The crystallographic and magnetic characteristics of Sr2CrO4 (K2NiF4-type) and Sr10(CrO4)6F2 (apatite-type)

Solid-state reaction between SrCO₃, Cr₂O₃ and SrF₂ has produced the apatite phase Sr₁₀(CrO₄)₆F₂ and Sr₂CrO₄ which adopts the K₂NiF₄-type structure. The reaction outcome was very sensitive to the heating rate with rapid rise times favouring the formation of Sr₂CrO₄, which has been synthesised at ambient pressure for the first time. Powder X-ray diffraction and electron diffraction confirmed that Sr₂CrO₄ adopts a body centred tetragonal cell (space group I4/mmm) with lattice parameters a=3.8357(1) Å and c=12.7169(1) Å, while a combination of neutron and X-ray diffraction verified Sr₁₀(CrO₄)₆F₂ is hexagonal (space group P6₃/m) with lattice parameters a=9.9570(1) Å and c=7.4292(1) Å. X-ray photoelectron spectroscopy and magnetic measurements were used to characterise the oxidation states of chromium contained within these phases.     

Neutron powder diffraction experiments on the layered triangular-lattice antiferromagnets RbFe(MoO4)2 and CsFe(SO4)2

Powder specimens of the layered triangular-lattice antiferromagnets RbFe(MoO₄)₂ and CsFe(SO₄)₂ were prepared and neutron powder diffraction experiments were carried out in order to determine the magnetic structure. The magnetic structure of both compounds is the so-called 120^° structure in the triangular plane and is incommensurate between the planes. The ordered moments are confined in the basal ab-plane. It is also found that RbFe(MoO₄)₂ exhibits structural phase transition at around 190 K from to .     

Crystal structure of the parent misfit-layered cobalt oxide [Sr2O2]qCoO2

The crystal structure of our newly discovered Sr-Co-O phase is investigated in detail through high-resolution electron microscopy (HREM) techniques. Electron diffraction (ED) measurement together with energy dispersive X-ray spectroscopy (EDS) analysis show that an ampoule-synthesized sample contains an unknown Sr-Co-O ternary phase with monoclinic symmetry and the cation ratio of Sr/Co=1. From HREM images a layered structure with a regular stacking of a CdI₂-type CoO₂ sheet and a rock-salt-type Sr₂O₂ double-layered block is observed, which confirms that the phase is the parent of the more complex “misfit-layered (ML)” cobalt oxides of [M_mA₂O_m+2]_qCoO₂ with the formula of [Sr₂O₂]_qCoO₂, i.e. m=0. It is revealed that the misfit parameter q is 0.5, i.e. the two sublattices of the CoO₂ sheet and the Sr₂O₂ block coexist to form a commensurate composite structure. We propose a structural model with monoclinic P2₁/m symmetry, which is supported by simulations of ED patterns and HREM images based on dynamical diffraction theory.     

Synthesis, crystal structure and magnetic properties of the Sr2Al0.78Mn1.22O5.2 anion-deficient layered perovskite

A new layered perovskite Sr₂Al_0.78Mn_1.22O_5.2 has been synthesized by solid state reaction in a sealed evacuated silica tube. The crystal structure has been determined using electron diffraction, high-resolution electron microscopy, and high-angle annular dark field imaging and refined from X-ray powder diffraction data (space group P4/mmm, a=3.89023(5) Å, c=7.8034(1) Å, R_I=0.023, R_P=0.015). The structure is characterized by an alternation of MnO₂ and (Al_0.78Mn_0.22)O_1.2 layers. Oxygen atoms and vacancies, as well as the Al and Mn atoms in the (Al_0.78Mn_0.22)O_1.2 layers are disordered. The local atomic arrangement in these layers is suggested to consist of short fragments of brownmillerite-type tetrahedral chains of corner-sharing AlO₄ tetrahedra interrupted by MnO₆ octahedra, at which the chain fragments rotate over 90°. This results in an averaged tetragonal symmetry. This is confirmed by the valence state of Mn measured by EELS. The relationship between the Sr₂Al_0.78Mn_1.22O_5.2 tetragonal perovskite and the parent Sr₂Al_1.07Mn_0.93O₅ brownmillerite is discussed. Magnetic susceptibility measurements indicate spin glass behavior of Sr₂Al_0.78Mn_1.22O_5.2. The lack of long-range magnetic ordering contrasts with Mn-containing brownmillerites and is likely caused by the frustration of interlayer interactions due to presence of the Mn atoms in the (Al_0.78Mn_0.22)O_1.2 layers.     

In[NC5H3(CO2)2](OH2)F: A new layered indium-organic framework material (NC5H3(CO2)2=2,6-pyridinedicarboxylate)

A new layered indium-organic framework material, In[NC₅H₃(CO₂)₂](OH₂)F has been synthesized by a hydrothermal reaction using In₂O₃, NH₄F, 2,6-NC₅H₃(CO₂H)₂ (2,6-pyridinedicarboxylic acid), HF, and water at 200 °C. Single-crystal X-ray diffraction was used to determine the structure of the reported material. In[NC₅H₃(CO₂)₂](OH₂)F has a novel layered structure consisting of InO₅NF polyhedra and the pyridinedicarboxylate organic linker. Detailed structural analyses with full characterization including infrared spectrum, thermogravimetric analysis, elemental analysis, exchange reactions for the coordinated water molecule, and gas adsorption experiments are reported.     

The structure and ordering of ε-MnO2

The presence of ε-MnO₂ as a major component of electrolytic manganese dioxide (EMD) has been demonstrated by a combined X-ray diffraction/transmission electron microscopy (TEM) study. ε-MnO₂ usually has a partially ordered defect NiAs structure containing 50% cation vacancies; these vacancies can be fully ordered by a low temperature (200 °C) heat treatment to form a pseudohexagonal but monoclinic superlattice.Numerous fine-scale anti-phase domain boundaries are present in ordered ε-MnO₂ and cause extensive peak broadening and a massive shift of a very intense, 0.37 nm superlattice peak. This suggests a radically different explanation of the ubiquitous, very broad ∼0.42 nm peak (∼21-22° 2θ, CuKα radiation) in EMDs, which heretofore has been attributed to Ramsdellite containing numerous planar defects. This work confirms the multi-phase model of equiaxed EMDs proposed by Heuer et al. [ITE Lett. 1(6) (2000) B50; Proc. Seventh Int. Symp. Adv. Phys. Fields 92 (2001)], rather than the defective single-phase model of Chabre and Pannetier [Prog. Solid State Chem. 23 (1995) 1] and Bowden et al. [ITE Lett. 4(1) (2003) B1].     

New La2/3TiO3 Derivatives: Structure and Impedance Spectroscopy

New phases which arise from inserting Na cations on the vacant A-sites of the compound La_2/3TiO₃ have been obtained, giving rise to the series La_4/3−xNa_3xTi₂O₆ (for x=0.16 and 0.28). These phases adopt a perovskite-type structure as deduced from their characterization by electron microscopy and neutron diffraction. Rietveld analyses show that the symmetry is orthorhombic (S.G. Ibmm). Electrical conductivity was determined by impedance spectroscopy, as a function of temperature. A similar behavior is observed for both samples, which behave as ionic conductors with activation energies of 0.92(3) and 0.92(5) eV, respectively.     

Syntheses, crystal structures and Si solubilities of new layered carbides Zr2Al4C5 and Zr3Al4C6

Two types of new ternary carbides, Zr₂Al₄C₅ and Zr₃Al₄C₆, have been synthesized and characterized by X-ray powder diffraction. The crystal structures were refined from laboratory X-ray powder diffraction data (CuKα₁) using the Rietveld method. These carbides form a homologous series with the general formula (ZrC)_mAl₄C₃ (m=2 and 3). The crystal structures can be regarded as intergrowth structures where the Al₄C₃-type [Al₄C₄] layers are the same, while the NaCl-type [Zr_mC_m+1] layers increase in thickness with increasing m value. The new carbides are most probably the end members of continuous solid-solutions (ZrC)_m[Al_4−xSi_x]C₃ with 0?x?0.44.     

Structure determination from powder diffraction data and thermal behaviour of layered lead nitrate oxalate hydrate, Pb2(NO3)2(C2O4).2H2O

The mixed lead nitrate oxalate, Pb₂(NO₃)₂(C₂O₄).2H₂O, has been obtained in a polycrystalline form in the course of a study on precursors of nanocrystalline PZT-type oxides. Its crystal structure has been solved from powder diffraction data collected using a monochromatic radiation from a conventional X-ray source. The symmetry is monoclinic, space group P2₁/c (No. 14), the cell dimensions are a=10.623(2) Å, b=7.9559(9) Å, c=6.1932(5) Å, β=104.49(1)° and Z=4. The structure consists of a stacking of complex double sheets parallel to (1 0 0), forming layers held together by hydrogen bonds. The sheets result from the condensation of PbO₁₀ polyhedra, in which the oxalate and nitrate groups, as well as water molecules, play a major role. The structure is discussed in terms of Pb---O distances, polyhedra shape and lead coordination, with emphasis on the dimensional polymerisation role of water molecules. The thermal behaviour of this layered compound is carefully described from temperature-dependent powder diffraction and thermogravimetric measurements. The enthalpy, Δ_rH=232(3) kJ mol⁻¹, and entropy, Δ_rS=532(8) J K⁻¹ mol⁻¹, of the dehydration reaction have been determined. The high value of Δ_rH demonstrates that the water molecules are strongly bonded in the structure. The complex decomposition proceeds through the crystallisation and decomposition of Pb(NO₃)₂(C₂O₄) into Pb(NO₃)₂ and PbC₂O₄, and, finally, various lead oxides.     

Magnetic structure and properties of Cu3(OH)4SO4 made of triple chains of spins s=1/2

Cu₃(OH)₄SO₄, obtained by hydrothermal synthesis from copper sulfate and soda in aqueous medium, is isostructural with the corresponding antlerite mineral, orthorhombic, space group Pnma (62), with a=8.289(1) b=6.079(1) and c=12.057(1) Å, V=607.5(2) ų, Z=4. Its crystalline structure has been refined from X-ray single crystal and powder neutron diffraction data at room temperature. It consists of copper (II) triple chains, running in the b-axis direction and connected to each other by sulfate groups. The magnetic structure, solved from powder neutron diffraction data at 1.4 K below the transition at 5 K evidenced by susceptibility and specific measurements, reveals that, inside a triple chain, the magnetic moments of the copper ions (μ_B=0.88(5) at 1.4 K) belonging to outer chains are oriented along the c-axis of the nuclear cell, with ferromagnetic order inside a chain and antiferromagnetic order between the two outer chains. No long-range magnetic order is obtained along the central chain with an idle spin behavior.     

La6Mo8O33: a new ordered defect scheelite superstructure

The structure of La₆Mo₈O₃₃ has been determined from a triple pattern powder diffraction analysis. Two high-resolution neutron diffraction patterns collected at 1.594 and 2.398 Å and one X-rays were used. This molybdate crystallizes in a non-centrosymmetric monoclinic space group P2₁(N°4), Z=2,a=10.7411(3) Å, b=11.9678(3) Å, c=11.7722(3) Å, β=116.062 (1)°. La₆Mo₈O₃₃ is an unusual ordered defect Scheelite. Hence, it should be described with cation vacancies and an extra oxygen atom following the formula: La₆□₂Mo₈O₃₂₊₁. This extra oxygen atom leads to a pyramidal environment, whereas the other molybdenum atoms present tetrahedral environment. A molybdenum tetrahedral is connecting to the pyramid, forming an [Mo₂O₉] unit.     

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.     

Sr4AlNbO8: A new crystal structure type determined from powder X-ray data

Sr₄AlNbO₈ was synthesized at 1500 °C in air. The crystal structure was initially determined from powder X-ray diffraction data, and later refined with combined X-ray and neutron diffraction data (P2₁/c; a=7.17592(2) Å, b=5.80261(2) Å, c=19.7408(1) Å; β=97.5470(1)°, V=814.869(3) Å³, Z=4, R_p/R_wp=10.04%/13.18% for X-ray data, 4.40%/5.67% for neutron data, and 7.71%/10.74% in total with χ² of 3.76, 23 °C). The crystal structure is a new structure type and may be described as a three-dimensional polyhedral network resulting from the corner-sharing of NbO₆ and Sr1O₆ octahedra and AlO₄ tetrahedra. Also, the other strontium atoms (Sr2, Sr3, and Sr4) occupy the larger cavities surrounded by oxygen atoms to form nine, eight, and 11 coordination, respectively. Considering that Sr, Al, and Nb atoms are crystallographically distinct in terms of interatomic distances and polyhedral coordination, Sr₄AlNbO₈ can be regarded as a stoichiometric compound.     

Bi2−xLnxWO6: a novel layered structure type related to the Aurivillius phases

The crystal structure of Bi_0.7Yb_1.3WO₆ (a representative of the isomorphous series Bi_2−xLn_xWO₆; 0.3<x<1.3, for most lanthanides) has been determined. Contrary to previous suggestions, this structure type (space group A2; a=8.1070(3) Å, b=3.7048(2) Å, c=15.8379(8) Å, β=103.548(4)°) contains layers of stoichiometry WO₄, containing WO₆ octahedra sharing both edges and corners. These layers alternate with fluorite-like (Bi/Yb)₂O₂ sheets; this is a novel and unexpected arrangement and contrasts dramatically with the purely corner-sharing octahedral WO₄-layer in the parent Aurivillius phase Bi₂WO₆.     

Structural order and disorder in Co-based layered cuprates CoSr2(Y,Ce)sCu2O5+2s (s=1-3)

Crystal structures of a homologous series of Co-based layered cuprates, CoSr₂(Y,Ce)_sCu₂O_5+2s (s=1-3), have been investigated by high-resolution electron microscopy (HREM) and electron diffraction (ED) techniques. For all the three phases ED patterns showed double periodicity along a direction parallel to the CoO layers, indicating a regular alternation of two types of CoO₄-tetrahedra chains within the layers. Also seen was ordering of the chains along the layer-stacking direction for the s=1 phase (Co-1212); ED patterns simulated based on the proposed superstructure model well reproduced the observed patterns. For the s=2 (Co-1222) and s=3 (Co-1232) phases in which an additional fluorite-type layer-block is inserted between two CuO₂ planes, HREM and ED analysis revealed complete disorder of the CoO₄ chains along the layer-stacking direction. This implies that the interlayer ordering is mainly controlled by the distance between the neighboring CoO layers.     

Coupled anion and cation ordering in Sr3RFe4O10.5 (R=Y, Ho, Dy) anion-deficientperovskites

The Sr₃RFe₄O_10.5 (R=Y, Ho, Dy) anion-deficient perovskites were prepared using a solid-state reaction in evacuated sealed silica tubes. Transmission electron microscopy and ⁵⁷Fe Mössbauer spectroscopy evidenced a complete A-cations and oxygen vacancies ordering. The structure model was further refined by ab initio structure relaxation, based on density functional theory calculations. The compounds crystallize in a tetragonal a≈2√2a_p≈11.3 Å, с≈4с_p≈16 Å unit cell (a_p: parameter of the perovskite subcell) with the P4₂/mnm space group. Oxygen vacancies reside in the (FeO_5/4□_3/4) layers, comprising corner-sharing FeO₄ tetrahedra and FeO₅ tetragonal pyramids, which are sandwiched between the layers of the FeO₆ octahedra. Smaller R atoms occupy the 9-fold coordinated position, whereas the 10-fold coordinated positions are occupied by larger Sr atoms. The Fe sublattice is ordered aniferromagnetically up to at least 500 K, while the rare-earth sublattice remains disordered down to 2 K.     

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.     

Synthetic Cu0.507(5)Pb8.73(9)Sb8.15(8)I1.6S20.0(2) nanowires

Nanowires of an iodine containing Pb-Sb-sulfosalt have been synthesized by chemical vapor transport. Their structure was studied using high-resolution transmission electron microscopy and X-ray powder diffraction. The lattice parameters show values equal to a=4.9801(4) nm, b=0.41132(8) nm (with two-fold superstructure), c=2.1989(1) nm and β=99.918(6)°. These parameters and the results of a multislice simulation are in good agreement with the mineral pillaite, Cu_0.10Pb_9.16Sb_9.84S_22.94Cl_1.06O_0.5 (space group C2/m, a=4.949(1) nm, b=0.41259(8) nm, c=2.1828(4) nm, and β=99.62(3)°). Microprobe and EDX analyses yielded a chemical composition of Cu_0.507(5)Pb_8.73(9)Sb_8.15(8)I_1.6S_20.0(2) which is close to natural pillaite but contains no oxygen and iodine instead of chlorine. The structure of the investigated material is based on chains of M-S polyhedra (M=Pb or Sb) typical for the architecture of sulfosalts implying iodine atoms in trigonal prismatic coordination with Pb atoms from the M-S polyhedra of neighboring chains. The [010] superstructure of the specimen was found to be unstable under electron beam irradiation with a rapid decrease of the b lattice parameter from 0.8 to 0.4 nm within 5 min.     

Synthesis and crystal structure of gallosilicate- and aluminogermanate tetrahydroborate sodalites Na8[GaSiO4]6(BH4)2 and Na8[AlGeO4]6(BH4)2

Tetrahydroborate enclathrated sodalites with gallosilicate and aluminogermanate host framework were synthesized under mild hydrothermal conditions and characterized by X-ray powder diffraction and IR spectroscopy. Crystal structures were refined in the space group P-43n from X-ray powder data using the Rietveld method. Na₈[GaSiO₄]₆(BH₄)₂: a=895.90(1) pm, V=0.71909(3)×10⁻⁶ nm³, R_P=0.074, R_B=0.022, Na₈[AlGeO₄]₆(BH₄)₂: a=905.89(2) pm, V=0.74340(6)×10⁻⁶ nm³, R_P=0.082, R_B=0.026. The tetrahedral framework T-atoms are completely ordered in each case and the boron atoms are located at the centre of the sodalite cages. The hydrogen atoms of the enclathrated anions were refined on x, x, x positions, restraining them to boron-hydrogen distances of 116.8 pm as found in NaBD₄.The IR-absorption spectra of the novel phases show the typical bands of the tetrahedral group as found in the spectrum of pure sodium boron hydride.The new sodalites are discussed as interesting -containing model compounds which could release pure hydrogen.     

The Crystal Structures, Microstructure and Ionic Conductivity of Ba2In2O5 and Ba(InxZr1−x)O3−x/2

This paper describes the results of electron microscopy, high-temperature powder neutron diffraction, and impedance spectroscopy studies of brownmillerite-structured Ba₂In₂O₅ and perovskite structured Ba(In_xZr_1−x)O_3−x/2. The ambient temperature structure of Ba₂In₂O₅ is found to adopt Icmm symmetry, with disorder of the tetrahedrally coordinated (In³⁺) ions of the type observed previously in Sr₂Fe₂O₅. Ba₂In₂O₅ undergoes a ∼6-fold increase in its ionic conductivity over the narrow temperature range from ∼1140 K to ∼1230 K, in broad agreement with previous studies. This transition corresponds to a change from the brownmillerite structure to a cubic perovskite arrangement with disordered anions. Electron microscopy investigations showed the presence of extended defects in all the crystals analyzed. Ba(In_xZr_1−x)O_3−x/2 samples with x=0.1 to 0.9 adopt the cubic perovskite structure, with the lattice parameter increasing with x.