High Melting Points of Tantalum in a Laser-Heated Diamond Anvil Cell

In situ x-ray diffraction has been used to characterize the structural modifications of tantalum samples under intense laser irradiation, up to 135 GPa in a diamond anvil cell. Melting data points are obtained that do not confirm the previously reported anomalously low melting curve. Two effects are identified that might alter the melting determination of refractory metals such as Ta under high static pressures. First, a strong chemical reactivity of Ta with the pressure transmitting media and with carbon diffusing out from the surface of the anvils is observed. Second, pyrometry measurements can be distorted when the pressure medium melts. The strong divergence between ab initio calculations, shock measurements and static determination is resolved here and hence many theoretical interpretations are ruled out. Finally, the body-centered cubic phase is stable over the pressure-temperature range investigated.

Figure 1

Previously reported melting curves or points of Ta [7, 8, 11, 12, 27, 28] and of pressure media used in this study [18, 24, 29, 30, 31].

Figure 2

Reactivity of Ta with diamond anvils. Monochromatic ($\lambda =0.3738\AA$) XRD spectra of a Ta sample in a MgO pressure medium recorded during one heating series at $\simeq 90\mathrm{GPa}$. The MAR-CCD bidimensionnal spectra have been circularly integrated. The diffraction peaks of TaC appear around 3600 K. Inset: temperature measured by pyrometry as a function of time (and laser’s power). The numbered filled squares correspond to the numbered XRD spectra. After the reaction between Ta and diamond, the temperature cannot be increased.

Figure 3

Reactivity of Ta with pressure medium. XRD spectra of Ta before or after heating above 4000 K in Ar (a), ${\mathrm{Al}}_2{\mathrm{O}}_3$ (b) and NaCl (c) pressure media. The circles, squares and plus signs, respectively, identify Ta, pressure medium, and TaC. Our interpretation of these spectra is: tantalum forms a amorphous alloy with Ar and chemically reacts with ${\mathrm{Al}}_2{\mathrm{O}}_3$ and NaCl (new unassigned peaks).

Figure 4

Difficulties in pyrometry measurements. Pyrometry temperature ($T_{\mathrm{pyr}}$) and temperature estimated using Ta lattice parameter ($T_{\mathrm{Ta}}$) as a function of time during a heating series in the argon pressure medium at $P=90.5\mathrm{GPa}$. Fast recrystallization of Ta single crystals has been observed between $01\mathrm{\text{:}}12\mathrm{\text{:}}12$ and $01\mathrm{\text{:}}15\mathrm{\text{:}}28$. The XRD signal from a liquid has been recorded between $01\mathrm{\text{:}}12\mathrm{\text{:}}58$ and $01\mathrm{\text{:}}15\mathrm{\text{:}}02$, during a plateau of $T_{\mathrm{Ta}}\simeq 4950\mathrm{K}$.

Figure 5

(a) Approximate $P\mathrm{\text{−}}T$ paths in the current study, compared to the LHDAC melting curves previously published. The colors of the vertical lines correspond to different pressure media. The circles indicate XRD evidence of melting (closed circles: $T_{\mathrm{pyr}}$, open circles: $T_{\mathrm{Ta}}$, for NaCl pressure medium). (b) Ta phase diagram.