Synthesis and crystal structures of the layered uranyl tellurites A2[(UO2)3(TeO3)2O2] (A=K, Rb, Cs)

The reactions of UO₃ and TeO₃ with KCl, RbCl, or CsCl at 800 °C for 5 d yield single crystals of A₂[(UO₂)₃(TeO₃)₂O₂] (A=K (1), Rb (2), and Cs (3)). These compounds are isostructural with one another, and their structures consist of two-dimensional sheets arranged in a stair-like topology separated by alkali metal cations. These sheets are comprised of zigzagging uranium(VI) oxide chains bridged by corner-sharing trigonal pyramidal TeO₃²⁻ anions. The chains are composed of dimeric, edge-sharing, pentagonal bipyramidal UO₇ moieties joined by edge-sharing tetragonal bipyramidal UO₆ units. The lone-pair of electrons from the TeO₃ groups are oriented in opposite directions with respect to one another on each side of the sheets rendering each individual sheet non-polar. The alkali metal cations form contacts with nearby tellurite oxygen atoms as well as with oxygen atoms from the uranyl moieties. Crystallographic data (193 K, MoKα, ): 1, triclinic, space group , , , , α=101.852(1)°, β=102.974(1)°, γ=100.081(1)°, , Z=2, R(F)=2.70% for 98 parameters and 1697 reflections with I>2σ(I); 2, triclinic, space group , , , , α=105.590(2)°, β=101.760(2)°, γ=99.456(2)°, , Z=2, R(F)=2.36% for 98 parameters and 1817 reflections with I>2σ(I); 3, triclinic, space group , , , , α=109.301(1)°, β=100.573(1)°, γ=99.504(1)°, , Z=2, R(F)=2.61% for 98 parameters and 1965 reflections with I>2σ(I).     

The disordered structures and low temperature dielectric relaxation properties of two misplaced-displacive cubic pyrochlores found in the Bi2O3-MO-Nb2O5 (M=Mg, Ni) systems

The disordered structures and low temperature dielectric relaxation properties of Bi_1.667Mg_0.70Nb_1.52O₇ (BMN) and Bi_1.67Ni_0.75Nb_1.50O₇ (BNN) misplaced-displacive cubic pyrochlores found in the Bi₂O₃-M^IIO-Nb₂O₅ (M=Mg, Ni) systems are reported. As for other recently reported Bi-pyrochlores, the metal ion vacancies are found to be confined to the pyrochlore A site. The B₂O₆ octahedral sub-structure is found to be fully occupied and well-ordered. Considerable displacive disorder, however, is found associated with the O′A₂ tetrahedral sub-structure in both cases. The A-site ions were displaced from Wyckoff position 16d (, , ) to 96 h (, , ) while the O′ oxygen was shifted from position 8b (, , ) to Wyckoff position 32e (, , ). The refined displacement magnitudes off the 16d and 8b sites for the A and O′ sites were 0.408 Å/0.423 Å and 0.350 Å/0.369 Å for BMN/BNN, respectively.     

Synthesis and characterization of the rare-earth dicyanamides Ln[N(CN)2]3 with Ln=La, Ce, Pr, Nd, Sm, and Eu

The rare-earth dicyanamides Ln[N(CN)₂]₃ (Ln=La, Ce, Pr, Nd, Sm, Eu) were obtained via ion exchange in aqueous medium and subsequent drying: The crystal structures were solved and refined based on X-ray powder diffraction data and they were found to be isotypic: Ln[N(CN)₂]₃; Cmcm (no. 63), Z=4, Ln=La: , , ; Ce: , , ; Pr: , , ; Nd: , , ; Sm: , , ; Eu: , , ). The compounds represent the first dicyanamides with trivalent cations. The Ln³⁺ ions are coordinated by three bridging N atoms and six terminal N atoms of the dicyanamide ions forming a three capped trigonal prism. The structure type is related to that of PuBr₃. The novel compounds Ln[N(CN)₂]₃ have been characterized by IR and Raman spectroscopy (Ln=La) and the thermal behavior has been monitored by differential scanning calorimetry (Ln=Ce, Nd, Eu).     

Three-dimensional framework of uranium-centered polyhedra with non-intersecting channels in the uranyl oxy-vanadates A2(UO2)3(VO4)2O (A=Li, Na)

The uranyl vanadates A₂(UO₂)₃(VO₄)₂O (A=Li, Na) have been synthesized by solid-state reaction and the structure of the Li compound was solved from single-crystal X-ray diffraction. The crystal structure is built from chains of edge-shared U(2)O₇ pentagonal bipyramids alternatively parallel to - and -axis and further connected together to form a three-dimensional (3-D) arrangement. The perpendicular chains are hung on both sides of a sheet parallel to (001), formed by U(1)O₆ square bipyramids connected by VO₄ tetrahedra, and derived from the autunite-type sheet. The resulting 3-D framework creates non-intersecting channels running down the - and -axis formed by empty face-shared oxygen octahedra, the Li⁺ ions are displaced from the center of the channels and occupy the middle of one edge of the common face. The peculiar position of the Li⁺ ion together with the full occupancy explain the low conductivity of Li₂(UO₂)₃(VO₄)₂O compared with that of Na(UO₂)₄(VO₄)₃ containing the same type of channels half occupied by Na⁺ ions in the octahedral sites.Crystallographic data for Li₂(UO₂)₃(VO₄)₂O: tetragonal, space group I41/amd, , , , Z=4, ρ_mes=5.32(2) g/cm³, ρ_cal=5.36(3) g/cm³, full-matrix least-squares refinement basis on F² yielded, R₁=0.032, wR₂=0.085 for 37 refined parameters with 364 independent reflections with I?2σ(I).     

Electrical conductivity of MOXO4 (M=V, Nb; X=P, As) compounds intercalated with H2O and H3XO4

Layered compounds with the general formula MOXO₄·yH₂O (M=V, Nb; X=P, As) were prepared. The content of water y was controlled by keeping the samples in an atmosphere with various relative humidities (RH). Depending on RH, the formation of several hydrates of niobyl phosphate and arsenate was observed and their basal spacings (d) were determined, namely, NbOPO₄·H₂O, , at 11% RH and lower, NbOPO₄·2H₂O, , at 22-33% RH, NbOPO₄·3H₂O, , at 43-84% RH, and NbOPO₄·5H₂O, , at 92% RH and above; NbOAsO₄·H₂O, , at 0-16% RH and NbOAsO₄·3H₂O, at 33% RH and above. As follows from ac and dc conductivity data, NbOXO₄·yH₂O compounds are practically pure protonic conductors, whereas VOXO₄·yH₂O compounds are mixed protonic-electronic conductors and the protonic component increases with y. Two intercalates of MOXO₄·yH₂O with inorganic acids were prepared. A new intercalate of H₃AsO₄ into VOAsO₄·yH₂O with the formula VOAsO₄·0.5H₃AsO₄·yH₂O (y=0.5-0.8) has the cell parameters a=6.37 and at 0-22% RH. Above 22% RH, the intercalate decomposes and the parent VOAsO₄·yH₂O with H₃AsO₄ adsorbed on the surface is formed. Another intercalate with formula NbOPO₄·H₃PO₄·yH₂O (y=2-4 at 0-75% RH) has the cell parameters a=6.43 and at RH from 0% to 5% and a=6.48 and at RH from 33% to 75%. Both intercalates are more conductive than their MOXO₄·yH₂O hosts and their conductivity increases with increasing RH of the surrounding atmosphere. Like NbOPO₄·yH₂O, also NbOPO₄·H₃PO₄·yH₂O can be considered pure proton conductor and its conductivity at 20 °C reaches 5×10⁻³ S cm⁻¹ for y=4.     

Crystal structures of the ionic conductors Bi46M8O89 (M=P, V) related to the fluorite-type structure

The crystal structures of the two oxides Bi₄₆M₈O₈₉ (M=P, V) have been solved from single crystals X-ray data at room temperature. Bi₄₆P₈O₈₉ crystallizes in the monoclinic symmetry (space group C2/m) with the cell parameters , , and β=112.14(3)°. The symmetry of Bi₄₆V₈O₈₉ is also monoclinic but the space group is P2₁/c with the unit-cell parameters: , , and β=107.27(3)°. Both structures derive from an oxygen deficient fluorite-type structure where the Bi and M cations (M=P, V) are ordered in the framework. The structures are characterised by isolated MO₄ tetrahedra (M=P, V) which contradicts the previous results. The difference between the two structures is only due to a different order of the M atoms (M=P, V) in the fluorite-type superstructure. It will be shown that some oxygen sites are partially occupied in both structures which can explain the ion conduction properties of these phases. A structural building principle will be proposed that can explain the large domain of solid solution related to the fluorite-type observed in both systems.     

Crystal structures of Rb2U2O7 and Rb8U9O31, a new layered rubidium uranate

Two alkali metal uranates Rb₂U₂O₇ and Rb₈U₉O₃₁ have been synthesized by solid state reaction at high temperature and their crystal structures determined from single crystal X-ray diffraction data, collected with a three circles Brucker SMART diffractometer equipped by Mo(Kα) radiation and a charge-coupled device (CCD) detector. Their structures were solved using direct methods and Fourier difference techniques and refined by a least-square method on the basis of F² for all unique reflections, with R₁=0.043 for 53 parameters and 746 independent reflections with I?2σ(I) for Rb₂U₂O₇, monoclinic symmetry, space group P2₁/c, , , , β=108.81(1)°, , , Z=2 and R₁=0.036 for 141 parameters and 2065 independent reflections with I?2σ(I) for Rb₈U₉O₃₁, orthorhombic, space group Pbna, , , , , , Z=4.The Rb₂U₂O₇ structure presents a strong analogy with that of K₂U₂O₇ and can be described by layers of distorted UO₂(O₄) octahedra built from dimeric units of edge shared octahedra further linked together by opposite corners. In Rb₈U₉O₃₁ puckered layers are formed by the association of two different uranium polyhedra, pentagonal bipyramids and distorted octahedra. The structure of Rb₈U₉O₃₁ is built from a regular succession of infinite ribbons similar to those observed in diuranates M₂U₂O₇ (MK, Rb) and infinite three polyhedra wide ribbons , to create an original undulated sheets .For both compounds Rb⁺ ions occupy the interlayer space and exhibit comparable mobility with conductivity measurements indicating an Arrhenius-type behavior.     

Crystal structures and magnetic properties of 6H-perovskite-type oxides Ba3MIr2O9 (M=Mg, Ca, Sc, Ti, Zn, Sr, Zr, Cd and In)

Crystal structures and magnetic properties of quaternary oxides Ba₃MIr₂O₉ (M=Mg, Ca, Sc, Ti, Zn, Sr, Zr, Cd and In) were investigated. Rietveld analyses of their X-ray diffraction data indicate that they adopt the 6H-perovskite-type structure with space group P6₃/mmc or, in the case of M=Ca, Sr and Cd, a monoclinically distorted structure with space group C2/c. The Ir valence configurations are (M=Mg, Ca, Zn, Sr and Cd), (M=Sc and In) and (M=Ti and Zr). Magnetic susceptibility and specific heat measurements were carried out. In the , the Ir⁵⁺ ions have a non-magnetic ground state and the magnetic behavior for these compounds is explained by the Kotani's theory. For , the effective magnetic moment of these compounds is significantly small, although the Ir⁴⁺ ions have magnetic moment, which indicates the existence of the strong antiferromagnetic interaction between Ir⁴⁺ ions in the Ir⁴⁺₂O₉ face-shared bioctahedra. In the case of , a specific heat anomaly was found at about 10 K (M=Sc) and 1.6 K (M=In), which suggests the magnetic ordering of the magnetic moments of Ir⁴⁺ in the (Ir⁴⁺Ir⁵⁺)O₉ bioctahedra.     

Synthesis and structure of three manganese oxalates: MnC2O4·2H2O, [C4H8(NH2)2][Mn2(C2O4)3] and Mn2(C2O4)(OH)2

Three manganese oxalates have been hydrothermally synthesized, and their structures determined by single-crystal X-ray diffraction. MnC₂O₄·2H₂O (I) is orthorhombic, P2₁2₁2₁, , , , , Z=4, final R, R_w=0.0832, 0.1017 for 561 observed data (I>3σ(I)). The one-dimensional structure consists of chains of oxalate-bridged manganese centers. [C₄H₈(NH₂)₂][Mn₂(C₂O₄)₃] (II) is triclinic, , , , , α=81.489(2)°, β=81.045(2)°, γ=86.076(2)°, , Z=1, final R, R_w=0.0467, 0.0596 for 1773 observed data (I > 3σ (I)). The three-dimensional framework is constructed from seven coordinate manganese and oxalate anions. The material contains extra-framework diprotonated piperazine cations. Mn₂(C₂O₄)(OH)₂ (III) is monoclinic, P2₁/c, , , , β=91.10(3)°, , Z=1, final R₁, wR2=0.0710, 0.1378 for 268 observed data (I>2σ (I)). The structure is also three dimensional, with layers of MnO₆ octahedra pillared by oxalate anions. The hydroxide group is found bonded to three manganese centers resulting in a four coordinate oxygen.     

Two aluminotriphosphates with closely related intersecting tunnel structures involving tetrahedral “AlP” chains and layers: AAl3(P3O10)2, A=Rb, Cs

Two new aluminotriphosphates, RbAl₃(P₃O₁₀)₂ and CsAl₃(P₃O₁₀)₂, were synthesized by solid-state reaction. They crystallize in non-centrosymmetric space groups: C222₁ with , , , (Z=4) for RbAl₃(P₃O₁₀)₂ and C2ce with , , , (Z=4) for CsAl₃(P₃O₁₀)₂. Their 3D frameworks, built up of corner sharing P₃O₁₀ groups, AlO₄ tetrahedra and AlO₆ octahedra, exhibit several remarkable features. The AlO₄ tetrahedra and P₃O₁₀ groups are directly associated through the corners, forming helical columns in the Rb-phase and “helicoid” layers in the Cs-phase. The simultaneous presence of AlO₄ and AlO₆ species, rather rare in phosphates, leads to the formation of closely related [Al₃P₆O₂₄]_∞ layers in both structures, which differ by their stacking mode. The presence of intersecting tunnels running along 〈110〉 and [001] directions, with Rb⁺ and Cs⁺ sitting at the intersections, shows the opened character of these two structures.     

Structure and magnetism of NaRu2O4 and Na2.7Ru4O9

The structures of NaRu₂O₄ and Na_2.7Ru₄O₉ are refined using neutron diffraction. NaRu₂O₄ is a stoichiometric compound consisting of double chains of edge sharing RuO₆ octahedra. Na_2.7Ru₄O₉ is a non-stoichiometric compound with partial occupancy of the Na sublattice. The structure is a mixture of single, double and triple chains of edge-shared RuO₆ octahedra. NaRu₂O₄ displays temperature independent paramagnetism with . Na_2.7Ru₄O₉ is paramagnetic, χ₀= with and a Curie constant of 0.0119 emu/mol Oe K. Specific heat measurements reveal a small upturn at low temperatures, similar to the upturn observed in La₄Ru₆O₁₉. The electronic contribution to the specific heat (γ) for Na_2.7Ru₄O₉ was determined to be15 mJ/mole_Ru K².     

The first rubidium rare-earth(III) thiophosphates: Rb3M3[PS4]4 (M=Pr, Er)

The two non-isotypical rubidium rare-earth(III) thiophosphates Rb₃M₃[PS₄]₄ of praseodymium and erbium can easily be obtained by the stoichiometric reaction of the respective rare-earth metal, red phosphorus and sulfur with an excess of rubidium bromide (RbBr) as flux and rubidium source at 950°C for 14 days in evacuated silica tubes. The pale green platelet-shaped single crystals of Rb₃Pr₃[PS₄]₄ as well as the pink rods of Rb₃Er₃[PS₄]₄ are moisture sensitive. Rb₃Pr₃[PS₄]₄ crystallizes triclinically in the space group (, , , α=84.329(4)°, β=88.008(4)°, γ=80.704(4)°; Z=2), Rb₃Er₃[PS₄]₄ monoclinically in the space group P2₁/n (, , , β=95.601(6)°; Z=4). In both structures, there are three crystallographically different rare-earth cations present. (M1)³⁺ is eightfold coordinated in the shape of a square antiprism, (M2)³⁺ and (M3)³⁺ are both surrounded by eight sulfur atoms as bicapped trigonal prisms each with a coordination number of eight as well as for the praseodymium, but better described as CN=7+1 in the case of the erbium compound. These [MS₈]¹³⁻ polyhedra form a layer according to by sharing edges with the isolated [PS₄]³⁻ tetrahedra (d(P-S)=200-209 pm, ?(S-P-S)=102-116°). These layers are stacked with a repetition period of three in the case of the praseodymium compound, but of only two for the erbium analog. The rubidium cation (Rb1)⁺ is located in cavities of these layers and tenfold coordinated in the shape of a tetracapped trigonal antiprism. The also tenfold but more irregularly coordinated rubidium cations (Rb2)⁺ and (Rb3)⁺ reside between the layers.     

Synthesis, crystal structure and vibrational spectra characterization of MLa(PO3)4 (M=Na, Ag)

The single crystals of lanthanum metaphosphate MLa(PO₃)₄ (M=Na, Ag) have been synthesized and studied by a combination of X-ray crystal diffraction and vibrational spectroscopy. The sodium and silver compounds crystallize in the same monoclinic P2₁/n space group ( factor group) with the following respective unit cell dimensions: a=7.255(2), b=13.186(3), , β=90.40(2)°, , Z=4 and a=7.300(5), b=13.211(9), , β=90.47(4)°, , Z=4. This three-dimensional framework is built of twisted zig-zag chains running along a direction and made up of PO₄ tetrahedra sharing two corners, connected to the LaO₈ and NaO₇ or AgO₇ polyhedra by common oxygen atoms to the chains. The infrared and Raman vibrational spectra have been investigated. A group factor analysis leads to the determination of internal modes of (PO₃)⁻ anion in the phosphate chain.     

M1−x[W2O2X6] with M=K, Tl, Ag, Hg, Pb; X=Cl, Br—A class of mixed valence tungsten (IV,V) compounds with layered structures, W-W bonds and high conductivity

The crystal structure of WOCl₃, determined on the basis of powder diffraction data (tetragonal, P4₂/mnm, a=10.6856(6), c=3.8537(2)), is isotypic to WOI₃ and contains one-dimensional strands of edge-sharing double-octahedral W₂O_4/2Cl₆ groups connected via common corners in trans position. A W-W bond of 2.99 Å is present within the planar W₂Cl₆ groups. A series of non-stochiometric, mixed valence W(IV,V) compounds M_1−x[W₂O₂Cl₆] can be obtained from WOCl₃ by reaction with metal halides (TlCl, KCl, PbCl₂) or by reaction of elemental Hg with WOCl₄. All were characterized by single crystal structure determinations and EDX measurements (Tl_0.981(2)[W₂O₂Cl₆]: monoclinic, C2/m, a=12.7050(4), b=3.7797(1), , β=107.656(1)°; K_0.84(2)[W₂O₂Cl₆]: monoclinic, C2/m, a=12.812(3), b=3.7779(6), , β=107.422(8)°; Pb_0.549(3)[W₂O₂Cl₆]: orthorhombic, Immm,a=3.7659(1), b=9.8975(4), ; Hg_0.554(6)[W₂O₂Cl₆]: monoclinic, C2/m, a=12.8361(8), b=3.7622(3), , β=113.645(3)°). Two representatives of this family of compounds have already been reported: Na[W₂O₂Br₆] [Y.-Q. Zhang, K. Peters, H.G. von Schnering, Z. Anorg. Allg. Chem. 624 (1998) 1415-1418] and Ag_0.74[W₂O₂Br₆] [S. Imhaïne, C. Perrin, M. Sergent, Mat. Res. Bull. 33 (1998) 927-933]. The Ag containing compound can be obtained from elemental Ag and WOBr₃. The crystal structure, originally reported in the triclinic system, was redetermined and shown to be monoclinic with space group C2/m (a=13.7338(10), b=3.7769(3), , β=112.401(3)°). The crystal structures of these compounds are in close relationship to the structure of WOCl₃ and all contain W₂O_4/2X₆ (X=Cl, Br) double strands with the mono and divalent cations coordinated by the terminal halogen atoms of the W₂X₆ groups and a short W-W bond (2.85 Å for X=Cl). A cube-shaped coordination environment is present for M=Tl, K and a trigonal-prismatic coordination for M=Ag, Hg. Hg_0.55[W₂O₂Cl₆] is a semiconductor with a non-Arrhenius behaviour, high specific conductivity of 0.05 Ω^-1 cm⁻¹ and a very small activation energy of 0.03 eV. Hg_0.55[W₂O₂Cl₆] and Ag_0.8[W₂O₂Br₆] show a temperature independent paramagnetism with a magnetic moment around 300×10^-6 cm³ mol^-1.     

X-ray powder diffraction studies and thermal behaviour of NaK2B9O15, Na(Na.17K.83)2B9O15, and (Na.80K.20)K2B9O15

The crystal structures of NaK₂B₉O₁₅ (, , , β=94.080(1)°, R_p=0.047, R_wp=0.059, R_B=0.026), Na(Na_.17K_.83)₂B₉O₁₅ (, , , β=94.228(2)°, R_p=0.053, R_wp=0.068, R_B=0.026), and (Na_.80K_.20)K₂B₉O₁₅ (, , , β=94.071(1)°, Z=4, R_p=0.041, R_wp=0.052, R_B=0.023) were refined in the monoclinic space groups P2₁/c(Z=4) using X-ray powder diffraction data and the Rietveld method. These nonaborates are isostructural to K₃B₉O₁₅. Their crystal structure consists of a three-dimensional open framework built up from three crystallographically independent triborate groups. The alkali metal cations are located on three different sites in the voids of the framework. High-temperature X-ray diffraction studies show that NaK₂B₉O₁₅ decomposes at about 700 °C in accordance with the peritectic reaction NaK₂B₉O₁₅↔K₅B₁₉O₃₁+liquid. The thermal expansion of NaK₂B₉O₁₅ and Na(Na_.17K_.83)₂B₉O₁₅ is highly anisotropic. A similarity of the thermal and compositional (Na-K substitution) deformations of NaK₂B₉O₁₅ is revealed: heating of NaK₂B₉O₁₅ by 1 °C leads to the same deformations of the crystal structure as increasing the amount of K atoms in (Na_1−xK_x)₃B₉O₁₅ by 0.04 at% K.     

Crystal growth, transport, and magnetic properties of Ln3Co4Sn13 (Ln=La, Ce) with a perovskite-like structure

Ln₃Co₄Sn₁₃ (Ln=La, Ce) have been synthesized by flux growth and characterized by single crystal X-ray diffraction. These compounds adopt the Yb₃Rh₄Sn₁₃-type structure and crystallize in the cubic space group (No. 223) with Z=2. Lattice parameters at 298 K are , , and , for the La and Ce analogues, respectively. The crystal structure consists of an Sn-centered icosahedron at the origin of the unit cell, which shares faces with eight Co trigonal prisms and 12 Ln-centered cuboctahedra. Magnetization data at 0.1 T show paramagnetic behavior down to 1.8 K for Ce₃Co₄Sn₁₃, with per Ce³⁺, while conventional type II superconductivity appears below 2.85 K in the La compound. Electrical resistivity and specific heat data for the La compound show a corresponding sharp superconducting transition at T_c∼2.85 K. The entropy and resistivity data for Ce₃Co₄Sn₁₃ show the existence of the Kondo effect with a complicated semiconducting-like behavior in the resistivity data. In addition, a large enhanced specific heat coefficient at low T with a low magnetic transition temperature suggests a heavy-fermionic character for the Ce compound. Herein, the structure and physical properties of Ln₃Co₄Sn₁₃ (Ln=La, Ce) are discussed.     

Rare-earth-rich tellurides: Gd4NiTe2 and Er5M2Te2 (M=Co, Ni)

Three new rare earth metal-rich compounds, Gd₄NiTe₂, and Er₅M₂Te₂ (M=Ni, Co), were synthesized in direct reactions using R, R₃M, and R₂Te₃ (R=Gd, Er; M=Co, Ni) and single-crystal structures were determined. Gd₄NiTe₂ is orthorhombic and crystallizes in space group Pnma with four formula units per cell. Lattice parameters at 110(2) K are a=15.548(9), b=4.113(2), . Er₅Ni₂Te₂ and Er₅Co₂Te₂ are isostructural and crystallize in the orthorhombic space group Cmcm with two formula units per cell. Lattice parameters at 110(2) K are a=3.934(1), b=14.811(4), , and a=3.898(1), b=14.920(3), , respectively. Metal-metal bonding correlations were analyzed using the empirical Pauling bond order concept.