Crystal structures of group IVa metals at ultrahigh pressures

Titanium Alloys at Extreme Pressure Conditions

“…The reported intermediate orthorhombic  phase (transforming between ~ 116 GPa -128 GPa) possessed a distorted hcp crystal structure (space group Cmcm), and the orthorhombic  phase (transforming at ~ 145 GPa) was of a distorted bcc type structure (space group Cmcm) (Akahama et al, 2001). These observations for Ti are at variance with the other Titanium Group transition metals Zr and Hf, which have been observed to follow the     pathway at RT (Jayaraman et al, 1963;Xia et al, 1990a;Xia et al, 1990b). However, Ahuja et al have observed the coexistence of a bcc-like structure (referred to by the authors as ') with the ω phase, during the compression of Ti at RT from 40 GPa to 80 GPa (Ahuja et al, 2004).…”

Pressure-induced novel compounds in the Hf-O system from first-principles calculations

“…Under ambient temperature, experiments [6,7] indicated that pure Hf is stable in the α-phase (hexagonal close-packed structure, space group: P 6 3 /mmc) and transforms to ω-phase (hexagonal structure, space group: P 6/mmm) at 46-58 GPa and then to β-Hf (body centered cubic, space group: Im3m) at 71.1 GPa-78.4 GPa. Our GGA calculated results indicate the transition pressures: α-phase → ω-phase at 49 GPa and ω-phase → β-Hf at 70 GPa, which are in accord with above experimental results.…”

Experimental and theoretical study of Ti-6Al-4V to 220 GPa

“…On static volume compression, Ti does not follow the α → ω → β transformation sequence predicted for, and observed in, both Zr and Hf [9][10][11][12] . The transition to the ω-phase occurs at 3-9 GPa on pressure increase [13][14][15][16][17][18] , and the ω-phase can be retained as a metastable phase at ambient pressure; the reverse transition back to the α phase in Ti occurs only on heating for extended periods at 380 K 13 .…”