Peter K. Dorhout

The synthesis and structural and physical characterization of a new family of rare-earth metal chalcoantimonates(III): K2(RE)2-xSb4+xSb4Se12, RE= La, Ce, Pr and Gd.

Abstract A family of rare-earth metal selenoantimonates(III) has been prepared by the reaction of the elements in alkali metal polyselenide fluxes or by the metathesis of rare-earth chlorides and alkali metal chalcoantimonates(III). The products are black, fine needles of K2(RE)2−xSb4+[itxSe12 where RE=La, Ce, Pr, and Gd and x~0.33. The compounds were analyzed by single crystal and powder X-ray diffraction, and the praseodymium compound by diffuse reflectance spectroscopy, and magnetic susceptibility. The structure, optical spectrum, and susceptibility data of the praseodymium compound are reported.

YF[MoO4] and YCl[MoO4]: Two halide derivatives of yttrium ortho-oxomolybdate: Syntheses, structures, and luminescence properties

The halide derivatives of yttrium ortho-oxomolybdate YX[MoO 4] (X = F, Cl) both crystallize in the monoclinic system with four formula units per unit cell. YF[MoO 4] exhibits a primitive cell setting (space group P21/ c; a = 519.62(2) pm, b = 1225.14(7) pm, c = 663.30(3) pm, beta = 112.851(4) degrees ), whereas the lattice of YCl[MoO 4] shows face-centering (space group C2/m; a = 1019.02(5) pm, b = 720.67(4) pm, c = 681.50(3) pm, beta = 107.130(4) degrees ). The two compounds each contain crystallographically unique Y (3+) cations, which are found to have a coordination environment of six oxide and two halide anions. In the case of YF[MoO 4], the coordination environment is seen as square antiprisms, and for YCl[MoO 4], trigon-dodecahedra are found. The discrete tetrahedral [MoO 4] (2-) units of the fluoride derivative are exclusively bound by six terminal Y (3+) cations, while those of the chloride compound show a 5-fold coordination around the tetrahedra with one edge-bridging and four terminal Y (3+) cations. The halide anions in each compound exhibit a coordination number of two, building up isolated planar rhombus-shaped units according to [Y 2F 2] (4+) in YF[MoO 4] and [Y 2Cl 2] (4+) in YCl[MoO 4], respectively. Both compounds were synthesized at high temperatures using Y2O3, MoO3, and the corresponding yttrium trihalide in a molar ratio of 1:3:1. Single crystals of both are insensitive to moist air and are found to be coarse shaped and colorless with optical band gaps situated in the near UV around 3.78 eV for the fluoride and 3.82 eV for the chloride derivative. Furthermore, YF[MoO 4] seems to be a suitable material for doping to obtain luminescent materials because the Eu (3+)-doped compound shows an intense red luminescence, which has been spectroscopically investigated.

X-ray characterization of annealed iridium films

The changes in the mechanical and structural properties of sputter-deposited iridium films are described as a function of annealing temperature from 673 to 1073 K. Glancing angle x-ray diffraction and x-ray reflectivity measurements indicated the growth of an IrO2 overlayer by annealing at 873 K. An increased annealing temperature of 1073 K led to the oxidation of the underlying iridium layer yielding a film comprising IrO2 (major) and Ir (minor) phases. Annealing the films at 873 and 1073 K also led to an increased surface roughness of the films. As-deposited as well as films annealed at 673 and 873 K exhibited tensile stresses along the normal to the plane in consideration. Annealing at 673 K and cooling within a tube furnace relaxed normal and shear stress present in as-deposited 20 nm Ir metal films. Slow cooling, formation of oxygen–iridium bonding, and increased roughness caused the preferential relaxation of the shear stresses for the film annealed at 873 K. The oscillations observed in the strain-sin2 ψ curve for the films annealed at 1073 K could be due to texture in the film and/or stress gradient across the thickness of the film. The residual stress has, therefore, not been evaluated for these films. X-ray photoelectron spectroscopic measurements on the iridium film annealed at 1073 K show that the film is oxygen rich at the surface and oxygen deficient near the substrate.The changes in the mechanical and structural properties of sputter-deposited iridium films are described as a function of annealing temperature from 673 to 1073 K. Glancing angle x-ray diffraction and x-ray reflectivity measurements indicated the growth of an IrO2 overlayer by annealing at 873 K. An increased annealing temperature of 1073 K led to the oxidation of the underlying iridium layer yielding a film comprising IrO2 (major) and Ir (minor) phases. Annealing the films at 873 and 1073 K also led to an increased surface roughness of the films. As-deposited as well as films annealed at 673 and 873 K exhibited tensile stresses along the normal to the plane in consideration. Annealing at 673 K and cooling within a tube furnace relaxed normal and shear stress present in as-deposited 20 nm Ir metal films. Slow cooling, formation of oxygen–iridium bonding, and increased roughness caused the preferential relaxation of the shear stresses for the film annealed at 873 K. The oscillations observed in the strain-...

Rare-Earth Metal(III) Oxide Selenides M4O4Se[Se2] (M = La, Ce, Pr, Nd, Sm) with Discrete Diselenide Units: Crystal Structures, Magnetic Frustration, and Other Properties

The rare-earth metal(III) oxide selenides of the formula La4O4Se[Se2], Ce4O4Se[Se2], Pr4O4Se[Se2], Nd4O4Se[Se2], and Sm4O4Se[Se2] were synthesized from a mixture of the elements with selenium dioxide as the oxygen source at 750 degrees C. Single crystal X-ray diffraction was used to determine their crystal structures. The isostructural compounds M4O4Se[Se2] (M=La, Ce, Pr, Nd, Sm) crystallize in the orthorhombic space group Amm2 with cell dimensions a=857.94(7), b=409.44(4), c=1316.49(8) pm for M=La; a=851.37(6), b=404.82(3), c=1296.83(9) pm for M=Ce; a=849.92(6), b=402.78(3), c=1292.57(9) pm for M=Pr; a=845.68(4), b=398.83(2), c=1282.45(7) pm for M=Nd; and a=840.08(5), b=394.04(3), c=1263.83(6) pm for M=Sm (Z=2). In their crystal structures, Se2- anions as well as [Se-Se]2- dumbbells interconnect {[M4O4]4+} infinity 2 layers. These layers are composed of three crystallographically different, distorted [OM4]10+ tetrahedra, which are linked via four common edges. The compounds exhibit strong Raman active modes at around 215 cm(-1), which can be assigned to the Se-Se stretching vibration. Optical band gaps for La4O4Se[Se2], Ce4O4Se[Se2], Pr4O4Se[Se2], Nd4O4Se[Se2], and Sm4O4Se[Se2] were derived from diffuse reflectance spectra. The energy values at which absorption takes place are typical for semiconducting materials. For the compounds M4O4Se[Se2] (M=La, Pr, Nd, Sm) the fundamental band gaps, caused by transitions from the valence band to the conduction band (VB-CB), lie around 1.9 eV, while for M=Ce an absorption edge occurs at around 1.7 eV, which can be assigned to f-d transitions of Ce3+. Magnetic susceptibility measurements of Ce4O4Se[Se2] and Nd4O4Se[Se2] show Curie-Weiss behavior above 150 K with derived experimental magnetic moments of 2.5 micro B/Ce and 3.7 micro B/Nd and Weiss constants of theta p=-64.9 K and theta p=-27.8 K for the cerium and neodymium compounds, respectively. Down to 1.8 K no long-range magnetic ordering could be detected. Thus, the large negative values for theta p indicate the presence of strong magnetic frustration within the compounds, which is due to the geometric arrangement of the magnetic sublattice in form of [OM4]10+ tetrahedra.

Two families of lamellar, luminescent solid solutions: The intercalative conversion of hydrogen uranyl phosphate arsenates to uranyl phosphate arsenates

Abstract Hydrogen uranyl phosphate (HUP) and hydrogen uranyl arsenate (HUAs) form solid solutions of composition HUO 2 (PO 4 ) 1− x (AsO 4 ) x ·4H 2 O (HUPAs;0⩽ x ⩽1), representing a family of lamellar, luminescent solids that can serve as hosts for intercalation chemistry; structural and spectroscopic data are all consistent with the formation of single-phase solid solutions from aqueous co-precipitation reactions of UO 2+ 2 with mixtures of PO 3- 4 and AsO 3- 4 . Hydrated, lamellar uranyl phosphate arsenates, (UO 2 ) 3 (PO 4 ) 2−2 x (AsO 4 ) 2 x (UPAs;0⩽ x ⩽1), can be prepared form the corresponding HUPAs solids by either of two routes: (1) intercalative exchange of UO 2+ 2 ions into the HUPAs solids; and (2) thermal decomposition of HUPAs solids in aqueous slurries. Power X-ray diffraction of the UPAs solids reveals them to be single phases, whose lattice constants increase monotonically with x ; the interlamellar spacings increase by about 2.5 A (to ≈ 11 A) compared to the HUPAs starting material. Samples of UPAs slowly revert to their corresponding HUPAs precursors upon cooling to room temperature in appropriate aqueous media. Unlike the highly emissive HUPAs solids (radiative quantum yields o r >0.2; lifetimes τ> 100 μs), the UPAs solids are only weakly emissive at 295 K (o r ≈0.003; τ 3 s −1 ), the difference in photophysical properties arises from an ≈ 100-fold increasein the nonradiative rate constant caused by the introduction of the interlamellar UO 2+ 2 ions. Rates for the intercalation and thermal conversion reactions have been measured for HUP and HUAs as a function of temperature (0−27°C for intercalation; 40–80°C for thermal decomposition), using the structural and optical changes accompanying the conversions.

Synthesis, structure, and optical properties of the quaternary seleno-gallates NaLnGa4Se8 (Ln = La, Ce, Nd) and their comparison with the isostructural thio-gallates.

Three new quaternary seleno-gallates containing rare-earth metals and sodium cations, have been synthesized by a solid-state route in evacuated quartz ampoules: Na LnGa 4Se 8 ( Ln = La( I), Ce ( II) and Nd ( III)). The synthesis involved the stoichiometric combination of sodium polyselenides, rare-earth metal, Ga 2Se 3, and Se or elemental Ga in place of Ga 2Se 3. Single-crystal structure analysis indicated that the compounds are isostructural to the thio-analogue, NaNdGa 4S 8. The structures of I- III are described in terms of layers of GaSe 4 tetrahedra joined by corner- and edge-sharing; the alkali-metal cations and the trivalent rare-earth metal cations occupy square antiprismatic sites between the layers. The optical properties of the compounds have been investigated and compared with the isostructural thio-gallate. The band gap of I was located around 2.65 eV. The band gaps of II and III were 2.66 and 2.73 eV, respectively, considerably narrower than their thio-analogues ( approximately 3.4 eV). The contraction of the band gap was attributed to the shift of the valence band to higher energy due to the involvement of higher energy (4p) Se orbitals. The 4f --> 5d gap of II is found to be located around 2.32 eV, which is 0.26 eV narrower than the thio-analogue is due to a greater dispersion of the Ln-(5d) band caused by more covalent Ce-Se bonds as well as rising of the f level energy.

Na 1.515 EuGeS 4 , a Three-Dimensional Crystalline Assembly of Empty Nanotubules Constructed with Europium(II/III) Mixed Valence Ions

A new compound, Na(1.515)EuGeS(4), has been synthesized at 750 °C from a reaction of elemental europium, germanium, and sulfur and Na(2)S. The compound crystallizes in the trigonal system with Z = 18 and the R3c space group with a = 23.322(3) Å, c = 6.838(1) Å, and V = 3221.2(9) Å(3). Na(1.515)EuGeS(4), which is isostructural with Na(2)EuSiSe(4), contains quasi-infinite nanoscale (∞)[EuGeS(4)](2-) tubules that are held together by sodium cations through electrostatic interactions. The tubules consist of a complex network of monoface-capped EuS(7) trigonal prisms and GeS(4) tetrahedra. The most striking structural feature of Na(1.515)EuGeS(4) is the absence of sodium cations inside the tubules, an absence that is balanced by the presence of mixed valence europium(II/III) ions. This mixed valence is confirmed by europium-151 Mössbauer spectroscopy, which indicates discrete mixed-valence europium ions at least up to 295 K. The stoichiometry has been determined by a fit of χ(M)T measured between 20 and 300 K with a combination of europium(II) ions, with a Curie constant of 7.877 emu K/mol, and europium(III) ions whose contribution to χ(M)T has been fit by using the Van Vleck expression for its molar susceptibility. The best fit corresponds to 51.5% of europium(II), 48.5% of europium(III), a stoichiometry of Na(1.515(5))EuGeS(4), and a splitting, E, between the J = 0 and the first excited J = 1 state of europium(III) of 360(6) cm(-1). The field dependence of the 1.8 K magnetization is in perfect agreement with a S = 7/2 Brillouin function with g = 2.00 and yields a saturation magnetization of 7 Nβ at 5 T.

Comparison of nanometer-thick films by x-ray reflectivity and spectroscopic ellipsometry

Films of tantalum pentoxide (Ta2O5) with thickness of 10–100 nm were deposited on Si wafers and have been compared using spectroscopic ellipsometry (SE) and x-ray reflectivity (XRR). (Ta2O5) was chosen for comparison work based on various criterions for material selection outlined in this article. Measurements were performed at six positions across the sample area to take into consideration thickness and composition inhomogeneity. SE and XRR fitted curves required the incorporation of a linearly graded interface layer. SE systematically measured higher values of film thickness as compared to XRR. A linear equation was established between the thickness measurements using SE and XRR. The slope of the linear equation established was found to be 1.02±0.01. However, the intercepts were found to be 1.7±0.2 and 2.6±0.3 when the interface was excluded and included, respectively. These differences in the values of intercepts were attributed to the uncertainties in the determination of the interface layer.

Fabrication and characterization of silicon nanocrystals by thermal oxidation of a-Si:H films in air

Hydrogenated amorphous silicon (a-Si:H) and Si–O–H heterogeneous thin films have been examined for their potential to photoluminesce. In this study, 50 nm a-Si:H films were deposited and oxidized to understand how film morphology affects their optical properties. Glancing angle x-ray diffraction (XRD), x-ray reflectivity, x-ray photoelectron spectroscopy, optical absorption spectroscopy in the wavelength range 250–1000 nm, and Fourier transform infrared measurements were used to complement room temperature photoluminescence (PL) studies. The results are discussed in light of the standard models for room temperature visible PL for a-SiOx:H films and silicon nanocrystals. The PL peak at 1.6 eV arises from silicon nanocrystals. Modeling this band to estimate the quantum dot size indicates that the mean silicon crystallite diameter is ∼5 nm, while XRD analysis gives ∼9±1 nm. The discrepancy in the estimation of crystallite size by the XRD method and PL analysis is attributed to the columnar growth of the sili...

Alkali-Metal Thiogermanates: Sodium Channels and Variations on the La3CuSiS7 Structure Type

Five new isotypic quaternary chalcogenides containing rare-earth metal atoms crystallizing in the hexagonal noncentrosymmetric space group P6(3) (No. 173) with the La(3)CuSiS(7) structure type have been synthesized by reacting the appropriate anhydrous rare-earth trichloride with sodium thiogermanate, Na(2)GeS(3). The reaction between LnCl(3) and Na(2)GeS(3) in an evacuated fused-silica ampule produced high yields of good-quality crystals of NaLn(3)GeS(7) [Ln = Ce (I), Nd (II), Sm (III), Gd (IV), and Yb (V)], while a similar reaction between EuCl(3) and Na(2)GeS(3) yielded a quinary chloride thiogermanate, Na(1.2)Eu(3.4)Cl(2)Ge(3)S(9) (VI), incorporating a cyclic trimeric Ge(3)S(9) building unit and adopting a structure related to La(3)CuSiS(7). The crystal structure of the compounds comprises a complex network of bicapped trigonal-prismatic LnS(8) and GeS(4) tetrahedra, which creates channels along the [001] direction. The Na(+) cations reside in these channels within trigonally distorted octahedral coordination environments, surrounded by six S atoms. For compounds III-V, the temperature dependence of the magnetic susceptibility indicates that these compounds are paramagnetic with μ(eff). = 1.86, 8.01, and 3.87 μ(B), for III-V, respectively. The experimental μ(eff) for IV is close to the theoretical value of 7.94 for free Gd(3+) ions, while μ(eff) values for III and V deviate from their theoretical values of 0.86 and 4.54 μ(B) for Sm(3+) and Yb(3+) ions, respectively. These compounds are semiconductors with optical band gaps of around 1.3 eV for III and V. Extended Hückel calculations suggest that the valence band comprises primarily S 3p and the bottom of the conduction band is dominated by empty rare-earth 5d orbitals. Compound VI exhibits a sharp optical absorption of around 2.18 eV, which is attributed to the f → d transition of Eu(II). The effective magnetic moment of 7.94 μ(B)/Eu is in excellent agreement with the theoretical value of 7.94 μ(B) for the free Eu(2+) ion.