Bulk ferromagnetism in cleavable van der Waals telluride NbFeTe2

A van der Waals telluride, NbFeTe2, has been synthesized using chemical vapor transport reactions. The optimized synthetic conditions yield high-quality single crystals with a novel monoclinic crystal structure. Monoclinic NbFeTe2 demonstrates a (100) cleavage plane, bulk ferromagnetism below 87 K, and a metallic ground state-the necessary prerequisites for needed spintronics technologies.

Mn2Ga2S5 and Mn2Al2Se5 van der Waals Chalcogenides: A Source of Atomically Thin Nanomaterials

Layered chalcogenides containing 3d transition metals are promising for the development of two-dimensional nanomaterials with interesting magnetic properties. Both mechanical and solution-based exfoliation of atomically thin layers is possible due to the low-energy van der Waals bonds. In this paper, we present the synthesis and crystal structures of the Mn2Ga2S5 and Mn2Al2Se5 layered chalcogenides. For Mn2Ga2S5, we report magnetic properties, as well as the exfoliation of nanofilms and nanoscrolls. The synthesis of both polycrystalline phases and single crystals is described, and their chemical stability in air is studied. Crystal structures are probed via powder X-ray diffraction and high-resolution transmission electron microscopy. The new compound Mn2Al2Se5 is isomorphous with Mn2Ga2S5 crystallizing in the Mg2Al2Se5 structure type. The crystal structure is built by the ABCBCA sequence of hexagonal close-packing layers of chalcogen atoms, where Mn2+ and Al3+/Ga3+ species preferentially occupy octahedral and tetrahedral voids, respectively. Mn2Ga2S5 exhibits an antiferromagnetic-like transition at 13 K accompanied by the ferromagnetic hysteresis of magnetization. Significant frustration of the magnetic system may yield spin-glass behavior at low temperatures. The exfoliation of Mn2Ga2S5 layers was performed in a non-polar solvent. Nanolayers and nanoscrolls were observed using high-resolution transmission electron microscopy. Fragments of micron-sized crystallites with a thickness of 70-100 nanometers were deposited on a glass surface, as evidenced by atomic force microscopy.

Interplay of two magnetic sublattices in related compounds Sm2Mn1-xGa6-yGey (x = 0.1-0.3, y = 0.6-1.0) and Sm4MnGa12-yGey (y = 3.0-3.5) with different ordering of empty and filled (Ga,Ge)6 octahedra

Single crystals of two new intermetallic phases Sm2Mn1-xGa6-yGey (x = 0.1-0.3, y = 0.6-1.0) and Sm4MnGa12-yGey (y = 3.0-3.5) were grown using a self-flux technique. According to single crystal X-ray diffraction data, Sm4MnGa12-yGey is characterised by the Y4PdGa12 structure type (a ∼ 8.65 Å; Im3̄m), while Sm2Mn1-xGa6-yGey formally adopts the K2PtCl6 structure type (a ∼ 8.71 Å; Fm3̄m). The general features of both compounds with rather similar crystal structures are represented by the alternation of empty and Mn-filled p-element octahedra, the order of which is determined by the Mn concentration. The diffraction data for Sm2Mn1-xGa6-yGey reveal a large concentration of Mn vacancies (x ∼ 0.3), which affects adjacent Ga/Ge atoms leading to their shift towards the vacancy. Both compounds demonstrate two ferromagnetic-like transitions and the presence of two interacting Mn and Sm magnetic sublattices. The Mn sublattice orders at TC1 of 143 K and 318 K, while the Sm one orders at lower temperatures at TC2 of 50 K and 280 K for Sm4MnGa8.6Ge3.4 and Sm2Mn0.74Ga5.1Ge0.9, respectively. The increase in Mn content not only increases the ordering temperatures, but also dramatically decreases the coercivity μ0HC from 230 mT to just 6.5 mT at 2 K. Despite the presence of two magnetically active sublattices in Sm2Mn0.74Ga5.1Ge0.9, the magnetic entropy change is quite low and only reaches 0.3 J kg-1 K-1 at T = 300 K and μ0H = 5 T, while the estimated relative cooling power (RCP) is about 36 J kg-1 at 5 T.

Impact of Ge Doping on Structural and Magnetic Ordering in RMnxGa3 and R4Mn1-xGa12-yGey (R = Tb, Dy; x ≤ 0.25, y ≈ 1.0-3.3)

Single crystals of RMnxGa3 and their new quaternary derivatives R4Mn1-xGa12-yGey (R = Tb, Dy, x ≤ 0.25, y ≈ 1.0-3.3) were grown from a Ga flux. The compounds are derivatives of cubic RGa3 phases, with Mn atoms filling the Ga6 voids. RMnxGa3 formally adopts a cubic ABO3 perovskite structure, in which the presence of Mn atoms results in a shift of the neighboring Ga atoms from their ideal position. A partial substitution of Ga by Ge leads to a higher Mn content, resulting in structural ordering of the latter and the formation of the superstructure phases R4Mn1-xGa12-yGey, which can be formally described in the Y4PdGa12 structure type. The presence of Mn vacancies, which was observed for R = Tb, and Ga/Ge mixing lead to a noticeable deviation from the idealized structure. The compounds contain two magnetic sublattices: the R sublattice, which orders antiferromagnetically near 20 K, and the Mn sublattice, which orders ferromagnetically at TC = 125-225 K with the Ge doping resulting in higher TC. The two sublattices are not independent, as the Mn sublattice induces partial ferromagnetic ordering of the rare earth atoms below TC, at least for the Ge-doped phases. Near TN, both magnetic susceptibility and heat capacity reveal complex behavior, indicating changes in magnetic structures below TN.

Itinerant ferromagnet Re4-xMnxGe7-δ (x = 0.9-1.5, δ = 0.42-0.44) with incommensurate chimney-ladder structure stabilised at ambient pressure

Re_4-xMn_xGe_7-δ (x = 0.9-1.5, δ = 0.42-0.44) is a new member of the Nowotny chimney-ladder family of compounds and features an incommensurate composite structure of transition element T (Re and Mn) and Ge substructures. Our theoretical calculations indicate metallic conductivity and ferromagnetic ordering, the latter being experimentally observed below 157 K.

Layered van der Waals Chalcogenides FeAl2Se4, MnAl2S4, and MnAl2Se4: Atomically Thin Triangular Arrangement of Transition-Metal Atoms

Layered van der Waals (vdW) chalcogenides of 3d transition metals are a rich source of two-dimensional (2D) nanomaterials, in which atomically thin layers with the terminating chalcogen atoms exhibit promising functionality for novel spintronic devices. Here, we report on the synthesis, crystal growth, and magnetic properties of FeAl₂Se₄, MnAl₂S₄, and MnAl₂Se₄ ternary chalcogenides. Crystal structures are probed by powder X-ray diffraction, Mössbauer spectroscopy, and high-resolution transmission electron microscopy. We improve the structural models of FeAl₂Se₄ and MnAl₂S₄ and show that isostructural MnAl₂S₄ and MnAl₂Se₄ crystallize in the centrosymmetric R3̅̅m space group. In the crystal structure, transition metal and Al atoms mutually occupy the octahedral and tetrahedral voids of four close-packing chalcogen layers terminated by vdW gaps. The transition-metal atoms form a triangular arrangement inside the close-packing layers. As a result, FeAl₂Se₄ and MnAl₂S₄ show no long-range magnetic order in the studied temperature range. In the paramagnetic state, Fe and Mn possess effective magnetic moments of 4.99(2) and 5.405(6) μ_B, respectively. Furthermore, FeAl₂Se₄ enters a frozen spin-disordered state below 12 K.

Cleavable crystals, crystal structure, and magnetic properties of the NbFe1+xTe3 layered van der Waals telluride

Transition metal-based two-dimensional nanomaterials with competing magnetic states are at the cutting edge of spintronic and low-power memory devices. In this paper, we present a Fe-rich NbFe_1+xTe₃ layered telluride (x ≈ 0.5), which shows an interplay of spin-glass and antiferromagnetic states below the Néel temperature of 179 K. The compound has a layered crystal structure, where the NbFeTe₃ layers are terminated by the Te atoms and van der Waals gaps. Bulk single crystals grown by chemical vapor transport reactions possess the (1̄01) cleavage plane suitable for the exfoliation of two-dimensional nanomaterials. Combination of high-resolution transmission electron microscopy and powder X-ray diffraction reveals the zigzag ladders of Fe atoms inside the structural layers, as well as complementary zigzag chains of the partially occupied Fe positions in the interstitial region. Fe atoms carry large effective magnetic moment of 4.85(3)μ_B per atom in the paramagnetic state yielding intriguing magnetic properties of NbFe_1+xTe₃. They include frozen spin-glass state at low temperatures and spin-flop transition in high magnetic fields indicating promising flexibility of the magnetic system and its potential control by magnetic field or gate tuning in the spintronic devices and heterostructures.

Effect of antifluorite layer on the magnetic order in Eu-based 1111 compounds, EuTAsF (T = Zn, Mn, and Fe)

The 1111 compounds with an alternating sequence of fluorite and antifluorite layers serve as structural hosts for the vast family of Fe-based superconductors. Here, we use neutron powder diffraction and density-functional-theory (DFT) band-structure calculations to study magnetic order of Eu²⁺ in the [EuF]⁺ fluorite layers depending on the nature of the [TAs]^- antifluorite layer that can be non-magnetic semiconducting (T = Zn), magnetic semiconducting (T = Mn), or magnetic metallic (T = Fe). Antiferromagnetic transitions at T_N ∼ 2.4-3 K due to an ordering of the Eu²⁺ magnetic moments were confirmed in all three EuTAsF compounds. Whereas in EuTAsF (T = Zn and Mn), the commensurate k₁ = (½ ½ 0) stripe order pattern with magnetic moments within the a-b plane is observed, the order in EuFeAsF is incommensurate with k = (0 0.961(1) ½) and represents a cycloid of Eu²⁺ magnetic moments confined within the bc-plane. Additionally, the Mn²⁺ sublattice in EuMnAsF features a robust G-type antiferromagnetic order that persists at least up to room temperature, with magnetic moments along the c-direction. Although DFT calculations suggest stripe antiferromagnetic order in the Fe-sublattice of EuFeAsF as the ground state, neutron diffraction reveals no evidence of long-range magnetic order associated with Fe. We show that the frustrating interplane interaction J₃ between the adjacent [EuF]⁺ layers is comparable with in-plane J₁-J₂ interactions already in the case of semiconducting fluorite layers [TAs]^- (T = Zn and Mn) and becomes dominant in the case of the metallic [FeAs]^- ones. The latter, along with a slight orthorhombic distortion, is proposed to be the origin of the incommensurate magnetic structure observed in EuFeAsF.

Controlled Reduction of Sn4+ in the Complex Iodide Cs2SnI6 with Metallic Gallium

Metal gallium as a low-melting solid was applied in a mixture with elemental iodine to substitute tin(IV) in a promising light-harvesting phase of Cs₂SnI₆ by a reactive sintering method. The reducing power of gallium was applied to influence the optoelectronic properties of the Cs₂SnI₆ phase via partial reduction of tin(IV) and, very likely, substitute partially Sn⁴⁺ by Ga³⁺. The reduction of Sn⁴⁺ to Sn²⁺ in the Cs₂SnI₆ phase contributes to the switching from p-type conductivity to n-type, thereby improving the total concentration and mobility of negative-charge carriers. The phase composition of the samples obtained was studied by X-ray diffraction (XRD) and ¹¹⁹Sn Mössbauer spectroscopy (MS). It is shown that the excess of metal gallium in a reaction melt leads to the two-phase product containing Cs₂SnI₆ with Sn⁴⁺ and β-CsSnI₃ with Sn²⁺. UV-visible absorption spectroscopy shows a high absorption coefficient of the composite material.

Fe-Rich Ferromagnetic Cleavable Van der Waals Telluride Fe5AsTe2

Transition metal-based layered compounds with van der Waals gaps between the structural layers are a rich source of magnetic materials for spintronic applications. Bulk crystals can be cleaved, providing high-quality two-dimensional nanomaterials, which are promising for the manipulation of spins in spintronic devices and low power quantum logic interfaces. The layered van der Waals telluride Fe₅AsTe₂ can be synthesized by the high-temperature reaction of elements. In the crystal structure, Fe-rich structural layers with the composition of Fe_4.58(4)AsTe₂ are separated by the van der Waals gaps with no atoms in the interstitial region. Crystal growth employing chemical vapor transport reactions yields bulk cleavable crystals, which exhibit weak inherent ferromagnetism below the Curie temperature of T_C = 48 K. In the ordered state, the magnetization shows a dual-slope behavior in low magnetic fields, indicating the compensated or canted nature of magnetism. Magnetic susceptibility and magnetization measurements reveal perpendicular magnetic anisotropy. The large Rhodes-Wohlfarth ratio of 4.6 indicates the itinerant nature of ferromagnetism in Fe₅AsTe₂.

Ferromagnetic correlations in the layered van der Waals sulfide FeAl2S4

Transition metal-based layered compounds with van der Waals gaps between the adjacent layers are a source of two-dimensional (2D) nanomaterials with nontrivial transport and magnetic properties. 2D ferromagnets, both metals and semiconductors, can be leveraged to produce spin-polarized current in spintronic devices with tailored functionalities. Here, we report on the synthesis, crystal growth, crystal and electronic structure, and magnetic properties of the Fe-based FeAl₂S₄ layered sulfide. In the crystal structure, Fe and Al atoms mix on octahedral and tetrahedral sites between hexagonal layers of S atoms, which are terminated by the van der Waals gaps. Band structure calculations reveal strong electronic correlations within the semiconducting ground state, which induce ferromagnetism with the magnetic moment of 0.12μ_B per formula unit for a Hubbard interaction U = 5 eV and Hund's rule coupling J = 0.8 eV. Crystal growth employing chemical vapor transport reactions results in bulk cleavable crystals, which show paramagnetic Curie-Weiss behavior at high temperatures with the Fe²⁺ magnetic centers. At low temperatures, an anomaly is observed on the magnetic susceptibility curve, below which the magnetization shows ferromagnetic hysteresis, indicating the presence of ferromagnetic correlations in FeAl₂S₄.

Intermetallic Compound Re2Ga9Ge with Re- and Ge-Embedded Gallium Clusters: Synthesis, Crystal Structure, Chemical Bonding, and Physical Properties

Transition metal-based endohedral cluster intermetallic compounds are interesting electron phases, which frequently exhibit superconductivity with a peculiar interplay between the critical temperature and valence electron count. We present a new Re-based endohedral gallium cluster compound, Re₂Ga₉Ge. Its unique crystal structure (P4₂/mmc space group, a = 8.0452(3) Å, c = 6.7132(2) Å) is built by two types of gallium polyhedra: monocapped Archimedean antiprisms centered by rhenium atoms and tetrahedra containing a main-group element inside. The analysis of chemical bonding shows the presence of localized pairwise interactions between the p-block elements and the formation of multicenter bonds with the participation of d-orbitals of rhenium. In the electronic band structure, the Fermi level is located in a narrow pseudogap indicating the optimum band filling and thus explaining the virtual absence of a homogeneity range. The compound exhibits Pauli paramagnetism and metallic properties with unexpectedly low thermal conductivity. A sharp anomaly observed on the magnetic susceptibility and resistivity curves presumably indicates the electronic phase transition accompanied by charge ordering at the characteristic temperature of T * = 271 K in zero magnetic field.

Molecular and Supramolecular Structures of Triiodides and Polyiodobismuthates of Phenylenediammonium and Its N,N-dimethyl Derivative

Despite remarkable progress in photoconversion efficiency, the toxicity of lead-based hybrid perovskites remains an important issue hindering their applications in consumer optoelectronic devices, such as solar cells, LED displays, and photodetectors. For that reason, lead-free metal halide complexes have attracted great attention as alternative optoelectronic materials. In this work, we demonstrate that reactions of two aromatic diamines with iodine in hydroiodic acid produced phenylenediammonium (PDA) and N,N-dimethyl-phenylenediammonium (DMPDA) triiodides, PDA(I₃)₂⋅2H₂O and DMPDA(I₃)I, respectively. If the source of bismuth was added, they were converted into previously reported PDA(BiI₄)₂⋅I₂ and new (DMPDA)₂(BiI₆)(I₃)⋅2H₂O, having band gaps of 1.45 and 1.7 eV, respectively, which are in the optimal range for efficient solar light absorbers. All four compounds presented organic–inorganic hybrids, whose supramolecular structures were based on a variety of intermolecular forces, including (N)H⋅⋅⋅I and (N)H⋅⋅⋅O hydrogen bonds as well as I⋅⋅⋅I secondary and weak interactions. Details of their molecular and supramolecular structures are discussed based on single-crystal X-ray diffraction data, thermal analysis, and Raman and optical spectroscopy.

Endohedral cluster intermetallic superconductors: at the frontier between chemistry and physics

When a transition metal combines with an excess of a p-metal, the latter forms endohedral clusters with the number of vertices up to 14. These clusters are the building units of endohedral cluster intermetallic compounds. Although discovered a few decades ago, they have gained renewed interest due to their peculiar crystal and electronic structures and frequently observed superconducting properties. Advances over recent years reveal that endohedral cluster architectures are flexible enough, enabling chemical substitutions and the formation of a series of structurally related phases, where the same clusters can be arranged in different ways. Within the structural series, the superconducting-state parameters, including critical temperature and magnetic field, can be controlled and finely tuned. Herein, we present the most recent results in the chemical properties and superconductivity of endohedral cluster intermetallics and provide an outlook for the field.

Intermetallic Fe6Ge5 formation and decay of a core-shell structure during the oxygen evolution reaction

Herein, we report on intermetallic iron germanide (Fe6Ge5) as a novel oxygen evolution reaction (OER) precatalyst with a Tafel slope of 32 mV dec-1 and an overpotential of 272 mV at 100 mA cm-2 in alkaline media. Furthermore, we uncover the in situ formation of a core-shell like structure that slowly collapses under OER conditions.

Magnetic structures of Fe32+δGe33As2 and Fe32+δ'Ge35-xPx intermetallic compounds: a neutron diffraction and 57Fe Mössbauer spectroscopy study

Fe_32+δGe₃₃As₂ and Fe_32+δ'Ge_35-xP_x are quasi-binary intermetallic compounds that possess a rare variant of intergrowth-type crystal structure, which is a combination of the column shaped Co₂Al₅ and MgFe₆Ge₆ structure type blocks. The compounds are antiferromagnets with the Néel temperatures around 125 K. Neutron powder diffraction experiments on the samples with δ≈ 0.1, δ'≈ 0.5 and x≈ 3 reveal commensurate magnetic ordering of low symmetry in both compounds and a non-monotonic change in the intensities of magnetic reflections. On the other hand, temperature dependence of the hyperfine fields obtained from ⁵⁷Fe Mössbauer spectroscopy indicates a gradual, monotonic increase in local magnetic fields upon cooling. We interpret these results as a spin reorientation within the Co₂Al₅-type block of the crystal structure, with the possible formation of a non-collinear magnetic order at low temperatures. Between the compounds, the reorientation occurs at significantly different temperatures, however the resulting magnetic structures themselves are similar as well as the average values of the magnetic moments and the hyperfine fields.

Synthesis and supramolecular organization of the iodide and triiodides of a polycyclic adamantane-based diammonium cation: the effects of hydrogen bonds and weak I⋯I interactions

Adamantane-like divalent building blocks and iodide or polyiodide anions combine into supramolecular architectures with the help of various noncovalent forces ranging from strong hydrogen bonds to secondary and weak I⋯I interactions.

Semiconducting and superconducting Mo–Ga frameworks: total energy and chemical bonding

The superconducting Mo₄Ga₂₁ structure type is derived from the electron-precise MoGa₄ cubic framework.

Electron-Precise Semiconducting ReGa2Ge: Extending the IrIn3 Structure Type to Group 7 of the Periodic Table

Intermetallic compounds with semiconducting properties are rare, but they give rise to advanced materials for energy conversion and saving applications. Here, we present ReGa₂Ge, a new electron-precise narrow-gap intermetallic semiconductor. The compound crystallizes in the IrIn₃ structure type (space group P4₂/mnm, a = 6.5734(3) Å, c = 6.7450(8) Å, and Z = 4), where Re atoms occupy the Ir site, while Ga and Ge jointly populate the In sites. ^69,71Ga nuclear quadrupole resonance spectroscopy indicates nonstatistical partially ordered distribution of Ga and Ge over two available crystallographic sites; however, the Ga:Ge ratio is exactly 2:1 without noticeable homogeneity range. The stoichiometry of ReGa₂Ge ensures its precise valence electron count, which is 17 e^- per formula unit. Accordingly, a narrow energy gap opens up at the Fermi energy in the electronic structure. Electrical resistivity, Seebeck coefficient, and thermal conductivity are in agreement with the semiconducting behavior deduced from the electronic structure calculations and point to prospective thermoelectric properties at high temperatures. Bonding analysis reveals dominant covalency in Re-E (E = Ga, Ge) and Re-Re interactions.

Indium Doping of Lead-Free Perovskite Cs2SnI6

Structure and properties of an inorganic perovskite Cs₂SnI₆ demonstrated its potential as a light-harvester or electron-hole transport material; however, its optoelectronic properties are poorer than those of lead-based perovskites. Here, we report the way of light tuning of absorption and transport properties of cesium iodostannate(IV) Cs₂SnI₆ via partial heterovalent substitution of tin for indium. Light absorption and optical bandgaps of materials have been investigated by UV-vis absorption and photoluminescent spectroscopies. Low-temperature electron paramagnetic resonance spectroscopy was used to study the kind of paramagnetic centers in materials.

The specific features of phononic and magnetic subsystems of type-VII clathrate EuNi2P4

A type-VII clathrate with a Eu2+ guest embedded into a Ni-P covalent framework, EuNi2P4, was synthesized by a standard two-stage ampoule synthesis and confirmed to crystallize in the orthorhombic space group Fddd with unit cell parameters a = 5.1829(1) Å, b = 9.4765(1) Å, and c = 18.9900(1) Å. A general technique for studying the lattice and magnetic properties of REE containing compounds is proposed. The temperature and field dependences of electrical resistivity ρ(T,H), magnetization M(T,H), magnetic susceptibility χ(T,H), heat capacity Cp(T), and unit cell parameters a(T), b(T), c(T), and volume V(T) were experimentally studied and analyzed at different pressures in the temperature range of 2-300 K. A cascade of anomalies in the studied dependences was identified and attributed to the magnetic phase transformation and peculiar lattice contributions at temperatures below 20 K. As a result of comparison with an isostructural clathrate SrNi2P4, the parameters of the magnetic and lattice contributions were determined. It is characteristic that the phase transition from the paramagnetic to the magnetically ordered state is not reflected in the temperature changes of the lattice parameters due to weak bonds between guest europium atoms and the Ni-P host matrix. We have constructed a tentative H-T phase diagram based on the M(T) and M(H) data, which includes 6 different phases. It is established that the anomalous lattice contribution to the clathrate heat capacity CTLS(T) appears due to the effect of two-level systems (TLS) in the Eu2+ subsystem on the thermodynamic properties of EuNi2P4. The values of TLS parameters as well as the parameters of the magnetic subsystem of the clathrate were determined.

Soft chemistry of pure silver as unique plasmonic metal of the Periodic Table of Elements

The International Year of The Periodic Table of Chemical Elements revealed that the Table remains both a deeply fundamental paradigm for various branches of chemistry and a universal practical tool for predictable design of new materials. Silver is a notable “nanoelement” particularly known by its plasmonic properties. A key advantage of this metal is an easily achievable morphological variety of nanostructured materials. This element represents a research branch of precise engineering of shapes and sizes of nanoparticle ensembles and smart hierarchic nanostructures. In the review, unique features of silver are discussed with respect to the development of novel analytical methods for forthcoming applications of surface-enhanced Raman spectroscopy (SERS) in ecology, biology and medicine.

Synthesis, electronic structure and physical properties of two new layered compounds, EuFAgSe and EuFAg1-δTe, featuring the active redox pair Eu2+/Ag. Dalton Trans 2020;49:7426-7435. [PMID: 32432284 DOI: 10.1039/d0dt01504k]

Systematic studies of the ZrCuSiAs (also LaOAgS or 1111) structure type resulted in the synthesis of two new fluoride chalcogenides, EuFAgSe and EuFAg1-δTe, whereas their sulfide analog, EuFAgS, could not be obtained. Both new compounds are tetragonal, P4/nmm, with cell parameters a = 4.1542(1) Å, c = 9.2182(1) Å for the selenide and a = 4.3255(1) Å, c = 9.5486(1) Å for the telluride. Rietveld refinement reveals a significant silver deficiency in the telluride (δ = 0.05), while the selenide is nearly stoichiometric. Both compounds are semiconductors as shown by diffuse reflectance spectroscopy and confirmed by density-functional calculations of the band structure. Magnetism of both compounds is predominantly driven by Eu2+, as indicated by magnetic susceptibility measurements and corroborated by 151Eu Mössbauer spectroscopy. EuFAg1-δTe and EuFAgSe are paramagnetic down to 1.8 K.

Crystal lattice disorder and characteristic features of the low-temperature thermal properties of higher borides

Heat capacity C_P(T) and lattice parameters a(T), b(T) and c(T) of LuB₄₄Si_3.5 borosilicide are experimentally studied as a function of temperature in the range of 2-300 K. The results are compared with those of pseudo-isostructural LuB₅₀ boride. At the lowest temperatures, it is shown that the C_P(T) dependence of borosilicide changes linearly with temperature. This is attributed to the effect of glass-like behaviour of the heat capacity due to the disorder in the sublattice of non-metals. The presence of defects in the B-Si sublattice and the irregular form of the cages in the B-Si matrix, which are occupied by Lu³⁺ ions, lead to the formation of two-level systems (TLS) in the Lu³⁺ subsystem. The TLS make a characteristic bell-like low-temperature contribution to the heat capacity of borosilicide. We show that there is a wide temperature range (5-150 K) of negative thermal expansion of borosilicide, which is attributed to the influence of quasi-independent vibrations of Lu³⁺ ions in the cages of the borosilicide crystal structure.

EuNi2P4, the first magnetic unconventional clathrate prepared via a mechanochemically assisted route

The first magnetic unconventional clathrate EuNi₂P₄ has been prepared and its thermodynamic properties have been investigated.

Endohedral Cluster Superconductors in the Mo-Ga-Sn System Explored by the Joint Flux Technique

Endohedral Ga cluster compounds feature nontrivial superconducting states including the two-gap superconductivity similar in nature to MgB₂. We use the joint flux synthetic technique to introduce Sn into the Ga matrix and tune the valence electron count in the two new endohedral cluster superconductors Mo₈Ga_41-xSn_x and Mo₄Ga_21-x-δSn_x with critical temperatures of T_c = 8.7 and 5.85 K, respectively. While the former compound is a derivative of the previously known Mo₈Ga₄₁ superconductor, where Sn atoms are enclosed inside the Sn@Ga₆ octahedral clusters, the latter is a new architecture built upon Mo@Ga₉Sn clusters, Ga@Ga₁₂ cuboctahedra, and Sn₄ squares. We show that this novel Mo₄Ga_21-x-δSn_x superconductor features strong electron-phonon coupling with the large ratio of 2Δ(0)/(k_BT_c) = 4.1 similar to that of the Mo₈Ga₄₁ superconductor with the closely related crystal structure.

Boosting Water Oxidation through In Situ Electroconversion of Manganese Gallide: An Intermetallic Precursor Approach

For the first time, the manganese gallide (MnGa₄) served as an intermetallic precursor, which upon in situ electroconversion in alkaline media produced high‐performance and long‐term‐stable MnO_x‐based electrocatalysts for water oxidation. Unexpectedly, its electrocorrosion (with the concomitant loss of Ga) leads simultaneously to three crystalline types of MnO_x minerals with distinct structures and induced defects: birnessite δ‐MnO₂, feitknechtite β‐MnOOH, and hausmannite α‐Mn₃O₄. The abundance and intrinsic stabilization of Mn^III/Mn^IV active sites in the three MnO_x phases explains the superior efficiency and durability of the system for electrocatalytic water oxidation. After electrophoretic deposition of the MnGa₄ precursor on conductive nickel foam (NF), a low overpotential of 291 mV, comparable to that of precious‐metal‐based catalysts, could be achieved at a current density of 10 mA cm⁻² with a durability of more than five days.

Synthesis, crystal and electronic structures of Pt-rich phosphides EuPt3P and EuPt6P2

Two new ternary Pt-rich phosphides, EuPt6P2 and EuPt3P, have been prepared via a two-step solid state reaction. Their crystal structures have been determined from powder XRD data. EuPt6P2 is isostructural to SrPt6P2 (cubic, Pa3[combining macron], a = 8.4603(1) Å); its crystal structure comprises corner-sharing Pt6P trigonal prisms hosting Eu2+ cations in the cuboctahedral voids of the framework. EuPt3P is isostructural to the SrPt3P anti-perovskite (P4/nmm, a = 5.7452(1) Å and c = 5.4212(1) Å). Magnetization measurements reveal the magnetic response caused by the Eu2+(4f7) cations. EuPt6P2 is paramagnetic exhibiting no phase transitions down to 1.8 K, whereas EuPt3P orders ferromagnetically below 19 K. Similar to SrPt6P2 and SrPt3P, the new compounds are metallic with states near the Fermi level predominantly formed by the 5d orbitals of Pt.

Single-gap superconductivity in Mo8Ga41

In this paper, the potential existence of two-gap superconductivity in Mo₈Ga₄₁ is addressed in detail by means of thermodynamic and spectroscopic measurements. A combination of highly sensitive bulk and surface probes, specifically ac-calorimetry and scanning tunneling spectroscopy (STS), are utilized on the same piece of crystal and reveal the presence of only one intrinsic gap in the system featuring strong electron-phonon coupling. Minute traces of additional superconducting phases detected by STS and also in the heat capacity measured in high magnetic fields on a high-quality and seemingly single-phase crystal might mimic the multigap superconductivity of Mo₈Ga₄₁ suggested recently in several studies.

From endohedral cluster superconductors to approximant phases: synthesis, crystal and electronic structure, and physical properties of Mo8Ga41-xZnx and Mo7Ga52-xZnx

Using the crystal-growth joint flux technique based on the combination of two aliovalent low-melt metals, gallium and zinc, we adjust the gross valence electron count in the Mo-Ga-Zn system and produce the Mo₈Ga_41-xZn_x and Mo₇Ga_52-xZn_x intermetallic compounds. Gradual reduction in the valence electron count first leads to the Zn for Ga substitution in the Mo₈Ga₄₁ endohedral cluster superconductor, accompanied by the formation of Zn-containing clusters in the crystal structure and by the gradual suppression of superconductivity. Mo₈Ga_41-xZn_x with x = 7.2(2) exhibits superconducting properties below T_C = 4 K, whereas there is no superconducting transition at temperatures above 2 K for the limiting composition of x = 11.3(2). Further, the Mo₇Ga_52-xZn_x phase is formed from the flux with a higher content of Zn. Mo₇Ga_52-xZn_x crystallizes in the Mo₇Sn₁₂Zn₄₀ structure type with a narrow homogeneity range and exhibits metallic behavior with no sign of superconductivity down to at least 1.8 K. Its experimental valence electron count of 2.9 e per atom is below that of endohedral gallium cluster superconductors. Electronic structures of Mo₈Ga_41-xZn_x and Mo₇Ga_52-xZn_x feature the opening of a pseudogap slightly below the Fermi level indicating the specific stability of these structure types at the valence electron count of 3.2 e per atom and 2.7 e per atom, respectively.

ReGaGe2: an intermetallic compound with semiconducting properties and localized bonding

ReGaGe2 is a new member of the family of intermetallic compounds with non-metallic properties. It displays highly localized covalent bonding patterns. Its electronic structure is governed by mixing of Re d orbitals with the s and p orbitals of Ga and Ge and features the Fermi level falling into the opened band gap, ensuring experimentally confirmed semiconducting properties.

Layered Compounds BaFMgPn (Pn = P, As, Sb, and Bi), Transition-Metal-Free Representatives of the 1111 Structure Type

Four new transition metal-free pnictide representatives of the LaOAgS structure type were predicted by DFT calculations and found in the BaFMgPn (Pn = P, As, Sb and Bi) family. The compounds adopt the tetragonal space group P4 /nmm with the unit cell parameters a/ c 4.3097(1) Å/9.5032(1) Å, 4.3855(1) Å/9.5918(1) Å, 4.5733(1) Å/9.8184(1) Å, and 4.6359(1) Å/9.8599(1) Å, respectively. According to the DFT calculations, these new compounds are semiconductors with band gaps steadily decreasing from Pn = P ( ca. 2 eV) to Pn = Bi ( ca. 1 eV). The corresponding strontium fluoride and rare-earth oxide analogs are unlikely to exist and have not been observed yet. The trends of the stability within 1111 and structurally and/or chemically related compounds based on a combined consideration of geometry and DFT calculations are discussed.

ReGa0.4Ge0.6: Intermetallic Compound with Pronounced Covalency in the Bonding Pattern

We report synthesis, crystal and electronic structure, and transport properties of new intermetallic compound ReGa_0.4Ge_0.6, which was obtained by two-step ampule method from the elements. ReGa_0.4Ge_0.6 crystallizes in its own structure type (space group I4/ mmm, a = 2.89222(3) Å, c = 15.1663(3) Å, and Z = 4) which can be described as a sequential alternation of blocks of rhenium atoms and blocks of gallium and germanium atoms. Chemical bonding analysis reveals pronounced covalency of Re-Re, Re-E, and E-E (E = Ga and Ge) interactions and an interesting bonding pattern that includes many variations of localized bonding within a single compound, including pairwise homo- and heterometallic bonding, three-centered homometallic and four-centered bonding, and possibly even more delocalized bonding, which is not often encountered in such a simple intermetallic compound. Metallic behavior is confirmed by electronic structure calculations and by measurements of electrical resistivity.

Crystal Growth of Intermetallics from the Joint Flux: Exploratory Synthesis through the Control of Valence Electron Count

In this study, we modify the flux-growth method for the purpose of exploratory synthesis of ternary intermetallic compounds. Our concept is based on the assumption that valence electron count plays a crucial role in the stability of polar intermetallic compounds of different structure types. Control of the valence electron count parameter is made possible through the use of an excess of two metals having a different number of valence electrons. By gradually changing the ratio between these metals in the joint flux, we scan the gross number of valence electrons and explore the crystallization of new compounds. In the ternary system Re-Ga-Zn, we detect compounds belonging to three structure types, ReGa₅, PtHg₄, and V₈Ga₄₁, while gradually increasing the content of Zn metal in the flux. Two new compounds, ReGa₃Zn and Re₈Ga_{41- x}Zn _x with x = 21.2(5), are obtained in the form of high-quality single crystals, and the former compound shows the narrow-gap semiconducting behavior favorable for high thermoelectric performance.

Effect of the cation sublattice composition of tin-based type-I clathrates on their low-temperature thermal properties

We performed an experimental study on thermal properties of the Sn18In6As21.5I8 clathrate by measuring temperature dependencies of its heat capacity (2-300 K) and thermal expansion (5-300 K). By comparing the results with those published previously for Sn-based clathrates Sn24P19.2I8, Sn20Zn4P20.8I8, and Sn17Zn7P22I8, we established that partial replacement of tin and phosphorus by heavier indium and arsenic, respectively, leads to lowering vibration frequencies in both host and guest substructures. Deviation of the observed thermal properties at low temperatures from those predicted by the Einstein-Debye model is caused by the Schottky-like contribution of two-level systems to heat capacity and thermal expansion. These systems form owing to transitions of guest atoms in non-spherical 24-vertex cages between stationary states with close energies.

From Isolated Anions to Polymer Structures through Linking with I2: Synthesis, Structure, and Properties of Two Complex Bismuth(III) Iodine Iodides

We report the synthesis, crystal structures, and optical properties of two new compounds, K₁₈Bi₈I₄₂(I₂)_0.5·14H₂O (1) and (NH₄)₇Bi₃I₁₆(I₂)_0.5·4.5H₂O (2), as well as the electronic structure of the latter. They crystallize in tetragonal space group P4/ mmm with the unit cell parameters a = 12.974(1) and c = 20.821(3) Å for 1 and a = 13.061(3) and c = 15.162(7) Å for 2. Though 1 and 2 are not isomorphous, their crystal structures display the same structural organization; namely, the BiI₆ octahedra are linked by I₂ units to form disordered layers in 1 and perfectly ordered chains in 2. The I-I bond distances in the thus formed I-I-I-I linear links are not uniform; the central bond is only slightly longer than in a standalone I₂ molecule, whereas the peripheral bonds are significantly shorter than longer bonds typical for various polyiodides, which is confirmed by Raman spectroscopy. The analysis of the electronic structure shows that the atoms forming the I-I-I-I subunits transfer electron density from their occupied 5p orbitals onto their vacant states as well as onto 6s orbitals of bismuth atoms that center the BiI₆ octahedra. This leads to low direct band gaps that were found to be 1.57 and 1.27 eV for 1 and 2, respectively, by optical absorption spectroscopy. Luminescent radiative relaxation was observed in the near-IR region with emission maxima of 1.39 and 1.24 eV for 1 and 2, respectively, in good agreement with the band structure, despite the strong quenching propensity of I₂ moieties.

Dynamics of the crystal structure of tin-based type-I clathrates with different degrees of disorder in their cationic frameworks

The temperature dependencies of heat capacity, C_P(T), and cubic unit cell parameter, a(T), were experimentally obtained in the range of 2-300 K for the compounds Sn₂₄P_19.2I₈, Sn₂₀Zn₄P_20.8I₈, and Sn₁₇Zn₇P₂₂I₈, which belong to a family of type-I clathrates. The experimental data were analyzed in the frames of the Debye-Einstein approximation, further accounting for the contributions of positional disorder in the clathrate frameworks as well as those of defect modes arising from the distribution of guest atoms over unequal in energy but close in space positions inside the framework cages. By fitting the experimental data, the Debye and Einstein characteristic temperatures describing the dynamics of the framework and guest atoms, respectively, were obtained. Their analysis revealed peculiar dependencies of the characteristic temperatures upon the number of substituted zinc atoms and the concentration of vacancies in the framework, which are discussed in this paper.

Structural irregularities and peculiarities of low-temperature thermal properties of Sn24P19.4Br8 clathrate

Temperature changes of the heat capacity and unit cell parameters of Sn₂₄P_19.4Br₈ clathrate were experimentally determined in the temperature range of 2 to 300 K. The data obtained were analyzed using Debye-Einstein approximation and taking into account the impact of both disorder in the host matrix and the presence of vacancies in the framework. Anomalous negative contribution to the thermal expansion was revealed and related to the defect mode influence on the clathrate thermal properties as a result of vibrations of two-level systems (TLS). The guest atoms that have the opportunity to occupy spatially close yet energetically non-equivalent positions in the asymmetric environment of the host matrix atoms play a principal role in the TLS formation. The results are compared with those previously obtained for semiclathrate Ge₃₁P₁₅Se₈.

Nontrivial Recurrent Intergrowth Structure and Unusual Magnetic Behavior of Intermetallic Compound Fe32+δGe33As2

A new phase Fe_32+δGe₃₃As₂ (δ ≤ 0.136) was obtained by two-step synthesis from the elements. Fe_32+δGe₃₃As₂ crystallizes in its own structure type (space group P6/mmm, Z = 1, a = 11.919(3) Å, c = 7.558(4) Å) that can be described as a recurrent two-dimensional intergrowth of two intermetallic structure types, MgFe₆Ge₆ and Co₂Al₅. Their blocks are represented by infinite columns in the structure. No visible structural changes were observed in the temperature range from 10 to 300 K. At 125 K, Fe_32+δGe₃₃As₂ undergoes an antiferromagnetic-like transition, while above 150 K it shows a typical Curie-Weiss paramagnetic behavior. Below the transition temperature, a peculiar field-dependent magnetic susceptibility, that shows a significant increase of the susceptibility upon increasing the magnetic field, and a change in transport properties have been observed. Above 140 K, Fe_32+δGe₃₃As₂ reveals a metallic behavior, in agreement with electronic structure calculation, while below this point the resistivity nonmonotonically increases upon cooling. The Seebeck coefficient is positive, indicating that holes are the major charge carriers, and shows a broad maximum around 57 K.

Structural and Thermodynamic Stability of the "1111" Structure Type: A Case Study of the EuFZnPn Series

Two new compounds with the LaOAgS structure, EuFZnAs (1) and EuFZnSb (2), were obtained via solid state reaction. Both compounds are tetragonal (P4/nmm) with the cell parameters a = 4.1000(1) Å and c = 9.0811(1) Å for 1 and a = 4.2852(1)Å and c = 9.4238(1)Å for 2. The absence of their phosphide analog can be explained based on crystal chemical considerations as well as on quantum-chemical estimates of their thermodynamic stability with respect to EuF₂ and EuZn₂Pn₂. The magnetic response of 1 and 2 is ascribed to the presence of Eu²⁺ ions. Both compounds are paramagnetic down to low temperatures, where they order antiferromagnetically at ∼5 K and ∼3 K, respectively. They are narrow-gap semiconductors, and EuFZnSb demonstrates a relatively high value of the Seebeck coefficient.

New Fe-based layered telluride Fe3-δAs1-yTe2: synthesis, crystal structure and physical properties

A new ternary telluride, Fe_3-δAs_1-yTe₂, was synthesized from elements at 600 °C. It crystallizes in the hexagonal P6₃/mmc space group with the unit cell parameters a = 3.85091(9) Å and c = 17.1367(4) Å for δ = 0.3 and y = 0.04. Its layered crystal structure contains partially occupied intralayer and interlayer Fe positions, which give rise to significant nonstoichiometry: Fe_3-δAs_1-yTe₂ was found to possess the homogeneity range of 0.25 < δ < 0.45 and y = 0.04. Regions of local vacancy ordering alternate with regions of randomly distributed vacancies, so that the ordering of Fe atoms and vacancies is not complete in the average structure. Clear evidence of the magnetic phase transition is obtained by thermodynamic measurements, Mössbauer spectroscopy, and neutron powder diffraction. Magnetic susceptibility measurements reveal weak ferromagnetism below T_C = 123 K with a net moment of M_S∼ 0.1μ_B/Fe at T = 2 K. This transition is confirmed by differential scanning calorimetry. Additionally, neutron powder diffraction indicates the onset of a complex AFM-like magnetic ordering below 100 K.

Iodobismuthates Containing One-Dimensional BiI4(-) Anions as Prospective Light-Harvesting Materials: Synthesis, Crystal and Electronic Structure, and Optical Properties

Four iodobismuthates, LiBiI4·5H2O (1), MgBi2I8·8H2O (2), MnBi2I8·8H2O (3), and KBiI4·H2O (4), were prepared by a facile solution route and revealed thermal stability in air up to 120 °C. Crystal structures of compounds 1-4 were solved by a single crystal X-ray diffraction method. 1: space group C2/c, a = 12.535(2), b = 16.0294(18), c = 7.6214(9) Å, β = 107.189(11)°, Z = 4, R = 0.029. 2: space group P21/c, a = 7.559(2), b = 13.1225(15), c = 13.927(4) Å, β = 97.14(3)°, Z = 2, R = 0.031. 3: space group P21/c, a = 7.606(3), b = 13.137(3), c = 14.026(5) Å, β = 97.14(3)°, Z = 2, R = 0.056. 4: space group P21/n, a = 7.9050(16), b = 7.7718(16), c = 18.233(4) Å, β = 97.45(3)°, Z = 4, R = 0.043. All solid state structures feature one-dimensional (BiI4)(-) anionic chains built of [BiI6] octahedra that share two opposite edges in such a fashion that two iodine atoms in cis-positions remain terminal. The calculated electronic structures and observed optical properties confirmed that compounds 1-4 are semiconductors with direct band gaps of 1.70-1.76 eV, which correspond to their intense red color. It was shown that the cations do not affect the optical properties, and the optical absorption is primarily associated with the charge transfer from the I 5p orbitals at the top of the valence band to the Bi 6p orbitals at the bottom of the conduction band. Based on their properties and facile synthesis, the title compounds are proposed as promising light-harvesting materials for all-solid solar cells.