M. L. Sanjuán

Cubic phases of garnet-type Li7La3Zr2O12: the role of hydration

We address the controversial issue of the structural stability of Li7La3Zr2O12 garnets, focusing on the mechanisms that result in the transformation from tetragonal to cubic symmetry. We show that undoped tetragonal Li7La3Zr2O12 not exposed to humidity at any moment undergoes a reversible phase transition to cubic symmetry at Tc ≃ 645 °C that we ascribe to lithium dynamic effects. On the other hand, a close correlation has been found between the appearance of a cubic phase between 100 and 200 °C in X-ray diffractograms and the presence of water, either in the atmosphere in which experiments are performed or already in the starting material. The natures of the high and low-temperature cubic garnets are totally different: the one found above the phase transition does not involve any change in the stoichiometry, whereas the cubic phase formed at low temperature is a hydrated, lithium defective phase, due to the combined effect of water insertion into the garnet structure and the H+/Li+ exchange mechanism. Differences in the actual compositions of the samples depending on their thermal history are corroborated by TG-MS experiments. Chemical reactions and phases formed along the thermal evolution are elucidated with the help of Raman spectroscopy.

Apatite germanates doped with tungsten: synthesis, structure, and conductivity

High oxygen content apatite germanates, La(10)Ge(6-x)W(x)O(27+x), have been prepared by doping on the Ge site with W. In addition to increasing the oxygen content, this doping strategy is shown to result in stabilisation of the hexagonal lattice, and yield high conductivities. Structural studies of La(10)Ge(5.5)W(0.5)O(27.5) show that the interstitial oxygen sites are associated to a different degree with the Ge/WO(4) tetrahedra, leading to five coordinate Ge/W and significant disorder for the oxygen sites associated with these units. Raman spectroscopy studies suggest that in the case of the WO(5) units, the interstitial oxygen is more tightly bonded and therefore not as mobile as in the case of the GeO(5) units, thus not contributing significantly to the conduction process.

Raman spectroscopy studies of apatite-type germanate oxide ion conductors: correlation with interstitial oxide ion location and conduction

A Raman spectroscopy study of the apatite series La8+xBa2−x(GeO4)6O2+x/2 is presented. The results show the presence of a new Raman band appearing at ∼645 cm−1, whose intensity increases with increasing interstitial oxide ion content. This new band is also observed in samples containing cation vacancies, consistent with previous suggestions that the presence of cation vacancies enhances Frenkel-type defect formation. The fact that the new band is in the stretching region of the spectra, rather than the bending region as observed for the silicate analogues, is consistent with the interstitial oxide ions being more closely associated with the Ge. This band is attributed to the presence of interstitial oxide ions leading to the formation of five coordinate Ge, in agreement with recent neutron diffraction and modelling studies. From the observation of a reduction in the intensity of this band with increasing temperature, it is suggested that the activation energy for conduction in these apatite germanates is a combination of the energy to “free” the interstitial oxide ions from the five coordinate Ge, and the energy for their subsequent migration. The former process is ascribed to the observed reduction in Raman intensity with an activation energy of 0.32 ± 0.06 eV. Thus the higher activation energy for the germanate apatites over the related silicates can be ascribed to the defect trapping associated with the closer association of the interstitial oxide ion with the tetrahedra in the former.

NMR study of Li distribution in Li7−xHxLa3Zr2O12 garnets

Despite the large number of NMR studies performed on lithium conductors with a garnet-type structure, the distribution of the lithium ions in Li7La3Zr2O12 (LLZO), and their contribution to ionic conductivity are still a matter of controversy. In this work we present a magic-angle spinning (MAS) NMR study of enriched 6Li7−xHxLa3Zr2O12 (0 ≤ x ≤ 5) garnets with the aim of identifying the bands arising from the different lithium sites occupied in the garnet lattice. Taking advantage of the known sensitivity of this material to moisture and facile proton-for-lithium exchange, we have been able to alter the relative population of tetrahedral and octahedral sites (the exchange is favoured in the latter) by submitting the samples to different post-treatments to obtain samples with varying lithium content. This has allowed the identification of three different bands that we ascribe to Li in different environments within the garnet structure. In addition, variable temperature measurements have indicated the presence of dynamic exchange processes between the octahedral and tetrahedral Li sites. Protons inserted in the garnet structure were analyzed using 1H-MAS-NMR and Raman spectroscopies. 6Li-1H-CP-MAS experiments have allowed the investigation of the relative distribution of protons and lithium ions in partially exchanged samples.

The Configuration of β‐Carotene in the Photosystem II Reaction Center

Different pigment extraction procedures and HPLC methods were tested to investigate the geometric configuration of the β‐carotene in two forms of the photosystem II reaction center (Dl‐D2‐cytochrome (Cyt) b559) complex containing one and two β‐carotene molecules per two pheophytin a. All the handling steps and HPLC analyses were done in darkness at room temperature and at 4°C. Two different pigment extracts were analyzed, a mixture of chlorophyll a, pheophytin a and 3‐carotene, and the isolated p‐carotene from that mixture. In both cases only the all‐frans‐β‐carotene was detected. The chromatographic profiles were similar at both temperatures only differing in the retention times that were longer at 4°C. This result was independent of the concentration of photosynthetic starting material. Furthermore, no differences were observed between Dl‐D2‐Cyt b559 complexes with one and two p‐carotene molecules per reaction center. The analysis of the β‐carotene chromatographic peak indicated no 15‐cis to all‐fraws isom‐erization occurred during the HPLC chromatography in our experimental conditions. Resonance Raman spectra were also recorded in the isolated Dl‐D2‐Cyt b559 complex at room and liquid nitrogen temperature with excitation at 514.5 nm from an Ar+ laser. Spectra of control preparations showed main bands at 1532, 1264, 1213, 1185, 1154 and 1003 cm−1 corresponding to the M‐trans isomer and confirm previous results. The presence in the reaction center suspension of artificial electron acceptors such as silicomolybdate or 2,5‐dibromo‐3‐methyl‐6‐iso‐propyl‐p‐benzoquinone that are able to quench 3P680 did not modify the resonance Raman spectra of the native Dl‐D2‐Cyt b559 complex. The results suggest that no isomerization takes place during the laser irradiation.

Influence of Li+ and H+ Distribution on the Crystal Structure of Li7–xHxLa3Zr2O12 (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.

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.

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.

'Laser chemistry' synthesis, physicochemical properties, and chemical processing of nanostructured carbon foams

Laser ablation of selected coordination complexes can lead to the production of metal-carbon hybrid materials, whose composition and structure can be tailored by suitably choosing the chemical composition of the irradiated targets. This ‘laser chemistry’ approach, initially applied by our group to the synthesis of P-containing nanostructured carbon foams (NCFs) from triphenylphosphine-based Au and Cu compounds, is broadened in this study to the production of other metal-NCFs and P-free NCFs. Thus, our results show that P-free coordination compounds and commercial organic precursors can act as efficient carbon source for the growth of NCFs. Physicochemical characterization reveals that NCFs are low-density mesoporous materials with relatively low specific surface areas and thermally stable in air up to around 600°C. Moreover, NCFs disperse well in a variety of solvents and can be successfully chemically processed to enable their handling and provide NCF-containing biocomposite fibers by a wet-chemical spinning process. These promising results may open new and interesting avenues toward the use of NCFs for technological applications.

Structural transition in orthorhombic Li5−xHxLa3Nb2O12 garnets induced by a concerted lithium and proton diffusion mechanism

Proton-exchanged Li-conducting garnets present novel properties compared with their parent compounds, among which proton conductivity may have potential applications. Here we study the structural and dynamical properties of Li5−xHxLa3Nb2O12 garnets with an intermediate proton content (x ≈ 2.8) using neutron and X-ray diffraction, Raman and NMR techniques. At RT the structure is noncentrosymmetric and orthorhombic but transforms reversibly at ∼150 °C into a cubic, still noncentrosymmetric phase. In the low temperature phase Li is split among six different tetrahedral sites and protons form triplet-wise hydroxyl entities around two of such tetrahedra. In the high temperature phase Li occupies two tetrahedral sites, protons being dynamically disordered around half of the tetrahedra. The occupancy of octahedral Li sites is very low. Spectroscopic techniques evidence that lithium exchange between tetrahedral and octahedral sites and proton reorientation among oxygen sites occur already in the low temperature phase but these processes are restricted at a local scale due to the blocking effect of protons for lithium diffusion and vice versa. We suggest that the phase transition is the consequence of a long range Li and proton redistribution through a concerted hopping mechanism. The relationship between the compound stoichiometry, structure and proton mobility is discussed. The occurrence of proton mobility below 200 °C for this specific composition may be of practical interest.