Siting of antimony dopants and gallium in Ba(8)Ga(16)Ge(30) clathrates grown from gallium flux

Flux Growth of Cerium Nickel Gallides Studied by In Situ Neutron Diffraction

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 CeNi_0.74Ga_3.26 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 CeNi_0.74Ga_3.26 for further study. Similar reactions replacing Ce with La and quenching above 750 °C yielded LaNi_0.35Ga_3.65 crystals. Magnetic susceptibility studies on CeNi_0.74Ga_3.26 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 LaNi_0.35Ga_3.65 and CeNi_0.74Ga_3.26 exhibit metallic behavior. Density of states calculations support this and indicate that Ni/Ga mixing in the compound stabilizes the structure.

High-efficiency thermoelectric Ba8Cu14Ge6P26: bridging the gap between tetrel-based and tetrel-free clathrates

Synergy between tetrel- and pnictide-based clathrates: synthesis, crystal structure, and transport properties of a Ba₈Cu₁₄Ge₆P₂₆.

A new type-I clathrate, Ba₈Cu₁₄Ge₆P₂₆, was synthesized by solid-state methods as a polycrystalline powder and grown as a cm-sized single crystal via the vertical Bridgman method. Single-crystal and powder X-ray diffraction show that Ba₈Cu₁₄Ge₆P₂₆ crystallizes in the cubic space group Pm3n (no. 223). Ba₈Cu₁₄Ge₆P₂₆ is the first representative of anionic clathrates whose framework is composed of three atom types of very different chemical natures: a transition metal, tetrel element, and pnicogen. Uniform distribution of the Cu, Ge, and P atoms over the framework sites and the absence of any superstructural or local ordering in Ba₈Cu₁₄Ge₆P₂₆ were confirmed by synchrotron X-ray diffraction, electron diffraction and high-angle annular dark field scanning transmission electron microscopy, and neutron and X-ray pair distribution function analyses. Characterization of the transport properties demonstrate that Ba₈Cu₁₄Ge₆P₂₆ is a p-type semiconductor with an intrinsically low thermal conductivity of 0.72 W m^–1 K^–1 at 812 K. The thermoelectric figure of merit, ZT, for a slice of the Bridgman-grown crystal of Ba₈Cu₁₄Ge₆P₂₆ approaches 0.63 at 812 K due to a high power factor of 5.62 μW cm^–1 K^–2. The thermoelectric efficiency of Ba₈Cu₁₄Ge₆P₂₆ is on par with the best optimized p-type Ge-based clathrates and outperforms the majority of clathrates in the 700–850 K temperature region, including all tetrel-free clathrates. Ba₈Cu₁₄Ge₆P₂₆ expands clathrate chemistry by bridging conventional tetrel-based and tetrel-free clathrates. Advanced transport properties, in combination with earth-abundant framework elements and congruent melting make Ba₈Cu₁₄Ge₆P₂₆ a strong candidate as a novel and efficient thermoelectric material.

Clathrates and semiclathrates of Type-I: crystal structure and superstructures

The review surveys the crystal chemistry of inorganic compounds belonging to the structure type of clathrate-I. The compounds of this family exhibit an unexpected variety of both chemical composition and features of the crystal structure. The basic crystal structure of clathrate-I and different variants of its modification, such as siting, position splitting, and superstructure formation including transformation to semiclathrates, are considered in terms of the group-subgroup relationship.

Structure and Thermoelectric Characterization of Ba8Al14Si31

A molten Al flux method was used to grow single crystals of the type I clathrate compound Ba8Al14Si31. Single-crystal neutron diffraction data for Ba8Al14Si31 were collected at room temperature using the SCD instrument at the Intense Pulsed Neutron Source, Argonne National Laboratory. Single-crystal neutron diffraction of Ba8Al14Si31 confirms that the Al partially occupies all of the framework sites (R1 = 0.0435, wR2 = 0.0687). Stoichiometry was determined by electron microprobe analysis, density measurements, and neutron diffraction analysis. Solid-state (27)Al NMR provides additional evidence for site preferences within the framework. This phase is best described as a framework-deficient solid solution Ba8Al14Si31, with the general formula, Ba(8)Al(x)Si(42-3/4x)[](4-1/4x) ([] indicates lattice defects). DSC measurements and powder X-ray diffraction data indicate that this is a congruently melting phase at 1416 K. Temperature-dependent resistivity reveals metallic behavior. The negative Seebeck coefficient indicates transport processes dominated by electrons as carriers.

Crystal Structures, Raman Spectroscopy, and Magnetic Properties of Ba7.5Al13Si29 and Eu0.27Ba7.22Al13Si29

The framework-deficient clathrate phases Ba7.5Al13Si29 and Eu0.27Ba7.22Al13Si29 were prepared using a molten Al flux. Single-crystal X-ray diffraction confirmed the two phases to be clathrate type I (space group Pmn). For Eu0.27Ba7.22Al13Si29, single-crystal X-ray diffraction revealed the Eu to partially occupy the 2a position. Microprobe analysis of single crystals provided the stoichiometry, and Raman spectroscopy was used to investigate the guest framework interactions. The Raman spectra are consistent with both Ba7.5Al13Si29 and Eu0.27Ba7.22Al13Si29 having minimal guest-host interactions. Magnetic susceptibility data for Eu0.27Ba7.22Al13Si29 imply weak magnetic ordering and indicate a 2+ oxidation state for the Eu ion.

Intermetallic Compounds with Near Zintl Phase Behavior: RE2Zn3Ge6 (RE = La, Ce, Pr, Nd) Grown from Liquid Indium

A series of compounds has been discovered while investigating reactions of rare earth, transition metals, and Ge in excess indium. These compounds, RE2Zn3Ge6 (RE = La, Ce, Pr, Nd), are isostructural, crystallizing in the orthorhombic space group Cmcm with lattice parameters a = 5.9691(9) angstroms, b = 24.987(4) angstroms, and c = 5.9575(9) angstroms for La2Zn3Ge6, a = 5.9503(5) angstroms, b = 24.761(2) angstroms, and c = 5.9477(5) angstroms for the Ce analogue, a =5.938(2) angstroms, b = 24.708(8) angstroms, and c = 5.936(2) angstroms for Pr2Zn3Ge6, and a = 5.9094(7) angstroms, b = 24.619(3) angstroms, and c = 5.9063(5) angstroms for the Nd analogue. The structure is composed of PbO-like ZnGe layers and ZnGe4 cage layers and is related to the Ce4Zn8Ge(11-x) structure type. The bonding in the system can be rationalized using the Zintl concept resulting in a material that is expected to be a valence precise semiconductor, although its behavior is more consistent with it being a semimetal, making it an intermediate case. The results of band structure calculations and magnetic measurements of these compounds are discussed.