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.

Verchenko V, Shevelkov AV, Driess M. 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.