Indium Flux-Growth of Eu2AuGe3: A New Germanide with an AlB2 Superstructure

Understanding and Expanding the Prospects for Electrosynthesis with Liquid Metal Electrodes

ConspectusThis Account describes and summarizes the latest work from our laboratory on developing and maturing strategies based on low-temperature liquid metals as reaction environments for materials synthesis. The electrochemical liquid-liquid-solid (ec-LLS) crystal growth concept is a hybrid method that combines electrodeposition and melt crystal growth. Using liquid metals as both electrodes and solvents for the purpose of producing inorganic crystals and materials, a simple and environmentally friendly process is possible. The impetus is to address the key deficiency in the inorganic crystalline materials that are the basis of modern optoelectronics and renewable energy capture/conversion systems. Specifically, existing methods for synthesizing crystalline inorganic materials for these purposes are largely energy- and resource-intensive, with a substantial impact on the environment when scaled. A long-term goal of our work with ec-LLS is to realize a materials synthetic process that is matured without requiring intensive resources or negatively impacting the environment. To this end, the factors that both limit and govern ec-LLS processes must be identified and understood. To date, questions regarding the factors that affect crystal nucleation and growth, form factors, and overall composition remain.Previous work established concretely ec-LLS as a versatile method for synthesizing and producing crystalline semiconductors at low temperatures as either particles, nanowires, or microwires. Subsequent experiments have focused on two tiers. First, the microscopic details of the liquid metal and its interfaces that dictate materials synthesis and crystal growth must be identified. Second, strategies that widen the attainable material form factors to facilitate device architectures must be realized. Hence, this Account describes results aimed at answering three questions: (1) What are the consequences of reaching supersaturation by an electrochemical rather than a thermal driving force for crystal growth in ec-LLS? (2) Can the location of nucleation and subsequent crystal growth be controlled? (3) Does the atomic structure of the liquid metal affect product formation in ec-LLS? The science described herein illustrates the value of in situ methods spanning transmission electron microscopy, X-ray diffraction, and X-ray reflectance for revealing the role that liquid metal composition and structure can play in ec-LLS. Additionally, we summarize work that shows for the first time that it is possible to produce both single-crystalline epitaxial films and complex intermetallic compounds through ec-LLS by tuning the cell design, electrochemical excitation waveform, and composition of the liquid metal electrodes.The cumulative findings described here substantially enrich our understanding of the ec-LLS concept while simultaneously motivating further questions moving forward. Is it possible to attain complete control over the crystalline quality and composition of ec-LLS products? Can the materials produced by ec-LLS provide tailored functional properties for targeted applications? Can the ec-LLS strategy be further refined to allow material synthesis and deposition at precise locations with deterministically chosen form factors? What synthetic pathways are accessible when even more sophisticated electrochemical waveforms and cell designs are used? Our hope is that this Account will spur additional researchers to help answer such questions.

Liquid metals: fundamentals and applications in chemistry

Post-transition elements, together with zinc-group metals and their alloys belong to an emerging class of materials with fascinating characteristics originating from their simultaneous metallic and liquid natures. These metals and alloys are characterised by having low melting points (i.e. between room temperature and 300 °C), making their liquid state accessible to practical applications in various fields of physical chemistry and synthesis. These materials can offer extraordinary capabilities in the synthesis of new materials, catalysis and can also enable novel applications including microfluidics, flexible electronics and drug delivery. However, surprisingly liquid metals have been somewhat neglected by the wider research community. In this review, we provide a comprehensive overview of the fundamentals underlying liquid metal research, including liquid metal synthesis, surface functionalisation and liquid metal enabled chemistry. Furthermore, we discuss phenomena that warrant further investigations in relevant fields and outline how liquid metals can contribute to exciting future applications.

Ca- and Sc-based ternary AlB2-like crystals: a first-principles study

The aluminum diboride (AlB₂) crystal structure comprises intercalated metal atoms between honeycomb sheets. In addition to metal diborides, which represent the most common family of AlB₂-like structures, many more materials are known to crystallize in this geometry. Here we use first-principles calculations to probe the structural and electronic properties of several such systems. Specifically, we investigate the stability of various polymorphs of CaAuAs, CaAuP, CaCuP, ScAuGe, ScAuSi, Ca₂AgSi₃ and Ca₂AuGe₃ and find lattice parameters in excellent agreement with available experimental data. The analysis of densities of states and band structure diagrams show that all materials are metallic. However, the details of band dispersion vary significantly, from typical metals such as CaAuP, to almost semi-metallic behaviour in CaCuP.

Synthetically tuned structural variations in CePdxGe2−x(x = 0.21, 0.32, 0.69) towards diverse physical properties

Three structural variations of CePd_xGe_2−xwith versatile properties were synthesized by varying the Pd : Ge ratio.

Ce2PtGe3: a new ordered orthorhombic superstructure in the AlB2family with spin glass behavior

A new compound Ce₂PtGe₃crystallizing in a new ordered superstructure of the AlB₂type with an orthorhombicCmcespace group was found to be a spin glass material at low temperature.

Metal Flux Growth, Structural Relations, and Physical Properties of EuCu2Ge2 and Eu3T2In9 (T = Cu and Ag)

Single crystals (SCs) of the compounds Eu₃Ag₂In₉ and EuCu₂Ge₂ were synthesized through the reactions run in liquid indium. Eu₃Ag₂In₉ crystallizes in the La₃Al₁₁ structure type [orthorhombic space group (SG) Immm] with the lattice parameters: a = 4.8370(1) Å, b = 10.6078(3) Å, and c = 13.9195(4) Å. EuCu₂Ge₂ crystallizes in the tetragonal ThCr₂Si₂ structure type (SG I4/mmm) with the lattice parameters: a = b = 4.2218(1) Å, and c = 10.3394(5) Å. The crystal structure of Eu₃Ag₂In₉ is comprised of edge-shared hexagonal rings consisting of indium. The one-dimensional chains of In₆ rings are shared through the edges, which are further interconnected with other six-membered rings forming a three-dimensional (3D) stable crystal structure along the bc plane. The crystal structure of EuCu₂Ge₂ can be explained as the complex [CuGe]^(2+δ)- polyanionic network embedded with Eu ions. These polyanionic networks present in the crystal structure of EuCu₂Ge₂ are shared through the edges of the 011 plane containing Cu and Ge atoms, resulting in a 3D network. The structural relationship between Eu₃T₂In₉ and EuCu₂Ge₂ has been discussed in detail, and we conclude that Eu₃T₂In₉ is the metal deficient variant of EuCu₂Ge₂. The magnetic susceptibilities of Eu₃T₂In₉ (T = Cu and Ag) and EuCu₂Ge₂ were measured between 2 and 300 K. In all cases, magnetic susceptibility data followed Curie-Weiss law above 150 K. Magnetic moment values obtained from the measurements indicate the probable mixed/intermediate valent behavior of the europium atoms, which was further confirmed by X-ray absorption studies and bond distances around the Eu atoms. Electrical resistivity measurements suggest that Eu₃T₂In₉ and EuCu₂Ge₂ are metallic in nature.

Novel Au- and Ge-based two-dimensional materials formed through topotactic transitions of AlB2-like structures

The topotactic reaction of a layered compound, for example CaGe2, with HCl solution is a common and facile method to produce two-dimensional (2D) materials. In this work we demonstrate with first-principles calculations that this technique can potentially lead to a whole new family of 2D materials starting from three-dimensional crystals with AlB2-like structures. As representative cases, we show here that the de-intercalation of Sc and Ca atoms from ScAuGe and Ca2AuGe3 crystals is strongly exothermic and produces the stable 2D monolayers AuGeH and AuGe3H3, respectively. Remarkably, both metals (AuGeH) and semiconductors (AuGe3H3) can be prepared by this method. Based on the broad availability of AlB2-like structures with varying stoichiometries, there are several possibilities to prepare novel functional 2D materials with suitable topotactic transitions.

Eu3Ir2In15: A Mixed-Valent and Vacancy-Filled Variant of the Sc5Co4Si10 Structure Type with Anomalous Magnetic Properties

A new compound, Eu3Ir2In15, has been synthesized using indium as an active metal flux. The compound crystallizes in the tetragonal P4/mbm space group with lattice parameters a = 14.8580(4) Å, b = 14.8580(4) Å, and c = 4.3901(2) Å. It was further characterized by SEM-EDX studies. The effective magnetic moment (μeff) of this compound is 7.35 μB/Eu ion with a paramagnetic Curie temperature (θp) of -28 K, suggesting antiferromagnetic interaction. The mixed-valent nature of Eu observed in magnetic measurements was confirmed by XANES measurements. The compound undergoes demagnetization at a low magnetic field (10 Oe), which is quite unusual for Eu-based intermetallic compounds. Temperature-dependent resistivity studies reveal that the compound is metallic in nature. A comparative study was made between Eu3Ir2In15 and hypothetical vacancy-variant Eu5Ir4In10, which also crystallizes in the same crystal structure. However, our computational studies along with control experiments suggest that the latter is thermodynamically less feasible compared to the former, and hence we propose that it is highly unlikely that an RE5T4X10 would exist with X as a group 13 element.

Synthesis, Structure, and Rigid Unit Mode-like Anisotropic Thermal Expansion of BaIr2In9

This Article reports the synthesis of large single crystals of BaIr2In9 using In flux and their characterization by variable-temperature single-crystal and synchrotron powder X-ray diffraction, resistivity, and magnetization measurements. The title compound adopts the BaFe2Al9-type structure in the space group P6/mmm with room temperature unit cell parameters a = 8.8548(6) Å and c = 4.2696(4) Å. BaIr2In9 exhibits anisotropic thermal expansion behavior with linear expansion along the c axis more than 3 times larger than expansion in the ab plane between 90 and 400 K. This anisotropic expansion originates from a rigid unit mode-like mechanism similar to the mechanism of zero and negative thermal expansion observed in many anomalous thermal expansion materials such as ZrW2O8 and ScF3.

Yb7Ni4InGe12: a quaternary compound having mixed valent Yb atoms grown from indium flux

The new intermetallic compound Yb7Ni4InGe12 was obtained as large silver needle shaped single crystals from reactive indium flux. Single crystal X-ray diffraction suggests that Yb7Ni4InGe12 crystallizes in the Yb7Co4InGe12 structure type, and tetragonal space group P4/m and lattice constants are a = b = 10.291(2) Å and c = 4.1460(8) Å. The crystal structure of Yb7Ni4InGe12 consists of columnar units of three different types of channels filled with the Yb atoms. The crystal structure of Yb7Ni4InGe12 is closely related to Yb5Ni4Ge10. The effective magnetic moment obtained from the magnetic susceptibility measurements in the temperature range 200-300 K is 3.66μB/Yb suggests mixed/intermediate valence behavior of ytterbium atoms. X-ray absorption near edge spectroscopy (XANES) confirms that Yb7Ni4InGe12 exhibits mixed valence.

Surfactant-thermal method to synthesize a novel two-dimensional oxochalcogenide

A new two-dimensional (2D) oxosulfide, (N2H4)2Mn3Sb4S8(μ3-OH)2 (1), has been successfully synthesized under surfactant-thermal conditions with hexadecyltributylphosphonium bromide as the surfactant. Compound 1 has a layered structure and contains a novel [Mn3(μ3-OH)2]n chain along the b-axis. The photocatalytic activity for compound 1 has been demonstrated under visible-light irradiation and continuous H2 evolution was observed. Our results indicate that surfactant-thermal synthesis could be a promising method for growing novel crystalline oxochalcogenides with interesting structures and properties.

Structure and unusual magnetic properties of YbMn0.17Si1.88

YbMn0.17Si1.88 was synthesized from the reaction of ytterbium, manganese, and silicon using indium as a flux. The average structure of YbMn0.17Si1.88 was refined in the monoclinic space group P21, with a = 4.0107(8) Å, b = 3.8380(8) Å, c = 14.458(3) Å, β = 97.97(3)°, R1/wR2 = 0.0296/0.0720. The structure can be described as the intergrowth of three AlB2-type layers and one BaAl4-type layer. Magnetic susceptibility measurements suggest that the ytterbium atoms in YbMnxSi2-x exist in a mixed valent or intermediate valent state. YbMn0.17Si1.88 shows weak antiferromagnetic ordering below ∼4.5 K. The magnetic interactions between the Mn and Yb atoms in YbMn0.17Si1.88 are evident from the magnetic susceptibility measurements performed at low field. A negative magnetization is observed on warming and a positive magnetization on cooling. The heat capacity data suggest moderate heavy fermion behavior.

Structures and phase transitions of CePd3+xGa8-x: new variants of the BaHg11 structure type

New distorted variants of the cubic BaHg11 structure type have been synthesized in Ga flux. Multiple phases of CePd3+xGa8-x, which include an orthorhombic Pmmn structure (x = 3.21(2)), a rhombohedral R3m structure (x = 3.13(4)), and a cubic Fm3m superstructure (x = 2.69(6)), form preferentially depending on reaction cooling rate and isolation temperature. Differential thermal analysis and in situ temperature-dependent powder X-ray diffraction patterns show a reversible phase transition at approximately 640 °C between the low temperature orthorhombic and rhombohedral structures and the high temperature cubic superstructure. Single crystal X-ray diffraction experiments indicate that the general structure of BaHg11, including the intersecting planes of a kagomé-type arrangement of Ce atoms, is only slightly distorted in the low temperature phases. A combination of Kondo, crystal electric field, and magnetic frustration effects may be present, resulting in low temperature anomalies in magnetic susceptibility, electrical resistivity, and heat capacity measurements. In addition to CePd3+xGa8-x, the rare earth analogues REPd3+xGa8-x, RE = La, Nd, Sm, Tm, and Yb, were successfully synthesized and also crystallize in one of the lower symmetry space groups.

Synthesis and properties of new multinary silicides R5Mg5Fe4Al(x)Si(18-x) (R = Gd, Dy, Y, x ≈ 12) grown in Mg/Al flux

Reactions of iron, silicon, and R = Gd, Dy, or Y in 1:1 Mg/Al mixed flux produce well-formed crystals of R(5)Mg(5)Fe(4)Al(x)Si(18-x) (x ≈ 12). These phases have a new structure type in tetragonal space group P4/mmm (a = 11.655(2) Å, c = 4.0668(8) Å, Z = 1 and R(1) = 0.0155 for the Dy analogue). The structure features two rare earth sites and one iron site; the latter is in monocapped trigonal prismatic coordination surrounded by silicon and aluminum atoms. Siting of Al and Si was investigated using bond length analysis and (27)Al and (29)Si MAS NMR studies. The magnetic properties are determined by the R elements, with the Gd and Dy analogues exhibiting antiferromagnetic ordering at T(N) = 11.9 and 6.9 K respectively; both phases exhibit complex metamagnetic behavior with varying field.