Cr Crystals Research Articles

Scanning Capacitance Microscopy of Triglycine Sulfate Crystals with the Profile Chromium Distribution

The spatial capacitance distribution, domain wall configuration, and impurity composition of triglycine sulfate TGS–TGS + Cr crystals with a growth periodic impurity structure have been investigated using scanning capacitance microscopy and X-ray fluorescence and topography. The chromium ion concentration in the strips emerging to the surface has been determined, and the periodic impurity distribution has been established. The difference between the chromium concentrations in nominally pure and impurity strips was found to be ~0.08 wt %, which is reflected in a variation in the capacitance image contrast by 0.17%. It is shown that capacitance images carry information about localization of the impurity gradient regions and domain walls and make it possible to establish a correlation between the defect and domain structures of a ferroelectric crystal.

Effect of Magnetic-Lattice Transformations on Low-Temperature Plasticity and Fracture of Chromium Single Crystals

Analysis of the literature indicates an interconnection of the lattice and magnetic subsystems of chromium, which suggests the further study of changes in low-temperature plasticity and fracture of chromium resulting from magneto-lattice transformations during thermal cycling in the range of 373↔77 K. For the initial microalloyed Eu single crystals of Cr (the content of impurities Ci ≈ 10–3 wt.%) with orientations [110] and [112] along the compression axes, a monotonous growth of the yield point was observed in the temperature range from room temperature to T = Tx ≈ 170 K (Tx is the temperature of cold-brittleness or the temperature of ductile-brittle transition). When the temperature was lowered in the range of Tx–4.2 K, the samples became brittle, with a decrease in the fracture stress, which is due to the growth of local stresses at the concentrators. As a result of thermal cycling accompanied by multiple phase transformations from the paramagnetic to the antiferromagnetic AF1 and AF2 states and vice versa, a decrease in the Tx value of chromium was observed of ≈ 20 K. The effect is related to the occurrence of relaxation processes near stress concentrators due to the displacement of the walls of domain structures and also cyclic striction deformations that compensate for the volume change caused by overheating and supercooling of the sample due to a rapid change in temperature.

The effect of surface texture on the oxidation behaviour of polycrystalline Fe-Cr

Grain-direction dependent oxidation at the surface of polycrystalline Fe-Cr steel is not well understood, as most of the described systems in literature focus on single crystals of either Fe or Cr. We found through electron backscatter diffraction that surface oxidation in air at temperatures between 260 and 450 °C depends severely on grain orientations at the outer surface. Subsequently electron microscopy was combined with X-ray photoelectron spectroscopy (XPS) and X-ray Diffraction (XRD) to characterize the oxide film further in detail. In particular we have observed the following sequence in oxidation rate of crystal planes parallel to the surface for Fe-Cr steel, {0 0 1} < {1 1 1} & {1 0 1}, which was not reported in literature before.

Crystal growth and optical characterization of chromium-doped l-arginine phosphate monohydrate

l-arginine phosphate monohydrate single crystals doped with Cr3+ ions (LAP:Cr) were grown by slow cooling method, with dimensions up to 10 × 10 × 8 mm3. Chromium ions induce a drastic reduction of overall growth rate and produce a drastic change in LAP growth morphology. The functional groups of LAP:Cr crystals were confirmed by FTIR analysis and compared with pure LAP. The optical band gap of LAP:Cr was determined from UV-VIS-NIR transmittance spectrum. Optical absorption spectrum at room temperature and 10 K was measured consisting of two large bands due to the 4A2 → 4T2 and 4A2 → 4T1 transitions and a narrower band attributed to the 4A2 → 2E spin-forbidden transition, characteristic of Cr3+ ions. Luminescence spectra of pure and Cr3+-doped samples were collected with excitation at wavelength of 350 nm and 335 nm. Emissions from host LAP matrix and Cr3+ ions were observed. Excited with 350 nm the spectra reveal a broad intense band from 21000 to 26000 cm−1 region due to electronic transition from LAP ligand. The emission spectra of doped sample present two broad bands centered at 12346 cm−1 and 13412 cm−1 from 2E → 4A2 and 4T2 → 4A2 transitions, characteristics of luminescence from Cr3+ ions. From these results the Racah and crystal-field Cr3+ parameters were evaluated and comparing with those obtained in similar systems.

On radiative lifetime measurement of chromium transitions in Cr doped ZnSe and ZnTe crystals using the frequency domain modulated infrared radiometry

The radiative lifetimes of chromium transitions in ZnSe:Cr crystal were determined by the modulated infrared radiometry method and the accuracy of the obtained data were analysed together with the recently studied from ZnTe:Cr crystal. The radiative lifetimes of chromium transitions in ZnSe:Cr crystal can be estimated from the photoluminescence response, additionally filtered out using an Inrasil window, with very good precision. Studying the influence of the light intensity on the estimation of the lifetime it was observed that with increasing light intensity the lifetime decreases. Also in ZnTe:Cr crystal it had been found, that with increasing light intensity the lifetimes decrease. However, whereas in ZnSe:Cr the accuracy of estimation error is independent on the light intensity in ZnTe:Cr the estimation error increases with the increasing light intensity.

Structure of the LiNbO3:Mg,Cr crystal and its properties at visible and terahertz wavelengths

Optical and radiophysical studies on a gradient-activated lithium niobate crystal with a double concentration profile of chromium (a smooth change in concentration from 0.036 at.% to 0.035 at.%) and magnesium (a change in the concentration from 7 at.% to 3 at.%) were performed with the aim of determining the defect structure and the radiation resistance of the test sample.

Integration of chromium terephthalate metal-organic frameworks with reduced graphene oxide for voltammetry of 4-nonylphenol

Metal-organic frameworks (MOFs) are attractive materials for electrochemical applications because of their large surface areas and rich pore structures. However, their poor electrical conductivity hinders these applications. In this contribution, the decoration of chromium terephthalate MOFs (MIL-101(Cr) crystals) with reduced graphene oxide (rGO) is proposed to improve the electrical conductivity of MIL-101(Cr) crystals. These nanocomposites (MIL-101(Cr)@rGO) were prepared using a simple chemical reduction method. Their morphology, interfacial properties, and electrochemical applications were investigated using electron microscopy, electrochemical, and related techniques. With an introduction of rGO into MIL-101(Cr) crystals, the conductivity of MIL-101(Cr)@rGO nanocomposites was much improved. As a case study of electrochemical applications of MIL-101(Cr)@rGO nanocomposites, the MIL-101(Cr)@rGO nanocomposite coated electrodes were employed for the voltammetric study of 4-nonylphenol. An irreversible, adsorption-controlled, one electron and one proton involved oxidation process was observed. By use of the recorded anodic peak current as the physical parameter, the sensitive and reproducible monitoring of 4-nonylphenol was realized in the range of 0.1–12.5 μM with a detection limit of 33 nM. The MIL-101(Cr)@rGO nanocomposites are thus useful electrode materials. The proposed strategy provides a simple and efficient approach to expand easily the applications of MOFs into various electrochemical fields.

Cr(III) removal from synthetic and real tanning effluents using an electro-precipitation method

An electro-precipitation method is proposed for the treatment of synthetic (2654.4 mg L−1 total Cr, pH 3.9) and real tanning effluents (2775 mg L−1 total Cr, pH 4.0) containing elevated concentrations of Cr(III). Aluminum and copper anodes are thermodynamically (Pourbaix-type diagrams) and electrochemically evaluated in the synthetic electrolyte, as potential candidates to generate the chromium precipitation. Copper is selected due to better dissolution properties and avoiding passivation. The choice of copper anode is effective in both solutions, generating Cu+2 species which displace the chromium species from Cr(III), and obtaining chromium recoveries >99% for these cases. The total Cr concentration is decreased down to 0.12 mg L−1 in the real wastewater. Visual inspection of the treated synthetic solution and FTIR analysis reveal that a chromium hydroxide is formed as insoluble compound after the treatment, while cupric sulfate remains in the supernatant. A similar result is obtained for the real effluent, but other impurities and compounds different than chromium or copper are present in the precipitate. Although these species are not identified due to their complexity, they could be related to the nature of the tanning process, being derived from hide, surfactants, fixers and dyes. X-ray diffraction conducted for the sludge electro-generated from the treatment of the real tanning effluent support the FTIR and electrochemical evaluations, and this technique enables to identify that a very hydrated chromium hydroxide is formed after the treatment. The drying of this insoluble compound produces a distinctive hexagonal crystal of Cr(OH)3·3H2O, and dehydrated phases of chromium oxides (e.g. Cr2O3). These findings suggest that the electro-precipitation method herein proposed is efficacious to decrease the total chromium concentration below the maximum permissible limits (0.5 mg L−1).

Template-directed fabrication of MIL-101(Cr)/mesoporous silica composite: Layer-packed structure and enhanced performance for CO2 capture

A novel hybrid material constituted of MIL-101(Cr) and mesoporous silica was successfully assembled through an in-situ hydrothermal method. The MCM−41 with well-ordered mesopores acted as the structure-directing agent, which regulated the growth of MIL-101(Cr) crystals along a certain direction and restricted the expansion of framework. Meanwhile, the hydroxyl groups existed in MCM−41 preferentially coordinated with the Cr3+ centers in MOF, followed by the layer-packed arrangement of MIL-101(Cr) nanocrystals on the surface of matrix. The structural characterizations further revealed that the introduction of MCM−41 could increase the micropore volume and specific surface area. The MIL-101(Cr)@MCM−41 exhibited higher CO2 uptake capacity and adsorption rate compared with the original MIL-101(Cr) at 298 K and 1 bar. Via ideal adsorbed solution theory (IAST), it could be further predicted that the composite was more inclined to adsorb CO2 than N2. The calculated isosteric heats of CO2 adsorption and desorption activation energy demonstrated that the interaction between CO2 molecules and the composite was also enhanced. The as-prepared MIL-101(Cr)@MCM−41 showed good reusability and could be easily regenerated without any reduction in its CO2 adsorption capacity. Hence, this study opened up a new pathway for designing hierarchical porous structured MOF-based materials with advanced gas separation performance.

The truth is out there: the metal-π interactions in crystal of Cr(CO)3(pcp) as revealed by the study of vibrational smearing of electron density

Abstract The vibrational smearing of electron density was studied in the crystal of complex of Cr(CO)3 with [2.2]paracyclophane. The combination of theoretical and experimental methods, including periodic calculations and screening of DFT calculated and multipole-decomposed electron densities, was utilized to reveal the vibrational smearing of electron density and its influence on the multipole-constructed electron density. The multipole model, commonly used to treat the high-resolution X-ray diffraction data, was shown to be rather inaccurate in description of electron density and its vibrational smearing in metal-π complex where the interchange between diatomic interactions can occur. Namely, some bond critical points can be hidden while analyzing multipole-decomposed electron density with proved effects of vibrational smearing even if the deconvolution problem is overcome by using the invariom approach. On the contrary, the recently proposed “clouds of critical point variation” (CCPV) approach is demonstrated as the route to gather all reasonable bonding trends and to reconstruct static electron density pattern in metal-π complexes.

Evolution mechanism of interface cohesion for the coating inducing by laser cladding YSZ@Ni core-shell nanoparticles: Experimental and theoretical research

Abstract Stainless steel coated by multi-layers of YSZ@Ni core-shell nanoparticles was sintered by laser cladding. Microstructure shows that the coating by laser cladding YSZ@Ni core-shell nanoparticles has a nanometer microstructure with stratification feature. Therein to cladding layer, amount of chromium atoms appear, and the Ni atoms are diffused largely in matrix layer, which results in the original YSZ@Ni core-shell nanoparticles translated into Cr crystal arming YSZ ceramics crystal. Such influence of variable bonding mechanism on ceramics/metal interface was studied deeply by first principle and molecular dynamics simulation. The results show that in diffusion process of solid state, the element diffusion of Zr↔Ni and Fe↔Ni are both hard to happen. But in Cr↔Ni diffusion process, it is hard only when they are at the nearest neighbor. Diffusion process in cooling process before liquid→solid phase transformation, the diffusion coefficient of Fe, Cr and Ni is DFe > DCr > DNi. But in nucleation process, its trend becomes DNi > DFe ≈ DCr, although all of them are too small. Deeply investigating on their electronic structure, the different distribution of Fe and Cr in cladding layer is originated from their trend of Mulliken population QO-Cr > QO-Fe. As a result, such weaken bonding of ZrO2/Cr has destroyed the initial purpose on the Ni as the excellent transition layers between ceramics enforcing phase and metal matrix phase. Thus comparing the connective and diffusive processes, our findings open an avenue for detailed and comprehensive studies of laser cladding process in producing composites.

Size dependence of discrete change in magnetization in single crystal of chiral magnet Cr1/3NbS2

The single crystal of a chiral magnet Cr1/3NbS2 exhibits discrete changes in magnetization (M) in response to changes in magnetic field (H) triggered by the formation of a chiral soliton lattice (CSL). In order to provide evidence of this phenomenon, the study of the size effect is indispensable. We investigated the effects of size on this phenomenon by the use of two single crystals, (A) and (B), whose crystal sizes along the c-axis were 110 μm and 60 μm, respectively. First, in (A), the large jumps of M observed in the process of decreasing H exhibited inconsistent features, whereas the largest and second-largest jumps in (B) exhibited reproducibility for both the value of H and the magnitude of the M jumps. This confirms that these large jumps do not originate from the Barkhausen effect, as this effect would result in M jumps appearing at random values of H. When the system size of a sample becomes smaller, the features of the Barkhausen effect are suppressed. Second, as for the successive jumps observed in the H region where the rapid change in M is entirely seen for both the samples, the number of observed M jumps for (B) is more than that for (A). Indeed, the number of the domain wall due to the 2π-soliton in the CSL increases, as the c-axis length of the single crystal increases. However, a series of M jumps must appear in a limited H region below the critical field of the order of 2 kOe. The greater the number of M jumps, the more difficult the detection of the M jump will be in the H resolution of the present setup. Thus, the effects of size on the M jumps observed in the present setup can be understood within the framework of the CSL formation.

Identification of the photoluminescence response in the frequency domain modulated infrared radiometry signal of ZnTe:Cr bulk crystal

In this work we investigated the photoluminescence response in the frequency domain modulated infrared radiometry signal observed of ZnTe:Cr bulk crystal. In mid-infrared range, three characteristic phenomena are observed in ZnTe:Cr crystal: absorption and emission of IR photons (2–3μm) and the free carrier absorption. This implies that the modulated infrared radiometry signal yields information about the effective infrared absorption coefficient (photothermal response) as well about the recombination lifetime of carriers related with the infrared photoluminescence emission. In this paper, the frequency equivalence of the two-term independent exponential photoluminescence decay model in order to explain the measured frequency characteristics is proposed. The measured recombination lifetimes (2.3μs for two exponential decay model and 1.5μs for one exponential decay model) are in good agreement with the values given by other authors (about 2.5–3.0μs). Moreover, we found that the photothermal response is uncorrelated with the photoluminescence one, in contrast, to the photocarrier response.

Optical studies of the uniaxial stress-induced orbital alignment of the Cr(2+) centers in KZnF3 single crystal.

Observation of an intense optical linear dichroism arising in cubic KZnF3:Cr crystal at low temperatures under uniaxial stress applied along the four-fold axis is reported. Dichroism occurs in the range of the wide vibronic absorption band corresponding to (5)Eg → (5)T2g transition of the Cr(2+) ions. Strain dependences of the dichroism value were studied at the temperatures of 2.0 K, 4.2 K, and 77 K. We associate our observations with the Jahn-Teller effect in the (5)Eg ground state of the Cr(2+) ion. The model is proposed based on a redistribution of the centers between the minima of the E⨂e problem adiabatic potential that become inequivalent under uniaxial stress applied along the four-fold axis of the crystal. It is shown that random strains in the sample have to be taken into account to achieve the quantitative agreement of the model predictions with experimental data. It is found that random strains in the studied sample originate predominantly from point defects. Obtained parameter values are inversion splitting δ = 9.2 ± 1.6 cm(-1), electron-strain coupling constant qVES = 16 500 ± 600 cm(-1), width of the random strain distribution w = (6.9 ± 0.5) ⋅ 10(-5). It is shown also that the minima of the [CrF6](4-) cluster adiabatic potential correspond to the elongated along the four-fold axes configurations.

Numerical adiabatic potentials of orthorhombic Jahn-Teller effects retrieved from ultrasound attenuation experiments. Application to the SrF2:Cr crystal

A methodology is worked out to retrieve the numerical values of all the main parameters of the six-dimensional adiabatic potential energy surface (APES) of a polyatomic system with a quadratic T-term Jahn-Teller effect (JTE) from the ultrasound experiments. The method is based on a verified assumption that ultrasound attenuation and speed encounter anomalies when the direction of propagation and polarization of its wave of strain coincides with the characteristic directions of symmetry breaking in the JTE. For the SrF2:Cr crystal, employed as a basic example, we observed anomaly peaks in the temperature dependence of attenuation of ultrasound at frequencies of 50–160 MHz in the temperature interval of 40–60 K for the wave propagating along the [110] direction, for both the longitudinal and the shear modes, the latter with two polarizations along the [001] and [11¯0] axes, respectively. We show that these anomalies are due to the ultrasound relaxation by the system of non-interacting Cr2+ JT centers with orthorhombic local distortions. The interpretation of the experimental findings is based on the T2g⊗(eg+t2g) JTE problem including the linear and the quadratic terms of vibronic interactions in the Hamiltonian and the same-symmetry modes reduced to one interaction mode. Combining the experimental results with a theoretical analysis, we show that on the complicated six-dimensional APES of this system with three tetragonal, four trigonal, and six orthorhombic extrema points, the latter are global minima, while the former are saddle points, and we estimate numerically all the main parameters of this surface, including the linear and quadratic vibronic coupling constants, the primary force constants, the coordinates of all the extrema points and their energies, the energy barrier between the orthorhombic minima, and the tunneling splitting of the ground vibrational states. To our knowledge, such a based-on-experimental-data numerical reconstruction of the APES of a JTE problem in the five-dimensional space of all active tetragonal and trigonal displacements has not been reported before.

Numerical Investigations of Laser Diode End-pumped Solid-state Cr\(^{3+}\):LiSAF Lasers Passively Q-switched with Cr\(^{4+}\):YSO Crystal

We have numerically investigated, for the first time, the optical performance and characteristics of diode-end-pumped passively Cr\(^{4+}\):YSO Q-switched solid-state Cr\(^{3 + }\):LiSAF lasers using a rate equation system extended to multi-wavelength. Homogeneously broadened emission spectra of the laser crystal and broad absorption spectra of Cr\(^{4+}\):YSO saturable absorber have been respected in the investigation. A tunable passively Q-switched laser operation is also developed with a birefringent filter plate used as an intracavity selective element. As a result, improved understanding of passively Q-switched Cr\(^{4+}\):YSO solid-state Cr\(^{3+}\):LiSAF lasers has been obtained in broadband and tunable laser operations. Characteristics of the tunable passively Q-switched Cr\(^{3 + }\):LiSAF laser were demonstrated to be clearly dependent on wavelength. The spectro-temporal evolution of a single pulse emitted from the passively Q-switched Cr\(^{3 + }\):LiSAF laser has been reported.

PHOTOCONDUCTIVITY AND PHOTOLUMINESCENCE OF ZnSe:Cr CRYSTALS IN THE VISIBLE SPECTRAL REGION

The photoconductivity and photoluminescence of ZnSe:Cr crystals in the visible spectra region are studied. The scheme of optical transitions within Cr2+ impurity centers is established. It is shown that the high-temperature impurity photoconductivity of ZnSe:Cr crystals is controlled by optical transitions of electrons from the 5T2(D) ground state to the higher levels of excited states of Cr2+ ions, with subsequent thermal activation of the electrons to the conduction band. Efficient excitation of intracenter luminescence of ZnSe:Cr crystals is attained with light corresponding to the region of impurity absorption.

Influence of trimethylsilane flow on the microstructure, mechanical and tribological properties of CrSiCN coatings in water lubrication

CrSiCN coatings with different silicon and carbon contents were deposited on silicon wafers and 316L stainless steels using unbalanced magnetron sputtering via adjusting trimethylsilane (TMS) flow, and their microstructure and mechanical properties were characterized by X-ray diffraction (XRD), scanning electron microscopy(SEM), X-ray photoelectrons spectroscopy(XPS) and nano-indenter, respectively. The tribological properties of CrSiCN coatings sliding against SiC balls in water were investigated using ball-on-disk tribometer. The results showed that the CrSiCN coatings had fine composite microstructure consisting of nanocrystallites of Cr(C, N) crystal and amorphous phases such as a-Si3N4 and a-C(a-CNx). The typical columnar structures changed from fine cluster to coarse ones when the Si content was beyond 3.4at.%. With an increase in the TMS flow, the hardness and Young's modulus of Corsican coatings all first increased, and then rapidly decreased, but the compressive stress in the coatings varied in the range of 2.8–4.8GPa. When the TMS flow was 10sccm, the CrSiCN coatings exhibited the highest hardness of 21.3GPa and the lowest friction coefficient (0.11) and wear rate (8.4×10−8mm3/Nm). But when the TMS flow was beyond 15sccm, the tribological properties of CrSiCN coatings in water became poor.

Formation of a regular domain structure in TGS–TGS + Cr crystals with a profile impurity distribution

A complex investigation of TGS–TGS + Cr crystals with a profile impurity distribution of chromium ions Cr3+ has been carried out at the macrolevel (measurement of dielectric properties by the method of nematic liquid crystals) and microlevel (domain structure according to atomic force microscopy data). It is established that periodic doped layers are formed only in individual growth pyramids in the regions where the polarization vector has a nonzero component along the normal to the growth faces rather than throughout the entire crystal volume. The domain configuration at the boundary of growth layers with different impurity compositions has been studied by piezoelectric force microscopy. The static unipolarity of layers with and without chromium impurity is approximately identical, whereas the domain-wall density in doped regions is higher than that in undoped ones by a factor of about 7.

Microstrip Resonators and Broadband Lines for X-band EPR Spectroscopy of Molecular Nanomagnets

We present a practical setup to perform continuous-wave X-band electron paramagnetic resonance spectroscopy by using planar microstrip lines and general purpose instrumentation. We fabricated Ag/alumina and Nb/sapphire microstrip resonators and transmission lines and compared their performance down to 2 K and under applied magnetic field. We used these devices to study single crystals of molecular Cr\(_{3}\) nanomagnets. By means of X-band planar resonators we measured angle-dependent spectra at fixed frequency, while broadband transmission lines were used to measure continuous wave spectra with varying frequency in the range 2–25 GHz. The spectra acquired at low temperatures allowed to extract the essential parameters of the low-lying energy levels of Cr\(_{3}\) and demonstrate that this method is particularly suitable to study small crystals of molecular nanomagnets.