Alpha-Beta And Beta-Gamma Phase Boundaries Of Solid Oxygen Observed By Adiabatic Magnetocaloric Effect

The magnetic-field-temperature phase diagram of solid oxygen is investigated by the adiabatic magnetocaloric effect (MCE) measurement with pulsed magnetic fields. Relatively large temperature decrease with hysteresis is observed at just below the beta-gamma and alpha-beta phase-transition temperatures owing to the field-induced transitions. The magnetic field dependence of these phase boundaries are obtained as T-beta gamma(H) = 43.8-1.55 x 10(-3)H(2) K and T-alpha beta(H) = 23.9 - 0.73 x 10(-3)H(2) K. The magnetic Clausius-Clapeyron equation quantitatively explains the H dependence of T-beta gamma, but does not T-alpha beta. The MCE curve at T-beta gamma is of typical first order, while the curve at T-alpha beta seems to have both characteristics of first-and second-order transitions. We discuss the order of the alpha-beta phase transition and propose possible reasons for the unusual behaviors.