Ni₃Cr₂P₂Q₉ (Q = S, Se): New Quaternary Transition Metal Chalcogenides with a Unique Layered Structure

The new transition metal chalcogenides Ni3Cr2P2S9 and Ni3Cr2P2Se9 have been discovered and characterized. Single-crystal X-ray diffraction studies of the selenide reveal a new layered structure type (space group P63/m, a = 6.244(4) Å, c = 18.479(19) Å, Z = 2, R1 = 0.0235). Powder X-ray diffraction and electron microprobe analysis suggest that the sulfide is isostructural to the selenide and that a solid solution forms between them. The layers are composed of transition metal centered octahedra with chalcogenides at the vertices which are joined in pairs by face sharing along the c-direction. These units share edges to form honeycomb layers in the ab-plane with linear P−Ni−P units in the holes of the honeycomb nets. Transport properties measurements from 80−300 K on single crystals with compositions Ni3Cr2P2S9-xSex (x = 0, 3, 6) revealed activated behavior in electrical resistivity (Ea = 0.02−0.03 eV, ρ300K = 50−220 mΩ·cm) and positive values of the Seebeck coefficient (S300K = 80−225 μV/K), showing that these compounds behave like p-type semiconductors. Magnetization measurements on Ni3Cr2P2S9 single crystals from 5 to 400 K reveal antiferromagnetic interactions between the transition metal ions and an ordering transition near 105 K

Ni₃Cr₂P₂Q₉ (Q = S, Se): New Quaternary Transition Metal Chalcogenides with a Unique Layered Structure

The new transition metal chalcogenides Ni3Cr2P2S9 and Ni3Cr2P2Se9 have been discovered and characterized. Single-crystal X-ray diffraction studies of the selenide reveal a new layered structure type (space group P63/m, a = 6.244(4) Å, c = 18.479(19) Å, Z = 2, R1 = 0.0235). Powder X-ray diffraction and electron microprobe analysis suggest that the sulfide is isostructural to the selenide and that a solid solution forms between them. The layers are composed of transition metal centered octahedra with chalcogenides at the vertices which are joined in pairs by face sharing along the c-direction. These units share edges to form honeycomb layers in the ab-plane with linear P−Ni−P units in the holes of the honeycomb nets. Transport properties measurements from 80−300 K on single crystals with compositions Ni3Cr2P2S9-xSex (x = 0, 3, 6) revealed activated behavior in electrical resistivity (Ea = 0.02−0.03 eV, ρ300K = 50−220 mΩ·cm) and positive values of the Seebeck coefficient (S300K = 80−225 μV/K), showing that these compounds behave like p-type semiconductors. Magnetization measurements on Ni3Cr2P2S9 single crystals from 5 to 400 K reveal antiferromagnetic interactions between the transition metal ions and an ordering transition near 105 K

Cu₄Mo₆Se₈: Synthesis, Crystal Structure, and Electronic Structure of a New Chevrel Phase Structure Type

Cu4Mo6Se8 has been synthesized by intercalation of Cu into Cu2Mo6Se8 at room temperature, and its crystal structure has been determined. This compound crystallizes in the triclinic space group P1̄, with a = 6.7609(8) Å, b = 6.8122(7) Å, c = 7.9355(10) Å, α = 70.739(4)°, β = 72.669(4)°, γ = 84.555(5)°, and Z = 1. Instead of residing in the voids between corners or edges of Mo6Se8 clusters as in the classic R3̄ Chevrel structure, the Cu atoms in Cu4Mo6Se8 fully occupy four sites between faces of two adjacent Mo6Se8 clusters. Thus, two of the six Mo atoms in each cluster do not have capping Se atoms from neighboring clusters. This represents a new triclinic structure type for Chevrel phases. In addition to the synthesis and crystal structure, we present and discuss results from electronic structure calculations using both extended Hückel and density functional theory. These calculations predict Cu4Mo6Se8 to be metallic. We also report results from Cu intercalation into Chevrel phase sulfides and tellurides. Preliminary experiments suggest that a telluride analogue of Cu4Mo6Se8 exists

Handling Hazards Using Continuous Flow Chemistry: Synthesis of <i>N</i>¹‑Aryl-[1,2,3]-triazoles from Anilines via Telescoped Three-Step Diazotization, Azidodediazotization, and [3 + 2] Dipolar Cycloaddition Processes

The conversion of commercially available anilines into triazole products was realized using a telescoped three-reactor flow diazotization, azidodediazotization, and [3 + 2] dipolar cycloaddition process. The diazotization–azidodediazotization sequence was accelerated by means of an ultrasonic bath resulting in a degassed, segmented effluent. An automated continuous flow unit controlled by custom software created in-house was used to collect the aryl azide stream and restore it to a continuous column of the reagent. When combined with a variety of dipolarophiles, 1-aryl-[1,2,3]-triazoles were thus assembled by either copper catalyzed alkyne–azide cycloaddition (CuAAC) or Huisgen cycloaddition reactions.

Crystallographic and Magnetic Phase Transitions in the Layered Ruthenium Oxyarsenides TbRuAsO and DyRuAsO

The crystallographic and physical properties of TbRuAsO and DyRuAsO at and below room temperature are reported, including full structure refinements from powder X-ray diffraction data and measured electrical and thermal transport properties, magnetic susceptibility, and heat capacity. Both compounds are isostructural to LaFeAsO (ZrCuSiAs-type, <i>P</i>4/<i>nmm</i>) at room temperature. However, DyRuAsO undergoes a symmetry-lowering crystallographic phase transition near 25 K, and adopts an orthorhombic structure (<i>Pmmn</i>) below this temperature. This structural distortion is unlike those observed in the analogous Fe compounds. Magnetic phase transitions are observed in both compounds which suggest antiferromagnetic ordering of lanthanide moments occurs near 7.0 K in TbRuAsO and 10.5 K in DyRuAsO. The nature of the structural distortion as well as thermal conductivity and heat capacity behaviors indicate strong coupling between the magnetism and the lattice. The behaviors of both materials show magnetic ordering of small moments on Ru may occur at low temperatures

Exploring Thallium Compounds as Thermoelectric Materials: Seventeen New Thallium Chalcogenides

We have begun investigating thallium-containing compounds as candidate thermoelectric materials. Alkali metal chalcogenide systems have produced compounds with promising thermoelectric properties, and the chemistry of the Tl+ cation is similar to that of the alkali metals. However, Tl is less electropositive and heavier than all alkali metals (except Fr). This may lead to compounds with lower electrical resistivity and lower thermal conductivity, both important for thermoelectric performance. Unlike the alkali metals, Tl+ has a lone pair of electrons which can be stereochemically active and can influence coordination geometries. Here we report the results of our initial study of quaternary Tl chalcogenide systems. We present the synthesis and structure of 17 new compounds. Some are isostructural to known alkali metal compounds, and some are new structure types. We discuss the activity of the Tl lone pairs. In addition, we have measured the thermoelectric properties (electrical resistivity, thermal conductivity, and thermopower) and band gaps of some of these materials. All of the measured thermal conductivities are extremely low; however, most of the materials are too resistive for thermoelectric applications, at least as currently prepared

Exploring Thallium Compounds as Thermoelectric Materials: Seventeen New Thallium Chalcogenides

We have begun investigating thallium-containing compounds as candidate thermoelectric materials. Alkali metal chalcogenide systems have produced compounds with promising thermoelectric properties, and the chemistry of the Tl+ cation is similar to that of the alkali metals. However, Tl is less electropositive and heavier than all alkali metals (except Fr). This may lead to compounds with lower electrical resistivity and lower thermal conductivity, both important for thermoelectric performance. Unlike the alkali metals, Tl+ has a lone pair of electrons which can be stereochemically active and can influence coordination geometries. Here we report the results of our initial study of quaternary Tl chalcogenide systems. We present the synthesis and structure of 17 new compounds. Some are isostructural to known alkali metal compounds, and some are new structure types. We discuss the activity of the Tl lone pairs. In addition, we have measured the thermoelectric properties (electrical resistivity, thermal conductivity, and thermopower) and band gaps of some of these materials. All of the measured thermal conductivities are extremely low; however, most of the materials are too resistive for thermoelectric applications, at least as currently prepared

Ferromagnetic Spin-1/2 Dimers with Strong Anisotropy in MoCl₅

The pentachloride MoCl5 adopts several molecular crystal structures, all comprising isolated Mo2Cl10 units with well-separated Mo–Mo magnetic dimers. Using magnetization measurements, single-crystal X-ray diffraction, and first-principles calculations, we confirm ferromagnetism with strong anisotropy below a Curie temperature of 22 K in α-MoCl5, and report a fifth polymorph, ϵ-MoCl5 , that we find to be ferromagnetic below 14 K. Magnetization measurements indicate unquenched orbital moments antialigned with the spins. This is confirmed by first-principles calculations, which also predict an unusually strong magnetocrystalline anisotropy in α-MoCl5 arising from spin–orbit coupling. An anisotropy field near 80 T is calculated, while a smaller but still substantial anisotropy field exceeding 12 T is realized experimentally. Further, increased anisotropy and Curie temperature are predicted when W is substituted for Mo. Similarly, strong magnetism and anisotropy are predicted for isolated Mo2Cl10 molecules, indicating the potential for true molecular magnetism. Together, these results identify Mo1–xWxCl5 as novel molecular crystals that combine spin 1/2 with strong magnetic anisotropy and exhibit surprisingly high Curie temperatures, considering their molecular nature

The Crystal Structure and Magnetic Behavior of Quinary Osmate and Ruthenate Double Perovskites La<i>ABB</i>′O₆ (<i>A</i> = Ca, Sr; <i>B</i> = Co, Ni; <i>B</i>′ = Ru, Os)

Six LaABB′O6 (A = Ca, Sr; B = Co, Ni; B′ = Ru, Os) double perovskites were synthesized, several for the first time, and their crystal structures and magnetic behavior were characterized with neutron powder diffraction and direct-current and alternating-current magnetometry. All six compounds crystallize with P21/n space group symmetry, resulting from a–a–c+ octahedral tilting and complete rock salt ordering of transition-metal ions. Despite the electronic configurations of the transition-metal ions, either d8–d3 or d7–d3, not one of the six compounds shows ferromagnetism as predicted by the Goodenough–Kanamori rules. LaSrNiOsO6, LaSrNiRuO6, and LaCaNiRuO6 display long-range antiferromagnetic order, while LaCaNiOsO6, LaCaCoOsO6, and LaSrCoOsO6 exhibit spin-glass behavior. These compounds are compared to the previously studied LaCaCoRuO6 and LaSrCoRuO6, both of which order antiferromagnetically. The observed variations in magnetic properties can be attributed largely to the response of competing superexchange pathways due to changes in B–O–B′ bond angles, differences in the radial extent of the 4d (B′ = Ru) and 5d (B′ = Os) orbitals, and filling of the t2g orbitals of the 3d ion

Ferromagnetic Spin-1/2 Dimers with Strong Anisotropy in MoCl₅

The pentachloride MoCl5 adopts several molecular crystal structures, all comprising isolated Mo2Cl10 units with well-separated Mo–Mo magnetic dimers. Using magnetization measurements, single-crystal X-ray diffraction, and first-principles calculations, we confirm ferromagnetism with strong anisotropy below a Curie temperature of 22 K in α-MoCl5, and report a fifth polymorph, ϵ-MoCl5 , that we find to be ferromagnetic below 14 K. Magnetization measurements indicate unquenched orbital moments antialigned with the spins. This is confirmed by first-principles calculations, which also predict an unusually strong magnetocrystalline anisotropy in α-MoCl5 arising from spin–orbit coupling. An anisotropy field near 80 T is calculated, while a smaller but still substantial anisotropy field exceeding 12 T is realized experimentally. Further, increased anisotropy and Curie temperature are predicted when W is substituted for Mo. Similarly, strong magnetism and anisotropy are predicted for isolated Mo2Cl10 molecules, indicating the potential for true molecular magnetism. Together, these results identify Mo1–xWxCl5 as novel molecular crystals that combine spin 1/2 with strong magnetic anisotropy and exhibit surprisingly high Curie temperatures, considering their molecular nature