Room-temperature synthesis and conductivity of the pyrochlore type Dy2(Ti1−yZry)2O7 (0?y?1) solid solution

Different compositions in a solid solution of general formula Dy₂(Ti_1−yZr_y)₂O₇, showing high oxygen ion conductivity, have been successfully prepared at room temperature via mechanochemical synthesis. Stoichiometric mixtures of the constituent oxides were dry milled together in a planetary ball mill by using zirconia vials and balls. Chemical changes in the powder mixtures as a function of composition and milling time were followed by X-ray diffraction and revealed that, in all cases and after milling for 19 h, the powder mixtures consisted of a single phase. Electrical properties were measured on sintered pellets as a function of frequency, temperature and zirconium content, revealing an increase in conductivity of more than one order of magnitude for y?0.4, which, as observed in the similar Y₂(Ti_1−yZr_y)₂O₇, has been related with the onset of disordering of the anion sublattice. Despite increasing structural disorder with increasing Zr content, conductivity remains almost constant for y>0.6, reaching a maximum value of ∼5×10⁻³ for Dy₂Zr₂O₇ at 900 °C.     

Synthesis and characterization of Zn1−xNixFe2O4 spinels prepared by a citrate precursor

Nanocrystalline single-phase samples of Zn_1−xNi_xFe₂O₄ ferrites (0<x<1) have been obtained via a soft-chemistry method based on citrate-ethylene glycol precursors, at a relatively low temperature (650 °C). The influence of the nickel and zinc contents as well as that of heat treatments were investigated by means of X-ray powder diffraction, Brunauer-Emmett-Teller (BET) surface area, scanning electron microscopy (SEM) and Fourier Transform Infrared (FTIR) Spectroscopy. Higher Ni content increases the surface areas, the largest one (∼20 m²/g) being obtained for NiFe₂O₄ annealed at 650 °C for 15 h. For all compositions, the surface area decreases for prolonged annealing at 650 °C and for higher annealing temperatures. Those results were correlated to the particle size evolution; the smallest particles (∼50 nm) observed in the NiFe₂O₄ sample (650 °C, 15 h) steadily increase as Ni ions were replaced by Zn, reaching ∼100 nm in the ZnFe₂O₄ sample (650 °C, 15 h). For all the Zn_1−xNi_xFe₂O₄ samples and, whatever the heat treatments was, the FTIR spectra show two fundamental absorption bands in the range 650-400 cm⁻¹, characteristics of metal vibrations, without any superstructure stating for cation ordering. The highest ν₁-tetrahedral stretching, observed at ∼615 cm⁻¹ in NiFe₂O₄, shifts towards lower values with increasing Zn, whereas the ν₂-octahedral vibration, observed at 408 cm⁻¹ in NiFe₂O₄, moves towards higher wavenumbers, reaching 453 cm⁻¹ in ZnFe₂O₄.     

On α-β phase transition in cristobalite-type Al1−xGaxPO4 (0.00?x?1.00)

The results of variable temperature powder X-ray diffraction and differential thermal analysis (DTA) studies on the orthorhombic (α) low-cristobalite to cubic (β) high-cristobalite phase transition for Al_1−xGa_xPO₄, (0.00?x?1.00) are presented. These studies reveal that all these compositions undergo reversible phase transitions from orthorhombic to cubic form at higher temperature. The high-temperature behavior of GaPO₄ is observed to have a different behavior compared to all other compositions in this series. Orthorhombic low-cristobalite-type GaPO₄ transforms to cubic high-cristobalite form at ∼605 °C. Above ∼700 °C, the cubic high-cristobalite-type GaPO₄ slowly transforms to trigonal quartz type structure. At about 960 °C, the quartz type GaPO₄ transforms back to the cubic high-cristobalite form. During cooling cycles the cubic phase of GaPO₄ reverts to trigonal quartz type phase. However, annealing of GaPO₄ at higher temperatures for longer duration can stabilize the orthorhombic low cristobalite phase. The phase transition temperatures and associated enthalpies are related to the change in unit cell volume and the orthorhombicity of the respective low cristobalite lattice.     

Structure and phase composition of nanocrystalline Ce1−xLuxO2−y

The microstructure and phase stability of nanocrystalline mixed oxide Lu_xCe_1−xO_2−y (x=0-1) are described. Nano-sized (3-4 nm) oxide particles were prepared by the reverse microemulsion method. Morphological and structural changes upon heat treatment in an oxidizing atmosphere were studied by transmission electron microscopy (TEM), X-ray diffraction (XRD), Raman and Yb³⁺ emission spectroscopy, the latter ion being present as an impurity in the Lu₂O₃ starting material. Up to 950 °C, the samples were single phase, with structure changing smoothly with Lu content from fluorite type (F) to bixbyite type (C). For the samples heated at 1100 °C phase separation into coexisting F- and C-type structures was observed for 0.35<x<0.7. It was also found that addition of Lu strongly hinders the crystallite growth of ceria during heat treatment at 800 and 950 °C.     

Crystal structure and electrical conductivity of lanthanum-calcium chromites-titanates La1−xCaxCr1−yTiyO3−δ (x=0-1, y=0-1)

Five series of perovskite-type compounds in the system La_1−xCa_xCr_1−yTi_yO₃ with the nominal compositions y=0, x=0-0.5; y=0.2, x=0.2-0.8; y=0.5, x=0.5-1.0; y=0.8, x=0.6-1.0 and y=1, x=0.8-1 were synthesized by a ceramic technique in air (final heating 1350 °C). On the basis of the X-ray analysis of the samples with (Ca/Ti)?1, the phase diagram of the CaTiO₃-LaCr^IIIO₃-CaCr^IVO₃ quasi-ternary system was constructed. Extended solid solution with a wide homogeneity range is formed in the quasi-ternary system CaCr^IVO₃-CaTiO₃-LaCr^IIIO₃. The solid solution La_{(1−x′−y)}Ca_(x′+y)Cr^IV_x′Cr^III_{(1−x′−y)}Ti_yO₃ exists by up to 0.6-0.7 mol fractions of CaCr^IVO₃ (x^′<0.6-0.7) at the experimental conditions. The crystal structure of the compounds is orthorhombic in the space group Pbnm at room temperature. The lattice parameters and the average interatomic distances of the samples within the solid solution ranges decrease uniformly with increasing Ca content. Outside the quasi-ternary system, the nominal compositions La_0.1Ca_0.9TiO₃, La_0.2Ca_0.8TiO₃, La_0.4Ca_0.6Cr_0.2Ti_0.8O₃ and La_0.3Ca_0.7Cr_0.2Ti_0.8O₃ in the system La_1−xCa_xCr_1−yTi_yO₃ were found as single phases with an orthorhombic structure. In the temperature range between 850 and 1000 °C, the synthesized single-phase compositions are stable at pO₂=6×10⁻¹⁶-0.21×10⁵ Pa. Oxygen stoichiometry and electrical conductivity of the separate compounds were investigated as functions of temperature and oxygen partial pressure. The chemical stability of these oxides with respect to oxygen release during thermal dissociation decreases with increasing Ca-content. At 900 °C and oxygen partial pressure 1×10⁻¹⁵-0.21×10⁵ Pa, the compounds with x>y (acceptor doped) are p-type semiconductors and those with x<y (donor doped) and x=y are n-type semiconductors. The type and level of electrical conductivity are functions of the concentration ratios of cations occupying the B-sites of the perovskite structures: [Cr³⁺]/[Cr⁴⁺] and [Ti⁴⁺]/[Ti³⁺]. The maximum electrical conductivity at 900 °C and pO₂=10⁻¹⁵ Pa was found for the composition La_0.1Ca_0.9TiO₃ (near 50 S/cm) and in air at 900 °C for La_0.5Ca_0.5CrO₃ (close to 100 S/cm).     

Formation of LiAlyNi1−yO2 solid solutions under high and atmospheric pressure

Three methods were used for the synthesis of LiAl_yNi_1−yO₂ solid solutions with layered crystal structure: citrate and hydroxide precursor methods at atmospheric pressure and high-pressure synthesis in oxygen-rich atmosphere (3 GPa). Structural characterization of the oxides was performed by powder XRD analysis and electron paramagnetic resonance (EPR) spectroscopy. Irrespective of the different preparation techniques used, it was found that LiAl_yNi_1−yO₂ solid solutions can be formed in the limited concentration range of 0?y?0.5 and 0.75?y?1.0. The unit cell parameter a decreases linearly with the Al content whereas the unit cell parameter c increases sharper as compared to the linear interpolation of the c parameter calculated for the two end compositions LiNiO₂ and LiAlO₂. In these compositions, aluminum substitutes for Ni in the NiO₂-layer, the mean Al_yNi_1−y-O bond length decreasing. The extent of the trigonal distortion of Al_yNi_1−yO₆ and LiO₆-octahedra varies with the aluminum content and depends on the synthesis procedure used. The LiO₆-octahedra are more flexible to tolerate the increased trigonal distortion as compared to the Al_yNi_1−yO₆-octahedra. High-pressure synthesis favors the formation of oxides with a higher extent of trigonal distortion of both Al_yNi_1−yO₆ and LiO₆-octahedra. From EPR measurements, it was shown that local cationic distribution in LiAl_yNi_1−yO₂ depends on the synthesis temperature. At atmospheric pressure, higher synthesis temperatures promote the reaction of cation mixing between the layers.     

Composition and structure of acid leached LiMn2−yTiyO4 (0.2≤y≤1.5) spinels

Lithium manganese titanium spinels, LiMn_2−yTi_yO₄, (0.2≤y≤1.5) have been synthesized by solid-state reaction between TiO₂ (anatase), Li₂CO₃ and MnCO₃. Li⁺ was leached from the powdered reaction products by treatment in excess of 0.2 N HCl at 85 °C for 6 h, under reflux. The elemental composition of the acidic solution and solid residues of leaching has been determined by complexometric titration, atomic absorption spectroscopy and X-ray fluorescence analysis. Powder X-ray diffraction was used for structural characterization of the crystalline fraction of the solid residues. It has been found that the amount of Li⁺ leached from LiMn_2−yTi_yO₄ decreases monotonically with increasing y in the interval 0.2≤y≤1.0 and abruptly drops to negligibly small values for y>1.0. The content of Mn and Li in the liquid phase and of Mn and Ti in the solid (amorphous plus crystalline) residue, were related to the composition and cation distribution in the pristine compounds. A new formal chemical equation describing the process of leaching and a mechanism of the structural transformation undergone by the initial solids as a result of Li⁺ removal has been proposed.     

Synthesis and characterization of NixMg1−xAl2O4 nano ceramic pigments via a combustion route

MgAl₂O₄ was successfully used as a crystalline host network for the synthesis of nickel-based nano cyan refractory ceramic pigments. Different compositions of Ni_xMg_1−xAl₂O₄ (0.1 ? x ? 0.8) powders have been prepared by using a low temperature combustion reaction (LTCR) of the corresponding metal nitrates with urea (U) as a fuel at 300 °C in an open air furnace. The as-synthesized samples were characterized by thermal analysis (TG-DTG/DTA), X-ray powder diffraction (XRD), Fourier transform infrared spectroscopy (FTIR) and transmission electron microscopy (TEM). UV-Vis and diffuse reflectance spectroscopy (DRS) using CIE- L^∗a^∗b^∗ parameters methods have been used for color measurements. The results show that the Ni_xMg_1−xAl₂O₄ samples are the crystalline phase with a particle size of 8.85-43.66 nm in the temperature range 500-1200 °C. The density, particle size, shape and color are determined for all the prepared samples with different calcination times and temperatures.     

High-temperature neutron diffraction study of the cation ordered perovskites TbBaMn2O5+x and TbBaMn2O5.5−y

The miscibility of TbBaMn₂O_5+x and TbBaMn₂O_5.5−y has been investigated at 100-600 °C using in situ powder neutron diffraction. No miscibility is observed, and the two phases remain oxygen stoichiometric (x,y=0) at 600 °C. Structure refinement results show that neither material undergoes a phase transition in this temperature range. TbBaMn₂O₅ is Mn²⁺/Mn³⁺ charge ordered and any charge melting transition is >600 °C. This symmetry-broken charge ordering is remarkably robust in comparison to that in other oxides.     

Crystal structure, microstructure and reducibility of LaNixCo1−xO3 and LaFexCo1−xO3 Perovskites (0<x≤0.5)

Nickel and iron substituted LaCoO₃ with rhombohedrally distorted perovskite structure were obtained in the temperature range of 600-900 °C by thermal decomposition of freeze-dried citrates and by the Pechini method. The crystal structure, morphology and defective structure of LaCo_1−xNi_xO₃ and LaCo_1−xFe_xO₃ were characterized by X-ray diffraction and neutron powder diffraction, TEM and SEM analyses and electron paramagnetic resonance spectroscopy. The reducibility was tested by temperature programmed reduction with hydrogen. The products of the partial and complete reduction were determined by ex-situ XRD experiments. The replacement of Co by Ni and Fe led to lattice expansion of the perovskite structure. For perovskites annealed at 900 °C, there was a random Ni, Fe and Co distribution. The morphology of the perovskites does not depend on the Ni and Fe content, nor does it depend on the type of the precursor used. LaCo_1−xNi_xO₃ perovskites (x>0.1) annealed at 900 °C are reduced to Co/Ni transition metal and La₂O₃ via the formation of oxygen deficient Brownmillerite-type compositions. For LaCo_1−xNi_xO₃ annealed at 600 °C, Co/Ni metal, in addition to oxygen-deficient perovskites, was formed as an intermediate product at the initial stage of the reduction. The interaction of LaCo_1−xFe_xO₃ with H₂ occurs by reduction of Co³⁺ to Co²⁺ prior to the Fe³⁺ ions. The reducibility of Fe-substituted perovskites is less sensitive towards the synthesis procedure in comparison with that of Ni substituted perovskites.     

Formation of apatite oxynitrides by the reaction between apatite-type oxide ion conductors, La8+xSr2−x(Si/Ge)6O26+x/2, and ammonia

Following growing interest in the use of ammonia as a fuel in solid oxide fuel cells (SOFCs), we have investigated the possible reaction between the apatite silicate/germanate electrolytes, La_8+xSr_2−x(Si/Ge)₆O_26+x/2, and NH₃ gas. We examine how the composition of the apatite phase affects the reaction with ammonia. For the silicate series, the results showed a small degree of N incorporation at 600 °C, while at higher temperatures (800 °C), substantial N incorporation was observed. For the germanate series, partial decomposition was observed after heating in ammonia at 800 °C, while at the lower temperature (600 °C), significant N incorporation was observed. For both series, the N content in the resulting apatite oxynitride was shown to increase with increasing interstitial oxide ion content (x/2) in the starting oxide. The results suggest that the driving force for the nitridation process is to remove the interstitial anion content, such that for the silicates the total anion (O+N) content in the oxynitrides approximates to 26.0, the value for an anion stoichiometric apatite. For the germanates, lower total anion contents are observed in some cases, consistent with the ability of the germanates to accommodate anion vacancies. The removal of the mobile interstitial oxide ions on nitridation suggests problems with the use of apatite-type electrolytes in SOFCs utilising NH₃ at elevated temperatures.     

Structural and conductivity studies of Y10−xLaxW2O21

The aim of this work was to determine structural parameters of the Y_10−xLa_xW₂O₂₁ (x=0-10) solid solution series and investigate their electric properties. Crystallographic data shows a gradual increase in symmetry with increasing La content, as the structure evolves from orthorhombic, Y₁₀W₂O₂₁, towards the pseudo-cubic structure of Y₅La₅W₂O₂₁. The solubility limit of La₂O₃ was found to be 50% (x=5). Above this level two phases were observed, La₆W₂O₁₅ and (La,Y)_10+xW_2−xO_21−δ. The conductivity of Y rich samples was very low, with σ of the order 2×10⁻⁵-5×10⁻⁵ S cm⁻¹ at 1000 °C, whilst ionic conductivity was observed for most La rich doped samples. The highest conductivity was observed for La₁₀W₂O₂₁ and its doped analogues, at 1×10⁻³-5×10⁻³ S cm⁻¹ at 1000 °C. Unit cell parameters were determined as a function of temperature from 0 to 1000°C, and thermal expansion of these materials was determined from temperature studies carried out at the Australian Synchrotron facility in Melbourne, Victoria, Australia.     

O NMR studies of local structure and phase evolution for materials in the Y2Ti2O7-ZrTiO4 binary system

¹⁷O MAS NMR and XRD studies of precursor-derived Y_1.6Zr_0.4Ti₂O_7.2 and Y_1.2Zr_0.8Ti₂O_7.4 have been performed to investigate the development of local and long-range order in these materials as they evolve from a metastable amorphous state upon heating. Zirconium titanate (ZrTiO₄) was also investigated to help interpret the ¹⁷O NMR spectra of the ternary compositions. Consistent with earlier studies, crystallization was observed at 800 °C to form a fluorite structure and a small amount of rutile; weak broad reflections were also observed which were ascribed to the presence of small pyrochlore-like ordered domains or particles within the fluorite phase. As the temperature was increased further, the sizes of these domains grew along with the concentration of rutile. At the highest temperature studied (1300 °C), the reflections of the thermodynamic phases, pyrochlore and zirconium titanate (ZrTiO₄), dominated the XRD pattern. The ¹⁷O NMR spectra revealed a series of different peaks that were assigned to different 3- and 4-coordinate O local environments. The data were consistent with the formation of a metastable phase Y_2−xZr_xTi_2−yZr_yO_7+x with pyrochlore-like ordering but with Zr substitution on both cation sites of the pyrochlore structure. At low temperatures, doping on the A (Y³⁺) sites predominates (i.e., x>y), consistent with the fact that the pyrochlore develops out of a more disordered fluorite-like, phase. As the temperature is raised, the Zr doping on the A site decreases and the metastable phase at this temperature can now be written as Y_2−x′Zr_x′Ti_2−y′Zr_y′O_7+x′ (i.e., x′<y′); TiO₂ is also observed, consistent with this suggestion. At high temperatures, doping on the B site decreases and the resonances due to the stoichiometric pyrochlore yttrium titanate (Y₂Ti₂O₇) dominate the NMR spectra. Weaker ¹⁷O NMR resonances due zirconium titanate (ZrTiO₄) are also observed.     

Oxygen permeability, thermal expansion and mixed conductivity of GdxCe0.8−xPr0.2O2−δ, x=0, 0.15, 0.2

The non-linear thermal expansion behaviour observed in Ce_1−yPr_yO_2−δ materials can be substantially controlled by Gd substitution. Coulometric titration shows that the charge compensation mechanism changes with increasing x, in the system Gd_xCe_0.8−xPr_0.2O_2−δ. For x=0.15, charge compensation is by vacancy formation and destabilises the presence of Pr⁴⁺. At x=0.2, further Gd substitution is charge compensated by additionally raising the oxidation state of Pr rather than solely the creation of further oxygen ion vacancies. Oxygen concentration cell e.m.f. measurements in an oxygen/air potential gradient show that increasing Gd content decreases ionic and electronic conductivities. Ion transference numbers measured under these conditions show a positive temperature dependence, with typical values t_o=0.90,0.98 and 0.80 for x=0,0.15 and 0.2, respectively, at 950 °C. These observations are discussed in terms of defect association. Oxygen permeation fluxes are limited by both bulk ambipolar conductivity and surface exchange. However, the composition dependent trends in permeability are shown to be dominated by ambipolar conductivities, and limited by the level of electronic conductivity. At the highest temperatures, oxygen permeability of composition x=0.2 approaches that of composition x=0, Ce_0.8Pr_0.2O_2−δ, with specific oxygen permeability values approximately 2×10⁻⁹ mol s⁻¹ cm⁻¹ at 950 °C, but offering much better thermal expansion properties.     

Magnetic and thermoelectric properties of layered LixNayCoO2

The magnetic, thermoelectric, and structural properties of Li_xNa_yCoO₂, prepared by intercalation and deintercalation chemistry from the thermodynamically stable phase Li_0.41Na_0.31CoO₂, which has an alternating Li/Na sequence along the c-axis, are reported. For the high Li-Na content phases Li_0.41Na_0.31CoO₂ and Li_0.40Na_0.43CoO₂, a sudden increase in susceptibility is seen below 50 K, whereas for Li_0.21Na_0.14CoO₂ an antiferromagnetic-like transition is seen at 10 K, in spite of a change from dominantly antiferromagnetic to dominantly ferromagnetic interactions with decreasing alkali content. The Curie constant decreases linearly with decreasing alkali content, at the same time the temperature-independent contribution to the susceptibility increases, indicating that as the Co becomes more oxidized the electronic states become more delocalized. Consistent with this observation, the low alkali containing phases have metallic-like resistivities. The 300 K thermopowers fall between 30 μV/K (x+y=0.31) and 150 μV/K (x+y=0.83).     

Reaction of perfluoro-1-ethylindan with SiO2/SbF5 and skeletal transformations of perfluoro-3-ethylindan-1-one under the action of SbF5 and SiO2/SbF5

Perfluoro-1-ethylindan heated with excess of SiO₂ in an SbF₅ medium at 75 °C and then treated with water, gives 4-carboxy-perfluoro-3-methylisochromen-1-one. Perfluoro-3-ethylindan-1-one is converted, under the action of SbF₅ at 70 °C, to perfluoro-2-(but-2-en-2-yl)benzoic acid as a mixture of E- and Z-isomers. When the reaction temperature is raised to 125 °C, a solution of salts of perfluoro-3,4-dimethyl-1H-isochromen-1-yl and perfluoro-4-ethyl-1H-isochromen-1-yl cations is obtained. Increase in the reaction time lowers the content of a salt of the latter cation in the solution. Hydrolysis of the solution of the salts gives perfluoro-3,4-dimethylisochromen-1-one and perfluoro-4-ethylisochromen-1-one.     

The lanthanoid polyantimonides with the ideal compositions Pr5Sb12 and Nd5Sb12 crystallizing with a new structure type and Ho2Sb5 with Dy2Sb5-type structure

The title compounds were isolated in well-crystallized form from samples with a substantial excess of antimony, annealed at temperatures slightly below the melting point of that element. Their crystal structures were determined from single-crystal diffractometer data. Pr_9-xSb_21-y and Nd_9-xSb_21-y crystallize with a new monoclinic structure type, Pearson symbol mS(62-5.4), space group Cm, Z=2 with a=2859.1(4) pm, b=426.3(1) pm, c=1356.1(2) pm, β=95.52(1)°, R=0.034 for 4351 structure factors and 188 variable parameters for Pr_9-xSb_21-y and a=2845(2) pm, b=424.7(8) pm, c=1345.9(9) pm, β=95.42(7)°, R=0.069 for 2928 F values and 188 variables for Nd_9-xSb_21-y. Of the 30 atomic sites, three show fractional occupancy corresponding to the compositions Pr_8.303(5)Sb_20.03(1) and Nd_8.30(2)Sb_19.98(9), respectively. A model for the order of occupied atomic sites with a tripled b-axis is proposed resulting in the ideal compositions Pr₅Sb₁₂ and Nd₅Sb₁₂. The holmium compound Ho₂Sb₅ has a Dy₂Sb₅-type structure: mP28, P2₁/m, a=1301.8(3) pm, b=414.9(1) pm, c=1451.1(2) pm, β=102.14(1)°, R=0.028 for 2573 F values and 86 variables. In both structure types most rare earth atoms have nine antimony neighbors forming tricapped trigonal prisms. The coordination polyhedra of the antimony atoms show a great variety, with a trigonal prism of rare earth atoms as one extreme case. The other extreme coordination of an antimony atom is a distorted octahedron formed by six antimony atoms. The differences and similarities of both structures are discussed. Chemical bonding within the antimony polyanions is analyzed on the basis of an extended Zintl-Klemm concept using bond-length-bond-strength relationships.     

Local structure in solid solutions of stabilised zirconia with actinide dioxides (UO2, NpO2)

The local structure of (Zr,Lu,U)O_2−x and (Zr,Y,Np)O_2−x solid solutions has been investigated by extended X-ray absorption fine structure (EXAFS). Samples were prepared by mixing reactive (Zr,Lu)O_2−x and (Zr,Y)O_2−x precursor materials with the actinide oxide powders, respectively. Sintering at 1600 °C in Ar/H₂ yields a fluorite structure with U(IV) and Np(IV). As typical for stabilised zirconia the metal-oxygen and metal-metal distances are characteristic for the different metal ions. The bond lengths increase with actinide concentration, whereas highest adaptation to the bulk stabilised zirconia structure was observed for UO and NpO bonds. The ZrO bond shows only a slight increase from 2.14 Å at 6 mol% actinide to 2.18 Å at infinite dilution in UO₂ and NpO₂. The short interatomic distance between Zr and the surrounding oxygen and metal atoms indicate a low relaxation of Zr with respect to the bulk structure, i.e. a strong Pauling behaviour.     

Structural behavior and thermoelectric properties of the brownmillerite system Ca2(ZnxFe2−x)O5

The Ca₂(Zn_xFe_2−x)O₅ series was synthesized and characterized to determine the influence of zinc dopant on the brownmillerite structure for thermoelectric applications. All single-phase compounds exhibited Pnma symmetry at room temperature up to the solubility limit at x=0.10. High-temperature X-ray powder diffraction was used to show that the nature of the Pnma-Imma(0 0 γ)s00 transition in Ca₂Fe₂O₅ is modified by the presence of zinc. While the Zn-free composition transitions to an incommensurate phase, the Zn-containing phases transition instead to a commensurate phase, Imma(0 0 γ)s00 with γ=1/2. Both the Néel temperature and the onset temperature of the Pnma-Imma(0 0 γ)s00 phase transition decreased with increasing zinc concentration. Rietveld analysis of the in situ diffraction pattern for the x=0 sample at 1300 °C demonstrates that the structure contains statistically disordered chain orientations as described by space group Imma. Thermoelectric properties were analyzed in air from 100 to 800 °C. The positive Seebeck coefficient revealed hole-type conduction for all compositions. Doped samples exhibited electrical conductivities up to 3.4 S/cm and thermal conductivity of 1.5 W/mK. Transport analysis revealed thermally activated mobility consistent with polaron conduction behavior for all compositions.     

Phase transition in the Ruddlesden-Popper layered perovskite Li2SrTa2O7

The crystal structure of the Ruddlesden-Popper layered perovskite Li₂SrTa₂O₇ has been characterized at various temperatures between −185 and 300 °C by several techniques: X-ray and neutron powder diffraction, single crystal diffraction, transmission electron microscopy and Raman spectroscopy. The low temperature structure has been confirmed to be orthorhombic Cmcm with a small octahedra antiphase tilting (ΦΦ0) (ΦΦ0) inside the perovskite blocks. With temperature, the tilting progressively vanishes leading around 230 °C to a tetragonal symmetry (S.G. I4/mmm). This reversible phase transition, followed by X-ray and neutron thermodiffraction and thermal Raman measurements, is considered as of second order. An attribution of the Raman bands based on normal mode analysis is proposed.