Interstitial Oxide Ion Distribution and Transport Mechanism in Aluminum-Doped Neodymium Silicate Apatite Electrolytes.

Rare earth silicate apatites are one-dimensional channel structures that show potential as electrolytes for solid oxide fuel cells (SOFC) due to their high ionic conductivity at intermediate temperatures (500-700 °C). This advantageous property can be attributed to the presence of both interstitial oxygen and cation vacancies, that create diffusion paths which computational studies suggest are less tortuous and have lower activation energies for migration than in stoichiometric compounds. In this work, neutron diffraction of Nd(28+x)/3AlxSi6-xO26 (0 ≤ x ≤ 1.5) single crystals identified the locations of oxygen interstitials, and allowed the deduction of a dual-path conduction mechanism that is a natural extension of the single-path sinusoidal channel trajectory arrived at through computation. This discovery provides the most thorough understanding of the O(2-) transport mechanism along the channels to date, clarifies the mode of interchannel motion, and presents a complete picture of O(2-) percolation through apatite. Previously reported crystallographic and conductivity measurements are re-examined in the light of these new findings.

Influence of Li(+) and H(+) Distribution on the Crystal Structure of Li(7-x)H(x)La3Zr2O12 (0 ≤ x ≤ 5) Garnets.

With appropriate doping or processing, Li7La3Zr2O12 (LLZO) is an excellent candidate to be used in Li batteries either as a solid electrolyte or as a separator between the Li anode and a liquid electrolyte. For both uses, the reactivity with water either from the air or in aqueous media is a matter of interest. We address here the structural changes undergone by LLZO as a result of H(+)/Li(+) exchange and relate them with the amount of H content and atomic distribution. Neutron diffraction is performed to elucidate Li and H location. Two different cubic phases derive from LLZO through H(+)/Li(+) exchange: Deep hydration up to 150 °C yields a noncentrosymmetric I4̅3d phase in which octahedral Li ions are exchanged by H ions, tetrahedral Li ions split into two sites with very different occupancies, and H ions form O4H4 entities around the less occupied tetrahedral site. Annealing above 300 °C results in a centrosymmetric Ia3̅d phase with lower H content in which Li ions occupy the usual sites of the cubic garnets and H ions occupy a split pseudooctahedral site. The centrosymmetric or noncentrosymmetric character is determined by the temperature at which exchange is performed and the H content. Both factors are not independent: at low temperature, the high H content favors H ordering around the vacant tetrahedra, while low H content and higher mobility at 350 °C lead to a disordered configuration of Li and H ions. The deeply hydrated garnets are stable up to at least 300 °C and also upon aging at room temperature.

Structural study of the apatite Nd8Sr2Si6O26 by Laue neutron diffraction and single-crystal Raman spectroscopy.

A single-crystal structure determination of Nd8Sr2Si6O26 apatite, a prototype intermediate-temperature electrolyte for solid oxide fuel cells grown by the floating-zone method, was completed using the combination of Laue neutron diffraction and Raman spectroscopy. While neutron diffraction was in good agreement with P63/m symmetry, the possibility of P63 could not be convincingly excluded. This ambiguity was removed by the collection of orientation-dependent Raman spectra that could only be consistent with P63/m. The composition of Nd8Sr2Si6O26 was independently verified by powder X-ray diffraction in combination with electron probe microanalysis, with the latter confirming a homogeneous distribution of Sr and the absence of chemical zonation commonly observed in apatites. This comprehensive crystallochemical description of Nd8Sr2Si6O26 provides a baseline to quantify the efficacy of cation vacancies, oxygen superstoichiometry, and symmetry modification for promoting oxygen-ion mobility.

Insights of the formation mechanism of nanostructured titanium oxide polymorphs from different macromolecular metal-complex precursors.

The insight into the mechanism of the unprecedented formation of pure anatase TiO2 from the macromolecular (Chitosan)•(TiOSO4)n precursor has been investigated using micro Raman spectroscopy, Scanning Electron Microscopy (SEM) and thermogravimetric/differential thermal analysis (TGA/DTA). The formation of a graphitic film was observed upon annealing of the macromolecular precursor, reaching a maximum at about 500 °C due to decomposition of the polymeric chain of the Chitosan and (PS-co-4-PVP) polymers. The proposed mechanism is the nucleation and growth of TiO2 nanoparticles over this graphitic substrate. SEM and Raman measurements confirm the formation of TiO2 anatase around 400 °C. The observation of an exothermic peak around 260 °C in the TGA/DTA measurements confirms the decomposition of carbon chains to form graphite. Another exothermic peak around 560 °C corresponds to the loss of additional carbonaceous residues.

Pro-angiogenic and osteogenic composite scaffolds of fibrin, alginate and calcium phosphate for bone tissue engineering.

Due to the limitations of bone autografts, we aimed to develop new composite biomaterials with pro-angiogenic and osteogenic properties to be used as scaffolds in bone tissue engineering applications. We used a porous, cross-linked and slowly biodegradable fibrin/alginate scaffold originally developed in our laboratory for wound healing, throughout which deposits of calcium phosphate (CaP) were evenly incorporated using an established biomimetic method. Material characterisation revealed the porous nature and confirmed the deposition of CaP precursor phases throughout the scaffolds. MC3T3-E1 cells adhered to the scaffolds, proliferated, migrated and differentiated down the osteogenic pathway during the culture period. Chick chorioallantoic membrane (CAM) assay results showed that the scaffolds were pro-angiogenic and biocompatible. The work presented here gave useful insights into the potential of these pro-angiogenic and osteogenic scaffolds for bone tissue engineering and merits further research in a pre-clinical model prior to its clinical translation.

Suitability of strontium and cobalt-free perovskite cathodes with La9.67Si5AlO26 apatite electrolyte for intermediate temperature solid oxide fuel cells.

Aluminium-doped lanthanum silicate (LSAO) apatite-type compounds have been considered as promising candidates for substituting yttria-stabilized zirconia (YSZ) as electrolytes for intermediate temperature solid oxide fuel cells (IT-SOFC). Nevertheless, not many materials have been reported to work as cathodes in a LSAO apatite-based cell. In the present work, eight different strontium and cobalt-free compounds with a perovskite-type structure and the general composition LaM1-xNxO3-δ (where M = Fe, Cr, Mn; N = Cu, Ni; and x = 0.2, 0.3) have been tested. This study includes the synthesis and structural characterization of the compounds, as well as thermomechanical and chemical compatibility tests between them. Functional characterization of the individual components has been performed by electrochemical impedance spectroscopy (EIS). Apatite/perovskite symmetrical cells were used to measure area-specific resistance (ASR) of the half cell in an intermediate temperature range (500-850 °C) both with and without DC bias. According to its electrochemical behaviour, LaFe0.8Cu0.2O3-δ is the most promising material for IT-SOFC among the compositions tested since its ASR is similar to that of the traditional (LaxSr1-x)MnO3 (LSM) cathode.

Insight into ramsdellite LI(2)Ti(3)O(7) and its proton-exchange derivative.

Despite being proven to be a good lithium-ion conductor 30 years ago, the crystal structure of the ramsdellite-like Li(2)Ti(3)O(7) has remained uncertain, with two potential models for locating the lithium ions in the structure. Although the model presently accepted states that both lithium and titanium occupy the octahedral sites in the framework, evidence against this model are provided by (6)Li and (7)Li MAS NMR spectroscopy. Thus, about 14% of these octahedral positions are empty since no lithium in octahedral coordination is present in the material. When Li(2)Ti(3)O(7)-ramsdellite is treated with nitric acid a complete exchange of lithium by protons is produced to yield H(2)Ti(3)O(7). The crystal structure of this proton-exchanged ramsdellite has been re-examined combining X-ray diffraction (XRD), neutron powder diffraction (NPD), and spectroscopic ((1)H and (7)Li MAS NMR) techniques. Two kinds of protons are present in this material with different acidity because of the local environments of oxygen atoms to which protons are bonded, namely, low acidic protons strongly bonded to highly charged oxygen atoms (coordinated to two Ti(4+) and a vacancy); and protons linked to low charged oxygen atoms (bonded to three Ti(4+) ions) which will display a more acidic behavior. H(2)Ti(3)O(7) absorbs water; proton mobility is enhanced by the presence of absorbed water, giving rise to a large improvement of its electrical conductivity in wet atmospheres. Thus, it seems that water molecules enter the tunnels in the structure providing a vehicle mechanism for proton diffusion.

Synthesis of new Ln4(Al2O6F2)O2 (Ln = Sm, Eu, Gd) phases with a cuspidine-related structure.

The first fluorination of the cuspidine-related phases of Ln4(Al2O7□)O2 (where Ln = Sm, Eu, Gd) is reported. A low-temperature reaction with poly(vinyl-idene difluoride) lead to the fluorine being substituted in place of oxygen and inserted into the vacant position between the dialuminate groups. X-ray photoelectron spectroscopy shows the presence of the F 1s photoelectron together with an increase in Al 2p and rare-earth 4d binding energies supporting F incorporation. Energy-dispersive X-ray spectroscopy analyses are consistent with the formula Ln4(Al2O6F2)O2, confirming that substitution of one oxygen by two fluoride atoms has been achieved. Rietveld refinements show an expansion in the cell upon fluorination and confirm that the incorporation of fluoride in the Ln4(Al2O7□)O2 structure results in changes in Al coordination from four to five. Thus, the isolated tetrahedral dialuminate Al2O7 groups are converted to chains of distorted square-based pyramids. These structural results are also discussed based on Raman spectra.

Biomimetic surface functionalization of clinically relevant metals used as orthopaedic and dental implants.

Titanium and its alloys or tantalum (Ta) are materials used in orthopaedic and dental implants due to their excellent mechanical properties and biocompatibility. However, their bioactivity and osteoconductivity is low. With a view to improving the bioactivity of these materials we hypothesised that the surface of Ta and TiAl6V4 can be functionalised with biomimetic, amorphous nano-sized calcium phosphate (CaP) apatite-like deposits, instead of creating uniform coatings, which can lead to flaking, delamination and poor adherence. We used Ta and TiAl6V4 metal discs with smooth and rough surfaces. Amorphous CaP apatite-like particles were deposited on the different surfaces by a biomimetic rapid two-step soaking method using concentrated simulated body fluid (SBF) solutions without a pre-treatment of the metal surfaces to induce CaP deposition. Immersion times in the second SBF solution of 48 and 18 h for Ta and TiAl6V4 respectively produced CaP deposits composed of amorphous globular nano-sized particles that also contained Mg, C and O. Longer immersion times produced more uniform coatings as well as an undesired calcite mineral phase. Prediction of in vivo behaviour by immersion in regular SBF showed that the obtained CaP deposits would act as a catalyst to rapidly form a Ca deficient CaP layer that also incorporates Mg. The amorphous CaP apatite-like deposits promoted initial attachment, proliferation and osteogenic differentiation of bone marrow derived mesenchymal stem cells. Finally, we used our method to functionalise 3D porous structures of titanium alloy made by selective laser sintering. Our study uses a novel and cost-effective approach to functionalise clinically relevant metal surfaces in order to increase the bioactivity of these materials, which could improve their clinical performance.

Insight into the local structure of barium indate oxide-ion conductors: an X-ray total scattering study.

In this paper we presented the X-ray PDF investigation of orthorhombic Ba(2)In(2)O(5) and cubic Ba(2)In(1.7)P(0.3)O(5.3) and Ba(2)In(1.7)S(0.3)O(5.45) samples. Pure Ba(2)In(2)O(5) was found to be properly described-at the local scale-by the orthorhombic average structure. Ba(2)In(1.7)P(0.3)O(5.3) and Ba(2)In(1.7)S(0.3)O(5.45) cannot be described, at the local scale, by a cubic symmetry. The PDFs of these two samples clearly showed a distorted atom arrangement in the short-range which can be described again with the orthorhombic symmetry found in pure barium indate.

Local structure investigation of oxide ion and proton defects in Ge-apatites by pair distribution function analysis.

In this communication we provide a direct insight into the local structure and defects of oxygen excess Ge-apatites, in both dry and deuterated states, by means of pair distribution function analysis.

Enhancement of the conductivity of Ba(2)In(2)O(5) through phosphate doping.

In this communication, we demonstrate the successful incorporation of phosphate into Ba(2)In(2)O(5), which leads to the conversion from an orthorhombic to a cubic unit cell. The resulting increased oxygen vacancy disorder leads to an enhancement in the oxide ion conductivity at low temperatures. In addition, in wet atmospheres, significant proton conduction is observed.

Pseudomorphic 2A--> 2M--> 2H phase transitions in lanthanum strontium germanate electrolyte apatites.

Apatite-like materials are of considerable interest as potential solid oxide fuel cell electrolytes, although their structural vagaries continue to attract significant discussion. Understanding these features is crucial both to explain the oxide ion conduction process and to optimise it. As the composition of putative P6(3)/m apatites with ideal formula [A(I)(4)][A(II)(6)][(BO(4))(6)][X](2) is varied the [A(I)(4)(BO(4))(6)] framework will flex to better accommodate the [A(II)(6)X(2)] tunnel component through adjustment of the A(I)O(6) metaprism twist angle (varphi). The space group theory prescribes that framework adaptation during phase changes must lead to one of the maximal non-isomorphic subgroups of P6(3)/m (P2(1), P2(1)/m, P1[combining macron]). These adaptations correlate with oxygen ion conduction, and become crucial especially when the tunnels are filled by relatively small ions and/or partially occupied, and if interstitial oxygens are located in the framework. Detecting and completely describing these lower symmetry structures can be challenging, as it is difficult to precisely control apatite stoichiometry and small departures from the hexagonal metric may be near the limits of detection. Using a combination of diffraction and spectroscopic techniques it is shown that lanthanum strontium germanate oxide electrolytes crystallise as triclinic (A), monoclinic (M) and hexagonal (H) bi-layer pseudomorphs with the composition ranges: [La(10-x)Sr(x)][(GeO(4))(5+x/2)(GeO(5))(1-x/2)][O(2)] (0