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Reinvestigation of high pressure polymorphism in Hafnium metal

There has been a recent controversy about the high pressure polymorphism of Hafnium (Hf). Unlike, the earlier known α $\rightarrow$ ω structural transition at 38 $\pm$ 8 GPa, Hrubiak et al (2012) did not observe it till 51 GPa. We have reinvestigated the room temperature phase diagram of Hf, employing x-ray diffraction (XRD) and DFT based first principles calculations. Experimental investigations have been carried out on several pure and impure Hf samples and also with different pressure transmitting media. Besides establishing the significant role of impurity levels on the high pressure phase diagram of Hf, our studies do not support the results of Hrubiak et al. (2012). The structural sequence, transition pressures, the lattice parameters, the c/a ratio and its variation with compression for the α and ω phases as predicted by our ab-initio scalar relativistic (SR) calculations are found to be in good agreement with our experimental results of pure Hf.