Melting curve of elemental zirconium

Abstract

Melting experiments require rapid data acquisition due to instabilities of the molten sample and optical drifting due to the high required laser power. In this work, the melting curve of zirconium has been determined for the first time up to 80 GPa and 4000 K using in-situ fast x-ray diffraction (XRD) in a laser-heated diamond anvil cell (LH-DAC). The main method used for melt detection was the direct observance of liquid diffuse scattering (LDS) in the XRD patterns and it has been proven to be a reliable melting diagnostic. The effectiveness of other melting criteria such as the appearance of temperature plateaus with increasing laser power is also discussed. PACS numbers: 92.60.hv, 61.50.Ks, 74.62.Fj, 81.30.Bx Zirconium (Zr) and its alloys have a very wide range of applications, from the chemical processing (as corrosion resistant materials) to the semiconductor industry 1 . Moreover, its good strength and ductility at high temperatures and the low thermal neutron cross-section absorption make it an ideal material for use as cladding at nuclear reactors 2 . Alloys of Zr with Cu, Al, Ti and Ni have been demonstrated to exhibit extraordinary glass forming ability 3 , while metallic glass formation in singleelement zirconium has also been discovered, with a wide stability in high pressure and temperature conditions 4 .Zirconium is a d-orbital transition metal with a rich and interesting phase diagram. At ambient conditions it crystallizes to an hcp structure (α-phase), while at temperatures higher than 1136 K it transforms to a bcc (β-) phase. By increasing pressure at ambient temperature it transforms to another hexagonal, but not close-packed, called the ω-phase and then back to β-phase around 35 GPa 5-7 . Similar transitions also occur in other group IV transition metals, such as Ti and Hf, and it seems that the electronic transfer between the broad sp band and the much narrower d band is the driving force behind those structural transitions.The high melting point (2128 K) of Zr often classifies it as a refractory metal. Although there are some works in the high temperature behavior of zirconium at high pressures 8-11 , its melting curve has not yet been investigated and this absence of experimental data has strongly motivated this study. On the other hand, the high pressure melting of transition metals has always been a subject of intense debate, because of the large uncertainties in the temperature measurements and the criteria used to identify the melting, so that different approaches can yield very different results. In most cases shock wave (SW) experiments and molecular dynamics (MD) calculations provide dramatically steeper curves than those obtained with the laser speckle method in a LH-DAC, where the melting is visually detected by observing the movements on the sample surface during heating. Tantalum is a good example of such a controversy, with melt-ing temperatures that differ thousands of K at 100 GPa by applying different experimental techniques [12][13][14][15] . Another more recent example is...

Melting and refreezing of zirconium observed using ultrafast x-ray diffraction

“…Good agreement is observed between the experimentally determined and the Lindemann-derived melting lines. A recent study of the melting line of Zr under static conditions [23], using, essentially, the same method we used here gave a near-perfect agreement in the 30-50 GPa overlap range.…”

A Review of the Melting Curves of Transition Metals at High Pressures Using Static Compression Techniques

“…For most metals, the density of the liquid is lower than that of the solid, while the entropy is higher, leading to mostly positive melting slopes (i.e., the melting temperature increases with pressure) [16,22,24,29,[35][36][37]39,40,63]. The situation is much more intricate for alkali metals, however, whose phase diagrams reveal complex liquid melting-curve maxima and negative melting slopes [64][65][66][67][68][69], since these elements exhibit several density discontinuities and phase transitions already in the solid phase.…”

“…In noble gases such as Ar, Kr, or Xe, the closed shell configuration (s 2 p 6 ) is responsible for a steep melting curve, and something similar happens with noble metals such as Cu, Ag, or Au. However, in many cases the Lindemann law overestimates the melting curve of d-transition metals with partially filled cells, especially at higher pressures, no matter the experimental approach [30,37,74,75]. Moreover, the fact that the Lindemann law takes into account only the thermodynamic parameters of the solid phase, neglecting the liquid, can also lead to inaccurate predictions.…”

“…Figure 1a plots together the melting lines of several transition metals, obtained in an LH-DAC [15,22,24,[35][36][37]63,77,83,84]. The melting curves are normalized to the ambient melting temperature T 0 of each element.…”

“…Despite the numerous technological advancements that have been made in both fields during the last two decades, reliable measurements remain very challenging, especially at the high end of P-T conditions. The diverse results for different experimental approaches concerning Ta [13,[20][21][22][23], Fe [14,[24][25][26], Mo [13,15,[27][28][29][30][31][32][33], V [34,35], Ti [13,36], Zr [37,38], or Ni [39][40][41] clearly exhibit the need for a universally established methodology.…”