Photoelectrochemical study of lanthanide zirconium oxides, Ln2Zr2O7 (Ln = La, Ce, Nd and Sm)

Photocatalytic activities for hydrogen evolution of lanthanide zirconium oxides, Ln₂Zr₂O₇ (Ln = La, Ce, Nd and Sm) prepared by a solution reaction method were investigated. Under the illumination of 500 W Xenon lamps, hydrogen gas was clearly evolved in a distilled water suspension of La₂Zr₂O₇, Sm₂Zr₂O₇ and Nd₂Zr₂O₇. Under the visible-light illumination, hydrogen gas was evolved in a distilled water suspension of Nd₂Zr₂O₇ and Sm₂Zr₂O₇. From the photoelectrochemical measurements, the values of the flat band potential were estimated to be −0.64, −0.52, −0.31 and +0.04 eV for La₂Zr₂O₇, Sm₂Zr₂O₇, Nd₂Zr₂O₇ and Ce₂Zr₂O₇, respectively, versus the normal hydrogen electrode (NHE). The values of the band gap energy were calculated to be about 3.52, 2.86, 2.67 and 2.53 eV for La₂Zr₂O₇, Sm₂Zr₂O₇, Nd₂Zr₂O₇ and Ce₂Zr₂O₇, respectively. Due to the effect of 4f orbital electrons, the band gap energy of these compounds becomes narrower than in ZrO₂ and as a consequence, Sm₂Zr₂O₇ and Nd₂Zr₂O₇ show the photocatalytic activity under the visible-light.     

Synthesis, structure determination and magnetic susceptibilities of the oxides Ln3Li5Sb2O12 (Ln ≠ Pr, Nd, Sm)

The oxides Ln₃Li₆Sb₂O₁₂ (Ln≠Pr, Nd, Sm) were prepared in air by heating a mixture of Ln₂O₃, LiNO₃and Sb₂O₃ in the temperature range 1023–1243 K. Lattice parameters as well as atomic coordinates in the space group 12,3 (Z≠8) are established from X-ray powder diffraction data by the Rietveld method. Magnetic susceptibilities from 4.2 to 300 K follow a Curie-Weiss law above 50 K (Ln≠Pr) or 70 K (Ln≠Nd). This is attributed to the splitting of the ground state associated with the Ln³⁺ ions by the influence of the crystal field. The magnetic moments, 3.54 and 3.61 μ_D for praseodymium and neodymium respectively, agree with those calculated by Hund's formula. The Sm³⁺ behaviour can be explained by taking into account that the splitting of the multiplets is not too large compared to kT. The magnetic moment observed at room temperature for this cation is 1.6 μ_B.     

The cubic-tetragonal phase equilibria in the ZrO2-R2O3 (R = Y, Gd, Sm, Nd) systems

The cubic-tetragonal (c-t) phase equilibria in the ZrO₂-R₂O₃ (R = Nd, Sm, Gd, Y) systems are examined both experimentally and theoretically. The width of the c-t two-phase field is not simply described as a function of oxygen vacancies as proposed by Hillert and Sakuma (Ref 6) but is dependent on ionic radius of trivalent cations. The larger the ionic radius, the wider the two-phase field. The result is satisfactorily explained by the addition of the excess energy term caused by strain energy in cubic solid solutions to the original model.     

Comparative study for mutual separation characteristics of binary mixed oxides Y2O3–Ln2O3 (Ln = Dy, Ho and Er) and Ho2O3–Er2O3 using stepwise chlorination–chemical vapor transport reaction mediated by vapor complexes KLnCl4

Mutual separation characteristics for binary oxide mixtures Y₂O₃–Dy₂O₃, Y₂O₃–Er₂O₃ and Ho₂O₃–Er₂O₃, in which these four kinds of rare earth ion(III) have very similar ion radius values, using a stepwise chlorination–chemical vapor transport (SC–CVT) reaction mediated by vapor complexes KLnCl₄ have been investigated in different temperature gradients. The unexpected results, together with that for Y₂O₃–Ho₂O₃ reported previously, are used to make a comparative analysis for the effect of ion radius values on the SC–CVT reaction for mutual separation of rare earths. Both the main deposition temperature region tendency and total transport amounts of chlorides for YCl₃ with respect to the ion radius of Y(III) were exceptional compared with those of LnCl₃ (Ln = Dy, Ho and Er), which were observed both in the degressive temperature gradient and the wave-type temperature gradient. The main deposition temperature of the chlorides produced from the oxide mixtures was in the order of DyCl₃ > YCl₃, HoCl₃ < YCl₃, ErCl₃ < YCl₃ and HoCl₃ > ErCl₃, total transported yields of the chlorides was in the order of DyCl₃ > YCl₃, HoCl₃ > YCl₃, ErCl₃ > YCl₃ and HoCl₃ > ErCl₃, and the largest separation factors 11.49 for Dy:Y, 15.28 for Ho:Y, 6.37 for Er:Y and 2.04 for Ho:Er in the lower temperature region were observed in the degressive temperature gradient, respectively. The results were discussed on the difference of ionic structure of Y on the one side and 4f lanthanoid elements of Dy, Ho and Er on the other hand, and verified that the ionic radius of the rare earth is one of the decisive factors of CVT reaction only for lanthanoid elements, not for Y. Furthermore, the improved separation factor values of 4.22 for Ho:Er and 3.20 for Er:Ho were obtained in the wave type temperature gradient due to variation of the dynamic conditions of CVT.     

Direct synthesis of A2(Ti(1 − y)Zry)2O7 (A = Gd, Y) solid solutions by ball milling constituent oxides

Different compositions in two solid solutions, A₂(Ti_(1 − y)Zr_y)₂O₇ (A = Gd³⁺, Y³⁺), with high oxygen ion conductivity, have been successfully prepared at room temperature via mechano-chemical synthesis. Stoichiometric mixtures of the constituent oxides were milled in a planetary ball mill by using zirconia vials and balls. Chemical changes in the powder mixtures as a function of composition and milling time were followed by using X-ray diffraction showing that in all cases and after milling for 19 h, the powders consisted of a single phase. Powders were also examined by scanning electron microscopy (SEM) finding out that they basically consist of sub-micron size agglomerates and aggregates of nanoparticles.     

Thermal-shock resistance of LnMgAl11O19 (Ln = La, Nd, Sm, Gd) with magnetoplumbite structure

Lanthanide hexaaluminates including LaMgAl₁₁O₁₉, NdMgAl₁₁O₁₉, SmMgAl₁₁O₁₉ and GdMgAl₁₁O₁₉ were synthesized via Sol–Gel method. Due to the anisotropic crystal growth, these oxides crystallize in the form of platelets and the platelet thickness increases with the decrease of rare-earth ionic radius. It was observed that the thermal-shock resistances of LaMgAl₁₁O₁₉, NdMgAl₁₁O₁₉ and SmMgAl₁₁O₁₉ oxides were superior to 8YSZ as proved by water quenching tests. In addition, the thinner the platelet, the more interstices are retained in the sintered specimen, and the better thermal-shock resistance the oxide has. Based on SEM images, it can be seen that the SmMgAl₁₁O₁₉ sample exhibits a mixture of the intergranular and transgranular fracture after thermal cycling failure.     

Synthesis, characterization and magnetic properties of complex oxides Ln2Mn2/3Re4/3O7 (Ln = Er, Y) and Y2Zn2/3Re4/3O7

Complex oxides Ln₂Mn_2/3Re_4/3O₇ (Ln = Y, Er) and Y₂Zn_2/3Re_4/3O₇ with a zirkelite structure and hexagonal unit cells (space group P3₁21, z = 6) have been obtained. Static and dynamic magnetic susceptibility measurements show that these oxides possess spin-glass behavior at low temperatures. Valence combinations of d-metals in the oxides are Mn²⁺(Zn²⁺)–Re⁵⁺. It is supposed that the examined specimens Ln₂Mn_2/3Re_4/3O₇ (Ln = Y, Er) contain the second magnetic phase of an unknown composition.     

A general hydrothermal route to synthesis of nanocrystalline lanthanide stannates: Ln2Sn2O7 (Ln = Y, La–Yb)

A general hydrothermal process with use of lanthanide (III) nitrates and Na₂SnO₃ as precursors has been proposed for synthesizing nanocrystalline lanthanide stannates with general formula: Ln₂Sn₂O₇ (Ln = Y, La–Yb). Stannates of all lanthanides except for the radioactive promethium were successfully synthesized. Characterization by XRD and TEM revealed that all the products were phase-pure nanocrystalline lanthanide stannates with pyrochlore-type structure. Photoluminescent properties of three samples (i.e. Tb₂Sn₂O₇, Dy₂Sn₂O₇ and Yb₂Sn₂O₇) are also presented. The mole ratio of Ln(NO₃)₃:Na₂SnO₃ and hydrothermal temperature were two key factors for this general hydrothermal route.     

Effect of Ln2O3 (Ln = La, Pr, Eu, Gd) addition on structure and electrical properties of (Na0.5Bi0.5)0.93Ba0.07TiO3 ceramics

(Na_0.5Bi_0.5)_0.93Ba_0.07TiO₃ ceramics added with 0.2 wt.% Ln₂O₃ (Ln = La, Pr, Eu, Gd) were prepared by a citrate method, and the structure and electrical properties of the ceramics were investigated with respect to the size of the lanthanide. All the specimens maintain a coexistence of rhombohedral and tetragonal phases in crystal structure, while no remarkable evolution in microstructure with the lanthanide addition was observed. Compared with (Na_0.5Bi_0.5)_0.93Ba_0.07TiO₃, the lanthanide addition resulted in an increased diffuseness in phase transition and a decrease in depolarization temperature (T_d). The variation in dielectric, piezoelectric and ferroelectric properties with the lanthanide addition presents an evident lanthanide size dependence. The addition of La₂O₃ or Pr₂O₃ tailored the electrical properties basically following a soft doping effect, with the specimens added with La₂O₃ and Pr₂O₃ attaining high piezoelectric constants (d₃₃) of 188 and 184 pC/N, respectively. By contrast, the Eu₂O₃ or Gd₂O₃ addition led to an abnormal change in the electrical properties, which was qualitatively interpreted by an internal stress effect.     

Synthesis of Er2Ti2O7 nanocrystals and its electrochemical hydrogen storage behavior

Ultrafine Er₂Ti₂O₇ was synthesized at 700 °C within 2 h by a soft-chemistry route named citric acid sol–gel method (CAM). The obtained Er₂Ti₂O₇ with high dispersibility was square-like and the average size was about 70 nm. The prepared Er₂Ti₂O₇ nanocrystals in 6 M KOH aqueous solutions were investigated as a hydrogen storage material. It was found that the Er₂Ti₂O₇ powders would function as electrochemical hydrogen storage, showed fair electrochemical reversibility, and considerably high charge–discharge capacity. The reversible discharge capacity of the Er₂Ti₂O₇ electrode was found to exceed 320 mAh/g and adsorption capability of hydrogen is up to 1.27% at a current rate of 100 mA/g. In addition, the cycling ability and high rate capability of the Er₂Ti₂O₇ electrode are fairly good with only 4% capacity decay after 25 cycles. Cyclic voltammograms (CVs) were carried out to further examine the electrochemical hydrogen storage mechanism of Er₂Ti₂O₇.     

Structure and hydrogen storage properties of MgH2 catalysed with La2O3

The catalytic effect of the addition of lanthanum oxide (La₂O₃), in the range 0.5–2.0 mol%, on the hydrogen storage properties of MgH₂ prepared by ball milling has been studied. The addition of La₂O₃ reduces the formation during milling of the metastable orthorhombic γ-MgH₂ phase. The desorption rate of samples with 1 and 2 mol% La₂O₃ comes out to be about 0.010 wt% per second at 573 K under an hydrogen pressure of 0.3 bar, better than for sample with 0.5 mol% La₂O₃. The presence of LaH₃ after hydrogenation/dehydrogenation cycles has been observed in all samples. The sample with 1 mol% of La₂O₃ gives a lower hysteresis factor compared with sample with 2 mol%.     

Thermal and structural properties of KPO3·Ln(PO3)3 glasses (Ln=rare earth ion)

Glass transition temperatures T_g and Raman spectra of KPO₃·Ln(PO₃)₃ (Ln=rare earth ion) glasses were measured for all rare earth members (except Pm). From the series behavior of the T_g and Raman data, it is concluded that the coordination number around rare earth ions changes, probably from nine to eight, in the middle of the rare earth series.     

Luminescence properties of SrIn2O4:Eu incorporated with Gd or Sm ions

The photoluminescence (PL) properties of SrIn₂O₄:Eu³⁺,Gd³⁺ and SrIn₂O₄:Eu³⁺,Sm³⁺ are investigated in this work. When the Gd³⁺ ions are introduced in this compound, the average distance metal-oxygen is increased, and then the vibration of lattice is decreased. It results in that the nonradiation of Eu³⁺ is decreased. Therefore, the emissions of SrIn₂O₄:Eu³⁺ are increased. However, little of energy transfer occurs from Gd³⁺ to Eu³⁺ ions. When the Sm³⁺ ions are introduced into SrIn₂O₄:Eu³⁺, the energy transfers occur from the CTS of O²⁻-Sm³⁺ to Sm³⁺ and Eu³⁺ ions, from the host absorption to Eu³⁺ ions, and from Sm³⁺ to Eu³⁺ ions, but not from the host absorption to Sm³⁺ ions.     

Red-emitting Ln2−xEuxTe2O6:RE (Ln = La, Y; RE = Sm, Gd) phosphors prepared by the Pechini sol-gel method

La_1.90Eu_0.10TeO₆:RE³⁺ (RE = Gd, Sm) and Y₂TeO₆:Eu³⁺nanophosphors were prepared by the Pechini sol-gel process, using lanthanide sesquioxides and telluric acid as precursors. X-ray diffraction (XRD), scanning electron microscopy (SEM), photoluminescence spectra (PL) and fluorescence lifetime were used to characterize the resulting phosphors. The results of XRD indicate that all samples crystallized completely at 1073 K and are isostructural with orthorhombic Ln₂TeO₆. The SEM study reveals that the samples have a strong tendency to form agglomerates with an average size ranging from 40 to 65 nm. The luminescence decay curves suggest for all samples a monoexponential behavior. The photoluminescence intensity and chromaticity were improved for excitation at 395 nm when the co-doping concentration reaches the 1% mol. The optimized phosphorsLa_1.88Eu_0.10Gd_0.02TeO₆and La_1.88Eu_0.10Sm_0.02TeO₆, could be considered an efficient red-emitting phosphor for solid-state lighting devices based on InGaN LEDs.     

Synthesis, characterization, shape-preserved transformation, and optical properties of La(OH)3, La2O2CO3, and La2O3 nanorods

A simple method to directly synthesize stable and crystalline pure phase La(OH)₃ nanorods, with a diameter of around 15 nm and lengths in the range of 120-200 nm, was developed using cationic surfactant (cetyltrimethylammonium bromide, CTAB). The obtained La(OH)₃ nanorods can be successfully converted to La₂O₂CO₃ and La₂O₃ nanorods via calcination under appropriate conditions. Analytical methods such as X-ray diffraction (XRD) spectra, Fourier transformed infrared (FTIR) spectrum, differential scanning calorimetry and thermogravimetric analysis (DSC-TGA), transmission electron microscopy (TEM), and high-resolution TEM (HRTEM) were employed to characterize the morphology and microstructure of the final products. The results reveal that La(OH)₃ nanorods were shape-preserved and transformed to La₂O₂CO₃ nanorods at 400 °C for 2 h and to La₂O₃ nanorods at 800 °C for 2 h, respectively. TEM images indicate that the as-obtained La₂O₂CO₃ and La₂O₃ entirely consist of uniform nanorods in high yield with diameters of about 15 nm and 23 nm, lengths of 200-300 nm and 300-500 nm, respectively. The formation mechanism of the La(OH)₃, La₂O₂CO₃ and La₂O₃ nanorods was investigated. Room-temperature photoluminescence (RTPL) properties were investigated under the excitation of 275 nm. The ⁵D₃ → ⁷F_j (j = 2-6) emission peaks at the wavelength below 500 nm were found in the RTPL spectra.     

Kinetic and spectral aspects in chemiluminescence system Eu(III)/HCO3/H2O2

Chemiluminescence (CL) of the systems containing Eu₂⁺ or HCO₃⁻ ions and hydrogen peroxide was studied. The kinetic curves and CL emission spectra of the systems were discussed. The emission spectrum of the system containing carbonates revealed two emission maxima: the first directly after initiation of the reaction by hydrogen peroxide and the second in several seconds after the first. On the basis of the characteristic bands in the CL spectrum the emitters in the system Eu₂⁺/HCO₃⁻/H₂O₂ were identified as: excited Eu³⁺ ions and excited products of carbonate decomposition. The emission bands for λ=600 and 420 nm appeared in a time distance of a few ten seconds.     

Improvement in hydrogen sorption properties of Mg by reactive mechanical grinding with Cr2O3, Al2O3 and CeO2

We tried to improve the hydrogen sorption properties of Mg by mechanical grinding under H₂ (reactive mechanical grinding) with oxides Cr₂O₃, Al₂O₃ and CeO₂. The hydriding rates of Mg are reportedly controlled by the diffusion of hydrogen through a growing Mg hydride layer. The added oxides can help pulverization of Mg during mechanical grinding. A part of Mg is transformed into MgH₂ during reactive mechanical grinding. The Mg+10wt.%Cr₂O₃ powder has the largest transformed fraction 0.215, followed in order by Mg+10wt.%CeO₂ and Mg+10wt.%Al₂O₃. The Mg+10wt.%Cr₂O₃ powder has the largest hydriding rates at the first and fifth hydriding cycle, followed in order by Mg+10wt.%Al₂O₃ and Mg+10wt.%CeO₂. Mg+10wt.%Cr₂O₃ absorbs 5.87wt.% H at 573 K, 11 bar H₂ during 60 min at the first cycle. The Mg+10wt.%Cr₂O₃ powder has the largest dehydriding rates at the first and fifth dehydriding cycle, followed by Mg+10wt.%CeO₂ and Mg+10wt.%Al₂O₃. It desorbs 4.44 wt.% H at 573 K, 0.5 bar H₂ during 60 min at the first cycle. All the samples absorb and desorb less hydrogen at the fifth cycle than at the first cycle. It is considered that this results from the agglomeration of the particles during hydriding–dehydriding cycling. The average particle sizes of the as-milled and cycled powders increase in the order of Mg+10wt.%Cr₂O₃, Mg+10wt.%Al₂O₃ and Mg+10wt.%CeO₂. The quantities of hydrogen absorbed or desorbed for 1 h for the first and fifth cycles decrease in the order of Mg+10wt.%Cr₂O₃, Mg+10wt.%Al₂O₃ and Mg+10wt.%CeO₂. The quantities of absorbed or desorbed hydrogen increase as the average particle sizes decrease. As the particle size decreases, the diffusion distance shortens. This leads to the larger hydriding and dehydriding rates. The Cr₂O₃ in the Mg+10wt.%Cr₂O₃ powder is reduced after hydriding–dehydriding cycling. The much larger chemical affinity of Mg than Cr for oxygen leads to a reduction of Cr₂O₃ after cycling.     

Synthesis and structure of a potassium nitridoditungstenate (K6W2N4O3), a potassium digermanate (K6Ge2O7) and a rubidium digermanate (Rb6Ge2O7)

Light yellow single crystals of potassium nitridoditungstate (K₆W₂N₄O₃) and pale single crystals of potassium digermanate (K₆Ge₂O₇) were obtained by the reaction of the metal oxides WO₃ (molar ratio, 1 : 15.7) or GeO₂ (molar ratio, 1 : 2) in alkali metal amide melts in an autoclave at 530–600 °C for 6–8 days. Colourless single crystals of rubidium digermanate (Rb₆Ge₂O₇) were prepared by the reaction of GeO₂ with rubidium amide (molar ratio, 1 : 2) in ammonia at 350 °C in a high-pressure autoclave (H. Jacobs and D. Schmidt, in E. Kaldis (ed.), High-pressure Ammonolysis in Solid State Chemistry, Current Topics in Materials Science, Vol. 8, North Holland, Amsterdam, 1981, p. 379) (p(NH₃) = 5.5 kbar) for 10 days. In all three cases other nitrogen-containing products were present.

The structures of the title compounds were determined on the basis of single-crystal data. They are isotypic or structurally closely related to each other: K₆W₂N₄O₃: P2₁/n, a = 6.720(2) Å, b = 9.473(1) Å, c = 9.581(2) Å, β = 91.99(2)°, Z = 2, R/R_w = 0.040/0.048, N(I) > 3σ(I) 2057, N(Var.) = 71. K₆Ge₂O₇: Pn, a = 6.529(2) Å, b = 9.079(4) Å, c = 9.162(6)Å, β = 91.85(4)°, Z = 2, R/R_w = 0.022/0.024, N(I) 3σ(I) = 1486, N(Var.) = 135. Rb₆Ge₂O₇: P2₁/n, a = 6.839(4) Å, b = 9.437(6) Å, c = 9.460(6) Å, β = 91.53(5)°, Z = 2, R/R_w = 0.061/0.074, N(I) 3σ(I) = 1055, N(Var.) = 71.     

Influence of La precursors on structure and properties of CeO2-ZrO2-Al2O3 composite oxides

Three La-doped CeO₂-ZrO₂-Al₂O₃ (CZA) composite oxide samples, namely, CZA-I, CZA-II and CZA-III, were prepared following a co-precipitation method in the presence of La₂O₃, La(NO₃)₃·6H₂O and H[La(EDTA)]·16H₂O precursors, respectively. When the precursor samples are sintered at 1000 °C, the as-prepared composite oxides mainly exhibit the CeO₂-ZrO₂ cubic fluorite phase, while the γ-Al₂O₃ and δ-Al₂O₃ phases appear when the precursor samples are subjected to sintering at 1100 and 1200 °C. CZA-III exhibits improved redox properties after high-temperature treatment compared with CZA-I and CZA-II. CZA-III presents the largest surface area of 97.46 m²/g among the three CZAs when the CZA-III precursor sample is sintered at 1000 °C. Furthermore, the corresponding oxygen storage capacity (OSC) is the largest with value of 400.27 μmol/g when CZA-III precursor sample is sintered at 1000 °C. Additionally, CZA-III exhibits the best thermal stability and the highest reduction temperature. However, by increasing the sintering temperature to 1200 °C, there is a dramatic decline in the properties of surface area and OSC. And a decrease for CZA-III in surface area by 58.94% and a decrease of the OSC value by 74.56% are observed.     

Sonochemical synthesis and characterization of uniform lanthanide orthophosphate(LnPO_4, Ln=La and Ce) nanorods

Uniform lanthanide orthophosphate(Ln PO4,Ln = La and Ce) nanorods were successfully synthesized by a simple ultrasonic irradiation method using lanthanide nitrate salt(Ln(NO3)3 6H2 O, Ln = La and Ce) and sodium phosphate(Na3PO4) in aqueous solutions with the p H of1–3. The products were characterized by X-ray diffraction(XRD), scanning electron microscopy(SEM), transmission electron microscopy(TEM), high-resolution transmission electron microscopy(HRTEM), Fourier transform infrared spectrometer(FT-IR), and UV–visible(UV-Vis) spectroscopy. In this research, the products are nanorods of monoclinic La PO4 and hexagonal Ce PO4 structures and the vibration modes of PO43-, including the strong peaks at227 nm for La PO4, and at 225 and 278 nm for Ce PO4.