Core level photoemission spectroscopy and chemical bonding in Sr2Ta2O7

Electronic parameters of constituent element core levels of strontium pyrotantalate (Sr₂Ta₂O₇) were measured with X-ray photoelectron spectroscopy (XPS). The Sr₂Ta₂O₇ powder sample was synthesized using standard solid state method. The valence electron transfer on the formation of the Sr–O and Ta–O bonds was characterized by the binding energy differences between the O 1s and cation core levels, Δ(O–Sr) = BE(O 1s) − BE(Sr 3d_5/2) and Δ(O–Ta) = BE(O 1s) − BE(Ta 4f_7/2). The chemical bonding effects were considered on the basis of our XPS results for Sr₂Ta₂O₇ and earlier published structural and XPS data for other Sr- and Ta-containing oxide compounds. The new data point for Sr₂Ta₂O₇ is consistent with the previously derived relationship for a set of Sr-bearing oxides. The binding energy difference Δ(O–Sr) was found to decrease with increasing bond distance L(Sr–O).     

Structure distortion and magnetism in double perovskites Ca2−xLaxFeReO6 (0≤x≤0.8)

The crystal structure and magnetism of Ca_2−xLa_xFeReO₆ (0≤x≤0.8) double perovskites have been investigated. The samples with low doping (x≤0.4) are found to crystallize with the monoclinic P2₁/n superstructure, while those in the high doping ones (x≥0.6) have orthorhombic Pbnm superstructure. With the increase of an La doping, the anti-site defects increases, giving rise to highly disordered samples at the Fe and Re positions. At the low doping region (x≤0.4), the compounds undergo a simultaneous structural and magnetic transition accompanying a slight increase of the Curie temperature. The increase of Curie temperature is discussed in terms of the structural change with doping.     

Ferroelectric and dielectric properties of bismuth-layered structural Sr2Bi4−xLnxTi5O18 (Ln=La, Nd, Sm and Dy) ceramics

La, Nd, Sm, and Dy-doped Sr₂Bi₄Ti₅O₁₈ (SBTi) ceramic samples have been prepared by the solid-state reaction method. The X-ray diffraction reveals that all of the ceramic samples are single phase compounds. Their remnant polarization (2P_r) increases at first, and then decreases with the increase of doping content. When doping content is 0.01, Sm and Dy-doped SBTi samples exhibit the maximum 2P_r of 18.2 and 20.1 μC/cm², respectively. While La and Nd-doped SBTi samples display the maximum 2P_r value of 18.4 and 19.1 μC/cm² with doping content of 0.05 and 0.10, respectively. The ferroelectric properties of Sr₂Bi_4−xLn_xTi₅O₁₈ are found to be dominated by the competition of the decrease of oxygen vacancy concentration and the relief of structural distortion.     

Analysis of structural and magnetic phase transition behaviors of La1−xSrxCrO3 by measurement of heat capacity with thermal relaxation technique

The heat capacity, C_p, of the La_1−xSr_xCrO₃ system and its temperature dependence have been measured by a thermal relaxation technique. Both structural and magnetic phase transitions were detected at temperatures that can be surmised from the phase diagram proposed in previous studies. The observed variation in enthalpy after the first-order structural phase transition, ΔH, showed agreement with those measured by differential scanning calorimetry (DSC). A decrease in the variation in C_p in the second-order magnetic phase transition, ΔC_p, with an increase in Sr content was detected, which can be attributed to a decrease in electronic spin configuration entropy with an increase in Sr content. In the dependence of ΔC_p on Sr content, a bending point was also observed at x  0.12, at which the crystal system varies from an orthorhombic-distorted perovskite structure to a rhombohedral-distorted perovskite structure.     

Evaluation of Bond Covalency and Bond Valence in Sr-Doped Perovskite-Type Compounds

Formulas for decomposing of complex crystals to a sum of binary crystals are described and applied to the study of bond covalency in La_1−xSr_xFeO₃ (0.0≤x≤0.9) and Ca_1−xSr_xMnO₃ (0.0≤x≤0.5). The bond valence is treated by bond-valence sums scheme. The results indicate that, for both compounds, with the increasing doping level, the bond covalency and bond valence show the same trend, namely, larger bond covalency corresponds to higher bond valence. For La_1−xSr_xFeO₃, with the increase of doping level, the bond covalency of La−O, Ca−O decreases in the orthorhombic (0.0≤x≤0.2) and rhombohedral (0.4≤x≤0.7) systems, then increases slightly for the cubic (0.8≤x≤0.9) system, but that of Fe−O increases for all crystal systems. A sharp decrease in bond covalency was observed where the crystal changes from orthorhombic to rhombohedral, while a smooth trend was seen for the rhombohedral-to-cubic transition. On the other hand, for orthorhombic Ca_1−xSr_xMnO₃, the bond covalency of Ca−O, Sr−O, and Mn−O (4-coordinate site) decreases with the increasing doping level, that of Mn−O (2-coordinate site) increases.     

Analysis of thermo-magnetic fluctuations above the glass-transition temperature in Bi1.6Pb0.5Sr2−xEuxCa1.1Cu2.1O8+δ (0.000 ≤ x ≤ 0.180) system

The resistivity of Bi_1.6Pb_0.5Sr_2−xEu_xCa_1.1Cu_2.1O_8+δ (0.000 ≤ x ≤ 0.180) superconductor has been measured as a function of temperature and magnetic field. The resistivity shows a glassy behavior even at higher temperatures and magnetic fields for the Eu-doped samples as compared with the Eu free sample. The values of glass-transition temperature [T_g], magnetic field dependent activation energy [U₀(B)] and the temperature and magnetic field dependent activation energy [U₀(B,T)] are found to be maximum for optimal doping levels (x = 0.135) which shows that the flux lines are effectively pinned in this sample. Also for temperatures below the superconducting transition temperature (T_C), a scaling of measured resistivity curves in magnetic field (B = 0.4 and 0.8 T) is obtained and this scaling is quite useful for better understanding of the behavior of the flux vortices in high temperature superconductors.     

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.     

Epitaxial Growth of Sr x Ba1 − x Nb2O6 Thin Films Prepared from Sol-Gel Process

In this study, we prepared Sr _x Ba_{1 – x} Nb₂O₆ (x = 0.3, 0.5 and 0.7) thin film on 0.75 wt% La doped SrTiO₃ (100) and (110) single crystal substrates. A homogeneous coating solution was prepared with Sr and Ba acetates and Nb(OEt)₅ as raw materials, and acetic acid and diethlene glycol monomethyl ether as solvents. The substrates were coated with the solution by spin coating method. As-coated thin films were heated from 973 to 1273 K in air. The grains of the thin film on La doped SrTiO₃ (100) were pillar shaped and arranged in right angle to each other. On the other hand, the grains of these thin films on La doped SrTiO₃ were pillar shape and arranged in one direction. The crystallographic relationship of the thin film between Sr _x Ba_{1 – x} Nb₂O₆ and substrate that the 130 and 310 direction of the thin film on the substrate were oriented with c-axis in parallel to the substrate surface. On the other hand, (hk0) phase diffractions of Sr _x Ba_{1 – x} Nb₂O₆ thin film on the substrate (110) were investigated in the XRD theta-2theta measurement. It is expected that the Sr _x Ba_{1 – x} Nb₂O₆ (x = 0.3, 0.5 and 0.7) were highly oriented or epitaxial growing on La doped SrTiO₃ (110) single crystal substrate.     

Crystal structure and magnetic properties of Sr4Mn2NiO9

The crystal structure of Sr₄Mn₂NiO₉ has been refined on single crystal. This phase belongs to the series A_1+x(A^′_xB_1–x)O₃ (x=1/3) related to the 2H-hexagonal perovskite. The structure contains transition metals in chains of oxide polyhedra (trigonal prisms and octahedra); neighboring chains are separated from each other by the Sr atoms. The sequence of the face sharing polyhedra along the chains is two octahedra + one trigonal prism. Mn occupies the octahedra and Ni is disordered in the trigonal prism with ≈80% in the pseudo square faces of the prism and ≈20% at the centre. This result has been confirmed by XANES experiments at Mn K and Ni K edges, respectively. Sr₄Mn₂NiO₉ is antiferromagnetic with a Néel temperature at T=3 K. The Curie constant measured at high temperature is in good agreement with ≈80% of the Ni²⁺ ions in the spin state configuration S=0.     

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.     

Compounds and phase compatibilities in the system La2O3---SrO---CuO at 950°C

The subsolidus phase diagram of the system La₂O₃---SrO---CuO at 950°C under 1 bar of pure oxygen has been investigated and a new ternary compound, La_1+xSr_2−xCu₂O_5.5+δ with 0.05 ≤ x ≤ 0.15, was isolated. This compound crystallizes in an orthorhombic unit cell with lattice constants related to the lattice constant of the perovskite cubic unit cell, a_p, by a = 3.80 Å a_p, B = 11.48 Å 3a_p, and c = 20.23 Å 5a_p. The structure is isotypical to that of LnSr₂Cu₂O_5.5+δ with Ln = Sm, Eu, or Gd. Reported data on the crystal chemistry of the equilibrium compounds in the system La₂O₃---SrO---CuO have been summarized and compared with the present data. The structure of all compounds is built up of a La---O rock-salt layer separated by a number of LaCuO₃ perovskite layers. The general formula is (La---O)(LaCuO₃)_n where La can be replaced either partly or completely by Sr. Compounds are found for n = 1, 2, and ∞. The structures of the compounds show different types of oxygen vacancy ordering.     

High-pressure structural evolution of a perovskite solid solution (La1−x,Ndx)GaO3

The structural evolution with pressure of six perovskites in the system La_1−xNd_xGaO₃ with x=0.00, 0.06, 0.12, 0.20, 0.62 and 1.00 have been determined by single-crystal diffraction. At room pressure, all six samples have Pbnm symmetry. The room-pressure bulk moduli vary only slightly with composition, between K_0T=169(4) and 177(2) GPa, with . As pressure is increased there is significant compression of the octahedral Ga–O bonds, the tilts of the GaO₆ octahedra decrease and the structures evolve towards higher symmetry. At room conditions the average Ga–O bond length increases with increasing compositional parameter x. However, the GaO₆ become stiffer with increasing x; the Ga–O bonds thus become stiffer as they become longer. Bond strengths in the octahedra in perovskites are therefore not a simple function of bond lengths but depend also upon the extra-framework cation.Phase transitions to R-3c symmetry occur at 2.2 GPa in end-member LaGaO₃, at 5.5 GPa in the x=0.06 sample, at 7.8 GPa for x=0.12, and at 12 GPa for x=0.20. No evidence of the transition in the x=0.62 or 1.00 samples was found by X-ray diffraction to 9.4 or 8.0 GPa, respectively, or by Raman measurements of NdGaO₃ up to 16 GPa. The transition pressure therefore increases with increasing Nd content (increasing x) at approximately 0.45 GPa per 0.01 increment in x, at least up to x=0.20. Compression of the R-3c phase of LaGaO₃ above the transition results in no significant changes in the tilt angle of the octahedra. The structural behavior of all six samples at high pressures is the result of the GaO₆ octahedra being softer than the extra-framework (La, Nd)O₁₂ site. The results therefore demonstrate that the evolution of solid-solution perovskites at high pressures follow the same general principles recently elucidated for end-member compositions.     

The oxycarbonates Sr4(Fe2−xMnx)1+y(CO3)1−3yO6(1+y): nano, micro and average structural approach

The possibility to synthesize layered oxycarbonates, with nominal composition Sr₄Fe_2−xMn_xO₆CO₃ involving trivalent manganese, with 0≤x≤1.5, is reported for the first time. The structural study of Sr₄FeMnO₆CO₃ using NPD, HREM, Mössbauer and XANES, shows that this phase is closely related to n=3 member of the Ruddlesden–Popper family. It derives from the latter by replacing the middle layer of transition metal octahedra by triangular CO₃ groups, with two different “flag” and “coat hanger” configurations. The magnetic order is antiferromagnetic and fundamentally different from the magnetic behavior of Sr₄Fe₂O₆CO₃.     

Electron doping versus antisite defects: Structural and magnetic properties of Sr2−xLaxCrMoO6 and Sr2−xLaxCr1+x/2Mo1−x/2O6 perovskites

This study reports the synthesis, structure and magnetic properties of Sr_2?xLa_xCrMoO₆ and Sr_2?xLa_xCr_1+x/2Mo_1?x/2O₆ samples. Although both series exhibit similar crystal structures, Sr_2?xLa_xCrMoO₆ samples present an effective electron doping revealed by a significant expansion of the unit cell with increasing x. In Sr_2?xLa_xCr_1+x/2Mo_1?x/2O₆ samples instead, the Cr-excess leads to a non electron doped system. Both series show a large amount of antisite defects whose number increases as La-content increases. Neutron diffraction patterns reveal the existence of long-range magnetic ordering for all samples but the magnetic peaks are very broad for Sr₂CrMoO₆ indicating a short coherence length of the magnetic ordering. This coherence length is increased upon replacing Sr by La. In both systems there is a clear increase of the magnetic transition temperature with increasing the La-content. The samples show ferromagnetic contributions at low temperature as deduced from the magnetic hysteresis loops typical of hard ferromagnetic materials. However, magnetic saturation is not achieved even at 5 T and the magnetic moment at this field is small. The ac magnetic susceptibility reveals the existence of several anomalies suggesting that these compounds are magnetically inhomogeneous. This is probably due to the presence of the large amount of structural defects not homogenously distributed.     

Tl2Ba2−xSrxCuO6: A system exhibiting a compositionally controlled superconductor-metal transition

Strontium-substituted thallium cuprates of the type Tl₂Ba_2−xSr_xCuO₆ have been synthesized for various values of x. Single phases were obtained for x ≤ 1.2. The compounds crystallize in the tetragonal structure with space group I4/mmm as the parent Tl₂Ba₂CuO₆ oxide. Both a and c lattice parameter decrease with increase in x. T_c decreases gradually from 92 to 77 K (x = 0.8). Beyond x = 1.0 superconductivity is lost. The x = 1.2 comparison is metallic down to 4.2 K.     

Preparation and Dielectric Properties of Mn-Doped Ba0.6Sr0.4TiO3-MgTiO3 Composite Ceramics

Barium strontium titanate (Ba_0.6Sr_0.4TiO₃, BST) nano-powders were prepared using Ba(NO₃)₂, Sr(NO₃)₂, oxalic acid dehydrate, and tetrabutyl titanate (Ti(OC4H9)4) as precursors by the chemical co-precipitation method. The product was characterized by thermogravimetry-differential scanning calorimetry (TG-DSC) thermal analyses, X-ray diffraction (XRD), and scanning electron microscopy (SEM). The experimental results indicated that the resulting Ba_0.6Sr_0.4TiO₃ nano-powders were homogeneous with agglomerated nature. The Ba_0.6Sr_0.4TiO₃-MgTiO₃ (BST-MT) bulk composite ceramics doped by Mn were obtained by the traditional solid phase method. The XRD patterns demonstrated that Mn-doped BST was unable to change the perovskite crystalline structure of BST materials. SEM photographs revealed that the crystalline grains became larger with increasing the content of doping Mn (<1.5% (x, molar fraction)) and then the size of grains decreased after the Mn content exceeded 1.5% in the BST ceramics, suggesting the effect of Mn doping on the morphologies of BST-MT composites. The dielectric properties of BST-MT composite ceramics doped with 0.1%-2.0% (x) Mn were investigated systematically. Two effects of Mn doping on the dielectric properties of the BST-MT composite ceramics were observed. At low Mn doping concentrations (<1.5%), Mn mainly acted as an acceptor dopant to replace Ti at the B site of ABO3 perovskite structure, leading to a diffused phase transition. It was also observed that the grain size increased drastically as the Mn content increased and thus caused the decrease of dielectric loss. At higher Mn doping concentrations (>1.5%), the grain size decreased and the suppression of permittivity and the drastic increase of the dielectric losses were observed, which indicated a “composite” mixing effect.     

Effects of Mn substitution on the structure and properties of chalcopyrite-type CuInSe2

Mn-doped CuInSe₂ compounds (CuIn_1−xMn_xSe₂, x=0.0125–0.20 and Cu_1−yIn_1−yMn_2ySe₂, 2y=0.0125–0.60) were synthesized by high-temperature solid-state reactions. Single phase materials with chalcopyrite structure persist up to 0.10 and 0.20 doping for CuIn_1−xMn_xSe₂ and Cu_1−yIn_1−yMn_2ySe₂, respectively. The chalcopyrite and sphalerite phases co-exist in the Cu_1−yIn_1−yMn_2ySe₂ system for 2y=0.25–0.50. Attempts to introduce greater manganese content, x=0.15–0.20 for CuIn_1−xMn_xSe₂ and 2y=0.60 for Cu_1−yIn_1−yMn_2ySe₂, result in partial phase segregation. For the single-phase samples, the lattice parameters of both systems increase linearly with manganese concentration and thus follow Vegard's law. The temperature of the chalcopyrite–sphalerite phase transition is decreased by manganese substitution for all single-phase samples. The bandgap of the materials remains around 0.9 eV. Additionally, the Mn-doped CuInSe₂ compounds display paramagnetic behavior, whereas pure CuInSe₂ is diamagnetic at 5–300 K. All the CuIn_1−xMn_xSe₂ and Cu_1−yIn_1−yMn_2ySe₂ compounds with chalcopyrite structure show antiferromagnetic coupling and measured effective magnetic moments up to 5.8 μ_B/Mn.     

Analysis of magnetic and structural phase transition behaviors of La1−xSrxCrO3 for preparation of phase diagram

The phase transition behavior of perovskite-type compounds, La_1−xSr_xCrO₃, was investigated by differential scanning calorimetry (DSC), dilatometry, dc magnetic susceptibility measurement and X-ray diffraction analysis. Both second-order magnetic phase transition from antiferromagnetic to paramagnetic and first-order structural phase transition from orthorhombic to rhombohedral were observed in the DSC or dilatometric curve of every specimen. The temperatures of both these magnetic and structural phase transitions decreased linearly with an increase in Sr content. The structural phase transition temperature of La_1−xSr_xCrO₃ with x less than 0.11 is higher than the magnetic phase transition temperature; however, a larger decrease in structural phase transition temperature than in magnetic phase transition temperature was observed with an increase in Sr content, resulting in a structural phase transition temperature lower than the magnetic phase transition temperature for La_1−xSr_xCrO₃ with x of more than 0.12. It was also observed that the heat of absorption of the structural phase transition decreased with an increase in x. In the dependence of dc magnetic susceptibility on temperature, variations by not only magnetic but also structural phase transitions were observed. It was also revealed that thermal expansion coefficient is affected not only by structural phase transition but also magnetic phase transition. Magnetic and structural phase diagram of La_1−xSr_xCrO₃, suggesting the existence of two Sr contents and temperatures at which triple phases coexist, was proposed.     

XAFS characterization of La1−xSrxMnO3±δ catalysts prepared by Pechini’s method

XAFS experiments at the Mn and Sr K-edges were carried out in order to investigate the short-range arrangement of Mn and Sr sites on La_1−xSr_xMnO_3±δ highly doped perovskites (x = 0, 0.2, 0.4 and 0.6). The Mn K-edge EXAFS spectra show a static Jahn–Teller distortion of the MnO₆ for x = 0 and 0.2, which is drastically reduced as x increases. The distortion of perovskite, characterized by the Mn–O–Mn tilt angle, progressively decreases with increasing Sr contents. Sr K-edge results indicated a decrease on the Sr–Mn coordination number upon Sr doping. Based on this and TPD results, a charge compensating mechanism is proposed suggesting a partial Mn oxidation and formation of Mn defect vacancies due to the introduction of Sr.     

Heat capacity, enthalpy and entropy of strontium niobate Sr2Nb2O7 and calcium niobate Ca2Nb2O7

The heat capacity and the enthalpy increments of strontium niobate Sr₂Nb₂O₇ and calcium niobate Ca₂Nb₂O₇ were measured by the relaxation time method (2–300 K), DSC (260–360 K) and drop calorimetry (720–1370 K). Temperature dependencies of the molar heat capacity in the form C_pm = 248.0 + 0.04350T − 3.948 × 10⁶/T² J K⁻¹ mol⁻¹ for Sr₂Nb₂O₇ and C_pm = 257.2 + 0.03621T − 4.434 × 10⁶/T² J K⁻¹ mol⁻¹ for Ca₂Nb₂O₇ were derived by the least-square method from the experimental data. The molar entropies at 298.15 K, S_m°(298.15 K) = 238.5 ± 1.3 J K⁻¹ mol⁻¹ for Sr₂Nb₂O₇ and S_m°(298.15 K) = 212.4 ± 1.2 J K⁻¹ mol⁻¹ for Ca₂Nb₂O₇, were evaluated from the low-temperature heat capacity measurements.