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Oxyapatites A10(GeO4)6O2 (A – alkaline, rare‐earth elements) are considered promising materials for applications in solid‐state lighting and non‐contact thermometry. The temperature dynamics of the oxygen sublattice have a crucial influence on the luminescent properties of these optical hosts. Here, the structural evolution with temperature of germanates with the general formula ALa9(GeO4)6O2 (A – Li, Na, K, Rb) is investigated, using X‐ray and neutron powder diffraction methods. These compounds crystallize in space‐group P63/m. NPD experiments helped to determine the oxygen and alkaline metal positions. Thermally‐induced reentrant structural transitions relating to the oxygen in the apatite channel are discovered for the first time. In the example of Eu‐doped RbLa9(GeO4)6O2 and KLa9(GeO4)6O2, it is demonstrated that the luminescence of the Eu3+ ion is influenced by small changes in the crystal structure. [ABSTRACT FROM AUTHOR]

A new series of BaRE6(Ge2O7)2(Ge3O10) (RE = Tm, Yb, Lu) germanates and activated phases BaYb6(Ge2O7)2(Ge3O10):xTm3+ and BaLu6(Ge2O7)2(Ge3O10):12yYb3+,yTm3+ have been prepared using a solid-state reaction. An XRPD study has revealed that the compounds crystallize in the monoclinic system (space group P21/m, Z = 2). The crystal lattice consists of zigzag chains of edge-sharing distorted REO6 octahedra, bowed trigermanate [Ge3O10] units, [Ge2O7] groups, and eight-coordinated Ba atoms. The density functional theory calculations have confirmed a high thermodynamic stability of the synthesized solid solutions. According to the results of vibrational spectroscopy studies and diffuse reflectance measurements, the BaRE6(Ge2O7)2(Ge3O10) germanates are promising compounds for the creation of efficient lanthanide ion activated phosphors. Under 980 nm laser diode excitation, the BaYb6(Ge2O7)2(Ge3O10):xTm3+ and BaLu6(Ge2O7)2(Ge3O10):12yYb3+,yTm3+ samples exhibit upconversion luminescence corresponding to the characteristic 1G4 → 3H6 (455–500 nm), 1G4 → 3F4 (645–673 nm) and 3H4 → 3H6 (750–850 nm) transitions in Tm3+ ions. Heating of the BaLu6(Ge2O7)2(Ge3O10):12yYb3+,yTm3+ phosphor with the optimal composition up to 498 K leads to the enhancement of a broad band at 673–730 nm, caused by 3F2,3 → 3H6 transitions. It has been revealed that the fluorescence intensity ratio between this band and the band at 750–850 nm may be used for temperature sensing. The absolute and relative sensitivities in the studied temperature range reach 0.021% K−1 and 1.94% K−1, respectively. [ABSTRACT FROM AUTHOR]

The (NH4)3Sc(SO4)3:0.005Eu3+ phosphor has been synthesized as rod-like microparticles by crystallization from an aqueous solution. Its crystal structure belongs to the monoclinic system, space group P211/C, = 4. An enantiotropic first-order phase transition from monoclinic to high-temperature rhombohedral structure (space group R3c) occurs at 350--355 K and compound decomposition begins only above 573 K. Under 248 nm excitation, the compound exhibits luminescence corresponding to the characteristic 5D0 → 7F2 transitions in Eu3+ ions. The absence of an inversion center in the sites occupied by Eu3+ causes a high intensity of spectral lines at 602--642 nm associated with hypersensitive induced electric dipole 5D0 → 7F2 transitions. Heating of the (NH4)3Sc(SO4)3:0.005Eu3+ sample to a temperature of 393 K is accompanied by considerable distortion of the Eu3+ coordination by the phase transition, manifesting a gradual increase in the asymmetry ratio ( ) between the integral intensities of the lines at 602--642 nm (5 D4 → 7F2 transition) and at 581--602 nm (5 D0 → 7F1). When the heating is suppressed, the level returns to the original value, confirming the reversibility of the structural transformation. A good reproducibility of the obtained values has been revealed during 4-fold thermal cycling. [ABSTRACT FROM AUTHOR]

Some phyllosilicate compounds have the ability of spontaneous scrolling because of the size mismatch between the covalently bounded metal oxide and silica sheets. Their unique structure and high theoretically predicted Young's modulus (around 210–230 GPa) induce phyllosilicates' application as reinforcing fillers. However, previous nanomechanical experiments with individual phyllosilicate nanoscrolls are in poor agreement with theory. The main reason for this is the low accuracy of experiments, which leads to large measurement errors compared to measured average values. Here, the study of the mechanical properties of synthetic (Mg1–xNix)3Si2O5(OH)4 phyllosilicates is reported by testing a suspended nanoobject (a nanobridge) with an atomic force microscope (AFM). The Young's modulus of corresponding phyllosilicate model layers is also calculated by means of the density functional theory (DFT). The original AFM approach makes it possible to account for the probe slipping off the nanobridge and determine its boundary conditions. The measured Young's modulus values are considered within the models of surface tension and shear strain contributions. The shear strain appears to have a decisive impact on the measured Young's modulus (from 150 ± 70 GPa to 200 ± 210 GPa) and its spread. [ABSTRACT FROM AUTHOR]

Imogolite—a congregation of aluminosilicate nanotubes—has shown a great promise for fabrication of advanced materials. Imogolite nanotubes hold the record narrow diameters distribution and demonstrate one of the highest curvatures among noncarbon nanotubes. Current experimental and theoretical prospects for their modification are related mostly to surface functionalization or to progressive Si substitution on Ge, whereas a doping of Al sublattice is steadily neglected. The empirical rules of Hume–Rothery disallow such doping due to a large size mismatch between Al3+ and the majority of metal cations. Using the quantum‐chemical calculations, herein, the difference in coordination environment is unveiled for 12 sorts of trivalent cations at octahedral Al sites in both a highly curved imogolite layer and the flat gibbsite layer. The curvature of natural imogolite acts provoking for violation of the Hume–Rothery rules, sterically promoting the doping of Al sublattice by cations with radii 1.0–1.2 Å typical for rare‐earth elements. Theoretically, the synthesis of a metal‐doped imogolite is restricted only by the overlap of the phase stability fields for precursor reactants or by kinetic factors. This study might be inciting for the synthesis of chemically modified imogolite and related Ge‐imogolites. [ABSTRACT FROM AUTHOR]

A new series of Ba2RE2Ge4O13 (RE = Pr, Nd, Gd, Dy) germanates and Ba2Gd2−xEuxGe4O13 (x = 0.1–0.8) solid solutions have been synthesized using the solid-state reaction technique and characterized by X-ray powder diffraction. All compounds crystallize in the monoclinic system, space group C2/c, Z = 4. The crystal lattice consists of RE2O12 dimers, zigzag C2-symmetric [Ge4O13]10− tetramers, and ten-coordinated Ba atoms located in voids between polyhedra. The density-functional theory (DFT) calculations performed on a rich set of Ba2RE2Ge4O13 compounds have confirmed the high thermodynamic stability of monoclinic modification. Under ultraviolet (UV) light excitation Ba2Gd2−xEuxGe4O13 phosphors exhibit an orange-red emission corresponding to the characteristic f–f transitions in Eu3+ ions. The highest intensity of lines at 580 nm (5D0 → 7F0), 582–602 nm (5D0 → 7F1), 602–640 nm (5D0 → 7F2), 648–660 nm (5D0 → 7F3), and 680–715 nm (5D0 → 7F4) is observed for the samples with x = 0.4–0.6. The possibility of their application has been assessed by studying their color characteristics, quantum efficiency, and thermal stability. The obtained data indicate that Ba2Gd2−xEuxGe4O13 solids can be considered as promising materials for UV-excited phosphor-converted light-emitting diodes (LEDs) if an aluminum nitride substrate (λex = 255 nm) is used as a semiconductor chip. [ABSTRACT FROM AUTHOR]

A new series of BaY2−xEuxGe3O10 (x = 0.1–0.8) trigermanates has been synthesized using the solid-state reaction method. A powder XRD study has revealed that the compounds crystallize in the monoclinic system, space group P21/m, Z = 2. The crystal lattice consists of distorted Y/EuO6 octahedra, trigermanate [Ge3O10]8− groups and Ba atoms located in eight-fold coordinated irregular sites. Under UV and 465 nm light excitation, the samples exhibit reddish-orange emission corresponding to the characteristic 5D0 → 7FJ transitions in Eu3+ ions. Concentration dependence study has shown that the powders with europium content x = 0.4–0.6 possess the highest luminescence intensity under 393 nm and 465 nm excitation. It has also been revealed that the predominant mechanism causing concentration quenching in the BaY2−xEuxGe3O10 series is due to multipolar interactions. The investigations on colour characteristics, quantum efficiency and thermal stability performance of BaY2−xEuxGe3O10 phosphors were carried out for the feasibility study of their practical application. The results indicate that BaY2−xEuxGe3O10 can be considered as a promising material for blue-excited phosphor-converted LEDs. [ABSTRACT FROM AUTHOR]

Layered transition metal dichalcogenides (TMDCs) are an emerging family of catalysts. Included in this group is tungsten diselenide (WSe2), which has attracted recent attention for electro-optical applications. Tungsten is opportune because it is the heaviest transition metal in the TMDC family and is widely abundant on Earth. Moreover, WS2 and WSe2 are more benign in nature than their molybdenum counterparts. Despite this, WSe2 has been relatively unexplored as a catalyst. We report the synthesis of WSe2 doped with various transition metals (Fe, Co, Nb, Ni and Zr). Among the doped catalysts, Ni–WSe2 has been found to be the most promising electrocatalyst for the hydrogen evolution reaction (HER) as it possesses the smallest charge transfer resistance and thus facilitates a faster catalytic reaction. Upon doping with Ni, catalytic enhancement results from improved hydrogen adsorption (Hads). Beyond this threshold of Ni loading, the improved activity in alkaline medium is due to the optimized interaction of the OH/surface active sites. Using density functional theory calculations, we identified that the catalytic sites are Se atoms either bound to a substitutional Ni dopant or constituting a small patch of NiSe grafted on the WSe2 surface. [ABSTRACT FROM AUTHOR]

Sulfur‐rich transition metal polysulfides MS5 (M=Mo, W) are synthesized by a low‐temperature solution method from corresponding carbonyls M(CO)6 and elemental sulfur. Extensive characterization reveals that all sulfur atoms are assembled into disulfide ligands (S−S) within the structure of the amorphous spherical particles. Their thermodynamic stabilities are estimated for the first time using density functional theory (DFT) calculations, indicating two stable chain models composed either of binuclear [M2S8] or trinuclear [M3S12] fragments linked through S−S units. Molecular dynamics (MD) DFTB simulation proves that the S−S bridges predetermine the supreme flexibility of the polysulfide chains as primary structures of MS5 and their globular secondary arrangements. Interestingly, this type of structural organization is reminiscent of that for classical polymers. Thus, the reasons for MS5 forming exclusively as amorphous phases are uncovered, which may be extended to many other sulfur‐rich polysulfides. The potential of these materials as increased capacity cathodes for lithium‐ion batteries is shown. [ABSTRACT FROM AUTHOR]

A new series of BaRE2Ge3O10 (RE = Y, Sc, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu) trigermanates has been prepared using a solid-state reaction. Powder XRD study has revealed that the compounds crystallize in the monoclinic system, space group P21/m, Z = 2. The crystal lattice consists of edge-sharing REO6 octahedra, bowed trigermanate [Ge3O10]8− groups, and Ba atoms located in eight-fold coordinated irregular sites. The geometry of [Ge3O10]8− anions persisting within different trigermanates has been examined with a special focus on the values of Ge–Ge–Ge angles and the types of conformation between the central and terminal GeO4 tetrahedra. The arrangement of [Ge3O10]8− anions in BaRE2Ge3O10 (RE = Y, Sc, Gd–Lu) crystal lattices is found to be unique. DFT calculations of different polymorphs for BaY2Ge3O10 and BaLa2Ge3O10 compounds have confirmed that our fabrication protocol yields all the products in their most thermodynamically stable forms. According to the results of vibrational spectroscopy studies, diffuse reflectance and photoluminescence measurements, the BaRE2Ge3O10 trigermanates are promising compounds for the preparation of efficient lanthanide ion activated phosphors. [ABSTRACT FROM AUTHOR]

We study the photolytic degradation of planar (PCB 2, PCB 12, PCB 13, PCB 15) and non-planar (PCB 8, PCB 29, PCB 31) polychlorobiphenyls in an aqueous alcohol under UV irradiation (λ = 240–320 nm) at ambient temperature and atmospheric pressure for 25 h on nanocrystalline TiO2 and CdS/TiO2. It is found that a conversion of PCB 2 (12.5%), PCB 12 (42.3%) and PCB 29 (98.0%) is more intense in the presence of the CdS/TiO2 composite, whereas mixture of the congeners PCB 8, PCB 13, PCB 15 and the congener PCB 31 can be photolyzed better in the presence of TiO2 with conversion 35.2% and 96.1%, respectively. The different conversions of the PCB congeners are explained considering the structures of the chloroaromatic radicals formed in situ as a result of the primary photolysis process and are confirmed by means of estimation of dipole moments calculated for singlet states of PCBs. [ABSTRACT FROM AUTHOR]

The extensive family of transition metal chalcogenides has been comprehensively investigated owing to their diverse useful properties. However, even among them, there are ones that have received comparatively less attention; in particular, these are molybdenum and niobium sulfides and selenides with the composition of M : Q = 2 : 3 (M = Mo, Nb; Q = S, Se). Mo or Nb chalcogenides with this stoichiometry may adopt one of two structures: (i) sesquichalcogenides M2Q3, where important structural elements are infinite metal chains, or (ii) self-intercalated compounds M1.33Q2, in which extra M atoms are inserted between MQ2 layers. Depending on the M–Q combination, in practice, either none, one, or both of them may exist. The reasons for chemical dissimilarity in the series of seemingly related compounds haven’t been addressed until the present work. Here, we present the first generalized comparative study of these chalcogenides by quantum-chemical computations verified by laboratory experiments. High-temperature phases of Mo2S3 and Nb2Se3 may be stably isolated at room temperature, while “Nb2S3” and “Mo2Se3” had not been obtained, nor were they expected to exist from DFT data. The structure-determining motifs of sesquichalcogenides M2Q3 are metallic chains, and thus, apparently, if metal's electron deficiency (or excess) prevents the formation of M–M chains, then the M2Q3-type structure cannot form. If the metal has an adequate electron density and the structure does form at high temperature (as it happens for Mo2S3 and Nb2Se3), then it can be kinetically stabilized by quenching, and stored under laboratory conditions for long times. However, if Nb2Se3 is left to cool down slowly, it undergoes phase transition to iso-stoichiometric intercalate Nb1.333Se2, in good agreement with DFT predictions of the close values of their free energies. Isostructural intercalate Nb1.333S2 is found to be the only experimental product in the Nb–S system, in full accordance with DFT prediction. Effective stabilization of self-intercalated phases is provided by significant charge transfer from intercalated Nb atoms to the NbQ2 layers, as confirmed by DFT. The obtained data may serve to get insight into polymorphism of some less-studied transition metal chalcogenides and to promote their use for future functional materials. [ABSTRACT FROM AUTHOR]

The apatite-like NaLa9(GeO4)6O2:Nd3+,Ho3+ phosphor is prepared using the solid-state method. Rietveld refinement of high-resolution time-of-flight neutron powder diffraction measurements indicate that this compound crystallizes in the hexagonal system with space group P63/m, Z = 1 and unit cell parameters a = 9.88903(6) Å, c = 7.25602(5) Å, V = 614.521(7) Å3 at room temperature. The 4f sites are statistically occupied by La, Nd and Na, while 6h sites are occupied by La and Nd. Luminescence in the near- and middle-IR range caused by the transitions in neodymium and holmium ions is excited under 808 nm laser diode radiation. The highest emission intensity in NaLa9−x−yNdxHoy(GeO4)6O2 is attained at trace amounts of holmium, and it decreases sharply when y increases to 0.01. The IR phosphors have a good thermal stability and exhibit a very weak upconversion emission in the red spectral range upon 808 nm excitation. A scheme of excitation and emission pathways involving ground/excited state absorption, energy transfer, cross-relaxation, nonradiative multiphonon relaxation processes in Nd3+ and Ho3+ ions has been proposed. The data analysis indicates that Nd3+ ions serve as sensitizers for Ho3+ ions in these compounds, stimulating intense 2.1 μm and 2.7 μm emissions. These apatite-related germanate phosphors are promising materials for near- and middle-infrared solid-state lighting applications. [ABSTRACT FROM AUTHOR]

A new series of Sr3RE2(Ge3O9)2 (RE = La, Pr, Nd, Sm, Eu, Gd) cyclogermanates has been prepared using an ethylenediaminetetraacetic acid assisted route. The powder XRD study reveals that the compounds crystallize in the monoclinic space group C2/c, Z = 4. A decrease in the crystal radius of RE3+ ions in Sr3RE2(Ge3O9)2 (RE = La–Gd) causes a morphotropic phase transition followed by a step-like change of the lattice parameters. The transition is accompanied by internal reorganization, including the redistribution of rare earth and strontium ions among three symmetry independent Sr/RE sites, a conformation change of [Ge3O9]6− rings and a variation of Sr/RE(2) atom coordination from seven-fold to nine-fold. Density functional theory calculations reveal that Sr3RE2(Ge3O9)2 compounds are indirect bandgap materials with a band gap around 3.0 eV. The magnetic property measurements indicate that Sr3RE2(Ge3O9)2 (RE = Pr, Sm, Gd) are Curie–Weiss paramagnets up to 2 K. For Sr3Eu2(Ge3O9)2 an anomaly of magnetic susceptibility is found at about 60 K. A photoluminescence study of the Sr3Sm2(Ge3O9)2 and Sr3Eu2(Ge3O9)2 samples reveals an emission in the visible spectral range, corresponding to the characteristic 4f–4f transitions in rare earth ions. [ABSTRACT FROM AUTHOR]

The Mo–S phase diagram exhibits two stable compounds: well-recognized layered molybdenum disulfide MoS2 and less studied molybdenum sesquisulfide Mo2S3. Here, we build a theoretical framework for the exfoliation of Mo2S3 based on density functional theory calculations and further experimentally prove its dispersibility under ultrasonic treatment and demonstrate its potential for use in gas detectors. First, we report computational studies of both Mo2S3 exfoliation and molecular adsorption on the Mo2S3 surface. The calculations reveal the preferential direction for the fragmentation of the Mo2S3 crystal along the (1̅01) plane and, likely, chemisorptive interaction of solvent molecules, such as H2O or DMSO, with the (1̅01) Mo2S3 surface. Next, we experimentally study the sonication of bulk Mo2S3 samples in organic solvents and show that, indeed, they may be converted to colloidal nanosized sheets. Solid particles in the dispersions retain the initial Mo2S3 crystal structure and have the shape of plates with typical thicknesses of 5–30 nm and lateral sizes of 100–400 nm. Finally, we investigate the gas sensing properties of the Mo2S3 films deposited from the dispersions towards moisture gas and several representative volatile organic compounds. [ABSTRACT FROM AUTHOR]