1304-29-6

The Ba substitution by Sr has been studied in two Hg-2212 series: Hg 2(Ba1-ySry)2YCu2O 8-δ and Hg2(Ba1-ySry) 2(Y0.80Ca0.20)Cu2O 8-δ. In both series a Tc enhancement of about 40 K is observed when Sr substitutes Ba from y=0 to 1.0. The y=0 compound of the first series is the non-superconducting Hg2Ba2YCu 2O8-δ prototype. In the second series, this y=0 compound is already superconducting at 21 K. Indeed the members of this series present a higher charge carrier density in their CuO2 superconducting planes than their homologues of the first series due to the doping introduced by the substitution of 20% of Y by Ca. The compounds of both series were synthesized in high pressure (3.5 GPa)-high temperature (950-1050 °C) conditions. In both cases Sr substitution was successful up to the full replacement of Ba (y=1.0). The Hg-2212 phases were characterized by XRD, SEM, EDX and a.c. susceptibility.

HgBa2Ca3Cu4O10+δ and HgBa2Ca4Cu5O12+δ were prepared by high pressure/high temperature technique. The phase composition in the Hg-Ba-Ca-Cu-O system was shown to be very sensitive to synthesis conditions. A variation of the oxygen content in the starting oxide mixture by changing the BaO/BaO2 ratio allowed the acquisition of Hg-1234 and Hg-1245 in the overdoped state. The heat treatments (nitrogen and oxygen flows, oxygen pressure 100 bar and 2000 bar) resulted in a change of Tcs from initial overdoped (116 and 100 K for Hg-1234 and Hg-1245, respectively) to underdoped (95 and 85 K) and, finally, to optimally-doped states (125 and 111 K). The dependencies of Tc vs. a-parameter (and, consequently, oxygen content) for these phases exhibit a cupola-shaped behavior, as it was previously found for the lower (n ≤ 3) members of the HgBa2Can-1CunO2n+2+δ series.

The Hg-1223 and Hg-1234 phases were synthesized using the program-controlled high pressure chamber. The variation of the oxygen content in the starting oxides mixture `HgBa2Ca3Cu4Ox', obtained by changing BaO/BaO2 and Cu2O/CuO ratios, influences significantly the phase formation. By varying the pressure (Ps), temperature (Ts), time (ts) and oxygen content (x), the optimum synthesis conditions of the Hg-1234 phase were determined with Ps = 2 GPa, Ts = 900 °C, ts = 2 h and x = 10.2. Using the initial mixture with the high oxygen content (x = 10.6) only Hg-1223 with the lattice constants a = 3.8474(9) angstroms, c = 15.79(1) angstroms was obtained. Hg-1223 thus prepared is strongly overdoped and exhibits superconductivity at 118 K, while the treatment in oxygen flow enhanced Tc up to 134 K and cell parameters to a = 3.852(1) angstroms, c = 15.84(1) angstroms. Dependence of Tc vs. a-parameter (and consequently, oxygen content δ) for the HgBa2Ca2Cu3O8+δ superconductor has a cupola-shaped character, as found previously for the lower members of the homologous series.

Barium(II)bis(oxalato)barium(II)dihydrate (BOD), Ba[Ba(C2O4)2]·2H2O and lead(II)bis(oxalato)lead(II)monohydrate (LOM), Pb[Pb(C2O4)2]·H2O have been synthesized and characterized by elemental analysis, conductance measurements, IR spectral, reflectance and X-ray powder diffraction studies. Thermal decomposition studies (TG, DTG and DTA) in air showed that at ca. 1000°C, a mixture of BaO2 and BaCO3 is generated from the compound, BOD, through the formation of BaO2 and BaC2O4 at around 514°C as intermediates. LOM gave Pb2O3 as final product at ca. 390°C. DSC study in nitrogen showed a different decomposition pattern from that in air for both compounds. Using seven mechanistic equations, the rate controlling process of the dehydration and decomposition mechanism of BOD is inferred to be one- and three-dimensional diffusion, respectively. The decomposition mechanism of LOM is a phase boundary reaction having cylindrical symmetry. Some of the decomposition products were identified by analytical, IR spectral and X-ray powder diffraction studies. A tentative reaction mechanism for the thermal decomposition of both the complexes is proposed.

We prepared a new oxycarbonate superconductor, (Cu0.5C0.5)2Ba3Ca2Cu3O11 at 1250 °C under 5 GPa. This phase is the m = 2 and n = 3 member of a homologous series, (Cu0.5C0.5)mBam+1Can-1CunO 2m+2n+1 reported previously. Its X-ray pattern could be indexed assuming a tetragonal subcell with a = 3.852 (1), c = 18.61 (2) angstroms. Electron microscopic observations indicated ordering between Cu and C in the (Cu, C) plane, resulting in a superlattice, as = 2a, bs = b and cs = 2c with respect to the tetragonal subcell. This new member showed a superconducting transition at 91 K.

Occurrence and products of solid/solid interactions in equimolar-mixed BaCO3 and Cr2O3 powders were examined isothermally (700-1000 °C) and non-isothermally (25-1200 °C) under different gas atmospheres, employing thermogra

The heat capacity of BaO2 from 13 to 600 K was measured by adiabatic calorimetry and differential scanning calorimetry (DSC). No anomalies in Cp were observed. The following standard values of thermodynamic parameters were obtained: Cp0 (298.15 K) = 66.28 ± 0.13 J/(mol K), S0(298.15 K) = 87.55 ± 0.18 J/(mol K), H0(298.15 K) - H0(0) = 13040 ± 26 J/mol.

Pulsed-laser ablated Ca, Sr, and Ba atoms have been reacted with O2 in excess argon during condensation at 10 K.Infrared spectra and oxygen isotopic substitution show that the major products are the symmetrical OMO metal dioxide and the rhombic metal oxid

The role of the divalent nature of barium atoms in chemical reactivity was explored using crossed molecular beams.Angular and velocity distributions of products from reactions of Ba(1S) with NO2 and O3 indicate the existence of long lived collision intermediates despite very large reaction exothermicities.The existence of these intermediates results from barriers to transfer of the second electron necessary to form ground state products.Although BaO was the dominant product in both reactions, two previously unknown channels were observed: Ba + NO2 -> BaON + O and Ba + O3 -> BaO2 + O.We obtained bond dissociation energies of D0(Ba-ON) = 270 +/- 80 kJ/mol and D0(Ba-O2) = 500 +/- 80 kJ/mol for these molecules.The dependence of the cross sections for the ion channels, Ba(1P) + Cl2 -> Ba+ + Cl2- and Ba(1P) + Cl2 -> BaCl+ + Cl- on Ba(1P) orbital alignment and collision energy was used to probe the course of the reaction through intersections between the ionic and covalent potential energy surfaces.