Anti-ThCr2Si2-type RE2O2Bi (RE = rare earth) with a Bi square net is known to show an insulator–metal transition by substituting RE. In this study, La2O2Bi polycrystals with different oxygen nonstoichiometry were synthesized. As the amount of oxygen in La2O2Bi increased, the c-axis length was expanded due to the generation of an additional 4e site for excess oxygen, while the a-axis length remained almost constant, indicating the separation of Bi square nets by oxygen intercalation. Concomitantly, transformation of insulating La2O2Bi into metallic La2O2Bi occurred with the change in carrier polarity from the n- to p-type. Despite its polycrystalline form, La2O2Bi with the largest amount of oxygen showed a rather high hole mobility of 85 cm2 V−1 s−1 among other layered oxypnictides and oxychalcogenides.

A solution model, based on the regular solution theory coupled with Flory-Huggins entropy term, was developed for the calculation of solubilities of inorganic compounds in water vapor under high temperatures and pressures. The solubilities of sodium chloride (NaCl), potassium hydroxide (KOH), sodium sulfate (Na 2 SO 4 ), lead oxide (PbO), silicon oxide (SiO 2 ), lithium nitrate (LiNO 3 ), sodium nitrate (NaNO 3 ) and potassium nitrate (KNO 3 ) were correlated by optimizing internal energies and molar volumes of inorganic compounds which give their solubility parameters.

A flow type apparatus was designed and constructed to measure the solubilities of salts in water vapor at high temperature and pressure. The apparatus was equipped with an additional pure water line to prevent the clogging by precipitated solid salts at the outlet of an equilibrium cell.

We report a series of layered superconductors, anti-ThCr2Si2-type RE2O2Bi (RE = rare earth), composed of electrically conductive Bi square nets and magnetic insulating RE2O2 layers. The superconductivity was induced by separating Bi square nets as a result of excess oxygen incorporation, irrespective of the presence of magnetic ordering in RE2O2 layers. Intriguingly, the transition temperature of all RE2O2Bi including nonmagnetic Y2O2Bi was approximately scaled by the unit cell tetragonality (c/a), implying a key role of relative separation of the Bi square nets to induce the superconductivity.