purely Frenkel exciton is a matter of definition, we show using an advanced first principles parameter-free approach that these excitons in CrX3, in both its bulk and monolayer variants, have band origin and it is the dp hybridization between Cr and X that primarily acts as the symmetry-breaking mechanism that relaxes the Laporte rule. We show that the character of these excitons is mostly determined by the Cr-d orbital manifold, nevertheless, the fractions of the spectral weight shared with the ligand halogen states increases as the dp hybridization enhances. The hybridization enhances as the halogen atom becomes heavier, bringing the X-p states closer to the Cr-d states in the sequence Cl → Br → I, with an attendant increase in exciton intensity and a decrease in binding energy. By applying a range of different kinds of perturbations that qualitatively mimics the effects originating from the missing vertex in self-energy, we show that moderate changes to the two-particle Hamiltonian that essentially modifies the Cr-d-X-p hybridization, can alter both the intensities and positions of the exciton peaks. A detailed analysis of several deep-lying excitons, with and without strain, elucidates the fact that the exciton is most Frenkel-like in CrCl3 and CrBr3 and acquires mixed Frenkel–Wannier character in CrI3, making the excitons in CrI3 most susceptible to environmental screening and spin–orbit coupling.

Van der Waals Superstructure and Twisting in Self-Intercalated Magnet with Near Room-Temperature Perpendicular Ferromagnetism.

The emergence of van der Waals (vdW) magnets has created unprecedented opportunities to manipulate magnetism for advanced spintronics based upon all-vdW heterostructures. Among various vdW magnets, Cr1+δTe2 possesses high temperature ferromagnetism along with possible topological spin textures. As this system can support self-intercalation in the vdW gap, it is crucial to precisely pinpoint the exact intercalation to understand the intrinsic magnetism of the system. Here, we developed an iterative method to determine the self-intercalated structures and show evidence of vdW "superstructures" in individual Cr1+δTe2 nanoplates exhibiting magnetic behaviors distinct from bulk chromium tellurides. Among 26,332 possible configurations, we unambiguously identified the Cr-intercalated structure as 3-fold symmetry broken Cr1.5Te2 segmented by vdW gaps. Moreover, a twisted Cr-intercalated layered structure is observed. The spontaneous formation of twisted vdW "superstructures" not only provides insight into the diverse magnetic properties of intercalated vdW magnets but may also add complementary building blocks to vdW-based spintronics.

Anomalous Hall effect in antiferromagnetic Cr thin films

Bulk chromium is the simplest antiferromagnet in our general material database, which has been firmly believed to exhibit a collinear spin structure and a consequent vanishing anomalous Hall effect (AHE). In this work, we report an unexpected weak AHE in polycrystalline thin films of chromium. We attribute this phenomenon to the noncollinear spin textures induced by the local spin frustration and rearrangement in certain areas with high residual strain and defect densities. Moreover, a dominant underlying mechanism of intrinsic Berry phase is speculated. This could be a general feature for all the collinear antiferromagnetic thin-film materials with moderately high defect concentrations, making them promising candidates for emergent antiferromagnetic spintronics.

Defect-Induced Magnetic Skyrmion in a Two-Dimensional Chromium Triiodide Monolayer.

Chromium iodide monolayers, which have different magnetic properties in comparison to the bulk chromium iodide, have been shown to form skyrmionic states in applied electromagnetic fields or in Janus-layer devices. In this work, we demonstrate that spin-canted solutions can be induced into monolayer chromium iodide by select substitution of iodide atoms with isovalent impurities. Several concentrations and spatial configurations of halide substitutional defects are selected to probe the coupling between the local defect-induced geometric distortions and orientation of chromium magnetic moments. This work provides atomic-level insight into how atomically precise chemical doping can be used to create and control complex magnetic patterns in chromium iodide layers and lays out the foundation for investigating the field- and geometric-dependent magnetic properties in similar two-dimensional materials.

Study and optimization of epitaxial films of Cr and Pt/Cr bilayers on MgO

We report a systematic study of the fabrication and chemical and structural characterization of epitaxial Cr films and Pt/Cr bilayers on MgO substrates. Cr films of different thicknesses were grown by molecular beam epitaxy at 373 K and then post-annealed at different temperatures. These films are chemically stable, present a well-ordered tetragonal structure, with only a low degree of oxidation distributed throughout the films. The lattice parameter can be finely tuned by varying the film thickness and the post-annealing temperature, as the elastic energy is progressively relaxed by edge dislocations, so that the film can approach the structural and electrical properties of bulk chromium. Moreover, the chemical and structural properties of Cr are not affected by a Pt capping layer, which grows in a well-ordered Pt[1 1 0]||Cr[1 0 0] orientation, holding potential for applications in recently proposed Pt/Cr-based antiferromagnet spintronics and for the realization of epitaxial Pt films on MgO by using a thin buffer layer of chromium.

Experimental Simulation of Processes of Ore Leaching from Lyubov Deposit (Transbaikalian Region)

To identify the mobile chemical elements in the processes of acidic weathering in the landscapes of Lyubov gold deposit (Transbaikalian Region), experiments on sulfuric acid leaching of gold–quartz ores (up to 2% of sulfides content) from the deposit are carried out. The chemical composition of ore samples is determined, and the major and minor minerals are described. The compositions of the ore and rock-forming minerals are studied by the methods of X-ray diffraction and chemical analyses and electron microscopy. According to the experimental results, a group of mobile chalcophile elements (Pb, Zn, Cu, Cd), elements of V group (As, Sb, Bi) attributed to sulfides and sulfosalts, and a group of rare-earths and trace elements are revealed. Various dynamics of leaching these groups of chemical elements is recorded. It has been found that even if the contents are below bulk earth values, lanthanides, cesium, chromium, and germanium rapidly move to the acidic solution.

Energetics and electronic structure of native point defects in antiferromagnetic CrN

We present a detailed analysis of the role of native point defects in the antiferromagnetic (AFM) phases of bulk chromium nitride (CrN). We perform first-principles calculations by using the local spin-density approximation, including local interaction effects $(\text{LSDA}+U)$, to study the two lowest energy AFM models expected to describe the low-temperature phase of the material. We study the formation energies, lattice deformations, and electronic and magnetic structure introduced by native point defects. We find that, as expected, nitrogen vacancies are the most likely defect present in the material at low temperatures. Nitrogen vacancies present different charged states in the cubic AFM model, exhibiting two transition energies, which could be measured by thermometry experiments and could help identify the AFM structure in a sample. These vacancies also result in partial spin polarization of the induced impurity band, which would have interesting consequences in experiments. Other point defects have also signature electronic and magnetic structure that could be identified in scanning probe experiments.

Evidence for a multilayered internal structure of the chondritic acapulcoite-lodranite parent asteroid

We report a petrography, mineral chemistry, oxygen and chromium isotopic study of Grove Mountains (GRV) 020043 together with a subset of other acapulcoites and lodranites. GRV 020043 is a petrologic type 4 chondrite, with chondrules of diverse types and sizes, and is composed of low-Ca pyroxene (40 vol.%), olivine (24 vol.%), diopside (8 vol.%), plagioclase (10 vol.%), Fe-Ni metal (kamacite and taenite), troilite and some accessory minerals (chromite and apatite). The olivine in GRV 020043 has an average fayalite content (Fa) of 10.7 mol.% with the low-Ca pyroxene having an average ferrosilite content (Fs) of 10.8 mol.%. The whole rock oxygen isotopic composition of GRV 020043 is +3.226 ± 0.267‰, +0.797 ± 0.131‰, and −0.927 ± 0.017‰ for δ18O, δ17O, and Δ17O, respectively, with a bulk chromium isotopic compositions of ε54Cr = −0.48 ± 0.10. These characteristics of GRV 020043 are different from all established or ungrouped chondrites but agree with those of the acapulcoite-lodranite clan. We therefore suggest that GRV 020043 represents the chondritic precursor of acapulcoite-lodranite parent body.The similarity of bulk oxygen and chromium isotopic compositions among GRV 020043, Acapulco, Northwest Africa (NWA) 468 (metal-rich lodranite), NWA 8118 (lodranite), NWA 8287 (acapulcoite), and NWA 8422 (lodranite) indicates that they originated from a common oxygen and chromium reservoir in the protoplanetary disk or may have derived from a parent body with a differentiated multilayer structure.

Formation of Slantwise Surface Ripples by Femtosecond Laser Irradiation.

We report on the formation of slantwise-oriented periodic subwavelength ripple structures on chromium surfaces irradiated by single-beam femtosecond laser pulses at normal incidence. Unexpectedly, the ripples slanted in opposite directions on each side the laser-scanned area, neither perpendicular nor parallel to the laser polarization. The modulation depth was also found to change from one ripple to the next ripple. A theoretical model is provided to explain our observations, and excellent agreement is shown between the simulations and the experimental results. Moreover, the validity of our theory is also confirmed on bulk chromium surfaces. Our study provides insights for better understanding and control of femtosecond laser nanostructuring.

Realization of deep 3D metal electrodes in diamond radiation detectors

A fabrication technique to create 3D diamond detectors is presented. Deep reactive ion etching was used to create an array of through-diamond vias (TDVs) in a 2 × 2 × 0.15 mm3 electronic grade single crystal diamond detector. The diameter of the TDVs was nominally 30 μm with a pitch of 100 μm between them. The TDVs were filled with chromium using hexavalent chromium electroplating to create 3D electrodes, which were connected electrically by interdigitated electrodes. The fabricated 3D diamond detector responded to both alpha particles and X-rays, exhibiting a charge collection efficiency of 52.3% at 200 V. Comparing to a diamond detector with the same interdigitated electrodes, but no 3D electrodes, confirms that the 3D electrodes are electrically active within the device. The average resistivity of the 3D electrodes is 2.89 ± 0.03 × 10−5 Ω cm, near that of bulk chromium. These results indicate that this fabrication technique is a potential option for 3D diamond detector fabrication.

Synthesis of bulk chromium hydrides under pressure of up to 120 GPa

Stable compounds in the Cr-H system have been synthesized through a direct reaction of chromium and hydrogen in a laser-heated diamond-anvil cell and investigated using synchrotron x-ray diffraction up to $120\phantom{\rule{0.28em}{0ex}}\mathrm{GPa}$. The sequence of hydrides CrH, ${\mathrm{Cr}}_{2}{\mathrm{H}}_{3}$, and ${\mathrm{CrH}}_{2}$ has been observed by increasing pressure. The known $\ensuremath{\epsilon}\ensuremath{-}hcp$-CrH hydride is formed above $3\phantom{\rule{0.28em}{0ex}}\mathrm{GPa}$. A ${\mathrm{Cr}}_{2}{\mathrm{H}}_{3}$ hydride with a $C2/m$ structure appears spontaneously above $19\phantom{\rule{0.28em}{0ex}}\mathrm{GPa}$, as a result of the filling of the tetrahedral sites of $\ensuremath{\epsilon}$-CrH. YAG laser heating helps dissolve more hydrogen inside the $hcp$ chromium structure to synthesize a ${\mathrm{CrH}}_{2}$ compound with a $Pnma$ structure from $30\phantom{\rule{0.28em}{0ex}}\mathrm{GPa}$ on. The volume expansion per hydrogen atom in octahedral and tetrahedral sites is measured up to the 100-GPa pressure range. The formation pressures and structures of these chromium interstitial hydrides are in very good agreement with DFT calculations. However, despite multiple heating attempts up to $100\phantom{\rule{0.28em}{0ex}}\mathrm{GPa}$, no evidence of the stability of the predicted ${\mathrm{CrH}}_{3}$ compound could be found.

Ligand-field helical luminescence in a 2D ferromagnetic insulator

Bulk chromium triiodide (CrI$_3$) has long been known as a layered van der Waals ferromagnet. However, its monolayer form was only recently isolated and confirmed to be a truly two-dimensional (2D) ferromagnet, providing a new platform for investigating light-matter interactions and magneto-optical phenomena in the atomically thin limit. Here, we report spontaneous circularly polarized photoluminescence in monolayer CrI$_3$ under linearly polarized excitation, with helicity determined by the monolayer magnetization direction. In contrast, the bilayer CrI$_3$ photoluminescence exhibits vanishing circular polarization, supporting the recently uncovered anomalous antiferromagnetic interlayer coupling in CrI$_3$ bilayers. Distinct from the Wannier-Mott excitons that dominate the optical response in well-known 2D van der Waals semiconductors, our absorption and layer-dependent photoluminescence measurements reveal the importance of ligand-field and charge-transfer transitions to the optoelectronic response of atomically thin CrI$_3$. We attribute the photoluminescence to a parity-forbidden d-d transition characteristic of Cr$^{3+}$ complexes, which displays broad linewidth due to strong vibronic coupling and thickness-independent peak energy due to its localized molecular orbital nature.

Oxidation and creep properties at 1200 °C of cast quaternary Ni-Cr-C-Ti alloys

Three nickel-based alloys rich in chromium (25wt%) and containing carbon and titanium in quantities chosen to promote the appearance of titanium carbides (0.25C-1Ti, 0.50C-1Ti and 0.50C-2Ti, in wt%) were elaborated by casting in inert atmosphere. Their microstructures and their thermodynamic, chemical and mechanical behaviours at high temperature were studied. The as-cast microstructures of the obtained alloys are composed of a dendritic nickel matrix and of interdendritic carbides. These carbides are not TiC as expected, but Cr7C3, the whole titanium remaining in solid solution in the matrix. The applied {46h, 1200°C}-exposure in air led to the development of an external duplex oxide scale constituted of a thick inner chromia scale and a thin outer rutile scale. These oxide layers resisted spallation more than the ones of Ti-free ternary reference alloys in similar conditions. The bulk's chromium carbides became rounder and coarsened, with as result a decrease in room temperature hardness. The {5 or 20MPa, 1200°C}−3 points flexural creep tests showed a rather good behaviour, regarding the high test temperature. It is concluded that titanium seems hardening these bases of alloys at low and high temperatures. However the effects on the oxidation behaviour may be deleterious for the oxidation rate and subsurface Cr-impoverishment. In contrast Ti appears as beneficial for the resistances against spallation and possibly chromia volatilization. This needs to be further investigated.

Influence of trivalent doping on point and Frenkel defect formation in bulk chromium (III) oxide

Substitutional doping in metal oxides is a well-known approach to modifying properties such as ionic conductivity or activity in catalysis. Chromium (III) oxide is an attractive reducible material that has potential use as an oxygenation catalyst, and the fact that it maintains its integrity at high operating temperatures makes it useful for high temperature methanol synthesis and solid oxide fuel cells. Understanding the defect chemistry of Cr2O3 is important for rational catalyst design, in particular when the material is modified by isovalent doping. Density functional theory calculations with a Hubbard +U correction applied to the Cr 3d and O 2p states are used to investigate isovalent doping with Al3+, Fe3+ and La3+ cations. Point defects including Cr and O vacancies, and Frenkel defects are investigated and the effect of cation doping on the defect formation energies in Cr2O3 is explored. Our calculations show that Cr Frenkel and peroxide point defects are the most favourable defects in undoped Cr2O3. Al3+ doping in Cr2O3 does not change the defect formation energies over undoped Cr2O3, and Fe3+ doping improves oxygen vacancy formation while greatly increasing the formation energies of other defects that are potential competing processes that may kill ionic conductivity. La3+ doping in Cr2O3 is found to also improve oxygen vacancy formation and induces a considerable decrease in the cost to form Cr vacancies and Frenkel defects. The modifications to the defect formation energies induced by these dopants in Cr2O3 can be used to impede formation of undesirable defects, thus enhancing ionic conductivity for oxygenation catalysis.

Enhancing the oxygen vacancy formation and migration in bulk chromium(iii) oxide by alkali metal doping: a change from isotropic to anisotropic oxygen diffusion

Oxygen vacancy formation and migration are vital properties for reducible oxides such as TiO2, CeO2 and Cr2O3 as the oxygen storage capacity (OSC) of these materials are important for a wide range of applications in photovoltaics, oxidative catalysis and solid oxide fuel cells.

Laser-Induced Charge-Density-Wave Transient Depinning in Chromium.

We report on time-resolved x-ray diffraction measurements following femtosecond laser excitation in pure bulk chromium. Comparing the evolution of incommensurate charge-density-wave (CDW) and atomic lattice reflections, we show that, a few nanoseconds after laser excitation, the CDW undergoes different structural changes than the atomic lattice. We give evidence for a transient CDW shear strain that breaks the lattice point symmetry. This strain is characteristic of sliding CDWs, as observed in other incommensurate CDW systems, suggesting the laser-induced CDW sliding capability in 3D systems. This first evidence opens perspectives for unconventional laser-assisted transport of correlated charges.