Determination of triple points in the phase diagram of praseodymium

The phase diagram for praseodymium under pressures up to 23 GPa and temperatures up to 573 K is studied by in situ energy-dispersive x-ray diffraction with an externally heated diamond-anvil cell apparatus along the phase boundaries between Pr(II) (fcc), Pr(III) (distorted fcc), and Pr(IV) ({α}-uranium-type structure). The triple point of these three phases is located around 21 GPa and 537 K, representing the end point of the apparently second-order phase transition between Pr(II) and Pr(III) at the transition to the high-pressure phase Pr(IV). Together with the compression data for Pr at ambient temperature, the present study extends the P-T phase diagram up to the melting lines.