Much more to explore with an oxidation state of nearly four: Pr valence instability in intermetallic m-Pr2Co3Ge5

For some intermetallic compounds containing lanthanides, structural transitions can result in intermediate electronic states between trivalency and tetravalency; however, this is rarely observed for praseodymium compounds. The dominant trivalency of praseodymium limits potential discoveries of emergent quantum states in itinerant 4f1 systems accessible using Pr4+-based compounds. Here, we use in situ powder x-ray diffraction and in situ electron energy-loss spectroscopy (EELS) to identify an intermetallic example of a dominantly Pr4+ state in the polymorphic system Pr2Co3Ge5. The structure-valence transition from a nearly full Pr4+ electronic state to a typical Pr3+ state shows the potential of Pr-based intermetallic compounds to host valence-unstable states and provides an opportunity to discover previously unknown quantum phenomena. In addition, this work emphasizes the need for complementary techniques like EELS when evaluating the magnetic and electronic properties of Pr intermetallic systems to reveal details easily overlooked when relying on bulk magnetic measurements alone.

Optimization of Chemical Bonding through Defect Formation and Ordering─The Case of Mg7Pt4Ge4

The new phase Mg₇Pt₄Ge₄ (≡Mg₈□₁Pt₄Ge₄; □ = vacancy) was prepared by reacting a mixture of the corresponding elements at high temperatures. According to single crystal X-ray diffraction data, it adopts a defect variant of the lighter analogue Mg₂PtSi (≡Mg₈Pt₄Si₄), reported in the Li₂CuAs structure. An ordering of the Mg vacancies results in a stoichiometric phase, Mg₇Pt₄Ge₄. However, the high content of Mg vacancies results in a violation of the 18-valence electron rule, which appears to hold for Mg₂PtSi. First principle density functional theory calculations on a hypothetical, vacancy-free "Mg₂PtGe" reveal potential electronic instabilities at E_F in the band structure and significant occupancy of states with an antibonding character resulting from unfavorable Pt-Ge interactions. These antibonding interactions can be eliminated through introduction of Mg defects, which reduce the valence electron count, leaving the antibonding states empty. Mg itself does not participate in these interactions. Instead, the Mg contribution to the overall bonding comes from electron back-donation from the (Pt, Ge) anionic network to Mg cations. These findings may help to understand how the interplay of structural and electronic factors leads to the "hydrogen pump effect" observed in the closely related Mg₃Pt, for which the electronic band structure shows a significant amount of unoccupied bonding states, indicating an electron deficient system.

Flux Growth, Crystal Structure, and Chemical Bonding of Yb2PdGe3, an AlB2 Superstructure within the Rare-Earth Series

The complete structure revision of the RE₂PdGe₃ (RE = rare-earth metal) series revealed that Yb₂PdGe₃ is the only AlB₂ ordered superstructure. Good-quality single crystals of this compound were successfully grown from molten indium flux, enabling accurate single-crystal investigations. Yb₂PdGe₃ crystallizes with the Ce₂CoSi₃-type structure in the hexagonal space group P6/mmm (no. 191) with lattice parameters a = 8.468(1) Å and c = 4.0747(7) Å. This structure is a four-order derivative of AlB₂, composed of planar _∞²[PdGe₃] honeycomb layers spaced by Yb species, located at the center of Ge₆ and Ge₄Pd₂ hexagons. A superconducting transition is observed below the critical temperature of 4 K. A divalent state of Yb is deduced from magnetic susceptibility measurements below room temperature, which indicate an almost nonmagnetic behavior. A charge transfer from Yb to Pd and Ge was evidenced by the Quantum Theory of Atoms in Molecules (QTAIM) effective charges; polar four-atomic Ge-Pd/Yb and two-atomic Pd-Yb bonds were observed from the ELI-D (electron localizability indicator), partial ELI-D, and ELI-D/QTAIM intersections. The bonding interactions between Ge atoms within regular Ge₆ hexagons are found to be intermediate between single bonds, as in elemental Ge, and higher-order bonds in the hypothetic Ge₆H₆ and Ge₆^6- aromatic molecules.

New Insights into the Crystal Chemistry of FeB-Type Compounds: The Case of CeGe

Several alkaline earth or rare earth binary monosilicides and -germanides possess complex bonding properties, such as polycation formation exceeding the scope of classical electron counting rules. In this study, we present characterization by powder and single-crystal diffraction and thermal analysis of CeGe, one of the few monogermanides crystallizing in the FeB-type structure. Comparative computational investigations for structure types experimentally observed for monogermanides and alternative structures with different structural motifs were performed to gain energetical insights into this family of compounds, underlining the preference for infinite germanium chains over other structural motifs. Formation enthalpy calculations and structural chemical analysis highlight the special position of FeB-type compounds among the monogermanides.

Crystal Structures of AlCr2 and MoSi2 : Same Structure Type vs. Different Bonding Pattern

In a joint effort utilizing modified sample preparation, microscopy, X-ray diffraction and micro-fabrication, it became possible to prepare single crystals of the "hidden" phase AlCr2 . High-resolution X-ray diffraction analysis is described in detail for two crystals with the similar overall composition, but different degree of disorder, which seems to be the main cause for the differing unit cell parameters. Chemical bonding analysis of AlCr2 in comparison to prototypical MoSi2 shows pronounced differences reflecting the interchange of main group element vs. transition metal as majority component.

Mg3Pt2: Anionic Chains in a Eu3Ga2-Type Structure

The binary phase Mg₃Pt₂ was prepared by direct reaction between the elements or by spark-plasma synthesis starting with MgH₂ and PtCl₂. The compound crystallizes in the monoclinic space group C2/c with a = 7.2096(3) Å, b = 7.1912(4) Å, c = 6.8977(3) Å, and β = 106.072(3)° and is isotypic to Eu₃Ga₂. Analysis of the electron density within the quantum theory of atoms in molecules shows a significant charge transfer from Mg to Pt in agreement with the electronegativity difference. Further study of the chemical bonding with the electron localizability approach reveals the formation of Pt chains stabilized by a complex system of multicenter interactions involving Mg and Pt species. The metallic character of Mg₃Pt₂ is confirmed by electronic structure calculations and physical measurements.

Mg₃Pt₂ is a new phase in the Mg−Pt system with a crystal structure built up by anionic Pt chains, which are stabilized by an interplay of multiatomic bonding interactions of Mg with Pt. Mg₃Pt₂ is a metallic diamagnet.

Crystal Structure and Magnetism of Noncentrosymmetric Eu2Pd2Sn

The new intermetallic compound Eu₂Pd₂Sn has been investigated. A single crystal was selected from the alloy and was analyzed by single-crystal X-ray diffraction, revealing that this compound possesses the noncentrosymmetric Ca₂Pd₂Ge structure type being, so far, the only rare-earth-based representative. Bonding analysis, performed on the basis of DOS and (I)COHP, reveals the presence of strong covalent Sn–Pd bonds in addition to linear and equidistant Pd–Pd chains. The incomplete ionization of Eu leads to its participation in weaker covalent interactions. The magnetic effective moment, extracted from the magnetic susceptibility χ(T) is μ_eff = 7.87 μ_B, close to the free ion Eu²⁺ value (μ_eff = 7.94 μ_B). The maximum of χ(T) at T_N ∼ 13 K indicates an antiferromagnetic behavior below this temperature. A coincident sharp anomaly in the specific heat C_P(T) emerges from a broad anomaly centered at around 10 K. From the reduced jump in the heat capacity at T_N a scenario of a transition to an incommensurate antiferromagnetic phase below T_N followed by a commensurate configuration below 10 K is suggested.

A new intermetallic compound, Eu₂Pd₂Sn, is so far the only rare-earth representative of the Ca₂Pd₂Ge structure type. Synthesis and characterization of the structure, together with magnetic properties, are discussed.

"Excess" electrons in LuGe

The monogermanide LuGe is obtained via high‐pressure high‐temperature synthesis (5–15 GPa, 1023–1423 K). The crystal structure is solved from single‐crystal X‐ray diffraction data (structure type FeB, space group Pnma, a=7.660(2) Å, b=3.875(1) Å, and c=5.715(2) Å, R_F=0.036 for 206 symmetry independent reflections). The analysis of chemical bonding applying quantum‐chemical techniques in position space was performed. It revealed—beside the expected 2c‐Ge‐Ge bonds in the germanium polyanion—rather unexpected four‐atomic bonds between lutetium atoms indicating the formation of a polycation by the excess electrons in the system Lu³⁺(2b)Ge²⁻×1 e⁻. Despite the reduced VEC of 3.5, lutetium monogermanide is following the extended 8‐N rule with the trend to form lutetium‐lutetium bonds utilizing the electrons left after satisfying the bonding needs in the anionic Ge‐Ge zigzag chain.

La2Pd3Ge5 and Nd2Pd3Ge5 Compounds: Chemical Bonding and Physical Properties

The two La₂Pd₃Ge₅ and Nd₂Pd₃Ge₅ compounds, crystallizing in the oI40-U₂Co₃Ge₅ crystal structure, were targeted for analysis of their chemical bonding and physical properties. The compounds of interest were obtained by arc melting and characterized by differential thermal analysis, scanning electron microscopy, and X-ray diffraction both on powder and on a single crystal (for the La analogue), to ensure the high quality of the samples and accurate crystallographic data. Chemical bonding was studied by analyzing the electronic structure and effective QTAIM charges of La₂Pd₃Ge₅. A significant charge transfer mainly occurs from La to Pd so that Ge species assume tiny negative charges. This result, together with the -(I)COHP analysis, suggests that, in addition to the expected homopolar Ge bonds within zigzag chains, heteropolar interactions between Ge and the surrounding La and Pd occur with multicenter character. Covalent La–Pd interactions increase the complexity of chemical bonding, which could not be adequately described by the simplified, formally obeyed, Zintl–Klemm scheme. Electric resistivity, specific heat, magnetization, and magnetic susceptibility as a function of temperature indicate for both compounds a metallic-like behavior. For Nd₂Pd₃Ge₅, two low-temperature phase transitions are detected, leading to an antiferromagnetic ground state.

The chemical bonding and physical properties of the two isotypic R₂Pd₃Ge₅ (R = La and Nd) intermetallics are presented. La₂Pd₃Ge₅ shows polar Ge−Pd/La multicenter interactions in addition to covalent Ge−Ge bonds. The bonding scenario is further complicated by the fact that Pd and La are also covalently interacting. For Nd₂Pd₃Ge₅, an antiferromagnetic ground state is established after a long-range magnetic ordering (at ∼7.5 K) followed by a spin-reorientation transition (at ∼6.2 K).

Uncovering new transition metal Zintl phases by cation substitution: the crystal chemistry of Ca3CuGe3 and Ca2+nMnxAg2−x+zGe2+n−z (n = 3, 4)

Crystal engineering in transition metal Zintl phases.

Solid state interactions in the La-Au-Mg system: phase equilibria, novel compounds and chemical bonding

Gold intermetallic chemistry is very rich, covering different classes of compounds ranging from the Hume-Rothery to Zintl phases to polar intermetallics to quasicrystals. Au's relativistic effects are frequently mentioned as responsible for the peculiar structural and physical properties of its compounds, nonetheless the aspects of chemical bonding are far to be clearly understood. In this work, the La-Au-Mg system was targeted for the discovery of new gold intermetallics and their structural and chemical bonding characterization. Studies on solid state interactions resulted in the construction of a partial La-Au-Mg isothermal section at 400 °C. The high reactivity between the constituents is reflected by the formation of five intermetallic compounds in the concentration range of less than 50 at% of Au. A complete crystallographic study was conducted for four of them, namely La1.82Au3+xMg14.36-x (0 ≤ x ≤ 0.90, hP42-3.64-CeMg10.3), La3Au4-xMg12+x (0 ≤ x ≤ 0.75, hP38-Gd3Ru4Al12), LaAuMg2 (oS16-MgCuAl2) and LaAu1+xMg1-x (0 ≤ x ≤ 0.15, hP9-ZrNiAl). A unifying description based on the different stacking sequences of equal slabs along the c-axis is proposed for these intermetallics. Chemical bonding in LaAuMg2 was studied by following the position space approach and including relativistic effects. Among the peculiarities of this LaMg2Au auride, there are two-atomic La-Au bonds showing a classical polar covalent character and that form distorted hexagonal planar layers and multi-atomic bonds involving Mg species. One of these is interpreted as a Mg-Mg bond supported by the neighbouring La and Au atoms, explaining the Mg reduced oxidation state (close to +1) in this compound.

In-Cage Interactions in the Clathrate Superconductor Sr8 Si46

The clathrate I superconductor Sr₈Si₄₆ is obtained under high‐pressure high‐temperature conditions, at 5 GPa and temperatures in the range of 1273 to 1373 K. At ambient pressure, the compound decomposes upon heating at T=796(5) K into Si and SrSi₂. The crystal structure of the clathrate is isotypic to that of Na₈Si₄₆. Chemical bonding analysis reveals conventional covalent bonding within the silicon network as well as additional multi‐atomic interactions between Sr and Si within the framework cages. Physical measurements indicate a bulk BCS type II superconducting state below T_c=3.8(3) K.

Unconventional Metal-Framework Interaction in MgSi5

The silicon‐rich cage compound MgSi₅ was obtained by high‐pressure high‐temperature synthesis. Initial crystal structure determination by electron diffraction tomography provided the basis for phase analyses in the process of synthesis optimization, finally facilitating the growth of single crystals suitable for X‐ray diffraction experiments. The crystal structure of MgSi₅ (space group Cmme, Pearson notation oS24, a=4.4868(2) Å, b=10.1066(5) Å, and c=9.0753(4) Å) constitutes a new type of framework of four‐bonded silicon atoms forming Si₁₅ cages enclosing the Mg atoms. Two types of smaller Si₈ cages remain empty. The atomic interactions are characterized by two‐center two‐electron bonds within the silicon framework. In addition, there is evidence for multi‐center Mg−Si bonding in the large cavities of the framework and for lone‐pair‐like interactions in the smaller empty voids.

SrPt3In2– an orthorhombically distorted coloring variant of SrIn5

The new intermetallic phase SrPt₃In₂was synthesized by induction-melting of the elements in a sealed tantalum ampoule followed by long-term annealing for crystal growth.