X-Ray powder diffraction laboratory @ University of Málaga (UMA)

X-Ray Powder Diffraction (XRPD) laboratory is a facility placed at Servicios Centrales de apoyo a la Investigación (SCAI) at University of Malaga (UMA) http://www.scai.uma.es/. This facility has three XRPD diffractometers and a diffractometer to measure high-resolution thin-films. X´Pert PRO MPD from PANalytical. This is a bragg-brentano (theta/2theta) with reflection geometry diffractometer which allows to obtain high resolution XRPD data with strictly monochromatic CuKα1 radiation (λ=1.54059Å) [Ge(111) primary monochromator] and an automatic sample charger. Moreover, it has parallel monochromatic CuKα1 radiation (λ=1.54059Å) with an hybrid Ge(220) monochromator for capillary and multiproposal (bulk samples up to 1 Kg) sample holders. The HTK1200N chamber from Anton Paar allows collecting high resolution high temperature patterns. EMPYREAN from PANalytical. This diffractometer works in reflection and transmission geometries with theta/theta goniometer, using CuKα1,2 radiation (λ=1.5418Å), a focusing X-ray mirror and a ultra-fast PIXCEL 3D detector with 1D and 2D collection data modes (microstructural and preferred orientation analysis). Moreover, the TTK450N chamber allows low temperature and up to 450ºC studies. A D8 ADVANCE (BRUKER) was installed in April 2014. It is the first diffractometer in Europe equipped with a Johansson Ge(111) primary monochromator, which gives a strictly monochromatic Mo radiation (λ=0.7093 Å) [1]. It works in transmission mode (with a sample charger) with this high resolution configuration. XRPD data suitable for PDF (Pair Distribution Function) analysis can be collected with a capillary sample holder, due to the high energy and high resolution capabilities of this diffractometer. Moreover, it has a humidity chamber MHC-trans from Anton Paar working on transmission mode with MoKα1 (measurements can be collected at 5 to 95% of relative humidity (from 20 to 80 ºC) and up to 150ºC [2]). Furthermore, this diffractometer also has a reaction chamber XRK900 from Anton Paar (which uses CuKα1,2 radiation in reflection mode), which allows data collection from room temperature to 900ºC with up to 10 bar of different gases. Finally, a D8 DISVOVER A25 from BRUKER was installed on December 2014. It has a five axis Euler cradler and optics devices suitable for high resolution thin film data collection collected in in-plane and out-of-plane modes. To sum up, high-resolution thin films, microstructural, rocking-curve, Small Angle X-ray Scattering (SAXS), Grazing incident SAXS (GISAXS), Ultra Grazing incident diffraction (Ultra-GID) and microdiffraction measurements can be performed with the appropriated optics and sample holders. [1] L. León-Reina, M. García-Maté, G. Álvarez-Pinazo, I. Santacruz, O. Vallcorba, A.G. De la Torre, M.A.G. Aranda “Accuracy in Rietveld quantitative phase analysis: a comparative study of strictly monochromatic Mo and Cu radiations” J. Appl. Crystallogr. 2016, 49, 722-735. [2] J. Aríñez-Soriano, J. Albalad, C. Vila-Parrondo, J. Pérez-Carvajal, S. Rodríguez-Hermida, A. Cabeza, F. Busqué, J. Juanhuix, I. Imaz, Daniel Maspoch “Single-crystal and humidity-controlled powder diffraction study of the breathing effect in a metal-organic framework upon water adsorption/desorption” Chem. Commun., 2016, DOI: 10.1039/C6CC02908F.

In-situ Molibdenum X-ray powder diffraction study of the early hydration of cementitious systems on a humidity chamber

The durability of cement-based construction materials depends on the environmental conditions during their service life. A further factor is the microstructure of the cement bulk, established by formation of cement hydrates. The development of the phases and microstructure under given conditions is responsible of the high strength of cementitious materials. The investigation on the early hydration behavior of cements and cementing systems has been for a long time a very important area of research: understanding the chemical reactions that lead to hardening is fundamental for the prediction of performances and durability of the materials. The production of 1 ton of Ordinary Portland Cement, OPC, releases into the atmosphere ~0.97 tons of CO2. This implies that the overall CO2 emissions from the cement industry are 6% of all anthropogenic carbon dioxide. An alternative to reduce the CO2 footprint consists on the development of eco-cements composed by less calcite demanding phases, such as belite and ye'elimite. That is the case of Belite-Ye’elimite cements (BY). Since the reactivity of belite is not quick enough, these materials develop low mechanical strengths at intermediate hydration ages. A possible solution to this problem goes through the production of cements which jointly contain alite with the two previously mentioned phases, named as Belite-Alite-Ye’elimite (BAY) cements. The reaction of alite and ye'elimite with water will develop cements with high mechanical strengths at early ages, while belite will contribute to later values. The final goal is to understand the hydration mechanisms of a variety of cementing systems (OPC, BAY and pure phases) as a function of water content, superplasticizer additives and type and content of sulfate source. In order to do so, in-situ laboratory humidity chambers with Molybdenum X-ray Powder diffraction are employed. In the first 2h of hydration, reaction degree (α) of ye'elimite had been decreased for superplasticizer.

Applications of synchrotron x-ray powder diffraction in hydrated cements: high-resolution and high-pressure studies

The main aim of this study is to apply synchrotron radiation techniques for the study of hydrated cement pastes. In particular, the tetracalcium aluminoferrite phase, C4AF in cement nomenclature, is the major iron-containing phase in Ordinary Portland Cement (OPC) and in iron rich belite calcium sulfoaluminate cements. In a first study, the hydration mechanism of pure tetracalcium aluminoferrite phase with water-to-solid ratio of 1.0 has been investigated by HR-SXRPD (high resolution synchrotron X-ray powder diffraction). C4AF in the presence of water hydrates to form mainly an iron-containing hydrogarnet-type (katoite) phase, C3A0.84F0.16H6, as single crystalline phase. Its crystal structure and stoichiometry were determined by the Rietveld method and the final disagreement factors were RWP=8.1% and RF=4.8% [1]. As the iron content in the product is lower than that in C4AF, it is assumed that part of the iron also goes to an amorphous iron rich gel, like the hydrated alumina-type gel, as hydration proceeds. Further results from the high-resolution study will be discussed. In a second study, the behavior of pure and iron-containing katoites (C3AH6 and C3A0.84F0.16H6) under pressure have been analyzed by SXRPD using a diamond anvil cell (DAC) and then their bulk moduli were determined. The role of the pressure transmitting medium (PTM) has also been studied. In this case, silicone oil as well as methanol/ethanol mixtures have been used as PTM. Some “new peaks” were detected in the pattern for C3A0.84F0.16H6 as pressure increases, when using ethanol/methanol as PTM. These new peaks were still present at ambient pressure after releasing the applied pressure. They may correspond to crystalline nordstrandite or doyleite from the crystallization of amorphous aluminium hydroxide. The results from the high-pressure study will also be discussed.

Location of hydrogen atoms in hydronium jarosite

Various models for the crystal structure of hydronium jarosite were determined from Rietveld refinements against neutron powder diffraction patterns collected at ambient temperature and also single-crystal X-ray diffraction data. The possibility of a lower symmetry space group for hydronium jarosite that has been suggested by the literature was investigated. It was found the space group is best described as R3¯m, the same for other jarosite minerals. The hydronium oxygen atom was found to occupy the 3¯m site (3a Wyckoff site). Inadequately refined hydronium bond angles and bond distances without the use of restraints are due to thermal motion and disorder of the hydronium hydrogen atoms across numerous orientations. However, the acquired data do not permit a precise determination of these orientations; the main feature up/down disorder of hydronium is clear. Thus, the highest symmetry model with the least disorder necessary to explain all data was chosen: The hydronium hydrogen atoms were modeled to occupy an m (18 h Wyckoff site) with 50 % fractional occupancy, leading to disorder across two orientations. A rigid body description of the hydronium ion rotated by 60° with H–O–H bond angles of 112° and O–H distances of 0.96 Å was optimal. This rigid body refinement suggests that hydrogen bonds between hydronium hydrogen atoms and basal sulfate oxygen atoms are not predominant. Instead, hydrogen bonds are formed between hydronium hydrogen atoms and hydroxyl oxygen atoms. The structure of hydronium alunite is expected to be similar given that alunite supergroup minerals are isostructural.

Magneto-structural study of a Cr-doped CaRuO3

Temperature-dependent neutron powder diffraction, magnetization and XPS studies were carried out on an optimally Cr-doped CaRuO3, i.e. CaRu0.85Cr0.15O3 (CRC-15). XPS data revealed that Cr exist in 3+ and 6+ oxidation states. The charge dissociation preserves the overall 4+ nominal charge of the Ru site. Although ferromagnetic correlations develop around 100 K, the system exhibits a large coercive field below 50 K. The unit cell volume exhibits negative thermal expansion below 50 K since the lattice expansion due to the magnetostrictive effect outweighs the thermal contraction due to the phonon-driven mechanism.

Degenerate rhombohedral and orthorhombic states in Ca-substituted Na0.5Bi0.5TiO3

Neutron powder diffraction and temperature dependent dielectric studies were carried out on Ca-substituted Na0.5Bi0.5TiO3, i.e., (Na0.5Bi0.5)(1-x)CaxTiO3. Stabilization of an orthorhombic phase even at a low Ca concentration (0.05 < x < 0.10) suggests that Na0.5Bi0.5TiO3 (NBT) is susceptible to orthorhombic distortion. The orthorhombic and rhombohedral phases coexist for x=0.10, suggesting these phases to be nearly degenerate. The orthorhombic distortion favoring tendency of Ca assists in promoting the inherent instability with regard to this structure in pure NBT, which was reported recently.

Phases in the (1-x)Na0.5Bi0.5TiO3-(x)CaTiO3 system

The structures of (1 - x) Na0.5Bi0.5TiO3-(x) CaTiO3 at room temperature have been investigated using neutron powder diffraction and dielectric studies. The system exhibits an orthorhombic (Pbnm) structure for x >= 0.15 and rhombohedral (R3c) for x <= 0.05. For x = 0.10, though the neutron diffraction pattern shows features of the orthorhombic (Pbnm) structure, Rietveld refinement using this structure shows a drastic reduction in the in-phase tilt angle (similar to 4 degrees) as compared to the corresponding value (similar to 8 degrees) for a neighbouring composition x = 0.15. The neutron diffraction pattern of x = 0.10 could be fitted equally well using a two-phase model (R3c + Pbnm) with orthorhombic as the minor phase (22%), without the need for a drastic decrease in the in-phase tilt angle. The dielectric studies of x = 0.10 revealed the presence of the polar R3c phase, thereby favouring the phase coexistence model, instead of a single-phase Pbnm structure, for this composition.

Spin-orbital ordering and mesoscopic phase separation in the double perovskite Ca2FeReO6

Neutron powder diffraction measurements on Ca2FeReO6 reveal that this double perovskite orders ferrimagnetically and shows anomalous lattice parameter behavior below T-C=521 K. Below similar to300 K and similar to160 K we observe that the high-T monoclinic crystal structure separates into two and three monoclinic phases, respectively. A magnetic field suppresses the additional phases at low T in favor of the highest-T phase. These manifestations of the orbital degree of freedom of Re 5d electrons indicate that these electrons are strongly correlated and the title compound is a Mott insulator, with competing spin-orbitally ordered states.

Complex structural phase transitions in slightly Ca modified Na0.5Bi0.5TiO3

A temperature dependent neutron powder diffraction study, in conjunction with dielectric and ferroelectric characterization, of slightly Ca modified Na0.5Bi0.5TiO3 (NBT) revealed an instability with regard to a non-polar orthorhombic (Pbnm) distortion above room temperature. This intermediate orthorhombic phase has earlier been reported for unmodified NBT by electron diffraction studies, but has never been captured by global (x-ray/neutron) diffraction techniques. Calcium substitution seems to amplify the magnitude of this intermediate orthorhombic distortion thereby making the corresponding superlattice reflections become visible in the neutron diffraction pattern. The study revealed the following sequence of very complex structural evolution with temperature: Cc -> Cc + Pbnm -> Pbnm + P4/mbm -> P4/mbm -> Pm (3) over barm.

Competing structural phase transition scenarios in the giant tetragonality ferroelectric BiFeO3-PbTiO3: Isostructural vs multiphase transition

A systematic x-ray and neutron powder diffraction study of the giant tetragonality multiferroic (1-x) BiFeO3-(x) PbTiO3 have revealed that the compositions close to the morphotropic phase boundary present two different structural phase transition scenarios on cooling from the cubic phase: (i) cubic -> tetragonal (T-2) + tetragonal (T-1) -> tetragonal (T-1) and (ii) cubic -> tetragonal (T-2) + tetragonal (T-1) + rhombohedral (R3c) -> tetragonal (T-1) + rhombohedral (R3c). The comparatively larger tetragonality (c/a - 1) of the T-1 phase as compared to the coexisting isostructural T-2 phase is shown to be a result of significantly greater degree of overlap of the Pb/Bi-6s and Ti/Fe-3d with the O-2p orbitals as compared to that in the T-2 phase. The formation/suppression of the minor metastable rhombohedral phase seems to be governed by subtle play of local kinetic factors. In the scenario when the minor rhombohedral (R) phase is formed along with the tetragonal phases it is able to accommodate the large transformation stress in the system due to formation of the tetragonal phases, and prevent the solid from disintegration into powder after sintering. When the metastable rhombohedral phase is not formed, the large transformation strain ruptures the grain boundaries leading to fragmentation of the dense solid to powder. (C) 2013 American Institute of Physics. http://dx.doi.org/10.1063/1.4792215]

Lead-free piezoelectric system (Na0.5Bi0.5)TiO3-BaTiO3: Equilibrium structures and irreversible structural transformations driven by electric field and mechanical impact

The structure-property correlation in the lead-free piezoelectric (1 - x)(Na0.5Bi0.5)TiO3-(x)BaTiO3 has been systematically investigated in detail as a function of composition (0 < x <= 0.11), temperature, electric field, and mechanical impact by Raman scattering, ferroelectric, piezoelectric measurement, x-ray, and neutron powder diffraction methods. Although x-ray diffraction study revealed three distinct composition ranges characterizing different structural features in the equilibrium state at room temperature: (i) monoclinic (Cc) + rhombohedral (R3c) for the precritical compositions, 0 <= x <= 0.05, (ii) cubiclike for 0.06 <= x <= 0.0675, and (iii) morphotropic phase boundary (MPB) like for 0.07 <= x < 0.10, Raman and neutron powder diffraction studies revealed identical symmetry for the cubiclike and the MPB compositions. The cubiclike structure undergoes irreversible phase separation by electric poling as well as by pure mechanical impact. This cubiclike phase exhibits relaxor ferroelectricity in its equilibrium state. The short coherence length (similar to 50A degrees) of the out-of-phase octahedral tilts does not allow the normal ferroelectric state to develop below the dipolar freezing temperature, forcing the system to remain in a dipolar glass state at room temperature. Electric poling helps the dipolar glass state to transform to a normal ferroelectric state with a concomitant enhancement in the correlation length of the out-of-phase octahedral tilt.

Polymorphic phase boundaries and enhanced piezoelectric response in extended composition range in the lead free ferroelectric BaTi1-xZrxO3

Neutron powder diffraction study of Ba(Ti1-xZrx)O-3 at close composition intervals has revealed coexistence of ferroelectric phases: orthorhombic (Amm2) + tetragonal (P4mm) for 0.02 <= x <= 0.05 and rhombohedral (R3m) + orthorhombic (Amm2) for 0.07 <= x < 0.09. These compositions exhibit relatively enhanced piezoelectric properties as compared to their single phase counterparts outside this composition region, confirming the polymorphic phase boundary nature of the phase coexistence regions. (C) 2013 AIP Publishing LLC.

Magnetic Structure and Properties of the Rechargeable Battery Insertion Compound Na2FePO4F

The magnetic structure and properties of sodium iron fluorophosphate Na2FePO4F (space group Pbcn), a cathode material for rechargeable batteries, were studied using magnetometry and neutron powder diffraction. The material, which can be described as a quasi-layered structure with zigzag Fe-octahedral chains, develops a long-range antiferromagnetic order below similar to 3.4 K. The magnetic structure is rationalized as a super-exchange-driven ferromagnetic ordering of chains running along the a-axis, coupled antiferromagnetically by super-super-exchange via phosphate groups along the c-axis, with ordering along the b-axis likely due to the contribution of dipole dipole interactions.