Polymorphism in Bi2Sn2O7

Single crystals of Bi₂Sn₂O₇ were grown in a Bi₂O₃ flux. Phase transitions were identified at about 90 and 680° using X-ray, SHG, DSC, dielectric, and optical data. γ-Bi₂Sn₂O₇, which exists above 680°C is centric and cubic with a = 10.73 Å at 700°, and it probably has the ideal pyrochlore structure. β-Bi₂Sn₂O₇, which exists between 680° and about 90°C, is acentric but remains cubic with a = 21.40 Å. α-Bi₂Sn₂O₇, which exists from about 90°C to below room temperature, is acentric and noncubic, probably tetragonal with a = 21.328 and c = 21.545 Å. The α-β transition is first order, and the β-γ transition appears to be second order. Substitutions of Pb²⁺ or Cd²⁺ for Bi³⁺ and of Ga³⁺, Rh³⁺ Sc³⁺, In³⁺, Sb⁵⁺ Nb⁵⁺ or Ta⁵⁺ for Sn⁴⁺ lower the α-β transition temperature.     

High-temperature superconductivity in the BaPb1-xBixO3 systems

Phases of the type BaPb_1-xBi_xO₃ have been prepared for the first time. These phases all have perovskite related structures, and superconductivity was observed over the range $\text{x ≌ 0.05-0.3}$. The highest critical temperature is 13 K which is exceptionally high for an oxide and is much higher than that previously observed for any superconductor not containing a transition element. Semiconducting behavior is observed from x = 1 to about 0.35.     

Preparation of ferric tungstate and its catalytic behavior toward methanol

Pure ferric tungstate, Fe₂(WO₄)₃, has been prepared and characterized for the first time. Ferric tungstate has a structure very similar to that of ferric molybdate with a unit cell volume about 1.5% larger. Decomposition to Fe₂WO₆ and WO₃ occurs at about 600°C. Ferric tungstate was tested as a catalyst for the selective oxidation of methanol and shown to have very different properties from ferric molybdate for this reaction. Whereas over the molybdate the predominant reaction is oxidation of methanol to formaldehyde, over the tungstate it is dehydration to dimethyl ether.     

High-Pressure Synthesis, Crystal Structure, and Metal–Semiconductor Transitions in the Tl2Ru2O7−δPyrochlore

Thallium ruthenium oxides, Tl₂Ru₂O_7−δ, with the pyrochlore structure were synthesized under a pressure of 1–5 GPa and 1173 K and characterized by resistivity, magnetization, and TOF neutron-diffraction measurements. The oxygen vacancy,δ, varied with the synthesis conditions and significantly affected their electrical properties. The pyrochlores synthesized at high pressure and atmospheric pressure are classified into four groups which depend on their oxygen nonstoichiometry. (i) Nonstoichiometric Tl₂Ru₂O_6.71shows a metallic conductivity with almost temperature-independent magnetization. (ii) Stoichiometric Tl₂Ru₂O₇synthesized under high oxygen pressure using KClO₄shows a metallic–semiconducting transition at 120 K with magnetization anomalies at 120 and 40 K. (iii) Slightly nonstoichiometric Tl₂Ru₂O_6.96shows spin-glass-like behavior around 40 K accompanying a resistivity increase at the transition. (iv) Tl₂Ru₂O₇synthesized at 773 K and atmospheric pressure is semiconducting with magnetization anomalies at 120 and 40 K. The change from the metallic to semiconducting state is discussed from the viewpoint of structure changes.     

The hunt for LaFeSbO: Synthesis of La2SbO2 and a case of mistaken identity

The phase La₂SbO₂ has been identified and characterized in the course of our efforts to realize LaFeSbO, a composition of great interest to the superconductor community. The compound La₂SbO₂ is a tetragonal layered oxypnictide containing La₂O₂ layers similar to LaMPnO compositions (where M=transition metal, Pn=pnictide) separated by pnictide anion layers. In order to better understand why LaFeSbO has remained elusive, density functional theory calculations have been used to determine the ground state heats of formation for LaFeSbO, La₂SbO₂, and other competing phases within the La–Fe–Sb–O system, as well as the phonon spectrum for LaFeSbO. These efforts suggest that LaFeSbO is a potentially metastable composition.     

A new ferroelastic transition in some A2(MO4)3 molybdates and tungstates

We have found for the first time a ferroelastic transition in many molybdates and tungstates with the Sc₂(MoO₄)₃-type structure. Below the transition these phases are monoclinic ($\text{P2}{}_1\text{a}$), and above the transition they are orthorhombic (Pnca). Observed transition temperatures are: Al₂(MoO₄)₃, 200°C; Al₂(WO₄)₃, ?6°C; Cr₂(MoO₄)₃, 385°C; Fe₂(MoO₄)₃, 499°C; In₂(MoO₄)₃, 335°C; In₂(WO₄)₃, 252°C; and Sc₂(MoO₄)₃, 9°C.