Structures,thermal expansion properties and phase transitions of ErxFe2−x(MoO4)3 (0.0 ≤ x ≤ 2.0)

Structures, thermal expansion properties and phase transitions of Er_xFe_2−x(MoO₄)₃ (0.0 ≤ x ≤ 2.0) have been investigated by X-ray diffraction and differential thermal analysis. The partial substitution of Er³⁺ for Fe³⁺ induces pronounced decreases in the phase transition temperature from monoclinic to orthorhombic structure. Rietveld analysis of the XRD data shows that both the monoclinic and orthorhombic Fe₂(MoO₄)₃, as well as the orthorhombic Er_xFe_2−x(MoO₄)₃ (x ≤ 0.8) have positive thermal expansion coefficients. However, the linear thermal expansion coefficients of Er_xFe_2−x(MoO₄)₃ (x = 0.6–2.0) decrease with increasing content of Er³⁺ and for x ≥ 1.0, compounds Er_xFe_2−x(MoO₄)₃ show negative thermal expansion properties. Attempts for making zero thermal expansion coefficient materials result in that very low negative thermal expansion coefficient of −0.60 × 10⁻⁶/°C in Er_1.0Fe_1.0(MoO₄)₃ is observed in the temperature range of 180–400 °C, and zero thermal expansion is observed in Er_0.8Fe_1.2(MoO₄)₃ in the temperature range of 350–450 °C. In addition, anisotropic thermal expansions are found for all the orthorhombic Er_xFe_2−x(MoO₄)₃ compounds, with negative thermal expansion coefficients along the a axes.     

Controllable thermal expansion and phase transition in Yb2−xCrxMo3O12

The thermal expansion and phase transition of solid solutions Yb_2?xCr_xMo₃O₁₂ have been investigated by X-ray powder diffraction and differential thermal analysis. The XRD patterns and the results of Rietveld refinement of Yb_2?xCr_xMo₃O₁₂ indicate that the solid solution limit was in the composition range of 0.0 ≤ x ≤ 0.4 and 1.7 ≤ x ≤ 2.0. Yb_2?xCr_xMo₃O₁₂ (0.0 ≤ x ≤ 0.4) has an orthorhombic structure and exhibits negative thermal expansion between 200 °C and 800 °C. Yb_2?xCr_xMo₃O₁₂ (1.7 ≤ x ≤ 2.0) crystallizes in monoclinic below the phase transition and above, transforms to orthorhombic. Both monoclinic and orthorhombic compounds Yb_2?xCr_xMo₃O₁₂ (1.7 ≤ x ≤ 2.0) present positive thermal expansion. Orthorhombic Yb_2?xCr_xMo₃O₁₂ exhibit anisotropic thermal expansion with the contraction of a and c axes, and the linear thermal expansion coefficients range from negative to positive with increasing chromium content. Partial substitution of Yb³⁺ for Cr³⁺ exhibits depressed monoclinic to orthorhombic phase transition.     

Phase transitions of CaTiO3 ceramics sintered with the aid of NaF and MgF2

New phases with initial composition (1 ? x)CaTiO₃ ? xNaF ? xMgF₂ (0 ≤ x ≤ 0.20) have been prepared at low temperature (950 °C) from mixtures of CaTiO₃ and fluorides NaF and MgF₂. The oxyfluorides obtained have been characterized by X-ray diffraction at room temperature and indexed by isotypy with orthorhombic CaTiO₃. The microstructures of these phases are observed by scanning electron microscopy. Dielectric measurements have been carried out during cooling cycle from 500 °C to room temperature at two frequencies (100 Hz, 1 kHz). Differential scanning calorimetry (DSC), thermogravimetry (TG) and differential thermogravimetry (DTG) analyses have been performed, respectively, from room temperature up to 550 °C (DSC) and 920 °C (TG–DTG). The dielectric measurements revealed two anomalies which have been confirmed by DSC analyses. These phenomena are ascribed to second order phase transitions. The variation of the real permittivity with temperature is in agreement with the class I capacitor specifications. However, the dielectric losses have to be improved.     

Mechanochemical synthesis and ionic conductivity of lanthanum phosphosilicate oxyapatites

Lanthanum phosphosilicate apatites with the chemical formula Sr_10–xLa_x(PO₄)_6–x(SiO₄)_xO, where 0 ≤ x ≤ 6, usually prepared by a solid-state reaction at about 1400 °C, were synthesized via the mechanochemical method at room temperature. The samples were characterized using powder X-ray diffraction, infrared spectroscopy and thermal analysis. The results showed that the prepared products were carbonated apatites and no secondary phase was detected. The realization of the milling under a controlled atmosphere can lead to oxyapatites containing no carbonates. The ionic conductivity of the Sr₆La₄(PO₄)₂(SiO₄)₄O sample was investigated by using impedance spectroscopy. The highest ionic conductivity value of 1.522 × 10⁻⁶ S·cm⁻¹ was found at 800 °C. In the investigated temperature range, the activation energy is of 0.85 eV.     

Investigation of the LiCo1−xMgxPO4 (0 ≤ x ≤ 0.1)–graphitic carbon foam composites

LiCo_1−xMg_xPO₄–graphitic carbon foam (LCMP–GCF with 0 ≤ x ≤ 0.1) composites are prepared by Pechini-assisted sol-gel method and annealed with the 2-steps annealing process (T = 300 °C for 5 min in flowing air, then at T = 730 °C for t = 12 h in flowing nitrogen). The XRD analysis, performed on powders reveals LiCoPO₄ as major crystalline phase, Co₂P and Co₂P₂O₇ as secondary phases. The morphological investigation revealed the formation and growth of microcrystalline “islands” which consist of acicular crystallites with different dimensions (typically 5–50 μm). By addition of Mg-ions, CV-curves of LCMP–GCF composites show a decrease of the surface between anodic and cathodic sweeps by cycling and a stark contribution of faradaic processes due to the graphitic structured foam. The electrochemical measurements, at a discharge rate of C/10 at room temperature, show the decrease of the discharge specific capacity from 100 mAh g⁻¹ for x = 0.0 to ∼35 mAh g⁻¹ for 0.025 ≤ x ≤ 0.05, then an increase to 69 mAh g⁻¹ for x = 0.1. The electrochemical impedance spectroscopy data reveal a decrease of the electrical resistance and the improvement of the Li-ion conductivity at high Mg-ions content into the LiCoPO₄ phase (x ≥ 0.025).     

Hopping conductivity of distorted K2NiF4-type (Ca1+xNd1−x)CrO4

Orthorhombic distorted K₂NiF₄-type (Ca_1+xNd_1?x)CrO₄ (0.00 ≤ x ≤ 0.15) was synthesized using a standard ceramic technique. The cell parameters (a and c) decreased, whereas the cell parameter (b) increased with the increase in x. The variation in the global instability index (GII) indicated that the crystal stability of (Ca_1+xNd_1?x)CrO₄ was not influenced by the Cr⁴⁺ ion content. At all temperatures, the electrical conductivity (σ) of (Ca_1+xNd_1?x)CrO₄ increased with the increase in x. (Ca_1+xNd_1?x)CrO₄ was a p-type semiconductor and exhibited hopping conductivity in a small-polaron model in the temperature range of 290 K ≤ T ≤ 713 K. The Cr⁴⁺ ion acts as an acceptor, and the electron transfer through the Cr³⁺–O–Cr⁴⁺ path becomes active as a result of the Cr⁴⁺ ion content and the Cr–O(1) distance.     

Ferromagnetic to spin glass cross over in (La,Tb)2/3Ca1/3MnO3

In the series La_2/3?xTb_xCa_1/3MnO₃, it is known that the compositions are ferromagnetic for smaller values of x and show spin glass characteristics at larger values of x. Our studies on the magnetic properties of various compositions in the La_2/3?xTb_xCa_1/3MnO₃ series show that the cross over from ferromagnetic to spin glass region takes place above x ≈ 1/8. Also, a low temperature anomaly at 30 K, observed in the ac susceptibility curves, disappears for compositions above this critical value of x. A mixed phase region coexists in the narrow compositional range 0.1 ≤ x ≤ 0.125, indicating that the ferromagnetic to spin glass cross over is not abrupt.     

Structural and optical properties of sprayed Zn1−xMnxO films

In recent years the dilute magnetic semiconductors have received much attention due to the complementary properties of semiconductor and ferromagnetic behaviour. Zn_1−xMn_xO thin films have been synthesized by chemical spray pyrolysis at a substrate temperature of 400 °C with different manganese compositions that vary in the range, 0.0 ≤ x ≤ 0.25, on Corning 7059 glass substrates. The X-ray diffraction studies revealed that all the films were strongly oriented along the (002) orientation corresponding to the hexagonal wurtzite structure. The crystalline quality of the layers was found to decrease with the increase of x, however, no structural changes were observed over the ‘Mn’ composition range investigated. The optical absorption studies revealed that the energy band gap of the films followed the Vegard's law. The optical band gap of the films prepared at x = 0.15 was found to be ∼3.35 eV. The photoluminescence characteristics of Zn_1−xMn_xO films showed an emission peak at around 390 nm with a broad band about 530 nm. The details of these results were reported and discussed.     

Temperature-induced phase transitions in BaTbO3

The crystal structures of BaTbO₃ have been investigated over a wide temperature range between 40 and 773 K using high-resolution time-of-flight neutron powder diffraction. Two-phase transitions were observed. Below about 280 K, BaTbO₃ adopts an orthorhombic perovskite structure (space group Ibmm), which is characterized by rotation of TbO₆ octahedra about the pseudocubic two-fold axis. Above 280 K, BaTbO₃ undergoes a first-order phase transition to a tetragonal symmetry (space group I4/mcm), in which the tilting of the octahedra is around the pseudocubic four-fold axis. As the temperature is further increased, BaTbO₃ adopts the primitive cubic aristotype at about 623 K. This later phase transformation is characterized by a gradual decrease of the rotation angle, indicating a continuous phase transition, which is described by a critical exponent β=0.35.     

Ferromagnetism vs. antiferromagnetism of the dimorphic HoCrO4 oxide

This paper reports the specific conditions used in the preparation of the dimorphic phases of HoCrO₄ oxide. The scheelite form has been obtained from the room pressure stable HoCrO₄–zircon heated at 823 K at 40 kbar. The structures of both the polytypes have been refined from X-ray powder diffraction data using the Rietveld method. The zircon type of HoCrO₄ oxide crystallizes with tetragonal symmetry, S.G. I4₁/amd and lattice parameters a = 7.119(10) and c = 6.2557(5) Å; while the scheelite–HoCrO₄ derivative shows tetragonal symmetry, S.G. I4₁/a and lattice parameters a = 5.0017(1) and c = 11.2664(2) Å. Magnetic susceptibility measurements show that both zircon and scheelite forms of HoCrO₄ oxide present a very different magnetic behaviour at low temperatures. The zircon form behaves as a ferromagnet with a Curie temperature of 17.6 K, while the scheelite form is antiferromagnetic with a Néel temperature of 7.6 K. A metamagnetic transition for the scheelite form with a critical field of 1 T at 2 K has been also observed. The changes in the sign of the interactions have been also analyzed by considering the possible super-exchange mechanisms and the differences found in the Ho–O–Cr distances and bond angles in going from zircon to scheelite structural type.     

Crystal structure,electrical conductivity,thermal expansion and compatibility studies of Co-substituted lanthanum strontium manganite system

La_0.76Sr_0.19Mn_1?xCo_xO_3±δ, LSMCo_x (0 ≤ x ≤ 1) perovskite oxides were synthesized by conventional ceramic route. The effect of Co substitution for Mn on the crystal structure, electrical conductivity and thermal expansion properties was investigated. XRD indicated rhombohedral symmetry for the studied compositions at 1673 K. The lattice parameters so determined showed significant reduction in cell volume, which is attributed to smaller ionic radii of Co³⁺ ions. The results of electrical conductivity data indicated that the conductivity mechanism is by thermally activated hopping of small polarons between localized states corresponding to Mn or Mn and Co sites of different valence value. The conductivity decreases at all temperatures up to 40 mol% Co substitution while the energy of activation increases. This is possibly due to an increase in Jahn–Teller distortion, at an extent higher than the increase of the concentration of charge carriers. Thermal expansion coefficient values in the series increase with increasing Co content which has been explained on the basis of the changes in the spin states of the Co ions and the consequent changes in the ionic size with temperature. Solution route synthesis produces fine-size particles with better properties, consequently one composition from the above having enhanced requisite properties, viz. La_0.76Sr_0.19Mn_0.8Co_0.2O_3±δ was synthesized by sol–gel route. The sol–gel synthesized compound had crystallite size of ～30 nm at 1173 K obtained using Scherrer's equation. Thus the potential of these compounds as cathodes for solid oxide fuel cells (SOFCs) have been evaluated.As Ce_0.8RE_0.2O_2?δ (RE = Sm, Gd) are being investigated for their use as electrolytes in SOFCs, their mechanical compatibility as well as chemical compatibility with the potential cathode material from the above LSMCo_x series was also studied.     

Crystal structures,site occupations and phase equilibria in the system V–Zr–Ge

The V–Zr–Ge system was studied for two isothermal sections at 900 and 1200 °C. Three ternary compounds VZrGe (tI12, I4/mmm, CeScSi-type), V_xZr_5?xGe₄ (oP36, Pnma, Sm₅Ge₄-type) and V_4+xZr_2?xGe₅ (oI44, Ibam, Si₅V₆-type) were structurally characterized. Optical microscopy and powder X-ray diffraction (XRD) were used for initial sample characterization and electron probe microanalysis (EPMA) of the annealed samples was used to determine the exact phase compositions. The variation of the cell parameters of the various ternary solid solutions with the composition was determined. The three ternary phases were structurally characterized by means of single crystal and powder XRD. While VZrGe is almost a line compound, V_xZr_5?xGe₄ (0.2 ≤ x ≤ 3.0) and V_4+xZr_2?xGe₅ (0.06 ≤ x ≤ 1.2) are forming extended solid solution ranges stabilized by differential fractional site occupancy of V and Zr on the metal sites.     

Structural characterization,molecular dynamics,dielectric and spectroscopic properties of tetrakis(pyrazolium) bis(μ2-bromo-tetrabromobismuthate(III)) dihydrate, [C3N2H5]4[Bi2Br10]·2H2O

The structure of [C₃N₂H₅]₄[Bi₂Br₁₀]·2H₂O, (PBB) was determined by single crystal X-ray diffraction at 100 K. It crystallizes in the monoclinic space group C2/m, with a = 12.992(4) Å, b = 16.326(5) Å, c = 8.255(3) Å, β = 108.56°(3), V = 1659.9(9) Å³ and Z = 2. The structure consists of discrete binuclear [Bi₂Br₁₀]⁴⁻ anions, ordered pyrazolium cations and water molecules. The crystal packing is governed by strong N–H⋯O and weak O–H⋯Br hydrogen bonds. A sequence of structural phase transitions in PBB was established on the basis of differential scanning calorimetry and dilatometric studies. Two reversible first-order phase transitions were found: (I → II) at 381/371 K (on heating/cooling) and (II → III) at 348/338 K. Dielectric response near both phase transitions is characteristic of crystals with the “plastic-like” phases. Over the phase III a low frequency dielectric relaxator is disclosed. The possible molecular motions in the PBB compound are characterized by the ¹H NMR studies. The infrared spectra of polycrystalline compound in the temperature range 300–380 K are reported for the region 4000–400 cm⁻¹. The observed spectral changes through the structural phase transition III → II are attributed to an onset of motion both of the pyrazolium cations and water molecules.     

The local structure of bismuth germanate glasses and glass ceramics doped with europium ions evidenced by FT-IR spectroscopy

Glasses of the xEu₂O₃·(100 − x)[4Bi₂O₃·GeO₂] system, with 0 ≤ x ≤ 30 mol%, have been characterized by FT-IR spectroscopy measurements in order to obtain information about the influence of Eu₂O₃ on the local structure of the 4Bi₂O₃·GeO₂ glass matrix. FT-IR spectroscopy data suggest that the europium ions play the network modifier role in the studied glasses and both Bi₂O₃ and GeO₂ play the role of network formers. Melting at 1100 °C and the rapid cooling at room temperature permitted to obtain glass samples. In order to improve the local order and to develop crystalline phases the glass samples were kept at 700 °C for 17 h. Both the influences of the europium ions as well as of the heat treatment on the local order of 4Bi₂O₃·GeO₂ glass matrix have been discussed.     

A Raman scattering study of structural changes in LaMn1−xCoxO3+δ system

We present results of Raman scattering studies on LaMn_1−xCo_xO_3+δ over a wide range of doping content (x = 0.1–0.75) and temperature range of 20–300 K. Powder X-ray diffraction patterns show that there is a structural change from orthorhombic to rhombohedral at x = 0.5 as x increases. Raman spectra of all LaMn_1−xCo_xO_3+δ samples show peaks near 260, 500, and 650 cm⁻¹. However, the Raman spectra are not drastically different from each other across the structural phase transition at x = 0.5. On the other hand, the peak frequencies of the modes near 260 and 500 cm⁻¹ as functions of Co content (x) show slope changes at x = 0.5. The full-width at the half-maximum (FWHM) of the mode near 650 cm⁻¹ as a function of Co content (x) shows minimum at x = 0.5. Normally, larger values of FWHM are expected at near x = 0.5, if the mode were affected by the structural disorder at the phase boundary. Therefore, it is likely due to lowest charge concentration at x = 0.5, which results in lowest screening effect. This is consistent with the fact that the intensity of the phonons is strongest at x = 0.5. As the temperature decreases, the two peaks near 500 and 650 cm⁻¹ of different Co contents, related with octahedral distortions, are found to shift to lower frequencies unlike the usual temperature behavior. However, no abrupt change in the peak frequencies and the FWHM is observed across measured temperature range, regardless of the Co content.     

Experimental excess molar properties of binary mixtures of (3-amino-1-propanol + isobutanol, 2-propanol) at T = (293.15 to 333.15) K and modelling the excess molar volume by Prigogine–Flory–Patterson theory

Density and viscosity of binary mixtures of (x₁3-amino-1-propanol + x₂isobutanol) and (x₁3-amino-1-propanol + x₂2-propanol) were measured over the entire composition range and from temperatures (293.15 to 333.15) K at ambient pressure. The excess molar volumes and viscosity deviations were calculated and correlated by the Redlich–Kister (RK) equation. The thermal expansion coefficient and its excess value, isothermal coefficient of excess molar enthalpy, and excess partial molar volumes were determined by using the experimental values of density and are described as a function of composition and temperature. The excess molar volumes are negative over the entire mole fraction range for both mixtures and increase with increasing temperature. The excess molar volumes obtained were correlated by the Prigogine–Flory–Patterson (PFP) model. The viscosity deviations of the binary mixtures are negative over the entire composition range and decrease with increasing temperature.     

Study of the ionic conductivity of Ca6La4(PO4)2(SiO4)4F2 and Ca4La6(SiO4)6F2

In order to enhance our knowledge about the Ca_10−xLa_x(PO₄)_6−x(SiO₄)_xF₂ (0 ≤ x ≤ 6) series, whose chemical stability decreases as the substitution degree increases, Ca₆La₄(PO₄)₂(SiO₄)₄F₂ and Ca₄La₆(SiO₄)₆F₂ compounds were prepared through a solid-state reaction. Their ionic conductivity was measured by impedance spectroscopy. The results indicate that the conductivity increases with substitution, and fits the Arrhenius equation over the investigated temperature range. At high temperatures, a change in the activation energy has been observed, which has been related to the nature of the Ca/La–F bond, i.e. to the polarizability of lanthanum.     

Dual role of the six-coordinated molybdenum and lead ions in novel of photochromic properties of the molybdenum–lead–borate glasses

Transparent glasses were prepared by conventional melting–quenching method in the xMoO₃·(100 ? x)[3B₂O₃·PbO] system where 0 ≤ x ≤ 15 mol%. By increasing the MoO₃ content up to 20 mol% the PbMoO₄ crystalline phase appears. These systems exhibit a photochromic effect which can be induced through laser exposures (λ = 633 nm) directly on the bulk sample. Structural investigations by FTIR spectroscopy show that the photosensitive effect is due to a reduction of Mo⁶⁺ to Mo⁴⁺ and/or Mo⁵⁺ promoted by the oxidation of Pb²⁺ and some structural changes of the borate network.     

Doping effects of Yb3+ on the crystal structures,nanoparticle properties and electrical behaviors of ZrO2 derived from a facile urea-based hydrothermal route

Weakly-agglomerated nanocrystalline (ZrO₂)_1−x(Yb₂O₃)_x (x=0.02–0.2) powders with high surface area (109–151 m² g⁻¹) were synthesized by a two-step hydrothermal process in the presence of urea: a stock solution of metal nitrates and urea was heated at 80 °C for 24 h and then at 180 °C for 48 h. For x=0.04–0.2, the as-derived powders were an assembly of uniform nanoparticles with well-defined edges in the size between 6.1–8.4 nm. Before and after calcination at 800 °C, the lattice parameters, microstrain and surface area of the (ZrO₂)_1−x(Yb₂O₃)_x samples tended to increase with Yb³⁺ concentration; while, the average crystallite size decreased correspondingly. In the Arrhenius plots over the measurement temperature range of 400–800 °C, the bulk ionic conductivity of the compacts sintered at 1400 °C for 24 h showed a maximum value at the composition of x=0.08 in cubic structure, with an activation energy of 0.89 eV. At 800 °C, σ_b=0.049 S cm⁻¹ for x=0.08.     

Comments on stabilizing layered manganese oxide electrodes for Li batteries

An electrochemical study of structurally-integrated xLi₂MnO₃•(1 −x)LiMn_0.5Ni_0.5O₂ ‘composite’ materials has been undertaken to investigate the stability of electrochemically-activated electrodes at the Li₂MnO₃-rich end of the Li₂MnO₃–LiMn_0.5Ni_0.5O₂ tie-line, i.e., for 0.7 ≤ x ≤ 0.95. Excellent performance was observed for x = 0.7 in lithium half-cells; comparable to activated electrodes that have significantly lower values of x and are traditionally the preferred materials of choice. Electrodes with higher manganese content (x ≥ 0.8) showed significantly reduced performance. Implications for stabilizing low-cost, manganese-rich, layered lithium-metal-oxide electrode materials are discussed.