Nonstoichiometric Phase Research Articles

Photoluminescence of Eu3+-doped CaZrO3 red-emitting phosphors synthesized via microwave-assisted hydrothermal method

Eu-doped 1, 3 and 5 % CaZrO3 red-emitting phosphors were synthesized for the first time using the microwave-assisted hydrothermal method followed by heat treatment at 1200 °C. After the crystallization, the orthorhombic CaZrO3 and the non-stoichiometric cubic Ca0.15Zr0.85O 1.85 phases were identified for all samples, and the portion of the non-stoichiometric phase increases as the Eu3+ concentration increases. The excitation spectra of the phosphors display Eu3+f-f excitation bands (namely at 395 nm and 464 nm) and the Eu3+→O2- charge transfer band at 250 nm, confirming that the CaZrO3 host acts as sensitizer for the Eu3+ luminescence. Likewise, the emission spectra feature the typical Eu3+ emission within the red spectral region, displaying CIE color coordinates of (0.642; 0.332), (0.663; 0.330) and (0.668; 0.327) for the 1, 3 and 5 % doped samples, respectively, indicating high emission color purity. By using Eu3+ as spectroscopic probe, the high-resolution selective emission spectra (14 K) confirm that Eu3+ replaces Ca2+ and Zr4+ local sites in both CaZrO3 and the Ca0.15Zr0.85O1.85 phases. Finally, the high absolute emission quantum yield of the 3 %-doped phosphor found at 393 nm (0.29 ± 0.03) confirms its potential application as red-emitting phosphor, especially in rapid identification of latent fingerprints, security ink or as red-emitting coating of near-UV LEDs for WLED applications.

Electron microscopy characterization of fast reactor MOX Joint Oxyde-Gaine (JOG)

The composition and crystal structure of the “Joint Oxyde Gaine” (JOG) has been investigated by means of electron microscopy. Microstructural characterization reveals a highly heterogeneous porous structure with inclusions containing both fission products and cladding components. Major fission products detected, other than Cs and Mo, are Te, I, Zr and Ba. The layer is composed by sub-micrometric crystallites. The diffraction data refinement, together with chemical mapping, confirms the presence of Cs2MoO4, which is the major component of the JOG. However, combinatorial analyses reveal that other non-stoichiometric phases are possible, highlighting the complex nature of the crystalline structure of the JOG.Fe is found in metallic Pd-rich precipitates with structure compatible with the tetragonal structure of FePd alloy. Cr is found in different locations of the JOG, in oxide form, but no structural data could be obtained due to local beam sensitization of the sample in those areas.

STRUCTURAL-PHASE COMPOSITION OF TiC/Ti COMPOSITES AND POWDERS OBTAINED BY COMBUSTION OF Ti-C GREEN MIXTURES WITH A TITANIUM EXCESS

The formation of the structure of the TiC/Ti metal matrix composite in the process of wave combustion has been discussed, taking into account the occurrence of possible reactions. An analysis of the experimental data shows that under this high-temperature process, a set of nonstoichiometric titanium carbides is formed, where, at certain ratios of the reacting components, low-dimensional TiC0.5 structures are formed, which can be attributed to the so-called MXenes. The study was carried out using electron and optical microscopy, X-ray diffraction and Raman spectroscopy. It was shown that the use of this set of experimental techniques makes it possible to reveal, by indirect signs, the presence of nonstoichiometric carbide phases, similar to MXenes, based on structural and morphological features.

Raman spectroscopy-in situ characterization of reversibly intercalated oxygen vacancies in α-MoO3.

This work reports on the in situ strategy to reversibly generate or suppress oxygen vacancies on α-MoO3 which were probed by Raman spectroscopy. Reversible changes in two features of the α-MoO3 Raman spectrum could be correlated to the generation of oxygen vacancies: displacement of the Tb band frequency and the intensity decrease of the symmetrical stretching (νs) band. These two features could be used to qualitatively describe oxygen vacancies. Raman results also indicate that oxygen vacancies are located in the interlayer region of the α-MoO3 lattice. This observation is corroborated by in situ X-ray diffraction, which also indicates the absence of nonstoichiometric phase transitions.

Direct conversion of syngas to light olefins (C2–C3) over a tandem catalyst CrZn–SAPO-34: Tailoring activity and stability by varying the Cr/Zn ratio and calcination temperature

A series of CrZn oxide catalysts with different Cr/Zn ratios were synthesized by co-precipitation and calcined at different temperatures. The catalysts were tested in a physical mixture with SAPO-34 in the direct conversion of syngas to olefins. X-ray diffraction, x-ray photoelectron and absorption spectroscopy, and electron microscopy were used to evaluate the impact of the Cr/Zn ratio and calcination temperature on the physicochemical properties of the resultant materials. CrZn–SAPO-34 catalysts were able to selectively convert syngas to C2–C3 hydrocarbons with a high olefin/paraffin ratio and low methane yield. However, the catalytic performance of these systems exhibited a trade-off between activity and stability in the olefin yield over time. By tuning the Cr/Zn ratio, it is possible to improve the olefin production stability, but with some compromise in the overall activity. The nonstoichiometric and inversed ZnCr2O4 spinel phase was considered to be the active phase for CO hydrogenation, while the decay in olefin production over time is correlated with abundance of ZnO on the resultant catalyst. Spectroscopic analysis demonstrated that the inversed spinel evolves toward its normal form under reaction conditions, which is accompanied by the formation of segregated ZnO; this phase may also increase the olefin hydrogenation activity.The challenge for the rational design of an optimal CrZn–SAPO-34 system is the stabilization of inversed ZnCr2O4, while minimizing the presence of the ZnO phase in the catalyst formulation.

Amplitude/Polarization Shift Keying Based and Logic Gate in Polarization Dependent C-Rich SiC Ring Waveguide

The TE/TM-mode spectral-separated C-rich SiC ring waveguide is employed to demonstrate polarization-dependent amplitude/polarization shift keying (ASK/PolSK) “AND” logic gate. The plasma-enhanced chemical vapor deposition-grown C-rich SiC film exhibits a nonstoichiometric SiC nano-cluster phase structure with a C/Si atomic ratio of 1.28, which provides an efficient nonlinearity of γ = 4.3 × 102 W−1m−1 and n2 = 2.44 × 10−12 cm2/W to demonstrate the “AND” gating between incoming ASK and PolSK data streams. By carefully designing the polarization preference of the ring waveguide structure, the TE/TM-mode-dependent transmission combs are spectrally separated because of the high birefringence of the waveguide optically triggered by the incoming pulsed stream. This type of a polarization dependent spectral notch can be utilized to perform the polarization-dependent all-optical AND gating, which was done in the current study by inserting a dual-format modulated pump and a continuous-wave probe into a ring waveguide. Output of the probe can be cross-wavelength modulated to provide the exact “AND” gating between the ASK and PolSK streams only when the pump is set to the ON state and the polarization of the pump is parallel to the corresponding notch of the ring waveguide throughput. This type of C-rich SiC ASK/PolSK AND logic gate is fully compatible with silicon-based complementary metal-oxide-semiconductor (CMOS) technology, which also has the advantages of low power consumption and enables polarization engineering for future photonic data logics.

Effect of Nonmagnetic Ion Deficiency on Magnetic Structure: Density Functional Study of Sr2MnO2Cu2-xTe2, Sr2MO2Cu2Te2 (M = Co, Mn), and the Oxide-Hydrides Sr2VO3H, Sr3V2O5H2, and SrVO2H.

Two seemingly puzzling observations on two magnetic systems were analyzed. For the oxide-hydrides Sr2VO3H, Sr3V2O5H2, and SrVO2H, made up of VO4H2 octahedra, the spin orientations of the V3+ (d2, S = 1) ions were reported to be different, namely, perpendicular to the H-V-H bond in Sr2VO3H but parallel to the H-V-H bond in Sr3V2O5H2 and SrVO2H, despite that the d-state split patterns of the VO4H2 octahedra are similar in the three oxide-hydrides. Another puzzling observation is the contrasting magnetic structures of Sr2CoO2Cu2Te2 and Sr2MnO2Cu1.58Te2, consisting of the layers made up of corner-sharing MO4Te2 (M = Co, Mn) octahedra. The Co2+ spins in each CoO2Te2 layer are antiferromagnetically coupled with spins perpendicular to the Te-Co-Te bond, whereas the Mn3+/Mn2+ ions of each MnO2Te2 layer are ferromagnetically coupled with spins parallel to the Te-Mn-Te bonds. We investigated the cause for these observations by performing first-principles density functional theory (DFT) calculations for stoichiometric phases Sr2VO3H, Sr3V2O5H2, SrVO2H, Sr2CoO2Cu2Te2, and Sr2MnO2Cu2Te2, as well as nonstoichiometric phase Sr2MnO2Cu1.5Te2. Our study reveals that the V3+ ions in all three oxide-hydrides should have the spin orientation parallel to the H-V-H bond. The unusual magnetic structure of the MnO2Te2 layers of Sr2MnO2Cu1.52Te2 arises from the preference of a Mn3+ spin to be parallel to the Te-Mn-Te bond, the ferromagnetic spin exchange between adjacent Mn3+ and Mn2+ ions, and the nearly equal numbers of Mn3+ and Mn2+ ions in each MnO2Te2 layer. We show that the spin orientation of the magnetic ions in an antiferromagnetically coupled perovskite layer, expected in the absence of nonmagnetic ion vacancies, cannot be altered by the magnetic ions of higher oxidation that result from trace vacancies at the nonmagnetic ion sites.

Nonstoichiometric Phases of Two-Dimensional Transition-Metal Dichalcogenides: From Chalcogen Vacancies to Pure Metal Membranes.

Two-dimensional (2D) membranes consisting of a single layer of Mo atoms were recently manufactured [ Adv. Mater. 2018 , 30 , 1707281 ] from MoSe2 sheets by sputtering Se atoms using an electron beam in a transmission electron microscope. This is an unexpected result as formation of Mo clusters should energetically be more favorable. To get microscopic insights into the energetics of realistic Mo membranes and nonstoichiometric phases of transition-metal dichalcogenides (TMDs) MaXb, where M = Mo and W and X = S, Se, and Te, we carry out first-principles calculations and demonstrate that the membranes, which can be referred to as metallic quantum dots embedded into a semiconducting matrix, can be stabilized by charge transfer. We also show that an ideal neutral 2D Mo or W sheet is not flat but a corrugated structure, with a square lattice being the lowest-energy configuration. We further demonstrate that several intermediate nonstoichiometric phases of TMDs are possible as they have lower formation energies than pure metal membranes. Among them, the orthorhombic metallic 2D M4X4 phase is particularly stable. Finally, we study the properties of this phase in detail and discuss how it can be manufactured by the top-down approaches.

Stable Ordered Phases of Cuprous Iodide with Complexes of Copper Vacancies

We perform an exhaustive theoretical study of the phase diagram of Cu-I binaries, focusing on Cu-poor compositions, relevant for p-type transparent conduction. We find that the interaction between neighboring Cu vacancies is the determining factor that stabilizes non-stoichiometric zincblende phases. This interaction leads to defect complexes where Cu vacancies align preferentially along the [100] crystallographic direction. It turns out that these defect complexes have an important influence on hole conductivity, as they lead to dispersive conducting $p$-states that extend up to around 0.8 eV above the Fermi level. We furthermore observe a characteristic peak in the density of electronic states, which could provide an experimental signature for this type of defect complexes.

Acid-Exfoliated Metallic Co0.85Se Nanosheets as Cocatalyst on Cd0.5Zn0.5S for Photocatalytic Hydrogen Evolution under Visible-Light Irradiation

Co0.85Se, nonstoichiometric phase, as an excellent electrocatalytic activity, was used as a cocatalyst to modify Cd0.5Zn0.5S nano-photocatalyst. The Co0.85Se (1.5 wt %)/Cd0.5Zn0.5S Schottky junctio...

Optimization of the Compositions of Solid Electrolytes Cd1 – xRxF2 + x (R = La–Lu, Y) with a Fluorite-Type Structure for Conductivity and Thermal Stability

Optimization of the compositions of Cd1–xRxF2+x nonstoichiometric phases (CaF2 type, R is a rare-earth element) for the ionic conductivity and thermal stability is based on the temperature measurements of the electrical conductivity of single crystals depending on the R3+ ionic radii and RF3 content and on the investigation of phase diagrams of the CdF2–RF3 systems. It is shown that the conductivity of 20 compositions out of 30 studied Cd1–xRxF2+x crystals exceeds the conditional limit σ500K = 10–5 S/cm, below which the use of solid electrolytes in electrochemical devices is believed to be undesirable because of low conductivity. The maximum conductivities σ500K = (3.0–3.2) × 10–4 S/cm and σ293K = (1.5–2.3) × 10–8 S/cm are observed for solid electrolytes with R = Ho, Er, Tm, or Yb and x = 0.22–0.27.

Structure and tribological behavior of titanium-based coatings deposited by reactive magnetron sputtering

The structure and functional properties of coatings deposited by reactive magnetron sputtering of titanium in acetylene-nitrogen mixtures of different composition have been studied. It has been established that all the coatings contain the amorphous (diamond-like) carbon phase, and different types of stoichiometric and non-stoichiometric titanium carbide, nitride and oxide phases. The tribological tests for friction fatigue have demonstrated that the coefficient of friction values for coatings were rather low and that they remained operational at contact pressures up to 450 MPa.

Mechanical modification and damage mechanism evolution of TiN films subjected to cyclic nano-impact by adjusting N/Ti ratios

For the purpose of optimizing the anti-impact performance of TiN film, a series of TiN films with different N/Ti ratios were investigated. The phase evolution and mechanical properties of TiN films were explored. In particular, the cyclic nano-impact tests with impact energy ranging from 0.1 μJ to 0.9 μJ were conducted to evaluate the anti-impact performance of TiN films. It was found that the non-stoichiometric phases of TiN0.30, Ti2N and TiN0.61 reduced with increasing N/Ti ratio in TiN films. The hardness of film increased with increasing stoichiometric TiN phase in films, while as an indicator of toughness, the H3/E2 ratio decreased. The anti-impact performance of TiN films displayed a close relationship with both hardness and H3/E2 ratio, in which a high enough hardness was the prerequisite of outstanding impact resistance, and H3/E2 ratio was an important factor affecting the damage mechanism. As the H3/E2 ratio decreased, the damage mechanism of TiN film transformed gradually from plastic fatigue damage to brittle fracture failure. Especially, the TiN film featuring N/Ti ratio of 0.780 (TiN-16) was not only hard enough to resist penetration, but also tough enough to prevent the film from fracturing, thus it exhibited the best comprehensive anti-impact performance.

Mechanisms of olivine dissolution by rock-inhabiting fungi explored using magnesium stable isotopes

To unravel the dissolution mechanisms of olivine by a rock-inhabiting fungus we determined the stable isotope ratios of Mg on solutions released in a laboratory experiment. We found that in the presence of the fungus Knufia petricola the olivine dissolution rates were about seven-fold higher (1.04 × 10−15 mol cm−2 s−1) than those in the abiotic experiments (1.43 × 10−16 mol cm−2 s−1) conducted under the same experimental condition (pH 6, 25 °C, 94 days). Measured element concentrations and Mg isotope ratios in the supernatant solutions in both the biotic and the abiotic experiment followed a dissolution trend in the initial phase of the experiment, characterized by non-stoichiometric release of Mg and Si and preferential release of 24Mg over 26Mg. In a later phase, the data indicates stoichiometric release of Mg and Si, as well as isotopically congruent Mg release. We attribute the initial non-stoichiometric phase to the rapid replacement of Mg2+ in the olivine with H+ along with simultaneous polymerization of Si tetrahedra, resulting in high dissolution rates, and the stoichiometric phase to be influenced by the accumulation of a Si-rich amorphous layer that slowed olivine dissolution. We attribute the accelerated dissolution of olivine during the biotic experiment to physical attachment of K. petricola to the Si-rich amorphous layer of olivine which potentially results in its direct exposure to protons released by the fungal cells. These additional protons can diffuse through the Si-rich amorphous layer into the crystalline olivine. Our results also indicate the ability of K. petricola to dissolve Fe precipitates in the Si-rich amorphous layer either by protonation, or by Fe(III) chelation with siderophores. Such dissolution of Fe precipitates increases the porosity of the Si-rich amorphous layer and hence enhances olivine dissolution. The acceleration of mineral dissolution in the presence of a rock-dissolving fungus further suggests that its presence in surficial CO2 sequestration plants may aid to accelerate CO2 binding.

Bismuth Oxide Faceted Structures as a Photocatalyst Produced Using an Atmospheric Pressure Plasma Jet

The photocatalyst bismuth oxide, which is active under visual light, was deposited using an atmospheric pressure plasma jet (APPJ). Sixteen different samples were generated under different parameters of the APPJ to investigate their catalytic activity. The prepared samples were characterized using X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), laser scanning microscopy (LSM), and UV–vis diffuse reflectance absorption spectroscopy. The measured data, such as average sample thickness, coverage ratio, phase fraction, chemical composition, band gap, and photocatalytic performance were used for comparing the samples. The XRD analysis showed that the deposition process produced a mixed phase of monocline Bi2O3 and tetragonal Bi2O2.33. Using the Rietveld refinement method, phase fractions could be determined and compared with the XPS data. The non-stoichiometric phases were influenced by the introduction of nitrogen to the surface as a result of the deposition process. The band gap calculated from the diffuse absorption spectroscopy shows that Bi2O2.33 with 2.78 eV had a higher band gap compared to the phases with a high proportion of Bi2O3 (2.64 eV). Furthermore, it was shown that the band gap was dependent on the thickness of the sample and oxygen vacancies or loss of oxygen in the surface. All coatings had degraded methyl orange (MO) under irradiation by xenon lamps.

Identifying the general trend of activity of non-stoichiometric metal oxide phases for CO oxidation on Pd(111)

Oxidation state changes under reaction conditions are very common in heterogeneous catalysis. However, due to the limitation of experiment and computational methods, the relation between oxidation state and catalytic activity is not clear. Herein, we obtain the most stable structures of palladium oxide films with different oxidation states on palladium metal surfaces using density functional theory calculations and a state-of-the-art optimization method, namely the particle swarm optimization. These structures clearly show the process of palladium oxide film formation on metallic surfaces. Using CO oxidation as a model reaction, we find that the activities increase first and then decrease with the increase of oxidation states, peaking at Pd4O3. Our findings offer an understanding of the phase transformation and the activity of non-stoichiometric phases.

Изучение фазового состава продуктов электрохимического окисления сульфидных гранул системы Cu1.96S–Ni3S2–Cu–Ni

The crystallization rate of copper and nickel sulfides influences on the phase formation processes. The high crystallization rate (about 103 degrees/s), achieved through granulation of the sulfide copper-nickel melt, leads to the stabilization of non-stoichiometric phases, the formation of ultrafine structures, which are grains and partial dissolution of the metal component in the sulfide. The structure of the granules is formed by nickel sulfide (Cu1.96S) phases in the form of dendritic inclusions of 2-20 µm in size in the nickel sulfide phase (Ni3S2). According to the phase diagram of the state of Cu – Ni – S, a solid solution of Cu – Ni may be present in the composition of eutectic compounds with copper and nickel sulphides. The electrochemical oxidation of copper and nickel sulfides in a solution of sulfuric acid occurs through a series of successive phase transformations described in the work, during which the conversion of sulfides occurs in intermediate oxidation states oxidizing to the elemental state: Cu1.96S → Cu1.8S → Cu1.75S → CuS → S; Ni3S2 → NiS → S. The non-stoichiometric composition of compounds suggests the presence of excessive or deficient sulfur and metal contents in the crystal lattice, which can affect the mechanism and sequence of phase transformations during the electrochemical oxidation of sulfide granules. Dissolution occurs not only on the surface of the granules, but also along the grain boundaries. The leached areas form capillaries inside the granules, through which electrolyte enters the electrochemical reactions. Porous sulfur sulfide sludge forms on the surface of the granules. The phase composition of the sludge was studied. The main phase components of poorly soluble products are nickel granules Ni3+хS2–Cu2-хS.

Study of a Nanostructured Anatase Coating on the Rutile Surface

Nanosized titanium dioxide allows solving complex engineering problems. One such a problem is the development of materials and coatings that reduce the probability of nosocomial infections of the surface of orthopedic structures, including implant systems. This article presents the results of studying anatase ceramic coatings deposited according to the sol–gel technology on a sintered material based on nanosized titanium dioxide powder (rutile modification) by the Raman spectroscopy, X-ray diffraction analysis, and scanning electron microscopy. The resulting coating has a complex layered structure, which almost completely consists of titanium dioxide in the anatase phase according to the Raman spectroscopy data. The simultaneous occurrence of both phases is fixed in a coating. The identification of rutile in X-ray diffraction patterns is apparently associated with the fact that rutile with a varied peak intensity is preferentially formed at the first deposition stages of the coating on the polycrystalline rutile surface. The fact that nonstoichiometric phases are also present in X-ray diffraction patterns makes it possible to assume that the phase composition of the coating over the thickness is nonidentical and presented by a gradual layerwise transition from rutile to anatase. The coating thickness is 60 ± 15 μm. It is presented by lamellar blocks of various sizes. The thickness of a separate plate is in the limits of 60–80 nm. This procedure allows the deposition of an anatase coating not only on the ceramic samples based on titanium dioxide, but also on the surface of titanium implants when preliminarily forming the titanium dioxide layer in the form of rutile on the metal surface. The experiments on the investigation into the antibacterial properties and morphological characteristics of bone tissue contacting with the implant are performed at the Department of the Prosthetic Dentistry at the Perm State Medical University.

Nanostructured Crystals of Fluorite Phases Sr1 – xRxF2 + x (R Are Rare-Earth Elements) and Their Ordering. 14: Concentration Dependence of the Defect Structure of Nonstoichiometric Phases Sr1 – xNdxF2 + x As Grown (x = 0.10, 0.25, 0.40, 0.50)

The defect structure of as grown Sr1 – xNdxF2 + x (x = 0.10–0.50) crystals grown from a melt by directional solidification under identical conditions has been studied by X-ray diffraction. All crystals belong to the CaF2-type structure (sp. gr. $$Fm\bar {3}m$$ ). The change in the type of structural defect cluster with a change in the NdF3 content, which occurs in some range of x rather than stepwise, was traced for the first time in the entire homogeneity range of fluorite nonstoichiometric phase Sr1 – xNdxF2 + x. Clusters of two types coexist in crystals with х = 0.10 and 0.25: tetrahedral [Sr4 – nNdnF26] and cuboctahedral [Sr14 – nNdnF64 + n]. With a further increase in the NdF3 content in the Sr1 – xNdxF2 + x structure, a transition from two types of clusters to only one type occurs, and only tetrahedral clusters are formed in crystals with х > 0.40.

Comparative characterization of biogenic and chemical synthesized hydroxyapatite biomaterials for potential biomedical application

Abstract This present study illustrates the processing and comprehensive characterization of biogenic hydroxyapatite (FB-HAp) biomaterial from fish bone (Tuna fish) in a facile single step thermal decomposition procedure. Fish bones were calcined at 1000 °C in an air atmosphere for 1 h. The obtained FB-HAp flakes were ball milled for 30 h at a unidirectional milling speed of 300 rpm. A well-established wet chemical process was obtained to synthesize HAp nanoparticles (C-HAp). The synthesized non-stoichiometric (Ca/P = 1.87) FB-HAp phases were identified by X-ray diffraction study and compared with C-HAp. The thermal properties were studied by thermogravimetric and differential thermal analysis. Raman spectroscopy and Fourier transform infrared spectroscopy confirmed the structural and functional groups of FB-HAp and C-HAp. The morphological and elemental analysis was performed by transmission electron microscopy, energy dispersive X-ray spectroscopy and Inductive coupled plasma (ICP) spectroscopy. The synthesized FB-HAp and C-HAp were mixed with 2% starch porogen and compacted with ∼200 MPa unidirectional pressure to obtained small pellet scaffolds. The obtained scaffolds were sintered at 1000 °C in an air atmosphere for 2 h and further used for cell attachment and proliferation study. The FB-HAp biomaterials showed non-toxic behavior with MG-63 osteoblast cell lines similar to C-HAp. Additionally, FB-HAp scaffold showed enhanced cellular attachment and proliferation on its surface. The in-depth chemical, structural, and morphological analysis suggest that fish bone could be a promising sustainable biogenic source of HAp for biomedical applications.