Crystal chemistry,magnetic, and electrical properties of the tetragonal plutonium oxide telluride Pu2O2Te

We describe the preparation and some physical properties of Pu₂O₂Te. The plutonium oxide telluride is isostructural with the corresponding rare-earth oxide tellurides which crystallize in the tetragonal system of La₂O₂Te-type. Magnetic susceptibility data from 4 K to room temperature are reported together with resistivity measurements. Pu₂O₂Te is found to be an antiferromagnet below 56 K and a semiconductor with an intrinsic energy gap of 0.65 eV. The magnetic behavior is interpreted in terms of superexchange coupling interactions via nonmetal p orbitals, i.e., in terms of 5f-p overlaps. This conclusion is supported by crystal chemistry considerations by comparison of cell volumes of Pu₂O₂Te and Nd₂O₂Te. In Pu₂O₂Te, the Pu crystal radius is found to be much lower than that of Nd in Nd₂O₂Te, suggesting some 5f electron “delocalization” leading to a crystal radius shrinkage. As for the hexagonal Pu₂O₂X compounds, with X = O, S, Se, the measured gap may be considered as the energy separation between the chalcogen np band and the 6d-7s conduction band, the occupied 5f states lying just below the np band with some 5f-np overlap.     

Etude comparative des composés TiX2 (X = S,Se, Te). Structures de TiTe2 et TiSeTe

The evolution of crystal-structure and chemical-bond characteristics is studied for the series of isotypical compounds TiX₂, with X = S, Se, Te. The structure determination of TiTe₂ and TiSeTe confirms the existence of an increasing trigonal C_3v distortion of metal sites when the chalcogen electronegativity decreases. The ESCA study shows that this distortion is associated with augmentation of the covalent or metallic character of the bonds. The structural and spectroscopic differences appearing from TiS₂ to TiSe₂ can be interpreted by an orbital delocalization in TiSe₂ and TiTe₂, which would explain the semimetallic behavior of these compounds.     

Stabilization of Mo6S8 by halogens; new superconducting compounds: Mo6S6Br2, Mo6S6I2

We present a study of the properties of the series Mo₆X_8?xY_x (X = S, Se, Te; Y = Br, I) having the hexagonal rhombohedral structure of the PbMo₆S₈ type. For X = S we have found two new superconducting compounds Mo₆S₆Br₂ and Mo₆S₆I₂, having critical temperatures of 13.8 and 14.0°K, respectively. We further find that Mo₆Te₈ becomes superconducting (T_c ≈ 2.6°K) upon substitution of Te by small quantities of iodine, and that in the case of Mo₆Se₈ substitution of a Se atom by a halogen, raises T_c up to about 7.6°K.     

Electronic structures and optical properties of wurtzite type LiBSe2 (B=Al, Ga, In): A first-principles study

The electronic structures of three wurtzite type isostructural compounds LiBSe₂ (B=Al, Ga, In) are studied by the density functional theory (DFT). The results reveal that the presence of Li cations has direct influence on neither the band gaps (Eg) nor the bonding levels, but plays an important role in the stabilization of the structures. The band structures and densities of states (DOS) are analyzed in detail, and the band gaps of LiBSe₂ adhere to the following trend Eg_(LiAlSe2)>Eg_(LiGaSe2)>Eg_(LiInSe2), which is in agreement with the decrease of the bond energy of the corresponding Se 4p-B s antibonding orbitals. The role of the active s electrons of B element on the band gaps is also discussed. Finally, the optical properties are predicted, and the results would be a guide to understand the experiments.     

The Family of Ln2Ti2S2O5 Compounds (Ln=Nd, Sm, Gd, Tb, Dy, Ho, Er, and Y): Optical Properties

The optical properties of Ln₂Ti₂S₂O₅ compounds (Ln=Nd, Sm, Gd, Tb, Dy, Ho, Er, and Y) have been measured. Diffuse reflectance spectra revealed a strong absorption band above 2 eV, which explains the color of the compounds. Moreover, other bands, with a lower intensity, have been attributed to 4f-4f rare earth transitions. In the case of neodymium the derived energy level scheme is rich enough to determinate a set of phenomenological crystal field parameters that correctly reproduce the spectrum. These parameters were also calculated from the crystallographic structure, in a good agreement with the experiment. Finally, the paramagnetic susceptibility, well reproduced by the calculation, confirms that the rare earth is in a trivalent state.     

Phase relations in the TlXTl2Se systems (X = Cl,Br, I) and the crystal structure of Tl5Se2I

Each of the quasibinary systems TlClTl₂Se, TlBrTl₂Se, and TlITl₂Se contains a region of solid solution up to 18 mole% Tl₂Se, which decomposes peritectically. The mixed crystals can be explained by a statistical substitution of Se by two I atoms on the fourfold sites of the Tl₂Se lattice. Compounds of the type Tl₅Se₂X were derived by complete substitution. Crystals of Tl₅Se₂I, suitable for a crystal structure determination, were grown by the Bridgman technique. Tl₅Se₂I is tetragonal, $\text{I4}\text{mcm}\text{; a = 866.3}\text{pm}\text{, c = 1346.3}\text{pm}\text{, Z = 4}$. The structure is an ordered variation of the In₅Bi₃ structure and isopuntal to the Cr₅B₃ type. The structure is formed basically by layers of Tl₂Se, in which strings of TlI are introduced. The compounds Tl₅Se₂Br (a = 861.1 pm, c = 1292.2 pm) and Tl₅Se₂Cl (a = 856.5 pm, c = 1273.3 pm) have probably very similar structures. A tendency for immiscibility in the TlXTl₂Se systems is shown by the existence of a miscibility gap in the system TlClTl₂Se and by the endothermic enthalpies of mixing in the system TlBrTl₂Se. In the TlITl₂Se system the compound Tl₆Se₄I system was encountered.     

An investigation of structural parameters and magnetic and optical properties of EuLn2Q4 (Ln=Tb-Lu, Q=S, Se)

EuLn₂Q₄ (Ln=Tb-Lu; Q=S, Se) has been synthesized using Sb₂Q₃ (Q=S, Se) fluxes at 1000 °C. These compounds crystallize in a CaFe₂O₄-type three-dimensional channel structure that is built from edge-shared double rutile chains of [LnQ₆] octahedra running down the b-axis. Each double chain is connected at the vertices to four other double chains to form open channels where bicapped trigonal prismatic Eu²⁺ ions reside. All of these compounds show antiferromagnetic ordering with Neel temperatures, T_N∼3-4 K. The optical band gaps for EuTb₂Se₄, EuDy₂Se₄, EuHo₂Se₄, EuEr₂Se₄, EuTm₂Se₄, EuYb₂Se₄ EuLu₂Se₄, and EuYb₂S₄ are found to be 2.0, 1.8, 1.8, 1.7, 1.8, 1.3, 1.7, and 1.6 eV, respectively.     

Synthesis, crystal structure and characterization of new 12H hexagonal perovskite-related oxides Ba6M2Na2X2O17 (M=Ru, Nb, Ta, Sb; X=V, Cr, Mn, P, As)

The new Ba₆Ru₂Na₂X₂O₁₇ (X=V, Mn) compounds have been prepared by electrosynthesis in molten NaOH and their crystal structures have been refined from single crystals X-ray diffraction, space group P6₃/mmc, Z=2, for X=V: , , R₁=4.76%, for X=Mn : , , R₁=3.48%. The crystal structure is a 12H-type perovskite with a (c′cchcc)₂ stacking sequence of [BaO₃]_c, [BaO₃]_h and [BaO₂]_c′ layers. The tridimensional edifice is formed by blocks of Ru₂O₉ dimers that share corners with NaO₆ octahedra. These blocks sandwich double sheets of X⁵⁺O₄ tetrahedra. Several isotypic Ba₆M⁵⁺₂Na₂X⁵⁺₂O₁₇ materials (X=V, Cr, Mn, P, As) and (M=Ru, Nb, Ta, Sb) have been prepared by solid state reaction and characterized by Rietveld analysis. The magnetic and electric properties have been investigated and show besides the Ru⁵⁺₂O₉ typical intradimer antiferromagnetic couplings, discrepancies of both χ and ρ versus T at 50 and 100 K for Ba₆Ru₂Na₂X₂O₁₇ (X=V, As). In this work, a review of the identified Ru-hexagonal perovskite materials is also reported in order to overview the wide variety of possibilities in the field of new compounds synthesis.     

First-principle studies of Ca-X (X=Si,Ge,Sn,Pb) intermetallic compounds

The structural properties, elastic properties, heats of formation, electronic structures, and densities of states of 20 intermetallic compounds in the Ca-X (X=Si, Ge, Sn, Pb) systems have been systematically investigated by using first-principle calculations. Our computational results indicated that with increasing atomic weight of X, the bulk modulus of Ca-X intermetallic compounds decreases gradually. It was also found that Ca₃₆Sn₂₃ and CaPb are mechanically unstable phases. Results on the electronic energy band and densities of states also indicated that Ca₃Si₄ is an indirect band gap semiconductor with a band gap of 0.598 eV, and Ca₂Si, Ca₂Ge, Ca₂Sn, and Ca₂Pb are direct band gap semiconductors with band gaps of 0.324, 0.265, 0.06, and 0.07 eV, respectively. In addition, it is found that the absolute values of heats of formation for all Ca-X intermetallics are larger than 30 kJ/mol atom.     

Electronic Structure and Formation Heat of Pu3M and PuM3 (M=Ga, In, Sn and Ge) Compounds

The equilibrium structure, electronic structure, and formation heat of Pu₃M (PuM₃) (M=Ga, In, Sn, and Ge) compounds with AuCu₃ structure have been calculated using full potential linear augmented plane wave (FPLAPW) method within generalized-gradient approximation (GGA) including spin-orbit coupling (SOC) and spin polarization (SP). The calculated lattice parameters are in good agreement with the experimental values. Density of state analysis shows hybridization effects between Pu and M are governed by the competitions depending on the M amount: Pu 6d-Pu 5f, M p-Pu 6d, and M sp-M sp interactions. Electronegativity difference and electronic hybridization effect are two important factors to influence the formation heat and stability of Pu₃M (PuM₃) compounds. The larger is the electronegativity difference and the lower is M s-band or p-band center relative to the Fermi energy, the more negative is the formation heat and the more stable are Pu₃M (PuM₃) compounds.     

Electronic and structural properties of A Al2Se4 (A=Ag, Cu, Cd, Zn) chalcopyrite semiconductors

We have studied the structural and electronic properties of defect chalcopyrite semiconductors A Al₂Se₄ (A=Ag, Cu, Cd, Zn) using density functional theory (DFT) based first principle technique within tight binding linear muffin-tin orbital (TB-LMTO) method. Our calculated structural parameters such as lattice constants a and c, tetragonal distortion (η=c/2a) are in good agreement with experimental work. Anion displacement parameters, bond lengths and bulk modulus are also calculated. Our band structure calculation suggests that these compounds are direct band gap semiconductors having band gaps 2.40, 2.50, 2.46 and 2.82 eV for A Al₂Se₄ (A=Ag, Cu, Cd, Zn) respectively. Calculated band gaps are in good agreement with other experimental and theoretical works within LDA limitation. We have made a quantitative estimation of the effect of p-d hybridization and structural distortion on the electronic properties. The reduction in band gap due to p-d hybridization is 19.47%, 21.29%, 0% and 0.7% for A Al₂Se₄ (A=Ag, Cu, Cd, Zn) respectively. Increment of the band gap due to structural distortion is 11.62%, 2.45%, 2.92% and 9.30% in case of AgAl₂Se₄, CuAl₂Se₄, CdAl₂Se₄ and ZnAl₂Se₄ respectively. We have also discussed the bond nature of all four compounds.     

Reduction of Eu to Eu in MAl2Si2O8 (M=Ca, Sr, Ba) in air condition

In general, the reduction of Eu³⁺ to Eu²⁺ in solids needs an annealing process in a reducing atmosphere. In this paper, it is of great interest and importance to find that the reduction of Eu³⁺ to Eu²⁺ can be realized in a series of alkaline-earth metal aluminum silicates MAl₂Si₂O₈ (M=Ca, Sr, Ba) just in air condition. The Eu²⁺-doped MAl₂Si₂O₈ (M=Ca, Sr, Ba) powder samples were prepared in air atmosphere by Pechini-type sol-gel process. It was found that the strong band emissions of 4f⁶5d¹-4f⁷ from Eu²⁺ were observed at 417, 404 and 373 nm in air-annealed CaAl₂Si₂O₈, SrAl₂Si₂O₈ and BaAl₂Si₂O₈, respectively, under ultraviolet excitation although the Eu³⁺ precursors were employed. In addition, under low-voltage electron beam excitation, Eu²⁺-doped MAl₂Si₂O₈ also shows strong blue or ultraviolet emission corresponding to 4f⁶5d¹-4f⁷ transition. The reduction mechanism from Eu³⁺ to Eu²⁺ in these compounds has been discussed in detail.     

The antiferromagnetic structures of KFeS2, RbFeS2, KFeSe2, and RbFeSe2 and the correlation between magnetic moments and crystal field calculations

The ternary compounds KFeS₂, RbFeS₂, KFeSe₂, and RbFeSe₂ were prepared and their crystal structures were determined from single-crystal diffractometer data. The atomic arrangement in the isotypic compounds (space group C2/c) is characterized by tetrahedra of chalcogen atoms. Those tetrahedra are centered by iron ions and linked by edges, thus forming chains of ¹_∞[FeX⁻_4/2] frameworks (X S or Se). Susceptibility measurements are reported. Neutron difraction experiments on powdered samples revealed magnetic structures in the antiferromagnetically ordered state. The magnetic moments of the iron ions shows a significant dependence of the atomic arrangement. Therefore calculations based on a simple point charge model are discussed to correlate the magnetic moments and crystal field splittings.     

The stereochemistry of Tutton's salts X2[M(H2O)6](YO4)2

Neutron structure determinations have been made of Tutton's salts, X₂[M(H₂O)₆] (YO₄)₂, where Y = Se, X = K⁺, M = Cu²⁺; Y = S, X = K⁺, M = Ni²⁺, Cu²⁺, Zn²⁺; X = Rb⁺, Cs⁺, M = Cu²⁺. This work has shown that there are extensive hydrogen networks with almost linear hydrogen bonds from [M(H₂O)₆]²⁺ to (YO₄)^2?. The (H … O) distance increases in the Cu²⁺ series for X = K⁺ to Cs⁺ but there is no difference for the potassium copper salts when Y = Se or S. Three different distorted [M(H₂O)₆]²⁺ octahedra were found in the series (orthorhombic, tetragonal with two long and four short, or four long and two short bonds). The interatomic distances from X⁺ to the neighboring O in a distorted XO₈⁺ dodecahedron increases with increased cation size, implying that the X⁺ polyhedron is maintaining its shape.     

The luminescence of Pr3+ in BaY4Si5O17

The cell constants of four new monoclinic compounds BaR₄X₅O₁₇ (R = Y, Gd; X = Si, Ge) are given. The luminescence of various RE activators in the silicates is reported. Pr³⁺-activated BaY₄Si₅O₁₇ shows efficient ultraviolet 5d → 4f emission and weak 4f → 4f emission (mainly red luminescence from the ¹D₂ level). The 5d → 4f emission is ascribed to Pr³⁺ on Y sites, the 4f → 4f emission to Pr³⁺ on Ba sites. Energy transfer from Pr³⁺ to Gd³⁺ has been observed. Gd³⁺ plays an intermediate role in the energy transfer from Pr³⁺ to Sm³⁺ and to Dy³⁺ in BaGd₄Si₅O₁₇. Upon activation with Tb³⁺ the silicates show characteristic green Tb³⁺ luminescence with a quantum efficiency of 75% for ultraviolet excitation.     

Seven new rare-earth transition-metal oxychalcogenides: Syntheses and characterization of Ln4MnOSe6 (Ln=La, Ce, Nd), Ln4FeOSe6 (Ln=La, Ce, Sm), and La4MnOS6

The quaternary oxychalcogenides Ln₄MnOSe₆ (Ln=La, Ce, Nd), Ln₄FeOSe₆ (Ln=La, Ce, Sm), and La₄MnOS₆ have been synthesized by the reactions of Ln (Ln=La, Ce, Nd, Sm), M (M=Mn, Fe), Se, and SeO₂ at 1173 K for the selenides or by the reaction of La₂S₃ and MnO at 1173 K for the sulfide. Warning: These reactions frequently end in explosions. These isostructural compounds crystallize with two formula units in space group of the hexagonal system. The cell constants (a, c in Å) at 153 K are: La₄MnOSe₆, 9.7596(3), 7.0722(4); La₄FeOSe₆, 9.7388(4), 7.0512(5); Ce₄MnOSe₆, 9.6795(4), 7.0235(5); Ce₄FeOSe₆, 9.6405(6), 6.9888(4); Nd₄MnOSe₆, 9.5553(5), 6.9516(5); Sm₄FeOSe₆, 9.4489(5), 6.8784(5); and La₄MnOS₆, 9.4766(6), 6.8246(6). The structure of these Ln₄MOQ₆ compounds comprises a three-dimensional framework of interconnected LnOQ₇ bicapped trigonal prisms, MQ₆ octahedra, and the unusual LnOQ₆ tricapped tetrahedra.     

The phase diagrams of Ag2XAgY (X = S,Se, Te; Y = Cl,Br, I): Mixtures and the structure of Ag5Te2Cl

The phase diagrams of Ag₂SAgI, Ag₂SeAgI, Ag₂TeAgI, Ag₂TeAgBr, and Ag₂TeAgCl were investigated. The system Ag₂S-AgI shows two broad regions of solid solution which are based on the structure of the high-temperature phases of the constituent compounds. The high-temperature modification of Ag₃SI is part of one of these regions. The system Ag₂SeAgI resembles the system Ag₂TeAgI; both contain limited regions of terminal solid solutions. The AgI-based solid solutions decompose peritectically. In the system Ag₂TeAgBr a compound Ag₃TeBr was found. Ag₃TeBr undergoes a phase transition at 590 ± 20 K. The low-temperature form has hexagonal symmetry with the lattice parameters a = 748.8(1) pm and c = 4357.6(6) pm. The compound Ag₅Te₂Cl was found in the Ag₂TeAgCl system. In both systems a restricted terminal solid solution, based on the high-temperature form of Ag₂Te, was observed. Ag₅Te₂Cl has a reversible phase transformation at 329 ± 3 K with ΔH_tr = 9.82 ± 0.4 kJ mole^?1. β-Ag₅TeCl, the low-temperature form probably has the space group $\text{P2}{}_{\mathrm{<mtext>1</mtext><mtext>n</mtext>}}\text{, a = 1365.5(1), b = 1386.1(1), c = 764.23(2)}$, β = 90.201(1)°, and Z = 4, α-Ag₅Te₂Cl has the space group $\text{I4}{}_{\mathrm{mcm}}$ with a = 975.5(3), c = 783.0(1) pm, and Z = 4. The anion sublattice is built of octahedra, which share all their vertices with neighboring octahedra. The Ag⁺ ions are distributed over octahedral holes of this network. The phase is similar in behavior to Ag₈GeTe₆ and may be a silver-ion conductor.     

Sur les chalcogeno-iodures d'antimoine SbXI(X =S,Se, Te): Structures et spectroscopie Mo¨ssbauer de121Sb

The crystal structure of SbXI(X =Se, Te) compounds has been determined by means of three-dimensional intensity data. The crystal structure of SbSeI, orthorhombic, space groupPnma witha = 8.698(2), b = 4.127(1), c = 10.412(2) Å, was refined at several temperatures (180 K,R = 0.021; 293 K,R = 0.020; 320 K,R = 0.023) in correlation with the paraelectric structure or SbSI stable above 293 K. The crystal structure of SbTeI, triclinic, space groupP1¯, witha = 7.570(3), b = 7.159(3), c = 4.228(3) Å, α = 107.22(5), β = 106.18(4), γ = 77.19(3)° has been determined by symbolic addition method and refined to a finalR value of 0.035. These structures are built up from infinite weakly linked ribbons (SbX₂)_n of trigonal SbX₃ with SbX bonds of 2.605(1), 2.795(1)Å(X =Se), and 2.829(1), 2.953(1), 2.955(1)Å(X =Te). The nature of SbX and SbI bonds is discussed in terms of the S, Se, Te substitution. Antimony-121 Mo¨ssbauer spectra have been recorded at liquid helium temperature. The data are discussed with regard to the stereochimical activity of the antimony (III) lone pair of electrons. For SbTeI the Mo¨ssbauer parameters are interpreted in terms of direct population of conductance bands by nonbonding electron pairs.     

Expanded polycationic mercury-chalcogen networks in the layered compounds Hg3E2[MX6] (E=S, Se; M=Zr, Hf; X=Cl, Br)

The reactions of HgE (E=S, Se) with HgX₂ and MX₄ (M=Zr, Hf; X=Cl, Br) in evacuated glass ampoules lead to a series of isotypic compounds of the general formula Hg₃E₂[MX₆] in the form of colorless (X=Cl) and light-yellow (X=Br) air-sensitive crystals. The crystal structures of Hg₃S₂[ZrCl₆] (I), Hg₃S₂[HfCl₆] (II), Hg₃Se₂[ZrCl₆] (III), Hg₃Se₂[HfCl₆] (IV), Hg₃S₂[ZrBr₆] (V), and Hg₃Se₂[ZrBr₆] (VI) were refined based on single-crystal data. All compounds crystallize in the monoclinic space group P2₁/a with the lattice parameters a=662.18(2) pm, b=734.97(3) pm, c=1290.83(5) pm, β=91.755(2)° for (I) and and a=701.97(3) pm, b=756.79(3) pm, c=1350.99(6) pm, β=92.164(3)° for (VI). The structures are built of (Hg₃E₂)²⁺ layers stacked perpendicular to the c-axis. The polycationic layers consist of two-dimensionally linked 12-membered Hg₆E₆ rings in the chair conformation with linear coordinated Hg and trigonal pyramidal coordinated chalcogen atoms. Almost regular octahedral [MX₆]²⁻ ions are embedded between the layers. This arrangement is closely related to the structure of Hg₃S₂[SiF₆], which represents a higher symmetric congener. The structure relation is discussed using the supergroup-subgroup relation between space groups.     

Stoichiometry and physical properties of ternary molybdenum chalcogenides MxMo6X8 (X = S,Se; M = Li,Sn, Pb)

Chemical and electrochemical insertion of Li at room temperature, as well as insertion of lead and tin at moderate temperatures (500°C), into the binary phase Mo₆X₈ forms ternary molybdenum chalcogenides M_xMo₆X₈ (X = S, Se). Crystallographic parameters, superconducting properties, and magnetic susceptibility are reported. The stoichiometry x for lead and tin is shown not to exceed x = 1, while for Li, x can reach approximately 4.0. For the lead and tin sulfide series, the hexagonal lattice parameters and superconducting critical temperatures (T_c) are invariant to changes in the nominal composition of 0.8 < x < 1.2, while both an increase in T_c and a small decrease in c_h is observed for the selenides; a narrow homogeneity range exists near x = 1 below 500°C for both these sulfides and selenides, the single-phase region being somewhat larger in the selenides. In contrast, several single-phase regions and large unit cell changes are observed in Li_xMo₆X₈ (0 < x < 3.2). Magnetic susceptibility measurements of the lithiated compounds at x ～ 3.2 reveals a structural phase transition at 140 and 185 K for the sulfide and selenide, respectively; but neither superconducts down to 1.5 K. At lower lithium concentration near x ～ 1.0, the T_c of the sulfide is raised from that of Mo₆S₈ (1.8 K) to 5.2 K but the T_c of Mo₆Se₈ (6.5 K) is depressed to 3.9 K.