Some A6B5O18 cation-deficient perovskites in the BaO-La2O3-TiO2-Nb2O5 system

Some dielectric oxides have been synthesized and characterized in the BaO-La₂O₃-TiO₂-Nb₂O₅ system. Through Rietveld refinement of X-ray powder diffraction data, Ba₅LaTi₂Nb₃O₁₈ and Ba₄La₂Ti₃Nb₂O₁₈ are identified as the A_nB_n−1O_3n (n=6) type cation-deficient perovskites with space group and lattice constants , and for Ba₅LaTi₂Nb₃O₁₈; , and for Ba₄La₂Ti₃Nb₂O₁₈, respectively. Their ceramics exhibit high dielectric constant up to 57 and high quality factors (Qf) up to 21,273 GHz. The temperature coefficient of resonant frequency (τ_f) of these ceramics is decreased with the increase of B-site bond valence.     

Extension of the La7Mo7O30 structural type with La7Nb3W4O30 and La7Ta3W4O30 compounds

Two compounds of formula La₇A₃W₄O₃₀ (with A=Nb and Ta) were prepared by solid-state reaction at 1450 and 1490 °C. They crystallize in the rhombohedric space group R-3 (No. 148), with the hexagonal parameters: , and , . The structure of the materials was analyzed from X-ray, neutron and electronic diffraction. These oxides are isostructural of the reduced molybdenum compound La₇Mo₇O₃₀, which are formed of perovskite rod along [111]. An order between (Nb, Ta) and W is observed.     

Structural and magnetic properties of Ba3La3Mn2W3O18

A quaternary phase, Ba₃La₃Mn₂W₃O₁₈, was synthesized in reduced atmosphere (5% H₂/Ar) at 1200 °C and characterized by using powder X-ray diffraction, electron diffraction and high resolution TEM. Ba₃La₃Mn₂W₃O₁₈ crystallizes in rhombohedral space group with the cell parameters, and , and can be attributed to the n=6 member in the B-site deficient perovskite family, A_nB_n−1O_3n. The structure can be described as close-packed [La/BaO₃] arrays in the sequence of (hcccch)₃, wherein the B-site cations, W and Mn, occupy five octahedral layers in every six octahedral layers, which leave a vacant octahedral layers separating the 5-layer perovskite blocks. The B-cation layers in the perovskite block alternate along the c-axis in a sequence of W⁶⁺-Mn²⁺-W⁵⁺-Mn²⁺-W⁶⁺. The bond valence calculation and optical reflection spectrum confirm the presence of W⁵⁺. This compound behaves paramagnetically in wide temperature range and weak antiferromagnetic interaction only occurs at low temperatures.     

Inorganic-organic hybrid structure: Synthesis, structure and magnetic properties of a cobalt phosphite-oxalate, [C4N2H12][Co4(HPO3)2(C2O4)3]

A hydrothermal reaction of a mixture of cobalt (II) oxalate, phosphorous acid, piperazine and water at 150 °C for 96 h followed by heating at 180 °C for 24 h gave rise to a new inorganic-organic hybrid solid, [C₄N₂H₁₂][Co₄(HPO₃)₂(C₂O₄)₃], I. The structure consists of edge-shared CoO₆ octahedra forming a [Co₂O₁₀] dimers that are connected by HPO₃ and C₂O₄ units forming a three-dimensional structure with one-dimensional channels. The amine molecules are positioned within these channels. The oxalate units have a dual role of connecting within the plane of the layer as well as out of the plane. Magnetic susceptibility measurement shows the compound orders antiferromagnetically at low temperature (). Crystal data: I, monoclinic, space group=P2₁/c (No. 14). a=7.614(15), b=7.514(14), , β=97.351(3)°, , Z=2, , , R₁=0.0310 and wR₂=0.0807 data [I>2σ(I)].     

Crystal growth, characterization and physical properties of PrNiSb3, NdNiSb3 and SmNiSb3

The crystal structures of three new intermetallic ternary compounds in the LnNiSb₃ (Ln=Pr, Nd and Sm) family have been characterized by single crystal X-ray diffraction. PrNiSb₃, NdNiSb₃ and SmNiSb₃ all crystallize in an orthorhombic space group, Pbcm (No. 57), Z=12, with , , , and ; , , , and ; and , , , and , for Ln=Pr, Nd and Sm, respectively. These compounds consist of rare-earth atoms located above and below layers of nearly square, buckled Sb nets, along with layers of highly distorted edge- and face-sharing NiSb₆ octahedra. Resistivity data indicate metallic behavior for all three compounds. Magnetization measurements show antiferromagnetic behavior with (PrNiSb₃), 4.6 K (NdNiSb₃), and 2.9 K (SmNiSb₃). Effective moments of 3.62 μ_B, 3.90 μ_B and 0.80 μ_B are found for PrNiSb₃, NdNiSb₃ and SmNiSb₃, respectively, and are consistent with Pr³⁺ (f ²), Nd³⁺ (f ³), and Sm³⁺ (f ⁴).     

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.     

Molten salt flux synthesis and structure of the new layered uranyl tellurite, K4[(UO2)5(TeO3)2O5]

The reaction of UO₃ and TeO₃ with a KCl flux at 800 °C for 3 days yields single crystals of K₄[(UO₂)₅(TeO₃)₂O₅]. The structure of the title compound consists of layered, two-dimensional sheets arranged in a stair-like topology separated by potassium cations. Contained within these sheets are one-dimensional uranium oxide ribbons consisting of UO₇ pentagonal bipyramids and UO₆ tetragonal bipyramids. The ribbons are in turn linked by corner-sharing with trigonal pyramidal TeO₃ units to form sheets. 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 nonpolar. The potassium cations form contacts with nearby tellurite units and axial uranyl oxygen atoms. Crystallographic data (193 K, MoKα, ): triclinic, space group , , , , α=99.642(1)°, β=93.591(1)°, γ=100.506(1)°, , Z=1,R(F)=4.19% for 149 parameters and 2583 reflections with I>2σ(I).     

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.     

Structures of the reduced niobium oxides Nb12O29 and Nb22O54

The crystal structure of Nb₂₂O₅₄ is reported for the first time, and the structure of orthorhombic Nb₁₂O₂₉ is reexamined, resolving previous ambiguities. Single crystal X-ray and electron diffraction were employed. These compounds were found to crystallize in the space groups P2/m (, , , β=102.029(3)^°) and Cmcm (, , ), respectively and share a common structural unit, a 4×3 block of corner sharing NbO₆ octahedra. Despite different constraints imposed by symmetry these blocks are very similar in both compounds. Within a block, it is found that the niobium atoms are not located in the centers of the oxygen octahedra, but rather are displaced inward toward the center of the block forming an apparent antiferroelectric state. Bond valence sums and bond lengths do not show the presence of charge ordering, suggesting that all 4d electrons are delocalized in these compounds at the temperature studied, T=200 K.     

Synthesis, crystal structure and optical properties of BiMgVO5

The new vanadate BiMgVO₅ has been prepared and its structure has been determined by single crystal X-ray diffraction: space group P2₁/n, , , , β=107.38(5)°, wR₂=0.0447, R=0.0255. The structure consists of [Mg₂O₁₀] and [Bi₂O₁₀] dimers sharing their corners with [VO₄] tetrahedra. The ranges of bond lengths are 2.129-2.814 Å for Bi-O; 2.035-2.167 Å for Mg-O and 1.684-1.745 Å for V-O. V-O bond lengths determined from Raman band wavenumbers are between 1.679 and 1.747 Å. An emission band overlapping the entire visible region with a maximum around 650 nm is observed.     

Hydrothermal synthesis and crystal structure of Cs6[(UO2)4(W5O21)(OH)2(H2O)2]: a new polar uranyl tungstate

The hydrothermal reaction of UO₃, WO₃, and CsIO₄ leads to the formation of Cs₆[(UO₂)₄(W₅O₂₁)(OH)₂(H₂O)₂] and UO₂(IO₃)₂(H₂O). Cs₆[(UO₂)₄(W₅O₂₁)(OH)₂(H₂O)₂] is the first example of a hydrothermally synthesized uranyl tungstate. It's structure has been determined by single-crystal X-ray diffraction. Crystallographic data: tetragonal, space group I4 cm, , , Z=4, MoKα, , R(F)=2.84% for 135 parameters with 2300 reflections with I>2σ(I). The structure is comprised of two-dimensional anionic layers that are separated by Cs⁺ cations. The coordination polyhedra found in the novel layers consist of UO₇ pentagonal bipyramids, WO₆ distorted octahedra, and WO₅ square pyramids. The UO₇ polyhedra are formed from the binding of five equatorial oxygen atoms around a central uranyl, UO₂²⁺, unit. Both bridging and terminal oxo ligands are employed in forming the WO₅ square pyramidal units, while oxo, hydroxo, and aqua ligands are found in the WO₆ distorted octahedra. In the layers, four (UO₂)O₅ polyhedra corner share with equatorial oxygen atoms to form a U₄O₂₄ tetramer entity with a square site in the center; a tungsten atom populates the center of each of these sites to form a U₄WO₂₅ pentamer unit. The pentamer units that result are connected in two dimensions by edge-shared dimers of WO₆ octahedra to form the two-dimensional [(UO₂)₄(W₅O₂₁)(OH)₂(H₂O)₂]^6- layers. The lack of inversion symmetry in Cs₆[(UO₂)₄(W₅O₂₁)(OH)₂(H₂O)₂] can be directly contributed to the WO₅ square pyramids found in the pentamer units. In the structure, all of these polar polyhedra align their terminal oxygens in the same orientation, along the c axis, thus resulting in a polar compound.     

PbI4Cu2(PPh3)4: A heterometallic iodide with unusual cis-divacant octahedral coordination sphere for lead: Synthesis, structure, red-infrared fluorescence and theoretical studies

A new heterometallic iodide, PbI₄Cu₂(PPh₃)₄, was synthesized by reactions of PbI₂, CuI and triphenylphosphine (PPh₃) in DMF solution. The single-crystal X-ray diffraction analyses show that Pb(II) center adopts an unusual cis-divacant octahedral geometry. Crystal data: triclinic, space group , , , , α=106.623(4)°, β=103.478(6)°, γ=93.574(5)°, and Z=2. Density function theory (DFT) calculations and fragment orbital interaction analyses reveal the presence of a three-center four-electron (3c-4e) hypervalent bonding about lead; and the formation of the unusual cis-divacant [PbI₄]²⁻ octahedron is energetically favorable. The title yellow compound has an optical bandgap of 2.69 eV and shows remarkable red-infrared fluorescence emission at 732 nm with lifetime of 24 μs which is assigned as an iodine 5p-lead 6s to PPh₃-lead 6p charge transfer (XM-LM-CT).     

Synthesis, structure, and characterization of a new one-dimensional tellurite phosphate, Ba2TeO(PO4)2

A new one-dimensional tellurite phosphate, Ba₂TeO(PO₄)₂ has been synthesized by standard solid-state reaction techniques using BaCO₃, TeO₂, and (NH₄)H₂PO₄ as reagents. The structure of Ba₂TeO(PO₄)₂ was determined by single-crystal X-ray diffraction. Ba₂TeO(PO₄)₂ crystallizes in the triclinic space group P-1 (No. 2), with , , , α=76.843(4)°, β=79.933(4)°, γ=75.688(4)°, , and Z=2. Ba₂TeO(PO₄)₂ has a novel one-dimensional chain structure that is composed of PO₄ tetrahedra and TeO₅ polyhedra. Te⁴⁺ cations are in asymmetric coordination environments attributable to their lone pairs. The lone pairs on the Te⁴⁺ cations point in the [100] and [−100] direction and interact with the Ba²⁺ cations. Infrared, Raman, and UV-Vis diffuse reflectance spectroscopy, thermogravimetric analysis, and dipole moment calculations are also presented.     

A one-dimensional zirconium hydroxyfluoride, [Zr(OH)2F3][enH]

The first example of a unidimensional zirconium hydroxide fluoride was synthesized under mild solvothermal treatment and characterized by X-ray diffraction and thermal techniques. Monoprotonated ethylenediamine cations reside between the anionic chains. Crystal data for this material are as follows: [C₂N₂H₉][Zr(OH)₂F₃], M=243.35, orthorhombic, space group Pca2₁, a=6.8016(13), b=6.1393(12), , , , Z=4, , μ(Mo-Kα)=1.777 mm⁻¹, . The material transforms to an unknown layered material at ∼175 °C, a common occurrence for 1D structures where the chains are arranged in hydrogen-bonded layers and separated by interlayer organoammoniums. Collapse to the known condensed mineral phase Zr(FO)_2.7 occurs at ca. 275 °C before finally transforming to the baddeleyite form of ZrO₂ at ca. 460 °C.     

Structural characterization and ferroelectric ordering in (C3N2H5)5Sb2Br11

A ferroelectric crystal (C₃N₂H₅)₅Sb₂Br₁₁ has been synthesized. The single crystal X-ray diffraction studies (at 300, 155, 138 and 121 K) show that it is built up of discrete corner-sharing bioctahedra and highly disordered imidazolium cations. The room temperature crystal structure has been determined as monoclinic, space group, P2₁/n with: , and and β=96.19^°. The crystal undergoes three solid-solid phase transitions: ) discontinuous, continuous and discontinuous. The dielectric and pyroelectric measurements allow us to characterize the low temperature phases III and IV as ferroelectric with the Curie point at 145 K and the saturated spontaneous polarization value of the order of along the a-axis (135 K). The ferroelectric phase transition mechanism at 145 K is due to the dynamics of imidazolium cations.     

The effect of glass additives on the microwave dielectric properties of Ba(Mg1/3Ta2/3)O3 ceramics

The effect of glass additives on the densification, phase evolution, microstructure and microwave dielectric properties of Ba(Mg_1/3Ta_2/3)O₃ (BMT) was investigated. Different weight percentages of quenched glass such as B₂O₃, SiO₂, B₂O₃-SiO₂, ZnO-B₂O₃, 5ZnO-2B₂O₃, Al₂O₃-SiO₂, Na₂O-2B₂O₃·10H₂O, BaO-B₂O₃-SiO₂, MgO-B₂O₃-SiO₂, PbO-B₂O₃-SiO₂, ZnO-B₂O₃-SiO₂ and 2MgO-Al₂O₃-5SiO₂ were added to calcined BMT precursor. The sintering temperature of the glass-added BMT samples were lowered down to 1300 °C compared to solid-state sintering where the temperature was 1650 °C. The formation of high temperature satellite phases such as Ba₅Ta₄O₁₅ and Ba₇Ta₆O₂₂ were found to be suppressed by the glass addition. Addition of glass systems such as B₂O₃, ZnO-B₂O₃, 5ZnO-2B₂O₃ and ZnO-B₂O₃-SiO₂ improved the densification and microwave dielectric properties. Other glasses were found to react with BMT to form low-Q phases which prevented densification. The microwave dielectric properties of undoped BMT with a densification of 93.1% of the theoretical density were ε_r=24.8, and Q_u×f=80,000. The BMT doped with 1.0 wt% of B₂O₃ has Q_u×f=124,700, ε_r=24.2, and . The unloaded Q factor of 0.2 wt% ZnO-B₂O₃-doped BMT was 136,500 GHz while that of 1.0 wt% of 5ZnO-2B₂O₃ added ceramic was Q_u×f=141,800 GHz. The best microwave quality factor was observed for ZnO-B₂O₃-SiO₂ (ZBS) glass-added ceramics which can act as a perfect liquid-phase medium for the sintering of BMT. The microwave dielectric properties of 0.2 wt% ZBS-added BMT dielectric was Q_u×f=152,800 GHz, ε_r=25.5, and .     

Chemical twinning of the pyrochlore structure in the system Bi2O3-Fe2O3-Nb2O5

New ternary bismuth iron niobates having structures based on chemical twinning of pyrochlore are described. Bi_5.67Nb₁₀FeO₃₅ has hexagonal symmetry, P6₃/mmc, , , Z=2 and Bi_9.3Nb_16.9Fe_1.1O_57.8 has rhombohedral symmetry, R-3m, , , Z=3. The structures of both phases were determined and refined to R₁=0.04 using single-crystal X-ray data. They can be described as being derived from the pyrochlore structure by chemical twinning on (111)_py oxygen planes. The chemical twin operation produces pairs of corner-connected hexagonal tungsten bronze (HTB) layers as in the HTB structure, so the structures may alternatively be described as pyrochlore:HTB unit-cell intergrowth structures. In the hexagonal phase the pyrochlore blocks have a width of 12 Å, whereas the rhombohedral phase has pyrochlore blocks of two widths, 6 and 12 Å, alternating with HTB blocks. It is proposed that the previously reported binary 4Bi₂O₃:9Nb₂O₅ phase has a related structure containing pyrochlore blocks all of width 6 Å. A feature of the structures is partial occupancy (∼65%) of the Bi sites and displacement of the Bi atoms from the ideal pyrochlore A sites towards the surrounding oxygen atoms, as observed in Bi-containing pyrochlores.     

Synthesis, structure and properties of new chain cuprates, Na3Cu2O4 and Na8Cu5O10

Na₃Cu₂O₄ and Na₈Cu₅O₁₀ were prepared via the azide/nitrate route from stoichiometric mixtures of the precursors CuO, NaN₃ and NaNO₃. Single crystals have been grown by subsequent annealing of the as prepared powders at 500 °C for 2000 h in silver crucibles, which were sealed in glass ampoules under dried Ar. According to the X-ray analysis of the crystal structures (Na₃Cu₂O₄: P2₁/n, Z=4, a=5.7046(2), b=11.0591(4), c=8.0261(3) Å, β=108.389(1)°, 2516 independent reflections, R₁(all)=0.0813, wR₂ (all)=0.1223; Na₈Cu₅O₁₀: Cm, Z=2, a=8.228(1), b=13.929(2), , β=111.718(2)°, 2949 independent reflections, R₁(all)=0.0349, wR₂ (all)=0.0850), the main feature of both crystal structures are CuO₂ chains built up from planar, edge-sharing CuO₄ squares. From the analysis of the Cu-O bond lengths, the valence states of either +2 or +3 can be unambiguously assigned to each copper atom. In Na₃Cu₂O₄ these ions alternate in the chains, in Na₈Cu₅O₁₀ the periodically repeated part consists of five atoms according to Cu^II-Cu^II-Cu^III-Cu^II-Cu^III. The magnetic susceptibilities show the dominance of antiferromagnetic interactions. At high temperatures the compounds exhibit Curie-Weiss behaviour (Na₃Cu₂O₄: , , Na₈Cu₅O₁₀: , , magnetic moments per divalent copper ion). Antiferromagmetic ordering is observed to occur in these compounds below 13 K (Na₃Cu₂O₄) and 24 K (Na₈Cu₅O₁₀).