New perovskite-based manganite Pb2Mn2O5

A new perovskite based compound Pb₂Mn₂O₅ has been synthesized using a high pressure high temperature technique. The structure model of Pb₂Mn₂O₅ is proposed based on electron diffraction, high angle annular dark field scanning transmission electron microscopy and high resolution transmission electron microscopy. The compound crystallizes in an orthorhombic unit cell with parameters a=5.736(1) Å≈√2a_p, b=3.800(1) Å≈a_p, c=21.562(6) Å≈4√2a_p (a_p—the parameter of the perovskite subcell) and space group Pnma. The Pb₂Mn₂O₅ structure consists of quasi two-dimensional perovskite blocks separated by 1/2[110]_p(1?01)_p crystallographic shear planes. The blocks are connected to each other by chains of edge-sharing MnO₅ distorted tetragonal pyramids. The chains of MnO₅ pyramids and the MnO₆ octahedra of the perovskite blocks delimit six-sided tunnels accommodating double chains of Pb atoms. The tunnels and pyramidal chains adopt two mirror-related configurations (“left” L and “right” R) and layers consisting of chains and tunnels of the same configuration alternate in the structure according to an -L-R-L-R-sequence. The sequence is sometimes locally violated by the appearance of -L-L- or -R-R-fragments. A scheme is proposed with a Jahn-Teller distortion of the MnO₆ octahedra with two long and two short bonds lying in the a-c plane, along two perpendicular orientations within this plane, forming a d-type pattern.     

Crystal growth and X-ray data of the lead phosphates Pb4P2O9 and Pb8P2O13

Single crystals of the title compounds have been grown by the Czochralski technique. Pb₄P₂O₉ crystallizes in the space group $\text{P2}{}_1\text{c}$ with the parameters a = 9.4812 Å, b = 7.1303 Å, c = 14.390 Å, β = 104.51° and Pb₈P₂O₁₃ in $\text{C2}\text{m}$ with a = 10.641 Å, b = 10.206Å c = 14.342 Å, β = 98.34°.     

Original close-packed structure and magnetic properties of the Pb4Mn9O20 manganite

The crystal structure of the Pb₄Mn₉O₂₀ compound (previously known as “Pb_0.43MnO_2.18”) was solved from powder X-ray diffraction, electron diffraction, and high resolution electron microscopy data (S.G. Pnma, a=13.8888(2) Å, b=11.2665(2) Å, c=9.9867(1) Å, R_I=0.016, R_P=0.047). The structure is based on a 6H (cch)₂ close packing of pure oxygen “h”-type (O₁₆) layers alternating with mixed “c”-type (Pb₄O₁₂) layers. The Mn atoms occupy octahedral interstices formed by the oxygen atoms of the close-packed layers. The MnO₆ octahedra share edges within the layers, whereas the octahedra in neighboring layers are linked through corner sharing. The relationship with the closely related Pb₃Mn₇O₁₅ structure is discussed. Magnetization measurements reveal a peculiar magnetic behavior with a phase transition at 52 K, a small net magnetization below the transition temperature, and a tendency towards spin freezing.     

Synthesis, X-ray diffraction study, and ionic conductivity of new AB2O6 pyrochlores

The oxides A(Ti_0.5Te_1.5)O₆ (A = K, Rb, Cs, Tl), A(Ti_0.5W_1.5)O₆ (A = Rb, Cs, Tl), and Cs(B_0.5W_1.5)O₆ (B = Zr, Hf) have been obtained as polycrystalline powders giving X-ray diffraction patterns characteristic of defect cubic pyrochlores, space group (No. 227), Z = 8. The best discrepancy R factors, from 0.0265 for Rb(Ti_0.5Te_1.5)O₆ to 0.0554 for Cs(Zr_0.5W_1.5)O₆, were obtained for the B cations randomly distributed at 16(d), A ions at one quarter of 32(e), and oxygen atoms at 48(f) positions. A linear relationship is observed between the a unit cell parameters and the ionic radii of the A cations, as well as the average ionic radii of the B atoms. The results of electrical resistivity measurements for A(Ti_0.5Te_1.5)O₆ (A = K, Rb, Cs, Tl) are given.     

High-pressure silicate pyrochlores,Sc2Si2O7 and In2Si2O7

The silicate compounds Sc₂Si₂O₇ and In₂Si₂O₇ have been converted from thortveitite type to pyrochlore type at 1000°C, 120 kbar, with resulting cell constants of 9.287(3) and 9.413(3) Å, respectively. Invariant reflection intensities in the X-ray powder diffraction patterns allowed precise absorption corrections to be made, and refinement of thermal parameters and of the single structural parameter x gave values of 0.4313(21) and 0.4272(15), respectively. The corresponding six-coordinate SiO distances were 1.761(7) and 1.800(5) Å, and the average eight-coordinate distances for ScO₈ and InO₈ were 2.267 and 2.275 Å. Values of structure-refined bond lengths for compounds containing six-coordinate silicon are surveyed, and overall weighted average octahedral distances of 1.782(14) Å for SiO and 2.520(18) Å for OO are derived. Pyrochlore phases were not produced from rare-earth disilicate or monosilicate phases subjected to the same reaction conditions as the Sc and In compounds.     

Synthesis and structure of ternary transition-metal silicides Zr3Mn4Si6 and Hf3Mn4Si6

The isostructural ternary transition-metal silicides Zr₃Mn₄Si₆ and Hf₃Mn₄Si₆ can be prepared by direct reaction of the elemental components or by arc-melting. The single-crystal structure of Zr₃Mn₄Si₆ was determined by X-ray diffraction (Pearson symbol tP104, tetragonal, space group P4₂/mbc, Z=8, , ). Zr₃Mn₄Si₆ is isostructural to Nb₃Fe₃CrSi₆ and contains an essentially ordered arrangement of the transition-metal atoms. Square antiprismatic clusters with Zr and Mn atoms at the corners and Si atoms at the center share opposite faces to form one-dimensional columns extending along the c direction. These columns occupy channels that are outlined by a framework of edge- and face-sharing MnSi₆ octahedra. The extensive metal-metal interactions in the structure are complemented by Si-Si bonding in the form of dumbbells, linear chains, and zigzag chains.     

Synthesis and structural characterization of novel tin and titanium potassium silicates K4M2Si6O18

The synthesis of a new potassium titanosilicate, K₄Ti₂Si₆O₁₈ (Ti-AV-11), possessing the crystal structure of potassium stannosilicate AV-11, has been reported. The unit cell of this material is trigonal, space group R3 (no. 146), Z=3, a=10.012, c=14.8413 Å, γ=120°, V=1289 Å³. The structure of AV-11 is built up of MO₆ (M=Sn, Ti) octahedra and SiO₄ tetrahedra by sharing corners. The SiO₄ tetrahedra form helix chains, periodically repeating every six tetrahedra. These chains extend along the [001] direction and are linked by isolated MO₆ octahedra, thus producing a mixed octahedral-tetrahedral oxide framework. AV-11 materials have been further characterized by bulk chemical analysis, powder X-ray diffraction (XRD), scanning electron microscopy (SEM), ²⁹Si and ¹¹⁹Sn magic-angle spinning (MAS) NMR spectroscopy.     

Neutron diffraction and X-ray absorption study of Ag5Pb2O6

Neutron diffraction and X-ray absorption measurements were carried out for a silver-lead oxide Ag₅Pb₂O₆. The powder neutron diffraction patterns could be fitted to the trigonal structure, as was found by X-ray diffraction. From the bond-valence-sum (BVS) analysis, the valences of the Ag and Pb ions were estimated to be about 1+ and 3.7+, respectively. The X-ray absorption measurements indicated that the ionic state of Ag is close to 1+, while that of Pb stands between 3+ and 4+. The deviation of the valence of the Pb ion from 4+ suggests a contribution of Pb orbitals to the metallic conduction as well as the possible superconductivity of this material, consistently with a recent band-structure calculation.     

New manganites NdM3Sr3Mn4O12 and NdM3Ba3Mn4O12 (M = Li, Na, K): Synthesis and X-ray diffraction characteristics

Manganites NdM₃Sr₃Mn₄O₁₂ and NdM₃Ba₃Mn₄O₁₂ (M = Li, Na, K) were synthesized by a ceramic method from the corresponding oxides and carbonates. The X-ray diffraction analysis showed that all the compounds crystallized in the tetragonal crystal system with the following lattice parameters: NdLi₃Sr₃Mn₄O₁₂: a = 10.88 ?, c = 9.52 ?, V ^o = 1126.9 ?³, Z = 4, ρ_X = 4.95 g/cm³, ρ_pycn = 4.87 ± 0.05 g/cm³; NdNa₃Sr₃Mn₄O₁₂: a = 10.73 ?, c = 10.66 ?, V ^o = 1227.3 ?³, Z = 4, ρ_X = 4.80 g/cm³, ρ_pycn = 4.73 ± 0.07 g/cm³; NdK₃Sr₃Mn₄O₁₂: a = 10.87 ?, c = 11.71 ?, V ^o = 1382.6 ?³, Z = 4, ρ_X = 4.50 g/cm³, ρ_pycn = 4.43 ± 0.09 g/cm³; NdLi₃Ba₃Mn₄O₁₂: a = 10.97 ?, c = 10.34 ?, V ^o = 1244.3 ?³, Z = 4, ρ_X = 5.33 g/cm³, ρ_pycn = 5.23 ± 0.09 g/cm³; NdNa₃Ba₃Mn₄O₁₂: a = 10.99 ?, c = 11.15 ?, V ^o = 1346.7 ?³, Z = 4; ρ_X = 5.11 g/cm³, ρ_pycn = 5.05 ± 0.06 g/cm³; NdK₃Ba₃Mn₄O₁₂: a = 10.997 ?; c = 13.80 ?, V ^o = 1668.9 ?³, Z = 4, ρ_X = 4.32 g/cm³, ρ_pycn = 4.26 ± 0.07 g/cm³. Original Russian Text ? B.K. Kasenov, E.S. Mustafin, M.A. Akubaeva, S.T. Edil’baeva, Sh.B. Kasenova, Zh.I. Sagintaeva, S.Zh. Davrenbekov, 2009, published in Zhurnal Neorganicheskoi Khimii, 2009, Vol. 54, No. 3, pp. 424–427.     

High-temperature X-ray diffraction and specific heat studies on GdAlO3, Gd3Al5O12 and Gd4Al2O9

Gadolinium aluminates, GdAlO₃, Gd₃Al₅O₁₂ and Gd₄Al₂O₉ were synthesized by the solution combustion method. Very fine particles in the nanoparticle range of ∼10-20 nm could be prepared by this method as evidenced by surface area measurement by multipoint BET method. Thermal studies on these compounds were carried out using high-temperature X-ray diffraction (HT-XRD) and differential scanning calorimetry (DSC) methods. The thermal expansion coefficients of GdAlO₃, Gd₃Al₅O₁₂ and Gd₄Al₂O₉ were calculated from the lattice parameter data and specific heats were calculated from DSC data. The lattice parameters of GdAlO₃ and Gd₃Al₅O₁₂ were found to increase linearly with temperature whereas Gd₄Al₂O₉ did not show a linear trend. The specific heats of these compounds show an increasing trend with increase in aluminum atom fraction. Based on the thermodynamic data available in the literature and the specific heat data obtained in this study, oxygen potential diagram was constructed at 1000 K.     

Polymorphism in the Sc2Si2O7-Y2Si2O7 system

This paper examines the structural changes with temperature and composition in the Sc₂Si₂O₇-Y₂Si₂O₇ system; members of this system are expected to form in the intergranular region of Si₃N₄ and SiC structural ceramics when sintered with the aid of Y₂O₃ and Sc₂O₃ mixtures. A set of different compositions have been synthesized using the sol-gel method to obtain a xerogel, which has been calcined at temperatures between 1300 and 1750 °C during different times. The temperature-composition diagram of the system, obtained from powder XRD data, is dominated by the β-RE₂Si₂O₇ polymorph, with γ-RE₂Si₂O₇ and δ-RE₂Si₂O₇ showing very reduced stability fields. Isotherms at 1300 and 1600 °C have been analysed in detail to evaluate the solid solubility of the components. Although, the XRD data show a complete solid solubility of β-Sc₂Si₂O₇ in β-Y₂Si₂O₇ at 1300 °C, the ²⁹Si MAS-NMR spectra indicate a local structural change at x ca. 1.15 (Sc_2−xY_xSi₂O₇) related to the configuration of the Si tetrahedron, which does not affect the long-range order of the β-RE₂Si₂O₇ structure. Finally, it is interesting to note that, although Sc₂Si₂O₇ shows a unique stable polymorph (β), Sc³⁺ is able to replace Y³⁺ in γ-Y₂Si₂O₇ in the compositional range 1.86?x?2 (where x is Sc_2−xY_xSi₂O₇) as well as in δ-Y₂Si₂O₇ for compositions much closer to the pure Y₂Si₂O₇.     

An ab initio MO study on structures and energetics of CH2Si2, CH2Si2, and CH2Si2

The geometric structures and isomeric stabilities of various stationary points in CH₂Si₂ neutral, cation and anion are investigated at the coupled-cluster singles, doubles (triples) (CCSD(T)) level of theory. For the geometrical survey, the basis sets used are of the cc-pVTZ for the neutral and cation. The final energies are calculated by the use of the CCSD(T) level of theory with the aug-cc-pVTZ basis set at their optimized geometries. To the competitive two-anion isomers, the aug-cc-pVTZ basis sets are applied. The global minimum (N-1) of the CH₂Si₂ neutral has a quite different framework from those of the C₃H₂ (cyclopropenylidene) and Si₃H₂ (trisilacyclopropenylidene) neutrals. No competitive low-lying isomers are found in the CH₂Si₂ neutral. The attractive conformer (C-1) is predicted for the most stable cation, where its framework is quite different from that of the neutral N-1. Both H atoms are connected to the same C atom, but each C–H bond length is different from each other. Two competitive anion isomers with positive (real) electron affinities are predicted. The framework of the most stable anion A-1 is quite similar to that of the cation C-1, whereas both H atoms are equally connected to the same C atom. The framework of the anion isomer A-2 is the same as that in the neutral N-1. The vertical and adiabatic ionization potentials from the most stable neutral N-1 are 9.02 and 8.71 eV, respectively. The adiabatic electron affinity of the lowest lying isomer N-1 is only 0.43 eV and the vertical electron detachment energy form the global minimum anion (A-1) is 2.02 eV. The multi-centered Si–H–Si bonds are found in the neutral, cation, and anion.     

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.     

Local structure and disorder in crystalline Pb9Al8O21

Crystalline Pb₉Al₈O₂₁ is a model compound for the structure of non-linear optical glasses containing lone-pair ions, and its structure has been investigated by neutron powder diffraction and total scattering, and ²⁷Al magic angle spinning NMR. Rietveld analysis (space group (No. 205), a=13.25221(4) Å) shows that some of the Pb and O sites have partial occupancies, due to lead volatilisation during sample preparation, and the non-stoichiometric sample composition is Pb_9−δAl₈O_21−δ with δ=0.54. The NMR measurements show evidence for a correlation between the chemical shift and the variance of the bond angles at the aluminium sites. The neutron total correlation function shows that the true average Al-O bond length is 0.8% longer than the apparent bond length determined by Rietveld refinement. The thermal variation in bond length is much smaller than the thermal variation in longer interatomic distances determined by Rietveld refinement. The total correlation function is consistent with an interpretation in which AlO₃ groups with an Al-O bond length of 1.651 Å occur as a result of the oxygen vacancies in the structure. The width of the tetrahedral Al-O peak in the correlation function for the crystal is very similar to that for lead aluminate glass, indicating that the extent of static disorder is very similar in the two phases.     

Crystal growth and properties of lead pyrophosphate,Pb2P2O7

Single crystals of Pb₂P₂O₇ have been grown by the Czochralski technique. They have the triclinic space group $\text{P}\text{1}$ with cell dimensions a = 6.9627 Å, b = 6.9754Å, c = 12.764 Å, α = 96.78°, β = 91.16°, γ = 89.68°. There are four molecules per unit cell. Dielectric properties for this compound have been measured and are discussed.     

X-ray diffraction study of ErMFe2O5.5 (M = Ca, Sr, Ba) double ferrites

Double ferrites ErCaFe₂O_5.5, ErSrFe₂O_5.5, and ErBaFe₂O_5.5 were synthesized by solid-state reactions from erbium and iron(III) oxides and calcium, strontium, and barium carbonates. The compounds were found to crystallize in the orthorhombic system. Their unit cell parameters were determined by X-ray powder diffraction; their densities were measured.     

The crystal structure determinations and refinements of K2S2O7, KNaS2O7 and Na2S2O7 from X-ray powder and single crystal diffraction data

The crystal structures of K₂S₂O₇, KNaS₂O₇ and Na₂S₂O₇ have been solved and/or refined from X-ray synchrotron powder diffraction data and conventional single-crystal data. K₂S₂O₇: From powder diffraction data, monoclinic C2/c, Z=4, a=12.3653(2), b=7.3122(1), , β=93.0792(7)°, R_Bragg=0.096. KNaS₂O₇: From powder diffraction data; triclinic , Z=2, a=5.90476(9), b=7.2008(1), , α=101.7074(9), β=90.6960(7), γ=94.2403(9)°, R_Bragg=0.075. Na₂S₂O₇: From single-crystal data; triclinic , Z=2, a=6.7702(9), b=6.7975(10), , α=116.779(2), β=96.089(3), γ=84.000(3)°, R_F=0.033. The disulphate anions are essentially eclipsed. All three structures can be described as dichromate-like, where the alkali cations coordinate oxygens of the isolated disulphate groups in three-dimensional networks. The K-O and Na-O coordinations were determined from electron density topology and coordination geometry. The three structures have a cation-disulphate chain in common. In K₂S₂O₇ and Na₂S₂O₇ the neighbouring chains are antiparallel, while in KNaS₂O₇ the chains are parallel. The differences between the K₂S₂O₇ and Na₂S₂O₇ structures, with double-, respectively single-sided chain connections and straight, respectively, corrugated structural layers can be understood in terms of the differences in size and coordinating ability of the cations.     

Syntheses, crystal and electronic structures of two new lead indium phosphates: Pb2In4P6O23 and Pb2InP3O11

Two compounds Pb₂In₄P₆O₂₃ and Pb₂InP₃O₁₁ in the new family of lead indium phosphates were synthesized by high temperature solution growth (HTSG) method and structurally characterized by X-ray single crystal diffraction, powder diffraction and electron microscopy. Two title compounds display different types of 3D architectures with interesting tunnel structure are built up of the InO₆ octahedra and PO₄ tetrahedra, sharing the corners or edges, and the Pb²⁺ cations are sitting in the tunnel. The structure of Pb₂In₄P₆O₂₃ features a novel 3D open framework which can be considered as built from the layer of {In₄(P₂O₇)(PO₄)₂}²⁻ parallel to the ac plane interconnected by bridging the single PO₄. The structure of Pb₂InP₃O₁₁ can be described by the assemblage of [InP₂O₁₁] units with monophosphate groups. The stereochemical activity of the Pb^II lone pair has also been discussed. The electronic band structure calculations for the two compounds have also been performed with the density functional theory method. The study of calculations and optical diffuse reflectance absorption spectrum measurement show both compounds are indirect band-gap insulators.     

Synthesis and crystal chemistry of BaNd2Ti3O10, BaNd2Ti5O14, and Nd4Ti9O24

Two new ternary compounds BaNd₂Ti₃O₁₀ (1:1:3) and BaNd₂Ti₅O₁₄ (1:1:5) have been identified in the BaONd₂O₃TiO₂ system. Single crystals of the compounds were grown and unit cell dimensions and space group symmetry were determined. BaNd₂Ti₃O₁₀ is orthorhombic with a = 3.8655 ± 0.0003, b = 28.156 ± 0.003 and c = 7.6221 ± 0.0007 Å and possible space groups are Cmcm or Cmc2. The compound melts congruently at 1640 ± 20°C. BaNd₂Ti₅O₁₄ is also orthorhombic with a = 22.346 ± 0.002, b = 12.201 ± 0.001 and c = 3.8404 ± 0.0003 Å and possible space groups are Pbam and Pba2. This compound melts congruently at 1540 ± 20°C. Single crystals of the binary compound Nd₄Ti₉O₂₄ were also grown and found to be orthorhombic with a = 35.289 ± 0.003, b = 13.991 ± 0.001, c = 14.479 ± 0.001 Å, space group Fddd.     

表面活性剂辅助合成LiNi0.8Co0.1Mn0.1O2正极材料

通过添加烷基季铵盐类表面活性剂来调控材料形貌和粒径的改性方法,在LiNi_0.8Co_0.1Mn_0.1O₂前驱体合成过程中添加表面活性剂十二烷基三甲基溴化铵(DTAB)和十六烷基三甲基溴化铵(CTAB),利用尿素作为配合剂和沉淀剂,采用溶剂热法合成LiNi_0.8Co_0.1Mn_0.1O₂前驱体。最后,高温混锂煅烧合成椭球形的空心多孔材料。相比于不添加表面活性剂的样本,改性的材料有着更小的粒径和更加规整的形貌。电化学测试表明,添加DTAB和CTAB之后,首次充电容量分别达到223与251 mAh·g^-1(0.1C)。其中,添加CTAB的样品首次放电容量达到216 mAh·g^-1(0.1C),100次循环后容量保持率为85.1%,高于LiNi_0.8Co_0.1Mn_0.1O₂的81.7%(0.1C)。表面活性剂的改性显著提高了材料的电化学性能,为高镍三元正极材料的改性提供了一种新的思路。