Refinements of the six-layer structures of the R-type hexagonal ferrites BaTi2Fe4O11 and BaSn2Fe4O11

The structures of BaTi₂Fe₄O₁₁ and BaSn₂Fe₄O₁₁ have been determined from neutron powder diffraction data collected at 300 K using the Rietveld profile refinement. The compounds were found to be isostructural, space group $\text{P6}{}_3\text{mmc}$. BaTi₂Fe₄O₁₁: a = 5.8470(2) Å, c = 13.6116(9) Å, V = 403.01(5) ų, M = 632.6, Z = 2, D_calc. = 3.09 Mg m^?3, final R-factor = 3.77. BaSn₂Fe₄O₁₁: a = 5.9624(5) Å, c = 13.7468(14) Å, V = 423.23(10) ų, M = 774.2, Z = 2. D_calc. = 3.66 Mg m^?3, final R-factor = 2.41. The structure consists of h-stacked BaO₃ and O₄ layers in the ratio 1:2. The BaO₃ layers contain a mirror plane. Between the O₄ layers three octahedral sites are occupied, and between the BaO₃ and O₄ layers an octahedral site and a tetrahedral site are occupied. Because of the mirror plane in the BaO₃ plane the latter sites both share faces in the BaO₃ plane. The octahedral sites are occupied by Fe and Ti or Sn, the pair of tetrahedral sites is occupied by one Fe atom. This Fe atom may hop between these two tetrahedral sites. The structure is considered to be constructed by two R-blocks of the BaFe₁₂O₁₉ (M) structure. Unit-cell dimensions are given of a number of isostructural compounds of general formula A^IIB^IV₂C^III₃O₁₁. Mössbauer experiments on some of these compounds were focused on the tetrahedral positions that show an unusual quadrupole splitting. A brief review is given of the observed magnetic properties of some compounds with the R-structure.     

Structure of ten-layer orthorhombic Ba5Fe5O14 (BaFeO2.8) determined from single crystal X-ray diffraction

The structure of the mixed-valent Ba₅Fe₅O₁₄ (BaFeO_2.8), prepared using a molten KOH-Ba(OH)₂ flux, has been determined using single crystal X-ray diffraction. Ba₅Fe₅O₁₄ forms twinned crystals with the orthorhombic space group Cmcm, a=5.7615(8), b=9.9792(14) and c=24.347(3) Å, Z=4. The structure, which is closely related to the 10H BaFeO_2.65 perovskite, is composed of two oxygen-deficient BaO₂ layers and eight BaO₃ layers with a (hchhc)₂ stacking sequence, where h and c denote hexagonal or cubic layers. A displacement of barium and oxygen atoms in the BaO₂ layers from hexagonal special positions lowers the symmetry from hexagonal to orthorhombic. This combination of stacking and vacancies creates trimers of face-sharing FeO₆ octahedra pillared by dimers of corner-sharing FeO₄ tetrahedra. The Fe⁴⁺ atoms are located in the center of the trimer and in the tetrahedral sites. The magnetism of Ba₅Fe₅O₁₄, investigated using SQUID magnetometry, is characteristic of a strongly coupled antiferromagnet.     

New Ba5M5−xPtxClO13 (M=Fe, Co) oxychlorides with layered perovskite-related structure

Crystals of Ba₅Fe_5−xPt_xClO₁₃ and Ba₅Co_5−yPt_yClO₁₃ were prepared for x=1.31, 1.51, 1.57, 1.59 and y=0, 0.97 and 1.08 in a BaCl₂ flux and investigated by X-ray diffraction methods. These compounds adopt a 10H perovskite structure built from the stacking of BaO₃ and BaOCl layers in the sequence (BaO₃)₄(BaOCl) with space group P6₃/mmc. The cation sites within the trimeric unit of face-sharing octahedra are occupied by Co or Fe and Pt ions, while the tetrahedral sites formed between BaO₃ and BaOCl layers are only occupied by Fe or Co. Moreover, oxygen stoichiometry indicates an average oxidation state for Co and Fe higher than +III, indicating the stabilization of Co⁴⁺ and Fe⁴⁺.     

Local structure and hyperfine interactions of Fe and Sn atoms in brownmillerite-like ferrite Sr2Fe1.98Sn0.02O5+x

Mössbauer spectroscopy has been applied for studying local environment of ⁵⁷Fe and ¹¹⁹Sn probe atoms within tin-doped Sr₂Fe_1.98Sn_0.02O_5+x (x?0.02) ferrite with the brownmillerite-type structure. ⁵⁷Fe Mössbauer spectra indicate no appreciable local distortions induced by the tin dopant atoms. The ¹¹⁹Sn spectra recorded below the magnetic ordering temperature (T_N) can be described as a superposition of two Zeeman sextets, which indicate that Sn⁴⁺ dopant ions are located in two non-equivalent crystallographic and magnetic sites. The observed hyperfine parameters were discussed supposing Sn⁴⁺ cations to replace iron cations in the octahedral (Sn_O) and tetrahedral (Sn_T) sublattices. It has been supposed that Sn⁴⁺ cations being stabilized in the tetrahedral sublattice complete their nearest anion surrounding up to the octahedral oxygen coordination “Sn_T⁴⁺”. Annealing of the Sr₂Fe_1.98Sn_0.02O_5+x in helium flux conditions at 950°C leads to formation of divalent Sn²⁺ cations with a simultaneous decrease of the contribution for the Sn_T⁴⁺ sub-spectrum. The parameters of combined electric and magnetic hyperfine interactions of the ¹¹⁹Sn²⁺ sub-spectrum underline that impurity atoms are stabilized in the sp³d-hybrid state in the oxygen distorted tetragonal pyramid. The analysis of the ¹¹⁹Sn spectra indicates a chemical reversibility of the processes Sn_T²⁺?Sn_T⁴⁺ within the tetrahedral sublattice of the brownmillerite-type ferrite.     

Détermination de la structure cristalline du ferrite de baryum Ba2Fe6O11

The crystal structure of dibarium triferrite Ba₂Fe₆O₁₁ has been solved by direct methods, using intensity data collected by means of an automated diffractometer (MoKα radiation) and corrected for absorption. It crystallizes in the orthorhombic space group Pnnm: a = 23.024(10)Å, b = 5.181(3) Å, c = 8.900(4) Å, Z = 4. Program MULTAN was successfully used for locating Ba²⁺ and most of the Fe³⁺ ions. The structure was further refined by conventional Fourier and least-squares methods (full-matrix program) to a final R value of 0.045 for 1448 observed reflections. Fe³⁺ ions occur in both octahedral (FeO mean distance: 2.02 Å) and tetrahedral (FeO mean distance: 1.865 Å) coordination. Two types of Ba²⁺ ions are found, with six and seven neighboring oxygen atoms. The structure consists of sheets of edge-shared FeO₆ octahedra which are connected by means of corner-shared tetrahedra.     

Layered perovskite-related ruthenium oxychlorides: crystal structure of two new compounds Ba5Ru2Cl2O9 and Ba6Ru3Cl2O12

Single crystals of the title compounds were prepared using a BaCl₂ flux and investigated by X-ray diffraction methods using MoKα radiation and a charge coupled device (CCD) detector. The crystal structures of these two new compounds were solved and refined in the hexagonal symmetry with space group P6₃/mmc, a=5.851(1) Å, c=25.009(5) Å, ρ_cal=4.94 g cm⁻³, Z=2 to a final R₁=0.069 for 20 parameters with 312 reflections for Ba₅Ru₂Cl₂O₉ and space group , a=5.815(1) Å, c=14.915(3) Å, ρ_cal=5.28 g cm⁻³, Z=1 to a final R₁=0.039 for 24 parameters with 300 reflections for Ba₆Ru₃Cl₂O₁₂. The structure of Ba₅Ru₂Cl₂O₉ is formed by the periodic stacking along [001] of three hexagonal close-packed BaO₃ layers separated by a double layer of composition Ba₂Cl₂. The BaO₃ stacking creates binuclear face-sharing octahedra units Ru₂O₉ containing Ru(V). The structure of Ba₆Ru₃Cl₂O₁₂ is built up by the periodic stacking along [001] of four hexagonal close-packed BaO₃ layers separated by a double layer of composition Ba₂Cl₂. The ruthenium ions with a mean oxidation degree +4.67 occupy the octahedral interstices formed by the four layers hexagonal perovskite slab and then constitute isolated trinuclear Ru₃O₁₂ units. These two new oxychlorides belong to the family of compounds formulated as [Ba₂Cl₂][Ba_n+1Ru_nO_3n+3], where n represents the thickness of the octahedral string in hexagonal perovskite slabs.     

Structural Characterization of Divalent Magnesium-Dopedα-Fe2O3

The defect structure of divalent magnesium-dopedα-Fe₂O₃has been examined by Rietveld structure refinement of the X-ray powder diffraction data. The results show that the Mg²⁺ions occupy the vacant interstitial octahedral sites as well as substituting on the two adjacent octahedral Fe³⁺sites in the corundum-relatedα-Fe₂O₃structure. The structure therefore involves a linear cluster of three Mg²⁺ions replacing two Fe³⁺ions. Interatomic potential calculations indicate that this is the most energetically favorable defect cluster for the system.     

Preparation of the ordered perovskite-like compounds Ba4M3LiO12 (M = Ta,Nb): A powder neutron diffraction determination of the structure of Ba4Ta3LiO12

The hexagonal ordered perovskite-like compounds Ba₄Ta₃LiO₁₂ and Ba₄Nb₃LiO₁₂ have been prepared. The structure of Ba₄Ta₃LiO₁₂ has been determined by profile analysis of a powder neutron diffraction pattern. The structure is based on an eight layer (ccch) stacking sequence of the BaO₃ layers (space group $\text{P6}{}_3\text{mmc}$) with the lithium atoms confined to the face-shared octahedral sites. The relationship of this structure to that of Ba₅Ta₄O₁₅ is discussed.     

Nanocrystalline ferrites NixZn1−xFe2O4: Influence of cation distribution on acidic and gas sensing properties

This work is devoted to a detailed analysis of the interconnection between composition, cation distribution and acidic properties of the surface of nanocrystalline ferrites Ni_xZn_1−xFe₂O₄ obtained by aerosol pyrolysis. The detailed analysis of the Mössbauer spectra allows us to determine the distribution of cations between tetrahedral and octahedral positions in spinel structure. Depending on samples composition, the tetrahedral positions can be occupied by only Fe³⁺ cations (inverse spinel, x≥0.4) or by Fe³⁺ and Zn²⁺ cations (mixed spinel, x=0, 0.2). Increasing the nickel concentration in the ferrite leads to decrease in the number of strong acid centers on the surface. It was found that the decrease in the contribution of strong surface acid sites leads to an increase in sensory sensitivity of the ferrite towards ammonia. For ethanol detection an inverse relationship between sensor signal and surface acidity was observed.     

Unusual valences and fast electron exchange in MoFe2O4

The distribution of d electrons over the cations in MoFe₂O₄, which is represented by the formal valence assignment, is shown to be complicated by the equilibrium reactionsFe²⁺_B+Fe³⁺_A+Mo³⁺Fe³⁺_B+Fe²⁺_A+Mo⁴⁺We have used thermal treatment to confirm that the Mo are primarily on octahedral sites; Fe_A[Mo_BFe_B]O₄. K-shell absorption and Mössbauer data at T = 423 K > T_c demonstrate that the iron has an average valence near 2.5+ with fast electron transfer (τ_h < 10⁻⁸ sec) on both octahedral and tetrahedral sites. Paramagnetic susceptibility data give a Curie constant C_M = 7.95 ± 0.2 emu/mole and a Weiss constant θ_p = −445 K; magnetometer measurements confirm a compensation point near 160 K. Transport data give a surprisingly high electronic conductivity, but also give an activated mobility similar to that found in AlFe₂O₄ and CrFe₂O₄ where mixed Fe^3+/2+ valences on both A and B sites have been demonstrated. However, a positive Seebeck coefficient and a preexponential factor one order of magnitude higher in MoFe₂O₄ point to involvement of a fraction of the Mo atoms in electronic transport, which would be consistent with the observation of a τ_h < 10⁻⁸ sec on the A sites of a spinel. An energy diagram consistent with these data and other information about the relative redox potentials of these ions in oxides are proposed for this system.     

Thermodynamic properties and cation distribution of the ZnFe2O4Fe3O4 spinel solid solutions at 900°C

The thermodynamic properties of the Fe₃O₄ZnFe₂O₄ spinel solid solution were determined at 900°C by the use of the solid electrolyte galvanic cell Fe₂O₃ + Fe₃O₄|O^2?|Fe₂O₃ + Zn_xFe_3?xO₄The activity values obtained exhibit slight negative deviation from the ideal solution model. An analysis of the free energy of mixing of the spinel solid solution provided information on the distribution of cations between the tetrahedral and octahedral sites of the spinel lattice. This is the basis for the estimation of the free energy of formation of pure zinc ferrite from oxides. ΔG⁰_ZnFe2O4 = ?2740 ? 1.6 T cal mole^?1     

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.     

Sodium “stuffed” Ba2−xSrxFe4O8 ferrites: new cationic conductors

Sodium insertion in the tetrahedral layer structure of the ferrites Ba_2−xSr_xFe₄O₈ was performed by solid state reaction at 1220 K in air. Superstoichiometric oxides with the actual formula (Ba_2−xSr_x)_1−y/4Na_yFe₄O₈—y0.56; 0.60Ba/Sr1.67—were characterized by X-ray and neutron powder diffraction. The hexagonal unit-cell volume shows an increasing dependence on the sodium insertion when the Ba/Sr ratio reaches the largest values. The marked expansion of the c parameter is the likely signature of the location of the inserted sodium cations within the interlayer space. One-half of the sodium cations partly sits on the Sr(Ba) sites in octahedral coordination and the other half occupies extra octahedral and tetrahedral sites. ac conductivity measurements point to a cationic conductivity whose thermally activated regime—E_a 0.7 eV—evidenced from 570 K, is unsensitive to the sodium content. The bottleneck of the 2D sodium mobility regards the crossing of the oxygen triangular faces shared by the different polyhedra within the interlayer space.     

New transparent conductors with the M7O12 ordered oxygen-deficient fluorite structure: from In4Sn3O12 to In5.5Sb1.5O12

A new transparent conductor, containing pentavalent antimony, In_4+xSn_3−2xSb_xO₁₂, has been synthesized for 0?x?1.5. The latter exhibits an ordered oxygen-deficient fluorite structure with an ordered distribution of Sb⁵⁺ and In³⁺/Sn⁴⁺ species in the octahedral and seven-fold coordinated sites, respectively. More importantly, it is shown that the electronic conductivity of this transparent conducting oxide (TCO) at room temperature, is one order of magnitude larger for x=1 (In₅SnSbO₁₂) than for x=0 (In₄Sn₃O₁₂) and it turns to a semi-metallic behavior in contrast to In₄Sn₃O₁₂ which is a semi-conductor. The potential of this new material, as TCO, is also shown by its reflectance spectra, similar to In₄Sn₃O₁₂, involving only a small increase of the optical bandgap, by 0.15 eV.     

Determination des structures de deux ferrites mixtes nouveaux de formule BaLa2Fe2O7 et SrTb2Fe2O7

The structures of the two new ferrites BaLa₂Fe₂O₇ and SrTb₂Fe₂O₇ with tetragonal symmetry have been resolved by X-ray and neutron diffraction performed on powder samples. In both compounds the arrangement of atoms present a close resemblance with the idealized Sr₃Ti₂O₇ structure. It consists of a packing along the c axis of two different blocks. One is formed by the adjunction of two perovskite cells and the other one by a halved rocksalt Sr(Ba)O cell. The iron cation lattice is built by infinite double layers perpendicular to the c axis with the shortest Fe³⁺Fe³⁺ distances inside the double layers much shorter than between the layers. In BaLa₂Fe₂O₇, Ba²⁺ is located on a regular 12-coordinated site and La³⁺ in a regular 9-coordinated polyhedron. Fe³⁺ is surrounded by five oxygen neighbours at 1.98 Å, building a rather regular tetragonal pyramid, with a sixth oxygen at 2.25 Å. In SrTb₂Fe₂O₇, Sr has only eight close neighbours at ～2.80 Å and four more distant at 3.15 Å. Tb³⁺ has seven close neighbours, six building a distorted octahedron with the largest triangular face capped by the seventh oxygen. Fe³⁺ again has five neighbours, but due to the lowering of the symmetry, the square pyramid has become a distorted trigonal bipyramid.     

Electronic Spectrum and Antiferromagnetic A-B Interactions between Tetrahedral 3d and 3d or 3d Octahedral Cations in Oxidic Lithium Spinels

The compounds chosen to illustrate the interpretation of ligand field spectra of inorganic solids with A-B antiferromagnetic coupling between Fe³⁺ tetrahedral and Fe³⁺ or Cr³⁺ octahedral cations belong to the Li_0.5Fe_xGa_2.5-xO₄ and Li_0.5(FeCr)_xGa_2.5-2xO₄ systems. New features, such as the interpretation of the iron(III) electronic spectrum in ferrimagnetic spinels, the influence of the nature of the superexchange interactions of the pair excitation processes, and the growth of an electronic transition assigned to Cr³⁺ + Fe³⁺ → Cr⁴⁺ + Fe²⁺ intervalence charge transfer at 1.8 eV are reported in this study.     

Diffraction and DAFS Studies of a Ba4(BaxPt1−x)Pt2O9Twinned Crystal, a Member of the Bap(BaxPt1−x)Ptp−2O3p−3Series

Crystallographic studies of the Ba–Pt–O system have been undertaken using X-ray and electron diffraction techniques. The system is described by means of a Ba_p(Ba_xPt²⁺_1−x)Pt⁴⁺_p−2O_3p−3formula which corresponds to a BaO₃hexagonal based framework with Pt chains, whereprepresents the oxygen deficiency and the presence of both Pt⁴⁺and Pt²⁺cations in the compounds, andxa possible substitution of Pt²⁺by Ba²⁺in trigonal prismatic sites. The structure of a Ba₄(Ba_0.04Pt²⁺_0.96)Pt⁴⁺₂O₉crystal has been solved by using 5548 X-ray difraction reflections collected on a twinned crystal. Refinements were performed with two distinct models: an “average”P321 space group and an “orthorhombic”C2 space group with cell parametersa=17.460(4) Å,b=10.085(2) Å,c=8.614(3) Å. In this structure, two Pt⁴⁺and one Pt²⁺cations are distributed over four Ba planes and form chains along thecaxis, consisting of two face-sharing Pt⁴⁺O₆octahedra connected with one Pt²⁺O₆trigonal prism. A lattice misfit occurs between the rigid barium lattice and the PtO₃chains, giving rise to a composite structure. Twinning and domain configurations are described and taken into account in the refinement. This twinning is related to the presence of Pt²⁺cations, whose positions break the threefold axis symmetry. A diffraction anomalous fine structure (DAFS) study was also performed on this twinned single crystal. Anomalous scattering factorsf′ andf″ for platinum in this crystal were refined near the L_IIIPt absorption edge. They confirm the weak barium occupancy of the trigonal prismataic site and the Pt⁴⁺valence of the octahedral sites. Reflection overlaps, due to twinning, flatten the DAFS sensitivity to Pt atoms in the prismatic sites and did not allow their clear valence determination, but Pt–O bond lengths agree with the presence of Pt²⁺cations at the center of prismatic faces. Electron diffraction patterns of powders having slightly different composition show a continuous evolution of incommensurate Bragg peaks and a weak correlation between the PtO₃chains. They also confirm the composite nature and the one-dimensionality of the Ba_p(Ba_xPt²⁺_1−x)Pt⁴⁺_p−2O_3p−3series, which can produce highly anisotropic physical properties.     

Combinatorial Optimization of Ba/Fe-cordierite Solid Solution (Ba0.05Fe0.1Mg)2Al4Si5O18 for High Infrared Radiance Materials

A combinatorial chemistry method was employed to screen the Ba²⁺ and Fe³⁺ incorporated into cordierite structure (Ba_0.05Fe_0.1Mg)₂Al₄Si₅O₁₈ for exploring of high infrared radiance materials. A series of square-type sample array that consists of 7×7 compositions was syn-thesized by ink-jetting nitrate solutions into micro-reactor wells in a ceramic plate and then heat-treated at high temperatures. X-ray diffraction and infrared thermograph were used to analyze the effects of Ba²⁺/Fe³⁺ incorporating on the lattice distortion of cordierite and resultant changes in infrared radiance properties. Based on the results of X-ray phase analy-sis and radiance measurement of the scale-up prepared samples, the optimal Ba²⁺ and Fe³⁺ co-adding amount was determined to be 5%Ba²⁺ plus 10%Fe³⁺. Moreover, the infrared emissivity of the optimal composition at 100 oC was found to be higher than 0.8 in a wide wavelength range of 5-24 μm. The research work demonstrates a promising application of Ba²⁺/Fe³⁺ cordierite solid solution as a kind of infrared heating materials for energy saving.     

Cation ordering in the fluorite-like transparent conductors In4+xSn3−2xSbxO12 and In6TeO12

The cation ordering in the fluorite-like transparent conductors In_4+xSn_3−2xSb_xO₁₂ and In₆TeO₁₂, was investigated by Time of Flight Neutron Powder Diffraction and X-ray Powder Diffraction (tellurate). The structural results including atomic positions, cation distributions, metal-oxygen distances and metal-oxygen-metal angles point to a progressive cation ordering on both sites of the Tb₇O₁₂-type structure with a strong preference of the smaller 4d¹⁰ cations (Sn⁴⁺, Sb⁵⁺, Te⁶⁺) for the octahedral sites. The corresponding increase of the overall structure-bonding anisotropy is analyzed in terms of the crystal chemical properties of the OM₄ tetrahedral network of the antistructure. The relationships between the M₇O₁₂ and the M₂O₃ bixbyite-type structures are explored. Within the whole series of compositions In_4+xM_3−xO₁₂ (M=Sn, Sb, Te) there exists an increase of the symmetry gap between the more symmetrical bixbyite structure and the M₇O₁₂ type. This is tentatively correlated with the progressive weakening of thermal stability of these compositions from Sn to Te via Sb.