A primitive tetragonal intermediate in the orthorhombic-cubic phase transition of perovskite-type strontium niobate Sr0.92NbO3

The Sr deficient perovskite Sr_0.92NbO₃ was synthesized from Sr₅Nb₄O₁₅ and Nb and its crystal structure was determined using powder neutron diffraction. At room temperature the structure is orthorhombic in space group Pnma with both in-phase and out-of-phase tilting of the NbO₆ octahedra. High temperature measurements have shown that the oxide undergoes a sequence of phase transitions with increasing temperature: Pnma→P4/mbm→Pm3¯m. The intermediate tetragonal phase has only in-phase tilts of the NbO₆ octahedra, rather than the out-of-phase tilts present in the more commonly observed I4/mcm structure, due to initial softening at the M point rather than R point. The tetragonal phase exists only over a very narrow temperature range. The importance of M-M and M-O bonding in controlling the transition temperatures in SrMO₃ perovskites is discussed.     

Structures and phase diagram for the system CaTiO3-La2/3TiO3

High-resolution neutron and synchrotron X-ray powder diffraction have been used to examine various compositions across the system (1−x)CaTiO₃−xLa_2/3TiO₃. The structures at room temperature were determined according to composition: in Pbnm for 0?x?0.5, Ibmm for 0.5<x<0.7, then I4/mcm for 0.7?x<0.9, and finally in Cmmm for x?0.9. Although the four structures are the same as those proposed previously in an X-ray diffraction study, the phase boundaries are somewhat different, in particular the Pbnm↔Ibmm phase boundary has been extended from x?0.4 to x>0.5 in the current study based on our high-resolution neutron diffraction data. From in situ measurements to identify structures above room temperature, an approximate composition-temperature phase diagram has been constructed, involving four temperature-induced phase transitions: Pbnm→I4/mcm, Ibmm→I4/mcm, I4/mcm→Pm3¯m and Cmmm→P4/mmm.     

Crystal structure and magnetic properties of high-oxygen pressure annealed Sr1−xLaxCo0.5Fe0.5O3−δ (0?x?0.5)

Structural and magnetic studies are presented for the perovskite type Sr_1−xLa_xCo_0.5Fe_0.5O_3−δ (0?x?0.5) materials annealed under moderately high-oxygen pressures of ∼200 atm. A detailed analysis of the room temperature neutron time-of-flight diffraction data reveals that the crystal structure of the sample SrCo_0.5Fe_0.5O_2.89(1), previously described as vacancy-disordered cubic, is similar to the formerly reported, oxygen-vacancy ordered Sr₈Fe₈O₂₃ compound, i.e. Sr₈Co₄Fe₄O₂₃ is tetragonal with the I4/mmm symmetry. With an increase of the La content the studied materials become nearly oxygen stoichiometric and a lowering of the crystal symmetry is observed from cubic (x=0.1 and 0.2) to tetragonal I4/mcm (x=0.3 and 0.4), and finally to monoclinic I12/c1 (x=0.5). Low-temperature structural and magnetic measurements show a ferromagnetic ordering with the maximum Curie temperature near 290 K at x=0.2.     

CeAlO3 and Ce1−xRxAlO3 (R=La, Nd) solid solutions: Crystal structure, thermal expansion and phase transitions

The crystal structures of CeAlO₃ and the solid solutions Ce_1−xR_xAlO₃ (R=La, Nd), and their thermal behaviour in a wide temperature range of 12−1200 K have been precisely determined by means of in situ high-resolution X-ray powder diffraction technique applying synchrotron radiation, thermal analysis and magnetic measurements. The unique sequence of the reversible phase transitions I4/mcm↔Imma↔R3¯c↔Pm3¯m has been detected in CeAlO₃ and solid solutions formed in the pseudo-binary system CeAlO₃-LaAlO₃. In the Ce_1−xNd_xAlO₃ system, the samples with x=0.3 and 0.5 compositions undergo three phase transformations I2/m↔Imma↔R3¯c↔Pm3¯m, whereas for the Ce-rich sample Ce_0.9Nd_0.1AlO₃ four successive transitions are observed: I4/mcm↔I2/m↔Imma↔R3¯c↔Pm3¯m. Crystal structure parameters of all structural polymorphs of CeAlO₃ and solid solutions based on them as well as their thermal evolution are reported. Based on in situ powder diffraction and DTA/DSC data, the phase diagrams of the pseudo-binary systems CeAlO₃-LaAlO₃ and CeAlO₃-NdAlO₃ are constructed together with a combined phase diagram, where the transition temperatures are presented as a function of the average radius of rare-earth cations.     

Composition-induced structural phase transitions in the (Ba1−xLax)2In2O5+x (0?x?0.6) system

Composition-induced structural phase changes across the high temperature, fast oxide ion conducting (Ba_1−xLa_x)₂In₂O_5+x, 0?x?0.6, system have been carefully analysed using hard mode infrared (IR) powder absorption spectroscopy, X-ray powder diffraction and electron diffraction. An orthorhombic brownmillerite to three-dimensionally disordered cubic perovskite phase transition in this system is signalled by a drastic change in slope of both wavenumber and average line widths of IR spectra as a function of composition. Some evidence is found for the existence of an intermediate tetragonal phase (previously reported to exist from electron diffraction data) around x∼0.2. The new spectroscopic data have been used to compare microscopic and macroscopic strain parameters arising from variation in composition. The strain and spectroscopic data are consistent with first-order character for the tetragonal→orthorhombic transition, while the cubic→tetragonal transition could be continuous. Differences between the variation with composition of spectral parameters and of macroscopic strain parameters are consistent with a substantial order/disorder component for the transitions. There is also evidence for precursor effects within the cubic structure before symmetry is broken.     

p(O2)-stability of LaFe1−xNixO3−δ solid solutions at 1100 °C

LaFe_1−xNi_xO_3−δ (x=0.1−1.0) perovskites were synthesized via citrate route. The p(O₂)-stability of the perovskite phases LaFe_1−xNi_xO_3−δ has been evaluated at 1100 °C based on the results of XRD analysis of powder samples annealed at various p(O₂) and quenched to room temperature. The isothermal LaFeO_3−δ-“LaNiO_3−δ” cross-section of the phase diagram of the La-Fe-Ni-O system has been proposed in the range of oxygen partial pressure −15<log p(O₂)/atm≤0.68. The unit cell parameters of orthorhombic perovskites O-LaFe_1−xNi_xO_3−δ increase with decrease in p(O₂) at fixed composition x. This behavior is explained on the basis of size factor. The decomposition temperatures of rhombohedral phases R-LaFe_1−xNi_xO_3−δ for x=0.7, 0.8, 0.9 and 1.0 in air were determined as 1137, 1086, 1060 and 995 °C, respectively.     

Structure and magnetic order in the series BixRE1−xFe0.5Mn0.5O3 (RE=La,Nd)

The influence of Bi³⁺ on the structural and magnetic properties of the rare-earth-containing perovskites REFe_0.5Mn_0.5O₃ (RE=La,Nd) was studied, and the limit of bismuth substitution was determined to be x≤0.5 in Bi_xRE_1−xFe_0.5Mn_0.5O_3+δ (RE=La,Nd) at ambient pressure. Crystal structures in both La and Nd series were determined to be GdFeO₃-type Pnma with the exception of the Bi_0.3La_0.7Fe_0.5Mn_0.5O₃ sample, which is monoclinic I2/a in the a⁻b⁻b⁻ tilt scheme. The samples undergo a transition to G-type antiferromagnetic order along with a weak ferromagnetic component, mixed with cluster-glass type behavior. The substitution of bismuth into the lattice results in a drop in T_N relative to the lanthanide end-members. Long range ordering temperatures T_N in the range 240-255 K were observed, with a significantly lower ordered magnetic moment in the case of lanthanum (M∼1.7-1.9 μ_B) than in the case of neodymium (M∼2.1 μ_B).     

Phase transitions in A-site substituted perovskite compounds: The (Ca1-2xNaxLax)TiO3 (0?x?0.5) solid solution

The (Ca_1-2xNa_xLa_x)TiO₃ (0?x?0.5) A-site substituted perovskite compounds have been synthesized and characterized by XRD and Raman spectroscopy at room temperature. The XRD powder diffraction study suggests that the end-member Na_1/2La_1/2TiO₃ crystallizes in the tetragonal space group I4/mcm. The phase transition from Pbnm to I4/mcm is located between x=0.34 and 0.39 and is driven by the variation of ionic radii at the A-site. The observed Raman modes are in agreement with group theory analysis, and the relationships between the behavior of structural parameters (e.g. Ti-O-Ti bond angle), indicated by long-range order, and the corresponding Raman frequency shifts and intensity evolution, indicated by short-range order, are established and discussed in terms of the radius effect and the mass effect.     

LaxSr2−xFeyRu1−yO4±δ: a new family of K2NiF4 type oxides

The phases La_xSr_2−xFe_yRu_1−yO_4±δ (x=0.2-0.8; y=0.6-0.9) have been synthesized by solid-state techniques and yield tetragonal structures with I4/mmm symmetry. The oxygen stoichiometry and high-temperature structures have been examined using diffraction techniques and in situ Mössbauer spectroscopy at temperatures up to ∼600°C. Furthermore, new reduced phases that adopt structures with Immm symmetry have been discovered. Unusual coordination numbers have been determined for the most highly reduced samples with square planar coordination evident for the B site cations. The reduced orthorhombic Immm phases were found to readily reoxidize in air to the tetragonal I4/mmm structure at relatively low temperatures of only ∼500°C.     

Electrical conduction nature and phase transition in CaTi1−x FexO3−δ (x = 0.1–0.5)

Transport numbers for oxygen ions and protons are measured by an emf method in the system CaTi_1?x Fe_xO_3?δ (x = 0.1–0.5) in the oxidizing and reducing atmospheres in the temperature interval 973–1173 K. It is shown that the compounds under study are mixed ion-electron conductors at small iron concentrations and electron conductors, at large iron contents. The proton conductivity in the compounds is very poor and does not exceed 0.5% in air. On the basis of the temperature dependences of transport numbers for ions and linear expansion, it is established that the CaTi_0.9Fe_0.1O_3?δ system has a phase transition of a second order in a reducing environment at 1020–1050 K. The total and partial electron conductivities of CaTi_0.9Fe_0.1O_3?δ are studied as a function of the partial pressure of oxygen at 1173 K. The nature of electroconduction in CaTi_1?x Fe_xO_3?δ is discussed.     

Neutron diffraction study of the crystal structure and structural phase transition of La0.7Ca0.3−xSrxCrO3 (0≤x≤0.3): The relationship between thermodynamic behavior and crystal structure changes at the phase transition

The crystal structure of the La_0.7Ca_0.3−xSr_xCrO₃ series, including the compositional and temperature dependence of the structural parameters, has been studied by variable temperature neutron diffraction measurements. The extent of the distortions from the ideal cubic perovskite structure has been evaluated quantitatively using the average bond lengths and the mean volumes of the [CrO₆] octahedron and [(La/Ca/Sr)O₁₂] polyhedron, and has been shown to decrease with increase of Sr content or temperature. At the structural phase transition from the orthorhombic (Pnma) structure to the rhombohedral one, the volume of the [CrO₆] octahedron decreases whereas that of the [(La/Ca/Sr)O₁₂] polyhedron shows little difference, resulting in an overall decrease in the level of distortion. The change in the degree of distortion at the phase transition decreases with increase of Sr content, in agreement with the smaller variation of the enthalpy and volume for the specimens with higher Sr content.     

Solid solubility and phase transitions in the system LaNb1−xTaxo4

The solid solubility between LaNbO₄ and LaTaO₄ was investigated by X-ray diffraction, and a two-phase region was observed in the composition region LaNb_1−xTa_xO₄ where 0.4?x?0.8. Single-phase LaNb_1−xTa_xO₄ (0?x?0.4) with the monoclinic Fergusonite structure at ambient temperature, was observed to transform to a tetragonal Scheelite structure by in-situ high-temperature X-ray diffraction, and the phase transition temperature was shown to increase with increasing Ta-content. This ferroelastic to paraelastic second-order phase transition was described by Landau theory using spontaneous strain as an order parameter. The thermal expansion of LaNb_1−xTa_xO₄ (0?x0.4) was shown to be significantly higher below the phase transition than above. Single-phase LaNb_1−xTa_xO₄ (0.8?x?1) with another monoclinic crystal structure at ambient temperature was shown to transform to an orthorhombic crystal structure by X-ray diffraction and differential scanning calorimetry. The phase transition temperature was observed to decrease with decreasing Ta-content. Finally, orthorhombic LaTaO₄ could also be transformed to monoclinic LaTaO₄ at ambient temperature by applying a uniaxial pressure of 150-170 MPa, reflecting the lower molar volume of monoclinic LaTaO₄.     

Microdomain texture and microstructures of Fe-containing CaTi0.4Fe0.6O3−δ

A study of the structure of Fe^IV CaTi_0.4Fe_0.6O_3−δ is presented and compared to data on the Fe^III counterparts. The powder XRD pattern was dominated by a simple cubic perovskite unit cell; however, some small peaks indicated an orthorhombic distortion. All peaks could be indexed using a space group analogous to the Fe^III phase Ca₃TiFe₂O₈. From HRTEM the strong cubic peaks are well explained by the superposition of three equivalent and mutually perpendicular orthorhombic unit cells. TEM analysis further revealed a microdomain structure consisting of disordered intergrowths of CaTiO₃- and Ca₃TiFe₂O₈-like phases. Mössbauer spectra show that ca. 4% of the Fe cations are in the 4+ oxidation state. Results suggest that the Fe⁴⁺ cations are associated with octahedral coordination and hence are associated with the CaTiO₃-like regions, transition regions between the CaTiO₃- and Ca₃TiFe₂O₈ intergrown phases and the domain boundaries. Structural models for the intergrowths are proposed based on HRTEM image simulations.     

Sol-gel synthesized semiconducting LaCo0.8Fe0.2O3-based powder for thick film NH3 gas sensor

Perovskite type LaCo_xFe_1−xO₃ nanoparticles was synthesized by a sol-gel citrate method. The structural, electrical and sensing characteristics of the LaCo_xFe_1−xO₃ system were investigated. The structural characteristics were performed by using X-ray diffraction (XRD) and transmission electron microscopy (TEM) to examine the phase and morphology of the resultant powder. The XRD pattern shows nanocrystalline solid solution of LaCo_xFe_1−xO₃ with perovskite phase. Electrical properties of synthesized nanoparticles are studied by DC conductivity measurement. The sensor shows high response towards ammonia gas in spite of other reducing gases when x = 0.8. The effect of 0.3 wt.% Pd-doped LaCo_0.8Fe_0.2O₃ on the response and a recovery time was also addressed.     

Subsolidus phase relations and crystal structures of R-Ca-Cu-O (R=Nd, Sm, Gd, Tm) systems

The subsolidus phase relations of R₂O₃-CaO-CuO ternary systems (R=Nd, Sm, Gd, Tm) have been investigated by X-ray powder diffraction. All samples were synthesized at about 950° in air. There exists a ternary compound Ca_14−xR_xCu₂₄O₄₁ (x = 4 for R=Nd, Gd and x = 5 for R = Sm) and a ternary solid solution Ca_2+xR_2−xCu₅O₁₀ (R=Nd, Sm, Gd, Tm) with a wide composition range Δx of about 0.6. The compound Ca_14−xR_xCu₂₄O₄₁ possesses a layered orthorhombic structure and is isostructural to Sr_14−xCa_xCu₂₄O₄₁. The lattice parameters a and c of the compound are basically independent of the ionic radius of R, while the lattice parameter b and unit-cell volume V decrease substantially with the decrease of the ionic radii of R. The Ca_2+xR_2−xCu₅O₁₀ solid solution is isostructural to Ca_2+xY_2−xCu₅O₁₀, the structure of which is based on an orthorhombic “NaCuO₂-type” subcell containing infinite one-dimensional chains of edge-shared square planar cuprate groups crosslinked by the layered cations Ca and R that locate in the inter-chain tunnels.     

Preparation and magnetic properties of Fe-Nb co-doped SnO2

Fe³⁺-Nb⁵⁺ co-doped SnO₂ was prepared at 1200 °C by a simple chemical co-precipitation method. The Sn_1−2xFe_xNb_xO₂ solid solutions kept cassiterite structure in the range of 0<x?0.33, and their cell parameters decrease with increasing x. While x=0.40, a second phase with orthorhombic FeNbO₄ structure co-exists with the cassiterite phase, and the second phase becomes dominant while x?0.45. The magnetic measurements indicated that low doping ratio sample (x=0.03) exhibits paramagnetic behavior. A paramagnetic-to-antiferromagnetic phase transition was observed for the samples with higher doping ratio (x?0.15).     

Structural characterization, magnetic behavior and high-resolution EELS study of new perovskites Sr2Ru2−xCoxO6−δ (0.5?x?1.5)

New oxides of general formula Sr₂Ru_2−xCo_xO_6−δ (0.5?x?1.5) have been synthesized as polycrystalline materials and characterized structurally by X-ray diffraction. For 0.5?x<0.67 the orthorhombic, Pnma, perovskite structure of the end member, SrRuO₃, is found. At x=0.67 a phase separation into an Ru-rich Pnma phase and a Co-rich I2/c phase occurs. The I2/c form is also found for x=1.0 but another orthorhombic phase, Imma, obtains for x=1.33 and 1.5. Reductive weight losses indicate negligible oxygen non-stoichiometry, i.e., δ∼0, for all compositions even those rich in Co. High-resolution electron energy loss spectroscopy (EELS) indicates that cobalt is high-spin Co³⁺ or high-spin Co⁴⁺ for all x. Appropriate combinations of Ru⁴⁺, Ru⁵⁺, HS Co³⁺ and HS Co⁴⁺ are proposed for each x which are consistent with the observed Ru(Co)-O distances. Significant amounts of Co⁴⁺ must be present for large x values to explain the short observed distances. Broad maxima in the d.c. susceptibilities are found between 78 and 97 K with increasing x, along with zero-field-cooled (ZFC) and field-cooled (FC) divergences suggesting glassy magnetic freezing. A feature near 155 K for all samples indicates a residual amount of ferromagnetic SrRuO₃ not detected by X-ray diffraction.     

Enzyme mimics of spinel-type CoxNi1−xFe2O4 magnetic nanomaterial for eletroctrocatalytic oxidation of hydrogen peroxide

A series of spinel-type Co_xNi_1−xFe₂O₄ (x = 0, 0.2, 0.4, 0.5, 0.6, 0.8, 1.0) magnetic nanomaterials were solvothermally synthesized as enzyme mimics for the eletroctrocatalytic oxidation of H₂O₂. X-ray diffraction and scanning electron microscope were employed to characterize the composition, structure and morphology of the material. The electrochemical properties of spinel-type Co_xNi_1−xFe₂O₄ with different (Co/Ni) molar ratio toward H₂O₂ oxidation were investigated, and the results demonstrated that Co_0.5Ni_0.5Fe₂O₄ modified carbon paste electrode (Co_0.5Ni_0.5Fe₂O₄/CPE) possessed the best electrocatalytic activity for H₂O₂ oxidation. Under optimum conditions, the calibration curve for H₂O₂ determination on Co_0.5Ni_0.5Fe₂O₄/CPE was linear in a wide range of 1.0 × 10⁻⁸–1.0 × 10⁻³ M with low detection limit of 3.0 × 10⁻⁹ M (S/N = 3). The proposed Co_0.5Ni_0.5Fe₂O₄/CPE was also applied to the determination of H₂O₂ in commercial toothpastes with satisfactory results, indicating that Co_xNi_1−xFe₂O₄ is a promising hydrogen peroxidase mimics for the detection of H₂O₂.     

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.     

Evolution of structure and magnetic properties in electron-doped double perovskites, Sr2−xLaxMnWO6 (0?x?1)

Powder neutron and X-ray diffraction studies show that the double perovskites in the region 0?x?1 exhibit two crystallographic modifications at room temperature: monoclinic P2₁/n and tetragonal I4/m, with a boundary at 0.75<x<0.9. Magnetic susceptibility measurements indicate that for x=0 and 0.5 Sr_2−xLa_xMnWO₆ orders antiferromagnetically (AFM) at 15 and 25 K, respectively, for 0.75?x<1.0, a contribution of weak ferromagnetism (FM), probably due to canted-AFM order, increases with increasing x. The end point compound SrLaMnWO₆ shows the strongest FM cluster effect; however, no clear evidence of magnetic order is discernable down to 4.2 K. X-ray absorption spectroscopy (XAS) confirms Mn²⁺ and mixed-valent W^6+/5+ formal oxidation states in Sr_2−xLa_xMnWO₆.