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We use vortex photon fields with orbital and spin angular momentum to probe chiral fluctuations within liquid crystals. In the regime of iridescence with a well-defined pitch length of chirality, we find low energy Raman scattering that can be decomposed into helical and chiral components depending on the scattering vector and the topological charge of the incident photon field. Based on the observation of an anomalous dispersion we attribute quasi-elastic scattering to a transfer of angular momenta to roton-like quasiparticles. The latter are due to a competition of short-range repulsive and long-range dipolar interactions. Our approach using a transfer of orbital angular momentum opens up an avenue for the advanced characterization of chiral and optically active devices and materials., Comment: 5 pages, 3 figures, Physical Rev. Lett., in print (2022)

The ground state of Fe2+ (S = 2) in α- and β-FeMoO4 is investigated by experiments including X-ray diffraction, Raman scattering, and 57Fe–Mossbauer spectroscopy below 300 K and evaluated by theore...

The reactivity of FeMoO

The dimeric motif is the smallest unit for two interacting spin centers allowing for systematic investigations of cooperative interactions. As spin transition compounds, dinuclear complexes are of particular interest, since they potentially reveal a two-step spin crossover (SCO), switching between the high spin-high spin [HS-HS], the high spin-low spin [HS-LS], and the low spin-low spin [LS-LS] states. Herein, we report the synthesis and characterization of six dinuclear iron(II) complexes [FeII2(μ2-L1)2](BF4)4 (C1), [FeII2(μ2-L1)2](ClO4)4 (C2), [FeII2(μ2-L1)2](F3CSO3)4 (C3), [FeII2(μ2-L2)2](BF4)4 (C4), [FeII2(μ2-L2)2](BF4)4 (C5), and [FeII2(μ2-L2)2](BF4)4 (C6), based on the 1,3,4-thiadiazole bridging motif. The two novel bis-tridentate ligands (L1 = 2,5-bis{[(1H-imidazol-2-ylmethyl)-amino]-methyl}-1,3,4-thiadiazole and L2 = 2,5-bis{[(thiazol-2-ylmethyl)-amino]-methyl}-1,3,4-thiadiazole) were employed in the presence of iron(II) salts with the different counterions. Upon varying ligands and counterions, we were able to change the magnetic properties of the complexes from a temperature-independent [HS-HS] spin state over a one-step spin transition toward a two-step SCO. When cooled slowly from room temperature, the two-step SCO goes along with two distinct phase transitions, and in the intermediate mixed [HS-LS] state distinct HS/LS pairs can be identified unambiguously. In contrast, rapid cooling precludes a crystallographically observable phase transition. For the mixed [HS-LS] state Mossbauer spectroscopy confirms a statistical (random) orientation of adjacent [HS-LS]·[HS-LS]·[HS-LS] chains.

In order to expand and exploit the useful properties of d

Iron molybdate catalysts are applied in the industrial FormOx process to produce formaldehyde by oxidative dehydrogenation (ODH) of methanol. Only few studies are available about the applicability of iron molybdate catalysts for the ODH of ethanol to produce acetaldehyde. Herein, an iron molybdate synthesized by co-precipitation (p) and an iron molybdate prepared by a ball-milling solid-state synthesis (bm) are applied as ethanol ODH catalysts. Both materials show attractive acetaldehyde selectivites of >90 % (280 °C: p-Fe₂(MoO₄)₃: YAcH=90.3 %; bm-Fe₂(MoO₄)₃: YAcH=60.4 %) and a stable performance. The bulk composition and crystal structure could be confirmed by various characterization techniques and is maintained during ethanol ODH. XPS reveals an enrichment of Mo on the catalyst surface which is slightly decreasing after the catalytic tests. This observation could be a first sign of long-term deactivation like known from methanol ODH. Comparing the performance of both materials, p-Fe₂(MoO₄)₃ shows higher activity and aldehyde selectivity. We propose the higher Mo/Fe surface ratio and the lower acidity to be the reasons for these differences.

The geometrically frustrated diamond spin chain system has yielded materials with a diversity of interesting magnetic properties but is predominantly limited to compounds with single-spin components. Here, we report the compound [(CH

We present derivatives of the Al3Ti-type of structure Al8FeMo3 (Al9–xFexMo3) and Al8CuMo3. Structural features are a molybdenum-square lattice and a solid solution of iron (copper) on aluminum sites. By substitution of Al with 3d-transition metals outstanding magnetic properties are found. Typical Pauli paramagnetism is observed above a critical temperature Tc ≈ 93 K. Below Tc the series Al9–xFexMo3 (0.5 < xFe < 1) exhibits the rare coexistence of negative magnetization/diamagnetism in zero-field cooling and ferromagnetism in field-cooling mode. For Al8.33Fe0.67Mo3 the negative magnetization is estimated to the order of ∼50% of the diamagnetic response of superconducting lead. The contribution of Fe to the magnetization is evaluated by 57Fe Mossbauer spectroscopy in zero field and applied external fields. We show that ferromagnetic fluctuations originate from screened magnetic moments of Fe. The magnetic order presumably involves multibands originating from iron and molybdenum in the vicinity of the Fermi...

Chalcopyrite CuFeS2 was shown to be a promising thermoelectric material. Considering thermoelectric efficiency, its relatively high and temperature weakly dependent power factor, economic affordability and ecological benignity is counterbalanced by a high lattice thermal conductivity. Thus it is highly desirable to lower the thermal conductivity of chalcopyrite thermoelectric material without deterioration of other thermoelectric characteristics. In our study, we demonstrate that mechanosynthesis followed by appropriate sintering enables to prepare such nanostructured ceramics with a favourable thermoelectric response. Our study shows that mechanosynthesis is a low-cost technological route for the production of thermoelectric chalcopyrite ceramics. Chalkopyrit CuFeS2 se ukázal jako slibný termoelektrický materiál. Vzhledem k termoelektrické účinnosti je její relativně vysoký a teplotně slabě závislý účiník, ekonomická dostupnost a ekologická neškodnost vyvážena vysokou tepelnou vodivostí mřížky. Proto je velmi žádoucí snížit tepelnou vodivost termoelektrického materiálu chalkopyritu bez zhoršení dalších termoelektrických vlastností. V naší studii dokazujeme, že mechanosyntéza následovaná vhodným slinováním umožňuje připravit takovou nanostrukturovanou keramiku s příznivou termoelektrickou odezvou. Naše studie ukazuje, že mechanosyntéza je levnou technologickou cestou pro výrobu termoelektrické keramiky z chalkopyritu.

Iron oxide magnetic nanoparticles produced by chemical synthesis are usually composed of both magnetite and maghemite phases. Information about the phase composition is typically obtained using Mos...

Organized three-dimensional (3D) nanomaterial architectures are promising candidates for applications in optoelectronics, catalysis, or theranostics owing to their anisotropy and advanced structural features that allow tailoring their physical and chemical properties. The synthesis of such complex but well-organized nanomaterials is difficult because the interplay of interfacial strain and facet-specific reactivity must be considered. Especially the magnetic anisotropy with controlled size and morphology plays a decisive role for applications like magnetic resonance imaging (MRI) and advanced data storage. We present a solution phase seed mediated synthesis of colloidal, well dispersible iron oxide superparticles with flower- and hedgehog-like morphology starting from dispersible spherical maghemite (SPH) and nanoplate hematite (HEX) templates. In the superparticles the templates are epitaxially decorated with nanodomains and nanorods as shown by (high-resolution) transmission electron microscopy (TEM), o...

BaNa2Fe[VO4]2 contains a Jahn–Teller active ion (FeII, 3d6, high-spin) in an octahedral coordination. On the basis of a combination of temperature-dependent X-ray diffraction and Mossbauer and Rama...

In this study, wettability behavior of B4C substrate as well as B4C/crystalline rice husk ash and B4C/amorphous rice husk ash substrates with two aluminum alloys were studied. The electrical resistivity, thermal expansion coefficients, and thermal diffusivity of bilayer Al/B4C/rice husk ash composite fabricated by one-step pressureless infiltration were measured and the obtained data were systemically analyzed using the Taguchi method and analysis of variance. Boron carbide substrates after addition of amorphous or crystalline rice husk ash display good wettability with molten aluminum alloys. The results show that, electrical resistivity of Al/B4C/rice husk ash composites is mainly influenced by initial preform porosity, while the coefficient of thermal expansion of composites is determined by the chemical composition of infiltrated alloys. The measured values for coefficient of thermal expansion (10.5 × 10−6/℃) and electrical resistivity (0.60 × 10−5 Ω.m) of Al/B4C/rice husk ash composites, fabricated according to analysis of variance's optimal conditions are in good agreement with those of the projected values (11.02 × 10−6/℃ and 0.65 × 10−5 Ω.m, respectively). The difference between the corresponding values obtained from verification tests and projected values, for electrical resistivity and coefficient of thermal expansion are less than 5%. Finally, as a material selection approach, the strengths and weaknesses of the composites have been graphed in the form of radar diagrams.

The effect of surface functionalization on the structural and magnetic properties of catechol-functionalized iron oxide magnetic (γ-Fe2O3) nanocrystals was investigated. γ-Fe2O3 nanocrystals (NCs) were synthesized from iron acetyl acetonate in phenyl ether with 1,2-tetradecanediol, oleic acid, and oleylamine. X-ray powder diffraction in combination with Mossbauer spectroscopy revealed the presence of γ-Fe2O3 (maghemite) particles only. Replacement of oleic acid (OA) with catechol-type 3,4-dihydroxyhydrocinnamic acid (DHCA) or polydentate polydopamine acrylate (PDAm) surface ligands leads to a pronounced change of the magnetic behavior of the γ-Fe2O3 nanocrystals and separated them into two distinctive magnetic entities. XPS and Mossbauer spectroscopy revealed the shell to be reduced with a magnetite (Fe3O4) contribution of up to 33% of the total mass while the core remained maghemite (γ-Fe2O3). The magnetic interaction between the maghemite core and the magnetite shell strongly reduced the anisotropy constant of the nanocrystals and the effective magnetization. Our experiments show that the surface chemistry strongly affects the phase distribution and the macroscopic magnetic properties of iron oxide nanopowders.

Multicomponent nanostructures containing purely organic or inorganic as well as hybrid organic–inorganic components connected through a solid interface are, unlike conventional spherical particles, able to combine different or even incompatible properties within a single entity. They are multifunctional and resemble molecular amphiphiles, like surfactants or block copolymers, which makes them attractive for the self-assembly of complex structures, drug delivery, bioimaging, or catalysis. We have synthesized Pd@FexO heterodimer nanoparticles (NPs) to fabricate a macroporous, hydrophobic, magnetically active, three-dimensional (3D), and template-free hybrid foam capable of repeatedly separating oil contaminants from water. The Pd domains in the Pd@FexO heterodimers act as nanocatalysts for the hydrosilylation of polyhydrosiloxane and tetravinylsilane, while the FexO component confers magnetic properties to the final functional material. Pd@FexO heterodimers were synthesized by heterogeneous nucleation and g...

The thermal oxidation reactions of the intermetallic phases Al8Mo3 and AlMo3 were investigated and analyzed by ex-situ powder-x-ray diffraction (XRD), difference thermal analysis (DTA), thermogravimetry (TGA), and infrared spectroscopy (IR). The initial oxidation reactions in air were found to yield Al2O3 and AlMo3 in the case of Al8Mo3 (Tonset =725 °C), and MoO3 as well as Al8−xMo3 (Tonset =435 °C) for the pure intermetallic phase AlMo3, respectively. Thus, both intermetallic phases are coexisting in an equilibrium within a temperature range of 300 °C under oxidizing conditions. The formation of β-Al2(MoO4)3 followed the second oxidizing process of the respective minority component at elevated temperatures. Decomposition and evaporation of the volatile MoO3 yielded α-Al2O3 as the residue at 1000 °C.

The effect of following processing parameters on the electrical and thermomechanical properties of Al/Si3N4 deposited silica composites was investigated using the Taguchi method and analysis of variance (ANOVA): infiltration temperature and time, atmosphere, effect of Si3N4 coating, porosity content in the preforms, and magnesium content in the alloy. The contributions of each of the parameters to modulus of elasticity, electrical resistivity, coefficient of thermal expansion (CTE), and thermal diffusivity of the resulting composites were determined. The maximum modulus of elasticity and electrical resistivity of obtained composites were 265 GPa, and 1.37 × 10−3 Ω m, respectively. These values were achieved in composites fabricated from Si3N4 preforms under conditions of temperature (1200 ০C), time (120 min), porosity of 40 vol %, Al-9Mg-13Si alloy, in the atmosphere of N2. The minimum value of CTE obtained was 5.3 × 10−6/০C. This value was achieved in composites fabricated from Si3N4 coated silica under the same mentioned conditions. The thermal diffusivity of four fabricated samples (two samples with the highest and two samples with lowest electrical resistivity) was measured. The results show that the highest value of thermal diffusivity (20.4 mm2/s) among fabricated composites is belonged to the one with highest value of electrical resistivity.

The mechanical, electrical and thermal properties as well as thermal expansion of Al/SiC/RHA (rice husk ash) monolayer and bilayer composite have been studied using the Taguchi method and analysis of variance (ANOVA). The parameter that most significantly affects the modulus of elasticity of Al/SiC/RHA bilayer composites is processing time, with contribution percentages of 68 and 27% calculated from stress-strain graphs and ultrasonic method, respectively. However, the factor which mostly affects bending strength, CTE value and electrical resistivity of composites is process temperature with contribution percentages of 32, 28, and 22%, respectively. The projected values for modulus of elasticity (170 GPa), bending strength (369 MPa), CTE (8.9 × 10 −6 /°C) and electrical resistivity (0.0019 Ω m) of Al/SiC/RHA composites are in excellent agreement with those obtained in the verification tests under the optimal conditions according to ANOVA.

Compared to conventional deposition techniques for the epitaxial growth of metal oxide structures on a bulk metal substrate, wet-chemical synthesis based on a dispersible template offers advantages such as low cost, high throughput, and the capability to prepare metal/metal oxide nanostructures with controllable size and morphology. However, the synthesis of such organized multicomponent architectures is difficult because the size and morphology of the components are dictated by the interplay of interfacial strain and facet-specific reactivity. Here we show that solution-processable two-dimensional Pd nanotetrahedra and nanoplates can be used to direct the epitaxial growth of γ-Fe2O3 nanorods. The interfacial strain at the Pd−γ-Fe2O3 interface is minimized by the formation of an FexPd “buffer phase” facilitating the growth of the nanorods. The γ-Fe2O3 nanorods show a (111) orientation on the Pd(111) surface. Importantly, the Pd@γ-Fe2O3 hybrid nanomaterials exhibit enhanced peroxidase activity compared to ...

The compound InCu${}_{2/3}$V${}_{1/3}$O${}_{3}$ is a rare realization of the quasi-two-dimensional (2D) honeycomb spin lattice with XY anisotropy and strongly frustrated interlayer coupling. Despite substantial interest, its ground state has not been unambiguously identified. The authors, using NQR, NMR, and \ensuremath{\mu}SR spectroscopies and theoretical calculations, have ultimately settled the existing controversies. They find evidence for a quasi-2D static antiferromagnetic state below 39 K gradually transforming into a 3D N\'eel state at 15 K. Interestingly, signatures of the topological Berezinsky-Kosterlitz-Thouless transition were observed in the 2D regime of this compound.

BaAg2Cu[VO4]2 contains Cu(II) S=1/2 ions on a distorted two-dimensional triangular lattice interconnected via non-magnetic [VO4] entities. DFT band structure calculations, quantum Monte-Carlo simulations, and high-field magnetization measurements show that the magnetism of this compound is determined by a superposition of ferromagnetic (FM) and antiferromagnetic (AFM) uniform spin-1/2 chains with nearest neighbor exchange couplings of J FM=−19 K and J AFM=9.5 K (A. Tsirlin, A. Möller, B. Lorenz, Y. Skourski, H. Rosner, Phys. Rev. B 85 (2012) 014401). Here we report the study of BaAg2Cu[VO4]2 by high-field/frequency electron spin resonance (HF-ESR) and nuclear magnetic resonance (NMR) spectroscopies, which probe the local magnetic properties. In the HF-ESR measurements, we observe an anisotropic ESR spectrum typical for the Cu(II) ions and determine the g-tensor, g ||=2.38 and g ⊥=2.06. Moreover, we see a substantial change in the spectral shape of the ESR lines at low temperatures indicating the presence of short range magnetic correlations. The analysis of the low-temperature ESR spectra shows that its peculiar structure is due to the development of the anisotropic internal fields corresponding to FM and AFM correlations in the respective Cu spin chains. In the NMR spectra the signals from 51V nuclei in the two types of chains were identified. The analysis of the temperature evolution of these signals strongly supports the ESR findings on the occurrence of two types of Cu chains. Altogether, the HF-ESR and NMR results confirm theoretical predictions of the superposition of FM and AFM Cu(II) spin-1/2 chains in the studied material.

Two new uranium(IV) fluorides, Na 3.13 Mg 1.43 U 6 F 30 ( 1 ) and Na 2.50 Mn 1.75 U 6 F 30 ( 2 ), were synthesized through an in situ mild hydrothermal route, and were structurally characterized by single crystal X-ray diffraction. The compounds exhibit complex crystal structures composed of corner- or edge-shared UF 9 and M F 6 ( M =Mg, Mn) polyhedra, forming hexagonal channels in the three-dimensional framework, in which ordered or disordered divalent metal and sodium atoms reside. The large hexagonal voids contain the nearly regular M(II)F 6 octahedra and sodium ions, whereas the small hexagonal cavities include M(II) and sodium ions on a mixed-occupied site. Magnetic susceptibility measurements yielded effective magnetic moments of 8.36 and 11.6 µ B for 1 and 2 , respectively, confirming the presence and oxidation states of U(IV) and Mn(II). The large negative Weiss constants indicate the spin gap between a triplet and a singlet state in the U(IV). Magnetization data as a function of applied fields revealed that 2 exhibits paramagnetic behavior due to the nonmagnetic singlet ground state of U(IV) at low temperature. UV–vis diffuse reflectance and X-ray photoelectron spectroscopy data were also analyzed.

We have synthesized Ba2XCu(OH)[V2O7] with X=Cl, Br by hydrothermal methods. The isotypic structures (Pnma, Z=4, a≈15.1 A, b≈6.1 A, c≈9.6 A) contain distorted hexagonal layers of Ba and X in a BN-type arrangement. Each halide is further coordinated by one out-of plane Ba atom in an alternate up-down fashion resulting in an overall Ba 2 X 3 + ∞ 2 structural feature. The planar Ba–X hexagonal rings are centered by divanadate groups in an eclipsed orientation. Edge-sharing chains of CuO 2/2 (OH) 2/2 O 2/1 5 - ∞ 1 complement the structure. The magnetic properties are associated with the magnetic Cu2+ ions and can be described as an antiferromagnetic quasi 1D S=1/2 Heisenberg system. Confirmation is obtained from both magnetic and specific heat measurements. Furthermore, lattice dynamics are studied by DFT methods, IR, and Raman spectroscopy.

BaNa

The series of compounds $A{\mathrm{Ag}}_{2}\mathrm{Cr}{[{\mathrm{VO}}_{4}]}_{2}$, with $A=\mathrm{Ag}$, K, or Rb, is layered S = 3/2 triangular-lattice (TL) systems in which the magnetic exchange interactions between ${\mathrm{Cr}}^{3+}\phantom{\rule{4pt}{0ex}}(3{d}^{3})$ ions are mediated by nonmagnetic ${[{\mathrm{VO}}_{4}]}^{3\ensuremath{-}}$ entities. Here, the relative orientation of the vanadate is altered with respect to the TL as a function of the $A$ site, which corresponds to an induced symmetry change of the $[{\mathrm{CrO}}_{6}]$ complex. All members of this series of compounds belong to the class of frustrated TL antiferromagnets. We find that the distorted TL ($A=\mathrm{Ag}$) exhibits collinear antiferromagnetic long-range order (LRO) at ${T}_{N}\ensuremath{\approx}10$ K, whereas the high-symmetry cases ($A=\mathrm{K}$, Rb) evade LRO in zero field down to 0.03 K, the lowest temperature of our experiments. The latter members of the series belong to the undistorted TL and are candidates for spin-liquid ground states presumably not related to Ising anisotropy or dimerization.