Cathie L. Condron

Synthesis and Characterization of K8−x(H2)ySi46

A hydrogen-containing inorganic clathrate with the nominal composition, K(7)(H(2))(3)Si(46), has been prepared in 98% yield by the reaction of K(4)Si(4) with NH(4)Br. Rietveld refinement of the powder X-ray diffraction data is consistent with the clathrate type I structure. Elemental analysis and (1)H MAS NMR confirmed the presence of hydrogen in this material. Type I clathrate structure is built up from a Si framework with two types of cages where the guest species, in this case K and H(2), can reside: a large cage composed of 24 Si, in which the guest resides in the 6d position, and a smaller one composed of 20 Si, in which the guest occupies the 2a position (cubic space group Pm3n). Potassium occupancy was examined using spherical aberration (Cs) corrected scanning transmission electron microscopy (STEM). The high-angle annular dark-field (HAADF) STEM experimental and simulated images indicated that the K is deficient in both the 2a and the 6d sites. (1)H and (29)Si MAS NMR are consistent with the presence of H(2) in a restricted environment and the clathrate I structure, respectively. FTIR and (29)Si{(1)H} CP MAS NMR results show no evidence for a Si-H bond, suggesting that hydrogen is present as H(2) in interstitial sites. Thermal gravimetry (TG) mass spectrometry (MS) provide additional confirmation of H(2) with hydrogen loss at approximately 400 degrees C.

Flux growth and structure of two compounds with the EuIn2P2 structure type, AIn2P2 (A = Ca and Sr), and a new structure type, BaIn2P2

Single crystals of the new Zintl phases AIn2P2 [A = Ca (calcium indium phosphide), Sr (strontium indium phosphide) and Ba (barium indium phosphide)] have been synthesized from a reactive indium flux. CaIn2P2 and SrIn2P2 are isostructural with EuIn2P2 and crystallize in the space group P63/mmc. The alkaline earth cations A are located at a site with 3m symmetry; In and P are located at sites with 3m symmetry. The structure type consists of layers of A2+ cations separated by [In2P2]2- anions that contain [In2P6] eclipsed ethane-like units that are further connected by shared P atoms. This yields a double layer of six-membered rings in which the In-In bonds are parallel to the c axis and to one another. BaIn2P2 crystallizes in a new structure type in the space group P2(1)/m with Z = 4, with all atoms residing on sites of mirror symmetry. The structure contains layers of Ba2+ cations separated by [In2P2]2- layers of staggered [In2P6] units that form a mixture of four-, five- and six-membered rings. As a consequence of this more complicated layered structure, both the steric and electronic requirements of the large Ba2+ cation are met.

Charge Density Wave Formation in R(2)Te(5) (R=Nd, Sm, And Gd)

The rare earth (R) tellurides R{sub 2}Te{sub 5} have a crystal structure intermediate between that of RTe{sub 2} and RTe{sub 3}, consisting of alternating single and double Te planes sandwiched between RTe block layers. We have successfully grown single crystals of Nd{sub 2}Te{sub 5}, Sm{sub 2}Te{sub 5}, and Gd{sub 2}Te{sub 5} from a self-flux and we describe here evidence for charge density wave formation in these materials. The superlattice patterns for all three compounds are relatively complex, consisting at room temperature of at least two independent wave vectors. Consideration of the electronic structure indicates that, to a large extent, these wave vectors are separately associated with sheets of the Fermi surface which are principally derived from the single and double Te layers.

Pressure-induced quenching of the charge-density-wave state in rare-earth tritellurides observed by x-ray diffraction

We report an x-ray diffraction study on the charge-density-wave (CDW) LaTe{sub 3} and CeTe{sub 3} compounds as a function of pressure. We extract the lattice constants and the CDW modulation wave-vector, and provide direct evidence for a pressure-induced quenching of the CDW phase. We observe subtle differences between the chemical and mechanical compression of the lattice. We account for these with a scenario where the effective dimensionality in these CDW systems is dependent on the type of lattice compression and has a direct impact on the degree of Fermi surface nesting and on the strength of fluctuation effects.

Weak coupling magnetism in Ce4Pt12Sn25: a small exchange limit in the Doniach phase diagram

Magnetic susceptibility, magnetization, specific heat, and electrical resistivity studies on single crystals of Ce4Pt12Sn25 reveal an antiferromagnetic transition at T(N) = 0.19 K, which develops from a paramagnetic state with a very large specific heat coefficient (C/T) of 14 J mol(-1) K(-2)-Ce just above T(N). On the basis of its crystal structure and these measurements, we argue that a weak magnetic exchange interaction in Ce4Pt12Sn25 is responsible for its low ordering temperature and a negligible Kondo-derived contribution to physical properties above T(N). The anomalous enhancement of specific heat above T(N) is suggested to be related, in part, to weak geometric frustration of f-moments in this compound.

Structure determination of H-encapsulating clathrate compounds in aberration-corrected STEM

In the quest for alternative energy sources, hydrogen has long been touted as one of the most likely candidates to replace fossil fuels, provided practical solutions for its storage and transport can be found. Clathrate hydrates, amongst other nano-porous materials, have shown remarkable potential for hydrogen storage, adsorbing up to 7.5%wt H, albeit in extreme pressure and temperature conditions [1]. Upon crystallising, these compounds form a three-dimensional host matrix where guest atoms can be accommodated within distinct polyhedral “cages”. Recent work exhibited a sodium silicide clathrate NaxSi46, consisting of a silicon matrix with two distinct Na guest sites, labelled 2a and 6d, stable at room temperature and with promising hydrogen encapsulation properties [2]. Magic angle spinning nuclear magnetic resonance was initially used to show that in the growth conditions specific to this work some of the 6d sites were Na-deficient and H-rich.

Characterization of Hydrogen-Encapsulated Type-I Silicon Clathrate

This study describes a new stable inorganic clathrate compound encapsulating hydrogen in the Si cage structure. In this work, the characterization of the new type-I Si clathrate, Na6(H2)2Si46, is illustrated. Na6(H2)2Si46 was prepared from NaSi and NH4X (X = Cl and Br) under dynamic vacuum at 300 oC. Rietveld refinement of powder X-ray diffraction data was consistent with the clathrate type-I structure. Solid-state 1 H, 29 Si and 23 Na MAS NMR confirmed the presence of both hydrogen and sodium in the clatharte cages. 23 Na NMR indicated that the guest sites are not fully occupied by Na. In order to validate the structure and investigate the Na content, the synthesized particles were examined using spherical aberration (Cs) corrected VG HB501 scanning transmission electron microscope (STEM) operated at 100 kV. Fig. 2 shows a high-angle annular dark-field (HAADF) image taken along [100] with the convergence semi-angle of 20 mrad and collection semi-angle of 70-210 mrad. Image calculation was made with Kirkland’s code [3] with the thickness of 21 nm and defocus value of 22.5 nm. The calculated image inserted in Fig. 2 is in good agreement with the experimental one. The ring pattern and the center of the ring correspond to the Si24 cage and the 6d sites, respectively. When observed along [100], the 6d sites are aligned in an atomic column which is apart from the nearest columns by 0.23 nm so that the Na occupancy in the 6d sites can be determined by comparing the experimental intensity profiles with calculated ones. Fig. 3 shows an averaged experimental image using 2dx software [4] and calculated images with different Na occupancies in the 6d sites as well as intensity profiles along the line X-Y. The peak intensity at the