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Structural properties of rare earth niobates (RE3NbO7, RE = Y, Er, Yb, Lu) have been studied using selected area electron diffraction (SAED) and high resolution transmission electron microscopy (HRTEM). SAED patterns show diffuse scattering features along the [111] direction of the pure fluorite (Fm–3m) structure due to a short-range ordering of anion vacancies in the fluorite structure or the change of coordination number around some cations. The latter is related to the presence of small domains of pyrochlore phase (Fd-3m), which are observed in the HRTEM images.

A layered perovskite related compound with the Sr4CoTaO8 composition was isolated. This material constitutes the n = 1 member of the recently reported [Srn+1(Co0.5Ta0.5)nO3n+1] Ruddlesden–Popper series. XRPD, SAED and HREM reveal that this compound crystallizes as a K2NiF4-type with space group I4/mmm. The structure is formed by one Sr2CoTaO6 perovskite block that intergrows with one SrO rock salt layer; no ordering between the Co and Ta cations was established. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2007)

Two new oxides of the Ruddlesden-Popper series have been isolated and structurally characterized in the Sr-Co-Ta-O system. X-ray and electron diffraction and high-resolution electron microscopy show that polycrystalline Sr(3)CoTaO(7) constitutes the n=2 member of a new Sr(n+1)(CoTa)(n)O(3n+1) homologous series, the essential feature of which is the existence of two connected Co/Ta octahedral layers, separated by Sr atoms. Sr(2)CoTaO(6), the n=infinity member of the series, shows a particular short-range ordering of Co and Ta at the octahedral sites leading, as shown by high-resolution electron microscopy, to the disordered intergrowth of simple and double perovskite type domains. Strategies to stabilize new oxides of this series are discussed.

Polycrystalline Eu2Mo3O12 has been synthesized and characterized by X-ray and electron diffraction, high resolution electron microscopy and magnetic measurements. This oxide shows a complex scheelite superstructure whose essential feature is the existence of isolated Mo tetrahedra. Each Mo atom has four oxygen neighbors at distances ranging from 1.75 to 1.91 A, whereas each Eu atom has eight oxygen neighbors at 2.43 A. The temperature dependence of the inverse magnetic susceptibility shows that Eu2Mo3O12 behaves as a Curie–Weiss paramagnet at high temperature (160–300 K), whereas in the range 100–20 K it shows Van Vleck paramagnetism. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2005)

Two compositions in the La-Ba-Cu-Co-O system − La2Ba3Cu2Co3O13 and La2Ba3Cu3Co2O13 − have been synthesized and characterized by means of SAED (Selected Area Electron Diffraction) and HREM (High Resolution Electron Microscopy). The La2Ba3Cu2Co3O13 phase leads to a fivefold perovskite superstructure along the c axis, which can be described as formed by the recurrent intergrowth of the two alternating blocks ···PP··· and ···POhP··· (P = pyramid and Oh = octahedron). Pyramidal and octahedral sites are randomly occupied by Cu/Co cations, while La/Ba cations exhibit an ordered distribution. The La2Ba3Cu3Co2O13 composition gives rise to a mixture of phases in which a new phase showing a threefold superstructure due to the sequence ···POhP··· coexists with a fivefold superstructure with the same polyhedra sequence (···PPPOhP···) as the previous one, but a disordered arrangement of lanthanum-barium cations. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2003)

Two new members of the one-dimensional (A3NiMnO6)α(A3Mn3O9)β homologous series, with the compositions (Sr0.75Ba0.25)5NiMn3O12 and Sr9Ni2Mn5O21, have been synthesised. Their structures can each be described as a hexagonal array of infinite one-dimensional chains of face-sharing polyhedra, running parallel to the c axis and separated by the Sr/Ba cations. The structure of (Sr0.75Ba0.25)5NiMn3O12, which constitutes the (α = 3, β = 2) member of the series, is made up of three face-sharing octahedra linked by one trigonal prism. For Sr9Ni2Mn5O21 (α = 2, β = 1) the sequence of polyhedra along the chains corresponds to a motif consisting of two octahedra−one trigonal prism−three octahedra−one trigonal prism. The manganese atoms occupy the octahedral sites in both phases, while the Ni2+ cations are distributed in the trigonal-prismatic sites in a disordered way. Only a small fraction of these (close to 20%) is located at the centres of the trigonal prisms, 80% being displaced towards the rectangular faces of the polyhedra and giving rise to a square-planar-like coordination. Both oxides present twinned microstructures, as evidenced by SAED and HREM. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2003)

The (alpha =3, beta =2) member of the (A3ABO6) (A3B3O9) homologous series has been stabilised in the Sr-Rh-O system for a [Sr10(Sr0.5Rh1.5)TP(Rh6)Oh]O24 composition. The structural characterisation has been performed by powder X-ray and electron diffraction measurements and high-resolution electron microscopy. In this structure, three face-sharing [RhO6] octahedra linked by one [Rh/SrO6] trigonal prism comprise the infinite one-dimensional chain that runs parallel to the c axis of a trigonal unit cell (Pc1), with parameters a=9.6411(1) and c=21.2440(4) A.

The α = 3, β = 6 term of the series (A3B2O6)α (A3B3O9)β has been synthesized in polycrystalline form. The structural characterization by powder X-ray diffraction, electron diffraction and high-resolution electron microscopy indicates that this material is isostructural with Ba9Co8O24. In the structure, seven face-sharing octahedra alternate with a trigonal prism defining chains running parallel to the c axis of a rhombohedral unit cell (Rc), with parameters a = 1.00766(4) nm and c = 4.1571(2) nm. The structural refinement shows the presence of a small percentage of Rh vacancies leading to the composition Ba9Rh7.92O24. Pauli paramagnetic behaviour is observed. (© Wiley-VCH Verlag GmbH, 69451 Weinheim, Germany, 2002)

Study of the structural changes occurring during the reduction process of the Sr2RhO4+δ (214), n=1 term of the Ruddlesden and Popper series, shows that for δ

A structural study by means of X-ray and electron diffraction of intermediate oxides between Eu2Mo3O12 and Eu2W3O12 shows the existence of a new Eu2(Mo1−xWx)3O12 solid solution with the scheelite structural type. The essential feature of the x = 1/3 and x = 2/3 compounds is the presence of isolated oxygen tetrahedra, in which, according to X-ray and electron diffraction data, W and Mo are randomly distributed. Eu atoms occupy distorted square antiprisms sites.

The title compound is synthesized by solid state reaction of a stoichiometric mixture of Na3V2O6.5 (obtained by heating a 3:2 mixture of Na2CO3 and V2O5 at 700 °C for 6 h) and MnO (Au tube, 450 °C for 6 h, 600 °C for 12 h, 700 °C for 24 h).

The new title compounds are synthesized by solid state reaction of stoichiometric mixtures of La2O3 or Pr6O11, NH4H2AsO4, and NH4Br using NaBr as a flux (Pt tubes, 900 °C, 72 h).

The title solid solutions are prepared from stoichiometric mixtures of BaCO3, TiO2, and Co3O4 (Pt crucible, 1648 K for 2 d (y = 0.1), 1473 K for 3 d (y = 0.2), 1373 K for 3 d (y = 0.3), 1398 K for 6 d (y = 0.4)).

The new title compound is synthesized by solid state reaction of a stoichiometric mixture of NaVO3 and MnO (Au tube, 500 °C, 120 h and 650 °C, 36 h).

New phases with one-dimensional structures in the A−Co−O system (A = Ca, Sr, Ba) have been isolated and characterized by electron diffraction and high-resolution electron microscopy. (Ca0.4Sr0.6)9Co7O21, Sr14Co11O33, Sr21Co17O51, Ba15Co13O39, and Ba66Co59O177 are members of an homologous series of general formula (A3Co2O6)α(A3Co3O9)β. All of them are constituted by the ordered intergrowth of structural frames of the phases limiting this series: Ca3Co2O6 and 2H−BaCoO3. All of the members of this series are commensurate phases derived by a modulation vector k = (a*2H + b*2H)/3 + m2c*2H, with m being related to the numbers α and β and, consequently, giving directly the A/Co ratio. The ordered succession of blocks can unambiguously be obtained from the HREM images.

Polycrystalline Ba2CoO4 has been synthesized by the ceramic method. X-ray and electron diffraction shows this phase to be monoclinic with a=5.8878(4); b=7.6158(6); c=10.3916(8); β=90.738(2). This compound is isostructural to Ba2TiO4 and the structure has been refined from X-ray powder diffraction data by using the Rietveld method. High resolution images interpreted in comparison to simulated images confirm the structure determined by X-ray diffraction. A crystal chemical study based on the cation arrays allows to establish a new relationship between Ba2CoO4 and 2H BaCoO3. Moreover, the Ba subarray seems to be related to the structure of elemental Ba.

Study of the structural changes occurring during the reduction process of the Sr2RhO4+δ (214), n=1 term of the Ruddlesden and Popper series, shows that for δ

The α = 3, β = 6 term of the series (A3B2O6)α (A3B3O9)β has been synthesized in polycrystalline form. The structural characterization by powder X-ray diffraction, electron diffraction and high-resolution electron microscopy indicates that this material is isostructural with Ba9Co8O24. In the structure, seven face-sharing octahedra alternate with a trigonal prism defining chains running parallel to the c axis of a rhombohedral unit cell (Rc), with parameters a = 1.00766(4) nm and c = 4.1571(2) nm. The structural refinement shows the presence of a small percentage of Rh vacancies leading to the composition Ba9Rh7.92O24. Pauli paramagnetic behaviour is observed. (© Wiley-VCH Verlag GmbH, 69451 Weinheim, Germany, 2002)

The influence of the partial substitution of Sr by Ba in the SrCoO3−y on its structure and properties has been investigated. SrBaCo2O5 is stable at room temperature as a cubic phase with a high oxygen vacancy concentration. This structure is stable up to 800 K. Above this temperature, the phase decomposes into a monodimensional rhombohedral (SrBa)7Co6O18 phase. At 1200 K, a reversible rhombohedral → cubic transformation is observed, the rhombohedral phase remaining stable until room temperature. SrBaCo2O5 shows antiferromagnetic behavior with TN ≈ 525 K. As determined by neutron diffraction data, the magnetic Co spins align ferromagnetically on the (111) planes and these planes are stacked antiferromagnetically in the normal direction (type II antiferromagnet). The temperature dependence of the electrical conductivity indicates SrBaCo2O5 to be a semiconducting p-type material with an activation energy of ∼0.3 eV.

A combination of X-ray, neutron and electron diffraction, and high-resolution electron microscopy have been used to establish the 5H structural type of a new hexagonal-type perovskite BaMn0.2Co0.8O2.80. The structure can be described as a (cc′chh) 5H hexagonal polytype with ordered oxygen vacancies where the cubic c′ layer corresponds to a composition of [BaO2] as opposed to [BaO3]. The resulting layer structure consists of [MnCo2O12] blocks of three sharing faces octahedra linked by corners to two unconnected [CoO4] tetrahedra. Electron Energy Switch order Loss Spectroscopy shows Mn to be present only as Mn(+IV) and therefore Co is present as mixed +III/+IV. Mn(+IV) and Co(+III) ions are distributed over the face sharing octahedral sites whereas Co(+IV) ions are located on the tetrahedral sites. The magnetic behavior is more complex than is observed for BaCoO2.80 (a ferromagnet with Tc = 47 K) and can be described by a Stoner–Wohlfarth model of random-anisotropic, noninteracting monodomain ferromagnetic clusters. The ferromagnetic clusters occur below 35 K and are assigned to groups of Co ions in octahedral and/or tetrahedral sites; however, incorporation of Mn ions in the octahedral sites disrupts the transition into long-range three-dimensional ferromagnetic order. Impedance Spectroscopy data reveals semiconducting grain conductivity at room temperature (1 × 10−2 S cm−1); however, subambient data reveal an unusual temperature dependence with a smooth changeover from a thermally activated process (0.07 eV) in the range 40–300 K to a low-temperature state below 40 K with a near-zero activation energy. The data cannot be described by conventional Arrhenius or variable-range hopping conduction models and the conduction mechanism(s) remain unresolved. Several possible suggestions for the conductivity behavior are made, including Anderson localization, anisotropic conduction associated with the 5H crystal structure or some complex correlated mechanism between the magnetic and electronic transport properties. The electrical microstructure of BaMn0.2Co0.8O2.8 ceramics consist of semiconducting grains and constrictive grain boundaries and therefore exhibit internal barrier layer capacitor (IBLC) behavior, with a high and temperature-stable apparent permittivity of 10 000 (at 10 kHz) above 100 K.

A layered perovskite related compound with the Sr4CoTaO8 composition was isolated. This material constitutes the n = 1 member of the recently reported [Srn+1(Co0.5Ta0.5)nO3n+1] Ruddlesden–Popper series. XRPD, SAED and HREM reveal that this compound crystallizes as a K2NiF4-type with space group I4/mmm. The structure is formed by one Sr2CoTaO6 perovskite block that intergrows with one SrO rock salt layer; no ordering between the Co and Ta cations was established. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2007)

Polycrystalline Eu2Mo3O12 has been synthesized and characterized by X-ray and electron diffraction, high resolution electron microscopy and magnetic measurements. This oxide shows a complex scheelite superstructure whose essential feature is the existence of isolated Mo tetrahedra. Each Mo atom has four oxygen neighbors at distances ranging from 1.75 to 1.91 A, whereas each Eu atom has eight oxygen neighbors at 2.43 A. The temperature dependence of the inverse magnetic susceptibility shows that Eu2Mo3O12 behaves as a Curie–Weiss paramagnet at high temperature (160–300 K), whereas in the range 100–20 K it shows Van Vleck paramagnetism. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2005)

Polycrystalline Sr9Mn5Co2O21 has been synthesized and characterized by X-ray and electron diffraction, high-resolution electron microscopy, and magnetic measurements. This oxide constitutes the α =...

Polycrystalline Sr4Mn2CoO9 has been synthesized and characterized by X-ray and electron diffraction, high-resolution electron microscopy, and magnetic measurements. This oxide constitutes the α = 3, β = 1 term of the homologous series (A3B2O6)α(A3B3O9)β. The essential feature of this structure is the presence of isolated rows of polyhedra sharing faces along the c-axis and formed by the alternation of one trigonal prism occupied by a Co cation and two Mn cations in octahedral coordination; the chains are separated by Sr atoms. The magnetic properties seem to indicate strong antiferromagnetic correlations along the chains, but no three-dimensional (3D) magnetic long range order is observed. The global magnetic behavior indicates a partially ordered system, close to a magnetic spin glass.

The α = 3, β = 1 term of the homologous series (A3A‘BO6)α(A3B3O9)β has been stabilized for the Sr3.3Ca0.7CoRh2O9 composition. The structural characterization by powder X-ray and electron diffraction and high-resolution electron microscopy indicates this material is isostructural to Sr4Ni3O9. In this structure, two face-sharing octahedra alternate to one trigonal prism forming chains of one-dimensional polyhedra running parallel to the c-axis of a trigonal unit cell (P321) of parameters a = 9.5767(7), c = 7.9720(4) A. The temperature dependence of the magnetic susceptibility of the Sr3.3Ca0.7CoRh2O9 is consistent with a weak antiferromagnetic interaction between magnetic cations. At low temperature our data seem to indicate the existence of a partially ordered magnetic state, as deduced from the small value of the magnetic part of the specific heat as well as the low entropy associated with the magnetic correlations.

New forms of Ba-Co oxides with one-dimensional structures related to the 2H hexagonal perovskite have been synthesized and characterized by electron diffraction and high-resolution electron microscopy. All of them exhibit orthorhombic unit cells and can be considered as members of a new homologous series (Ba 8 Co 6 O 18 ) α (Ba8Co8024) β . Their structures can be derived from the hcp stacking sequence of Ba 8 Co 2 O 18 and Ba 8 O 24 layers. Such a description allows very close structural relationships between this orthorhombic series and the rhombohedral (Ba 3 Co 2 O 6 ) α (Ba 3 Co 3 O 9 ) β family, also formed by a hcp stacking of mixed layers, but with different symmetry and stoichiometry, Ba 3 CoO 6 and Ba 3 O 9 layers. The ability of cobalt to be stabilized in all these different types of layers allows phases belonging to both rhombohedral and orthorhombic families to be isolated thus, leading to the presence of polymorphism, for some compositions.