Three-layer Aurivillius phases containing magnetic transition metal cations: Bi2−xSr2+x(Nb,Ta)2+xM1−xO12, M=Ru, Ir, Mn, x≈0.5

We report the synthesis of Aurivillius-type phases incorporating magnetic M⁴⁺ cations (M=Mn, Ru, Ir), based on the substitution of M⁴⁺ for Ti⁴⁺ in Bi₂Sr₂(Nb,Ta)₂TiO₁₂. The key to incorporating these magnetic transition metal cations appears to be the partial substitution of Sr²⁺ for Bi³⁺ in the α-PbO-type layer of the Aurivillius phase, leading to a concomitant decrease in the M⁴⁺ content; i.e., the composition of the prepared compounds was Bi_2−xSr_2+x(Nb,Ta)_2+xM_1−xO₁₂, x≈0.5. These compounds only exist over a narrow range of x, between an apparent minimum (x≈0.4) Sr²⁺ content in the α-PbO-type [Bi₂O₂] layer required for Aurivillius phases to form with magnetic M⁴⁺ cations, and an apparent maximum (x≈0.6) Sr²⁺ substitution in this [Bi₂O₂] layer. Rietveld-refinement of synchrotron X-ray powder diffraction data making use of anomalous dispersion at the Nb and Ru K edges show that the overwhelming majority of the incorporated M cations occupy the central of the three MO₆ octahedral layers in the perovskite-type block. Magnetic susceptibility measurements are presented and discussed in the context of the potential for multiferroic (magnetoelectric) properties in these materials.     

A structural study of the perovskite series Ca1−xNaxTi1−xTaxO3

Compounds in the solid solution series Ca_1−xNa_xTi_1−xTa_xO₃ were synthesized at 1300 °C, followed by annealing at 850 °C or 800 °C with quenching and/or slow cooling to room temperature. Rietveld refinement of their powder X-ray diffraction patterns show that all compounds are single-phase ternary perovskites which adopt the space group Pbnm (a≈b≈√2a_p; c≈2a_p; Z=4) at ambient conditions. The unit cell parameters and cell volumes of the compounds increase regularly with increasing values of x. The coordination of the A-site cations changes throughout the series from eight for CaTiO₃ to nine for NaTaO₃. Compounds with 0?x ?0.4 have A-site cations in eight fold coordination, whereas the coordination of those with 0.4<x<0.9 is ambiguous. Analysis of the crystal chemistry of the compounds shows that the change in coordination at x=0.4 is related to the departure of the B-site cations from the second coordination sphere of the A-site cations, as in compounds with x>0.4 the A-^IIO distances become less than the A-B intercation distances. Contemporaneous with these coordination changes, the tilt angles of the BO₆ polyhedra decrease with increasing values of x. This solid solution series is unusual in that these structural and coordination changes occur regardless that Goldschmidt tolerance factors remain essentially constant at approximately 0.89, and observed tolerance factors, assuming eight fold coordination of the A-site cations, range only from 0.91 to 0.93 (0?x?0.8).     

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.     

Structural aspects of Pr1−xSrxFeO3−w

Structural aspects of the distorted perovskite ABO₃ phase Pr_1−xSr_xFeO_3−w,x=0.00-0.80,w=0.000-0.332, were studied by powder X-ray diffraction, powder neutron diffraction, Mössbauer spectroscopy, and Fe K-, Sr K-, and Pr L_III-edge EXAFS techniques. The diffraction data revealed no indications for ordering of Pr and Sr at the A site, nor for oxygen vacancy ordering at O sites for heavily reduced samples. Mössbauer spectroscopy showed octahedral, square pyramidal, and tetrahedral Fe coordinations with relative amounts closely following the predictions for a binomial distribution of oxygen vacancies. In addition to Fe³⁺ and Fe⁴⁺, also Fe⁵⁺ appears at 77 K for (G-type) antiferromagnetic samples with high average Fe valence. This suggests dynamic 2 Fe⁴⁺↔Fe³⁺+Fe⁵⁺ fluctuations. At 296 K, a mixed valence Fe⁽³⁺ⁿ⁾⁺ component significantly improved the fit of Mössbauer spectra for the most oxidized paramagnetic samples. The qualitative EXAFS study shows that the local environments for Fe, Pr, and Sr strongly depend on x and w. The local Pr- and Sr-site geometries differ significantly from the cubic average structure for Pr_0.50Sr_0.50FeO_2.746.     

Phase segregation in the Gd1−xSrxFeO3−δ series

Gd_1−xSr_xFeO_3−δ ferrites have been studied by means of X-ray powder diffraction in the whole composition range. Single-phase solid solution is found for x<0.09 and for x>0.63. At intermediate Sr content, phase segregation takes place. Compounds with x?0.05 crystallize in the orthorhombic structure, space group Pbnm. Oxygen-deficient Gd_1−xSr_xFeO_3−δ with x?2/3 are cubic or nearly cubic. The oxygen vacancies stabilize the cubic phase for x=2/3 whereas highly oxidized samples show an orthorhombic distortion, which has not been observed earlier. Magnetic and electrical properties have been measured for the single-phase solid solutions. Gd_1−xSr_xFeO_3−δ compounds with x?2/3 order antiferromagnetically below ∼100 K. In the paramagnetic region, their susceptibility follows the Curie-Weiss law in all but SrFeO_2.96 compound. These ferrites show semiconducting behavior in the electrical transport likely related to atomic disorder. We find that the conductivity activation energy becomes larger by increasing either the Gd content or the oxygen vacancies.     

Crystal structures of the double perovskites Ba2Sr1−xCaxWO6

Structures of the double perovskites Ba₂Sr_1−xCa_xWO₆ have been studied by the profile analysis of X-ray diffraction data. The end members, Ba₂SrWO₆ and Ba₂CaWO₆, have the space group I2/m (tilt system a⁰b⁻b⁻) and Fm3¯m (tilt system a⁰a⁰a⁰), respectively. By increasing the Ca concentration, the monoclinic structure transforms to the cubic one via the rhombohedral R3¯ phase (tilt system a⁻a⁻a⁻) instead of the tetragonal I4/m phase (tilt system a⁰a⁰c⁻). This observation supports the idea that the rhombohedral structure is favoured by increasing the covalency of the octahedral cations in Ba₂MM′O₆-type double perovskites, and disagrees with a recent proposal that the formation of the π-bonding, e.g., d⁰-ion, determines the tetragonal symmetry in preference to the rhombohedral one.     

Microstructural and transport properties in substituted Bi2Sr2CaCu2O8+δ-modulated compounds

X-ray powder diffraction and resistivity measurements were performed on Bi₂Sr₂CaCu₂O_8+δ ceramics substituted by Y and Zn for Ca and Cu sites, respectively. X-ray diffraction patterns show an incommensurate modulated structure along the b-axis. The structural refinements were carried out using the four-dimensional space group Bbmb(0β1)0 0 0. From the X-ray peak profiles analysis, an anisotropic line-shape broadening was observed. The use of the “Williamson and Hall” method allows distinguishing the origin of broadening as mainly due to microstrains. A large transition from a metallic to semiconductor behaviour is observed on the resistivity curves at x≈0.4 for Bi₂Sr₂Ca_1−xY_xCu₂O_8+δ and at x^′≈0.36 for Bi₂Sr₂Ca_1−xY_xCu_1.94Zn_0.06O_8+δ, which can be also correlated to the defects. Oppositely to the metallic behaviour, which satisfies the Mathiessen's rule, the semiconducting one can be modelled by a variable range hopping process.     

The nature of the orthorhombic to tetragonal phase transition in Sr1−xCaxMnO3

The orthorhombic-tetragonal phase transition in the perovskite series Sr_1−xCa_xMnO₃ 0.4?x?0.6 has been studied by synchrotron X-ray powder diffraction. At room temperature the Ca rich oxides x?0.45 have the orthorhombic Pbnm superstructure whereas Sr_0.6Ca_0.4MnO₃ is two phases with both tetragonal I4/mcm and orthorhombic Pbnm. Analysis of the octahedral tilts suggest the co-existence of these two phases is a consequence of a first-order I4/mcm to Pbnm transition. The evolution of the structure of Sr_0.5Ca_0.5MnO₃ with temperature is also described and this is found to evolve from orthorhombic to tetragonal and ultimately cubic.     

Crystal structure of lanthanum bismuth silicate Bi2−xLaxSiO5 (x∼0.1)

A melting and glass recrystallization route was carried out to stabilize a new tetragonal form of Bi₂SiO₅ with bismuth partially substituted by lanthanum. The crystal structure of Bi_2−xLa_xSiO₅ (x∼0.1) was determined from powder X-ray and neutron diffraction data (space group I4/mmm, , c=15.227(1) Å, V=224.18 Å³, Z=2; reliability factors: R_Bragg=5.65%, R_p=14.6%, R_wp=16.8%, R_exp=8.3%, χ²=8.3 (X-ray) and R_Bragg=2.40%, R_p=8.1%, R_wp=7.5%, R_exp=4.2%, χ²=3.3 (neutrons); 11 structural parameters refined).The main effect of lanthanum substitution is to introduce, by removing randomly some bismuth 6s² lone pairs, a structural disorder in the surroundings of (Bi₂O₂)²⁺ layers, that is in the (SiO₃)²⁻ pyroxene files arrangement. It results in a symmetry increase relatively to the parent compound Bi₂SiO₅, which is orthorhombic. The two structures are compared.     

Subsolidus phase equilibria and properties in the system Bi2O3:Mn2O3±x:Nb2O5

Subsolidus phase relations have been determined for the Bi-Mn-Nb-O system in air (750-900 °C). Phases containing Mn²⁺, Mn³⁺, and Mn⁴⁺ were all observed. Ternary compound formation was limited to pyrochlore (A₂B₂O₆O′), which formed a substantial solid solution region at Bi-deficient stoichiometries (relative to Bi₂(Mn,Nb)₂O₇) suggesting that ≈14-30% of the A-sites are occupied by Mn (likely Mn²⁺). X-ray powder diffraction data confirmed that all Bi-Mn-Nb-O pyrochlores form with structural displacements, as found for the analogous pyrochlores with Mn replaced by Zn, Fe, or Co. A structural refinement of the pyrochlore 0.4000:0.3000:0.3000 Bi₂O₃:Mn₂O_3±x:Nb₂O₅ using neutron powder diffraction data is reported with the A and O′ atoms displaced (0.36 and 0.33 Å, respectively) from ideal positions to 96g sites, and with Mn²⁺ on A-sites and Mn³⁺ on B-sites (Bi_1.6Mn²⁺_0.4(Mn³⁺_0.8Nb_1.2)O₇, (?227), a=10.478(1) Å); evidence of A or O′ vacancies was not found. The displacive disorder is crystallographically analogous to that reported for Bi_1.5Zn_0.92Nb_1.5O_6.92, which has a similar concentration of small B-type ions on the A-sites. EELS spectra for this pyrochlore were consistent with an Mn oxidation between 2+ and 3+. Bi-Mn-Nb-O pyrochlores exhibited overall paramagnetic behavior with negative Curie-Weiss temperature intercepts, slight superparamagnetic effects, and depressed observed moments compared to high-spin, spin-only values. At 300 K and 1 MHz the relative dielectric permittivity of Bi_1.600Mn_1.200Nb_1.200O₇ was ≈128 with tan δ=0.05; however, at lower frequencies the sample was conductive which is consistent with the presence of mixed-valent Mn. Low-temperature dielectric relaxation such as that observed for Bi_1.5Zn_0.92Nb_1.5O_6.92 and other bismuth-based pyrochlores was not observed. Bi-Mn-Nb-O pyrochlores were readily obtained as single crystals and also as textured thin films using pulsed laser deposition.     

Thermoelectric properties of polycrystalline La1−xSrxCoO3

Thermoelectric properties of polycrystalline La_1−xSr_xCoO₃, where Sr²⁺ is substituted in La³⁺ site in perovskite-type LaCoO₃, have been investigated. Sr-doping increases the electrical conductivity (σ) of La_1−xSr_xCoO₃, and also decreases the Seebeck coefficient (S) for 0.01?x?0.40. A Hall coefficient measurement reveals that the increase in electrical conductivity arises from increases in both carrier concentration and the Hall mobility. The decrease in the Seebeck coefficient is caused by a decrease in carrier effective mass as well as increase in carrier concentration. The highest power factor (σS²) is 3.7×10⁻⁴ W m⁻¹ K⁻² at 250 K for x=0.10. The thermal conductivity (κ) is about 2 W m⁻¹ K⁻¹ at 300 K for 0?x?0.04, and increases for x?0.05 because of an increase in heat transport by conductive carrier. The thermoelectric properties of La_1−xSr_xCoO₃ are improved by Sr-doping, and the figure of merit (Z=σS² κ⁻¹) reaches 1.6×10⁻⁴ K⁻¹ for x=0.06 at 300 K (ZT=0.048). For heavily Sr-doped samples, the thermoelectric properties diminish mainly because of the decrease in the Seebeck coefficient and the increase in thermal conductivity.     

Oxygen-ion and electron conductivity in Sr2(Fe1−xGax)2O5

High-temperature electrical conductivity measurements, structural data from powder X-ray diffraction and ⁵⁷Fe Mössbauer spectroscopy were combined to study the interrelationship of oxygen ion transport and p- and n-type transport in Sr₂(Fe_1−xGa_x)₂O₅, where x=0, 0.1 and 0.2. Although gallium substitution generally decreases the total ion-electron transport, the transition of the orthorhombic brownmillerite structure to a cubic phase on heating results in the recurrence of the conductivity to the same high level as in the parent ferrite (x=0). The changes of the partial contributions to the total conductivity as a function of x are shown to reflect a complicated interplay of the disordering processes that develop in the oxygen sublattice on heating in response to replacement of iron with gallium.     

Crystal structure of Bi1−xTbxFeO3 from high-resolution neutron diffraction

Samples of Bi_1−xTb_xFeO₃, with x=0.05, 0.10, 0.15, 0.20 and 0.25, have been synthesised by solid state reaction. The crystal structures of the perovskite phases, characterised via Rietveld analysis of high resolution powder neutron diffraction data, reveal a structural transition from the R3c symmetry of the parent phase BiFeO₃ to orthorhombic Pnma symmetry, which is complete for x=0.20. The x=0.10 and 0.15 samples are bi-phasic. The transition from a rhombohedral to orthorhombic unit cell is suggested to be driven by the dilution of the stereochemistry of the Bi³⁺ lone pair at the A-site. The G-type antiferromagnetic spin structure, the size of the ordered magnetic moment (∼3.8 μ_B) and the T_N (∼375 °C) are relatively insensitive to increasing Tb concentrations at the A-site.     

Structural and magnetic characterisation of Aurivillius material Bi2Sr2Nb2.5Fe0.5O12

The n=3 Aurivillius material Bi₂Sr₂Nb_2.5Fe_0.5O₁₂ is investigated and combined structural refinements using neutron powder diffraction (NPD) and X-ray powder diffraction data (XRPD) data reveal that the material adopts a disordered, tetragonal (I4/mmm) structure at temperatures down to 2 K. Significant ordering of Fe³⁺ and Nb⁵⁺ over the two B sites is observed and possible driving forces for this ordering are discussed. Some disorder of Sr²⁺ and Bi³⁺ over the M and A sites is found and is consistent with relieving strain due to size mismatch. Highly anisotropic thermal parameters for some oxygen sites suggest that the local structure may be slightly distorted with some rotation of the octahedra. Magnetic measurements show that the material behaves as a Curie-Weiss paramagnet in the temperature range studied with no evidence of any long-range magnetic interactions. Solid solutions including Bi_3−xSr_xNb₂FeO₁₂, Bi₂Sr_2−xLa_xNb₂FeO₁₂ and Bi₂Sr₂Nb_3−xFe_xO₁₂ were investigated but single-phase materials were only successfully synthesised for a narrow composition range in the Bi₂Sr₂Nb_3−xFe_xO₁₂ system.     

Composition- and temperature-dependent phase transitions in 1:3 ordered perovskites Ba4−xSrxNaSb3O12

A series of 25 members of the 1:3 ordered perovskite family of the type Ba_4−xSr_xNaSb₃O₁₂ has been synthesized and their structures determined using synchrotron X-ray and neutron powder diffraction techniques. At room temperature the sample Ba₄NaSb₃O₁₂ has a cubic structure in space group with a=8.2821(1) Å, where the Na and Sb cations are ordered in the octahedral sites but there is no tilting of the (Na/Sb)O₆ octahedra. As the average size of the A-site cation decreases, through the progressive replacement of Ba by Sr, tilting of the octahedra is introduced firstly lowering the symmetry to tetragonal in P4/mnc then to orthorhombic in Cmca and ultimately a monoclinic structure in P2₁/n as seen for Sr₄NaSb₃O₁₂ with a=8.0960(2) Å, b=8.0926(2) Å, c=8.1003(1) Å and β=90.016(2)°. The powder neutron diffraction studies show that the orthorhombic and tetragonal phases in Cmca and P4/mnc co-exist at room temperature for samples with x between 1.5 and 2.     

Orthorhombic superstructures within the rare earth strontium-doped cobaltate perovskites: Ln1−xSrxCoO3−δ (Ln=Y, Dy-Yb; 0.750?x?0.875)

A combination of electron, synchrotron X-ray and neutron powder diffraction reveals a new orthorhombic structure type within the Sr-doped rare earth perovskite cobaltates Ln_1−xSr_xCoO_3−δ (Ln=Y³⁺, Dy³⁺, Ho³⁺, Er³⁺, Tm³⁺and Yb³⁺). Electron diffraction shows a C-centred cell based on a 2√2a_p×4a_p×4√2a_p superstructure of the basic perovskite unit. Not all of these very weak satellite reflections are evident in the synchrotron X-ray and neutron powder diffraction data and the average structure of each member of this series could only be refined based on Cmma symmetry and a 2√2a_p×4a_p×2√2a_p cell. The nature of structural and magnetic ordering in these phases relies on both oxygen vacancy and cation distribution. A small range of solid solution exists where this orthorhombic structure type is observed, centred roughly around the compositions Ln_0.2Sr_0.8CoO_3−δ. In the case of Yb³⁺ the pure orthorhombic phase was only observed for 0.850?x?0.875. Tetragonal (I4/mmm; 2a_p×2a_p×4a_p) superstructures were observed for compositions having higher or lower Sr-doping levels, or for compounds with rare earth ions larger than Dy³⁺. These orthorhombic phases show mixed valence (3+/4+) cobalt oxidation states between 3.2+ and 3.3+. DC magnetic susceptibility measurements show an additional magnetic transition for these orthorhombic phases compared to the associated tetragonal compounds with critical temperatures > 330 K.     

A new 2212-type stair like structure: Bi14Sr21Fe12O61, m=5 member of the generic [Bi2Sr3Fe2O9]m[Bi4Sr6Fe2O16] family

A new oxide, Bi₁₄Sr₂₁Fe₁₂O_61, with a layered structure derived from the 2212 modulated type structure Bi₂Sr₃Fe₂O₉, was isolated. It crystallizes in the I2 space group, with the following parameters: a=16.58(3) Å, b=5.496(1) Å, c=35.27(2) Å and β=90.62°. The single crystal X-ray structure determination, coupled with electron microscopy, shows that this ferrite is the m=5 member of the [Bi₂Sr₃Fe₂O₉]_m[Bi₄Sr₆Fe₂O₁₆] collapsed family. This new collapsed structure can be described as slices of 2212 structure of five bismuth polyhedra thick along , shifted with respect to each other and interconnected by means of [Bi₄Sr₆Fe₂O₁₆] slices. The latter are the place of numerous defects like iron or strontium for bismuth substitution; they can be correlated to intergrowth defects with other members of the family.     

Extension of the “1201” Family to Strontium-Rich Chromite and Ferrite, Bi0.4Sr2.5Cr1.1O4.9 and Bi0.4Sr2.5Fe1.1O5

Two new Sr-rich “1201”-type oxides, Bi_0.4Sr_2.5Cr_1.1O_4.9 and Bi_0.4Sr_2.5Fe_1.1O₅ have been synthesized. These compounds, intergrowths of double rock-salt layers with single perovskite layers, show a 1:1 ordering between (Bi,M) and Sr species within the intermediate rock-salt layer [Bi_0.4M_0.1Sr_0.5O]. The XANES study shows that bismuth is mainly trivalent, whereas iron is mixed valent containing 50% Fe³⁺ and 50% Fe⁴⁺ (also confirmed by Mössbauer), and chromium could be a mixture of Cr³⁺ and Cr⁶⁺ sitting in the perovskite and rock-salt-type sites, respectively. Both compounds exhibit antiferromagnetic interactions. The Cr-phase is a strong insulator, whereas the Fe-phase exhibits a semi-conductor-like resistivity whose value at room temperature is close to that of isotypic cobaltite.     

Crystal structure and magnetic properties of the solid-solution phase Ca3Co2−vScvO6

The two crystallographically non-equivalent Co atoms of the quasi-one-dimensional crystal structure of Ca₃Co₂O₆ form chains with alternating, face-sharing polyhedra of Co2O₆ trigonal prisms and Co1O₆ octahedra. This compound forms a substitutional solid-solution phase with Sc, in which the Sc atoms enter the Co2 sublattice exclusively. The homogeneity range of Ca₃Co_2−vSc_vO₆ (more specifically Ca₃Co1Co2_1−vSc_vO₆) extends up to v≈0.55. The crystal structure belongs to space group R3¯c with lattice parameters (in hexagonal setting): 9.0846(3)?a?9.1300(2) Å and 10.3885(4)?c?10.4677(4) Å. The magnetic moment decreases rapidly with increasing amount of the non-magnetic Sc solute in the lattice.     

Preparation and characterization of nanosize nickel-substituted cobalt ferrites (Co1−xNixFe2O4)

Nanosize nickel-substituted cobalt ferrites were prepared using aerosol route and characterized by TEM, XRD, magnetic and Mössbauer spectroscopy. The particle size of as obtained samples was found to be ∼10 nm which increases upto ∼80 nm on annealing at 1200 °C. The unit cell parameter ‘a’ decreases linearly with the nickel concentration due to smaller ionic radius of nickel. The saturation magnetization for all the samples after annealing at 1200 °C lies in the range 47.6-84.5 emu/g. Room temperature Mössbauer spectra of as obtained samples exhibit a broad doublet, suggesting super paramagnetic nature of the sample. The broad doublet is further resolved into two doublets corresponding to the iron atoms residing at the surface and internal regions of the particle. The samples annealed at 1200 °C showed broad sextet, which is resolved into two sextets, corresponding to tetrahedrally and octahedrally coordinated Fe cations. Cation distribution calculated using XRD and Mössbauer data indicates a decrease in Fe³⁺(oct.)/Fe³⁺(tet.) ratio with increasing nickel concentration.