KCl Flux Research Articles

Non-stoichimometric square sheets in RSe2−x (x ∼ 1/6) (R = Gd and Tb)

The application of optimal electron count has proven effective in predicting the existence of complex structural topologies containing electron-rich polyanionic networks, including linear and zig-zag chains, square planar structures, and simple cubic lattices of main group elements. In this study, we report the successful synthesis of a new family of magnetic compounds, RSe2−x (x ∼ 1/6) (R = Gd and Tb), using KCl flux. The resulting crystal structure of RSe2−x is tetragonal, exhibiting PbFCl-type symmetry with space group P4/nmm. Chemical composition analysis consistently reveals x ∼ 1/6, indicating chalcogen-deficient square sheets. This observation suggests that RSe2−x adheres to the 14-electron rule, as the total valence electrons per RSe2−x (x ∼ 1/6) is 14 e−. In GdSe2−x (x ∼ 1/6), the magnetic susceptibility measurement displays a lambda-shaped peak around 8 K, indicating an antiferromagnetic-type transition. However, the positive Curie–Weiss temperature (θCW) suggests the presence of an additional ferromagnetic interaction, which may result from the large magnetic moment of the Gd atoms. In contrast, TbSe2−x (x ∼ 1/6) exhibits antiferromagnetic ordering. Chemical bonding analysis also indicates that the strong Se–Se antibonding in the square net may be related to Se deficiency. Our work shed light on ellucidating the interplay between chemical rule, defects, and magnetism.

Deoxidation of Titanium Utilizing Thulium and Halide Flux

AbstractDevelopment of an efficient deoxidation method for titanium (Ti) is desired to recycle oxygen (O)-contaminated Ti scrap. In this study, the utilization of thulium (Tm) as a deoxidant for Ti in various halide fluxes was investigated. Tm is a rare-earth metal, which is a by-product of other rare-earth metals with high demand and has limited industrial uses. When NaCl or KCl flux was installed in the deoxidation experiments, the impurity oxygen in Ti was removed to the concentrations of 140–590 mass ppm O, which are lower than that achieved under the equilibrium between Tm and Tm2O3 (290–530 mass ppm O). The results show that the nominal activity of the deoxidation product (Tm2O3) was lowered by the presence of halide fluxes in the reaction systems. The combination of Tm and halide fluxes in a new deoxidation technique holds promising potential for both accelerating the Ti recycling and exploring novel applications for Tm. Graphical Abstract

Low-temperature flux synthesis of potassium hexatitanate whiskers: Growth mechanism and applications in high-temperature infrared thermal insulation

Potassium hexatitanate whiskers (PHWs, K2Ti6O13)are a promising functional material with high thermal insulation capacity. In this study, PHWs were successfully synthesized using the flux method at temperatures ranging from 725 °C to 1100 °C. The crystal growth mechanism of PHWs in the KCl flux follows a series-parallel pattern. The synthesized PHWs have a near-infrared reflectivity (NIR) exceeding 95%, and the cause of high reflectivity was theoretically analyzed based on the Kubelka-Munk equation. The high temperature stability of near-infrared reflectivity and the thermal stability of the PHWs were further assessed using TG-DSC, XRD, SEM, and UV–VIS-NIR analysis. It was found that PHWs prepared at 800 °C begin to decompose into TiO2 at approximately 1375 °C. Nevertheless, the near-infrared reflectivity remains stable even after heat treatment. Additionally, PHWs were prepared as a composite thermal insulating coating, showing thermal conductivity between 0.177 and 0.569 W/(m K) across the temperature range of 200 °C to 900 °C. The temperature difference between the hot and cold surfaces of the composite coating was significantly greater compared to the control groups at various temperatures, with a p-value of less than 0.01 in the paired t-test. This result demonstrates the great application potential of PHWs as a high-temperature near-infrared thermal insulation material.

Tolerance Factor Approach for the Design of Quaternary Materials as Applied to the A2Ln4Cu2nQ7+n Homologous Series.

The design of novel quaternary materials can be a chemically challenging endeavor. Here, we show that a new tolerance factor, we herein call the quaternary tolerance factor (QTF), akin to the Goldschmidt ratio, can be applied to a homologous series of quaternary materials to aid in the prediction of structural stability in new phases. Single crystals of nine new members of the A2Ln4Cu2nQ7+n homologous series were synthesized (A = K, Rb, Cs, Ln = Sm, Gd, Tb, Dy, Er, Yb, Lu, Q = S, Se) for n = 1, 2, and 3 using a reactive KCl flux method as the sole potassium source. This homologous series is notable for being the largest of all known chalcogenide homologies and thus serves as a case study for understanding quaternary structural relationships. We present a thorough structural analysis of these newly synthesized compounds and place them in the context of all previously reported members of the homologous series. We then report a generalizable tolerance factor capable of predicting structurally interrelated quaternary materials from an a priori sphere-packing approach for the design of new materials.

Upconversion luminescence of cubic and hexagonal structured SrTa4O11:Er3+/Yb3+ phosphors.

Cubic and hexagonal structured SrTa4O11(STO):Er3+/Yb3+ phosphors were synthesized by a solid state reaction (SSR) and molten salt synthesis (MSS). The upconversion luminescence (UCL) intensity of these samples was investigated. Hexagonal STO:Er3+/Yb3+ with much or a little β-Ta2O5 can be synthesized by SSR in air or vacuum, respectively, and the UCL intensity of the sample synthesized by SSR in a vacuum is higher. Cubic STO:Er3+/Yb3+ can be synthesized by MSS with KCl flux, and hexagonal STO:Er3+/Yb3+ can be synthesized by MSS with B2O3 flux, which has the strongest UCL intensity among all the samples compared with samples prepared by SSR. The sample by MSS with B2O3 flux was acid pickled (AP) with HCl solution, and the green UCL intensity increased by 2.18 times, which reached 32.95% for β-NaYF4:Er3+/Yb3+. The UCL intensity of the hexagonal STO:Er3+/Yb3+ is much higher than that of the cubic structure, which is due to the layered structure and the non-central symmetry of the Er3+/Yb3+ doped sites in hexagonal STO:Er3+/Yb3+. The temperature sensitivity of samples is evaluated by luminescence intensity ratio (LIR) technology. The maximum relative sensitivity is 0.0099 K-1 at 303 K. All the results show that hexagonal STO:Er3+/Yb3+ has excellent pure green UCL intensity and high temperature sensitivity, which can be used in UCL display and temperature sensing.

Molten salt synthesized Tb3+, Pr3+ or Dy3+ single doped CaTa4O11 with persistent luminescence

A series of Tb3+, Pr3+ and Dy3+ monodoped CaTa4O11(CTO) persistent luminescence (PersL) phosphors have been successfully synthesized by molten salt method with KCl flux. The PersL color is green and...

Molten salt synthesized LaTa7O19:Er3+/Yb3+ with superior upconversion luminescence by KCl flux

LaTa7O19(LTO):Er3+/Yb3+ upconversion luminescence (UCL) phosphors with hexagonal structure was successfully prepared by a molten salt synthesis using KCl as solvent. Increasing holding temperature from 825 to 1150oC, the LTO structure...

Ultra-low lattice thermal conductivity and high figure-of -merit in Cl and K co-dopped Bi2Se3 prepared by KCl flux

Bismuth telluride (Bi2Te3) is a widely used thermoelectric (TE) material in practical applications. However, it contains expensive and toxic tellurium, a promising TE material should include low-cost and non-toxic element. Bismuth selenide (Bi2Se3) is similar to Bi2Te3 in electronic and crystal structure. The electron effective mass of Bi2Se3 is smaller than that of Bi2Te3, while its electrical transport performance is inferior to that of Bi2Te3. Therefore, in this study, Cl and K co-doped Bi2Se3 thermoelectric material with high density, low thermal conductivity, and excellent thermoelectric performance was prepared using KCl as the solvent. It is found that the effective electron mass and carrier concentration of the doped sample is effectively improved, which significantly increases the doped sample's electrical transport properties and ZT value. Finally, the Bi2Se3(KCl)3.5 sample reached a maximum ZT value of 0.79 at 550 K, and the average ZT in the measured temperature range reached 0.45.

Synthesis of 1Y6Ce–ZrO2 nanoparticles with excellent sintering performance via novel Sol-Gel-Flux method

To alleviate agglomeration and refine the particle size, a novel sol-gel-flux method (SGFM) was applied to synthesise ZrO 2 (1Y6Ce–ZrO 2 ) nanoparticles (NPs) stabilised by 1 mol% Y 2 O 3 and 6 mol% CeO 2 . Both the NaCl and NaCl + KCl flux systems were investigated. The effects of calcination temperature and holding time on the phase composition and morphology of the NPs were studied, and the sintering performances of the 1Y6Ce–ZrO 2 NPs prepared in the two flux systems were compared. The results showed that the optimum temperature for the preparation of 1Y6Ce–ZrO 2 NPs by SGFM could be reduced from 800 to 700 °C if the NaCl system was replaced by the NaCl + KCl system. The lower calcination temperature resulted in well-dispersed tetragonal 1Y6Ce–ZrO 2 NPs with an average grain size of 14.5 nm and quasi-spherical morphology. 1Y6Ce–ZrO 2 NPs prepared in the NaCl + KCl system exhibited better sintering performance than 1Y6Ce–ZrO 2 NPs prepared in the NaCl system. The 1Y6Ce–ZrO 2 ceramics obtained by sintering the former showed higher stability at high temperatures, smaller grain sizes, and narrower grain size distributions, which are conducive to improving the mechanical properties of 1Y6Ce–ZrO 2 ceramics. Fully tetragonal 1Y6Ce-TZP ceramic with an average grain size of 0.68 μm and a relative density of 98.49% were obtained by pressureless sintering of the former at 1400 °C for 2 h.

Flux Growth of Single-Crystalline Hollandite-Type Potassium Ferrotitanate Microrods From KCl Flux.

Hollandite-type crystals have unique and interesting physical and chemical properties. Here, we report the flux growth of hollandite-type single-crystalline potassium ferrotitanate (KFTO) with faceted surface features from a KCl flux. We varied the flux growth conditions, including the kind of flux, holding temperature, and solute concentration for growing faceted crystallites. KCl was found to be the best flux to grow the single-crystalline KFTO particles, while heating at or above 900°C was needed to yield the KFTO single crystals. The crystal growth was only weakly dependent on the solute concentration. Next, we characterized the grown single crystals and discussed the manner of their growth from the KCl flux. TEM images with clear electron diffraction spots indicated that the KFTO crystals grew along the <001> direction to form microrods ~10 μm in size. DFT calculation results indicated that the surface energy of the (100) face is lower than that of the (001) face. Based on these characterization results, we proposed a possible growth mechanism of the KFTO crystals.

Platinum-loaded lanthanum-doped calcium titanate photocatalysts prepared by a flux method for photocatalytic steam reforming of methane

Abstract Calcium titanate (CaTiO3) samples were prepared by a flux method with various parameters such as selection of the flux, presence or absence of the flux, and solute concentration in the molten mixture with the flux. These conditions much influenced the structural and physical properties like morphology, particle size, specific surface area, and photoabsorption bands. The photocatalytic activity of the samples loaded with Pt-cocatalyst (Pt/CaTiO3) was evaluated for the photocatalytic steam reforming of methane around room temperature. It was found that the sample prepared with a KCl flux realized 3.3 times higher photocatalytic activity than that prepared by a conventional solid-state reaction method. Further, La-doping to the CaTiO3 photocatalyst (Pt/CaTiO3:La) was examined. It was revealed that the La cation was successfully doped at the Ca site in the CaTiO3 lattice. It is observed that small amount of La-doping anomalously enhanced the crystal growth although it lowered the photocatalytic activity. In contrast, the sample with a moderate amount of La-doping such as 1 mol% exhibited 1.6 times higher photocatalytic activity than the non-doped sample. Although most of the structural properties such as morphology, particle size and specific surface area could not simply explain the improvement of photocatalytic activity, at least it was revealed that the defects giving rise to the absorption bands in visible light region decreased the photocatalytic activity.

Comparative Analysis of the Methods for Obtaining Superconducting Fe1 + ySexTe1 – x Single Crystals

The Fe1 + ySexTe1 – x single crystals grown by the KCl flux and Bridgman methods have been compared using the same investigation techniques. It is shown that the KCl flux growth yields high-quality Fe1 + ySexTe1 – x superconducting single crystals. The oxygen diffusion coefficient in Fe1 + ySexTe1 – x has been estimated. During the Bridgman crystal growth, a stable nonsuperconducting phase can be formed, which suggests a need for refining the phase diagram of this compound.

Growth and transport properties under high pressure of PrOBiS2 single crystals

PrOBiS2 single crystals were successfully grown using a KCl flux. The obtained crystals had a plate-like shape with a typical size of 0.5–1.0 mm and well-developed ab-plane. The Pr valence of the grown crystals, as determined by an X-ray absorption fine structure spectroscopy analysis, was almost trivalent. The PrOBiS2 single crystals did not exhibit superconductivity down to 0.25 K. The transport properties under high pressures up to 50 GPa ensured semiconducting behavior for 2–300 K using a diamond anvil cell.

Function of Tetrahedral ZnS3O Building Blocks in the Formation of SrZn2S2O: A Phase Matchable Polar Oxysulfide with a Large Second Harmonic Generation Response

Single crystals of a new zinc oxysulfide SrZn2S2O were grown in a eutectic KF–KCl flux, and the structure was determined by single-crystal X-ray diffraction. SrZn2S2O crystallizes in the noncentrosymmetric polar space group Pmn21 with lattice parameters of a = 3.87440(10) A, b = 9.9847(3) A, and c = 6.0916(2) A. In the crystal structure, close-packed corrugated double layers of ZnS3O tetrahedra, which are derived from the wurtzite structure, are vertically separated by Sr2+ ions. In addition, the O/S anion ordered arrangement in each close-packed layer yields two distinct orientations of the Zn-centered tetrahedra. The crystals of SrZn2S2O are colorless and transparent, and the oxysulfide has a band gap of 3.86 eV, based on UV–vis–NIR diffuse reflectance measurements. Thermogravimetric measurements showed that SrZn2S2O is stable up to 650 °C in O2 gas atmosphere. First-principle calculations indicate that the valence band maximum is mainly composed of O-2p and S-3p states, whereas the conduction band mini...

Preparation of (11n) oriented Bi2Sr2CaCu2O8+x thin films by solution methods using NdGaO3 (100) substrates

We have prepared (11n) oriented Bi2Sr2CaCu2O8+x (Bi2212) thin films by KCl flux method which is one kind of solution method. The (100) substrate of NdGaO3 (NGO) was used. Heat treatment was performed with four different temperature profiles. Only the (11n) peak appeared in the θ-2θ X-ray diffraction pattern. From the SEM image, it was found that elongated plate-like crystal grains of Bi2212 were grown. From the viewpoint of lattice matching, this elongated crystal grain is considered to be (010) (or (100)) oriented crystal grains.

Development of a New Fabrication Route for High Quality Visible-Light-Driven Photocatalysts; Atmosphere Controlled Flux Growth for Oxynitride and Nitride Crystals

Photocatalysts and related materials have been attracted a number of research interests due to their various application such as decomposition of organic substances, superhydrophilic surfaces, dye-sensitized solar cells, and hydrogen production by water splitting. In particular, water-splitting by photocatalysts have been investigated because of expectation to supply clean and recyclable hydrogen energy. Generally, photocalysts, as represented by TiO2, are activated by only ultraviolet light illumination due to their wide band gap. Although these UV-light-driven photocatalysts can split water in a proper condition, the solar energy conversion efficiency is rather low because UV light energy is just several percent in total energy of sun light on the earth, and visible light accounts for almost half of the solar energy. From the viewpoint of increase the efficiency and industrial application of solar hydrogen production, visible-light-driven photocatalysts have intensively attracted research interests. In particular, oxynitride and nitride semiconductor photocatalysts are one of promising materials for construction of photocatalytic water splitting systems. Although visible-light-driven photocatalysts with sufficiently small band gap, an appropriate band gap position for water splitting, and stability during the reaction have been developed, it is still required to improve the quantum yield. Generally, lattice defects are not suitable for photocatalysts with high performance because they would become recombination centers of excited carriers. Flux method is a liquid phase crystal growth technique which uses molten metals or molten metal salts as solvents. Flux method can afford binary or multicomponent high-quality crystals at lower temperatures below their melting points. It is interesting that flux-grown crystals usually have idiomorphic shapes with well-developed facets, which represents their high crystallinity. In addition, it provides simple, low-cost and environmentally-benign pathway compared to conventional solid-state-reaction method. Therefore, flux growth is a promising way to prepare high-quality visible-light-driven photocatalyst crystals. Herein, we present growth of oxynitride and nitride crystals for photocatalysts through atmosphere controlled flux method. LaTiO2N, Ta3N5, SrTaO2N, BaTaO2N, and BaNbO2N (and many kinds of oxynitrides and nitrides) crystals were grown under NH3 flow with using flux. Especially LaTiO2N crystals were directly grown from KCl flux and exhibited higher oxygen evolution performance compared to those prepared by conventional methods. It is attributed to elongated lifetime of excited carriers which detected by time-resolve IR spectroscopy. The flux growth is applied to crystal layer (i.e., thin and thick films) fabrication technique and then various photoelectrode consisting of the above-mentioned photocatalysts and current collector were fabricated. The details of flux growth and evaluation (e.g., photocatalytic performance) of the oxynitride and nitride crystals will be presented in 233rd ECS Meeting. This research was supported in part by the Japan Technological Research Association of Artificial Photosynthetic Chemical Process (ARPChem), NEDO and JSPS Grant-in-Aid for Scientific Research (A) 25249089.

Improved visible-light activity of nitrogen-doped layered niobate photocatalysts by NH3-nitridation with KCl flux

In the present study, nitrogen doping into layered perovskite niobites, KCa2Nan–3NbnO3n+1 (n = 3 or 4), was attempted to enhance their response to visible light during photocatalytic water reduction and oxidation. Although conventional nitridation, i.e., heating in NH3 stream at a high temperature, produced black-colored samples due to the undesirable reduction of Nb5+ to Nb4+ (and/or Nb3+), the use of KCl flux with NH3 stream effectively prevented the reduction of Nb5+, resulting in yellow-colored samples that had a much lower concentration of reduced Nb species. The samples prepared with KCl flux exhibited stronger absorption (at λ = 350–550 nm) when compared to those prepared without KCl flux. This was due to the higher concentration of N3– anions, as confirmed by elemental analysis and the observed peak shift in XRD patterns. On the other hand, the absorption band with λ > 550 nm, which indicated the presence of reduced species or anion vacancies, were astonishingly smaller for the samples prepared with KCl than for those without KCl. This indicated that the formation of reduced Nb species was inhibited by KCl. XPS analysis also supported this effect. The use of KCl flux also prevented the problematic decrease in the concentration of potassium cations in the interlayer space during high-temperature treatment in NH3 flow. The samples prepared with KCl flux exhibited much higher O2 and H2 evolution rates under visible-light irradiation in the presence of an electron acceptor and a donor, respectively as compared to those prepared without KCl. This clear improvement in O2 and H2 evolution was probably due to the larger amount of doped nitrogen introduced and/or the suppressed formation of reduced Nb species.

Sodium Cation Substitution in Sr2KTa5O15 toward Enhancement of Photocatalytic Conversion of CO2 Using H2O as an Electron Donor.

The K and Sr cations (K+ and Sr2+) in a Sr2KTa5O15 photocatalyst were found to be easily substituted by Na cations (Na+) to form SrxKyNazTa5O15 by a facile one-pot flux method using a mixture of potassium chloride (KCl) and sodium chloride (NaCl). SrxKyNazTa5O15 fabricated using a mixture of KCl and NaCl with a Ag cocatalyst showed enhanced photocatalytic activity without apparent change in selectivity toward CO for the photocatalytic conversion of CO2 using H2O. The present study demonstrates that the flux treatment significantly affected the phase, morphology, band gap, and surface Sr composition of the catalyst owing to the substitution of K+ and Sr2+ for Na+. The stability and durability of the catalyst were also enhanced as compared to those of the photocatalyst fabricated using only KCl flux due to more stable Ag on the surface of SrxKyNazTa5O15.

Engaging the flux-grown La1−xSrxFe1−yTiyO3 crystals in visible-light-driven photocatalytic hydrogen generation

Perovskite LaFeO3 is regarded as one of the promising photocatalysts for solar splitting of water to hydrogen and oxygen due to its suitable band edge positions, visible light absorbance, and high chemical stability. To further improve its photocatalytic performance of LaFeO3, the effects of solute concentration in the KCl-flux growth and partial A– and B–site substitution are investigated in this work. Controlling the solute concentration in the range of 1–20 mol% is crucial to grow phase-pure LaFeO3 crystals with idiomorphic shape. With increasing the solute concentration, the flux-based growth route gradually changes to a solid state-based growth route because of a decrease in solubility and an increase in the crystallization core number. The La1−xSrxFe1−yTiyO3 (x,y = 0,0; 0,0.15; 0.15,0; 0.1125,0.0375; 0.0375,0.1125; and 0.075,0.075) crystals were also synthesized by a KCl-flux method to explore the effect of partial Sr2+-to-La3+ and/or Ti4+-to-Fe2−4+ substitution on photocatalytic performance of LaFeO3. The Sr2+–Ti4+ co-substitution is found to enhance the photocatalytic performance of LaFeO3 as compared with the corresponding individual substitution (Sr2+ or Ti4+). The highest photocatalytic hydrogen generation rate (83.2 μmol h−1) was observed for Pt-photodeposited La0.925Sr0.075Fe0.925Ti0.075O3 crystals in 5-h reaction due to the improvement of both bulk properties and photoactivity and the reduction in both grain boundaries and lattice defects stemmed from the Sr2+–Ti4+ co-substitution and KCl flux growth.

Synthesis and visible-light-induced sacrificial photocatalytic water oxidation of quinary oxynitride BaNb0.5Ta0.5O2N crystals

Quinary oxynitride BaNb0.5Ta0.5O2N crystals were fabricated through the partial nitridation and acidification of the KCl flux grown Ba5Nb2Ta2O15 crystals. The parameters of both the solute concentrations and cooling rates are optimized for the KCl flux growth of the larger Ba5Nb2Ta2O15 crystals with clearer crystal habits. Here, the optimal Ba5Nb2Ta2O15 crystals mainly have a hexagonal plate-like shape. After the partial nitridation and acidification, the porous BaNb0.5Ta0.5O2N crystals maintained the crystal shape of the oxide precursor and had a single-crystalline nature. For the Ba5Nb2Ta2O15 crystals, the wavelength of the absorption edge was about 707 nm. Especially, the CoOx-loaded BaNb0.5Ta0.5O2N photocatalyst demonstrated the comparatively high amount of O2 gas (150.7 μmol) during the 5 h visible-light-induced sacrificial water oxidation half-reaction, which might be achieved due to the high crystallinity and visible-light absorption property.