Structural studies of gadolinium at high pressure and temperature

High pressure and high temperature angle-dispersive x-ray diffraction measurements on gadolinium were performed up to $93\mathrm{GPa}$ and nearly $2500\mathrm{K}$ using argon as pressure medium. Our room temperature measurements confirmed the transition pressures reported in a previous study carried out using nitrogen as pressure medium, solving former discrepancies found in the literature. A new phase was observed above $1200\mathrm{K}$ between $39\mathrm{GPa}$ and $52\mathrm{GPa}$. Additionally, the structure of the previously observed dfcc and post-dfcc phases was determined, being the space group symmetry of the post-dfcc phase $(C2∕m)$, different from what was predicted before. Finally, the room temperature equation of state was determined for Gd and its pressure-temperature phase diagram redefined.

Figure 1

RT ADXRD patterns of Gd at different pressures: (a) fcc phase at $30\mathrm{GPa}$, (b) dfcc phase at $39\mathrm{GPa}$, (c) dfcc phase at $57\mathrm{GPa}$, and (d) Gd-VIII phase at $65\mathrm{GPa}$. Ar peaks are connected by dashed lines. The stars depict peaks of the hcp phase of Ar. The background was removed.

Figure 2

RT ADXRD pattern of dfcc phase of Gd. Black symbols: observations, solid line: refined model, and dashed line: residual of the Rietveld refinement. The ticks mark the position of the Gd and Ar peaks.

Figure 3

RT ADXRD pattern of Gd-VIII phase. Black symbols: observations, solid line: refined model, and dashed line: residual of the Rietveld refinement. The ticks mark the position of the Gd and Ar peaks.

Figure 4

Volume-pressure data of Gd. Solid squares: present data. Solid curve: fitted EOSs.

Figure 5

ADXRD patterns of Gd measured at $39\mathrm{GPa}$ and different temperatures. The high-temperature patters illustrate the appearance of the Gd-IX phase above $1200\mathrm{K}$. The Ar picks are indicated in the bottom trace.

Figure 6

P-T phase diagram of Gd. Different symbols indicate different phases observed in our measurements; squares: dhcp phase, up triangles: fcc phase, circles: dfcc phase, down triangles: Gd-VIII phase, hexagons: Gd-IX phase, and diamonds: bcc phase. The cross is a shock-wave data point (Ref. 36). Solid lines are from piston-cylinder experiments (Ref. 35) and melting experiments (Ref. 13). Dashed lines are the proposed phase boundaries.