Systematics of transition-metal melting

It is now understood that the crystal structures of metals exhibit sequences determined by changes in their electronic configuration. With increasing atomic number, at ambient conditions, the transition metals exhibit the structural sequence hcp-bcc-hcp-fcc as the d-electron bands become progressively filled. 1,2 This sequence can be explained by differences in the sum of one electron band-structure energies. While room temperature ~RT! high pressure diamond-anvilcell ~DAC! studies using x-ray diffraction methods have extended our knowledge of the structural changes to about 500 GPa ~Ref. 3! very little is known of the melting behavior of the transition metals. The only high pressure melting measurements for transition metals are for Co and Ni, 4 and Fe. 5,6 For the bcc transition metals in Groups VA and VIA there is virtually no melting data at high pressure. In the present paper we present new melting data for the transition metals Ti, V, Cr, Mo, W, Ta, Fe, Co, and Ni using a laser-heated DAC up to pressures near 100 GPa where melting temperatures approach 4000 K. The experimental technique has been described elsewhere. 7,8 In the present study we made further improvements by using diamondcoated tungsten gaskets, which further increased the height of the pressure chamber, thus improving thermal insulation from the diamonds and reducing temperature gradients in the laser-heated samples. These gaskets allowed a significant expansion in the pressure range for routine use of Ar as a pressure medium. Most data were obtained in Ar and in some cases compared with those using other pressure media. We also directly compared data using different melting criteria: The in situ laser speckle method 7,8 gave the same results as the changes in the surface texture observed on recovered or quenched samples. Figure 1 shows a Mo sample before ~top! and after melting ~bottom! at 17 GPa. The temperature difference between top and bottom was 50 K. The formation of a bead in the bottom picture from a polished surface ~top! can only be due to melting, thus eliminating arguments that the laser speckle method may indicate recrystallization of the sample. The highest pressures we attained are over an order of magnitude greater those in previous studies. 9 For the purpose of discussion we have divided the transition metals into those which melt from bcc structures and those melting from close-packed structures. The bcc transition metals are known to have very stable structures. Totalenergy calculations for Mo, W, Ta, and Cr predict the bcc structure to be stable to 420, 1250, 1000, and 700 GPa, respectively, followed by a transition to hcp. 10 DAC x-ray studies have confirmed the absence of RT phase transitions in Mo to 560 GPa and in W to 420 GPa. 3 The high structural stability of these materials provides an excellent opportunity to examine melting in the bcc phase over a wide pressure range. Figure 2 shows the melting curves of the bcc transition metals Mo, Ta, W, V, Cr, and Ti. Ti is hcp at RT, but melts