# $\alpha$-$\beta$ and $\beta$-$\gamma$ phase boundaries of solid oxygen observed by adiabatic magnetocaloric effect

## TL;DR

This study maps the magnetic-field-temperature phase diagram of solid oxygen using adiabatic magnetocaloric effect measurements, revealing field-dependent phase boundaries and transition characteristics at the $eta$-$eta$ and $eta$-$eta$ phase transitions.

## Contribution

It provides the first detailed magnetic-field dependence of phase boundaries in solid oxygen and analyzes the order of phase transitions using magnetocaloric effect data.

## Key findings

- Field-dependent phase boundaries are quantitatively determined.
- The $eta$-$eta$ transition is first-order, while $eta$-$eta$ shows mixed characteristics.
- The magnetic Clausius-Clapeyron equation explains the $eta$-$eta$ boundary behavior.

## Abstract

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 dependences of these phase boundaries are obtained as $T_\mathrm{\beta\gamma}(H)=43.8-1.55\times10^{-3}H^2$ K and $T_\mathrm{\alpha\beta}(H)=23.9-0.73\times10^{-3}H^2$ K. The magnetic Clausius-Clapeyron equation quantitatively explains the $H$ dependence of $T_\mathrm{\beta\gamma}$, meanwhile, does not $T_\mathrm{\alpha\beta}$. The MCE curve at $T_\mathrm{\beta\gamma}$ is of typical first-order, while the curve at $T_\mathrm{\alpha\beta}$ 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 behavior.