Metallization pressure for NaCl in the B2 structure
Local-density-approximation results for the electronic structure and elastic properties are reported for NaCl in the B2 structure at large volume compressions. Both linear augmented-plane-wave (LAPW) (with no shape approximations for the potential or charge density) and APW (within the muffin-tin approximation) methods were used as a check on both methods for determining the metallization volume. Total energies and virial pressures are also computed in the case of the LAPW method for the purpose of achieving values of the pressure- and tetragonal-strain-related elastic properties at these extreme compressions. With the use of Hedin-Lundqvist exchange and correlation, the LAPW result is that direct (M-point) band-gap closure occurs at V/V0=0.285 (p=0.455 TPa) and indirect (-M) band-gap closure occurs at V/V0=0.272 (p=0.524 TPa), where V0 is the experimental room-temperature equilibrium volume of (B1-structure) NaCl. For comparison, the Herzfeld criterion of metallization has been reported to yield V/V0=0.32. The APW results for the metallization volume depend somewhat on the muffin-tin radii but are generally in excellent agreement with these LAPW results. Finally, our elastic-strain-energy results suggest that the B2 structure is stable with respect to tetragonal strains at these compressions; we also present values of c11 and c12. Thus, although it has been rather widely suggested that alkali halides might have transitions to tetragonal structures at high pressure, we see no evidence for such a transition in NaCl. © 1990 The American Physical Society.
