Atomic Positional Parameters Research Articles

Synthesis, characterization and applications of cubic fluorite cerium oxide nanoparticles: A comprehensive study

Rare earth oxides are the most promising candidates for catalyst, environmental, biological, and medical uses. In this study, we report the synthesis, characterization, and applications of cerium oxide (CeO2) nanoparticles (NPs). CeO2 NPs were successfully synthesized by a simple and cost-effective chemical precipitation method. Several characterization techniques were employed to investigate the synthesized metal oxide nanoparticles. Structural properties of CeO2 NPs were analysed using powder X-ray diffraction (XRD) and Raman spectroscopy. The average crystallite size of NPs was calculated by using the Debye Scherrer formula. The atomic position and structural parameters of synthesized NPs were determined by rietveld refinement process of X-ray diffractogram. Energy dispersive X-ray analysis (EDAX) was used to confirm the chemical and elemental composition of the synthesized CeO2 NPs. High resolution transmission electron microscopy (HRTEM) was used to determine the average particle size of the CeO2 NPs. The polycrystalline nature of CeO2 NPs was confirmed by the ring pattern obtained in selected area electron diffraction (SAED). Photoluminescence (PL) spectroscopy was employed to identify defects in the sample. UV–Vis spectroscopy was used to study the optical properties and parameters of the CeO2 NPs, and the refractive index was calculated using various theoretical models. The photocatalytic activity of the synthesized CeO2 NPs was studied for the degradation of methylene blue (MB) dye at different catalyst doses under UV light irradiation. Additionally, the anticancer activity of the synthesized CeO2 NPs against human lung carcinoma cells was studied in detail.

Crystal structure of the new ternary aluminide ErAg0.77Al2.23

Сrystal structure of the ternary erbium and silver aluminide ErAg 0 . 77 Al 2 . 23 has been studied using powder X-ray diffraction data: space group R-3m , Pearson symbol h R36 , lattice parameters a = 0.55049(1) nm, c = 2.61415(6) nm, final residual values are R I = 0,0450; R P = 0,0314; wR P = 0,0430. Samples were prepared by an arc melting under purified argon atmosphere with nonconsumable tungsten electrode and water-cooled copper hearth of the elemental components with certified purities of 99.999 wt.% Er, 99.95 wt.% Ag and 99.99 wt.% Al. All samples were re-melted twice to ensure homogeneity, and then heat treated in evacuated silica tubes at 600 o C during 700 hours. Annealed alloys were quenched in cold water without breaking the tubes. Phase analysis of the prepared samples and crystal structure determination of new compound were carried out using X-ray powder diffraction patterns recorded on a automatic powder diffractometer Huber Imaging Plate Guinier Camera G670 (Cu K α1 -radiation, l = 0.154056 nm; 3.503 ≤ 2 θ ≤ 99.99°; step 0,01° of 2 θ , scan time is 250 sec per step). Refinement of the atomic positional and displacement parameters in the crystal structure of the compounds was carried out by the full profile Rietveld method in the range 2 θ =3.50­–99.99 o (Cu K α1 -radiation) using the WinCSD program package. Existence of the new ternary aluminide ErAg 0 . 77 Al 2 . 23 with the rhombohedral structure of the PuNi 3 -type was found in the two three-component samples of the starting compositions Er 0 . 25 Ag 0 . 20 Al 0 . 55 and Er 0 . 25 Ag 0 . 17 Al 0 . 58 . In both samples new aluminide was in thermodynamic equilibrium with the small admixtures of the earlier known ternary phases ErAg 2 . 5 Al 2 . 5 (DyAg 2 . 4 Al 2 . 6 -type structure) and Er 8 Ag 17 Al 49 (Yb 8 Cu 17 Al 49 -type structure). Partially ordered distribution of the smaller atoms Ag and Al was observed in the crystal structure of new intermetallide, in particular Wyckoff site 6 с is fully occupied by Ag atoms, whereas Wyckoff sites 3 b and 18 h are occupied by the statistical mixtures of Ag and Al atoms with a predominant content of aluminum atoms leading to the next formulas ErAg 0 . 77(1) Al 2 . 23(1) or Er(Ag 0 . 26 Al 0 . 74 ) 3 . Interatomic distances in the ErAg 0 . 77 Al 2 . 23 crystal structure are in good correlation with the respective sums of the atomic radii of the components indicating predominance of the metallic bonding. Structure type PuNi 3 (or NbBe 3 ) can be considered as formed from the structural units of the CaCu 5 and MgZn 2 structure types. Keywords: crystal structure, X-ray powder diffraction, aluminide, erbium, silver.

X-ray molecular orbital analysis. I. Quantum mechanical and crystallographic framework.

Molecular orbitals were obtained by X-ray molecular orbital analysis (XMO). The initial molecular orbitals (MOs) of the refinement were calculated by the ab initio self-consistent field (SCF) MO method. Well tempered basis functions were selected since they do not produce cusps at the atomic positions on the residual density maps. X-ray structure factors calculated from the MOs were fitted to observed structure factors by the least-squares method, keeping the orthonormal relationship between MOs. However, the MO coefficients correlate severely with each other, since basis functions are composed of similar Gaussian-type orbitals. Therefore, a method of selecting variables which do not correlate severely with each other in the least-squares refinement was devised. MOs were refined together with the other crystallographic parameters, although the refinement with the atomic positional parameters requires a lot of calculation time. The XMO method was applied to diformohydrazide, (NHCHO)2, without using polarization functions, and the electron-density distributions, including the maxima on the covalent bonds, were represented well. Therefore, from the viewpoint of X-ray diffraction, it is concluded that the MOs averaged by thermal vibrations of the atoms were obtained successfully by XMO analysis. The method of XMO analysis, combined with X-ray atomic orbital (AO) analysis, in principle enables one to obtain MOs or AOs without phase factors from X-ray diffraction experiments on most compounds from organic to rare earth compounds.

Feasibility of an optical fiber clock

We explore the feasibility of a compact high-precision Hg atomic clock based on a hollow core optical fiber. We evaluate the sensitivity of the $^1S_0$-$^3P_0$ clock transition in Hg and other divalent atoms to the fiber inner core surface at non-zero temperatures. The Casimir-Polder interaction induced $^1S_0$-$^3P_0$ transition frequency shift is calculated for the atom inside the hollow capillary as a function of atomic position, capillary material, and geometric parameters. For $^{199}\mathrm{Hg}$ atoms on the axis of a silica capillary with inner radius $\geq 15 \,\mu \mathrm{m}$ and optimally chosen thickness $d\sim 1 \,\mu \mathrm{m}$, the atom-surface interaction induced $^1S_0$-$^3P_0$ clock transition frequency shift can be kept on the level $\delta\nu/\nu_{\mathrm{Hg}} \sim10^{-19}$. We also estimate the atom loss and heating due to the collisions with the buffer gas, lattice intensity noise induced heating, spontaneous photon scattering, and residual birefringence induced frequency shifts.

Electron density, disorder and polymorphism: high-resolution diffraction studies of the highly polymorphic neuralgic drug carbamazepine.

Analysis of neutron and high-resolution X-ray diffraction data on form (III) of carbamazepine at 100 K using the atoms in molecules (AIM) topological approach afforded excellent agreement between the experimental results and theoretical densities from the optimized gas-phase structure and from multipole modelling of static theoretical structure factors. The charge density analysis provides experimental confirmation of the partially localized π-bonding suggested by the conventional structural formula, but the evidence for any significant C-N π bonding is not strong. Hirshfeld atom refinement (HAR) gives H atom positional and anisotropic displacement parameters that agree very well with the neutron parameters. X-ray and neutron diffraction data on the dihydrate of carbemazepine strongly indicate a disordered orthorhombic crystal structure in the space group Cmca, rather than a monoclinic crystal structure in space group P2(1)/c. This disorder in the dihydrate structure has implications for both experimental and theoretical studies of polymorphism.

Structure and stability of high pressure synthesized MgTM2H6 (TM = Zr, Nb) hydrides

The structure of two isomorphous high pressure synthesized metal hydrides MgZr2H6 and MgNb2H6 is described in the R-3m space group with the hexagonal unit cells a=3.3592(2)Å, c=25.131(3)Å and a=3.2901(9)Å, c=23.09(1)Å, respectively (Z=3).The metal atom structures were refined from powder XRD data, with Mg in the 3a site (0,0,0,) and Zr and Nb in the 6c site (0,0,z) with the only refineable atomic parameter z=0.225 and z=0.220, respectively.As only a minute sample could be produced in the anvil cell, a conventional structural determination with neutron diffraction on a deuterated sample could not be made. Instead formation energies of the all proposed structural models were calculated using density functional theory (DFT) for accessing the most stable structure of the lowest energy level. The measured lattice parameters and the refined atomic positional parameters from the powder X-ray diffraction pattern were reproduced well by the DFT calculation. The hydrogen atoms were found to fully occupy an octahedral site coordinated by 3Mg and 3Zr atoms, a tetrahedral site coordinated by 4Zr atoms and another tetrahedral site coordinated by 3Mg and 1Zr atoms. Chemical, mechanical and dynamical stabilities of the MgZr2H6 structure were further discussed based on a chemical potential equilibrium diagram constructed from the calculated formation energies, calculated elastic constants and phonon spectra respectively. It is also shown that a high pressure increases the stability of the new hydrides.

Research on the optical band positions, spin-Hamiltonian parameters and atom-position parameter of Co2+ ion in CdSe crystal

The optical band positions and EPR (or spin-Hamiltonian) parameters (g factors g//, g⊥ and zero-field splitting D) of Co2+ ion in the trigonally-distorted tetrahedral Cd2+ site of CdSe crystal are calculated simultaneously from the complete diagonalization (of energy matrix) method (CDM) based on the two-spin–orbit-parameter model. This model contains the contributions to the spectral data from both the spin–orbit parameters of central dn ion (i.e., one-spin–orbit-parameter model) in the conventional crystal-field theory and that of ligand ions via covalence effect. The calculated 12 optical band positions and three spin-Hamiltonian parameters using four adjustable parameters show reasonable agreement with the experimental values. The local atom-position parameter uloc (where the corresponding parameter u in the host CdSe crystal is unlikely) in the Co2+ center is also estimated from the calculations. The calculations of these spectral data from the CDM based on the conventional one-spin–orbit parameter model are also made for comparison. It is found that the calculated optical band positions are also close to the experimental values, but the calculated spin-Hamiltonian parameters are in poor agreement with the observed values. So, for the unified and reasonable calculations of optical and EPR data of dn ions in crystals (in particular, in the cases of ligand with large spin–orbit parameter), the CDM based on the two-spin–orbit-parameter model should be applied.

Redetermination of Dy3Ni from single-crystal X-ray data

The classification of the title compound, tridysprosium nickel, into the Fe3C (or Al3Ni) structure type has been deduced from powder X-ray diffraction data with lattice parameters reported in a previous study [Lemaire & Paccard (1967 ▶). Bull. Soc. Fr. Mineral. Cristallogr. 40, 311–315]. The current re-investigation of Dy3Ni based on single-crystal X-ray data revealed atomic positional parameters and anisotropic displacement parameters with high precision. The asymmetric unit consists of two Dy and one Ni atoms. One Dy atom has site symmetry .m. (Wyckoff position 4c) and is surrounded by twelve Dy and three Ni atoms. The other Dy atom (site symmetry 1, 8d) has eleven Dy and three Ni atoms as neighbours, forming a distorted Frank–Kasper polyhedron. The coordination polyhedron of the Ni atom (.m., 4c) is a tricapped trigonal prism formed by nine Dy atoms.

Characterization of a high pressure, high temperature modification of ammonia borane (BH3NH3)

At elevated pressures (above 1.5 GPa) dihydrogen bonded ammonia borane, BH3NH3, undergoes a solid-solid phase transition with increasing temperature. The high pressure, high temperature (HPHT) phase precedes decomposition and evolves from the known high pressure, low temperature form with space group symmetry Cmc21 (Z = 4). Structural changes of BH3NH3 with temperature were studied at around 6 GPa in a diamond anvil cell by synchrotron powder diffraction. At this pressure the Cmc21 phase transforms into the HPHT phase at around 140 °C. The crystal system, unit cell, and B and N atom position parameters of the HPHT phase were extracted from diffraction data, and a hydrogen ordered model with space group symmetry Pnma (Z = 4) subsequently established from density functional calculations. However, there is strong experimental evidence that HPHT-BH3NH3 is a hydrogen disordered rotator phase. A reverse transition to the Cmc21 phase is not observed. When releasing pressure at room temperature to below 1.5 GPa the ambient pressure (hydrogen disordered) I4mm phase of BH3NH3 is obtained.

Structural, elastic, electronic and thermodynamic properties of the filled skutterudite CeOs4Sb12 determined by density functional theory

Structural, elastic, electronic and thermodynamic properties of the ternary cubic filled skutterudite CeOs4Sb12 compound were calculated using the full-potential linear muffin-tin orbital implementation of density functional theory. The exchange-correlation potential was treated with the local density approximation. The calculated ground state quantities such as the lattice parameter, atomic position parameters of Sb atoms, bulk modulus and its pressure derivative are compared to the available experimental data. We have computed the elastic moduli and their pressure dependence, which have not been calculated or measured yet. The Debye temperature is estimated from the average sound velocity. From the elastic parameter behavior, it is inferred that this compound is elastically stable and brittle in nature. The electronic band structure calculations revealed metallic behavior for the herein studied compound at zero pressure, but under pressure effect, the metallic character disappears and the compound becomes a narrow indirect band gap semiconductor. Through the quasi-harmonic Debye model, in which phononic effects are considered, the effect of pressure P and temperature T on the lattice constant, bulk modulus, heat capacity, thermal expansion coefficient and Debye temperature are investigated.

Phase relations and structural features in the system Ni–Zn–B

Phase relations for the system Ni–Zn–B have been established in the isothermal section at 800°C based on X-ray powder diffraction and electron microprobe analyses of about 60 samples, which have been prepared by hot pressing of pre-annealed powder compacts of arc-melted master alloys NixBy and Zn-filings. Six ternary compounds (labeled τ1 to τ6) were found to exist, which in some cases exhibit considerable mutual solid solubilities mostly as an exchange of Ni–Zn at constant B-content, but in the case of the so-called τ-phase τ1-(Ni1−xZnx)21[Zn1−y−z□y(B4)z]2B6 (0.07≤x≤0.11, 0.07≤y≤0.53, 0≤z≤0.3) also with Zn/B substitution. Single crystal X-ray data analysis for the composition x=0.07, y=0.125, z=0.30, and a=1.05800(3) nm confirmed the Cr23C6 type with space group Fm3¯m, but revealed partial replacement of Zn-atoms by B4-tetrahedra (RF=0.014). Whereas Ni/Zn exchange (at constant B-content) ranges at about 4–5at% for τ4-Ni3ZnB2 and τ5-Ni48Zn32B20, it is below 3at% for τ2-Ni12ZnB8−x (x=0.43), τ3-Ni21Zn2B20 and τ6-Ni47Zn23B30. The crystal structures of τ2-Ni12ZnB8−x (x=0.43; Ni12AlB8 type), τ3-Ni21Zn2B20 (own type) and τ4-Ni3ZnB2 (own type) have been determined in our foregoing paper [8]. Due to the non-availability of suitable single crystals, the crystal structures of τ5-Ni48Zn32B20 (body-centered orthorhombic; a=1.6(2)nm, b=0.63(7)nm, and c=0.27(0)nm, determined from TEM) and of τ6-Ni47Zn23B30 have not been elucidated yet. Phase equilibria at 800°C are dominated by a large three-phase field (βB)+Ni2Zn11+τ3, as practically all ternary compounds form at compositions with less than about 50at% Ni. Furthermore, precise data on atom site distribution and positional parameters have been provided from X-ray single crystal refinements for Ni-borides, for which crystal structures hitherto have only been derived from X-ray diffraction photographs: o-Ni4B3−x (x=0.19; Pnma, a=1.9665(4)nm, b=0.29852(1)nm, c=0.65750(2)nm; RF=0.015) and m-Ni4B3 (C2/c, a=0.64356(3)nm, b=0.48867(3)nm, c=0.78267(3)nm, β=103.288(3)°, RF=0.021).

THE CRYSTAL STRUCTURE AND GENESIS OF KRENNERITE, Au3AgTe8

The crystal structure of a natural krennerite (Au 3 AgTe 8 ) from Cripple Creek, Colorado, USA has been refined in space group Pma 2 using 3,151 observed unique (acentric) reflections. The final R value for observed reflections was 0.022. Unit cell parameters are a 16.590(3) A, b 8.867(2) A, c 4.4886(8) A, V 660.29(13) A 3 , and Z = 2. The precision of the atom position parameters is improved by up to a factor of ten over previous studies. The three Au–Ag sites in the structure are ordered, with Ag = 43% occupancy at the Au1 (2 a ) position (point symmetry 2) and 59% at the Au2 (2 c ) position (point symmetry m ); the Au3 (4 d ) position (point symmetry 1) is 100% Au. The observed ordering in the structure appears to be due to avoidance of Ag–Te–Ag bonding. Ordering of Au and Ag in the structure may thus limit the composition range to being between those of calaverite and sylvanite. Detailed petrographic study of the sample in thin section, accompanied by electron microprobe data, demonstrates the paragenesis of the ore that provided the crystal for refinement. The petrographic study provides evidence for sylvanite exsolution from krennerite in natural systems, consistent with the narrow compositional range inferred from the crystallographic study.

Structural study of the coherent dehydration of wadsleyite

The coherent dehydration of pure-Mg wadsleyite has been investigated by single-crystal X‑ray diffraction at high temperature and room pressure. Hydrous wadsleyite with 2.8 wt% H 2 O has monoclinic unit-cell parameters of a = 5.6693(4), b = 11.571(1), c = 8.2407(5) Å, b = 90.209(3)°, and V = 540.59(7) Å 3 . Dehydration begins at 635 K with an abrupt increase in the a-axis and decrease in b. After dehydration is complete, the dehydrated sample is orthorhombic with a = 5.6995(3), b = 11.4589(8), c = 8.2556(5) Å, and V = 539.17(6) Å 3 at ambient conditions. Atom position and displacement parameters have been refined for both hydrous and dehydrated wadsleyite samples from intensity measurements conducted at high temperatures. The most significant changes during dehydration are systematic decreases in M2-O1 and M3-O1 bond lengths. After dehydration, M2-O1 and M3-O1 bonds decrease by 3% and 2.5%, respectively. While the length changes of the other M-O bonds are no more than 1%, consistent with the hydration mechanism being protonation of O1. For the monoclinic structure, the average thermal expansion coefficient is 38.4(3) × 10-6 K-1 before dehydration and 28.1(8) × 10 -6 K -1 for the dehydrated sample. The volumetric thermal expansion coefficients for MgO 6 octahedra in the hydrous sample, a0(V), are 36(4), 41(4), 49(5), and 35(4) (10 -6 K -1 ) for M1, M2, M3A, and M3B, respectively. In the dehydrated sample, they are 35(5), 34(4), and 36(2) (10 -6 K -1 ) for M1, M2, and M3, respectively. No significant thermal expansion is observed for SiO 4 tetrahedra of either the hydrous or dehydrated sample.

Spin-Hamiltonian parameters and defect structures for the trigonal Yb3+ centers in wurtzite-type ZnS and CdS crystals

A complete diagonalization (of the energy matrix, where the corresponding Hamiltonian consists of free-ion, crystal-field, Zeeman and hyperfine interaction terms) method is applied to calculate the spin-Hamiltonian parameters (g factors g||, g⊥ and hyperfine structure constants A∥, A⊥) for the trigonal Yb3+ centers in wurtzite-type ZnS and CdS crystals. The calculated results are in reasonable agreement with the experimental values. In the calculations, the crystal-field parameters are obtained from the superposition model, which enables correlation of the spin-Hamiltonian parameters with the defect structure of the studied impurity center. The defect structures of Yb3+ centers (characterized by the local atom-position parameters uloc, which differ from the corresponding parameter in the host crystal) in both crystals are therefore estimated from the calculations. The results are discussed.

The system Ce–Zn–B at 800 °C

The isothermal section for the system Ce–Zn–B has been established at 800 °C using electron microprobe analysis and X-ray powder diffraction. No ternary compounds exist and mutual solid solubilities of binary phases are negligible. In the concentration range of 10.0–10.5 at% Ce two structural modifications have been confirmed: high temperature βCe 2Zn 17 above ∼750 °C with the Th 2Zn 17 type ( R3̄ m, a=0.90916(4) nm, c=1.3286(1) nm) and low temperature αCeZn 7 (Ce 1− x Zn 5+2 x ; x∼0.33) up to 750 °C for which we attributed the TbCu 7 type ( P6 /mmm, a=0.52424(2), c=0.44274(1) nm). The crystal structure of CeZn 7 was derived from the Rietveld refinement of X-ray powder intensities. Precise data on atom site distribution and positional parameters have been furthermore provided from X-ray single crystal refinements for two compounds, for which crystal structures hitherto have only been derived from X-ray diffraction photographs: Ce 3Zn 11 ( Immm, a=0.45242(2) nm, b=0.88942(3) nm, c=1.34754(4) nm) and Ce 3Zn 22 ( I4 1 /amd; a=0.89363(2) nm, c=2.1804(5) nm).

Elucidating Negative Thermal Expansion in MOF-5

Multi-temperature X-ray diffraction studies show that twisting, rotation, and libration cause negative thermal expansion (NTE) of the nanoporous metal−organic framework MOF-5, Zn4O(1,4-benzenedicarboxylate)3. The near-linear lattice contraction is quantified in the temperature range 80−500 K using synchrotron powder X-ray diffraction. Vibrational motions causing the abnormal expansion behavior are evidenced by shortening of certain interatomic distances with increasing temperature according to single-crystal X-ray diffraction on a guest-free crystal over a broad temperature range. Detailed analysis of the atomic positional and displacement parameters suggests two contributions to cause the effect: (1) local twisting and vibrational motion of the carboxylate groups and (2) concerted transverse vibration of the linear linkers. The vibrational mechanism is confirmed by calculations of the dynamics in a molecular fragment of the framework.

ChemInform Abstract: Ferromagnetic Ordering of NdNiSi2 and PrNiSi2 Studied by Neutron Diffraction and Magnetic Measurements.

Abstract The magnetic properties of the compounds PrNiSi 2 and NdNiSi 2 were studied by means of magnetic measurements and neutron diffraction. Neutron diffraction experiments made in the paramagnetic regime confirmed the CeNiSi 2 structure reported earlier for these compounds. Refined atomic position parameters for both compounds are given. Neutron diffraction measurements performed in the magnetically ordered regime showed that the rare earth moments in PrNiSi 2 and NdNiSi 2 order ferromagnetically below T c = 20 K and T c =9.5 K respectively. The preferred moment direction is along the c axis in both cases. Ferromagnetic ordering was also concluded from magnetic measurements performed on these compounds.

Mode crystallography of distorted structures

The description of displacive distorted structures in terms of symmetry-adapted modes is reviewed. A specific parameterization of the symmetry-mode decomposition of these pseudosymmetric structures defined on the setting of the experimental space group is proposed. This approach closely follows crystallographic conventions and permits a straightforward transformation between symmetry-mode and conventional descriptions of the structures. Multiple examples are presented showing the insight provided by the symmetry-mode approach. The methodology is shown at work, illustrating its various possibilities for improving the characterization of distorted structures, for example: detection of hidden structural correlations, identification of fundamental and marginal degrees of freedom, reduction of the effective number of atomic positional parameters, quantitative comparison of structures with the same or different space group, detection of false refinement minima, systematic characterization of thermal behavior, rationalization of phase diagrams and various symmetries in families of compounds etc. The close relation of the symmetry-mode description with the superspace formalism applied to commensurate superstructures is also discussed. Finally, the application of this methodology in the field of ab initio or first-principles calculations is outlined. At present, there are several freely available user-friendly computer tools for performing automatic symmetry-mode analyses. The use of these programs does not require a deep knowledge of group theory and can be applied either a posteriori to analyze a given distorted structure or a priori to parameterize the structure to be determined. It is hoped that this article will encourage the use of these tools. All the examples presented here have been worked out using the program AMPLIMODES [Orobengoa et al. (2009). J. Appl. Cryst. 42, 820-833].

Synchrotron-based X-ray diffraction of the lead apatite series Pb10(PO4)6Cl2–Pb10(AsO4)6Cl2

A number of compounds of the pyromorphite-mimetite solid solution series were synthesized at room temperature and analyzed with infrared spectroscopy and powder X-ray diffraction. High-resolution high-quality powder diffraction data were obtained by means of the state-of-the-art instrument at the 11-BM beamline of the Advanced Photon Source at Argonne National Laboratory, Argonne, IL, USA. The unit-cell dimensions, atomic position and atomic displacement parameters, as well as site occupancy for analyzed phases, were refined by the Rietveld method and reported herein for the first time. The results of this study indicate that the pyromorphite-mimetite series is continuous in near-Earth-surface environments. Detailed structural changes caused by P(5+)-As(5+) substitutions in minerals from the pyromorphite-mimetite solid solution series are reported.

A Combined Solid‐State NMR and MD Characterization of the Stability and Dynamics of the HET‐s(218‐289) Prion in its Amyloid Conformation

The three-dimensional structure of amyloid fibrils of the prion-forming part of the HET-s protein [HET-s(218-289)], as determined by solid-state NMR, contains rigid and remarkably well-ordered parts, as witnessed by the narrow solid-state NMR line widths for this system. On the other hand, high-resolution magic-angle-spinning (HRMAS) NMR results have shown that HET-s(218-289) amyloid fibrils contain highly flexible parts as well. Here, we further explore this unexpected behaviour using solid-state NMR and molecular dynamics (MD). The NMR data provide new information on order and dynamics in the rigid and flexible parts of HET-s(218-289), respectively. The MD study addresses whether or not small multimers, in an amyloid conformation, are stable on the 10 ns timescale of the MD run and provides insight into the dynamic parameters on the nanosecond timescale. The atom-positional, root-mean-squared fluctuations (RMSFs) and order parameters S(2) obtained are in agreement with the NMR data. A flexible loop and the N terminus exhibit dynamics on the ps-ns timescale, whereas the hydrophobic core of HET-s(218-289) is rigid. The high degree of order in the core region of HET-s(218-289) amyloids, as observed in the MD simulations, is in agreement with the narrow, solid-state, NMR lines. Finally, we employed MD to predict the behaviour of the salt-bridge network in HET-s(218-289), which cannot be obtained easily by experiment. Simulations at different temperatures indicated that the network is highly dynamic and that it contributes to the thermostability of the HET-s(218-289) amyloids.