Daniel M. Wells

Syntheses, structures, physical properties, and electronic properties of some AMUQ3 compounds (A = alkali metal, M = Cu or Ag, Q = S or Se).

The seven new isostructural quaternary uranium chalcogenides KCuUS 3, RbCuUS 3, RbAgUS 3, CsCuUS 3, CsAgUS 3, RbAgUSe 3, and CsAgUSe 3 were prepared from solid-state reactions. These isostructural materials crystallize in the layered KZrCuS 3 structure type in the orthorhombic space group Cmcm. The structure is composed of UQ 6 octahedra and MQ 4 tetrahedra that share edges to form (2) infinity[UMQ 3 (-)] layers. These layers stack perpendicular to [010] and are separated by layers of face- and edge-sharing AQ 8 bicapped trigonal prisms. There are no Q-Q bonds in the structure, so the formal oxidation states of A/U/M/Q may be assigned as 1+/4+/1+/2-, respectively. CsCuUS 3 shows semiconducting behavior with thermal activation energy E a = 0.14 eV and sigma 298 = 0.3 S/cm. From single-crystal absorption measurements in the near IR range, the optical band gaps of these compounds are smaller than 0.73 eV. The more diffuse 5f electrons play a much more dominant role in the optical properties of the AMUQ 3 compounds than do the 4f electrons in the AMLnQ 3 compounds (Ln = rare earth). Periodic DFT spin band-structure calculations on CsCuUS 3 and CsAgUS 3 establish two energetically similar antiferromagnetic spin structures and show magnetic interactions within and between the layers of the structure. Density-of-states analysis shows M-Q orbital overlap in the valence band and U-Q orbital overlap in the conduction band.

Oxidation state of uranium in A6Cu12U2S15 (A = K, Rb, Cs) compounds.

Black single crystals of A(6)Cu(12)U(2)S(15) (A = K, Rb, Cs) have been synthesized by the reactive flux method. These isostructural compounds crystallize in the cubic space group Ia ̅3d at room temperature. The structure comprises a three-dimensional framework built from US(6) octahedra and CuS(3) trigonal planar units with A cations residing in the cavities. There are no S-S bonds in the structure. To elucidate the oxidation state of U in these compounds, various physical property measurements and characterization methods were carried out. Temperature-dependent electrical resistivity measurement on a single crystal of K(6)Cu(12)U(2)S(15) showed it to be a semiconductor. These three A(6)Cu(12)U(2)S(15) (A = K, Rb, Cs) compounds all exhibit small effective magnetic moments, < 0.58 μ(B)/U and band gaps of about 0.55(2) eV in their optical absorption spectra. From X-ray absorption near edge spectroscopy (XANES), the absorption edge of A(6)Cu(12)U(2)S(15) is very close to that of UO(3). Electronic band structure calculations at the density functional theory (DFT) level indicate a strong degree of covalency between U and S atoms, but theory was not conclusive about the formal oxidation state of U. All experimental data suggest that the A(6)Cu(12)U(2)S(15) family is best described as an intermediate U(5+)/U(6+) sulfide system of (A(+))(6)(Cu(+))(12)(U(5+))(2)(S(2-))(13)(S(-))(2) and (A(+))(6)(Cu(+))(12)(U(6+))(2)(S(2-))(15).

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.

Structural, Electronic, and Magnetic Properties of UFeS3 and UFeSe3

Black prisms of UFeS(3) and UFeSe(3) have been synthesized by solid-state reactions of U, Fe, and S or Se with CsCl as a flux at 1173 K. The structure of these isostructural compounds consists of layers of edge- and corner-sharing FeS(6) or FeSe(6) octahedra that are separated by layers of face- and edge-sharing US(8) or USe(8) bicapped trigonal prisms. The isomer shifts in the iron-57 Mössbauer spectra of both UFeS(3) and UFeSe(3) are consistent with the presence of high-spin iron(II) ions octahedrally coordinated to S or Se. The XANES spectra of UFeS(3) and UFeSe(3) are consistent with uranium(IV). Single-crystal magnetic susceptibility measurements along the three crystallographic axes of UFeSe(3) reveal a substantial magnetic anisotropy with a change of easy axis from the a-axis above 40 K to the b-axis below 40 K, a change that results from competition between the iron(II) and uranium(IV) anisotropies. The temperature dependence of the magnetic susceptibility along the three axes is characteristic of two-dimensional magnetism. A small shoulder-like anomaly is observed in the magnetic susceptibilities along the a- and b-axes at 96 and 107 K, respectively. Below 107 K, the iron-57 Mössbauer spectra of UFeS(3) and UFeSe(3) show that the iron nuclei experience a magnetic hyperfine field that results from long-range magnetic ordering of at least the iron(II) magnetic moments because the field exhibits Brillouin-like behavior. Below 40 K there is no significant change in the Mössbauer spectra as a result of change in magnetic anisotropy. The complexity of the iron-57 Mössbauer spectra and the temperature and field dependencies of the magnetic properties point toward a complex long-range magnetic structure of two independent iron(II) and uranium(IV) two-dimensional sublattices. The temperature dependence of the single-crystal resistivity of UFeSe(3) measured along the a-axis reveals semiconducting behavior between 30 and 300 K with an energy gap of about 0.03 eV below the 53 K maximum in susceptibility, of about 0.05 eV between 50 and 107 K, and of 0.03 eV above 107 K; a negative magnetoresistance was observed below 60 K.

Quaternary Neptunium Compounds: Syntheses and Characterization of KCuNpS3, RbCuNpS3, CsCuNpS3, KAgNpS3, and CsAgNpS3

The five quaternary neptunium compounds KCuNpS3, RbCuNpS3, CsCuNpS3, KAgNpS3, and CsAgNpS3 (AMNpS3) have been synthesized by the reaction of Np, Cu or Ag, S, and K2S or Rb2S3 or Cs2S3 at 793 K (Rb) or 873 K. These isostructural compounds crystallize as black rectangular plates in the KCuZrS3 structure type in space group Cmcm of the orthorhombic system. The structure comprises MS4 (M = Cu or Ag) tetrahedra and NpS6 octahedra that edge share to form infinity 2[MNpS3-] layers. These layers are separated by the alkali-metal cations. The Np-S bond lengths vary from 2.681(2) to 2.754(1) A. When compared to the corresponding isostructural Th and U compounds these bond distances obey the expected actinide contraction. As the structure contains no S-S bonds, formal oxidation states of +1/+1/+4/-2 may be assigned to A/M/Np/S, respectively. From these results a value of 2.57 for the bond-valence parameter r0 for Np(4+)-S(2-) has been derived and applied to the estimation of the formal oxidation states of Np in the binary NpxSy compounds whose structures are known.

Neptunium(III) copper(I) diselenide.

The title compound, NpCuSe2, is the first ternary neptunium transition-metal chalcogenide. It was synthesized from the elements at 873u2005K in an evacuated fused-silica tube. Single crystals were grown by vapor transport with I2. NpCuSe2 crystallizes in the LaCuS2 structure type and can be viewed as a stacking of layers of CuSe4 tetrahedra and of double layers of NpSe7 monocapped trigonal prisms along [100]. Because there are no Se—Se bonds in the structure, the formal oxidation states of Np/Cu/Se may be assigned as +III/+I/−II, respectively.

Structure, properties, and theoretical electronic structure of UCuOP and NpCuOP.

The compounds UCuOP and NpCuOP have been synthesized and their crystal structures were determined from low-temperature single-crystal X-ray data. These isostructural compounds crystallize with two formula units in space group P4/nmm of the tetragonal system. Each An atom (An = U or Np) is coordinated to four O and four P atoms in a distorted square antiprism; each Cu atom is coordinated to four P atoms in a distorted tetrahedron. Magnetic susceptibility measurements on crushed single crystals indicate that UCuOP orders antiferromagnetically at 224(2) K. Neutron diffraction experiments at 100 and 228 K show the magnetic structure of UCuOP to be type AFI (+ - + -) where ferromagnetically aligned sheets of U atoms in the (001) plane order antiferromagnetically along [001]. The electrical conductivity of UCuOP exhibits metallic character. Its electrical resistivity measured in the ordered region with the current flowing within the tetragonal plane is governed by the scattering of the conduction electrons on antiferromagnetic spin-wave excitations. The electrical resistivity of single-crystalline NpCuOP shows semimetallic character. It is dominated by a pronounced hump at low temperatures, which likely arises owing to long-range magnetic ordering below about 90 K. Density of state analyses using the local spin-density approximation show covalent overlap between AnO and CuP layers of the structure and dominant contributions from 5f-actinide orbitals at the Fermi level. Calculations on a 2 × 2 × 2 supercell of NpCuOP show ferromagnetic ordering within the Np sheets and complex coupling between these planes. Comparisons of the physical properties of these AnCuOP compounds are made with those of the family of related tetragonal uranium phosphide compounds.

Syntheses, crystal structures, and physical properties of La 5Cu6O4S7 and La5Cu 6.33O4S7

Single crystal and bulk powder samples of the quaternary lanthanum copper oxysulfides La5Cu6.33O4S7 and La5Cu6O4S7 have been prepared by means of high-temperature sealed-tube reactions and spark plasma sintering, respectively. In the structure of La 5Cu6.33O4S7, Cu atoms tie together the fluorite-like (2)infinity[La5O4S(5+)] and antifluorite-like (2) infinity[Cu6S6(5-)] layers of La5Cu6O4S7. The optical band gap, E g, of 2.0 eV was deduced from both diffuse reflectance spectra on a bulk sample of La5Cu6O4S7 and for the (010) crystal face of a La 5Cu6.33O4S7 single crystal. Transport measurements at 298 K on a bulk sample of La 5Cu 6O 4S 7 indicated p-type metallic electrical conduction with sigma electrical =2.18 S cm(-1), whereas measurements on a La 5Cu6.33O4S7 single crystal led to sigma electrical =4.5 10(-3) S cm(-1) along [100] and to semiconducting behavior. In going from La 5Cu6O4S7 to La5Cu6.33O4S7, the disruption of the (2)infinity[Cu6S6(5-)] layer and the decrease in the overall Cu(2+)(3d(9)) concentration lead to a significant decrease in the electrical conductivity.

Syntheses, crystal structures, and physical properties of La 5 Cu 6 O 4 S 7 and La 5 Cu 6.33 O 4 S 7

Single crystal and bulk powder samples of the quaternary lanthanum copper oxysulfides La5Cu6.33O4S7 and La5Cu6O4S7 have been prepared by means of high-temperature sealed-tube reactions and spark plasma sintering, respectively. In the structure of La 5Cu6.33O4S7, Cu atoms tie together the fluorite-like (2)infinity[La5O4S(5+)] and antifluorite-like (2) infinity[Cu6S6(5-)] layers of La5Cu6O4S7. The optical band gap, E g, of 2.0 eV was deduced from both diffuse reflectance spectra on a bulk sample of La5Cu6O4S7 and for the (010) crystal face of a La 5Cu6.33O4S7 single crystal. Transport measurements at 298 K on a bulk sample of La 5Cu 6O 4S 7 indicated p-type metallic electrical conduction with sigma electrical =2.18 S cm(-1), whereas measurements on a La 5Cu6.33O4S7 single crystal led to sigma electrical =4.5 10(-3) S cm(-1) along [100] and to semiconducting behavior. In going from La 5Cu6O4S7 to La5Cu6.33O4S7, the disruption of the (2)infinity[Cu6S6(5-)] layer and the decrease in the overall Cu(2+)(3d(9)) concentration lead to a significant decrease in the electrical conductivity.