Alessandro F. Gualtieri

Accuracy of XRPD QPA using the combined Rietveld–RIR method

QPA (quantitative phase analysis) of polycrystalline materials using XRPD (X-ray powder diffraction) can be performed using the combined Rietveld and reference intensity ratio (RIR) methods, providing an estimate of both the crystalline and the amorphous phase in a mixture. Although the accuracy of estimates of the phase composition of simple ternary or quaternary mixtures is generally very good, uncertainties remain and bias the accuracy of phase determinations in complex systems such as pyroclastic rocks, which may contain two or more zeolite species, a number of other phases (generally up to four or five), and glass. The contribution of the incoherent scattering, biasing accuracy, in the glass determination has been discussed in an earlier work on the modal analysis of pyroclastic rocks [Gualtieri (1996). Powder Diffr. 11, 97–106]. In this work, the assumption of the crystal structure, the influence of the spike addition, background and atomic (thermal) displacement parameters are discussed. It is shown that the structure-model assumption no longer holds for systems containing complex phases such as zeolites, as the accuracy is degraded and, as far as the influence of the added spike is concerned, there is an underestimation of the glass content with increasing amounts of added spike.

In situ study of the goethite-hematite phase transformation by real time synchrotron powder diffraction

Goethite (010), (110), and (111) growth faces and (010) cleavage surfaces of large, natural single crystals, as well as a high surface area synthetic sample were characterized using various surface sensitive microscopies and spectroscopies. Differential interference contrast and atomic force microscopy characterization of the natural single crystal faces showed microtopography indicative of growth, dissolution, and cleavage. Low energy electron diffraction patterns of the goethite (010) surface exhibit sharp, intense diffraction spots, indicating long-range order on this important surface. These patterns have two-dimensional point group symmetry 2mm, consistent with an undistorted surface structure and unitcell parameters a = 4.62 ± 0.14 Å and c = 2.99 ± 0.08 Å. These parameters equal the equivalent bulk cell dimensions given the uncertainties. Ultra-high vacuum scanning tunneling microscopy was performed on (010) cleavage faces, although the tunneling properties of the surface were very heterogeneous. Atomic resolution was not obtained; however, microtopographic images are identical to those collected with AFM. XPS spectra from the (010) faces of two natural samples as well as the synthetic powder all have peak maxima for Fe (2p3/2) at 711.5 ± 0.1 eV. The O(1s) line originating from the goethite can be fit with two peaks with a chemical shift of 1.3 eV. The peak at higher binding energy (531.3 eV ± 0.1 eV) represents the protonated oxygen in the structure, and the peak at lower binding energy (530.0 eV ± 0.1 eV) represents the proton-free oxygen in the structure. Ab initio and semi-empirical models of the (010) surface suggest that cleavage occurs through the hydroxide plane at 1/4 b in the structure. This is contrary to cleavage through the oxide plane at 1/2 b, which has been assumed in several previous studies.

The nature of disorder in montmorillonite by simulation of X-ray powder patterns

Abstract The planar disorder of Ca-montmorillonite (Fuller’s earth) has been investigated using structural simulations of X-ray powder patterns. A standard sample was fully characterized using chemical, microscopic, and diffraction methods. Earlier models of disorder taken from the literature and newly formulated combined models were used to generate simulated powder patterns to be compared with the experimental spectrum. A new model of disorder with random shifts of -a/3 and ±b/3, with a total density of defects of 75%, gives the best fit to the observed data. Thus, the sample cannot be classified as a turbostratic structure (fully disordered) and consequently turbostratic disorder does not invariably apply to all smectite samples. These findings open a debate on the nature and application of turbostratic disorder: is it possible for smectite samples to have intermediate degrees of disorder between a fully disordered stacking (turbostratic) and a highly faulted but well-defined stacking or is the result obtained for the Ca-montmorillonite just an exception? This model of disorder is useful for the quantitative phase analysis by X-ray powder diffraction based on the Rietveld method, which can now benefit from a more reliable initial structure model for Ca-montmorillonite and which will improve the accuracy of the weight-fraction estimates

Kinetic study of the kaolinite-mullite reaction sequence. Part I: Kaolinite dehydroxylation

The decomposition reaction of kaolinite has been investigated as a function of the defectivity of the starting material and the temperature of reaction. Time resolved energy-dispersive powder diffraction patterns have been measured using synchrotron radiation, both under a constant heating rate (heating rates from 10 to 100° C/min) and in isothermal conditions (in the temperature range 500 to 700° C). The apparent activation energy of the dehydroxylation process is different for kaolinites exhibiting a different degree of stacking fault density. The results of the analysis of the kinetic data indicate that the starting reaction mechanism is controlled by diffusion in the kaolinite particle. The diffusion process is dependent on the defective nature of both kaolinite and metakaolinite. At high temperatures, and at higher heating rates, the reaction mechanism changes and the resistance in the boundary layer outside the crystallites becomes the rate-limiting factor, and nucleation begins within the reacting particle. During the final stage of the dehydroxylation process the reaction is limited by heat or mass transfer, and this might be interpreted by the limited diffusion between the unreacted kaolinite domains and the metakaolinite matrix.

Kinetic study of the kaolinite-mullite reaction sequence. Part II: Mullite formation

The present work is a follow-up of the investigation on the decomposition reaction of kaolinite as a function of the defectivity of the starting material and the temperature of reaction. In the present work we study the high temperature reaction of mullite synthesis from kaolinite, from the starting point of the results obtained in the first part.Time resolved energy-dispersive powder diffraction patterns have been measured using synchrotron radiation in isothermal conditions. The apparent activation energy for mullite nucleation and growth is found to be related to the defective structure of the starting kaolinite, which thus must have an influence on the chemical homogeneity of the amorphous intermediate phase.The analysis of the kinetic data indicate that the initial reaction mechanism is controlled by mullite nucleation, while as the reaction proceeds it shifts towards a grain growth-limited process which is intermediate between phase boundary and diffusion controlled. The order of the reaction obtained from standard analysis of the isothermal kinetic data is lower in the case of the ordered kaolinite KGa-1, in agreement with a rate limiting process more strongly limited by diffusion.For each sample there is a small but significant decrease in the order of the reaction at higher temperature: we interpret the change as related to the variation of the diffusion process in the amorphous phase due to the growing grains of mullite and cristobalite.The values of the activation energies and induction times are comparable neither to a model of mullite formation from a monophasic gel, nor mullite formation from a diphasic gel, being intermediate between the two. We can infer that the amorphous precursors from natural kaolinites can be considered pseudo-monophasic gel-like phases, approaching the monophasic gel-like behaviour as the defectivity of the initial kaolinite increases.

Multipurpose imaging-plate camera for in situ powder XRD at the GILDA beamline

An Imaging-Plate (IP) camera for X-ray powder diffraction (XRPD) experiments was installed on the synchrotron radiation beamline GILDA at the ESRF. The IP camera can be used in fixed data-collection mode of the whole diffraction rings, or in translation mode for time-dependent experiments. The apparatus is ideal for collecting medium to relatively high-resolution diffraction data from diluted or weakly scattering samples and to investigate in situ phase changes induced by temperature and/or chemical reactions. The possibility to rapidly collect several good quality diffraction patterns coupled with tunable beam energy allow for multiwavelength experiments such as anomalous XRPD.

Simultaneous refinement of structure and microstructure of layered materials

The recursive description of stacking in layered crystals, originally developed by Treacy et al. [Proc. R. Soc. London Ser. A (1991), 433, 499–520] and implemented in the DIFFaX code, is enclosed in a non-linear least-squares minimization routine and combined with additional models (of specimen-related broadening and instrumental broadening) to allow the simultaneous refinement of both structural and microstructural parameters of a layered crystal. This implementation is named DIFFaX+. As examples, the refinements both of a simulated pattern of diamond, showing fault clustering, and of the observed powder pattern of a synthetic stoichiometric nanocrystalline chrysotile are reported.

Dehydration dynamics of analcime by in situ synchrotron powder diffraction

Abstract The continuous structural transformation of tetragonal analcime (Na15.87Al15.20Si32.64O96·16.3H2O) upon dehydration was studied, using Rietveld structure analysis of temperature- resolved powder diffraction data collected using synchrotron radiation. The variation of the a-c axis length difference and normalized intensity of the (200) reflection as a function of temperature suggest that tetragonal analcime evolves toward a cubic structure at high temperature. The removal of water was accompanied by a spreading of the initial Na sites into many positions bonded to the framework O atoms. The migration of H2O molecules through the [111] channels during dehydration caused the six-member ring apertures to open as widely as possible: this was accompanied by a twisting of the tetragonal prism, constituting the analcime framework, which led to an opposite tilting of tetrahedra connecting the prisms. These modifications induced by water diffusion are not energetically favored because they would increase the elastic energy of the system, and require a substantial thermal activation energy. The analcime framework reached a maximum distortion at about 650 K, the temperature of complete water loss, then underwent a relaxation process during which the T-O-T angles were restored to the starting value. The relative variation of cell volume associated with the opening of wide six-member ring channels during water migration, and then due to the framework relaxation process after complete dehydration, provides an explanation of the ‘‘negative thermal expansion’’ (i.e., volume contraction) effect in dehydrated analcime, which is complementary to that based on the Rigid Unit Modes theory.

Dehydration dynamics of stilbite using synchrotron X-ray powder diffraction

Abstract The continuous structural transformation of the natural zeolite stilbite (Na3.62K0.44Ba0.03Ca6.32Sr0.28Mg0.04[Fe3+001Al17.33Si54.64O144] · 60H2O) upon dehydration has been studied using Rietveld structure analysis of temperature-resolved powder diffraction data collected with synchrotron radiation. In the initial stage of heating, the monoclinic F2/m stilbite structure (the so-called A phase) behaves as a noncollapsible framework, featuring only a slight framework distortion and a slight cell-volume contraction. At about 420 K, a first-order phase transition occurs changing the symmetry to an orthorhombic Amma phase, whose framework is collapsible and shows a large cell-volume contraction with temperature. The cell contraction is related to the process of T-O-T bond breaking and leads to a high-temperature stilbite phase with the same Amma space group and a collapsed structure similar to the previously described B phase in stellerite and barrerite. The structural refinement indicates that the dynamics of bond breaking is related to the shift of the Ca cations in the channels to achieve optimal coordination after the release of the H2O molecules. Refined statistical occupancies of the tetrahedral atoms involved in the bondbreaking process (T1 and T1P) are consistent with a random rupture and re-formation of the T-O-T bonds. This is the first experimental study of the dynamic bond breaking of T-O-T bonds in a framework structure.

Nature of structural disorder in natural kaolinites : a new model based on computer simulation of powder diffraction data and electrostatic energy calculation

A new model for the description of the structural disorder in natural kaolinite materials is proposed, based on the stacking of two 1:1 layers and their enantiomorphs, and encompassing previously proposed models. The layers, where randomly stacked along the c axis (using probabilistic functions nested in recursive algorithms), correctly describe the observed powder diffraction patterns of natural kaolinites with any density of structural faults. The proposed model was evaluated using electrostatic energy calculations against earlier models of disorder based on layer shift, layer rotation, statistical occupancy of the Al octahedra, or enantiomorphic layers. The present 4-layer model has a minimum of potential energy with respect to the previous models. As expected, the fully ordered triclinic structure of kaolinite possesses the absolute minimum of potential energy.