The Study of Thermal Equilibrium and Highly Oxygenated State under High Oxygen Pressure

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In this study, we have developed a new apparatus by which thermogravimetry and differential thermal analysis (TG-DTA) can be performed under high gaseous pressure. By using this apparatus, we have investigated oxygen nonstoichiometry and phase transition in high temperature superconductors RBa_2Cu_3O_y (R=Y,Sm, Er) under high oxygen pressure up to 150 atm. We observed a reversible weight change between room temperature and 300ﾟC above 10 atm. From various experiments it was concluded that this behavior is due to buoyancy under a dense gas. The behavior was quantitatively explained by the calculation taking buoyancy into a consideration. The observed TG curves were corrected by using this calculation. RBa_2Cu_3O_y shows a transition from orthorhombic to tetragonal which is a kind of order-disorder transition. It has been unknown how is the transition in the region of high oxygen content. We first observed the transition in such high oxygen content region. The transition temperature (T_t) has a trend to saturate at 830ﾟC in YBa_2Cu_3O_y and 730ﾟC in SmBa_2Cu_3O_y around y=6.75, and in ErBa_2Cu_3O_y it tends to increase again above y=6.75. These behaviors suggest an existence of a new phase with an another arrangement of oxygen vacancies around y=6.75. And we found interesting pressure dependences of thermal decomposition in some oxalic acids. This machine is available for hydrogen atmosphere.