Non-random cation distribution in hexagonal Al0.5Ga0.5PO4

Based on powder X-ray diffraction and ³¹P Magic Angle Spinning Nuclear Magnetic Resonance (MAS NMR) investigations of mixed phosphate Al_0.5Ga_0.5PO₄, prepared by co-precipitation method followed by annealing at 900 °C for 24 h, it is shown that Al_0.5Ga_0.5PO₄ phase crystallizes in hexagonal form with lattice parameter a=0.491(2) and c=1.106(4) nm. This hexagonal phase of Al_0.5Ga_0.5PO₄ is similar to that of pure GaPO₄. The ³¹P MAS NMR spectrum of the mixed phosphate sample consists of five peaks with systematic variation of their chemical shift values and is arising due to existence of P structural units having varying number of the Al³⁺/Ga³⁺ cations as the next nearest neighbors in the solid solution. Based on the intensity analysis of the component NMR spectra of Al_0.5Ga_0.5PO₄, it is inferred that the distribution of Al³⁺ and Ga³⁺ cations is non-random for the hexagonal Al_0.5Ga_0.5PO₄ sample although XRD patterns showed a well-defined solid solution formation.     

Gd5Ni0.96Sb2.04 and Gd5Ni0.71Bi2.29: Crystal structure, magnetic properties and magnetocaloric effect. Structural transformation and magnetic properties of hexagonal Gd5Bi3

Nickel was successfully introduced into the Gd₅Sb₃ and Gd₅Bi₃ binaries to yield the Gd₅Ni_0.96(1)Sb_2.04(1) and Gd₅Ni_0.71(1)Bi_2.29(1) phases. Both Ni-substituted compounds adopt the orthorhombic Yb₅Sb₃-type structure. While the Gd₅Ni_0.71Bi_2.29 phase is thermodynamically stable at 800 °C and decomposes at lower temperatures upon annealing, it can be easily quenched to room temperature by rapid cooling from 800 °C. The Gd₅Ni_0.96Sb_2.04 phase is found to be thermodynamically stable till room temperature. Through annealing at different temperatures, Gd₅Bi₃ was proven to undergo the Mn₅Si₃-type (LT)↔Yb₅Sb₃-type (HT) transformation reversibly, whereas Gd₅Sb₃ was found to adopt only the hexagonal Mn₅Si₃-type structure. Orthorhombic Gd₅Ni_0.96Sb_2.04 and Gd₅Ni_0.71Bi_2.29 and low-temperature hexagonal Gd₅Bi₃ order ferromagnetically at 115, 162 and 112 K, respectively. In Gd₅Bi₃, the ferromagnetic ordering is followed by spin reorientation below 64 K. Magnetocaloric effect in terms of ΔS was evaluated from the magnetization data and found to reach the maximum values of −7.7 J/kgK for Gd₅Ni_0.96Sb_2.04 and −5.6 J/kgK for Gd₅Ni_0.71Bi_2.29 around their Curie temperatures.     

Competition of phase dissolution and crystallization in poly(ε-caprolactone)/poly(styrene-co-acrylonitrile) blend

Blend of poly(ε-caprolactone) (PCL) and poly(styrene-co-acrylonitrile) (SAN) containing 27.5 wt% of acrylonitrile was studied. The PCL/SAN blend having LCST (lower critical solution temperature) phase boundary above the melting point T_m of PCL offered an excellent opportunity to investigate the competition of liquid-solid phase transition (crystallization) and liquid-liquid phase transition (phase dissolution). A blend with the critical composition (80/20 PCL/SAN) underwent a temperature-jump above LCST to proceed spinodal decomposition, yielding a regularly phase-separated structure (SD structure). Then, it was quenched to the temperatures below T_m at which both the crystallization and the phase dissolution could occur. By transmission electron microscopy it was found that during isothermal annealing after quenching to high temperatures close to T_m (e.g. 51 °C), the SD structure gradually disappeared, and then the crystallization started from a single-phase mixture to yield normal crystalline structure similar to that of a neat crystalline polymer. At lower temperatures (e.g. 40 °C), crystallization quickly occurred and the SD structure was preserved, implying that the crystallization prevailed over the dissolution yielding a bi-continuous structure consisting of amorphous (SAN-rich) and crystalline (PCL-rich) regions. At intermediate temperatures (e.g. 45 °C), the phase dissolution competed with the crystallization, resulting in a bi-continuous structure with longer periodic distance and a broad boundary having a gradient in composition of amorphous region between PCL crystal lamellae. Light-scattering analysis quantitatively revealed a competition of the crystallization and the phase dissolution in terms of the crystallization rate (from H_v scattering) and the apparent diffusion coefficient for dissolution (from V_v scattering).     

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.     

Evolution of titania nanotubes-supported WOx species by in situ thermo-Raman spectroscopy, X-ray diffraction and high resolution transmission electron microscopy

Structural evolution of WO_x species on the surface of titania nanotubes was followed by in situ thermo-Raman spectroscopy. A total of 15 wt% of W atoms were loaded on the surface of a hydroxylated titania nanotubes by impregnation with ammonium metatungstate solution and then, the sample was thermally treated in a Linkam cell at different temperatures in nitrogen flow. The band characteristic of the WO bond was observed at 962 cm⁻¹ in the dried sample, which vanished between 300 and 700 °C, and reappear again after annealing at 800 °C, along with a broad band centered at 935 cm⁻¹, attributed to the v₁ vibration of WO in tetrahedral coordination. At 900 and 1000 °C, the broad band decomposed into four bands at 923, 934, 940 and 950 cm⁻¹, corresponding to the symmetric and asymmetric vibration of WO bonds in Na₂WO₄ and Na₂W₂O₇ phases as determined by X-ray diffraction and High resolution transmission electron microscopy (HRTEM). The structure of the nanotubular support was kept at temperatures below 450 °C, thereafter, it transformed into anatase being stabilized at temperatures as high as 900 °C. At 1000 °C, anatase phase partially converted into rutile. After annealing at 1000 °C, a core-shell model material was obtained, with a shell of ca. 5 nm thickness, composed of sodium tungstate nanoclusters, and a core composed mainly of rutile TiO₂ phase.     

Noncrystalline blue-emitting 9,10-diphenylanthracene end-capped with triphenylamine-substituted fluorene

Two blue-emitting oligomers, namely FDPA1 and FDPA2 containing 9,10-diphenylanthracene core end-capped with triphenylamine-substituted fluorene has been synthesized and characterized. The spiro-configuration end-capping groups imparts two compounds with pronounced morphological stability (T_g > 185 °C, T_d > 420 °C) and excellent hole injection ability (E_HOMO > −5.27 eV) with the advantageous optical characteristics of corresponding core. Scanning electron microscope (SEM) and X-ray diffraction (XRD) reveal that the two oligomers form excellent amorphous films and possess good morphological stability after annealing.     

The chemical and dielectric properties of epoxy/(Ba0.8Sr0.2)(Ti0.9Zr0.1)O3 composites for embedded capacitor application

The characteristics of epoxy/(Ba_0.8Sr_0.2)(Ti_0.9Zr_0.1)O₃ (BSTZ) composites are investigated for the further application in embedded capacitor device. The effects of BSTZ ceramic powder filler ratio on the chemical, physical and dielectric properties of epoxy/BSTZ composites are studied. Differential scanning calorimeter (DSC) thermal analysis is conducted to determine the optimum values of hardener agent, curing temperature, reaction heat, and glass transition temperature (T_g). The hardener reaction process starts at about 115 °C and completes at about 200 °C, for that it is appropriate to process of epoxy/BSTZ composites in the range of temperature. The highest glass transition temperature (T_g) of 155 °C is obtained at one equivalent weight ratio (hardener/epoxy). Only the BSTZ phase can be detected in the XRD patterns of epoxy/BSTZ composites. The more BSTZ ceramic powder is mixed with epoxy, the higher crystalline intensity of tetragonal BSTZ phase are revealed in the XRD patterns. The dielectric constant measured at 1 MHz increases from 5.8 to 23.6 as the content of BSTZ ceramic powder in the epoxy/BSTZ composites increases from 10 to 70 wt%. The loss tangents of the epoxy/BSTZ composites slightly increase with the increase of measurement frequency.     

A study of the reactivity of elemental Cr/Se/Te thin multilayers using X-ray reflectometry, in situ X-ray diffraction and X-ray absorption spectroscopy

The reactivity of [Cr/Se/Te] multilayers under annealing was investigated using X-ray reflectometry, in situ X-ray diffraction, X-ray absorption fine structure (XAFS) measurements and transmission electron microscopy. For all samples, interdiffusion was complete at temperatures between 100 and 300 °C, depending on the repeating tri-layer thickness. A crystalline phase nucleated approximately 20 °C above the temperature where interdiffusion was finished. The first crystalline phase in a binary Cr/Te sample was layered CrTe₃ nucleating at 230 °C. In ternary samples (Se:Te=0.6-1.2), the low-temperature nucleation of such a layered CrQ₃ (Q=Se, Te) phase is suppressed and instead the phase Cr₂Q₃ nucleates first. Interestingly, this phase decomposes around 500 °C into layered CrQ₃. In contrast, binary Cr/Se samples form stable amorphous alloys after interdiffusion and Cr₃Se₄ nucleates around 500 °C as the only crystalline phase. Evaluation of the XAFS data of annealed samples yield Se-Cr distances of 2.568(1) and 2.552(1) Å for Cr₂Q₃ and CrQ₃, respectively. In the latter sample, higher coordination shells around Se are seen accounting for the Se-Te contacts in the structure.     

Thermal chemiluminescence of fibrous proteins

Thermal chemiluminescence (TCL) from the fibrous proteins wool and feather keratin, silk fibroin and Type I collagen is reported for the first time. The proteins all emit TCL when heated in the atmosphere of O₂ or N₂ in the range 40-220 °C. Plotting non-isothermal CL data in O₂ in Arrhenius format showed an increase in the activation energy at temperatures in the range 129-161 °C for each protein. This may indicate that a different free radical oxidation process operates when the mobility of the amorphous phase of the protein is increased above its T_g. Wool, silk and collagen exhibited a luminescence peak at 130 °C (with feather keratin at 145 °C) during non-isothermal CL experiments in N₂, similar to that observed in many synthetic polymers and characteristic of polymer hydroperoxides.     

Formation and structural refinements of tunneled intergrowth phases in the Ga2O3-In2O3-SnO2-TiO2 system

Over 100 samples were prepared as (Ga,In)₄(Sn,Ti)_n−4O_2n−2, n=6, 7, and 9 by solid-state reaction at 1400 °C and characterized by X-ray diffraction. Nominally phase-pure beta-gallia-rutile intergrowths were observed in samples prepared with n=9 (0.17?x?0.35 and 0?y?0.4) as well as in a few samples prepared with n=6 and 7. Rietveld analysis of neutron time-of-flight powder diffraction data were conducted for three phase-pure samples. The n=6 phase Ga_3.24In_0.76Sn_1.6Ti_0.4O₁₀ is monoclinic, P2/m, with Z=2 and a=11.5934(3) Å, b=3.12529(9) Å, c=10.6549(3) Å, β=99.146(1)°. The n=7 phase Ga_3.24In_0.76Sn_2.4Ti_0.6O₁₂ is monoclinic, C2/m, with Z=2 and a=14.2644(1) Å, b=3.12751(2) Å, c=10.6251(8) Å, β=108.405(1)°. The n=9 phase Ga_3.16In_0.84Sn₄TiO₁₆ is monoclinic, C2/m, with Z=2 a=18.1754(2) Å, b=3.13388(3) Å, c=10.60671(9) Å, β=102.657(1)°. All of the structures are similar in that they possess distorted hexagonal tunnels parallel to the [010] vector.     

The new ternary silicide Gd5CoSi2: Structural, magnetic and magnetocaloric properties

Gd₅CoSi₂ was prepared by annealing at 1003 K. Its investigation by the X-ray powder diffraction shows that the ternary silicide crystallizes in a tetragonal structure deriving from the Cr₅B₃-type (I4/mcm space group; a=7.5799(4) and c=13.5091(12) Å as unit cell parameters). The Rietveld refinement shows a mixed occupancy on the (8h) site between Si and Co atoms. Magnetization and specific heat measurements performed on Gd₅CoSi₂ reveal a ferromagnetic behaviour below T_C=168 K. This magnetic ordering is associated to an interesting magnetocaloric effect; the adiabatic temperature change ΔT_ad is about 3.1 and 5.9 K, respectively, for a magnetic field change of 2 and 4.6 T.     

Temperature-dependent structural study of microporous CsAlSi5O12

CsAlSi₅O₁₂ crystals were synthesized at high temperature by slow cooling of a vanadium oxide flux. Single-crystal X-ray diffraction structure analysis and electron microprobe analyses yielded the microporous CAS zeolite framework structure of Cs_0.85Al_0.85Si_5.15O₁₂ composition. High-temperature single-crystal and powder X-ray diffraction studies were utilized to analyze anisotropic thermal expansion. Rietveld refined cell constants from powder diffraction data, measured in steps of 25 °C up to 700 °C, show a significant decrease in expansion above 500 °C. At 500 °C, a displacive, static disorder-dynamic disorder-type phase transition from the acentric low-temperature space group Ama2 to centrosymmetric Amam (Cmcm in standard setting) was found. Thermal expansion below the phase transition is governed by rigid-body TO₄ rotations accompanied by stretching of T-O-T angles. Above the phase transition at 500 °C all atoms, except one oxygen (O6), are fixed on mirror planes. Temperature-dependent polarized Raman single-crystal spectra between −270 and 300 °C and unpolarized spectra between room temperature and 1000 °C become increasingly less resolved with rising temperature confirming the disordered static-disordered dynamic type of the phase transition.     

Synthesis of anatase nanoparticles with extremely wide solid solution range and ScTiNbO6 with α-PbO2 structure

Anatase-type nanoparticles Sc_XTi_1−2XNb_XO₂ with wide solid solution range (X=0-0.35) were hydrothermally formed at 180 °C for 5 h. The lattice parameters a₀ and c₀, and the optical band gap of anatase gradually and linearly increased with the increase of the content of niobium and scandium from X=0 to 0.35. Their photocatalytic activity and adsorptivity by the measurement of the concentration of methylene blue (MB) that remained in the solution in the dark or under UV-light irradiation were evaluated. The anatase phase existed stably up to 900 °C for the samples with X=0.25-0.30 and 750 °C for that with X=0.35 during heat treatment in air. The phase with α-PbO₂ structure and the rutile phases coexisted in the samples with X=0.25-0.30 after heated at temperatures above 900-950 °C. The α-PbO₂ structure having composition ScTiNbO₆ with possibly some cation order similar to that seen in wolframite existed as almost completely single phase after heat treatment at temperatures 900-1500 °C through phase transformation from anatase-type ScTiNbO₆.     

Dependence of nitrogen doping on TiO2 precursor annealed under NH3 flow

N-doped TiO₂ photocatalysts were prepared by annealing two different precursors, P25 and a TiO₂ xerogel powder under NH₃/Ar flow at 500, 550, and 600 °C. The xerogel powder prepared by peptizing Ti(OH)₄ with HNO₃ was composed of nanoparticles and had large specific surface area. During the annealing process, the xerogel powder underwent increase in crystallinity, grain growth and phase transformation, whereas P25 did not show obvious changes. Compared with the N-doped TiO₂ photocatalysts from P25, the N-doped TiO₂ photocatalysts from the xerogel powder possessed higher concentrations of the substitutional nitrogen and exhibited more obvious absorption in the visible light region. The N-doped TiO₂ photocatalysts from the xerogel powder exhibited obvious visible-light activities for photodegrading methylene blue and the sample prepared at 500 °C achieved the best performance with a rate constant (k) about 0.44 h⁻¹, whereas those from P25 did not exhibit improved visible-light activities.     

Advances in the structure and microstructure determination of yttrium silicates using the Rietveld method

The Y₂O₃-SiO₂ 1:1 composition doped with a weak concentration of europium ions was prepared with the sol-gel technique and the products studied by X-ray diffraction as a function of temperature in the range from 900 to 1300 °C, using the method of Rietveld for quantitative evaluation of amorphous and crystalline evolving phases. The amorphous profile of the yttrium oxyorthosilicate glasses has been described following the “Rietveld for Disordered Materials” method and subsequently included in the patterns of semicrystalline samples that have been heat-treated for temperatures above 900 °C at 1000, 1100, 1150, 1200 and 1300 °C. The quantitative evaluation of the amorphous phase is obtainable from the Rietveld approach equivalent to the method after Ruland. This enabled us to study in fine detail the structural rearrangements and growth mechanisms that take place during the crystal-to-amorphous transformation in terms of coordination numbers, average interatomic distances, average crystallite size and microstrain and to identify the polymorphous transformation involving the Y₂SiO₅ phase from low-to-high-temperature forms, as well as some minor quantities of other phases namely α-Y₂Si₂O₇ phase, Y₂O₃ and Y_4.67(SiO₄)₃O.     

On the interaction of copper with defects in gallium arsenide

The behavior in gallium arsenide of copper introduced through the melt has been investigated. Measurements have been made on the electrical and photoluminescent properties of single crystals that had either been quenched from T_cryst, slowly cooled after crystallization or thermally annealed at 1100°C after completion of the growth. On quenching from T_cryst, formation of very shallow acceptors is observed. On slow cooling or annealing at 1100°C, acceptor levels are formed of the order of 0.12 and 0.02 eV. The concentration of carriers for both levels diminishes with lower temperatures of annealing and with concentration of copper added. The 0.12 eV level is assigned to the center (Cu_GaV_Ga)⁻.     

A MTDSC analysis of phase transition in polyurethane-organoclay nanocomposites

Modulated differential scanning calorimetry (MTDSC) was applied to investigate the phase transition behaviour of polyurethane/organoclay nanocomposites. The endotherm transitions located at 50-80 and 140 °C were re-analysed through revealed thermal features in the MTDSC reversing, non-reversing and dC_p/dT curves. It was proposed that the diffused interfacial phase exists between hard and soft phases in the polyurethane system. The assignment of endotherm at 140 °C is attributed to the hard microphase domain transition, which is similar to an order-disorder transition. The transition in the region of 50-80 °C reveals the relaxation of segments in the interface resulted by annealing. The addition of organoclay resulted in a reduction of hard domain ordering level. A simple method for quantitatively estimating the amount of polymer chains intercalated into layers of clay was introduced, and the relationship between the weight fraction (ω_p), of the polymer intercalated into layers and the weight fraction (ω_c), of clay was established.     

Synthesis and characterization of three-dimensionally ordered macroporous ternary oxide

A three-dimensionally ordered macroporous (3DOM) ternary oxide, CsAlTiO₄, with a framework related to ‘stuffed-tridymite’ has been synthesized at temperatures 500-700 °C using a sol-gel precursor solution and templating with polystyrene spheres. The 3DOM material displayed pore diameters of 0.5-0.8 μm with the walls composed of anhedral and acicular CsAlTiO₄ crystals whose dimensions ranged from 16 to 25 nm. Microanalysis confirmed near-stoichiometric proportions (1:1:1) of Cs, Al and Ti. The effect of sintering temperature on the macroporous structure and on the CsAlTiO₄ walls was studied. As the sintering temperature increased from 500 to 600 °C the unit cell parameters varied through dilation (a and b) and contraction (c-axis), followed by a reversal of these trends from 700 to 900 °C. This behaviour in non-equilibrated CsAlTiO₄ can be attributed to distortion of the (Al, Ti)O₄ tetrahedral framework, however at the highest temperature the cell constants stabilized close to those reported for single crystal CsAlTiO₄. X-ray amorphous content was significant in all materials varying from 73 wt% after 500^oC and reducing to 44 wt% at 900 °C.     

Preparation, crystal structure, and superconductive characteristics of new oxynitrides (Nb1−xMx)(N1−yOy) where M=Mg, Si, and x≈y

New niobium oxynitrides containing either magnesium or silicon were prepared at 1000 °C by ammonia nitridation of oxide precursors obtained via the citrate route. The products had rock-salt type crystal structures. Crystallinity was improved by annealing in 0.5 MPa N₂ and the final compositions were (Nb_0.95Mg_0.05)(N_0.92O_0.08) at 1500 °C and (Nb_0.87Si_0.09□_0.04)(N_0.87O_0.13) at 1200 °C. The magnesium and oxide ions partially co-substitute the niobium and nitride ions in the octahedral sites of the δ-NbN lattice, respectively. Silicon ions were also successfully doped together with oxide ions into the rock-salt type NbN lattice. The Si doped product exhibited relatively large displacement at the octahedral sites and was accompanied by a small amount of cation vacancies. Superconductivity was improved by annealing to obtain critical temperatures/volume fractions of T_c=17.6 K/100% for Mg- and T_c=16.2 K/95% for the Si-doped niobium oxynitrides.     

Hydrolytic degradation of carbohydrate-based polyamides of the AABB type derived from l-arabinose and d-xylose

The hydrolytic degradation of a series of aregic carbohydrate-based polyamides derived from l-arabinose and d-xylose is described. These polyamides are those that are fully sugar-based (PA-SuSu), those derived from aldaric acids and polyalkylene diamines (PA-mSu), and those derived from diamine sugars and polyalkylene dicarboxylic acids (PA-Sun). Their physical properties and crystal structures depend on their constitution and the configuration of the carbohydrate-based moiety. The feasibility of the hydrolysis of these polyamides was, in general, related with such structural properties. Thus, the fully sugar-based PA-SuSu were amorphous, water-soluble materials, and were hydrolysed in water at 70 °C. PA-mSu were crystalline and more resistant to hydrolysis — they were degraded at pH 2 and 70 °C [T_g(s) 60-90 °C]. PA-Sun were amorphous and highly hygroscopic materials — they were hydrolysed in water at 37 °C [T_g(s) 25-40 °C].