Each member of the mullite-type Pb-2(Pb1-xSnx)O-4 solid-solution can be described with the schafarzikite structure type. The endmember Pb2PbO4 (x = 0) crystallizes in space group P4(2)/mbc at ambient conditions, whereas the orthorhombic subgroup Pbam is needed for the structural description of the phases for x >= 0.3. On cooling, Pb2PbO4 undergoes a P4(2)/mbc -> Pbam phase transition between 171(1) K and 174(4) K confirmed by FTIR and Raman data analysis, respectively. On heating, the other endmember Pb2SnO4 (x = 1) shows the reverse Pbam -> P4(2)/mbc phase transition at 1221(19) K, confirmed by a Landau-type fit of the lattice parameter difference Delta(ab). During thermogravimetric analysis Pb2PbO4 decomposes into PbO and oxygen. However, samples with 0.3 < x < 0.8 undergo an exsolution of PbO along with an enrichment of the Sn-content in the respective mullite-type phases. Due to slow decomposition kinetics several exsolution steps are visible in the temperature-dependent unit-cell volumes for different stochiometric x-values. For all phases, the lattice thermal expansion was modeled using the Debye-Einstein-Anharmonicity-Gliding (DEAG) approach. A single Debye term well describes the internal energy contribution followed by different phase-change properties. The initial phase-change temperatures can be derived from the respective fits of the gliding components. The stoichiometries of the intermediate phases are estimated from the calculated 0K unit-cell volume of the DEAG model. Based on the structure analysis a polyhedral distortion sum was calculated from the weighted octahedral distortion of Pb4+O6 and the Wang-Liebau-eccentricity parameter of Pb2+O4E nido-pyramids. Whereas the latter one does not play an important role for the temperature-dependent behavior, the structural distortion originating from the size difference of the cations on the octahedra position is found to be the main reason for the miscibility-gap of the solid solution for 0.0 < x < 0.3, as can be derived from the sum distortion parameters relative to their structural average.

An extension of the Debye–Einstein–anharmonicity (DEA) model has been addressed to describe the lattice thermal expansion up to a temperature‐dependent phase transition. By using an intrinsic anharmonicity (A) term in the DEA model only, it is not possible to describe the continuous changes of the lattice thermal expansion leading to anomalies originating from second‐order phase transitions. Therefore, an extended formalism is empirically developed to model these anomalies of the lattice thermal expansion based on temperature‐dependent X‐ray powder diffraction data. Inspired by Landau's theory of second‐order phase transitions, a gliding function (G) that considers the excess energy above the DEA terms, necessary to drive the phase transition, is introduced. The G‐function has been considered as an additive constituent of the DEA model, leading to the DEA + G model that describes the temperature‐dependent internal energy of the unit cell implemented in the first‐order Mie‐Grüneisen zero pressure equation of state (MG‐EoS). The extended approach allows to describe the soft‐mode‐driven anharmonic internal energy contribution and to determine the critical temperature of both nuclear and magnetic phase transitions.

Plagioclase feldspars draw intensive research attention in planetary sciences because of their abundance in the Martian regolith. Crystal chemical studies on plagioclase feldspars would be of crucial importance for possible in situ resource utilization for future human settlement on Mars. This study focuses on the synthesis of representative plagioclase feldspars followed by simulation of mechanical weathering using ball milling. A series of (Ca1-xNa x )(Al2-xSi2+x)O8 plagioclase feldspars is synthesized perfoming the solid-state method, where the endmembers are the anorthite (CaAl2Si2O8) and albite (NaAlSi3O8). The bulk chemical composition, particularly the Al/Si ratio, of each member is determined from energy-dispersive X-ray spectroscopy, which is supported by X-ray powder diffraction data Rietveld refinements. Selective plagioclase members (x = 0.0, 0.4 and 1.0) are mechanically weathered using high-energy ball milling, leading to significant changes of microstructural features such as average crystallite size and micro-strain. Total scattering data are collected using in-house X-ray facilities and analyzed by pair distribution function refinements. The vibrational modes of the samples are evaluated by Raman spectroscopy, complementing the local structural description.

A procedure is presented to exactly obtain the apparent average crystallite size (ACS) of powder samples using standard in-house powder diffraction experiments without any restriction originating from the Scherrer equation. Additionally, the crystallite size distribution within the sample can be evaluated. To achieve this, powder diffractograms are background corrected and long-range radial distribution functions G(r) up to 300 nm are calculated from the diffraction data. The envelope function f env of G(r) is approximated by a procedure determining the absolute maxima of G(r) in a certain interval (r range). Fitting of an ACS distribution envelope function to this approximation gives the ACS and its distribution. The method is tested on diffractograms of LaB6 standard reference materials measured with different wavelengths to demonstrate the validity of the approach and to clarify the influence of the wavelength used. The latter results in a general description of the maximum observable average crystallite size, which depends on the instrument and wavelength used. The crystallite site distribution is compared with particle size distributions based on transmission electron microscopy investigations, providing an approximation of the average number of crystallites per particle.

Regolith draws intensive research attention because of its importance as the basis for fabricating materials for future human space exploration. Martian regolith is predicted to consist of defect-rich crystal structures due to long-term space weathering. The present report focuses on the structural differences between defect-rich and defect-poor forsterite (Mg2SiO4) - one of the major phases in Martian regolith. In this work, forsterites were synthesized using reverse strike co-precipitation and high-energy ball milling (BM). Subsequent post-processing was also carried out using BM to enhance the defects. The crystal structures of the samples were characterized by X-ray powder diffraction and total scattering using Cu and synchrotron radiation followed by Rietveld refinement and pair distribution function (PDF) analysis, respectively. The structural models were deduced by density functional theory assisted PDF refinements, describing both long-range and short-range order caused by defects. The Raman spectral features of the synthetic forsterites complement the ab initio simulation for an in-depth understanding of the associated structural defects.

Nanoporous gold (npAu) attracted increasing attention over the last 20 years as a highly active and selective oxidation catalyst in particular at low temperatures. Previous research mainly focused on npAu that was fabricated by corrosive dealloying of AuAg parent alloys. Yet, the use of other binary alloys, such as AuCu, promises interesting variations of the catalytic properties, when considering that residual amounts of the less noble metal were shown to be co-catalytically involved. Aiming at providing a platform for systematic studies in this direction for Cu, we not only dealt with strategies for a reliable and reproducible preparation of npAu(Cu) catalysts from AuCu, but also with their potential for CO oxidation in comparison to npAu(Ag). We were able to develop an approach based on thermally quenched Au0.3Cu0.7 alloys, providing distinct synthetic advantages as a starting material for the catalyst fabrication versus the thermodynamically more stable AuCu3 intermetallic compound. Using PCD (potentiostatically controlled dealloying), well-defined pore structures with ligament diameters of similar to 40 nm and variable residual Cu concentrations in the range between similar to 0.6 at % and similar to 1.2 at % could be straightforwardly obtained. After activating such catalysts at 150 degrees C, they reproducibly showed catalytic activity for aerobic CO oxidation in a broad temperature window between 40 degrees C and 250 degrees C. As opposed to npAu(Ag), the activity increased with decreasing residual Cu content, outperforming the former at temperatures above similar to 60 degrees C not only with respect to CO2 formation rates but also with respect to thermal stability. Based on X-ray photoelectron spectroscopic and transmission electron microscopic results, it was possible to conclude that Cu segregates to the surface and, with rising Cu bulk content, increasingly occurs in form of Cu2+ species at the surface. While the latter are expected to be catalytically inactive, Cu and Cu+ species are likely candidates for the activation of oxygen being not possible on pure Au. Nanoporous gold has emerged as a highly active oxidation catalyst, especially at low temperatures. While in the past it was predominantly synthesized from AuAg alloys, this study explores AuCu as starting material. Since residues of the less noble metal are co-catalytically involved, deviating properties are expected and were indeed observed. image

To fabricate metals from the base materials for future Mars exploration, synthesis of representative olivine phases and their structural and spectroscopic characterizations are of crucial importance. Using mechanochemical technique that mimics the mechanical weathering, a complete solid solution of (Mg1-xFex)(2)SiO4 has been synthesized to investigate the associated crystal chemical properties. X-ray powder diffraction data Rietveld analysis confirms that each polycrystalline sample crystallizes in space group Pbnm. The average crystallite size ranges between 80(1) nm and 223(4) nm. Each lattice parameter increases with increasing Fe-content due to the larger Fe2+ radius than that of Mg2+, following Vegard's rule. For a given nominal chemical composition, substitution of Mg with Fe at the M1-site (4a: 0,0,0) is preferred to the M2-site (4c: x,y,1/4). As a consequence, the average Fe-content lies slightly below the equivalence line for x = 0.2-0.8, indicating that the Fe/Mg ratio in the amorphous scattering content is most likely greater than unity. Characteristic Raman spectral features of the olivines have been explained in terms of the chemical composition (x). Simple regression models are demonstrated based on both X-ray diffraction and Raman spectroscopic data for the calculation of Mg/Fe in olivines. Diffuse reflectance UV/Vis spectra RATD analysis shows each olivine phase possesses direct band-gap between 3.38(3) eV and 4.90(3) eV. This study could keep valuable information to relevant databases for future human missions on Mars, in particular, for precise estimation of the representative olivines from the remote X-ray diffraction and spectroscopic data.

AbstractSodalites of the general type |Na8X2|[T1T2O4]6with X = Cl−, Br−, I−have been synthesized for Al–Si, Ga–Si, Al–Ge and Ga–Ge as T1–T2frameworks. The structures were examined using in-house and synchrotron X-ray diffraction, Raman spectroscopy, force-field structure optimizations and DFT basedab-initiomolecular dynamics (MD) computations. Calculated phonon density of states (PDOS) of the 12 compounds show only minor differences within a framework composition with a lowering of certain phonon energies with increasing anion size. Earlier published Debye and Einstein temperatures obtained with a Debye-Einstein-anharmonicity (DEA) model approach are confirmed using the determined low-temperature lattice parameters (18 K–293 K) and show no correlation with the respective PDOS. Small-box refinements against radial pair distribution functions (PDF) allowed the determination of anisotropic displacement ellipsoids (ADP) for Na+and O2−, indicating a strong dependency of the ADP of Na+on the chemical composition. Significantly lower thermal displacements from MD calculations suggested an influence of structural displacements. For compounds with an aspherical ADP for sodium, structural models could be refined in which the sodium is located on two8eor one24isite (both partially occupied), and also temperature-dependent (100 K–300 K) for the compounds with Ga–Ge framework. 3D-plots of the bond-valence sums of Na+further validate the structural differences. These results imply that the local structure of halide-sodalites in many cases is not best described by the known average structure and may even not be cubic.

A novel polymorph of ThB2O5, denoted as β-ThB2O5, was synthesised under high-temperature high-pressure (HT/HP) conditions. Via single crystal X-ray diffraction measurements, β-ThB2O5 was found to form a three-dimensional (3D) framework structure where thorium atoms are ten-fold oxygen coordinated forming tetra-capped trigonal prisms. The only other known polymorph of ThB2O5, denoted α, synthesised herein using a known borax, B2O3-Na2B4O7, high temperature solid method, was found to transform to the β polymorph when exposed to conditions of 4 GPa and ∼900 °C. Compared to the α polymorph, β-ThB2O5 has smaller molar volume by approximately 12%. Exposing a mixture of the α and β polymorphs to HT/HP conditions ex situ further demonstrated the preferred higher-pressure phase being β, with no α phase material being observed via Rietveld refinements against laboratory X-ray powder diffraction (PXRD) measurements. In situ heating PXRD measurements on α-ThB2O5 from RT to 1030 °C indicated that α-ThB2O5 transforms to the β variant at approximately 900 °C via a 1st order mechanism. β-ThB2O5 was found to exist only over a narrow temperature range, decomposing above 1050 °C. Ab initio calculations using density functional theory (DFT) with the Hubbard U parameter indicated, consistent with experimental observations, that β is both the preferred phase at higher temperatures and high pressures. Interestingly, it was found by switching from B2O3-Na2B4O7 to H3BO3-Li2CO3 flux using consistent high temperature solid state conditions for the synthesis of the α variant, β-ThB2O5 could be generated. Comparison of their single crystal measurements showed this was identical to that obtained from HT/HP conditions.

A new precursor for the formation of mullite-type visible-light active photocatalyst Bi 2 Al 4 O 9 has been identified. The crystal structure of the organic–inorganic hybrid perovskite can be described using the hexagonal setting of the rhombohedral unit cell with lattice parameters a = 1.1342(2) nm, c = 2.746(1) nm, and V = 3.059(2) nm 3 . The presence of di-nitro-glycerin as organic component, which is centered together with two bismuth atoms at the A-sites of the ABX 3 -type perovskite, suggests for doubling of the a - and c -lattice parameters compared to isostructural BiAlO 3 perovskite. The nano-crystalline precursor with the chemical composition [Bi 2 (C 3 H 5 N 2 O 7 )]Al 4 [O 9 (□ 1- x (H 2 O) x ) 3 ] (□: vacancies) decomposes at 540(10) K to a quantum-crystalline phase with an average crystallite size of 1.4(1) nm, refined from X-ray powder data Bragg reflections and confirmed by atomic pair distribution function data analysis. Further heating enables a controlled formation of quantum- or nano-crystalline mullite-type phases, depending on temperature and time. The same precursor structure could also be obtained as iron-containing phase and for Al/Fe solid-solution samples. UV/Vis diffuse reflectance spectroscopy suggests an indirect band-gap transition energy of 3.50(3) eV calculated by the Reflectance-Absorption-Tauc-DASF (RATD) method. Temperature-dependent UV/Vis allows to follow the change of band-gap energy across all associated phase transformations. The long- and short-range appearance of each phase has been presented using X-ray Bragg scattering and total scattering data analyses. This is supported by Raman and infrared spectroscopic investigations complemented by density functional theory (DFT) calculations. Moreover, the theoretical calculation confirms the incorporated di-nitro-glycerin. Thermal stabilities of the phases are investigated by using thermal analysis and temperature-dependent X-ray diffraction.

Abstract Twelve cubic sodalites |Na8X2|[T1T2O4]6 (T1 = Al3+, Ga3+; T2 = Si4+, Ge4+; X = Cl−, Br−, I−) were examined using high-temperature (HT) X-ray diffraction experiments and TGA-DSC measurements. Temperature-dependent structure data was obtained by Rietveld refinements. Decomposition temperatures were determined using TGA-DSC data for all compounds. The temperature-dependent volume expansion was used to determine Debye and Einstein temperatures using DEA fits. Distinct relations between thermal expansion, bond lengths and the decomposition temperature could not be found. Determination of Lindemann constants of all compounds enables a classification of the sodalites in three groups.

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Abstract The temperature-dependent structure-property relationships of the aluminosilicate perrhenate sodalite |Na8(ReO4)2|[AlSiO4]6 (ReO4-SOD) were analysed via powder X-ray diffraction (PXRD), Raman spectroscopy and heat capacity measurements. ReO4-SOD shows two phase transitions in the investigated temperature range (13 K < T < 1480 K). The first one at 218.6(1) K is correlated to the transition of dynamically ordered P4¯3n$P\overline{4}3n$ (> 218.6(1 K) to a statically disordered (<218.6(1) K) SOD template in P4¯3n$P\overline{4}3n$. The loss of the dynamics of the template anion during cooling causes an increase of disorder, indicated by an unusual intensity decrease of the 011-reflection and an increase of the Re-O2 bond length with decreasing temperature. Additionally, Raman spectroscopy shows a distortion of the ReO4 anion. Upon heating the thermal expansion of the sodalite cage originated in the tilt-mechanism causes the second phase transition at 442(1) K resulting in a symmetry-increase from P4¯3n$P\overline{4}3n$ to Pm3¯n$Pm\overline{3}n$, the structure with the sodalites full framework expansion. Noteworthy is the high decomposition temperature of 1320(10) K.

Abstract Synthesis, crystal structure and temperature-dependent behavior of Na2H4Ga2GeO8 are reported. This novel gallogermanate crystallizes in space group I41/acd with room-temperature powder diffraction lattice parameters of a = 1298.05(1) pm and c = 870.66(1) pm. The structure consists of MO4 (M = Ga, Ge) tetrahedra in four-ring chains, which are connected by two different (left- and right-handed) helical chains of NaO6 octahedra. Protons coordinating the oxygen atoms of the GaO4 tetrahedra not linked to germanium atoms ensure the charge balance. Structure solution and refinement are based on single crystal X-ray diffraction measurements. Proton positions are estimated using a combined approach of DFT calculations and NMR, FTIR and Raman spectroscopic techniques. The thermal expansion was examined in the range between T = 20(2) K and the compound’s decomposition temperature at 568(5) K, in which no phase transition could be observed, and Debye temperatures of 266(11) and 1566(65) K were determined for the volume expansion.

Spinel ferrites such as nickel ferrite are promising energy conversion photocatalysts as they are visible-light absorbers, chemically stable, earth abundant, and inexpensive. Nickel ferrite shows poor photocatalytic activity due to fast electron–hole recombination upon illumination. This study evaluates the capability of carbon dots (CDs) to improve charge-carrier separation in NiFe2O4. We report a facile solvothermal approach for synthesizing NiFe2O4 and CDs/NiFe2O4 nanoparticles at 200–215 °C. The photocatalysts were characterized using transmission and scanning electron microscopy, x-ray diffraction, Raman spectroscopy, Fourier transform infrared spectroscopy, UV-VIS-NIR spectroscopy, photoelectrochemical analysis, and laser flash photolysis. Photocatalytic oxidation of methanol to formaldehyde under visible light was employed to test the effect of CDs on the photocatalytic efficacy of NiFe2O4. UV-VIS-NIR spectroscopy depicted a total quenching of NIR absorption and a diminished absorption of a peak at ∼745 nm in CDs/NiFe2O4 compared with NiFe2O4, indicating a transfer of electrons from NiFe2O4 to CDs. A 12-fold increment in the incident-photon-to-charge-efficiency was achievable with CDs/NiFe2O4 (0.36%) compared with NiFe2O4 (0.03%). Impedance spectroscopy exhibited a more efficient charge separation and faster interfacial charge transfer in CDs/NiFe2O4 compared with pure NiFe2O4. This was accounted for by the lower initial quantity of charge carrier upon irradiation in CDs/NiFe2O4 compared with NiFe2O4 as detected from laser flash photolysis, indicating that CDs acted as electron acceptors and reservoirs in CDs/NiFe2O4. Compared with NiFe2O4, CDs/NiFe2O4 showed an enhanced photocatalytic activity toward formaldehyde formation. Consequently, CDs are good electron mediators for NiFe2O4, capable of improving charge-carrier separation and the photocatalytic activity of NiFe2O4.

Three-dimensional cellular automata (CA) models for cage template reactions in sodalites are developed. The CA's basic structure of cells and their neighborhood relations in sodalites is defined by their body-centered cubic net of cages. They are connected via four- and six-ring windows, with transport mechanism only allowed via the six-ring windows. Cage reactions are encoded in the CA's transformation rules. Two reactions are modelled: The decomposition of the permanganate ion in the sodalite cage at elevated temperatures and the transformation of the nitrite sodalite to the carbonate nosean in a CO2 atmosphere. In both cases parameters are extracted which can be compared with experimental results, showing a good agreement. Advantageous features of such CA are: The possibility to extract the contributions of single reaction steps, easy examination of the influence of crystallite shapes. Furthermore, the influence of the reaction's confinement on the cages and their connections are naturally included in these models.

An entry from the Inorganic Crystal Structure Database, the world’s repository for inorganic crystal structures. The entry contains experimental data from a crystal diffraction study. The deposited dataset for this entry is freely available from the joint CCDC and FIZ Karlsruhe Access Structures service and typically includes 3D coordinates, cell parameters, space group, experimental conditions and quality measures.

An entry from the Inorganic Crystal Structure Database, the world’s repository for inorganic crystal structures. The entry contains experimental data from a crystal diffraction study. The deposited dataset for this entry is freely available from the joint CCDC and FIZ Karlsruhe Access Structures service and typically includes 3D coordinates, cell parameters, space group, experimental conditions and quality measures.

Abstract The aluminosilicate iodide sodalite |Na8I2|[AlSiO4]6 was examined by temperature-dependent neutron time-of-flight powder diffraction from 5 K to 290 K and X-ray diffraction from 298 K to 1200 K. The temperature-dependent properties of the mean structure in space group P4̅3n were obtained by Rietveld analysis. A negative slope for the thermal expansion coefficient below 50 K could be observed, and the displacement parameters of the iodide ions indicate anharmonic effects. Local structure models (8×8×8 super cells) were refined against pair-distribution functions calculated from total scattering data collected at 5 K, 165 K and 240 K. The results indicate isotropic displacements for all atoms except for I-atoms, showing the effects of an anharmonic potential around this anion at very low temperatures.