Sebastian Bruque

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.

Rietveld quantitative amorphous content analysis

A procedure for Rietveld quantitative amorphous content analysis (RQACA) is outlined, in which the effects of systematic errors in the powder patterns are studied. The method derives the amorphous content from the small overestimation of an internal crystalline standard in a Rietveld refinement of an appropriate mixture. Of several standards studied, Al2O3 gave the best results. The statistical analysis of standard mixtures with a known amount of amorphous content indicated that this is a precise and accurate tool. It enables the measurement of the amorphous content with an accuracy close to 1%. Sample preparation and Rietveld analysis need to be optimized in order to minimize the systematic errors. The analysis of samples with phases displaying strong preferred orientation effects gives very high errors in the amorphous content. Samples with different absorption coefficients have also been studied in order to evaluate the importance of microabsorption. This plays an important role but it can be adequately corrected if the absorption coefficients of the standard and the sample are not very different. RQACA has been applied to tricalcium silicate, C3S, which is the main component of Portland cement. The average amorphous content of C3S, after microabsorption correction using two standards of higher and lower absorption coefficients, was found to be 19%.

The superstructure of C3S from synchrotron and neutron powder diffraction and its role in quantitative phase analyses

We have synthesised the room temperature MIII form of alite stabilised by doping with Mg and Al. The complex disordered superstructure of this tricalcium silicate [Ca3SiO5 (C3S)] sample has been studied by a joint Rietveld refinement of ultra-high-resolution synchrotron X-ray powder diffraction data, medium-resolution neutron powder diffraction data and soft constraints of interatomic distances. Alite crystallises in a monoclinic cell with dimensions a=33.1078(6) A u , b=7.0355(1) A u , c=18.5211(4) A u , b=94.137(1)� and V=4302.9(2) Au 3 . The final R factors were RWP=8.76% and RF(C3S)=3.45% for the synchrotron data and RWP=6.09% and RF(C3S)=5.10% for the neutron data. The reported superstructure is simpler than those previously reported, and it fits properly to a variety of Portland clinker and cement patterns. The Rietveld analyses of four clinkers with variable Mg contents have shown that the refinements are good. The Bogue approach gave quite poor results when compared to these state-of-the-art powder diffraction analyses. Bogue method slightly underestimates the C3S+C2S content, overestimates the C3A fraction and underestimates the C4AF content. Similar analyses of Portland cements with nine crystalline phases are shown to be feasible. D 2002 Elsevier Science Ltd. All rights reserved.

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.

New lead triphosphonates: synthesis, properties and crystal structures

Two new lead nitrilotris(methylene)triphosphonates, Pb[(H 2 O 3 PCH 2 )N(CH 2 PO 3 H) 2 ] I and Pb 2 [(O 3 PCH 2 )N(CH 2 PO 3 H) 2 ]·H 2 O II, have been synthesised. I is triclinic, space group P1, a=8.5077(2), b=11.2363(3), c=5.9484(2) A, α=98.802(2), β=104.955(1), γ=80.911(2)°, Z=2. II is monoclinic, space group Pn, a=7.3614(2), b=11.3889(2), c=7.2541(2) A, β=100.389(2)° and Z=2. Their structures have been solved from laboratory X-ray data by using ab initio powder diffraction methodology. The reliability factors were R wP =9.28%, R P =7.20% and R F =2.67%, for I, and R wP =12.51%, R P =9.38% and R F =4.45%, for II. I shows a new layered structure which has small cavities, inside an inorganic layer, in which the hydrogen phosphonate and the dihydrogen phosphonate groups are located. II is also layered with the water molecule situated in the interlamellar space. Thermal data, IR data and ion-exchange properties are also reported and discussed. II dehydrates to yield orthorhombic Pb 2 [(O 3 PCH 2 )N(CH 2 PO 3 H) 2 ] III. Hydrolysis of I leads to II.

Structural complexity and metal coordination flexibility in two acetophosphonates

Two divalent metal acetophosphonates, Pb 6 (O 3 PCH 2 CO 2 ) 4 and Mn 3 (O 3 PCH 2 CO 2 ) 2 , have been synthesised hydrothermally. They crystallise in the triclinic system, space group P, a=11.0064(1), b=12.3604(1), c=8.9783(1) A, α=98.632(1), β=90.474(1), γ=75.629(1)°, Z=2, for M=Pb, and a=10.0146(5), b=6.3942(4), c=8.4796(6) A, α=101.452(4), β=106.254(2), γ=96.431(4)°, Z=2, for M=Mn. The structures were solved ab initio using direct methods from synchrotron powder diffraction data (λ0.4 A) for M=Pb and from laboratory-ray data for M=Mn. The crystal structure of the Pb compound is very complex with 38 non-hydrogen atoms in general positions (114 refined positional parameters), it had been refined by Rietveld method using soft constraints, and converged to R WP =6.8% and R F =1.6%. The structure for M=Mn has a moderate complexity with 19 non-hydrogen atoms (57 refined positional parameters) which was also refined with soft constraints to R WP =8.3%,R F =3.9%. Both compounds show a framework built of alternate metal oxide inorganic layers, pillared by the organic groups. The metal environments in these materials are very distorted. Manganese atoms present three different distorted oxygen environments: four-, five- and six-coordinate. Thermal and IR data are also reported and discussed.

NASICON to scandium wolframate transition in Li1+xMxHf2−x(PO4)3 (M=Cr, Fe): structure and ionic conductivity

a ´´ ´ ´ ´ ´ Abstract The Li M Hf (PO ) (M5 Cr, Fe, Bi) systems have been studied and single phases have been isolated for M 5 Cr 11 xx 22x 43 and Fe. The samples have been characterized by X-ray powder diffraction, diffuse reflectance and impedance spectroscopy. There is a reconstructive transition between rombohedral NASICON and orthorhombic Sc (WO ) -type structures as a 24 3 function of x, at very low values, 0.2 and 0.1 for Cr and Fe, respectively. For the Cr series, a further subtle structural change has been observed for x values higher than 1.7. These phases have the Sc (WO ) -type framework, but the symmetry is 24 3 orthorhombic Pcnb at low values of x and monoclinic P2 / n at high values. The structural changes are discussed on the basis 1 of the sizes of the cavities left by the two frameworks and the lithium order / disorder in these voids. These materials are ionic conductors and their electrical behaviours are also discussed.

Structure and Electrons in Mayenite Electrides

One major goal in materials chemistry is to find inexpensive compounds with improved capabilities. Stable inorganic electrides, derived from nanoporous mayenite [Ca12Al14O32]O, are a new family that has very interesting properties such as electronic conductivity combined with transparency. However, an intriguing fundamental problem is to understand the structures of these cubic materials and to characterize their free-electron loadings. Here we report an accurate structural study for three members of the series [Ca12Al14O32]O(1-delta)e(2delta) (delta = 0, 0.15, and 0.45), from single-crystal low-temperature synchrotron X-ray diffraction. The complex structural disorder imposed by the presence of the oxide anions into the mayenite cages has been unravelled. Furthermore, the final electron density map for delta = 0.45 black mayenite has shown electron density localized into the center of the cages, which is the first experimental proof of their electride nature. The reported structural findings challenge theorists to improve predictive models in this new family of materials.

Layered acid arsenates α-M(HAsO4)2·H2O (M=Ti, Sn, Pb): synthesis optimization and crystal structures

Abstract The syntheses of α-M(HAsO 4 ) 2 ·H 2 O (M=Ti, Sn and Pb) have been optimized to prepare crystalline single phase materials. The crystal structures of M=Sn, Pb have been refined using X-ray powder diffraction data by the Rietveld method. A combined X-ray and neutron powder data refinement for M=Ti has allowed to obtain a very detailed picture of the structure including the hydrogen-bonding network. The results have been compared with those of the phosphates analogs. In addition, detailed characterization of the samples has been carried out by IR spectroscopy, thermal analysis and powder thermodiffractometry.

Effective Correction of Peak Asymmetry: Rietveld Refinement of High‐Resolution Synchrotron Powder Data of Li1.8(Hf1.2Fe0.8)(PO4)3

Rietveld refinement of powder diffraction data is a very popular and powerful technique. Several effects in the diffraction patterns, such as anisotropic peak broadening, preferred orientation or extinction, are properly taken into account; however, the asymmetry due to axial divergence remains problematic. Recently, a seminal paper by Finger, Cox & Jephcoat [J. Appl. Cryst. (1994), 27, 892–900] proved that this effect can be treated with parameters related to the diffractometer optic. Several refinements of asymmetric profiles obtained with synchrotron and laboratory X-ray and neutron radiation are reported. Furthermore, the refinement of the structure of Li1.8(Hf1.2Fe0.8)(PO4)3 with synchrotron data (λ ≃ 0.4 A) taken on the worlds highest resolution diffractometer (at BM16, ESRF) is presented. The fit of the pattern which has very asymmetric peaks is excellent, as indicated by the low value of Rwp = 8.1% and the very flat difference curve. It can be said that asymmetry due to axial divergence is no longer a problem.