FIRST-PRINCIPLES THEORY OF TA UP TO 10 MBAR PRESSURE - STRUCTURAL ANDMECHANICAL-PROPERTIES

Fundamental high-pressure structural and mechanical properties of Ta h ave been investigated theoretically over a wide pressure range, 0-10 M bar, by means of ab initio electronic-structure calculations. The calc ulations are fully relativistic and use a state-of-the-art treatment o f gradient corrections to the exchange-correlation potential and energ y within density-functional theory. The calculated zero-temperature eq uation of state for bcc Ta is in good agreement with diamond-anvil-cel l measurements up to 750 kbar and with reduced shock data to 2.3 Mbar. The crystal-structure stability among bcc, fee, hcp, and A15 phases h as been studied as a function of compression and the observed ambient- pressure bcc phase is found to be thermodynamically stable throughout the entire 0-10 Mbar range. At the upper end of this range, a metastab le fee phase develops with positive elastic moduli and a decreasing fc c-bcc energy difference, suggesting that at even higher pressures abov e 10 Mbar, fee Ta will become stable over the bcc phase. Elastic const ants, the H- and N-point zone-boundary phonons, and the ideal shear st rength have also been calculated for bcc Ta up to 10 Mbar pressure. Th e elastic moduli and phonons are in good agreement with experiment at ambient pressure and remain real and positive for all compressions stu died, demonstrating that the bcc phase is mechanically stable in this regime. The calculated elastic constants validate the assumed pressure scaling of the shear modulus in the Steinberg-Guinan strength model o f Ta, while the calculated values of ideal shear strength provide an u pper bound to the high-pressure yield stress. [S0163-1829(98)01117-5].