Synthesis and electrical transport properties of Lu2+xTi2−xO7−x/2 oxide-ion conductors

The Lu_2+xTi_2−xO_7−x/2 (x = 0; 0.052; 0.096; 0.286; 0.44; 0.63; 33.3–49 mol% Lu₂O₃) nanoceramics with partly disordered pyrochlore-type structure are prepared by sintering freeze-dried powders obtained by a co-precipitation technique with 1600 °C annealing. Similar to pyrochlore-like compositions in the zirconate system, some of the new titanates are good oxide-ion conductors in air. The new solid-state electrolytes have oxide-ion conductivity in the interval of 1.0 × 10^− 3 – 2.5 × 10^− 3 S/cm at 740 °C in air. This value of conductivity is comparable with that of ZrO₂/Y₂O₃ ceramics. The conductivity of Lu_2+xTi_2−xO_7−x/2 depends on the chemical composition. The highest ionic conductivity is exhibited by nearly stoichiometric Lu_2+xTi_2−xO_7−x/2 (x = 0.096; 35.5 mol% Lu₂O₃) material containing ∼ 4.8 at.% Lu_Ti anti-site defects.     

LiMn2−xTixO4 spinel-type compounds (x ≤ 1): Structural,electrical and magnetic properties

LiMn_2−xTi_xO₄ compounds with 0 ≤ x ≤ 1 were prepared by solid state reaction and Pechini technique. Powder X-ray diffraction showed that all samples crystallize with the spinel crystal structure (S.G. Fd3¯m). The cubic unit-cell parameter increases with the Ti content. The influence of the Ti content and cationic distribution on the magnetic properties of the compounds was studied by measuring the temperature and magnetic field dependences of the magnetization: substitution by non-magnetic d⁰ Ti⁴⁺ ions appeared to weaken the magnetic interactions between the manganese ions. The electrical properties of LiMnTiO₄ were studied by AC impedance spectroscopy and DC polarisation measurements, which revealed the electronic character of the conduction process.     

Oxygen-rich Ti1−xO2 pillar growth at a gold nanoparticle–TiO2 contact by O2 exposure

In-situ gas-injection transmission electron microscopy revealed that a pillar grew at the edge of the interface of a gold nanoparticle and a TiO₂ substrate during exposure to O₂ gas at 100 Pa. The pillar was found to have a titanium-deficient chemical composition of Ti_1 ? xO₂ (x > 0) by electron energy loss spectroscopy (EELS). The spectra showed a chemical shift of oxygen and titanium ions to have ionic states of Ti³⁺ and O^y? (y < 3/2). The formation of the Ti_1 ? xO₂ at the contact edge of gold–Ti_1 ? xO₂ interface is discussed from the perspective of an O₂ affinity, which plays an important role in CO oxidation process of supported gold particle.     

Synthesis and visible light photocatalysis water splitting property of chromium-doped Bi4Ti3O12

Photocatalysts Bi₄Ti_3 ? xCr_xO₁₂(x = 0.00, 0.06, 0.15, 0.30, 0.40, and 0.50) with perovskite structure were synthesized by sol–gel method and their electronic structures and photocatalytic activities were investigated. The Bi₄Ti_2.6Cr_0.4O₁₂ photocatalyst exhibited the highest performance of H₂ evolution in methanol aqueous solution (58.1 μmol h^? 1 g^? 1) under visible light irradiation (λ > 400 nm) without a co-catalyst, whereas no H₂ evolution is observed for Bi₄Ti₃O₁₂ under the same conditions. The UV–vis spectra indicated that the Bi₄Ti_2.6Cr_0.4O₁₂ had strong photoabsorption in the visible light region. The results of density functional theory (DFT) calculation illuminate that the conduction bands of Bi₄Ti₃O₁₂ are mainly attributable to the Ti 3d + Bi 6p orbitals, and the valence bands are composed of O 2p + Bi 6s hybrid orbitals, while the conduction bands of chromium-doped Bi₄Ti₃O₁₂ are mainly attributable to the Ti 3d + Bi 2p + Cr 3d orbitals, and the O 2p + Cr 3d hybrid obitals are the main contribution to the valence band.     

An XAS study of the defect structure of Ti-doped α-Cr2O3

The bulk defect structure in Cr_2−xTi_xO₃ (x = 0.05, 0.20 and 0.30) has been studied by X-ray absorption spectroscopy measurements at the Cr and Ti K-edges. The results show that the Ti is predominantly present in the IV oxidation state and resides on the normal Cr host lattice site. The dopant is charge compensated by Cr³⁺ vacancies and there is evidence for the formation of defect clusters; however, the detailed structure of these clusters could not be deduced.     

Structure and relaxor behavior of BaBi4Ti4−xZrxO15 ceramics

BaBi₄Ti_4−xZr_xO₁₅ with x = 0.1, 0.2, 0.3 and 0.5, has been synthesized via modified solid state reaction route. X-ray diffraction studies confirmed the formation of single phase Zr⁴⁺ substituted BaBi₄Ti₄O₁₅ up to x = 0.2. ZrO₂ and Bi₂O₃ based impurity phases were found at x = 0.3 and 0.5 substitutions. However, Rietveld refinement showed the increase in lattice parameters of BaBi₄Ti₄O₁₅ up to x = 0.5 substitutions. A broad dielectric peak associated with frequency dependence dielectric maximum temperature was observed at low substitutions. Relaxor behavior was suppressed at x = 0.5 substitution. A broadening and shifting of permittivity-temperature peak was found for the substitution. The high temperature slopes of dielectric peaks were analyzed by quadratic law for relaxors. The degree of relaxation and phase transformation diffusiveness were investigated at different substitutions.     

Microscopic magnetic properties of (V1−xTix)2O3 near the phase boundary of the metal–insulator transition

Magnetic susceptibility (χ) and ⁵¹V NMR have been measured in (V_1−xTi_x)₂O₃ near the phase boundary of the metal–insulator transition. It is established that the transition from antiferromagnetic insulating (AFI) to antiferromagnetic metallic phases near x_c≈0.05 is not quantum critical, but is discontinuous with a jump of the transition temperature. In the AFI phase at 4.2 K, we observed the satellite in the zero-field ⁵¹V NMR spectrum around 181 MHz in addition to the ‘host’ resonance around 203 MHz. The satellite is also observable in the paramagnetic metallic phase of the x=0.055 sample. We associated the satellite with the V sites near Ti, which are in the V³⁺-like oxidation state, but has different temperature dependence of the NMR shift from that of the host V site. The host d-spin susceptibility for x=0.055 decreases below ∼60 K, but remains finite in the low-temperature limit.     

NMR and electric impedance study of lithium mobility in fast ion conductors LiTi2 − xZrx(PO4)3 (0 ≤ x ≤ 2)

Materials of the LiTi_2 − xZr_x(PO₄)₃ series (0 ≤ x ≤ 2) were prepared and characterized by powder X-ray (XRD) and neutron diffraction (ND), ⁷Li and ³¹P Nuclear Magnetic Resonance (NMR) and Electric Impedance techniques. In samples with x < 1.8, XRD patterns were indexed with the rhombohedral R3̅c space group, but in samples with x ≥ 1.8, XRD patterns display the presence of rhombohedral and triclinic phases. The Rietveld analysis of the LiTi_1.4Zr_0.6(PO₄)₃ neutron diffraction (ND) pattern provided structural information about intermediate compositions. For low Zr contents, compositions deduced from ³¹P MAS-NMR spectra are similar to nominal ones, indicating that Zr⁴⁺ and Ti⁴⁺ cations are randomly distributed in the NASICON structure. At increasing Zr contents, differences between nominal and deduced compositions become significant, indicating some Zr segregation in the triclinic phase. The substitution of Ti⁴⁺ by Zr⁴⁺ stabilizes the rhombohedral phase; however, electrical performances are not improved in expanded networks of Zr-rich samples. Below 300 K, activation energy of all samples is near 0.36 eV; however, above 300 K, activation energy is near 0.23 eV in Ti-rich samples and close to 0.36 eV in Zr-rich samples. The analysis of electrical data suggests that the amount of charge carriers and entropic terms are higher in Zr-rich samples; however, the increment of both parameters does not compensate lower activation energy terms of these samples. As a consequence of different contributions, the bulk conductivity of Zr-rich samples, measured at room temperature, is one order of magnitude lower than that measured in Ti-rich samples.     

A single-crystal study of the electrochemically Li-ion intercalated spinel-type Li4Ti5O12

Single crystals of Li_4 + xTi₅O₁₂ were prepared by means of electrochemical Li-ion intercalation technique using parent Li₄Ti₅O₁₂ single crystals. The obtained Li_4 + xTi₅O₁₂ (x = 1.35) crystallizes in the cubic spinel-related type structure, space group Fd3?m, and lattice parameters of a = 8.346(2) Å and V = 581.3(5) Å³ and Z = 8. The Li-ion intercalated sites were successfully determined to be both the 8a and 16c sites by using the difference Fourier synthesis map. The structure was determined by single-crystal X-ray structure analysis and refined to the conventional value of R = 3.7% for 132 independent observed reflections. The chemical composition has been determined to be Li_5.35Ti₅O₁₂ from the result of site-population refinements. In addition, theoretical electron density distributions and total energy were calculated for three postulated compounds of “Li_4.5Ti_4.5O₁₂” and “Li_4.5 + xTi_4.5O₁₂” with x = 1.5 and 3.0.     

Reduction of thin-film ceria on Pt(111) by supported Pd nanoparticles probed with resonant photoemission

Local defects present in CeO_2 ? x films result in a mixture of Ce³⁺ and Ce⁴⁺ oxidation states. Previous studies of the Ce 3d region with XPS have shown that depositing metal nanoparticles on ceria films causes further reduction, with an increase in Ce³⁺ concentration. Here, we compare the use of XPS and resonant photoemission spectroscopy (RESPES) to estimate the concentration of Ce³⁺ and Ce⁴⁺ in CeO_2 ? x films grown on Pt (111), and the variation of this concentration as a function of Pd deposition. Due to the nature of the electronic structure of CeO_2 ? x, resonant peaks are observed for the 4d–4f transitions when the photon energy matches the resonant energy; (hν = 121.0 eV) for Ce³⁺ and (hν = 124.5 eV) for Ce⁴⁺. This results in two discrete resonant photoemission peaks in valence band spectra. The ratio of the difference of these peaks with off-resonance scans gives an indication of the relative contribution of Ce³⁺. Results from RESPES indicate reduction of CeO_2 ? x on deposition of Pd, confirming earlier findings from XPS studies.     

Magnetic properties and formation of Sr ferrite nanoparticle and Zn,Ti/Ir substituted phases

Strontium hexaferrite nanoparticles are prepared by the chemical sol–gel route. Specific saturation magnetization σ_s and coercive field strength H_c are determined depending on the heat treatment of the gel and iron/strontium ratio in the starting solution. These ultrafine powders with single-domain behavior have specific saturation magnetization σ_s=74 emu/g and coercive field strength H_c=6.4 kOe. Experimental results show that it is necessary to preheat the gel between 400 and 500°C for several hours . It can prevent the formation of intermediate γ-Fe₂O₃ and help to obtain ultrafine strontium ferrite single phase with narrow size distribution at a low annealing temperature. Additionally, the magnetic properties of sol–gel derived strontium ferrite with iron substituted by Zn²⁺, Ti⁴⁺ and Ir⁴⁺ are discussed. For an amount of substitution 0<x⩽0.6, the (Zn, Ti)_x substituted strontium ferrite shows higher values of both coercive field strength and saturation magnetization than the (Zn, Ir)_x substituted phase.     

Theoretical study of the (110) surface of Sn1 - xTixO2 solid solutions with different distribution and content of Ti

The composition and thermodynamic stability of the (110) surface of Sn_1 - xTi_xO₂ rutile solid solutions was investigated as a function of Ti-distribution and content up to the formation of a full TiO₂ surface monolayer. The bulk and (110) surface properties of Sn_1 - xTi_xO₂ were compared to that of the pure SnO₂ and TiO₂ crystal. A large supercell of 720 atoms and a localized basis set based on the Gaussian and plane wave scheme allowed the investigation of very low Ti-content and symmetry. For the bulk, optimization of the crystal structure confirmed that up to a Ti-content of 3.3 at.%, the lattice parameters (a, c) of SnO₂ do not change. Increasing further the Ti-content decreased both lattice parameters down to those of TiO₂. The surface energy of these solid solutions did not change for Ti-substitution in the bulk of up to 20 at.%. In contrast, substitution in the surface layer rapidly decreased the surface energy from 0.99 to 0.74 J/m² with increasing Ti-content from 0 to 20 at.%. As a result, systems with Ti atoms distributed in the surface (surface enrichment) had always lower energies and thus were thermodynamically more favorable than those with Ti homogeneously distributed in the bulk. This was attributed to the lower energy necessary to break the TiO bonds than SnO bonds in the surface layer. In fact, distributing the Ti atoms homogeneously or segregated in the (110) surface led to the same surface energy indicating that restructuring of the surface bond lengths has minimal impact on thermodynamic stability of these rutile systems. As a result, a first theoretical prediction of the composition of Sn_1 - xTi_xO₂ solid solutions is proposed.     

Cation site environment in ultrathin TiOx films grown on Pt(1 1 1) probed by X-ray absorption spectroscopy at the Ti 2p edge

We present the results of a XAS experiment carried out at the Ti 2p edge on well-ordered TiO_x ultrathin films grown on the Pt(1 1 1) surface. XAS at the Ti 2p edge has been extensively applied to the study of Ti bulk compounds, particularly to the study of titania. According to the literature, the corresponding Ti 2p edge spectral shape is related to the stoichiometry and crystal field symmetry at the Ti sites. In the present study we aim at extending the potential of the XAS technique by discussing Ti 2p spectra obtained on several, dimensionally confined, TiO_x phases in the form of ultrathin films. One of the main features of these films is their high degree of structural order. Furthermore, the results of previous studies provide valuable information about the chemistry and structure of the films, so that we are able to analyse the current XAS data in detail and to compare them with the appropriate Ti 2p XAS data of bulk oxides. We find that in the case of ultrathin film with a fully oxidised Ti⁴⁺ stoichiometry, the Ti 2p XAS data display features that are very similar to the ones observed in related bulk systems. The XAS data of the reduced TiO_x films (with x < 2) show a rather different and specific shape. By comparing the experimental spectra with an atomic type of model calculation we show that the Ti 2p XAS profiles can be attributed mainly to stoichiometry-symmetry effects.     

Synthesis and characterisation of Cr3+-containing NASICON-related phases

Chromium-containing NASICON-related phosphates of the type Na_(1+x)Cr_xM_(2−x)P₃O₁₂) (M = Ti, Hf, Zr) have been synthesised by solid state reaction and structurally characterised by Rietveld refinement of the powder X-ray diffraction data. Materials of composition A_(1+x)/2Cr_xZr_(2−x)P₃O₁₂ (A = Cd, Ca, Sr), have also been prepared and characterised. The crystal structure of Na_(1+x)Cr_xM_(2−x)P₃O₁₂ corresponds to R-3c symmetry for x values ranging from 0.15 to 2.00, whereas compounds of composition A_(1+x)/2Cr_xZr_(2−x)P₃O₁₂ corresponding to R-3c are obtained when x ≤ 1.00 for Sr²⁺ and Ca²⁺, and x ≤ 1.50 for Cd²⁺. The polarizing effect of the two different metal ions A and M on the phosphorus atom and the P–O bond was studied by both ³¹P MAS NMR and infrared spectroscopy and shows that the electron density on the phosphorus, and thus the strength of the P–O bonds, are affected by both the interstitial (A) and the structural (M) metal ions.     

Effects of Zr/Ti ratio on dielectric and ferroelectric properties of 0.8Pb(ZrxTi1−x)O3–0.2Pb(Co1/3Nb2/3)O3 ceramics

The influences of Zr/Ti ratio on electrical properties of the 0.8Pb(Zr_xTi_1−x)O₃–0.2Pb(Co_1/3Nb_2/3)O₃ ceramics prepared by a mixed-oxide method (with x = 0.46, 0.48, 0.50, 0.52, and 0.54) have been investigated in order to identify the morphotropic phase boundary composition in this system. With XRD analysis, the crystal structure of dense specimens appeared to change gradually from tetragonal to rhombohedral with increasing Zr content. The dielectric properties measurements showed a maximum dielectric constant at x = 0.50, while the transition temperature decreased with increasing Zr content in the system. Moreover, all ceramics showed diffused phase transition behaviors with a minimum diffusivity at x = 0.50. In addition, the Polarization–Electric field (P–E) hysteresis loops of the ceramic systems also changed significantly with the Zr content. Interestingly, the loop squareness parameter reached maximum around x = 0.50. Other ferroelectric hysteresis parameters showed noticeable change at x = 0.50. These results clearly showed the significance of Zr/Ti ratio in controlling the electrical properties of the PZT–PCN ceramic systems.     

Synthesis and photoluminescence properties of YVO4:Eu3+, Al3+ phosphor

The Y_0.95?xAl_xVO₄:5%Eu³⁺ (0≤x≤0.1) phosphors were successfully synthesized by solid state reaction at 900 °C for 6 h, and their luminescence properties were investigated under UV and VUV excitation. Monitoring at 619 nm, a strong broad absorption was enhanced by co-doping of Al³⁺ into the YVO₄:Eu³⁺ lattices at 256 nm under UV excitation. The VUV excitation spectra also showed the enhanced excitation bands at about 156 and 200 nm. Under 254 or 147 nm excitation, it was found that Y_0.95?xAl_xVO₄:Eu³⁺(0≤x≤0.1) phosphors showed strong red emission at about 619 nm corresponding to the electric dipole ⁵D₀^–7F₂ transition of Eu³⁺. The improvement of luminescence intensity of YVO₄:Eu³⁺ was also observed after partial substituting Y³⁺ by Al³⁺ and the optimal luminescence intensity appeared with incorporation of 2.5 mol% Al³⁺.     

Development of perovskite and phase transition in lead cobalt niobate modified lead zirconate titanate system

Ferroelectric lead zirconate titanate–lead cobalt niobate ceramics with the formula (1 − x)Pb(Zr_1/2Ti_1/2)O₃–xPb(Co_1/3Nb_2/3)O₃ where x = 0.0–0.5 were fabricated using a high temperature solid-state reaction method. The formation process, the structure and homogeneity of the obtained powders have been investigated by X-ray diffraction method as well as the simultaneous thermal analysis of both differential thermal analysis (DTA) and thermogravimetry analysis (TGA). It was observed that for the binary system (1 − x)Pb(Zr_1/2Ti_1/2)O₃–xPb(Co_1/3Nb_2/3)O₃, the change in the calcination temperature is approximately linear with respect to the PCoN content in the range x = 0.0–0.5. In addition, X-ray diffraction indicated a phase transformation from a tetragonal to a pseudo-cubic phase when the fraction of PCoN was increased. The dielectric permittivity is remarkably increased by increasing PCoN concentration. The maximum value of remnant polarization P_r (25.3 μC/cm²) was obtained for the 0.5PZT–0.5PCoN ceramic.     

Band overlap via chemical pressure control in double perovskite (Sr2−xCax)FeMoO6 (0 ⩽ x ⩽ 2.0) with TMR effect

The chemical pressure control in (Sr_2−xCa_x)FeMoO₆ (0 ⩽ x ⩽ 2.0) with double perovskite structure has been investigated systematically. We have performed first-principles total energy and electronic structure calculations for x = 0 and x = 2.0. The increasing Ca content in (Sr_2−xCa_x)FeMoO₆ samples increases the magnetic moment close to the theoretical value due to reduction of Fe/Mo anti-site disorder. An increasing Ca content results in increasing (Fe²⁺ + Mo⁶⁺)/(Fe³⁺ + Mo⁵⁺) band overlap rather than bandwidth changes. This is explained from simple ionic size arguments and is supported by X-ray absorption near edge structure (XANES) spectra and band structure calculations.     

Ti/CeOx(111) interfaces studied by XPS and STM

Low coverage of Ti was deposited on the well-ordered CeO_x(111) (1.5 < x < 2) thin films grown on Ru(0001) by physical vapor deposition at room temperature. The structure and interaction of Ti/ceria interfaces were investigated with X-ray photoelectron spectroscopy (XPS), low energy electron diffraction (LEED) and scanning tunneling microscopy (STM) techniques under ultrahigh vacuum conditions. XPS data indicate that the deposition of Ti on both oxidized and reduced ceria surfaces causes the partial reduction of Ce from + 4 to + 3 state. Ti is formally in the + 4 state. STM data show the formation of small atomic-like titania features at 300 K, which coalesce to form chain structures upon heating. It is demonstrated in the study that the deposition of Ti can form mixed metal oxides at the interface and modify both electronic and structural properties of the ceria support. The structural study of Ti/ceria interfaces can be a key for understanding the higher catalytic activity of the Ti–CeO_x mixed oxide catalysts as compared with the individual pure oxides.     

Electrical properties of yttrium doped strontium titanate with A-site deficiency as potential anode materials for solid oxide fuel cells

Yttrium doped strontium titanate with A-site deficiency ((Y_0.08Sr_0.92)_1 ? xTiO_3 ? δ) was synthesized by conventional solid state reaction. The deficiency limit of A-site in (Y_0.08Sr_0.92)_1 ? xTiO_3 ? δ is below 6 mol% in Ar/H₂ (5%) at 1500 °C. The sinterability of (Y_0.08Sr_0.92)_1 ? xTiO_3 ? δ samples decreases slightly with increasing A-site deficiency level (x). The ionic conductivity of (Y_0.08Sr_0.92)_1 ? xTiO_3 ? δ samples increases while the electronic conductivity decreases with increasing A-site deficient amount. The defect chemistry analysis indicates that the introduction of A-site deficiency results in not only the increase of oxygen vacancy concentration but also the decrease of Ti³⁺-ion concentration. The latter plays the main role in the electrical conduction. (Y_0.08Sr_0.92)_1 ? xTiO_3 ? δ shows good thermal-cyclic performance in electrical conductivity and has an excellent chemical compatibility with YSZ electrolyte below 1500 °C.