RESUMEN
Zintl phases have potential applications as thermoelectric materials for power generation and cooling owing to their complex crystal structures and unique electronic properties. We carried out reactions of silicon with barium and strontium in excess Mg/Zn flux to synthesize (Ba/Sr)5+xMg19-xSi12 Zintl phases, investigating the effect of varying Ba/Sr ratio on site mixing and thermoelectric properties. (Ba/Sr)5+xMg19-xSi12 compounds with 0 < x < 3 are charge-balanced Zintl phases which adopt the hexagonal Ho5Ni19P12 structure type (space group P6Ì 2m). Density of states calculations indicate that these materials are semimetals. Single-crystal X-ray diffraction data and elemental analysis for Ba5Mg19Si12, Ba4.86Sr2.94Mg16.20Si12, Ba3.63Sr4.20Mg16.17Si12, Ba1.93Sr5.99Mg16.08Si12, and Sr7.82Mg16.18Si12 show occupation of barium and strontium cations in Ho sites, while strontium mixes with magnesium on a specific Ni site. Powder XRD data of products show that they are single phase throughout the sample. Thermoelectric measurements indicate that increasing strontium content and mixing on three cation sites decreases thermal conductivity; it is hypothesized that improved overall thermoelectric behavior is likely due to the rattling of the Sr cations in their positions.
RESUMEN
Reactions of cerium and transition metals in excess molten gallium were carried out, exploring the formation of different cerium intermetallics as the flux reaction is cooled. Ce/T/Ga reactions with T = Ni, Cu, Pd, Ag, and Zn produce a high-temperature product, which converts into a low-temperature product in the flux. The phases present in the flux mixture were determined by quenching identical reactions at 750 and 300 °C and identifying the isolated products using elemental analysis and X-ray diffraction. The compounds CeGa2, CeCu0.37Ga3.63, CePd0.32Ga3.68, Ce5Ag1.76Ga17.29, and Ce5Zn1.37Ga17.73 were isolated by quenching at 750 °C. Upon cooling to 300 °C, the corresponding reactions instead yielded CeGa6, Ce2CuGa12, Ce2PdGa12, Ce2Ag0.7Ga9.1, and CeZnxGa7-x. All of these structures contain cerium in the ThCr2Si2-related layers. Large crystals of high-temperature products CeCu0.37Ga3.63, CePd0.32Ga3.68, Ce5Ag1.76Ga17.29, and Ce5Zn1.37Ga17.73 were used for magnetic susceptibility measurements. All of these materials show highly anisotropic ferromagnetic ordering of Ce3+ moments below 8 K, which is in contrast to the antiferromagnetism seen for the compounds that were isolated at 300 °C.
RESUMEN
Reactions of anthracene in Ln/T eutectic mixtures (Ln = La, Yb; T = Ni, Cu) have produced crystals of new complex lanthanide carbide hydride phases. The thermal decomposition of anthracene provides a source of both carbon and hydrogen. LaCHx forms in space group Pnma with unit cell parameters a = 7.2736(4) Å, b = 3.7218(2) Å, and c = 13.0727(7) Å. Yb2CHx forms in space group P-3m1 with unit cell parameters a = 3.5659(4) Å and c = 5.8000(8) Å, with a structure related to that of Ho2CF2. The presence of hydride in interstitial sites is supported by the formation of different compounds in the absence of hydrogen and by comparison to known hydride structures. 1H nuclear magnetic resonance (NMR) spectra collected on LaCHx show two unique hydride resonances, in agreement with the two available interstitial sites. Density of states calculations show that filling the tetrahedral sites in LaCHx with hydrogen increases metallic behavior, while adding hydrogen into the tetrahedral sites in Yb2CHx induces the formation of a band gap.
RESUMEN
La15(FeC6)4F2 was grown as large crystals by reacting iron in a La/Ni eutectic flux in the presence of decafluorobiphenyl (C12F10) which acts as both a carbon and fluoride source. This mild fluorinating technique enables the isolation of an intermetallic product containing fluoride interstitials, as opposed to forming ionic metal fluorides. The compound adopts a structure in the hexagonal crystal system with space group P6Ì which features FeC6 units composed of a central iron atom coordinated by three ethylenide units in a trigonal planar configuration. The structure is related to the previously reported La15(FeC6)4H, but with fluoride fully occupying the interstitial hydride positions, which induces partial occupancies and site splitting disorder in the adjacent layers of lanthanide ions. No supercell formation is observed.
RESUMEN
Reactions of cerium and nickel in excess molten gallium were monitored by neutron diffraction during heating and cooling. The formation of binary intermediates CeGa2 and Ni2Ga3 was observed during heating. During cooling of the molten mixture from 900 °C, precipitation of BaAl4-type CeNi0.74Ga3.26 occurred at 850 °C. Upon cooling to 650 °C, this compound reacted in the flux to form Ce2NiGa10 and then Ce2NiGa12, the latter of which persisted to room temperature. Making use of this information, subsequent reactions were quenched at 750 °C to isolate crystals of CeNi0.74Ga3.26 for further study. Similar reactions replacing Ce with La and quenching above 750 °C yielded LaNi0.35Ga3.65 crystals. Magnetic susceptibility studies on CeNi0.74Ga3.26 indicate that the cerium is trivalent; the Ce3+ moments undergo a strongly anisotropic ferromagnetic ordering with moment perpendicular to the c axis below 7 K. Heat capacity data show little evidence of heavy fermion behavior. Resistivity measurements show that both LaNi0.35Ga3.65 and CeNi0.74Ga3.26 exhibit metallic behavior. Density of states calculations support this and indicate that Ni/Ga mixing in the compound stabilizes the structure.
RESUMEN
An1.33T4Al8Si2 (An = Ce, Th, U, Np; T = Ni, Co) were synthesized in metal flux reactions carried out in aluminum/gallium melts. In previous work, U1.33T4Al8Si2 (T = Co, Ni) analogues were formed by arc-melting U:T:Si and reacting this mixture in Al/Ga flux. However, in the current work, all compounds were synthesized by using AnO2 reactants, taking advantage of the ability of the aluminum in the flux to act as both solvent and reducing agent. While reactions with T = Co yielded hexagonal Gd1.33Fe4Si10-type quaternary phases for all An, reactions with T = Ni produced these compounds only with An = U and Np. For reactions with An = Ce and Th, the reactions led instead to the formation of AnNi3-xSixAl4-yGay phases, with the tetragonal KCu3S4 structure type. Attempts to synthesize plutonium analogues Pu1.33T4Al8Si2 were also unsuccessful, producing the previously reported PuCoGa5 and Pu2Ni5Si6 instead. Magnetic data collected on the neptunium analogues Np1.33T4Al8Si2 (T = Ni, Co) show antiferromagnetic coupling at low temperatures and indicate a tetravalent state for the Np ions.
RESUMEN
La15(FeC6)4H was synthesized from the reaction of iron and anthracene in La/Ni eutectic flux. Anthracene was the source of both the carbon and hydrogen in the product. The structure of this metal carbide hydride features hydride ions in tetrahedral interstitial sites surrounded by lanthanum ions, which was confirmed by single-crystal neutron diffraction studies. The trigonal planar FeC6 units in which the central iron atom is coordinated by three ethylenide groups are similar to those found in La3.67FeC6, a previously reported compound that is formed in the absence of a hydride source. Magnetic susceptibility data confirm that the iron sites do not have magnetic moments. Density of states calculations indicate that La15(FeC6)4H is metallic and is stabilized by the incorporation of hydride anions.
RESUMEN
Reactions of boron, tellurium, and either iron or manganese in a praseodymium-nickel flux led to the production of Pr21M16Te6B30 (M = Fe or Mn) with a novel structure type that features M16B30 clusters surrounded by a Pr/Te framework. Due to disorder in the orientation of the transition metal boride clusters, these phases initially appear to form in the cubic space group Pm3Ì m. However, analysis of site occupancy, bond lengths, and local structure in the M16B30 sublattice indicates the local symmetry is P4Ì 3m. This space group symmetry is supported by transmission electron microscopy studies including selected area electron diffraction (SAED) and high angle annular dark field scanning transmission electron microscopy (HAADF-STEM), which indicate ordered regions. The M16B30 cluster twinning domain that could be as small as nanometer size inside a single crystal results in the misleading Pm3Ì m symmetry. Electronic structure calculations indicate the Pr21M16Te6B30 phases are metals. Magnetic susceptibility measurements show that both the praseodymium and the transition metal have magnetic moments in these compounds. Pr21Mn16Te6B30 exhibits antiferromagnetic ordering at TN = 15 K, and Pr21Fe16Te6B30 undergoes a likely ferrimagnetic transition at TC = 23 K.
RESUMEN
MgNi2Bi4 was grown from the reaction of magnesium and nickel in excess bismuth flux. It forms as large, malleable crystals with a new structure type in orthorhombic space group Cmcm. The structure contains a building block common to Ni-Bi binary phases-nickel zigzag chains running along one direction and surrounded by bismuth. Magnetic susceptibility and transport measurements indicate that the compound is metallic; this is supported by calculations of density of states. Crystal orbital Hamilton population analyses indicate that Ni-Bi interactions are the strongest bonding interactions in the structure, whereas Bi-Bi bonding between the layers is negligible, making MgNi2Bi4 a potential two-dimensional material.
RESUMEN
Two new quaternary analogs of the Gd1+xFe4Si10-y structure type were grown from the reaction of uranium, silicon, and a transition metal (nickel or cobalt) in an excess of aluminum/gallium flux. The use of a mixed flux was found to be necessary for the formation of U1.33T4Al8Si2 (T = Ni, Co). Single crystal X-ray diffraction data shows the presence of disordered U/Si layers that are characteristic of this structure type; precession photographs indicate partial formation of a superstructure and stacking disorder along the c-axis. This disorder may be the cause of the spin glass behavior that is particularly evident in the nickel analog, which exhibits a spin freezing transition at TF = 7 K. These compounds are resistant to chemical attack and oxidation and may be potential waste forms.
RESUMEN
The Ba/Yb/Mg/Si intermetallic system is extremely complex, with four competing structurally related compounds forming from reactions of barium, ytterbium, and silicon in magnesium-rich Mg/Al flux. In addition to the previously reported Ba2Yb0.9Mg11.1Si7, Ba5Yb2Mg17Si12, and Ba20Yb5Mg61Si43, a new compound has been found. Ba6Yb1.84Mg18.16Si13 crystallizes in the P6Ì space group, with the Zr6Ni20P13 structure type. Quenching experiments and in situ neutron powder diffraction studies were carried out to determine the reaction parameters that favor particular products. Under slow-cooling conditions, Ba5Yb2Mg17Si12 precipitates from the flux at 800 °C. A faster cooling rate of an identical reaction results in the formation of single crystals of Ba20Yb5Mg61Si43 in the flux at 640 °C. This indicates that the crystallization of products in this metal flux reaction does not involve precipitation and interconversion of different phases but instead depends on the rate of cooling across the supersaturated metastable zone in this system.
RESUMEN
Reactions of silicon, carbon, and iron in a low-melting flux mixture of praseodymium and nickel produced two competing intermetallic compounds. Pr62Fe21Si16C32 has a new structure type in tetragonal space group P4/ mmm ( a = 15.584(2) Å, c = 11.330(1) Å, Z = 1) that features trigonal planar FeC3 units that share corners to form a framework of cylindrical channels encompassing a network of silicon-centered praseodymium clusters. Slight variation of reactant ratio and heating profile produced Pr21Fe8Si7C12 instead; this compound has the previously reported cubic La21Fe8Sn7C12 structure type. Identical Pr/Si clusters and FeC3 subunit motifs are found in both structure types. In addition to reactant ratio and heating profile, size effects play a role in determining which structure forms. Replacing silicon with smaller phosphorus atoms produces only the tetragonal structure; replacement with larger elements (M = Ge, Sn) yields only cubic Pr21Fe8M7C12. Magnetic susceptibility measurements on single crystals of Pr62Fe21Si16C32 indicate antiferromagnetic ordering of the Pr moments below 17 K and no magnetic moment on iron atoms. The behavior of Pr21Fe8Si7C12 is more complex, revealing magnetic contributions from both Pr and Fe atoms and possible spin frustration.
RESUMEN
Metal flux synthesis involves the reaction of metals and metalloids in a large excess of a low-melting metal that acts as a solvent. This technique makes use of an unusual temperature regime (above the temperatures used for solvothermal methods and below the temperatures used in traditional solid state synthesis) and facilitates the growth of products as large crystals. It has proven to be a fruitful method to discover new intermetallic compounds. However, little is known about the chemistry occurring within a molten metal solvent; without an understanding of the nature of precursor formation and assembly, it is difficult to predict product structures and target properties. Organic chemists have a vast toolbox of well-known reagents and reaction mechanisms to use in directing their synthesis toward a desired molecular structure. This is not yet the case for the synthesis of inorganic extended structures. We have carried out extensive explorations of the growth of new magnetic intermetallic compounds from a variety of metal fluxes. This Account presents a review of some of our results and recent reports by other groups; this work indicates that products with common building blocks and homologous series with identical structural motifs are repeatedly seen in metal flux chemistry. For instance, fluorite-type layers comprised of transition metals coordinated by eight main group metal atoms are found in the Th2(AuxSi1-x)[AuAl2]nSi2 and R[AuAl2]nAl2(AuxSi1-x)2 series grown from aluminum flux, the CenPdIn3n+2 series grown from indium flux, and CePdGa6 and Ce2PdGa10 grown from gallium flux. Similarly, our investigations of reactions of heavy main group metals, M, in rare earth/transition metal eutectic fluxes reveal that the R/T/M/M' products usually feature M-centered rare earth clusters M@R8-12, which share faces to form layers and networks that surround transition metal building blocks. These structural trends, temperature dependence of products formed in the flux, and interconversions observed by differential scanning calorimetry support the idea that these clusters likely form in the melt, existing as precursors and assembling into different crystalline products depending on time, temperature, and reaction ratio. Proof of this mechanism will require future investigations using techniques such as pair distribution function analysis of flux melts to observe cluster formation and in situ diffraction during cooling to detect various phases as they crystallize and interconvert. These data will aid in understanding the parameters that control cluster formation and assembly in metal melts, allow for prediction of products of flux reactions, and will potentially enable the tailoring of reaction conditions to promote the formation of structures with desirable properties.
RESUMEN
Reactions of tin and manganese in a lanthanum/nickel eutectic melt in alumina crucibles produce La11Mn13-x-yNixAlySn4-δ (0 ≤ x ≤ 3.6; 2.5 ≤ y ≤ 4.9; 0.6 ≤ δ ≤ 1.1) phases with the stoichiometry dependent on the reactant ratio. These compounds crystallize in a new tetragonal structure type in space group P4/mbm, with a = 8.4197(1) Å, c = 19.2414(3) Å, and Z = 2 for La11Mn8.2Ni0.8Al4Sn3.3. The structure can be viewed as an intergrowth between La6Co11Ga3-type layers and Cr5B3-type La/Sn slabs. This system represents a unique playground to study the itinerant magnetism of diluted icosahedral Mn layers. The dilution of manganese sites in the Mn/Ni/Al layer with nonmagnetic elements has a significant effect on magnetic properties, with low Mn content analogues being paramagnetic and higher Mn content analogues such as La11Mn10Al3Sn3.4 exhibiting spin-glass behavior with a freezing transition at 20 K. The lack of long-range magnetic ordering is confirmed by heat capacity and resistivity measurements.
RESUMEN
Bismuth and antimony were reacted in sulfur/iodine flux mixtures at various temperatures and iodine concentrations to explore the effects of these variables on the synthesis and properties of Bi1-xSbxSI products. The products grow as crystals; microprobe elemental analysis and UV/vis/NIR spectroscopy of the Bi1-xSbxSI solid solutions indicate that substitution is homogeneous within individual crystals but varies up to 15% between crystals within each synthesis batch. Raman spectra show a two-mode behavior upon substitution, indicating covalent bonding within the structure, and TEM/SEM data confirm no presence of nanoclustering or segregation within the crystals.
RESUMEN
Reactions of indium and silicon with lithium nitride in Ca/Li flux produce two new nitridosilicates: Ca8In2SiN4 (orthorhombic, Ibam; a = 12.904(1) Å, b = 9.688(1) Å, c = 10.899(1) Å, Z = 4) and Ca3SiN3H (monoclinic, C2/c; a = 5.236(1) Å, b = 10.461(3) Å, c = 16.389(4) Å, ß = 91.182(4)°, Z = 8). Ca8In2SiN4 features isolated [SiN4]8- units and indium dimers surrounded by calcium atoms. Ca3SiN3H features infinite chains of corner-sharing SiN4 tetrahedra and distorted edge-sharing H@Ca6 octahedra. Optical properties and band structure calculations indicate that Ca8In2SiN4 is a void metal with calcium and indium states at the Fermi level and Ca3SiN3H is a semiconductor with a band gap of 3.1 eV.
RESUMEN
Two new tellurium-containing nitrides were grown from reactions in molten calcium and lithium. The compound Ca6Te3N2 crystallizes in space group R3Ì c (a = 12.000(3)Å, c = 13.147(4)Å; Z = 6); its structure is an anti-type of rinneite (K3NaFeCl6) and 2H perovskite related oxides such as Sr3Co2O6. The compound Ca6(LixFe1-x)Te2N3 where x ≈ 0.48 forms in space group P42/m (a = 8.718(3)Å, c = 6.719(2)Å; Z = 2) with a new stuffed anti-type variant of the Tl3BiCl6 structure. Band structure calculations and easily observable red/green dichroic behavior indicate that Ca6Te3N2 is a highly anisotropic direct band gap semiconductor (Eg = 2.5 eV). Ca6(LixFe1-x)Te2N3 features isolated linear N-Fe-N units with iron in the rare Fe(1+) state. The magnetic behavior of the iron site was characterized by magnetic susceptibility measurements, which indicate a very high magnetic moment (5.16µB) likely due to a high degree of spin-orbit coupling. Inherent disorder at the Fe/Li mixed site frustrates long-range communication between magnetic centers.
RESUMEN
Two new complex main-group metal carbides were synthesized from reactions of indium, carbon, and a metal hydride in metal flux mixtures of an alkaline earth (AE = Ca, Ba) and lithium. Ca(12)InC(13-x) and Ba(12)InC(18)H(4) both crystallize in cubic space group Im3Ì [a = 9.6055(8) and 11.1447(7) Å, respectively]. Their related structures are both built on a body-centered-cubic array of icosahedral clusters comprised of an indium atom and 12 surrounding alkaline-earth cations; these clusters are connected by bridging monatomic anions (either H(-) or C(4-)) and allenylide anions, C(3)(4-). The allenylide anions were characterized by Raman spectroscopy and hydrolysis studies. Density of states and crystal orbital Hamilton population calculations confirm that both compounds are metallic.
RESUMEN
The reaction of arsenic with sources of light elements in a Ca/Li melt leads to the formation of two new arsenide hydride phases. The predominant phase Ca14As6X7 (X = C(4-), N(3-), H(-)) exhibits a new tetragonal structure type in the space group P4/mbm (a = 15.749(1) Å, c = 9.1062(9) Å, Z = 4, R1 = 0.0150). The minor phase LiCa3As2H also has a new structure type in the orthorhombic space group Pnma (a = 11.4064(7) Å, b = 4.2702(3) Å, c = 11.8762(8)Å, Z = 4, R1 = 0.0135). Both phases feature hydride and arsenide anions separated by calcium cations. The red color of these compounds indicates they should be charge-balanced. DOS calculations on LiCa3As2H confirm a band gap of 1.4 eV; UV-vis spectroscopy on Ca14As6X7 shows a band gap of 1.6 eV. Single-crystal neutron diffraction studies were necessary to determine the mixed occupancy of carbon, nitrogen, and hydrogen anions on the six light-element sites in Ca14As6X7; these data indicated an overall stoichiometry of Ca14As6C(0.445)N(1.135)H(4.915).