Von Dreele Research Articles

POWTEX visits POWGEN.

The high-intensity time-of-flight (TOF) neutron diffractometer POWTEX for powder and texture analysis is currently being built prior to operation in the eastern guide hall of the research reactor FRM II at Garching close to Munich, Germany. Because of the world-wide 3He crisis in 2009, the authors promptly initiated the development of 3He-free detector alternatives that are tailor-made for the requirements of large-area diffractometers. Herein is reported the 2017 enterprise to operate one mounting unit of the final POWTEX detector on the neutron powder diffractometer POWGEN at the Spallation Neutron Source located at Oak Ridge National Laboratory, USA. As a result, presented here are the first angular- and wavelength-dependent data from the POWTEX detector, unfortunately damaged by a 50g shock but still operating, as well as the efforts made both to characterize the transport damage and to successfully recalibrate the voxel positions in order to yield nonetheless reliable measurements. Also described is the current data reduction process using the PowderReduceP2D algorithm implemented in Mantid [Arnold et al. (2014). Nucl. Instrum. Methods Phys. Res. A, 764, 156-166]. The final part of the data treatment chain, namely a novel multi-dimensional refinement using a modified version of the GSAS-II software suite [Toby & Von Dreele (2013). J. Appl. Cryst.46, 544-549], is compared with a standard data treatment of the same event data conventionally reduced as TOF diffraction patterns and refined with the unmodified version of GSAS-II. This involves both determining the instrumental resolution parameters using POWGEN's powdered diamond standard sample and the refinement of a friendly-user sample, BaZn(NCN)2. Although each structural parameter on its own looks similar upon comparing the conventional (1D) and multi-dimensional (2D) treatments, also in terms of precision, a closer view shows small but possibly significant differences. For example, the somewhat suspicious proximity of the a and b lattice parameters of BaZn(NCN)2 crystallizing in Pbca as resulting from the 1D refinement (0.008 Å) is five times less pronounced in the 2D refinement (0.038 Å). Similar features are found when comparing bond lengths and bond angles, e.g. the two N-C-N units are less differently bent in the 1D results (173 and 175°) than in the 2D results (167 and 173°). The results are of importance not only for POWTEX but also for other neutron TOF diffractometers with large-area detectors, like POWGEN at the SNS or the future DREAM beamline at the European Spallation Source.

Atomic-resolution STEM-EDS studies of cation ordering in Ti-Nb oxide crystals

Ternary metal oxide compounds, such as Ti-Nb and Nb-W oxides, have renewed research interest in energy storage materials because these oxides contain multivalent metal ions that may be able to control the ion transport in solid lithium batteries. One of these oxides is Ti2Nb10O29, which is composed of metal–oxygen octahedra connected through corner-sharing and edge-sharing to form “block structures”. In the early 1970s Von Dreele and Cheetham proposed a metal-atoms ordering in this oxide crystal using Rietveld refined neutron powder diffraction method. Most recent studies on these oxides, however, have not considered cation ordering in evaluating the battery electrode materials. In this paper, by utilizing the latest scanning transmission electron microscopy combined with energy dispersive X-ray spectroscopy imaging technology, the cation chemical ordering in those oxide crystals was directly revealed at atomic resolution.

Rietveld in 100 picoseconds.

A recent article by Von Dreele, Clarke & Walsh [J. Appl. Cryst. (2021), 54, https://doi.org/10.1107/S1600576720014624] introduces an entirely new paradigm in structure determination, where a complete structural measurement is made in a tenth of a nanosecond.

Phase and structural features of tubular halloysite (7 Å)

ABSTRACTDespite many reviews and original articles, the actual crystal structure of tubular halloysites remains unclear. Analysis of the structural features of defect-free kaolinite, refined by Bish & von Dreele (1989), shows that the ordered 1Tc kaolinite structure can be described equally well by the orthogonal layer cell {a0, b0, γ0} (γ0 = 90°) or by two enantiomorphic oblique layer cells {a1, b1, γ1} and {a2, b2, γ2}, related to each other by a mirror plane. To simulate diffraction effects for tubular halloysite, the parameters and atomic coordinates of the orthogonal layer unit cell and the layer-displacement vectors t1 and t2 responsible for formation of the kaolinite enantiomorphs were deduced by transformation of the parameters of the defect-free kaolinite refined by Bish & von Dreele (1989). Modelling X-ray diffraction patterns show that the samples consist of either single, two or three phases, with the number and their structural features depending on the morphology of the particles. Samples formed of prismatic particles consist of halloysite-like structure (HLS), kaolinite-like structure (KLS) and halloysite cylindrical structure (HCS) phases occurring in various proportions. Samples of proper cylindrical tubes consist of a single HCS phase, whilst samples formed by particles having morphologies intermediate between proper cylindrical and well-developed prismatic forms consist of the KLS and HCS phases. The KLS phase is comparable to low-ordered platy kaolinite with identical unit-cell parameters, layer-displacement vectors and arbitrary stacking faults, except that the layer displacements are not random as in kaolinite, but are distributed at R = 1 such that t1 and t2 displacements have a strong tendency to be segregated. Structural parameters describing the HLS and KLS phases are identical, but in the HLS phase there is a strong tendency to the regular alternation of the t1 and t2 displacements, and the HLS phases do not contain arbitrary stacking faults. A characteristic feature of the three-phase prismatic samples is that the stacking of the layers along the c* axis is periodic and the layer thicknesses are similar to those of platy kaolinite. In contrast, in the KLS phase formed in samples with particles of intermediate morphologies, the hydrated 10 and 7.25 Å layers are interstratified. The relationship between the structural and morphological features of the coexisting phases suggests a sequence of phase formation from the centre to the surface of halloysite tubes that progresses from the HCS to the HLS via the KLS phase. The results of this study demonstrate that all kaolinite and halloysite (7 Å) varieties are built by the same fundamental structural units.

Crystallographic Analysis of Structural Changes in Lithium Nickel Manganese Cobalt Oxides at Different Stages of Lithiation

Mixed transition metal oxides are among the most promising cathode materials for lithium ion batteries. The high specific capacity, low cost and thermal stability makes Li(Ni1/3Mn1/3Co1/3)O2, so-called NMC, an excellent material for power tool applications. Therefore, main leading battery manufacturer focus on the combination of these three transition metals to combine the strengths of each of them [1]. The development and design of battery materials require deep understanding of the relations between chemistry, structure and its properties. Nowadays, the commercialized NMC can only reach around 60 % of its theoretical capacity when cycled up to 4.2 V. Higher specific capacities might be reached when the upper voltage limit is raised up to 4.5 V [2], but the lifetime of the cathode then decreases drastically. The deintercalation of more than 0.6 moles of lithium in the charge process of the battery leads to irreversible capacity losses. Lithium nickel manganese cobalt oxides with different transition metal ratios were successfully synthesized via coprecipitation route. The cathode materials were characterized physico- and electrochemically and the composition was confirmed via inductive coupled plasma atomic emission spectroscopy (ICP-AES). Structural changes of non-symmetric NMC compounds at different stages of lithiation were recorded using x-ray diffraction. The resulting patterns were analyzed and the structures refined using the Rietveld method implemented in the GSAS (General Structure Analysis System) package [3]. The same procedure of crystallographic characterization of the NMC structure was carried out for different transition metal ratios cycled up to 4.5 V. The results are presented and comparison between symmetric and non-symmetric lithium nickel manganese cobalt oxides will give better insight into structural changes at higher voltages and the relation to their compositions. Acknowledgement This work was financially supported by the Austrian Federal Ministry for Transport, Innovation and Technology (bmvit) and the Austrian Research Promotion Agency (FFG). [1] N. Yabuuchi, T. Ohzuku, Journal of Power Sources 119-121 (2003): 171-174 [2] Q. Yu, Z. Chen, L. Xing, D. Chen, H. Rong, Q. Liu,W. Li, Electrochimica Acta 176 (2015): 919-925 [3] A. C. Larson, R. B. Von Dreele, Los Alamos National Laboratory Report, No. LAUR-86-748, (1987)

Accuracy in quantitative phase analysis of mixtures with large amorphous contents. The case of zircon-rich sanitary-ware glazes

The accuracy of quantitative phase analysis (QPA) of samples with dominant amorphous content, reproducing zircon-rich sanitary-ware glazes, has been investigated. X-ray powder diffraction (XRPD) methods were applied using both conventional Cu Kα radiation and high-resolution synchrotron data. In this work, a combination of the reference intensity ratio (RIR) and Rietveld methods was applied to an artificial mixture (90 wt% glass, 10 wt% zircon), taking into account some of the most common effects that may affect the accuracy in amorphous quantification, such as the degree of crystallinity of the phases, microabsorption and sample preparation. Certified NIST SRM 676a (α-Al2O3) [Cline, Von Dreele, Winburn, Stephens & Filliben (2011).Acta Cryst.A67, 357–367] was used to quantify the amorphous content in zircon and in the different internal standards commonly used when a certified standard is not available or not applicable: the results show that all of the phases invariably contain amorphous material in the range 2.0–15.0 wt%. If the amorphous content of the standard is taken into account, the accuracy of the QPA of the artificial mixture is improved. It was observed that the Brindley correction for microabsorption does not significantly improve the results. Care must be applied if grinding time is increased, since this may increase the amorphous content in the sample. Finally, the sensitivity of the RIR–Rietveld method to the addition of a small amount of zircon (∼1 wt%) has been considered, showing that accurate results can be achieved if great care is taken in the sample preparation and refinement strategy.

Macromolecular powder diffraction: Structure solution via molecular replacement

Macromolecular powder diffraction is a burgeoning technique for protein structure solution - ideally suited for cases where no suitable single crystals are available. Over the past seven years, pioneering work by Von Dreele et al. [1,2] and Margiolaki et al. [3,4] has demonstrated the viability of this approach for several protein structures. Among these initial powder studies, molecular replacement solutions of insulin and turkey lysozyme into alternate space groups were accomplished. Pressing the technique further, Margiolaki et al. [5] executed the first molecular replacement of an unknown protein structure: the SH3 domain of ponsin, using data from a multianalyzer diffractometer. To demonstrate that cross-species molecular replacement using image plate data is also possible, we present the solution of hen egg white lysozyme using the 60% identical human lysozyme (PDB code: 1LZ1) as the search model. Due to the high incidence of overlaps in powder patterns, especially in more complex structures, we have used extracted intensities from five data sets taken at different salt concentrations in a multi-pattern Pawley refinement. The use of image plates severely increases the overlap problem due to lower detector resolution, but radiation damage effects are minimized with shorter exposure times and the fact that the entiremore » pattern is obtained in a single exposure. This image plate solution establishes the robustness of powder molecular replacement resulting from different data collection techniques.« less

ExtSym: a program to aid space-group determination from powder diffraction data

Once unit-cell dimensions have been determined from a powder diffraction data set and therefore the crystal system is known (e.g.orthorhombic), the method presented by Markvardsen, David, Johnson & Shankland [Acta Cryst.(2001), A57, 47–54] can be used to generate a table ranking the extinction symbols of the given crystal system according to probability. Markvardsenet al.tested a computer program (ExtSym) implementing the method against Pawley refinement outputs generated using theTF12LSprogram [David, Ibberson & Matthewman (1992). Report RAL-92-032. Rutherford Appleton Laboratory, Chilton, Didcot, Oxon, UK]. Here, it is shown thatExtSymcan be used successfully with many well known powder diffraction analysis packages, namelyDASH[David, Shankland, van de Streek, Pidcock, Motherwell & Cole (2006).J. Appl. Cryst.39, 910–915],FullProf[Rodriguez-Carvajal (1993).Physica B,192, 55–69],GSAS[Larson & Von Dreele (1994). Report LAUR 86-748. Los Alamos National Laboratory, New Mexico, USA],PRODD[Wright (2004).Z. Kristallogr.219, 1–11] andTOPAS[Coelho (2003). Bruker AXS GmbH, Karlsruhe, Germany]. In addition, a precise description of the optimal input forExtSymis given to enable other software packages to interface withExtSymand to allow the improvement/modification of existing interfacing scripts.ExtSymtakes as input the powder data in the form of integrated intensities and error estimates for these intensities. The output returned byExtSymis demonstrated to be strongly dependent on the accuracy of these error estimates and the reason for this is explained.ExtSymis tested against a wide range of data sets, confirming the algorithm to be very successful at ranking the published extinction symbol as the most likely.

Powder diffraction from a continuous microjet of submicrometer protein crystals

Atomic-resolution structures from small proteins have recently been determined from high-quality powder diffraction patterns using a combination of stereochemical restraints and Rietveld refinement [Von Dreele (2007), J. Appl. Cryst. 40, 133-143; Margiolaki et al. (2007), J. Am. Chem. Soc. 129, 11865-11871]. While powder diffraction data have been obtained from batch samples of small crystal-suspensions, which are exposed to X-rays for long periods of time and undergo significant radiation damage, the proof-of-concept that protein powder diffraction data from nanocrystals of a membrane protein can be obtained using a continuous microjet is shown. This flow-focusing aerojet has been developed to deliver a solution of hydrated protein nanocrystals to an X-ray beam for diffraction analysis. This method requires neither the crushing of larger polycrystalline samples nor any techniques to avoid radiation damage such as cryocooling. Apparatus to record protein powder diffraction in this manner has been commissioned, and in this paper the first powder diffraction patterns from a membrane protein, photosystem I, with crystallite sizes of less than 500 nm are presented. These preliminary patterns show the lowest-order reflections, which agree quantitatively with theoretical calculations of the powder profile. The results also serve to test our aerojet injector system, with future application to femtosecond diffraction in free-electron X-ray laser schemes, and for serial crystallography using a single-file beam of aligned hydrated molecules.

A scanning angle energy-dispersive X-ray diffraction technique for high-pressure structure studies in diamond anvil cells

A scanning angle diffraction technique with an energy-dispersive solid-state detector (SSD) and white synchrotron radiation has been developed for high-pressure structure studies in diamond anvil cells (DACs). This technique is similar to the CAESAR technique [Y. Wang, T. Uchida, R. Von Dreele, M.L. Rivers, N. Nishiyama, K. Funakoshi, A. Nozawa, and H. Kaneko, A new technique for angle-dispersive powder diffraction using an energy-dispersive setup and synchrotron radiation, J. Appl. Cryst. 37 (2004), p. 947] developed for large-volume presses, but extended to DAC applications with high spatial resolution. The main feature of the technique is the well-defined collimation in the beam path to the detector, which improves the signal-to-noise ratio significantly, compared to routine monochromatic angle-dispersive powder diffraction with area detectors. This is particularly useful and essential for low-scattering materials and for amorphous and liquid diffraction/scattering studies using DACs. Data collected from crystalline and amorphous samples in DACs show that a coarse 2θ scan (0.1–0.2° for crystals and 0.5° for amorphous structure) is sufficient to obtain reasonable diffraction resolution. The scanning angle energy-dispersive X-ray diffraction technique provides angle-dispersive X-ray diffraction (ADXD) data in multiple energies. Such multi-energy ADXD data carry much more information than regular single-energy ADXD, which could provide site-specific atomic structure information for full structure refinement.

Powder crystallography on macromolecules

Following the seminal work of Von Dreele, powder X-ray diffraction studies on proteins are being established as a valuable complementary technique to single-crystal measurements. A wide range of small proteins have been found to give synchrotron powder diffraction profiles where the peak widths are essentially limited only by the instrumental resolution. The rich information contained in these profiles, combined with developments in data analysis, has stimulated research and development to apply the powder technique to microcrystalline protein samples. In the present work, progress in using powder diffraction for macromolecular crystallography is reported.

Powder diffraction studies on proteins: An overview of data collection approaches

Abstract. Following the seminal work of Von Dreele, high quality powder X-ray diffraction studies on proteins are being established as a valuable complementary technique to single-crystal measurements. Several studies using a variety of experiments approaches have been reported in the literature, including high-resolution studies employing parallel beam geome-try and high intensity measurements using position sensitive detectors. The choice of the optimum instrumental configuration depends on a number of competing factors such as the amount of sample available, its radiation sensitivity, and the quality of the data required for data analysis, e.g. angular resolution, the extent of the data in d-spacing, or the number of patterns required to explore the protein’s behaviour at different temperatures, or under differ-ent crystallisation conditions, etc. Here we discuss several advantages and disadvantages of different data collection methods followed for selected examples of small proteins.

D032 X-ray stress analysis employing the lattice parameter ellipsoid

If we assume a spherical element with a radius “r” within a specimen and the specimen is then subjected to a state of stress, the sphere will be deformed into an ellipsoid. The semi-axes of the ellipsoid will have the values of (r + ex), (r + ey), and (r + ez), which are replaced by dx, dy, and dz, or for the cubic case, ax, ay, and az. In this technique, at a particular Phi angle (see Figure 1), the two-theta position of a high angle (hkl) peak is determined at Psi angles of 0, 15, 30, and 45°. These measurements are repeated for 3 to 6 Phi angles in steps of 30°. The “d” or “a” values are then determined from the peak positions. This data is then fitted to the general quadratic equation for an ellipsoid by the method of least squares. From the coefficients of the quadratic equation, the angle between the laboratory and the specimen coordinates (direction of the principle stress) can be determined. Applying the general rotation of axes equations to the quadratic, the equation of the ellipse in the x-y plane is determined. The ax, ay, and az values for the principal axes of the lattice parameter ellipsoid are then evaluated. It is then possible to determine the unstressed ao value from Hooke’s Law using ax, ay, and az, and the magnitude of the principal stresses/strains. . INTRODUCTION In 1925 H. H. Lester and R. H. Aborn [1] published their paper entitled “The Behavior Under Stress of the Iron Crystals in Steel”. They could not have foreseen that during the next 81 years there would be such a dramatic change in the way the x-ray stresses measurements would be carried out. The use of computer controlled x-ray diffractometers, with sophisticated software to reduce the data and determine the residual stress/strain tensor has made stress analysis a routine technique. Examples of these developments include the work of B. B. He and K. L. Smith [2], where they introduce the use of the two-dimensional detector for x-ray stress analysis. The twodimensional x-ray detector is able to record a large fraction of the diffraction cone, which will be distorted by the residual stress in the sample. It is this relationship of the stress tensor and the distortion of the diffraction conic that make this a very powerful method for stress analysis. The work of D. Balzar, R. Von Dreele and H. Ledbetter[3] have employed the Rietveld refinement of the diffraction patterns collected at different specimen orientations, the refined lattice parameters yield the strain components in the specimen. At the present time the sin(ψ) method is probably the most popular technique used, it is very simple to use and the tabulation of the elastic constants for most materials of interest have been tabulated. This paper presents a different approach to x-ray stress analysis, which employs the lattice parameter ellipsoid. This method has proven to give results that are in good agreement with the conventional sin(ψ) technique. However, it has one advantage over the sin(ψ) technique, that is it integrates the data over many φ angles, where the sin(ψ) technique only evaluates the data in one plane at a time. 145 Copyright ©JCPDS-International Centre for Diffraction Data 2007 ISSN 1097-0002

Comparative evaluation of the March and generalized spherical harmonic preferred orientation models using X-ray diffraction data for molybdite and calcite powders

Preferred crystallographic orientation,i.e.texture in crystalline materials powder diffraction data, can cause serious systematic errors in phase composition analysis and also in crystal structure determination. The March model [Dollase (1986).J. Appl. Cryst.19, 267–272] has been used widely in Rietveld refinement for correcting powder diffraction intensities with respect to the effects of preferred orientation. In the present study, a comparative evaluation of the March model and the generalized spherical harmonic [Von Dreele (1997).J. Appl. Cryst.30, 517–525] description for preferred orientation was performed with X-ray powder diffraction data for molybdite (MoO3) and calcite (CaCO3) powders uniaxially pressed at five different pressures. Additional molybdite and calcite powders, to which 50% by weight silica gel had been added, were prepared to extend the range of preferred orientations considered. The patterns were analyzed initially assuming random orientation of the crystallites and subsequently the March model was used to correct the preferred orientation. The refinement results were compared with parallel refinements conducted with the generalized spherical harmonic [Sitepu (2002).J. Appl. Cryst.35,274–277]. The results obtained show that the generalized spherical harmonic description generally provided superior figures-of-merit compared with the March model results.

Rigid bodies in powder diffraction. A practical guide

Recipes are given to assist in setting up rigid bodies for common molecules and coordination polyhedra, to define satellite groups, to perform rotations around arbitrary axes through the origin of the rigid body, and to refine intramolecular degrees of freedom under consideration of the special needs of powder diffraction. To the greatest possible extent, the notation follows that of the well known Rietveld refinement program GSAS (Larson and Von Dreele, 1994).

ACA hosts “new directions in neutron scattering instrumentation for structural biology”

An all-day symposium on “New Directions i n Neutron Scattering Instrumentation for Protein and Membrane Crystallography” was held at the annual meeting of the ACA in Arlington, Virginia on July 23, 1998. The aims of the organizers, Benno Schoenborn and Bob von Dreele (LANL), were to promote and to make provisions for new technical developments which promise to broaden the application of neutron diffraction in structural biology. Ten speakers presented recent accomplishments in the application of neutrons to biology using a variety of different diffraction techniques to an audience of about 50.

Antiferromagnetic resonance in Bi2CuO4

Magnetic resonance of the low-frequency spin-wave branch in the Bi2CuO4 antiferromagnet with an easy-plane anisotropy has been studied. Angular, frequency, and temperature dependences of the position and width of the antiferromagnetic resonance (AFMR) line have been measured. Our measurements combined with earlier data [H. Ochta, K. Yoshida, T. Matsuya, T. Nanba, M. Motokawa, K. Yamada, Y. Endon, and S. Hosoya, J. Phys. Soc. Jpn. 61, 2921 (1992); E. W. Ong, G. H. Kwei, R. A. Robinson, B. L. Ramakrishna, and R. B. von Dreele, Phys. Rev. B 42, 4255 (1990) [ have allowed us to determine anisotropy constants of this material and to account for the unusual character of its static susceptibility anisotropy. The AFMR line shifts to the high-field side and broadens in a temperature range of 10–15 K, and the cause of this has remained unclear. In the low-temperature range the line shows a hysteresis corresponding to a static field magnitude several times as large as the spin-flop field. The position and width of the AFMR line depend sensitively on the sample preparation technique.

PROFVAL: functions to calculate powder-pattern peak profiles with axial-divergence asymmetry

The crystallographic problem: In a powder diffractometer, the curvature of the Debye---Scherrer rings causes an asymmetry in the peak profiles as the Bragg angle approaches either 0 or 180 °. For accurate Rietveld refinement, this asymmetry, which is known as axial divergence, must be modeled correctly. Traditionally, empirical ad hoc functions such as a split-Pearson-VII function or multiple overlapping Gaussian peaks have been employed. Finger e t a / . (1994) extended the method of van Laar & Yellon (1984) for convolution of the ring shape with an intrinsic peak profile a procedure that describes extremely asymmetric profiles and involves only physically meaningful parameters. To date, however, only the program GSAS (Larson &von Dreele, 1990) has incorporated this method. This contribution makes available tested code for the convolution of the asymmetry function with the intrinsic line function in the hope that more Rietveld codes will incorporate this method.

Neutron powder diffraction study of hydrogarnet to 9.0 GPa

Other| October 01, 1996 Neutron powder diffraction study of hydrogarnet to 9.0 GPa George A. Lager; George A. Lager University of Louisville, Department of Geography and Geosciences, Louisville, KY, United States Search for other works by this author on: GSW Google Scholar Robert B. Von Dreele Robert B. Von Dreele Los Alamos National Laboratory, United States Search for other works by this author on: GSW Google Scholar American Mineralogist (1996) 81 (9-10): 1097–1104. https://doi.org/10.2138/am-1996-9-1006 Article history first online: 02 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Twitter LinkedIn Tools Icon Tools Get Permissions Search Site Citation George A. Lager, Robert B. Von Dreele; Neutron powder diffraction study of hydrogarnet to 9.0 GPa. American Mineralogist 1996;; 81 (9-10): 1097–1104. doi: https://doi.org/10.2138/am-1996-9-1006 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu nav search search input Search input auto suggest search filter All ContentBy SocietyAmerican Mineralogist Search Advanced Search Abstract The crystal structure of synthetic, deuterated katoite [Ca3Al2(O4D4)3] has been refined at 0.0001, 0.78, 3.6, 6.1, 7.9, and 9.0 GPa using the opposed-anvil Paris-Edinburgh cell and the POLARIS powder diffractometer at the U.K. pulsed spallation source, ISIS. A constrained Birch-Mumaghan fit to the unit-cell volume yields K0 = 52(1) GPa for K0´⁠, = 4.0. The lower bulk modulus of katoite relative to anhydrous Ca-bearing garnets (K0 = 159-179 GPa) is due to the greater compressibility of the Ca dodecahedron and O4D4 tetrahedron. At high pressure, corner-sharing tetrahedra and octahedra undergo a rigid-body rotation, which causes a shift in the O position. This rotation, coupled with the large compression of the two tetrahedral edges shared with the dodecahedron, changes the relative lengths of the Ca-O bonds at high pressure, such that Ca2-0 < Cal-O for P > 6 GPa. With increasing pressure, the O-D· · ·O angles widen slightly, as the O-D vector rotates toward the tetrahedral face. Both O-D and O· · ·D bond lengths decrease as a function of pressure, which was unexpected in view of the results of recent high-pressure studies. Comparison of crystallographic and spectroscopic data collected at high pressure for hydrogamets suggests that empirical relationships derived for hydrogen-bond systems at ambient conditions, where a relaxed, equilibrium state is attained, may not always apply to O-D · · · O hydrogen bonds under compression. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not currently have access to this article.

27Al Satellite Transition MAS-NMR Spectroscopy of Kaolinite

AbstractHighly crystalline kaolinite has been studied by 27Al satellite transition magic angle-spinning (MAS) nuclear magnetic resonance (NMR) spectroscopy. This technique provides an additional narrowing of the MAS spectral lines by a factor of more than three compared with the width of the prominent central transition. Evidence for the presence of two crystallographically inequivalent hexacoordinated Al sites has been found. The results support the kaolinite crystal structure proposed by Bish &amp; Von Dreele (1989).