La2-xCexCu1-yZnyO4 perovskites for high temperature water-gas shift reaction

The performance of La2-xCexCu1-yZnyO4 perovskites as catalysts for the high temperature water-gas shift reaction (HT-WGSR) was inves-tigated. The catalysts were characterized by EDS, XRD, BET surface area, TPR, and XANES. The results showed that all the perovskites exhibited the La2CuO4 orthorhombic structure, so the Pechini method is suitable for the preparation of pure perovskite. However, the La1.90Ce0.10CuO4 perovskite alone, when calcined at 350/700℃, also showed a (La0.935Ce0.065)2CuO4 perovskite with tetragonal struc-ture, which produced a surface area higher than the other perovskites. The perovskites that exhibited the best catalytic performance were those calcined at 350/700 ℃ and, among these, La1.90Ce0.10CuO4 was outstanding, probably because of the high surface area associated with the presence of the (La0.935Ce0.065) 2CuO4 perovskite with tetragonal structure and orthorhombic La2CuO4 phase.     

Control on the Copper Valence and Properties by Oxygen Content Adjustment in the LaCuO3−ySystem (0≤y≤0.5)

The LaCuO_3−yperovskite is considered as the first member of the 01(n−1)nseries of “layered cuprates.” Highly oxidized, stoichiometric LaCuO₃is stabilized under very high oxygen pressures, and was synthesized in a cubic-anvil-type high-pressure apparatus at 5 GPa and 1400°C using excess amounts of KClO₄as an external oxidizing agent. Upon heating under ambient pressure the rhombohedral high-pressure phase loses oxygen yielding tetragonal, monoclinic, and orthorhombic forms of LaCuO_3−yas intermediate products before the final decomposition into La₂CuO₄and CuO or Cu₂O around 800°C. All three oxygen-deficient LaCuO_3−yphases could be isolated and their stability limits and corresponding oxygen contents conveniently investigated by annealing stoichiometric LaCuO₃in a thermobalance of high sensitivity in order toin situdetect the exact amount of oxygen loss. The nominal copper valence values calculated from the oxygen contents are compared and discussed with XPS data as well as with the results evaluated from magnetic susceptibility measurements.     

Effects of the partial replacement of La by M (M=Ce, Ca and Sr) in La2-xMxCuO4 perovskites on catalysis of the water-gas shift reaction

The performance of La2-xMxCuO4 perovskites (where M = Ce, Ca or Sr) as catalysts for the water-gas shift reaction was investigated at 290 ℃ and 360 ℃. The catalysts were characterized by EDS, XRD, N2 adsorption-desorption, XPS and XANES. The XRD results showed that all the perovskites exhibited a single phase (the presence of perovskite structure), suggesting the incorporation of metals in the perovskite structure. The XPS and XANES results showed the presence of Cu²⁺ on the surface. The perovskites that exhibited the best catalytic performance were La2-xCexCuO4 perovskites, with CO conversions of 85%-90%. Moreover, these perovskites have higher surface areas and larger amounts of Cu on the surface. And Ce has a higher filled energy level than the other metals, increasing the energy of the valence band of Ce and providing more electrons for the reaction. Besides, the La1.80Ca0.20CuO4 perovskite showed a good catalytic performance.     

Thermodynamics and phase relationships of the ternary lanthanum-uranium-oxygen system

In the selected regions La:(La + U) = 0.05 and O:(La + U) = 2.00 of the ternary system lanthanum-uranium-oxygen emf measurements on solid state galvanic cells, coulometric titrations, and X-ray diffraction techniques were used to obtain phase boundaries and thermodynamic data in the temperature range from 600 to 1000°C. For the first time order disorder transformations of La_1−yU_yO_2+x up to 15 mole% lanthanum are reported. The transformation temperature is 1415°K for UO_2.23; 1397°K for La_0.05U_0.95O_2.23, and 1449°K for La_0.15U_0.85O_2.23. The vibrational entropy component of excess oxygen in M_1−yU_yO_2+x is estimated.     

High performance anode-supported intermediate temperature solid oxide fuel cells (IT-SOFCs) with La0.8Sr0.2Ga0.8Mg0.2O3−δ electrolyte films prepared by electrophoretic deposition

Remarkable power density was obtained for anode-supported solid oxide fuel cells (SOFCs) based on La_0.8Sr_0.2Ga_0.8Mg_0.2O_3−δ (LSGM) electrolyte films, fabricated following an original procedure that allowed avoiding undesired reactions between LSGM and electrode materials, especially Ni. Electrophoretic deposition (EPD) was used for the fabrication of 30 μm-thick electrolyte films. Anode supports were made of La_0.4Ce_0.6O_2−x (LDC). The LSGM powder was deposited by EPD on an LDC green tape-cast membrane added with carbon powder, both as pore former and substrate conductivity booster. A subsequent co-firing step at 1490 °C produced dense electrolyte films on porous LDC skeletons. Then, a La_0.8Sr_0.2Fe_0.8Co_0.2O_3−δ (LSFC) cathode was applied by slurry-coating and calcined at 1100 °C. Finally, the porous LDC layer was impregnated with molten Ni nitrate to obtain, after calcination at 900 °C, a composite NiO–LDC anode. Maximum power densities of 780, 450, 275, 175, and 100 mW/cm² at 700, 650, 600, 550, and 500 °C, respectively, were obtained using H₂ as fuel and air as oxidant, demonstrating the success of the processing strategy. As a comparison, electrolyte-supported SOFCs made of the same materials were tested, showing a maximum power density of 150 mW/cm² at 700 °C, more than 5 times smaller than the anode-supported counterpart.     

Thermodynamic Quantities and Defect Structure of La0.6Sr0.4Co1−yFeyO3−δ(y=0–0.6) from High-Temperature Coulometric Titration Experiments

The partial energies and entropies of O₂in perovskite-type oxides La_0.6Sr_0.4Co_1−yFe_yO_3−δ(y=0, 0.1, 0.25, 0.4, 0.6) were determined as a function of nonstoichiometryδby coulometric titration of oxygen in the temperature range 650–950°C. An absolute reference value ofδwas obtained by thermogravimetry in air. The nonstoichiometry at a given oxygen pressure and temperature decreases with iron contenty. At low nonstoichiometries the oxygen chemical potential decreases withδ. The observed behavior can be interpreted by assuming random distribution of oxygen vacancies, an electronic structure with both localized donor states on Fe, and a partially filled itinerant electron band, of which the density of states at the Fermi level scales with the Co content. The energy of the Fe states is close to the energy at the Fermi level in the conduction band. The observed trends of the thermodynamic quantities can be interpreted in terms of the itinerant electron model only when the iron content is small. At high values ofδthe chemical potential of O₂becomes constant, indicating partial decomposition of the perovskite phase. The maximum value ofδat which the compositions are single-phase increases with temperature.     

Preparation and electrical properties of new oxide ion conductors Ce6−xDyxMoO15−δ (0.0 ≤ x ≤ 1.8)

Ce_6−xDy_xMoO_15−δ (0.0 ≤ x ≤ 1.8) were synthesized by modified sol–gel method. Structural and electrical properties were investigated by means of X-ray diffraction (XRD), Raman, X-ray photoelectron spectroscopy (XPS) and electrochemical impedance spectroscopy (EIS). The XRD patterns showed that the materials were single phase with a cubic fluorite structure. Impedance spectroscopy measurement in the temperature range between 350 °C and 800 °C indicated a sharp increase in conductivity for the system containing small amount of Dy₂O₃. The Ce_5.6Dy_0.4MoO_15−δ detected to be the best conducting phase with the highest conductivity (σ_t = 8.93 × 10⁻³ S cm⁻¹) is higher than that of Ce_5.6Sm_0.4MoO_15−δ (σ_t = 2.93 × 10⁻³ S cm⁻¹) at 800 °C, and the corresponding activation energy of Ce_5.6Dy_0.4MoO_15−δ (0.994 eV) is lower than that of Ce_5.6Sm_0.4MoO_15−δ (1.002 eV).     

Regions of stability of variable-composition perovskite phases La1?xSrxMn1?yMyO3 (M = Ti, Ni)

The formation of the orthorhombic solid solution series LaMn_1−y Ti _y O₃ with 0.0 ≤ y ≤ 0.15 and LaMn_1−y Ni _y O₃ with 0.0 ≤ y ≤ 0.4 was observed. The stability boundaries of the La_1−x Sr _x Mn_1−y M _y O₃ (M = Ti, Ni) perovskite phases were determined. Fragments of isobaric-isothermal sections of the phase diagrams of the La₂O₃-SrO-Mn₃O₄-TiO₂ and La₂O₃-SrO-Mn₃O₄-NiO systems in air at 1100°C were suggested. Original Russian Text ? E.A. Filonova, A.N. Demina, A.N. Petrov, 2007, published in Zhurnal Fizicheskoi Khimii, 2007, Vol. 81, No. 10, pp. 1787–1790.     

Compounds and phase compatibilities in the system La2O3---SrO---CuO at 950°C

The subsolidus phase diagram of the system La₂O₃---SrO---CuO at 950°C under 1 bar of pure oxygen has been investigated and a new ternary compound, La_1+xSr_2−xCu₂O_5.5+δ with 0.05 ≤ x ≤ 0.15, was isolated. This compound crystallizes in an orthorhombic unit cell with lattice constants related to the lattice constant of the perovskite cubic unit cell, a_p, by a = 3.80 Å a_p, B = 11.48 Å 3a_p, and c = 20.23 Å 5a_p. The structure is isotypical to that of LnSr₂Cu₂O_5.5+δ with Ln = Sm, Eu, or Gd. Reported data on the crystal chemistry of the equilibrium compounds in the system La₂O₃---SrO---CuO have been summarized and compared with the present data. The structure of all compounds is built up of a La---O rock-salt layer separated by a number of LaCuO₃ perovskite layers. The general formula is (La---O)(LaCuO₃)_n where La can be replaced either partly or completely by Sr. Compounds are found for n = 1, 2, and ∞. The structures of the compounds show different types of oxygen vacancy ordering.     

A study of the perovskite solid solution series LaxSr1−xRuO3 and LaxCa1−xRuO3 by ruthenium-99 Mössbauer spectroscopy

The magnetic, electronic, and structural properties of the solid solutions La_xSr_1−xRuO₃ and La_xCa_1−xRuO₃ have been studied by ⁹⁹Ru Mössbauer spectroscopy and other techniques. The La_xCa_1−xRuO₃ phases are reported for the first time and have been shown by powder X-ray diffraction measurements to be orthorhombically distorted perovskites. Electrical resistivity measurements on compacted powders show that all the phases are metallic with p 10⁻³, ohm-cm. Progressive substitution of Sr²⁺ by La³⁺ in ferromagnetic SrRuO₃ leads to a rapid collapse of the magnetic hyper-fine splitting at 4.2°K. For x = 0.25 some ruthenium ions still experience a magnetic field but for 0.4 x 0.75 only single, narrow resonance lines are observed, consistent both with the complete removal of the ferromagnetism and with the presence of an averaged ruthenium oxidation state in each phase, i.e., La_x³⁺Sr_1−x²⁺Ru^(4−x)+O₃ rather than La_x³⁺Sr_1−x²⁺Ru_x³⁺Ru_1−x⁴⁺O₃. LaRuO₃ and CaRuO₃ both give essentially single-line spectra at 4.2°K, indicating that the ruthenium ions in these oxides are not involved in long-range antiferromagnetic order but are paramagnetic. The solid solutions La_xCa_1−xRuO₃ (0 < x 0.6) give sharp symmetrical singlets with chemical isomer shifts (relative to the Ru metal) which move progressively from the value characteristic of Ru⁴⁺ (−0.303 mm sec⁻¹) toward the value for Ru³⁺ (−0.557 mm sec⁻¹), consistent with the presence of intermediate ruthenium oxidation states in these phases also.     

Evidence for nickel-(I)-rich mixed oxide with a defect K2NiF4-type structure

Reduced phases obtained from lanthanum mixed nickel oxide, i.e., La_2−xSr_xNiO_4−y, K₂NiF₄ type, have been studied. Reduction under controlled conditions led to the composition LaSrNiO_3.1 containing formally more than 80% of Ni in the valence state (I). Structural calculations and local studies by X-ray adsorption spectroscopy provide evidence for a lowering of the Ni octahedral coordination configuration. V-square-pyramidal and IV-square configurations are obtained, depending on the reduction level. The oxygen vacancies are highly ordered along the b axis of the orthorhombic unit cell, in agreement with recent defect modeling of this structure     

Oxygen-deficient SrTiO3−x, X = 0.28, 0.17, and 0.08. Crystal growth, crystal structure, magnetic, and transport properties

The grossly nonstoichiometric perovskites SrTiO_3−x with x = 0.28, 0.17, and 0.08 were prepared from a reaction of Sr₂TiO₄, TiO, and TiO₂ at 1500°C. For x = 0.28 relatively large single crystals were obtained. Also for this sample the crystal symmetry was found to depend on the rate of cooling from the reaction temperature and the annealing conditions. Rapidly cooled samples are tetragonal a = 3.9177(3) Å, c = 3.8878(5) Å. Samples annealed in vacuum at temperatures of 1000 to 600°C are cubic a = 3.9075(3) Å with no change in cell volume. Single crystal data from a tetragonal sample indicate slight preferential occupancy of one oxygen position in P4/mmm. No evidence for any supercell due to defect ordering could be seen by TEM in either cubic or tetragonal samples. The x = 0.28 crystals show metallic resistivity, (300 K) = 6 × 10⁻⁴ ohm-cm and temperature-independent paramagnetism, χ_m = 118 × 10⁻⁶ cm³ mole⁻¹. Hall effect data from 300 to 4.2 K analyzed on a single carrier model give a temperature-independent n-type carrier density of 2.4 × 10²¹ cm⁻³. This is a factor of 3.9 less than that expected if the creation of each oxygen vacancy results in the production of two carriers in a single band. Hall data for x = 0.17 and 0.08 samples give similar results corresponding to densities of 2.1 and 1.4 × 10²¹ cm⁻³, respectively, in the same temperature range. These densities are 2.7 and 1.9 times less than the expected single-band value, respectively. Such results point to a two-band model with a large effective mass in one of the bands.     

Phase equilibria in the system LaMnO<Subscript>3</Subscript>-SrMnO<Subscript>3</Subscript>-SrCrO<Subscript>4</Subscript>-LaCrO<Subscript>3</Subscript>

A continuous solid solution LaMn_1?y Cr _y O₃ with an orthorhombic structure is found to exist in the range of 0.0 ≤ y ≤ 1.0. An orthorhombic solid solution La_1?x Sr _x CrO₃ exists in the range of 0.0 ≤ x ≤ 0.1. The stability boundaries are determined for the perovskite phase La_1?x Sr _x Mn_1?y Cr _y O₃. An isobaric-isothermal section LaMnO₃-SrMnO₃-SrCrO₄-LaCrO₃ of the system La₂O₃-SrO-Mn₃O₄-Cr₂O₃ in air at 1100°C is designed.     

Influence of Ce0.35Zr0.55La0.10O1.95 Solid Solution on the Performance of Low Pt-Rh Three-way Catalysts

Ce_0.35Zr_0.55La_0.10O_1.95 solid solution was prepared by coprecipitation technique and characterized by specific surface area measurements (BET), X-ray diffraction (XRD), and temperature-programmed (TP) technique. Ce_0.35Zr_0.55La_0.10O_1.95 was used to prepare low Pt-Rh three-way catalyst (TWC) and its influence on the performance of TWC was investigated. The results showed that Ce_0.35Zr_0.55La_0.10O_1.95 had a cubic structure similar to Ce_0.50Zr_0.50O₂ and a large specific area after calcined at 600 °C for 5 h. Furthermore, after being aged at 1000 °C for 5 h, Ce_0.35Zr_0.55La_0.10O_1.95 still maintained a stable cubic structure and a specific surface area of 47.25 m²·g⁻¹. The results of H₂-TPR and O₂-TPO indicated that Ce_0.35Zr_0.55La_0.10O_1.95 had good redox properties. The catalyst containing Ce_0.35Zr_0.55La_0.10O_1.95 possessed a fairly wide range of three-way working-windows, good low-temperature light-off properties, and better ability of water-gas shift. Being hydrothermally aged at 1000 °C, the catalyst containing Ce_0.35Zr_0.55La_0.10O_1.95 still showed good catalytic activity in comparison with Ce_0.50Zr_0.50O₂ TWC, which indicated that Ce_0.35Zr_0.55La_0.10O_1.95 improved the anti-aging properties of the catalyst.     

High performance protonic ceramic membrane fuel cells (PCMFCs) with Ba0.5Sr0.5Zn0.2Fe0.8O3−δ perovskite cathode

Protonic ceramic membrane fuel cells (PCMFCs) based on proton-conducting electrolytes have attracted much attention because of many advantages, such as low activation energy and high energy efficiency. BaZr_0.1Ce_0.7Y_0.2O_3−δ (BZCY7) electrolyte based PCMFCs with stable Ba_0.5Sr_0.5Zn_0.2Fe_0.8O_3−δ (BSZF) perovskite cathode were investigated. Using thin membrane BZCY7 electrolyte (about 15 μm in thickness) synthesized by a modified Pechini method on NiO-BZCY7 anode support, PCMFCs were assembled and tested by selecting stable BSZF perovskite cathode. An open-circuit potential of 1.015 V, a maximum power density of 486 mW cm⁻², and a low polarization resistance of the electrodes of 0.08 Ω cm² was achieved at 700 °C. The results have indicated that BZCY7 proton-conducting electrolyte with BSZF cathode is a promising material system for the next generation solid oxide fuel cells.     

Preparation and Structure of Cerium Titanates Ce2TiO5, Ce2TiO7, and Ce4Ti9O24

Compounds Ce₂TiO₅, Ce₂Ti₂O₇, and Ce₄Ti₉O₂₄ were prepared by heating appropriate mixtures of solids containing Ce⁴⁺ and Ti³⁺ or Ti which were placed in a platinum-silica-ampoule combination at T = 1250°C (3d) under vacuum. The new compounds were characterized by powder patterns. We obtained Ce₂TiO₅ which is isotypic to La₂TiO₅ and crystallizes in the Y₂TiO₅-type (space group Pnma) with a = 10.877(6) Å, b = 3.893(1) Å, c = 11.389(8) Å, Z = 4. Ce₂Ti₂O₇ is isotypic to La₂Ti₂O₇ and crystallizes in the monoclinic Ca₂Nb₂O₇ type (space group P 2₁) with a = 7.776(6) Å, b = 5.515(4) Å, c = 12.999(6) Å, β = 98.36(5), Z = 4. The compound Ce₄Ti₉O₂₄ crystallizes orthorhombic with a = 14.082(4) Å, b = 35.419(8) Å, c = 14.516(4) Å, Z = 16. The new cerium titanate Ce₄Ti₉O₂₄ is isotypic to Nd₄Ti₉O₂₄ (space group Fddd (No. 70)) which represents a novel type of structure.     

Ce(iv)-centered charge-neutral perovskite layers topochemically derived from anionic [CeTa2O7]− layers

Layered perovskites have been extensively investigated in many research fields, such as electronics, catalysis, optics, energy, and magnetics, because of the fascinating chemical properties that are generated by the specific structural features of perovskite frameworks. Furthermore, the interlayers of these structures can be chemically modified through ion exchange to form nanosheets. To further expand the modification of layered perovskites, we have demonstrated an advance in the new structural concept of layered perovskite “charge-neutral perovskite layers” by manipulating the perovskite layer itself. A charge-neutral perovskite layer in [Ce^IVTa₂O₇] was synthesized through a soft chemical oxidative reaction based on anionic [Ce^IIITa₂O₇]⁻ layers. The Ce oxidation state for the charge-neutral [Ce^IVTa₂O₇] layers was found to be tetravalent by X-ray absorption fine structure (XAFS) analysis. The atomic arrangements were determined through scattering transmission electron microscopy and extended XAFS (EXAFS) analysis. The framework structure was simulated through density functional theory (DFT) calculations, the results of which were in good agreement with those of the EXAFS spectra quantitative analysis. The anionic [Ce^IIITa₂O₇]⁻ layers exhibited optical absorption in the near infrared (NIR) region at approximately 1000 nm, whereas the level of NIR absorption decreased in the [Ce^IVTa₂O₇] charge-neutral layer due to the disappearance of the Ce 4f electrons. In addition, the chemical reactivity of the charge-neutral [Ce^IVTa₂O₇] layers was investigated by chemical reduction with ascorbic acid, resulting in the reduction of the [Ce^IVTa₂O₇] layers to form anionic [Ce^IIITa₂O₇]⁻ layers. Furthermore, the anionic [Ce^IIITa₂O₇]⁻ layers exhibited redox activity which the Ce in the perovskite unit can be electrochemically oxidized and reduced. The synthesis of the “charge-neutral” perovskite layer indicated that diverse features were generated by systematically tuning the electronic structure through the redox control of Ce; such diverse features have not been found in conventional layered perovskites. This study could demonstrate the potential for developing innovative, unique functional materials with perovskite structures.

This study proposed a new layer modification technique, “layer charge control”, for layered perovskites, and the structures of the obtained charge neutral [CeTa₂O₇] perovskite sheet were characterized theoretically and experimentally.     

Oxygen defect K2NiF4-type oxides: The compounds La2−xSrxCuO4−x2+δ

Oxygen defect K₂NiF₄-type oxides $\text{La}{}_{\mathrm{2?x}}\text{Sr}{}_{\mathrm{x}}\text{CuO}{}_{\mathrm{<mtext>4?</mtext><mtext>x</mtext><mtext>2</mtext><mtext>+\delta </mtext>}}$ have been synthesized for a wide composition range: 0 ≤ x ≤ 1.34. From the X-ray and electron diffraction study three domains have been characterized: orthorhombic compounds with La₂CuO₄ structure for 0 ≤ x < 0.10, tetragonal oxides similar to LaSrCuO₄ for 0.10 ≤ x < 1 and several superstructures derived from the tetragonal cell ( with n = 3, 4, 4.5, 5, 6) for 1 ≤ x ≤ 1.34. The compounds corresponding to 0 < x < 1 differ from the other oxides in that they are characterized by the presence of copper with two oxidation states: + 2 and + 3. A model structure for La_0.8Sr_1.2CuλO_3.4, in which copper has only the + 2 oxidation state, and for which the actual cell is tegragonal—a = 18.80₄ Å and c = 12.94 Å—has been established. The particular structural evolution of these compounds is discussed in terms of a competition between the capability of Cu(II) to be oxidized to Cu(III) and the ordering of oxygen vacancies.     

Structure and dehydration of layered perovskite niobate with bilayer hydrates prepared by exfoliation/self-assembly process

The crystals of an H-form niobate of HCa₂Nb₃O₁₀·xH₂O (x=0.5) being tetragonal symmetry (space group P4/mbm) with unit cell parameters a=5.4521(6) and c=14.414(2) Å were exfoliated into nanosheets with the triple-layered perovskite structure. The colloid suspension of the nanosheets was put into dialysis membrane tubing and allowed self-assembly in a dilute KCl solution. By this method, a novel layered K-form niobate KCa₂Nb₃O₁₀·xH₂O (x=1.3, typically) with bilayer hydrates in the interlayer was produced. The Rieveld refinement and transmission electron microscope (TEM)/selected-area electron diffraction (SAED) observation indicated that the orientations of the a-/b-axis of each nanosheet as well as the c-axis are uniform, and the self-assembled compound had the same symmetry, tetragonal (P4/mbm) with a=5.453(2) and c=16.876(5) Å, as the H-form precursor; the exfoliation/self-assembly process does not markedly affect the two-dimensional lattice of the layer. The large basal spacing resulted from the interlayer K⁺ ions solvated by two layers of water molecules. The interlayer bilayers-water was gradually changed to monolayer when the temperatures higher than 100 °C, and all the water molecules lost when over 600 °C. Accompanying the dehydration, the crystal structure transformed from tetragonal to orthorhombic symmetry. Water molecules may take an important role for the layer layered compound to adjust the unit cell to tetragonal symmetry.     

Electrochemical characteristics of cathodes based on perovskites modified by ceria

A system consisting of a solid oxide electrolyte of the Ce_0.9Gd_0.1O_{2 − x} (CGO) composition in contact with a two-layer cathode based on a nonstoichiometric composition (La_0.8Sr_0.2)_0.95MnO_{3 ± δ} (LSM1) and a stoichiometric perovskite La_0.8Sr_0.2MnO_{3 ± δ} (LSM2) is prepared by the tape-casting process. It was shown that the best electrochemical characteristics are achieved for a three-layer system LSM2/{CGO-LSM1}/CGO sintered at 1410°C. The use of Ce-modified perovskites La_0.8Sr_0.2MnO_{3 ± δ} and La_0.6Sr_0.6CoO_{3 ± δ} as the collector layer of two-layer electrodes allows the electrochemical characteristics at moderately high temperatures (600–750°C) to be improved.