Laura León-Reina

Interstitial oxygen conduction in lanthanum oxy-apatite electrolytes

The La10 − x(SiO4)6O3 − 1.5x (9.33 ≤ 10 − x ≤ 9.73) apatite series has been prepared and hexagonal single phases were obtained in a narrow compositional range (9.33 ≤ 10 − x ≤ 9.60). The room temperature crystal structure of La9.55(SiO4)6O2.32 has been determined from joint Rietveld refinement of neutron and laboratory X-ray powder diffraction data: a = 9.7257(1) A, c = 7.1864(1) A, V = 588.68(1) A3, Z = 1, RwpN = 3.2%, RwpX = 7.7%, RFN = 1.8%, RFX = 1.9%. An interstitial site for the extra-oxygen has been determined in the position very recently predicted in a theoretical study using atomistic simulations. The high temperature crystal structures have been obtained from neutron powder diffraction, NPD, collected at 773 and 1173 K showing the thermal evolution of this interstitial site. Previously reported neutron data for La9.60(GeO4)6O2.40 have also been re-analysed establishing the existence, and thermal evolution, of this interstitial site. The electrical results suggest that the samples are oxide ion conductors. The plots of the imaginary parts of the impedance, Z″, and the electric modulus, M″, vs. log (frequency), possess maxima for both curves separated by two decades in frequency. Bulk conductivities have been obtained from the fitting of the complex impedance spectra with the appropriate equivalent circuit. Bulk activation energies have been determined from two Arrhenius plots, one representing the bulk conductivities and the other representing the frequencies of the modulus peak maxima, fmax(M″). A comparative discussion of the two series, La10 − x(TO4)6O3 − 1.5x (T = Si, Ge), is given.

Interstitial oxygen in oxygen-stoichiometric apatites

Several oxy-apatite materials La10−xSrx(TO4)6O3−0.5x (T = Ge, Si; 10−x = 9.00, 8.80, 8.65 and 8.00) and La9.33(Si1−xGexO4)6O2 (x = 0, 0.5, 0.67) have been prepared as highly crystalline phases. The impedance study showed that all samples are oxide ion conductors. However, bulk conductivities changed by more than 2 orders of magnitude at a given temperature for some compositions. A thorough study on the oxygen sublattices for oxygen-stoichiometric oxy-apatites has been carried out. Constant-wavelength neutron powder diffraction data have been collected for La9.33(SiO4)6O2. Time-of-flight neutron data have been collected for La9.33(Si0.5Ge0.5O4)6O2, La8Sr2(SiO4)6O2 and La8Sr2(GeO4)6O2. The room-temperature structures have been derived from joint Rietveld refinements of neutron and laboratory X-ray powder diffraction data. High temperature structures have been obtained only from Rietveld refinements of neutron powder diffraction data. The refinements show that La9.33(SiO4)6O2 and La9.33(Si0.5Ge0.5O4)6O2 contain interstitial oxygen, associated to vacancies at the oxygen channels. The amount of interstitial oxygen is negligible in La8Sr2(SiO4)6O2 and La8Sr2(GeO4)6O2. Hence, the novelty of this work is to explain the high oxide conductivity of the lanthanum-deficient samples which it is due to the presence of interstitial oxygens. Lanthanum stoichiometric samples do not have interstitial oxygens and, so, their conductivities are much lower.

Low temperature crystal structures of apatite oxygen-conductors containing interstitial oxygen

Oxygen-stoichiometric La(9.33) square(0.67)(Si(6)O(24))O2 and oxygen-excess La(8.65)Sr(1.35)(Ge(6)O(24))O(2.32) and La(8.65)Sr(1.35)(Si(6)O(24))O(2.32) oxy-apatites have been structurally characterized at low temperatures by the Rietveld method. Oxygen-interstitial distribution has been studied at 15 K for La(9.33) square(0.67)(Si(6)O(24))O2 and La(8.65)Sr(1.35)(Ge(6)O(24))O(2.32) by time-of-flight neutron powder diffraction and at 4 K for La(8.65)Sr(1.35)(Si(6)O(24))O(2.32) by constant-wavelength neutron powder diffraction. The low temperature structural study was undertaken in order to distinguish between the effects of static disorder, originated mainly from the presence of interstitial oxygens, and the anisotropic thermal vibrations. At such low temperatures, the influence of the anisotropic thermal vibrations is minimised. This structural study has firmly established the existence of interstitial oxygens in these materials, which may be useful as electrolytes for solid oxide fuel cells.

Glasses and crystalline A3Al2(PO4)3(A = Na, Li): an impedance and 31P, 27Al, 23Na and 7Li MAS-NMR study

A new vitreous phase with Na3Al2(PO4)3 composition has been prepared. Its crystallisation has been optimised by annealing at low temperature, giving crystalline c-Na3Al2(PO4)3 with Sc2(WO4)3-type structure. The direct synthesis of vitreous Li3Al2(PO4)3 has been attempted but it was unsuccessful. Ion exchange reaction has been carried out in Na3Al2(PO4)3 glass to obtain the lithiated vitreous solid, v-Li3Al2(PO4)3. On heating, v-Li3Al2(PO4)3 decomposes to yield crystalline AlPO4 and Li3PO4 phases. The three solids, with NASICON-like stoichiometries, have been thermal and electrically characterised. DTA and thermodiffractometric studies of the two glass solids have allowed the characterisation of their thermal behaviours including crystallisation. The grain boundary contributions to the total impedance are almost absent in the two vitreous phases. The electrical conductivity values are ∼ 10−6 S cm−1 at 150 °C. The 31P, 23Na, 27Al and 7Li MAS-NMR spectra for these materials provide some insight into the disordered constitution of the vitreous phases and the low mobility of the alkaline cations.

Accuracy in Rietveld quantitative phase analysis: a comparative study of strictly monochromatic Mo and Cu radiations

This work focuses on the comparison of Mo Kα1 and Cu Kα1 radiations for the Rietveld quantitative phase analysis of challenging crystalline mixtures including amorphous content determination. The quantifications have been carried out by using calibration curves with increasing amounts of a given phase. Mo Kα1 patterns were found to yield slightly more accurate analyses than those derived from Cu Kα1 radiation.

Rietveld quantitative phase analysis with molybdenum radiation

Building materials are very complex samples of worldwide importance; hence quantitative knowledge of their mineralogical composition is necessary to predict performances. Rietveld quantitative phase analysis (RQPA) allows a direct measurement of the crystalline phase contents of cements. We highlight in this paper the use of laboratory X-ray powder diffraction (LXRPD) employing high-energy radiation, molybdenum (Mo), for attaining the RQPA of cements. Firstly, we evaluate the accuracy of RQPA employing a commercial calcium sulfoaluminate clinker with gypsum. In addition to Mo Kα 1 and Mo Kα 1,2 radiations, Cu and synchrotron patterns are also analyzed for the sake of comparison. Secondly, the assessment of the accuracy of RQPA results obtained using different radiations (synchrotron, Mo, and Cu) and geometries (reflection and transmission) is performed by analyzing two well-known commercial samples. As expected, for LXRPD data, accuracy in the RQPA results improves as the irradiated volume increases. Finally, three very complex aged hydrated cements have been analyzed using Mo K α 1 -LXRPD and Synchrotron-XRPD. The main overall outcome of this work is the benefit for RQPA of using strictly monochromatic Mo Kα 1 radiation. Best laboratory results arise from Mo Kα 1 data as the effective tested volume is much increased but peak overlapping is not swelled.

Powder diffraction analysis of gemstone inclusions

Gemstones are pieces of materials that once cut and polished are used as jewels or adornments. Gemstones may be single crystal such as diamonds , polycrystalline such as lapis lazuli , or amorphous such as amber . In any case, gems may have inclusions that may yield a variety of optic effects. It is also important to unravel the crystal structure of the inclusion s in order to determine the origin of the gem and to help to understand their formation mechanism. Here, we expand the use of powder diffraction to identify crystalline inclusions in bulk gemstones highlighting Mo K radiation to penetrate within compact gems. Initially, rock crystal quartz with rutile needles was investigated and rutile diffraction peaks were more conspicuous in the Mo pattern than in the Cu pattern. Next, rock crystal quartz with beetle legs was characterized and the red iron oxide inclusion was identified as hematite. The study of a fake gem, glass showing aventurine effect, gave the diffraction peaks of metallic copper. Later, polycrystalline gems, moss agate, and aventurine quartz were also studied. The powder patterns of these compact gemstones could be successfully fitted using the Rietveld method. Finally, we discuss opportunities for further improvements in laboratory powder diffraction to characterize inclusions in compact gems.

A Self‐Folding Polymer Film Based on Swelling Metal–Organic Frameworks

Herein, we exploit the well-known swelling behaviour of metal-organic frameworks (MOFs) to create a self-folding polymer film. Namely, we show that incorporating crystals of the flexible MOF MIL-88A into a polyvinylidene difluoride (PVDF) matrix affords a polymer composite film that undergoes reversible shape transformations upon exposure to polar solvents and vapours. Since the self-folding properties of this film correlate directly with the swelling properties of the MIL-88A crystals, it selectively bends to certain solvents and its degree of folding can be controlled by controlling the relative humidity. Moreover, it shows a shape-memory effect at relative humidity values from 60 % to 90 %. As proof of concept, we demonstrate that these composite films can lift cargo and can be used to assemble 3D structures from 2D patterns. Our strategy is a straightforward method for designing autonomous soft materials with folding properties that can be tuned by judicious choice of the constituent flexible MOF.