Crystallography at non-ambient conditions and physical properties of the synthesized double perovskites, Sr2(Co1-xFex)TeO6.

Polycrystalline double perovskite-type Sr2(Co1-xFex)TeO6 with various stoichiometric compositions (x = 0, 0.25, 0.5, 0.75, and 1) were synthesized by solid-state reactions in air. The crystal structures and phase transitions of this series at different temperature intervals were determined by X-ray powder diffraction, and from the obtained data the crystal structures were refined. It has been proven that for the compositions x = 0.25, 0.50, and 0.75, the phases crystallize at room temperature in the monoclinic space group I2/m. Down to 100 K, depending on the composition, these structures experience a phase transition from I2/m to P21/n. At high temperatures up to 1100 K their crystal structures show two further phase transitions. The first one is a first-order phase transition, from monoclinic I2/m to tetragonal I4/m, followed by a second-order phase transition to cubic Fm3̄m. Therefore, the phase transition sequence of this series detected at temperatures ranging from 100 K to 1100 K is P21/n → I2/m → I4/m → Fm3̄m. The temperature-dependent vibrational features of the octahedral sites were investigated by Raman spectroscopy, which furthermore complements the XRD results. A decrease in the phase-transition temperature with increasing iron content has been observed for these compounds. This fact is explained by the progressive diminishing of the distortion of the double-perovskite structure in this series. Using room-temperature Mössbauer spectroscopy, the presence of two iron sites is confirmed. The two different transition metal cations Co and Fe at the B sites allow exploring their effect on the optical band-gap.

Effects of iron substitution and anti-site disorder on crystal structures, vibrational, optical and magnetic properties of double perovskites Sr2(Fe1-xNix)TeO6.

The double-perovskite series, Sr2(Fe1-xNix)TeO6 (x = 0, 0.25, 0.50, 0.75, and 1) has been synthesized in polycrystalline form by solid-state reaction at 1300 K in air. Their crystal structures were probed by powder X-ray diffraction at room temperature. Rietveld analysis revealed that all samples crystallize in the monoclinic space group I2/m. The double-perovskite structures ideally contain two alternating types of octahedra (Fe/Ni)2dO6 and (Te)2aO6, tilted in the system (a-a-c0). However, the refinements have shown a complex distribution of all three cations over the two available octahedral sites; 2d (½, ½, 0) and 2a (0, 0, 0). Raman spectroscopy further complements the obtained results, by revealing a tiny increase of the wavenumber of some Raman modes when Fe is substituted by Ni. The optical characteristics of the series were determined by fitting diffuse reflectance UV/Vis spectra enabling the optical band gaps to be derived from Tauc method and derivation of absorption spectra fitting (DASF) techniques. Analyses of the obtained 57Fe Mössbauer hyperfine parameters at room temperature of samples with compositions x = 0, 0.25, 0.50 and 0.75 reveal the presence of Fe3+ in high-spin state with an anti-site disorder of Fe-Ni-Te cations in distorted octahedral environments (site 2d and 2a). The results show that significant correlations exist between the crystal structures and physical properties of double perovskites containing B site transition elements of different charge and size. Temperature-dependent magnetic susceptibility data show magnetic transitions below 40(1) K (38(1) K, 31(1) K, 25(1) K, 20(1) K, and 35(1) K for x = 0, 0.25, 0.50, 0.75, and 1, respectively. A divergence between FC and ZFC curves for all compositions has been observed. The results show that the ground states of the doped materials might be spin glasses or magnetically ordered.

Understanding the role of flux, pressure and temperature on polymorphism in ThB2O5.

A novel polymorph of ThB2O5, denoted as ß-ThB2O5, was synthesised under high-temperature high-pressure (HT/HP) conditions. Via single crystal X-ray diffraction measurements, ß-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 ß polymorph when exposed to conditions of 4 GPa and ∼900 °C. Compared to the α polymorph, ß-ThB2O5 has smaller molar volume by approximately 12%. Exposing a mixture of the α and ß polymorphs to HT/HP conditions ex situ further demonstrated the preferred higher-pressure phase being ß, 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 ß variant at approximately 900 °C via a 1st order mechanism. ß-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 ß 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, ß-ThB2O5 could be generated. Comparison of their single crystal measurements showed this was identical to that obtained from HT/HP conditions.

KLi2RE(BO3)2 (RE = Dy, Ho, Er, Tm, Yb, and Y): Structural, Spectroscopic, And Thermogravimetric Studies on a Series of Mixed-Alkali Rare-Earth Orthoborates.

We report a detailed structural, spectroscopic, and thermogravimetric investigation of a new series of mixed-alkali rare-earth orthoborates KLi2RE(BO3)2 (RE = Dy, Ho, Er, Tm, Yb, and Y). Single crystals were directly prepared by a flux method as well as mechanically separated from the polycrystalline powder obtained from the conventional solid-state reactions. All KLi2RE(BO3)2 members are isotypic and crystallize in the space group P21/n. The novel structure type is comprised of [RE2(BO3)4O4]14- anionic clusters where the edge-sharing REO7 pentagonal bipyramids are connected by BO3 groups and both K+ and Li+ cations are located at the interstitial voids of the 3D network. The metric parameters and crystal structural features obtained from the single-crystal data are in excellent agreement with those refined from the powder data. The change of the lattice parameters and unit cell volumes can be explained in terms of the lanthanide contraction effect. A comparison between KLi2RE(BO3)2 and other rare-earth borates with similar chemical compositions indicates that the sum of the ionic radii of the alkali-metal cations governs the symmetry of the crystals. Diffuse reflectance UV-vis spectra display the characteristic absorption behaviors of the RE3+ cations and the fundamental absorption edge. Both the Tauc's and derivation of absorption spectrum fitting (DASF) methods were used to identify the magnitude and type of bandgap, respectively, which are compared with those obtained from density functional theory (DFT) calculations. The calculated phonon density of states and the vibrational frequency at the gamma point help explain the Fourier transform infrared and Raman spectra of KLi2RE(BO3)2. The incongruent melting behavior and the thermal stability of each member of the KLi2RE(BO3)2 series were also studied by thermogravimetric analyses.

Synthesis and Thermal Investigations of Eleven-Membered Ring Systems Containing One of the Heavier Group 14 Element Atoms Si, Ge, and Sn.

Unique eleven-membered rings containing silicon, germanium, and tin were synthesized in good yields by the reactions of the corresponding 1,2-bis((2-bromothiophen-3-yl)methoxy)benzenes with (C6H5)2ECl2 where E = Sn, Ge, Si. The Sn and Ge congeners were crystallized, but the conformers that these rings crystallized in, were quite different. As confirmed by Density Functional Theory (DFT) calculations, (C28H22O2S2Sn) assumes a unique crystal structure that leaves more room around the tetrel atom as compared to the crystal structure of the corresponding Ge compound. In the latter, the central cavity is quite open, whereas in the former, one of the methylene groups can fold inwards. Another consequence is the influence on the planes of the aromatic rings flanking the heterocycle. In the Ge case, the benzene ring is folded away from the central cavity, whereas in the Sn case, it is almost parallel to the imaginary axis through the center of the ring. Thermal analysis investigations (TGA and DSC methods) of these eleven-membered rings suggested the loss of a phenyl group in the first decomposition step. The decomposition temperature decreased from the Si containing heterocycle to Ge and was lowest for the Sn containing heterocycle.

An Old Dog with New Tricks - Additions to the Cesium Lithium Chloride System: Cs3Li2Cl5 and the Hydrated Cs3LiCl4 · 4H2O.

The CsCl/LiCl system has been studied for over a century now. Numerous phases have been predicted - only three have hitherto been found. We present the synthesis and single-crystal structure of the cesium lithium pentachloride Cs3Li2Cl5, predicted earlier but with a different structure. The anhydrous new phase readily reacts to Cs3LiCl4 · 4H2O in air. The tetrahydrate can also be obtained through the simplest, most inexpensive and green synthesis possible: an immediate, room-temperature mechanosynthesis from only CsCl and LiCl (3 : 1) in air. Differential scanning calorimetry (DSC) and thermogravimetric analyses (TGA), as well as in situ temperature-dependent powder X-ray diffraction studies on this second ever reported ternary alkali chloride hydrate allowed for a revision of the CsCl/LiCl phase diagram. Density of states and total energy calculations further elucidate the new alkali chlorides and update the relative stability of previous structure predictions.

Thermochromic Behavior of Yttrium-Substituted Bismuth Oxides.

High-temperature thermochromic materials are poorly explored in fundamental research, let alone applied research, although these materials may be used as low-cost, intuitively usable sensing materials in an industrial environment. Yet, only few of these materials have been described systematically. We describe a series of yttrium-substituted bismuth oxides (Bi1-xYx)2O3 (0.05 ≤ x ≤ 0.25) that show thermochromic behavior with a color change from yellow at low temperatures to various brown hues at high temperatures. The compounds were analyzed between 293 and 1050 K by X-ray powder diffraction, UV/vis spectroscopy, and differential scanning calorimetry. A combination of derived absorption spectral fitting and Tauc methods was applied to determine the band gap energies and band gap types from the diffuse UV/vis spectra, respectively. Two types of materials were found: one with x = 0.05 that exhibits the tetragonal ß-phase at room temperature, and the defect fluorite-type cubic δ-phase at temperatures above 920 K. This phase showed a reversible, gradual color change upon heating, followed by an abrupt color change at the phase-transformation temperature. The second type of material had higher yttrium contents (x > 0.10); these samples were cubic at room temperature and showed a continuous color change upon heating and cooling. In contrast to the material with x = 0.05, these latter phases show a reduced cycle stability and were gradually annealed to the hexagonal phase-I. The samples with x = 0.10 provided a mixture of the ß- and δ-phases, showing both, the reversible behavior for the ß- to δ-phase transition and the irreversible behavior concerning the ß2-phase. This points the way toward smart materials that can not only sense the actual thermal stress but also monitor cumulative thermal stresses over a certain material lifetime.

Structural, vibrational and electronic properties of SnMBO4 (M = Al, Ga): a predictive hybrid DFT study.

We propose two new members of the mullite-type family, SnAlBO4 and SnGaBO4, and carry out an in-depth study of their crystal properties using the hybrid method PW1PW. Both are isostructural to PbMBO4 (M = Fe, Mn, Al, Ga), which show axial negative linear compressibility (ANLC), among other interesting features. We find that, although Sn2+ is susceptible of being oxidized by oxygen, a suitable range of experimental parameters exists in which the compounds could be synthesized. We observe absence of ANLC below 20 GPa and explain it by the small space occupied by the lone electron pairs, as indicated by the small length of the corresponding Liebau Density Vectors. In agreement with this fact, the structures present a low number of negative mode-Grüneisen parameters, which may also suggest lack of negative thermal expansion. The electronic properties show a remarkable anisotropic behaviour, with a strong dependence of the absorption spectra on light polarization direction.

Nanoporous gold functionalized with praseodymia-titania mixed oxides as a stable catalyst for the water-gas shift reaction.

Dealloyed nanoporous metals hold great promise in the field of heterogeneous catalysis; however their tendency to coarsen at elevated temperatures or under catalytic reaction conditions sometimes limit further applications. Here, we report on a highly stable nanoporous gold catalyst (npAu) functionalized with praseodymia-titania mixed oxides as synthesized by a sol-gel method. Specifically, we used aberration-corrected transmission electron microscopy to study the morphology and the interface between the oxide deposits and the npAu substrate at the atomic level. Based on electron energy loss spectroscopy (EELS), it is concluded that Pr-TiOx mixed oxides form a solid solution. Flow reactor tests reveal that the Pr-TiOx functionalized nanoporous gold is not only highly active but also very stable for the water gas shift reaction in a large temperature range (180-400 °C). Our results demonstrate the potential of engineering the compositions of oxides coatings on npAu for advanced functional systems.

Effect of the degree of inversion on optical properties of spinel ZnFe2O4.

Spinel ferrites (T[M1-xFex]O[MxFe2-x]O4 with 0 ≤ x ≤ 1, where M is a bivalent metal ion and the superscripts denote tetrahedral and octahedral sites) are materials commonly used in electronics due to their outstanding magnetic properties. Thus, the effect of the degree of inversion, x, on these properties is well known. However, its effect on other properties of these materials has rarely been investigated in detail. Since ferrites gained much attention during the last decade as visible light active photocatalysts and photoelectrocatalysts, understanding the effect of the degree of inversion on the optical properties became necessary. Among photocatalytically and photoelectrocatalytically active spinel ferrites, zinc ferrite (ZnFe2O4, ZFO) is one of the most widely studied materials. In this work, five ZFO samples with degrees of inversion varying from 0.07 to 0.20 were prepared by a solid-state reaction employing different annealing temperatures and subsequent quenching. Raman and UV-Vis-NIR spectra were measured and analyzed together with theoretical results obtained from ab initio calculations. Changes in the UV-Vis-NIR spectra associated with electronic transitions of tetrahedrally and octahedrally coordinated Fe3+ ions are distinguished. However, the optical band gap of the material remains unchanged as the degree of inversion varies. Based on the experimental and theoretical results, a new assignment for the Raman active internal modes and the electronic transitions of ZFO is proposed.

Symmetry reduction due to gallium substitution in the garnet Li6.43(2)Ga0.52(3)La2.67(4)Zr2O12.

Single-crystal structure refinements on lithium lanthanum zirconate (LLZO; Li7La3Zr2O12) substituted with gallium were successfully carried out in the cubic symmetry space group I [Formula: see text]3d. Gallium was found on two lithium sites as well as on the lanthanum position. Due to the structural distortion of the resulting Li6.43(2)Ga0.52(3)La2.67(4)Zr2O12 (Ga-LLZO) single crystals, a reduction of the LLZO cubic garnet symmetry from Ia[Formula: see text] d to I [Formula: see text]3d was necessary, which could hardly be analysed from X-ray powder diffraction data.

Giant Volume Change and Topological Gaps in Temperature- and Pressure-Induced Phase Transitions: Experimental and Computational Study of ThMo2 O8.

By applying high temperature (1270 K) and high pressure (3.5 GPa), significant changes occur in the structural volume and crystal topology of ThMo2 O8 , allowing the formation of an unexpected new ThMo2 O8 polymorph (high-temperature/high-pressure (HT/HP) orthorhombic ThMo2 O8 ). Compared with the other three ThMo2 O8 polymorphs prepared at the ambient pressure (monoclinic, orthorhombic, and hexagonal phases), the molar volume for the quenched HT/HP-orthorhombic ThMo2 O8 is decreased by almost 20 %. As a result of such a dramatic structural transformation, a permanent high-pressure quenchable state is able to be sustained when the pressure is released. The crystal structures of the three ambient ThMo2 O8 phases are based on three-dimensional (3D) frameworks constructed from corner-sharing ThOx (x=6, 8, or 9) polyhedra and MoO4 tetrahedra. The HT/HP-orthorhombic ThMo2 O8 , however, crystallizes in a novel structural topology, exhibiting very dense arrangements of ThO11 and MoO4+1 polyhedra connecting along the crystallographic c axis. The phase transitions among all four of these ThMo2 O8 polymorphs are unveiled and fully characterized with regard to the structural transformation, thermal stability, and vibrational properties. The complementary first principles calculations of Gibbs free energies reveal the underlying energetics of the phase transition, which support the experimental findings.

Dinuclear Face-Sharing Bi-octahedral Tungsten(VI) Core and Unusual Thermal Behavior in Complex Th Tungstates.

Invited for the cover of this issue are the groups of Evgeny V. Alekseev at the Forchungszentrum Jülich and Thorstem M. Gesing at the University of Bremen. The image depicts the complex thorium tungstate polyanions, having a six-leafed lily cross-section, containing a rare confacial [W2 O9 ](6-) bioctahedral core. Read the full text of the article at 10.1002/chem.201500500.

Dinuclear Face-Sharing Bi-octahedral Tungsten(VI) Core and Unusual Thermal Behavior in Complex Th Tungstates.

Two new thorium tungstates A6 Th6 (WO4 )14 O (A=K and Rb) were synthesized by high-temperature solid-state reactions. The structures of both phases are based on a rare dinuclear confacial bi-octahedral [W2 O9 ](6-) core, encapsulated in a [Th6 W7 O46 (W2 O9 )](32-) cage showing a cross-section geometry similar to a six-leafed lily. The adjacent cages are connected in two dimensional layers by WO4 tetrahedral linkers. Due to the dissimilarities in mutual orientations of adjacent layers in these two structures, K6 Th6 (WO4 )14 O crystallizes in space group of R32 while Rb6 Th6 (WO4 )14 O stabilizes in P$\bar 6$2c. The high-temperature phase transition was observed in Rb6 Th6 (WO4 )14 O and investigated using high-temperature PXRD technique. The results demonstrate a very unusual thermal behavior of this compound. The Raman and IR spectra of both phases were analyzed with respect to their complex structures.

Morphotropy and temperature-driven polymorphism in A2Th(AsO4)2 (A = Li, Na, K, Rb, Cs) series.

A new alkaline thorium arsenate family was obtained and systematically investigated. The structures of A2Th(AsO4)2 (A = Li, Na, K, Rb, Cs) were determined from single crystal X-ray diffraction data. Li2Th(AsO4)2 and either isostructural K2Th(AsO4)2 and Rb2Th(AsO4)2 crystallize in the monoclinic crystal system. Na2Th(AsO4)2 and Cs2Th(AsO4)2 crystallize in the orthorhombic and tetragonal crystal systems, respectively. Li2Th(AsO4)2 consists of [Th(AsO4)2](2-) layers with Li atoms in the interlayer space. The rest of the compounds are based on 3D frameworks. Differences in local environments of ThO8 coordination polyhedra are described in relation to the symmetry. Despite different local environments of ThO8 coordination polyhedra and different structural symmetry, underlying nets of A2Th(AsO4)2 (A = Na, K, Rb, Cs) were shown to be the same. Single-crystal and powder Raman spectra were measured, and bands are assigned. DSC measurements showed phase transitions in K2Th(AsO4)2 and Rb2Th(AsO4)2, which were studied using high-temperature powder X-ray diffraction (HT-PXRD). The data of HT-PXRD demonstrates two high-temperature polymorphic modification of K2Th(AsO4)2 and only one for the isotypic Rb2Th(AsO4)2. The phase transitions in both K and Rb phases are reversible.

Sol-gel preparation of alumina stabilized rare earth areo- and xerogels and their use as oxidation catalysts.

A new sol-gel synthesis route for rare earth (Ce and Pr) alumina hybrid aero- and xerogels is presented which is based on the so-called epoxide addition method. The resulting materials are characterized by TEM, XRD and nitrogen adsorption. The results reveal a different crystallization behavior for the praseodymia/alumina and the ceria/alumina gel. Whereas the first remains amorphous until 875°C, small ceria domains form already after preparation in the second case which grow with increasing calcination temperature. The use of the calcined gels as CO oxidation catalysts was studied in a quartz tube (lab) reactor and in a (slit) microreactor and compared to reference catalysts consisting of the pure rare earth oxides. The Ce/Al hybrid gels exhibit a good catalytic activity and a thermal stability against sintering which was superior to the investigated reference catalyst. In contrast, the Pr/Al hybrid gels show lower CO oxidation activity which, due to the formation of PrAlO3, decreased with increasing calcination temperature.

High-temperature phase transitions, spectroscopic properties, and dimensionality reduction in rubidium thorium molybdate family.

Four new rubidium thorium molybdates have been synthesized by high-temperature solid-state reactions. The crystal structures of Rb8Th(MoO4)6, Rb2Th(MoO4)3, Rb4Th(MoO4)4, and Rb4Th5(MoO4)12 were determined using single-crystal X-ray diffraction. All these compounds construct from MoO4 tetrahedra and ThO8 square antiprisms. The studied compounds adopt the whole range of possible structure dimensionalities from zero-dimensional (0D) to three-dimensional (3D): finite clusters, chains, sheets, and frameworks. Rb8Th(MoO4)6 crystallizes in 0D containing clusters of [Th(MoO4)6](8-). The crystal structure of Rb2Th(MoO4)3 is based upon one-dimensional chains with configuration units of [Th(MoO4)3](2-). Two-dimensional sheets occur in compound Rb4Th(MoO4)4, and a 3D framework with channels formed by thorium and molybdate polyhedra has been observed in Rb4Th5(MoO4)12. The Raman and IR spectroscopic properties of these compounds are reported. Temperature-depended phase transition effects were observed in Rb2Th(MoO4)3 and Rb4Th(MoO4)4 using thermogravimetry-differential scanning calorimetry analysis and high-temperature powder diffraction methods.

Neutral and cation-free LTA-type aluminophosphate (AlPO(4)) molecular sieve membrane with high hydrogen permselectivity.

A novel neutral and cation-free LTA-type AlPO(4) membrane has been prepared on porous asymmetric ceramic supports. Hydrogen can be effectively separated from other gases by molecular sieving.

Guanidinium tetra-bromidomercurate(II).

The Hg atoms in the crystal structure of the title compound, (CH(6)N(3))(2)[HgBr(4)], are tetra-hedrally coordinated by four Br atoms and the resulting [HgBr(4)](2-) tetra-hedral ions are linked to the [C(NH(2))(3)](+) ions by bromine-hydrogen bonds, forming a three-dimensional network. In the structure, the anions are located on special positions. The two different Hg⋯Br distances of 2.664 (1) and 2.559 (1) Šobserved in the tetra-bromidomercurate unit are due to the connection of Br atoms to different number of H atoms.

Bis(guanidinium) tetra-iodidomercurate(II).

The Hg atom in the crystal structure of the title compound, (CH(6)N(3))(2)[HgI(4)], is tetra-hedrally coordinated by four I atoms. The [HgI(4)](2-) ions are inter-connected to the [C(NH(2))(3)](+) ions by N-H⋯I hydrogen bonds, forming a three-dimensional network. The four different observed Hg-I distances [2.760 (2), 2.7762 (15), 2.8098 (14) and 2.833 (2) Å] are consistent with four different (127)I NQR frequencies observed, showing the existence of four unique I atoms in the tetra-iodidomercurate unit.