Contraction Of Unit Cell Volume Research Articles

XRD analysis of nanocrystalline anatase powders prepared by various chemical routes: correlations between micro-structure and crystal structure parameters

Nanocrystalline anatase powders synthesised by various chemical processes as super/subcritical fluid extraction, sol-gel technique and hydrolysis of titanium alkoxides in hydrogen peroxide were studied by X-ray diffraction (XRD) whole profile modelling method (WPPM) in order to reveal correlations between structural and micro-structural parameters as well as sample treatment conditions. Anisotropy of the diffraction line broadening due to truncated bipyramidal shape of anatase crystals was discussed. The hkl-anisotropy can be very strong but also almost negligible in dependence on relative ratio of the crystallite dimensions. The latter was the case for the studied samples. The size of synthesised anatase nanoparticles was within the range 3–25 nm. The theoretical total surface area of crystallites calculated from XRD was in a good correlation with the surface area measured by the nitrogen physisorption up to the temperature 400–450 °C, when the particles started to agglomerate. At atomic scale a unit cell volume contraction with decreasing crystallite size and a significant deficiency in the Ti-site occupancy was observed. Both effects were attributed to the presence of Ti-vacancies and a linear coefficient between the relative cell volume contraction and the fraction of Ti-vacancies was estimated to (–0.017 ± 0.003).

Structural and optical properties of manganese substituted nanocrystalline bismuth ferrite thin films by sol–gel process

Abstract Manganese (Mn) substituted nanocrystalline Bismuth Ferrite (BiFe1−xMnxO3) thin films were deposited on quartz substrates by sol–gel process and the effect of the substitution on the structural and optical properties of BiFe1−xMnxO3 system was studied. With the increase in Mn content up to 30%, a distortion in crystal lattice constants and contraction in unit cell volume was observed without any formation of secondary phase. From the AFM photograph ran increase in grain size with the increase in Mn contents was observed. Optical transmittance spectra showed a decrease in transmittance (T%) with the increase in Mn content. Further, it was observed that the presence of Mn lead to the decrease in optical energy band gap of the samples. The effect of Mn substitution on the photoluminescent properties of BiFe1−xMnxO3 system was also studied in order to exploit their possible application in nanoscale optoelectronic devices.

Magnetic Properties of Phospho-Olivine Li(Fe$_{1-x}$Mn$_{x}$)PO$_{4}$Investigated With Mössbauer Spectroscopy

The crystalline structure of the Li(Fe1-xMnx)PO4 (x = 0.0, 0.2) samples was determined to be olivine-type orthorhombic with Pnma space group. The contraction ofunit cell volume from LiFe0.8Mn0.2PO4 (V = 293.296 Å3) to LiFePO4 (V = 291.445 Å3) can be explained by the different ionic radii of high-spin Fe2+ and Mn2+. The temperature-dependent magnetic susceptibility and Curie-Weiss fitted reciprocal susceptibility curves of Li(Fe1-xMnx)PO4 indicates that there are antiferromagnetic ordering with different values of magnetic Neel temperature and effective moment between LiFe0.8Mn0.2PO4 (TN = 49 K, μeff = 5.66μB) and LiFePO4 (TN = 51 K, μeff = 5.37 μB). From the Mossbauer analysis, The distribution of the Mn2+ (3d5) ions in M2 magnetic sites can lead to the reduction of magnetic hyperfine field (Hhf) of LiFe0.8Mn0.2PO4 in Fe2+ (3d6) sites. Also, the isomer shift (δ) and electric quadrupole splitting (ΔEQ) values, increasing with Mn2+ substitution, can be attributed to the decrease of charge density at Fe nucleus ρA (0) due to the 3d-electron penetrating closer to the Fe nucleus with asymmetric charge distribution by the presence of Mn2+ ions on the FeO6 octahedral sites.

Electrical, electrochemical, and thermomechanical properties of perovskite-type (La1−x Sr x )1−y Mn0.5Ti0.5O3−δ (x = 0.15–0.75, y = 0–0.05)

Strontium additions in (La1−xSrx)1−yMn0.5Ti0.5O3−δ (x00.15-0.75, y00-0.05) having a rhombohedrally distorted perovskite structure under oxidizing conditions lead to the unit cell volume contraction, whilst the total conductivity, thermal and chemical expansion, and steady-state oxygen permeationlimited bysurface exchangeincrease with increas- ing x. The oxygen partial pressure dependencies of the con- ductivity and Seebeck coefficient studied at 973-1223 K in the p(O2) range from 10 −19 to 0.5 atm suggest a dominant role of electron hole hopping and relatively stable Mn 3+ and Ti 4+ states. Due to low oxygen nonstoichiometry essentially con- stant in oxidizing and moderately reducing environments and to strong coulombic interaction between Ti 4+ cations and oxygen anions, the tracer diffusion coefficients measured by the 18 O/ 16 O isotopic exchange depth profile method with time-of-flight secondary-ion mass spectrometric analysis are lower compared to lanthanum-strontium manganites. The average thermal expansion coefficients determined by controlled-atmosphere dilatometry vary in the range 9.8- 15.0×10 −6 K −1 at 300-1370 K and oxygen pressures from 10 −21 to 0.21 atm. The anodic overpotentials of porous La0.5Sr0.5Mn0.5Ti0.5O3−δ electrodes with Ce0.8Gd0.2O2-δ inter- layers, applied ontoLaGaO3-basedsolid electrolyte, are lower compared to (La0.75Sr0.25)0.95Cr0.5Mn0.5O3−δ when no metal- lic current-collecting layers are introduced. However, the po- larization resistance is still high, ~2 Ω×cm 2 in humidified 10 % H2-90 % N2 atmosphere at 1073 K, in correlation with relatively low electronic conduction and isotopic exchange rates. The presence of H2 St races in H2-containing gas mix- tures did not result in detectable decomposition of the perov- skite phases.

Cobalt-doped ZnO nanowires on quartz: Synthesis by simple chemical method and characterization

The synthesis of cobalt-doped ZnO nanowires is achieved using a simple, metal salt decomposition growth technique. A sequence of drop casting on a quartz substrate held at 100°C and annealing results in the growth of nanowires of average (modal) length ∼200nm and diameter of 15±4nm and consequently an aspect ratio of ∼13. A variation in the synthesis process, where the solution of mixed salts is deposited on the substrate at 25°C, yields a grainy film structure which constitutes a useful comparator case. X-ray diffraction shows a preferred [0001] growth direction for the nanowires while a small unit cell volume contraction for Co-doped samples and data from Raman spectroscopy indicate incorporation of the Co dopant into the lattice; neither technique shows explicit evidence of cobalt oxides. Also the nanowire samples display excellent optical transmission across the entire visible range, as well as strong photoluminescence (exciton emission) in the near UV, centered at 3.25eV.

1,2-Cyclohexanedionedioxime as a useful co-ligand for fabrication of one-dimensional Zn(ii) and Cd(ii) coordination polymers with wheel-and-axle topology and luminescent properties

The combination of 1,2-cyclohexanedionedioxime (NioxH2) and carboxylate-containing ligands resulted in Zn(II) or Cd(II) robust metal building blocks which being coupled with the flexible pyridine ligands fabricated five one-dimensional (1D) coordination polymers {[Cd(CHOO)2(NioxH2)(bpe)]·DMF}∞ (1), [Cd(CHOO)2(NioxH2)(dps)]∞ (2), [Zn(CH3COO)2(NioxH2)(dps)]∞ (3), [Cd(CH3COO)2(NioxH2)(dps)]∞ (4), and {[Zn(pyta)2(NioxH2)]·DMF}∞ (5) (where bpe = 4,4′-bipyridine ethane, dps = 4,4′-dipyridylsulfide, Hpyta = (4-pyridylthio)acetic acid, DMF = N,N-dimethylformamide), whose crystal structures were determined by single crystal X-ray diffraction. The stability of compounds 1 and 5 to loss of DMF as a guest has been addressed experimentally, and the crystal-to-crystal transformation observed for 1 resulted in the guest free 1D polymer [Cd(CHOO)2(NioxH2)(bpe)]∞ (1a) which maintains its coordination network with the 10.3% unit cell volume contraction, while compound 5 remains unchangeable under the same conditions. Compounds 1, 5 and the dioxime-free 1D polymer [Cd(pyta)2(H2O)]∞ (6) display blue luminescence upon excitation with ultraviolet light. The emission intensity of the guest-loaded complex 1 is effectively reduced in comparison with the previously reported DMF free analogues [Zn(CH3COO)2(NioxH2)(bpe)(H2O)]∞ and [Cd(CH3COO)2(bpe)(H2O)]∞.

The unusual thermal expansion of pure silica sodalite probed by in situ time-resolved synchrotron powder diffraction

The thermal behaviour of a pure silica sodalite containing trioxane was studied using the in situ synchrotron X-ray powder-diffraction technique. Rietveld refinements show that temperature-induced transformations can be schematized into three main steps: (1) The relaxation of the host–guest interactions between trioxane and the surrounding framework in the 25–200 °C temperature range determines a regularization in the 6-membered rings and, consequently, a process of expansion. (2) The cell volume remains constant until 380 °C and then starts to contract when template molecule decomposition occurs. (3) When the trioxane molecule decomposition and expulsion process is completed at above 760 °C, unit cell volume contraction cannot be fully justified by the negligible mass loss observed on the TG curves, thus suggesting a negative thermal expansion (NTE) process. This behaviour is unusual for sodalite-type materials which typically show positive thermal expansion upon heating, thus indicating that the presence of organic molecules instead of metal ions in the zeolite cages strongly controls the thermal expansion mechanism.

In-situ dehydration studies of fully K-, Rb-, and Cs-exchanged natrolites

In-situ synchrotron X-ray powder diffraction studies of K-, Rb-, and Cs-exchanged natrolites between room temperature and 425 °C revealed that the dehydrated phases with collapsed frameworks start to form at 175, 150, and 100°C, respectively. The degree of the framework collapse indicated by the unit-cell volume contraction depends on the size of the non-framework cation: K-exchanged natrolite undergoes an 18.8% unit-cell volume contraction when dehydrated at 175 °C, whereas Rband Cs-exchanged natrolites show unit-cell volume contractions of 18.5 and 15.2% at 150 and 100°C, respectively. In the hydrated phases, the dehydration-induced unit-cell volume reduction diminishes as the cation size increases and reveals increasingly a negative slope as smaller cations are substituted into the pores of the natrolite structure. The thermal expansion of the unit-cell volumes of the dehydrated K-, Rb-, and Cs-phases have positive thermal expansion coefficients of 8.80 × 10−5 K−1, 1.03 × 10−4 K−1, and 5.06 × 10−5 K−1, respectively. Rietveld structure refinements of the dehydrated phases at 400 °C reveal that the framework collapses are due to an increase of the chain rotation angles, ψ, which narrow the channels to a more elliptical shape. Compared to their respective hydrated structures at ambient conditions, the dehydrated K-exchanged natrolite at 400°C shows a 2.2-fold increase in ψ, whereas the dehydrated Rb- and Cs-natrolites at 400°C reveal increases of ψ by ca. 3.7 and 7.3 times, respectively. The elliptical channel openings of the dehydrated K-, Rb-, to Cs-phases become larger as the cation size increases. The disordered non-framework cations in the hydrated K-, Rb-, and Csnatrolite order during dehydration and the subsequent framework collapse. The dehydrated phases of Rb- and Cs-natrolite can be stabilized at ambient conditions

Chemical and Hydrostatic Pressure in Natrolites: Pressure-Induced Hydration of an Aluminogermanate Natrolite

The ambient structure and pressure-induced structural changes of a synthetic sodium aluminogermanate with a natrolite (NAT) framework topology (Na−AlGe-NAT) were characterized by using Rietveld refinements of high-resolution synchrotron X-ray powder diffraction data at ambient and high pressures. Unlike a previously established model for Na8Al8Ge12O40·8H2O based on a single-crystal study, the ambient structure of the Na−AlGe-NAT is found to adopt a monoclinic space group Cc (or Fd) with a ca. 6% expanded unit cell. The refined ambient structure of Na8Al8Ge12O40·12H2O indicates an increased water content of 50%, compared to the single-crystal structure. The unit-cell volume and water-content relationships observed between the two Na−AlGe-NAT structures at ambient conditions with 8 and 12 H2O respectively seem to mirror the ones found under hydrostatic pressure between the Na8Al8Si12O40·8H2O and the parantrolite phase Na8Al8Si12O40·12H2O. Under hydrostatic pressures mediated by a pore-penetrating alcohol and water mixture, the monoclinic Na−AlGe-NAT exhibits a gradual decrease of the unit-cell volume up to ca. 2.0 GPa, where the unit-cell volume then contracts abruptly by ca. 4.6%. This is in marked contrast to what is observed in the Na−AlSi-NAT and Na−GaSi-NAT systems, where one observes a pressure-induced hydration and volume expansion due to the auxetic nature of the frameworks. Above 2 GPa, the monoclinic phase of Na−AlGe-NAT transforms into a tetragonal structure with the unit-cell composition of Na8Al8Ge12O40·16H2O, revealing pressure-induced hydration and a unit cell volume contraction. Unlike in the Na−Al,Si-paranatrolite phase, however, the sodium cations in the Na−AlGe-NAT maintain a 6-fold coordination in the monoclinic structure and only become 7-fold coordinated at higher pressures in the tetragonal structure. When comparing the pressure-induced hydration in the observed natrolite-type zeolites, Na−AlGe-NAT appears to have a nonauxetic framework and reveals the highest onset pressure for complete superhydration.

Paramagnetism in single-phase Sn1−xCoxO2 dilute magnetic semiconductors

Transition metal (TM) Co doped SnO2 dilute magnetic semiconductor (DMS) in both nano- and bulk state are prepared by solvothermal and mechanosyhtesis route, respectively. Contraction in unit cell volume of tetragonal rutile SnO2 after Co doping and redshift in energy band gap compared to that of undoped SnO2 ensures the incorporation of smaller Co2+ ions replacing larger host cations Sn4+. Vibrating sample magnetometer measurements show that paramagnetism is the intrinsic magnetic property in single-phase Sn1−xCoxO whereas non-DMS related ferromagnetism is associated only with the corresponding nanostructures. Paramagnetism is also confirmed by thermal dependence of magnetization M (T) and magnetic susceptibility (χ) measurement in between 80≤T≤350 K.

Topotactic Redox Chemistry of NaFeAs in Water and Air and Superconducting Behavior with Stoichiometry Change

We report experimental evidence that shows superconductivity in NaFeAs occurs when it is Na deficient. The oxidation of NaFeAs progresses differently in water and in air. In water the material oxidizes slowly and slightly retaining the original anti-PbFCl structure. In air NaFeAs oxidizes topotactically quickly and extensively transforming to the ThCr2Si2 structure type. Water acts as a mild oxidizing agent on the FeAs layer by extracting electrons and Na+ cations from the structure, while oxidation in air is more extensive and leads to change in structure type from NaFeAs to NaFe2As2. The superconducting transition temperature moves dramatically during the oxidation process. Exposed to water for an extended time period NaFeAs shows a substantial increase in Tc up to 25 K with contraction of unit cell volume. NaFe2As2, the air oxidized product, shows Tc of 12 K. We report detailed characterization of the redox chemistry and transformation of NaFeAs in water and air using single crystal and powder X-ray di...

Mixed conducting perovskite-like ceramics on the base of lanthanum gallate

Ceramic perovskite solid solutions (La 0.9Sr 0.1)[(Ga 1− x M x ) 0.8Mg 0.2]O 3− y , 0 ≤ x ≤ 0.5, M = Fe, Ni, Cr (systems I–III) and brownmillerite solid solutions (La 0.2Sr 1.8)[Ga(Fe 1− x Mg x )]O 5− z , 0 ≤ x ≤ 0.5, (system IV) have been prepared. The samples have been studied by X-ray diffraction and electron microscopy methods, dielectric spectroscopy and permeability measurements. The correlation between the composition, unit cell parameter changes, electrical transport and oxygen permeation properties has been revealed. Introduction of transition metals (Fe, Ni, or Cr), substituting for gallium, ensures the enhancement of the electronic constituent of the conductivity in the perovskite systems I–III. Stabilization of the transition metal high valence states 4+ or 5+ has been suggested for compositions I and III. This leads to a unit cell volume contraction and provides a decrease in the concentration of oxygen vacancies. The oxygen permeability reaches its maximum values in compositions I–III with x ∼ 0.3. On the contrary, increasing concentration of the doping element with lower valence state (magnesium), substituting for iron, determines the expansion of the brownmillerite unit cell volume and provides an increase of the oxygen vacancy concentration, which in turn, favors the enhancement of oxygen permeability of composition IV.

Structure and magnetic properties of (Nd1－xGdx)3 Fe27.31Ti1.69 compounds

The structure and magnetic properties of (Nd1-xGdx)3Fe27.31Ti1.69 compounds have been investigated by means of X-ray diffraction and magnetic measurements. All compounds with x in the range of x≤0.6 crystallize in Nd3(Fe, Ti)29-type structure. Substitution of Gd for Nd leads to a contraction of unit-cell volume. The Curie temperature TC and anisotropy field Ba of the (Nd1-xGdx)3Fe27.31Ti1.69 compounds increases monotonically with increasing Gd content. Saturation magnetization Ms of the (Nd1-xGdx)3Fe27.31Ti1.69 compounds decreases with increasing x. As temperature lowers from room temperature to 5K, a spin reorientation from the easy plane to the easy cone occurs for (Nd1-xGdx)3Fe27.31Ti1.69 compounds. Spin phase diagrams have been constructed for (Nd1-xGdx)3Fe27.31Ti1.69 compounds.

Isostructural phase transition in m-carboxyphenylammonium monohydrogenphosphite

Crystals of m-carboxyphenylammonium monohydrogenphosphite, C7H8NO2+.H2PO3- (m-CPAMP), space group P2(1)/c, grown from aqueous solution undergo a reversible first-order single-crystal phase transition at Tc = 246 (2) K with a hysteresis of 3.6 K. The thermal behaviour of the sample was characterized by differential scanning calorimetry (DSC) experiments. Variations of the unit-cell parameters versus temperature between 100 and 320 K are reported. The transition from the higher-temperature phase (HTP) to the lower-temperature phase (LTP) is characterized by a unit-cell volume contraction of 1.77%. The average structure and unit-cell packing of m-CPAMP at lower temperature (100 K) are reported from accurate X-ray data sets and compared with those of the higher-temperature phase (293 K) in order to investigate the mechanism of the phase transition. The reciprocal lattice reconstruction showed a few very weak satellite reflections which will be discussed in a forthcoming paper.

Evolution of the Crystal and Electronic Structures of Magnetic RuSr2-xLaxGdCu2O8 Superconductors upon La Substitution

We have investigated systematically the effects of La substitution on the chemical bonding nature and physical properties of magnetic RuSr(2-x)La(x)GdCu2O8 superconductors. X-ray diffraction and energy dispersive spectroscopic microprobe analyses reveal that a fraction of Sr ions can be successfully replaced by La ions with the contraction of unit cell volume. According to electrical resistance and dc magnetization measurements, the La substitution gives rise to a significant reduction of superconducting transition temperature (T(c)) but to an increase of magnetic ordering temperature with depressed remanent magnetization. Ru K- and Cu K-edge X-ray absorption spectroscopic results clarify that average Ru and Cu oxidation states decrease upon the La substitution. On the basis of the spectroscopic evidences presented here, we are able to attribute the T(c) reduction upon the La substitution to the depletion of the hole density in CuO2 layers and the accompanying variation of magnetic coupling behavior to the change of Ru oxidation state.

Critical current density and flux pinning in La 2− xPr xCa 2 x Ba 2Cu 4+2 xO z ( x=0.1–0.5) superconductors

Polycrystalline La 2− x Pr x Ca 2 x Ba 2Cu 4+2 x O z (LPCaBCO) compounds with x=0.1–0.5 were synthesized by solid-state reaction method and studied by room temperature X-ray diffraction, dc resistivity, dc magnetization and iodometry. The superconducting transition temperatures in these tetragonal triple perovskite compounds increases from 32 to 62 K ( T c onset values) with increasing dopant concentration. The mixing of rare earth La 3+ and Pr 3+/4+ ions at rare earth site (La 3+) along with substitution of divalent Ca 2+ results in the shrinkage of unit cell volume. The contraction of unit cell volume due to larger ion being substituted by smaller ions, gives rise to creation of pinning centres in the unit cell leading to increase in critical current density and flux pinning.

Critical current density and flux pinning in La2$minus;xPrxCa2xBa2Cu4+2xOz (x=0.1?0.5) superconductors

Polycrystalline La2−xPrxCa2xBa2Cu4+2xOz (LPCaBCO) compounds with x=0.1–0.5 were synthesized by solid-state reaction method and studied by room temperature X-ray diffraction, dc resistivity, dc magnetization and iodometry. The superconducting transition temperatures in these tetragonal triple perovskite compounds increases from 32 to 62 K (Tconset values) with increasing dopant concentration. The mixing of rare earth La3+ and Pr3+/4+ ions at rare earth site (La3+) along with substitution of divalent Ca2+ results in the shrinkage of unit cell volume. The contraction of unit cell volume due to larger ion being substituted by smaller ions, gives rise to creation of pinning centres in the unit cell leading to increase in critical current density and flux pinning.

Effect of Mn substitution on magnetoelectric properties of bismuth ferrite system

The effect of Mn substitution on ferroelectric and antiferromagnetic properties of the Bi0.9La0.1Fe1−xMnxO3 system has been studied. It is seen that, with the increase in Mn content, there is a contraction in unit cell volume. Since the cell volume contraction is isotropic, lattice distortion (c/a) remains the same and so is the ferroelectric transition temperature TC. Further, it is observed that the presence of Mn does not affect the loss tangent (tan δ) of the samples. On the contrary, with increase in Mn content, a small but linear enhancement in the magnetization is observed.

Structural and magnetic properties of (Nd1-xErx)3Fe273Ti17 compounds

Structural and magnetic properties of (Nd1-xErx)3Fe273Ti17 compounds have been investigated by means of x-ray diffraction and magnetic measurements. All the compounds with x in the range of 00—05 crystallize in a Nd3(Fe,Ti)29-type structure. Substitution of Er for Nd leads to a contraction of unit-cell volume. Curie temperature TC of the (Nd1-xErx)3Fe273Ti17 compounds decreases monotonically with increasing Er content. Saturation magnetization Ms of the (Nd1-xErx)3Fe273Ti17 compounds decreases with increasing x. As temperature decreases from room temperature to 5K, a spin reorientation from easy plane to easy cone occurs for (Nd1-xErx)3Fe273Ti17 compounds. The anisotropy field Ba for(Nd1-xErx)3Fe273Ti17 compounds decreases with increasing Er content. Spin-phase diagrams have been constructed for (Nd1-xErx)3Fe273Ti17 compounds.

Size effect study in magnetoelectric BiFeO3 system

In this paper, we report for the first time finite size effects on Neel temperature (T N) of magnetoelectric BiFeO3 system. Novel wet chemical route has been developed to produce fine particles of BiFeO3 with controlled size and size distribution. Unlike other oxide systems, lattice volume contraction has been observed with decrease in particle size. The decrease in T N is co-related to unit cell volume contraction occurring with reduction in particle size.