Barium titanate via thermal decomposition of Ba,Ti-precursor complexes: The nature of the intermediate phases

The thermal decomposition of Ba,Ti-precursor complexes, containing organic ligands and suitable for the single-source preparation of nanocrystalline BaTiO₃, leads firstly to the segregation of specific Ba-rich and Ti-rich phases. Quantitative electron energy loss spectroscopy and powder X-ray diffraction data indicated that the (i) Ba-rich phase is a BaO-stabilised variant of the calcite-type high-temperature modification of BaCO₃ and (ii) Ti-rich phases are represented by low crystalline barium titanates with the general Ba:Ti ratio close to 1:4. The subsequent solid state reaction between these phases results then in the formation of BaTiO₃.     

The effect of the silicon incorporation on the hydroxylapatite structure. A neutron diffraction study

Nominal stoichiometric hydroxylapatite, Ca₁₀(PO₄)₆(OH)₂ and silicon-substituted HA, containing 0.9 wt% in silicon, were prepared by the ceramic method. The samples were analyzed by elemental chemical analysis, Fourier transformed infrared spectroscopy (FTIR) and the structural study was carried out by Rietveld analysis combining powder X-ray (XRD) and neutron diffraction (ND). The refinements of the occupancies at the 4e Wyckoff position shows that the OH⁻/O²⁻ ratio is higher when SiO₄⁴⁻ substitutes PO₄³⁻. The observation of the Fourier maps points out that also the HPO₄²⁻ formation from PO₄³⁻ is also higher when Si is included in the HA structure. For the first time, the atomic coordinates for the H atoms of HPO₄²⁻ anions are provided.     

In-situ X-ray diffraction study of carbonate formation and decomposition in perovskite-type BCFZ

The effect of CO₂ on the two phase perovskite with average composition of BaCo_0.4Fe_0.4Zr_0.2O_3−δ is investigated by in-situ X-ray diffraction (XRD) as well as transmission electron microscopy (TEM). Partial decomposition of the BCFZ into high-temperature rhombohedral BaCO₃ polymorph was observed during annealing in an atmosphere, which contained 50 vol% CO₂/50 vol% N₂ at 1173 K. This carbonate structure is not quenchable and cannot be detected by ex-situ methods. Additionally, the reversible phase transition of BaCO₃ from orthorhombic to rhombohedral to cubic at different temperatures accompanied by formation of CoO was shown by in-situ XRD and TEM. Furthermore, complete regeneration of perovskite phase was obtained after high-temperature treatment under CO₂-free conditions.     

The formation of the Ba(CuOx)1−y(CO3)y

Carbocuprate compounds are generally described as multiple perovskites with CO ₃ ² - and Cu-O alternating layers containing Ba and/or Sr; they have gained an unexpected importance in the high temperature superconductivity field, because many compounds with transition temperature above 100 K belong to this class of materials.We have started a systematic study on phase formation and stabilisation in the Ba-Cu-C-O system in the temperature range 20-600°C, by using thermal analysis techniques. Starting from a BaCO₃-BaO₂-CuO mixture (311 mol), a new phase isomorphic with BaCO₃ formed after heat treatment above 390°C in air. TG, DSC, EGA and high temperature XRD were employed to follow the complex interaction of the reactants with the atmosphere and the formation of the new phase.     

Synthesis of barium and strontium carbonate crystals with unusual morphologies using an organic additive

In this paper, strontium carbonate (SrCO₃) and barium carbonate (BaCO₃) crystals were synthesized in the presence of an organic additive-hexamethylenetetramine (HMT) using two CO₂ sources. Scanning electron microscopy and X-ray powder diffractometry were used to characterize the products. The results showed that the morphologies of orthorhombic strontianite SrCO₃ transformed from branch-like to flower-like, and to capsicum-like at last, while the morphologies of BaCO₃ change from fiber-like to branchlike, and to rod-like finally with an increase of the molar ratio HMT/Sr²⁺ and HMT/Ba²⁺ from 0.2 to 10 using ammonium carbonate as CO₂ source. When using diethyl carbonate instead of ammonium carbonate as CO₂ source, SrCO₃ flowers aggregated by rods and BaCO₃ shuttles were formed. The possible formation mechanisms of SrCO₃ and BaCO₃ crystals obtained in different conditions were also discussed.     

The effect of calcination conditions on the superconducting properties of Y−Ba−Cu−O powders

Calcination conditions of the precursor powders, i.e. temperature, type of atmosphere and duration, were determined with a view to obtain superconducting powders with the most advantageous physico-chemical properties. Investigated were powders in the Y?Ba?Cu?O system prepared by the sol-gel method. Thermogravimetric examinations of the powders have revealed that the decomposition kinetics of BaCO₃ determines the formation rate of the superconducting YBa₂Cu₃O_7?x (‘123’) phase. It follows from the decomposition kinetics of BaCO₃ that the process is the most intensive in argon, whereas in static air and oxygen it is the slowest. The phase composition analysis (XRD) and low-temperature magnetic susceptibility measurements of the calcinated powders, confirm the above mentioned changes in the decomposition kinetics. The reaction of barium carbonate can be completed if the calcination process is conducted at the temperature of 850°C for 25 h, yielding easily sinterable powders for obtaining single-phase superconducting bulk samples with advantageous functional parameters.     

The effect of the Co-anion,SCN−, on macrocycle-facilitated selective transport of Cd (II) over Ag (I) in an emulsion membrane

Emulsion membrane systems consisting of (1) an aqueous source phase containing 0.001 M Cd (NO₃)₂ and/or 0.001 M AgNO₃ and varying concentrations of SCN⁻, (2) a toluene membrane containing dicyclohexano-18-crown-6 (DC18C6) (0.02 M) and the surfactant Span 80 (sorbitan monooleate) (3% v/v), and (3) an aqueous receiving phase containing MgS₂O₃ or Mg (NO₃)₂ were studied with respect to the disappearance of Cd (II) and/or Ag (I) from the source phase as a function of time. The transport rate of Cd (II) was highest when a maximum amount of the Cd(II) in the source phase was present as Cd(SCN)₂ ([SCN⁻] =0.4 M). Cadmium(II) was transported over Ag(I), which is present mainly as Ag(SCN)₄³⁻, by 5- and 55-fold in 5 minutes with 0.4 M SCN⁻ in the source phase and 0.3 M S₂O²⁻₃ and 0.3 M NO⁻₃, respectively, in the receiving phase. In these competitive experiments, the total percent of Cd (II) transported was 98 and 84, respectively. The results are explained using the various equilibrium constants for cation-DC18C6, cation-SCN⁻ and cation-S₂O²⁻₃ interactions. These results indicate that rapid transport occurs when a cation is present in the source phase as a neutral complex. Selectivity for neutral species can be designed into these membrane systems when other cations interact with the source-phase anion to form charged species. Emulsion systems like those above were studied with respect to the appearance of Li⁺ in the source phase as a measure of membrane breakage. Maximum membrane stability was obtained when the ionic strengths of the source and receiving phases were equal.     

Temperature‐Mediated Template Release: Facile Growth of Copper(I) Mixed Ethynediide/Isopropylethynide Nanoclusters

In the comproportionation reaction of Cu^IIX₂ and Cu⁰ with isopropylacetylene (iPr−C≡C−H), the ethynediide species C₂²⁻ is generated via concomitant C−H/C−C bond cleavage of the iPr−C≡C−H precursor under moderate temperature to direct the formation of Cu^I mixed ethynediide/isopropylethynide nanoclusters (potentially explosive). The active ethynediide dianion C₂²⁻ exhibits chameleon‐like templating behavior to form C₂@Cu_m (m =6 ( 3 , 4 ), 7 ( 2 , 4 ), 8 ( 1 )) central structural units for successive formation of {C₂²⁻⊂Cu₂₄} ( 1 , 2 ), {6 C₂²⁻⊂Cu₄₈} ( 3 ), and {18 C₂²⁻⊂Cu₉₂} ( 4 ) complexes. Bearing the highest C₂²⁻ content, complex 4 features an unprecedented nanoscale Cu₂C₂ kernel. Furthermore, 1 – 3 exhibit structure‐controlled photoluminescence in the solid state.     

Structural and dynamical studies of δ-Bi2O3 oxide ion conductors: III. Phase relationships in the system (Bi2O3)1−x(M2O3)x as a function of pressure and temperature (M = Y,Er, or Yb)

We report the results of a structural investigation of the nonstoichiometric solid solutions (Bi₂O₃)_1−x(M₂O₃)_x (M = Y, Er, or Yb) treated at temperatures of between 298 and 1023 K and at pressures of up to 4 GPa. For x = 0.25 and M = Er or Y, 4 GPa pressure at 873 K causes the fluorite-related phase stable under ambient conditions to transform to a monoclinic phase which on subsequent annealing transforms to a rhombohedral phase isostructural with that adopted by the solid solution (Bi₂O₃)_1−x(Sm₂O₃)_x under ambient conditions. For x = 0.4 and M = Er or Y, application of 4 GPa at 1073 K causes the fluorite phase to undergo a distortion to another rhombohedral structure with a smaller unit cell. No transitions were found in the Yb³⁺-doped system.     

The influence of partial substitution of phosphorus by arsenic in monoclinic CsH2PO4. X-ray single crystal,vibrational and phase transitions in the mixed CsH2(PO4)0.72(AsO4)0.28

Partial substitution of P by As, leading to the solid solution CsH₂(PO₄)_1−x(AsO₄)_x, with x=0.28 (abbreviated as CDAP) has been shown. The structural characteristics of the crystals were analyzed by means of X-ray diffraction, which revealed that the new title compound is nearly isomorphous with the monoclinic phase of CsH₂PO₄ (CDP). The structure was solved from 796 independent reflections with R₁=0.0292 and Rw₂=0.0702, refined with 59 parameters. The following results have been obtained: space group P2₁, a=4.9250(4) Å, b=6.4370(3) Å, c=7.9280(6) Å, β=107.316(3)°, V=239.94(3) Å³, Z=2 and ρ_cal=3.349 g cm⁻³. The hydrogen bonds are clearly distinguished in the electron density maps which display distributions corresponding to order of protons. The shorter bond (2.452(4) Å), links the phosphate–arsenate groups into chains running along the b-axis and the longer bond (2.531(3) Å), crosslinks the chains to form (001) layers. The Raman and infrared spectra of CDAP recorded at room temperature in the frequency ranges 15–1200 cm⁻¹ and 400–4000 cm⁻¹, respectively, confirm the presence of PO³⁻₄ and AsO³⁻₄ groups in the crystal. Differential scanning calorimetry traces show three phase transitions at 333, 449 and 490 K in this material, which are characterized by X-ray powder diffraction at high temperature.     

Deintercalation and reintercalation energetics of ammoniated titanium disulfide

The deintercalation and reintercalation processes in ammoniated TiS₂ have been studied by thermogravimetric analysis, differential scanning calorimetry, vapor-pressure measurements, and powder X-ray diffraction. The enthalpies determined calorimetrically for complete NH₃ and NH⁺₄ deintercalation of (NH⁺₄)_0.24(NH₃)_0.23TiS^0.24−₂ are approximately 10.5 and 22 kcal/mole, respectively. These enthalpies are in good agreement with those reevaluated from a previous calorimetric study of ammoniated TaS₂. Ammonia vapor-pressure curves for deintercalation and reintercalation of (NH⁺₄)_0.24(NH₃)_y″TiS^0.24−₂ exhibit hysteresis, and the enthalpies for these reactions are estimated to be 15.5 and −13 kcal/mole, respectively. The absolute values of these enthalpies decrease progressively as NH₃ is deintercalated and then reintercalated. The structural changes that accompany these processes are relatively complex and involve at least two phases. Further structural studies are necessary to help elucidate the energetics of these intercalation compounds.     

Possible explanation of unstable superconducting phase in KxC60 with Tc=21 K

EPR and MMMA methods revealed two K₃C₆₀ phases, the unstable one with T_c=21.0 K containing C¹⁻₆₀ centers and a stable one with T_c=18.0 K constituting of C₆₀ molecules in the form of C³⁻₆₀ ions. The metastable phase appears only in the first stage of potassium intercalation of fullerene at the C₆₀/K₃C₆₀ phase boundary. This phase is being transformed to a stable one on further doping to K₃C₆₀. The appearance of a metastable phase is related to the instability of the fullerene C₆₀ fcc structure during the process of octahedral sites being filled by potassium ions. The critical temperature T_c shift upwards from 18.5 to 21.0 K is ascribed to the possible process of potassium transient back charge transfer valency, i.e. K₃¹⁺C₆₀³⁻↔K₂¹⁺K¹⁻C₆₀¹⁻ [3K¹⁺+C³⁻₆₀⇔2K¹⁺+K¹⁻+C₆₀¹⁻], which could enlarge the unit cell necessary to shift a superconducting critical temerature according to T_c on volume dependence.     

Mechanosynthesis of nanopowders of the proton-conducting electrolyte material Ba(Zr, Y)O3−δ

The formation of perovskite nanopowders of the common proton-conducting, electrolyte material Ba(Zr_1−xY_x)O_3−δ is demonstrated by room temperature mechanosynthesis for the compositional range x=0, 0.058 and 0.148. This is achieved with a planetary ball mill at 650 rpm in zirconia vials, starting from BaO₂ with ZrO₂, (ZrO₂)_0.97(Y₂O₃)_0.03 or (ZrO₂)_0.92(Y₂O₃)_0.08 precursors, respectively. Powder X-ray diffraction (XRD) reveals the formation of the perovskite phase in the early stages of milling with phase purity being achieved after milling times of 240 min for composition x=0.058 whereas 420 min is necessary for composition x=0.148. In contrast, traces of ZrO₂ are apparent in composition x=0 even after milling times of 420 min. The use of BaCO₃ as precursor does not allow the formation of the perovskite phase for any composition. The perovskite crystallites are spherical in shape with an average size determined from XRD of ca. 30 nm in agreement with transmission electron microscopy observations. FTIR spectra demonstrate that contamination levels of BaCO₃ in the mechanosynthesized powders are very low. The spherical shape and nanoscale of the crystallites allow densification levels that are highly competitive when compared to BaZrO₃-based materials formed by alternative synthesis techniques documented in the literature.     

Stabilization of the γ form of the rare earth sesquisulfides at lower temperature by doping with Calcium

The preparation of several samples forming a solid solution that can be formulated as Ca_(3/2)yR_2−y_{0.25−(1/2)y}S₃ (R = Ce, Sm, Gd) (0 ≤ y ≤ 0.30) is reported, together with their structural characterization, mainly through transmission electron microscopy. The introduction of Ca²⁺ into the rare earth metal sesquisulfide matrix stabilizes the γ form phase at 900 °C. This effect can be related to the non-stoichiometric nature of this phase, R_3−x_xS₄, because the introduction of Ca²⁺ requires the elimination of cation vacancies from the structure: 2R³⁺ + → 3Ca²⁺ (R = rare earth metal;  = cation vacancies). However, a NaCl-type solid solution is formed for R = Eu, formulated as Eu_1−yCa_yS. Well-ordered crystals are found in every sample, as it is revealed by transmission electron microscopy images and diffraction patterns. The color properties of the samples have been evaluated with reflectance spectra in the visible range and with L*–a*–b* coordinates.     

Liquid–liquid extraction of palladium(II) and gold(III) with N-benzoyl-N′,N′-diethylthiourea and the synthesis of a palladium benzoylthiourea complex

N-Benzoylthioureas have been reported to form complexes with gold (III) and palladium (II) and other transition metals. In this study, an N-benzoyl-N′,N′-diethylthiourea (3f) ligand was used in the solvent extraction of palladium(II) and gold(III) from aqueous chloride media (0.1 mol l−1 NaCl). The distribution coefficient was determined as a function of both metal concentration in the aqueous phase and extractant concentration in the organic phase. The experimental distribution data were numerically analysed by letagrop-distr software in order to obtain the thermodynamic model corresponding to the metal extraction. It is found that pH does not affect the metal extraction process in the 1–2 pH range. Synthesis of the palladium benzoyl thiourea complexes was carried out by mixing quantities of metal and ligand solutions in methanol in a 1:2 ratio stoichiometric. Yields of 74 and 80.9% were obtained for the Pd-3c and Pd-3f complexes. In order to confirm the formation of the palladium complexes, NMR, FTIR and MS analyses were performed. From MS analyses a complex stoichiometry 1:2 (metal:ligand) was confirmed. The formation of crystals of palladium N-benzoyl-N′,N′-diethylthiourea complex (Pd-3f) in the methanolic solution allows the characterisation of the complex structure by XRD. The resulting structure is described and discussed. Bis(1,1,-diheptadecyl-3-benzoyl-thioureate)palladium(II) (Pd-3c) and bis(1,1,-diheptadecyl-3-benzoyl-thioureate)palladium(II) (Pd-3f) were used as ionophores in polymeric membrane electrodes. Their potentiometric responses to different anionic metal chlorocomplexes are evaluated and discussed taking into consideration the results obtained in the liquid–liquid distribution studies. A nernstian response was only obtained for AuCl4 − (PDL=8.8×10−8) and PdCl4 2− (PDL=1.5×10−4 M) with a selectivity coefficient of KAuCl4-, PdCl42−pot=−3.4, calculated taking AuCl4 − as being the primary anion.     

Strain correlated effect on structural,magnetic, and dielectric properties in Ti4+ substituted Bi0.8Ba0.2Fe1−xTixO3

Ti⁴⁺ substituted Bi_0.8Ba_0.2Fe_1−xTi_xO₃ for x = 0.0, 0.1 and 0.2 are prepared by modified solid state reaction method. The prepared samples sintered at 850 °C for 1 h show a single phase nature. A structural change was observed on Ti⁴⁺ substitutions are confirmed through X-ray Diffraction, Fourier Transform Infrared spectroscopy and Raman spectra. An anomalous phase transition is observed in Bi_0.8Ba_0.2FeO₃ at 1173 K. The absence of ferroelectric transition and enhancement of decomposition temperature is observed in the substituted samples from the thermal analysis. A dielectric spectroscopic measurement shows that on Ti⁴⁺ substitutions, the magnitude of dielectric constant and loss tangent (tan δ) value is decreased. Vibrating Sample Magnetometer (VSM) study shows both antiferromagnetic and ferromagnetic phases coexist in the M−H curve. On Ti⁴⁺ substitutions in Bi_0.8Ba_0.2FeO₃, the antiferromagnetism dominates over the ferromagnetic phase. In corroboration to magnetisation process, ZFC–FC measurement confirms it that on Ti⁴⁺ substitution, the antiferromagnetic behaviour gets dominated. The report suggests that the interplay of strain upon Ti⁴⁺ substitution causes the structural and magnetic phase transition.     

Reversible oxygen removal and uptake in the La2ZnMnO6 double perovskite: Performance in symmetrical SOFC cells

A polycrystalline oxygen-stoichiometric La₂ZnMnO₆ double-perovskite oxide has been prepared by soft-chemistry procedures, followed by annealing in air at 800 °C. A reduced specimen, with a La₂ZnMnO_6−δ composition, has been obtained by topotactical oxygen removal in an H₂/N₂ (5%/95%) flow at 600 °C. The structural characterization has been conducted from neutron powder diffraction (NPD) data, very sensitive to the contrast between Zn and Mn and the oxygen stoichiometry. Both perovskites (oxidized and reduced) crystallize in the monoclinic P2₁/n space group, exhibiting an antisite Zn/Mn disorder of about 15% and 14%, respectively. The partial reduction of Mn⁴⁺ to Mn³⁺ in the reduced phase is accompanied with the occurrence of oxygen vacancies, located at the axial octahedral sites. Thermogravimetric analysis (TGA) substantiates the oxygen stoichiometry and the stability range. Magnetic susceptibility measurements indicate an antiferromagnetic behaviour, confirming the presence of Mn³⁺ ions in the structure. The magnetic structure of the reduced phase, determined from NPD data at 3 K, shows an antiferromagnetic G-type coupling between Mn at 2c and 2d sites (promoted by the anti-site disorder); the ordered magnetic moment at Mn site is 0.789 μ_B at 3 K. Both phases display a semiconductor-like behaviour with a maximum conductivity of 0.052 S cm⁻¹ for the reduced phase at 650 °C, due to the occurrence of Mn³⁺–Mn⁴⁺ mixed valence. Moreover, the measured thermal expansion coefficients perfectly match with the values usually displayed by SOFC electrolytes. The reversibility and versatility of the present compounds as catalysts for oxygen reduction (cathode) or fuel oxidation (anode) were tested in single SOFC cells yielding power density spanning from 120 to 155 W/cm² using these perovskites as anode, cathode and symmetric electrodes for SOFC.     

Syntheses,solid state electronic and infrared spectra of the Ni(II), Pd(II) and Cu(II) complexes of N-(2-hydroxyethyl) and N-(3-hydroxypropyl) oxamide

The new complexes K₂[Ni(Hheo)₂], K₂[Cu(Hheo)₂]·H₂O, K₂[Ni(Hhpo)₂]·H₂O, K₂[M(Hhpo)₂]·0.5H₂O (M = Cu, Pd) and K₂[Cu₂(hpo)₂·0.5H₂O, where H₃heo = N-(2-hydroxyethyl)oxamide and H₃hpo = N-(3-hydroxypropyl)oxamide, have been prepared. Several synthetic routes were investigated and the complexes were characterized by analyses, conductivity measurements, thermogravimetry, magnetic susceptibility and spectroscopy (i.r. and far i.r., diffuse reflectance u.v.). Monomeric square planar structures are found for the [M(Hheo)₂]²⁻ and [M(Hhpo)₂]²⁻ complex anions, while the hpo³⁻ Cu(II) complex appears to be a square planar dimer. The doubly deprotonated Hheo²⁻ and Hhpo²⁻ ions exhibit a bidentate N(secondary amide), N′(tertiary amide)-coordination with the OH-group remaining uncoordinated, while the triply deprotonated hpo³⁻ ion behaves as a bridging N(secondary amide), N′(tertiary amide), O(deprotonated) ligand, while two Cu(II) centres are bridged by two alkoxide-O atoms. The vibrational analysis of the dehydrated complexes is carried out, using NH/ND, OH/OD, ⁵⁸Ni/⁶²Ni and ⁶³Cu/⁶⁵Cu substitutions.