Daniel E. Bugaris

Pentavalent and tetravalent uranium selenides, Tl3Cu4USe6 and Tl2Ag2USe4: syntheses, characterization, and structural comparison to other layered actinide chalcogenide compounds.

The compounds Tl(3)Cu(4)USe(6) and Tl(2)Ag(2)USe(4) were synthesized by the reaction of the elements in excess TlCl at 1123 K. Both compounds crystallize in new structure types, in space groups P2(1)/c and C2/m, respectively, of the monoclinic system. Each compound contains layers of USe(6) octahedra and MSe(4) (M = Cu, Ag) tetrahedra, separated by Tl(+) cations. The packing of the octahedra and the tetrahedra within the layers is compared to the packing arrangements found in other layered actinide chalcogenides. Tl(3)Cu(4)USe(6) displays peaks in its magnetic susceptibility at 5 and 70 K. It exhibits modified Curie-Weiss paramagnetic behavior with an effective magnetic moment of 1.58(1) μ(B) in the temperature range 72-300 K, whereas Tl(2)Ag(2)USe(4) exhibits modified Curie-Weiss paramagnetic behavior with μ(eff) = 3.4(1) μ(B) in the temperature range 100-300 K. X-ray absorption near-edge structure (XANES) results from scanning transmission X-ray spectromicroscopy confirm that Tl(3)Cu(4)USe(6) has Se bonding characteristic of discrete Se(2-) units, Cu bonding generally representative of Cu(+), and U bonding consistent with a U(4+) or U(5+) species. On the basis of these measurements, as well as bonding arguments, the formal oxidation states for U may be assigned as +5 in Tl(3)Cu(4)USe(6) and +4 in Tl(2)Ag(2)USe(4).

Ba8Hg3U3S18: a complex uranium(+4)/uranium(+5) sulfide.

The compound Ba(8)Hg(3)U(3)S(18) was obtained from the solid-state reaction at 1123 K of U, HgS, BaS, and S, with BaBr(2)/KBr or BaCl(2) as a flux. This material crystallizes in a new structure type in space group P6 of the hexagonal system with three formula units in a cell of dimensions a = 27.08(1) Å, c = 4.208(2) Å, and V = 2673(2) Å(3). The structure contains infinite chains of US(6) octahedra and nearly linear [S-Hg-S](2-) dithiomercurate anions, separated by Ba(2+) cations. In the temperature range 100-300 K, the paramagnetic behavior of Ba(8)Hg(3)U(3)S(18) can be fit to the Curie-Weiss law, resulting in μ(eff) = 5.40(4) μ(B), or 3.12(2) μ(B)/U. The compound displays an antiferromagnetic transition at T(N) = 59 K. Although the formal oxidation states of Ba, Hg, and S can be assigned as +2, +2, and -2, the oxidation state of U is less certain. On the basis of interatomic distance arguments and the magnetic susceptibility data, the compound is proposed to contain U in both +4 and +5 formal oxidation states.

2H-PEROVSKITE RELATED OXIDES: SYNTHESIS, STRUCTURES AND PREDICTIONS

The ability to synthesize new complex oxide materials that belong to any of the large number of known oxide structural families relies typically on a general understanding of the relationship between the specific structure type and the chemical composition of its members. However, before one can create such a structure-composition relationship that enables the synthesis of new members, one needs structural information about a sizable number of existing compositions belonging to this structural family, somewhat of a “chicken or the egg” problem. In this Highlight we will use one family of oxides, specifically oxides related to the hexagonal perovskite structure, to illustrate how exploratory crystal growth methods have been used successfully to synthesize enough diverse compositions to enable the formulation of a general structural description. Furthermore, by now it appears that enough members with different compositions have been synthesized so that one can attempt to create a structure-composition relationship that has predictive powers.

La2U2Se9: an ordered lanthanide/actinide chalcogenide with a novel structure type.

The compound La(2)U(2)Se(9) was obtained in high yield from the stoichiometric reaction of the elements in an Sb(2)Se(3) flux at 1123 K. The compound, which crystallizes in a new structure type in space group Pmma of the orthorhombic system, has a three-dimensional structure with alternating U/Se and La/Se layers attached via three independent, infinite polyselenide chains. The U atom has a monocapped square antiprismatic coordination of Se atoms, whereas one La atom is bicapped square prismatic and the other La atom is trigonal prismatic. La(2)U(2)Se(9) displays an antiferromagnetic transition at T(N) = 5 K; above 50 K, the paramagnetic behavior can be fit to the Curie-Weiss law, yielding a mu(eff) of 3.10(1) mu(B)/U. The low-temperature specific heat of La(2)U(2)Se(9) exhibits no anomalous behavior near the Neel temperature that might indicate long-range magnetic ordering or a phase transition. X-ray absorption near-edge structure (XANES) spectra have confirmed the assignment of formal oxidation states of +III for lanthanum and +IV for uranium in La(2)U(2)Se(9).

Hydroflux Crystal Growth of Platinum Group Metal Hydroxides: Sr6NaPd2(OH)17, Li2Pt(OH)6, Na2Pt(OH)6, Sr2Pt(OH)8, and Ba2Pt(OH)8

Crystals of five complex metal hydroxides containing platinum group metals were grown by a novel low-temperature hydroflux technique, a hybrid approach between the aqueous hydrothermal and the molten hydroxide flux techniques. Sr6NaPd2(OH)17 (1) crystallizes in orthorhombic space group Pbcn with lattice parameters a = 19.577(4) Å, b = 13.521(3) Å, and c = 6.885(1) Å. This compound has a three-dimensional framework structure with Sr(OH)n polyhedra, Na(OH)6 octahedra, and Pd(OH)4 square planes. Isostructural phases Li2Pt(OH)6 (2) and Na2Pt(OH)6 (3) crystallize in trigonal space group P-3 with lattice parameters of a = 5.3406(8) Å and c = 4.5684(9) Å and a = 5.7984(8) Å and c = 4.6755(9) Å, respectively. Structures of these materials consist of layers of A(OH)6 (A = Li (2), Na (3)) and Pt(OH)6 octahedra. Sr2Pt(OH)8 (4) crystallizes in monoclinic space group P2(1)/c with lattice parameters a = 5.9717(6) Å, b = 10.997(1) Å, c = 6.0158(6) Å, and β = 113.155(2)°, while Ba2Pt(OH)8 (5) crystallizes in orthorhombic space group Pbca with lattice parameters a = 8.574(2) Å, b = 8.673(2) Å, and c = 10.276(2) Å. Both of these compounds have three-dimensional structures composed of Pt(OH)6 octahedra surrounded by either Sr(OH)8 or Ba(OH)9 polyhedra. Decomposition of these materials into condensed metal oxides, which is of importance to possible catalytic applications, was monitored via thermogravimetric analysis. For example, Na2Pt(OH)6 (3) converts cleanly via dehydration into α-Na2PtO3.

Dichalcogenide bonding in seven alkali-metal actinide chalcogenides of the KTh2Se6 structure type.

The solid-state compounds CsTh(2)Se(6), Rb(0.85)U(1.74)S(6), RbU(2)Se(6), TlU(2)Se(6), Cs(0.88)(La(0.68)U(1.32))Se(6), KNp(2)Se(6), and CsNp(2)Se(6) of the AAn(2)Q(6) family (A = alkali metal or Tl; An = Th, U, Np; Q = S, Se, Te) have been synthesized by high-temperature techniques. All seven crystallize in space group Immm of the orthorhombic system in the KTh(2)Se(6) structure type. Evidence of long-range order and modulation were found in the X-ray diffraction patterns of TlU(2)Se(6) and CsNp(2)Se(6). A 4a × 4b supercell was found for TlU(2)Se(6) whereas a 5a × 5b × 5c supercell was found for CsNp(2)Se(6). All seven compounds exhibit Q-Q interactions and, depending on the radius ratio R(An)/R(A), disorder of the A cation over two sites. The electrical conductivity of RbU(2)Se(6), measured along [100], is 6 × 10(-5) S cm(-1) at 298 K. The interatomic distances, including those in the modulated structure of TlU(2)Se(6), and physical properties suggest the compounds may be formulated as containing tetravalent Th or U, but the formal oxidation state of Np in the modulated structure of CsNp(2)Se(6) is less certain. The actinide contraction from Th to U to Np is apparent in the interatomic distances.

Crystal growth of new hexahydroxometallates using a hydroflux.

A series of seven compounds, Sr2Mn(OH)6, Ba2Mn(OH)6, Sr2Co(OH)6, Ba2Co(OH)6, Sr2Ni(OH)6, Ba2Ni(OH)6, and Ba2Cu(OH)6, were synthesized using a low-melting hydroflux, a hybrid approach between aqueous hydrothermal and molten hydroxide flux techniques. Crystals of the hexahydroxometallates were obtained by dissolving appropriate amounts of alkaline-earth nitrates or hydroxides and transition-metal oxides, acetates, or chlorides in the hydroflux and reacting at 180-230 °C. The isostructural compounds all crystallize in the monoclinic space group P2(1/n). The monoclinic structure consists of isolated transition-metal octahedra within a three-dimensional framework of corner- and edge-shared eight-coordinate, alkaline-earth polyhedra. Magnetic susceptibility data show that all compounds are simple paramagnets. Thermogravimetric analysis indicates that these hydroxides lose water between 215 and 350 °C and transform into oxide products, the identity of which depends on the metal cations present in the parent hexahydroxometallates.

Investigation of the high-temperature redox chemistry of Sr2Fe1.5Mo0.5O6−δvia in situ neutron diffraction

Crystallographic structural changes were investigated for Sr2Fe1.5Mo0.5O6−δ, an electrode material for symmetric solid oxide fuel cells. The samples of this material were heated and cooled in wet hydrogen and wet oxygen atmospheres, to simulate the reducing and oxidizing conditions experienced under actual fuel cell operating conditions, and their structures and oxygen contents were determined using in situ powder neutron diffraction. The existence of a reversible tetragonal to cubic phase transition was established to occur between room temperature and 400 °C, both on heating and cooling in either oxygen or hydrogen. The oxygen content reaches a low value of 5.50(2) at 850 °C in wet hydrogen. Excellent correlations are observed between the oxygen content of the structure and the conductivities reported in the literature.

Dicaesium hexa­mercury hepta­sulfide

The title compound, Cs2Hg6S7, crystallizes in a new structure type that is closely related to that of K2Zn6O7. The structure comprises a three-dimensional mercury sulfide network that is composed of channels. These channels, which are along [001], are of two different diameters. The crystal structure contains one Cs, two Hg, and three S atoms in the asymmetric unit. The Cs, one Hg, and one S atom are at sites of symmetry m, whereas a second S atom is at a site of symmetry 2mm. The Hg atoms are bound to the S atoms in both three- and four-coordinate geometries. The caesium cations occupy the central spaces of the larger diameter channels and exhibit a coordination number of 7.

Li3Al(MoO4)3, a lyonsite molybdate.

Trilithium aluminium trimolybdate(VI), Li(3)Al(MoO(4))(3), has been grown as single crystals from α-Al(2)O(3) and MoO(3) in an Li(2)MoO(4) flux at 998 K. This compound is an example of the well known lyonsite structure type, the general formula of which can be written as A(16)B(12)O(48). Because this structure can accomodate cationic mixing as well as cationic vacancies, a wide range of chemical compositions can adopt this structure type. This has led to instances in the literature where membership in the lyonsite family has been overlooked when assigning the structure type to novel compounds. In the title compound, there are two octahedral sites with substitutional disorder between Li(+) and Al(3+), as well as a trigonal prismatic site fully occupied by Li(+). The (Li,Al)O(6) octahedra and LiO(6) trigonal prisms are linked to form hexagonal tunnels along the [100] axis. These polyhedra are connected by isolated MoO(4) tetrahedra. Infinite chains of face-sharing (Li,Al)O(6) octahedra extend through the centers of the tunnels. A mixed Li/Al site, an Li, an Mo, and two O atoms are located on mirror planes.