Structure and conductivity of strontium-doped cerium orthovanadates Ce1−xSrxVO4 (0≤x≤0.175)

A-site substituted cerium orthovanadates, Ce_1−xSr_xVO₄, were synthesised by solid-state reactions. It was found that the solid solution limit in Ce_1−xSr_xVO₄ is at x=0.175. The crystal structure was analysed by X-ray diffraction and it exhibits a tetragonal zircon structure of space group I4₁/amd (1 4 1) with a=7.3670 (3) and c=6.4894 (1) Å for Ce_0.825Sr_0.175VO₄. The UV-vis absorption spectra indicated that the compounds have band gaps at room temperature in the range 4.5-4.6 eV. Conductivity measurements were performed for the first time up to the strontium solid solution limit in air and in dry 5% H₂/Ar with conductivity values at 600 °C ranging from 0.3 to 30 mS cm⁻¹ in air to 30-45 mS cm⁻¹ in reduced atmosphere. Sample Ce_0.825Sr_0.175VO₄ is redox stable at a temperature below 600 °C although the conductivity is not high enough to be used as an electrode for solid oxide fuel cells.     

Crystal structures of the double perovskites Ba2Sr1−xCaxWO6

Structures of the double perovskites Ba₂Sr_1−xCa_xWO₆ have been studied by the profile analysis of X-ray diffraction data. The end members, Ba₂SrWO₆ and Ba₂CaWO₆, have the space group I2/m (tilt system a⁰b⁻b⁻) and Fm3¯m (tilt system a⁰a⁰a⁰), respectively. By increasing the Ca concentration, the monoclinic structure transforms to the cubic one via the rhombohedral R3¯ phase (tilt system a⁻a⁻a⁻) instead of the tetragonal I4/m phase (tilt system a⁰a⁰c⁻). This observation supports the idea that the rhombohedral structure is favoured by increasing the covalency of the octahedral cations in Ba₂MM′O₆-type double perovskites, and disagrees with a recent proposal that the formation of the π-bonding, e.g., d⁰-ion, determines the tetragonal symmetry in preference to the rhombohedral one.     

Structural and magnetic study of the cation-ordered perovskites Ba2−xSrxErMoO6

A series of perovskite phases have been prepared from the appropriate carbonates and oxides by heating under reducing conditions at temperatures up to 1300 °C. Complete ordering between ErO₆ and MoO₆ octahedra and a disordered distribution of Sr²⁺ and Ba²⁺ occur in all compounds. Neutron powder diffraction experiments show that the substitution of Sr²⁺ into Ba₂ErMoO₆ introduces a progressive reduction in symmetry from Fm3¯m (x=0) to I4/m (x=0.5, 0.8) to P2₁/n (x=1.25, 1.75, 2.0). Magnetic susceptibility measurements indicate that all of these compounds show Curie-Weiss paramagnetism and that for x<1.25 this behaviour persists down to 2 K. The monoclinically distorted compounds show magnetic transitions at low temperature and neutron diffraction has confirmed the presence of long-range antiferromagnetic order below 2.5 and 4 K in Ba_0.25Sr_1.75ErMoO₆ and Sr₂ErMoO₆, respectively. Ba_0.75Sr_1.25ErMoO₆, Ba_0.25Sr_1.75ErMoO₆ and Sr₂ErMoO₆ do not undergo structural distortion on cooling from room temperature.     

Synthesis of silicated hydroxyapatite Ca10(PO4)6−x(SiO4)x(OH)2−x

The preparation of silicated hydroxyapatite Ca₁₀(PO₄)_6−x(SiO₄)_x(OH)_2−x (SiHA) with 0?x?2 was investigated using a wet precipitation method followed by a heat treatment. X-ray diffraction and Rietveld refinement, Fourier transformed IR (FTIR) spectroscopy, elemental analyses, transmission electron microscopy and thermal analyses were used to characterize the samples. The raw materials were composed of a partially silicated and carbonated apatite and a secondary minor phase containing the excess silicon. Single phase silicated hydroxyapatites, with 0?x?1, could be synthesized after a thermal treatment of the raw powders above 700 °C. The presence of carbonate groups in the raw apatite played an important role in the incorporation of silicates during heating. From the different results, the mechanisms of formation of SiHA are discussed.     

Synthesis and characterization of Zn1−xNixFe2O4 spinels prepared by a citrate precursor

Nanocrystalline single-phase samples of Zn_1−xNi_xFe₂O₄ ferrites (0<x<1) have been obtained via a soft-chemistry method based on citrate-ethylene glycol precursors, at a relatively low temperature (650 °C). The influence of the nickel and zinc contents as well as that of heat treatments were investigated by means of X-ray powder diffraction, Brunauer-Emmett-Teller (BET) surface area, scanning electron microscopy (SEM) and Fourier Transform Infrared (FTIR) Spectroscopy. Higher Ni content increases the surface areas, the largest one (∼20 m²/g) being obtained for NiFe₂O₄ annealed at 650 °C for 15 h. For all compositions, the surface area decreases for prolonged annealing at 650 °C and for higher annealing temperatures. Those results were correlated to the particle size evolution; the smallest particles (∼50 nm) observed in the NiFe₂O₄ sample (650 °C, 15 h) steadily increase as Ni ions were replaced by Zn, reaching ∼100 nm in the ZnFe₂O₄ sample (650 °C, 15 h). For all the Zn_1−xNi_xFe₂O₄ samples and, whatever the heat treatments was, the FTIR spectra show two fundamental absorption bands in the range 650-400 cm⁻¹, characteristics of metal vibrations, without any superstructure stating for cation ordering. The highest ν₁-tetrahedral stretching, observed at ∼615 cm⁻¹ in NiFe₂O₄, shifts towards lower values with increasing Zn, whereas the ν₂-octahedral vibration, observed at 408 cm⁻¹ in NiFe₂O₄, moves towards higher wavenumbers, reaching 453 cm⁻¹ in ZnFe₂O₄.     

Crystal structure and electrical conductivity of lanthanum-calcium chromites-titanates La1−xCaxCr1−yTiyO3−δ (x=0-1, y=0-1)

Five series of perovskite-type compounds in the system La_1−xCa_xCr_1−yTi_yO₃ with the nominal compositions y=0, x=0-0.5; y=0.2, x=0.2-0.8; y=0.5, x=0.5-1.0; y=0.8, x=0.6-1.0 and y=1, x=0.8-1 were synthesized by a ceramic technique in air (final heating 1350 °C). On the basis of the X-ray analysis of the samples with (Ca/Ti)?1, the phase diagram of the CaTiO₃-LaCr^IIIO₃-CaCr^IVO₃ quasi-ternary system was constructed. Extended solid solution with a wide homogeneity range is formed in the quasi-ternary system CaCr^IVO₃-CaTiO₃-LaCr^IIIO₃. The solid solution La_{(1−x′−y)}Ca_(x′+y)Cr^IV_x′Cr^III_{(1−x′−y)}Ti_yO₃ exists by up to 0.6-0.7 mol fractions of CaCr^IVO₃ (x^′<0.6-0.7) at the experimental conditions. The crystal structure of the compounds is orthorhombic in the space group Pbnm at room temperature. The lattice parameters and the average interatomic distances of the samples within the solid solution ranges decrease uniformly with increasing Ca content. Outside the quasi-ternary system, the nominal compositions La_0.1Ca_0.9TiO₃, La_0.2Ca_0.8TiO₃, La_0.4Ca_0.6Cr_0.2Ti_0.8O₃ and La_0.3Ca_0.7Cr_0.2Ti_0.8O₃ in the system La_1−xCa_xCr_1−yTi_yO₃ were found as single phases with an orthorhombic structure. In the temperature range between 850 and 1000 °C, the synthesized single-phase compositions are stable at pO₂=6×10⁻¹⁶-0.21×10⁵ Pa. Oxygen stoichiometry and electrical conductivity of the separate compounds were investigated as functions of temperature and oxygen partial pressure. The chemical stability of these oxides with respect to oxygen release during thermal dissociation decreases with increasing Ca-content. At 900 °C and oxygen partial pressure 1×10⁻¹⁵-0.21×10⁵ Pa, the compounds with x>y (acceptor doped) are p-type semiconductors and those with x<y (donor doped) and x=y are n-type semiconductors. The type and level of electrical conductivity are functions of the concentration ratios of cations occupying the B-sites of the perovskite structures: [Cr³⁺]/[Cr⁴⁺] and [Ti⁴⁺]/[Ti³⁺]. The maximum electrical conductivity at 900 °C and pO₂=10⁻¹⁵ Pa was found for the composition La_0.1Ca_0.9TiO₃ (near 50 S/cm) and in air at 900 °C for La_0.5Ca_0.5CrO₃ (close to 100 S/cm).     

Structure and phase composition of nanocrystalline Ce1−xLuxO2−y

The microstructure and phase stability of nanocrystalline mixed oxide Lu_xCe_1−xO_2−y (x=0-1) are described. Nano-sized (3-4 nm) oxide particles were prepared by the reverse microemulsion method. Morphological and structural changes upon heat treatment in an oxidizing atmosphere were studied by transmission electron microscopy (TEM), X-ray diffraction (XRD), Raman and Yb³⁺ emission spectroscopy, the latter ion being present as an impurity in the Lu₂O₃ starting material. Up to 950 °C, the samples were single phase, with structure changing smoothly with Lu content from fluorite type (F) to bixbyite type (C). For the samples heated at 1100 °C phase separation into coexisting F- and C-type structures was observed for 0.35<x<0.7. It was also found that addition of Lu strongly hinders the crystallite growth of ceria during heat treatment at 800 and 950 °C.     

p-Type electron transport in La1−xSrxFeO3−δ at high temperatures

Electrical conductivity, thermopower and oxygen content were measured for La_1−xSr_xFeO_3−δ (x=0.2, 0.5, 0.9) within the oxygen partial pressure range 10⁻⁴-0.5 atm and at temperatures 750-950 °C. The dominating charge carriers under these experimental conditions are electron holes. The results of oxygen nonstoichiometry measurements are used to estimate the concentration of holes and to analyze data on conductivity and thermopower. The changes in thermopower are described by the model assuming that the number of sites accessible for migration of holes is independent on oxygen content. The mobility of electron holes is calculated, and it is found to be virtually independent on temperature in the compositions with x<0.5 while compositions with x>0.5 exhibit thermally activated mobility and mobility values about 0.1 cm² V⁻¹ s⁻¹ or smaller. It is suggested that the main contribution to the composition dependent variations in electron hole mobility are associated with Coulomb interactions at small x's, whereas at high degrees of doping the mobility of holes is most strongly affected by the increasing amount of oxygen vacancies.     

Thermoelectric properties of polycrystalline La1−xSrxCoO3

Thermoelectric properties of polycrystalline La_1−xSr_xCoO₃, where Sr²⁺ is substituted in La³⁺ site in perovskite-type LaCoO₃, have been investigated. Sr-doping increases the electrical conductivity (σ) of La_1−xSr_xCoO₃, and also decreases the Seebeck coefficient (S) for 0.01?x?0.40. A Hall coefficient measurement reveals that the increase in electrical conductivity arises from increases in both carrier concentration and the Hall mobility. The decrease in the Seebeck coefficient is caused by a decrease in carrier effective mass as well as increase in carrier concentration. The highest power factor (σS²) is 3.7×10⁻⁴ W m⁻¹ K⁻² at 250 K for x=0.10. The thermal conductivity (κ) is about 2 W m⁻¹ K⁻¹ at 300 K for 0?x?0.04, and increases for x?0.05 because of an increase in heat transport by conductive carrier. The thermoelectric properties of La_1−xSr_xCoO₃ are improved by Sr-doping, and the figure of merit (Z=σS² κ⁻¹) reaches 1.6×10⁻⁴ K⁻¹ for x=0.06 at 300 K (ZT=0.048). For heavily Sr-doped samples, the thermoelectric properties diminish mainly because of the decrease in the Seebeck coefficient and the increase in thermal conductivity.     

On α-β phase transition in cristobalite-type Al1−xGaxPO4 (0.00?x?1.00)

The results of variable temperature powder X-ray diffraction and differential thermal analysis (DTA) studies on the orthorhombic (α) low-cristobalite to cubic (β) high-cristobalite phase transition for Al_1−xGa_xPO₄, (0.00?x?1.00) are presented. These studies reveal that all these compositions undergo reversible phase transitions from orthorhombic to cubic form at higher temperature. The high-temperature behavior of GaPO₄ is observed to have a different behavior compared to all other compositions in this series. Orthorhombic low-cristobalite-type GaPO₄ transforms to cubic high-cristobalite form at ∼605 °C. Above ∼700 °C, the cubic high-cristobalite-type GaPO₄ slowly transforms to trigonal quartz type structure. At about 960 °C, the quartz type GaPO₄ transforms back to the cubic high-cristobalite form. During cooling cycles the cubic phase of GaPO₄ reverts to trigonal quartz type phase. However, annealing of GaPO₄ at higher temperatures for longer duration can stabilize the orthorhombic low cristobalite phase. The phase transition temperatures and associated enthalpies are related to the change in unit cell volume and the orthorhombicity of the respective low cristobalite lattice.     

Structural, spectroscopic and photoluminescence studies of LiEu(WO4)2−x(MoO4)x as a near-UV convertible phosphor

A series of lithium europium double tungsto-molybdate phosphors LiEu(WO₄)_2−x(MoO₄)_x (x=0, 0.4, 0.8, 1.2, 1.6, 2.0) have been synthesized by solid-state reactions and their crystal structure, optical and luminescent properties were studied. As the molybdate content increases, the intensity of the ⁵D₀→⁷F₂ emission of Eu³⁺ activated at wavelength of 396 nm was found to increase and reach a maximum when the relative ratio of Mo/W is 2:0. These changes were found to be accompanied with the changes in the spectral feature, which can be attributed to the crystal field splitting of the ⁵D₀→⁷F₂ transition. As the molybdate content increases the emission intensity of the 615 nm peak also increases. The intense red-emission of the tungstomolybdate phosphors under near-UV excitation suggests them to be potential candidate for white light generation by using near-UV LEDs. In this study the effect of chemical compositions and crystal structure on the photoluminescent properties of LiEu(WO₄)_2−x(MoO₄)_x is investigated and discussed.     

Hydrothermal preparation and visible-light photocatalytic activity of Bi2WO6 powders

Bi₂WO₆ powder photocatalyst was prepared using Bi(NO₃)₃ and Na₂WO₄ as raw materials by a simple hydrothermal method at 150 °C for 24 h, and then calcined at 300, 400, 500, 600 and 700 °C for 2 h, respectively. The as-prepared samples were characterized with UV-visible diffuse reflectance spectra, fourier transform infrared spectra (FTIR), X-ray diffraction (XRD), scanning electron microscopy (SEM) and N₂ adsorption-desorption measurement. The photocatalytic activity of the samples was evaluated using the photocatalytic oxidation of formaldehyde at room temperature under visible light irradiation. It was found that post-treatment temperature obviously influenced the visible-light photocatalytic activity and physical properties of Bi₂WO₆ powders. At 500 °C, Bi₂WO₆ powder photocatalyst showed the highest visible-light photocatalytic activity due to the samples with good crystallization and high BET surface area.     

A structural study of the perovskite series Ca1−xNaxTi1−xTaxO3

Compounds in the solid solution series Ca_1−xNa_xTi_1−xTa_xO₃ were synthesized at 1300 °C, followed by annealing at 850 °C or 800 °C with quenching and/or slow cooling to room temperature. Rietveld refinement of their powder X-ray diffraction patterns show that all compounds are single-phase ternary perovskites which adopt the space group Pbnm (a≈b≈√2a_p; c≈2a_p; Z=4) at ambient conditions. The unit cell parameters and cell volumes of the compounds increase regularly with increasing values of x. The coordination of the A-site cations changes throughout the series from eight for CaTiO₃ to nine for NaTaO₃. Compounds with 0?x ?0.4 have A-site cations in eight fold coordination, whereas the coordination of those with 0.4<x<0.9 is ambiguous. Analysis of the crystal chemistry of the compounds shows that the change in coordination at x=0.4 is related to the departure of the B-site cations from the second coordination sphere of the A-site cations, as in compounds with x>0.4 the A-^IIO distances become less than the A-B intercation distances. Contemporaneous with these coordination changes, the tilt angles of the BO₆ polyhedra decrease with increasing values of x. This solid solution series is unusual in that these structural and coordination changes occur regardless that Goldschmidt tolerance factors remain essentially constant at approximately 0.89, and observed tolerance factors, assuming eight fold coordination of the A-site cations, range only from 0.91 to 0.93 (0?x?0.8).     

Formation of apatite oxynitrides by the reaction between apatite-type oxide ion conductors, La8+xSr2−x(Si/Ge)6O26+x/2, and ammonia

Following growing interest in the use of ammonia as a fuel in solid oxide fuel cells (SOFCs), we have investigated the possible reaction between the apatite silicate/germanate electrolytes, La_8+xSr_2−x(Si/Ge)₆O_26+x/2, and NH₃ gas. We examine how the composition of the apatite phase affects the reaction with ammonia. For the silicate series, the results showed a small degree of N incorporation at 600 °C, while at higher temperatures (800 °C), substantial N incorporation was observed. For the germanate series, partial decomposition was observed after heating in ammonia at 800 °C, while at the lower temperature (600 °C), significant N incorporation was observed. For both series, the N content in the resulting apatite oxynitride was shown to increase with increasing interstitial oxide ion content (x/2) in the starting oxide. The results suggest that the driving force for the nitridation process is to remove the interstitial anion content, such that for the silicates the total anion (O+N) content in the oxynitrides approximates to 26.0, the value for an anion stoichiometric apatite. For the germanates, lower total anion contents are observed in some cases, consistent with the ability of the germanates to accommodate anion vacancies. The removal of the mobile interstitial oxide ions on nitridation suggests problems with the use of apatite-type electrolytes in SOFCs utilising NH₃ at elevated temperatures.     

La3TaO7 derivatives with Weberite structure type: Possible electrolytes for solid oxide fuel cells and high temperature electrolysers

In this study, with the aim to enhance the ionic conduction of known structures by defect chemistry, the La₂O₃-Ta₂O₅ system was considered with a focus on the La₃TaO₇ phase whose structure is of Weberite type. In order to predict possible preferential substitution sites and substitution elements, atomistic simulation was used as a first approach. A solid solution La_3−xSr_xTaO_7−x/2 was confirmed by X-ray diffraction and Raman spectroscopy; it extends for a substitution ratio up to x = 0.15. Whereas La₃TaO₇ is a poor oxide ion conductor (σ_700 °C = 2 × 10⁻⁵S.cm⁻¹), at 700 °C, its ionic conductivity is increased by more than one order of magnitude when 3.3% molar strontium is introduced in the structure (σ_700 °C = 2 × 10⁻⁴S.cm⁻¹).     

Electron-poor SrAuxIn4−x (0.5?x?1.2) and SrAuxSn4−x (1.3?x?2.2) phases with the BaAl4-type structure

Solid solutions SrAu_xIn_4−x (0.5?x?1.2) and SrAu_xSn_4−x (1.3?x?2.2) have been prepared at 700 °C and their structures characterized by powder and single-crystal X-ray diffraction. They adopt the tetragonal BaAl₄-type structure (space group I4/mmm, Z=2; SrAu_1.1(1)In_2.9(1), a=4.5841(2) Å, c=12.3725(5) Å; SrAu_1.4(1)Sn_2.6(1), a=4.6447(7) Å, c=11.403(2) Å), with Au atoms preferentially substituting into the apical over basal sites within the anionic network. The phase width inherent in these solid solutions implies that the BaAl₄-type structure can be stabilized over a range of valence electron counts (vec), 13.0-11.6 for SrAu_xIn_4−x and 14.1-11.4 for SrAu_xSn_4−x. They represent new examples of electron-poor BaAl₄-type compounds, which generally have a vec of 14. Band structure calculations confirm that substitution of Au, with its smaller size and fewer number of valence electrons, for In or Sn atoms enables the BaAl₄-type structure to be stabilized in the parent binaries SrIn₄ and SrSn₄, which adopt different structure types.     

Synthesis, crystal structure and thermal behavior of Sr3B2SiO8 borosilicate

Single crystals of Sr₃B₂SiO₈ were obtained by solid-state reaction of stoichiometric mixture at 1200 °C. The crystal structure of the compound has been solved by direct methods and refined to R1=0.064 (wR=0.133). It is orthorhombic, Pnma, a=12.361(4), b=3.927(1), c=5.419(1) Å, V=263.05(11) Å³. The structure contains zigzag pseudo-chains running along the b axis and built up from corner sharing (Si,B)−O polyhedra. Boron and silicon are statistically distributed over one site with their coordination strongly disordered. Sr atoms are located between the chains providing three-dimensional linkage of the structure.The formation of Sr₃B₂SiO₈ has been studied using annealing series in air at 900-1200 °C. According powder XRD, the probe contains pure Sr₃B₂SiO₈ over 1100 °C. The compound is not stable below 900 °C. In the pseudobinary Sr₂B₂O₅-Sr₃B₂SiO₈ system a new series of solid solutions Sr_3−xB₂Si_1−xO_8−3x (x=0-0.9) have been crystallized from melt. The thermal behavior of Sr₃B₂SiO₈ was investigated using powder high-temperature X-ray diffraction (HTXRD) in the temperature range 20-900 °C. The anisotropic character of thermal expansion has been observed: α_a= −1.3, α_b=23.5, α_c=13.9, and α_V=36.1×10⁻⁶ °C⁻¹ (25 °C); α_a= −1.3, α_b=23.2, α_c=5.2, and α_V=27.1×10⁻⁶ °C⁻¹ (650 °C). Maximal thermal expansion of the structure along of the chain direction [0 1 0] is caused by the partial straightening of chain zigzag. Hinge mechanism of thermal expansion is discussed.     

Synthesis and characterization of NixMg1−xAl2O4 nano ceramic pigments via a combustion route

MgAl₂O₄ was successfully used as a crystalline host network for the synthesis of nickel-based nano cyan refractory ceramic pigments. Different compositions of Ni_xMg_1−xAl₂O₄ (0.1 ? x ? 0.8) powders have been prepared by using a low temperature combustion reaction (LTCR) of the corresponding metal nitrates with urea (U) as a fuel at 300 °C in an open air furnace. The as-synthesized samples were characterized by thermal analysis (TG-DTG/DTA), X-ray powder diffraction (XRD), Fourier transform infrared spectroscopy (FTIR) and transmission electron microscopy (TEM). UV-Vis and diffuse reflectance spectroscopy (DRS) using CIE- L^∗a^∗b^∗ parameters methods have been used for color measurements. The results show that the Ni_xMg_1−xAl₂O₄ samples are the crystalline phase with a particle size of 8.85-43.66 nm in the temperature range 500-1200 °C. The density, particle size, shape and color are determined for all the prepared samples with different calcination times and temperatures.     

H2S sensing properties of nanocrystalline Sr2FeO.6NiO.4MoO6 thick film prepared by sol-gel citrate method

Nanocrystalline Sr₂FeMoO₆ (SFMO) belonging to the group of double perovskite oxides, was prepared by the sol-gel citrate method. The structural and microstructural characterization has been carried out with the help of X-ray diffraction (XRD) and transmission electron microscopy (TEM), respectively. XRD of Sr₂Fe_1−xNi_xMoO₆ (SFNMO) shows the formation of solid solution with average grain size of about 40 nm. A comparative study of gas sensing behaviour of Sr₂FeMoO₆ and Sr₂Fe_1−xNi_xMoO₆ with reducing gases like hydrogen sulfide (H₂S), liquid petroleum gas (LPG), hydrogen (H₂), ethanol (C₂H₅OH) and carbon monoxide (CO) were also discussed. The sensitivity is calculated by measuring the change the resistance of the sensor material in the presence of gas. Among the different composition of x (x = 0.2, 0.3, 0.4, 0.5), Sr₂Fe_0.6Ni_0.4MoO₆ (x = 0.4) shows better response to H₂S gas at 260 °C. Incorporation of palladium (Pd) improves the gas response, selectivity, response time and reduced the operating temperature from 260 to 220 °C for H₂S gas.     

Evolution of titania nanotubes-supported WOx species by in situ thermo-Raman spectroscopy, X-ray diffraction and high resolution transmission electron microscopy

Structural evolution of WO_x species on the surface of titania nanotubes was followed by in situ thermo-Raman spectroscopy. A total of 15 wt% of W atoms were loaded on the surface of a hydroxylated titania nanotubes by impregnation with ammonium metatungstate solution and then, the sample was thermally treated in a Linkam cell at different temperatures in nitrogen flow. The band characteristic of the WO bond was observed at 962 cm⁻¹ in the dried sample, which vanished between 300 and 700 °C, and reappear again after annealing at 800 °C, along with a broad band centered at 935 cm⁻¹, attributed to the v₁ vibration of WO in tetrahedral coordination. At 900 and 1000 °C, the broad band decomposed into four bands at 923, 934, 940 and 950 cm⁻¹, corresponding to the symmetric and asymmetric vibration of WO bonds in Na₂WO₄ and Na₂W₂O₇ phases as determined by X-ray diffraction and High resolution transmission electron microscopy (HRTEM). The structure of the nanotubular support was kept at temperatures below 450 °C, thereafter, it transformed into anatase being stabilized at temperatures as high as 900 °C. At 1000 °C, anatase phase partially converted into rutile. After annealing at 1000 °C, a core-shell model material was obtained, with a shell of ca. 5 nm thickness, composed of sodium tungstate nanoclusters, and a core composed mainly of rutile TiO₂ phase.