The stability, electronic structure, and positron-electron pair momentum of body-centered-cubic (bcc) copper, which is metastable, are theoretically studied and are compared with those of stable face-centered-cubic (fcc) copper and of ferromagnetic iron (bcc Fe). We first perform electronic structure calculations based on the local-density approximation or generalized gradient approximation (GGA) and find that the GGA well reproduces measured bulk properties, i.e., lattice constants, cohesive energies, and bulk moduli. The calculated cohesive energies of fcc and bcc coppers are very similar and the estimated lattice mismatch between bcc Cu and bcc Fe is very small $(\sim 1.4\%).$ These results support previous experimental suggestion that the lattice of bcc Cu precipitates in Fe matrix is nearly coherent to that of the matrix. Next, we calculate momentum-density distributions of positron-electron pairs using the two-component density-functional theory. It is found that the momentum-density distributions of bcc Cu and fcc Cu are very similar but are quite different from that of Fe, which indicates that an analysis of measured coincidence Doppler broadening (CDB) of positron annihilation radiation gives useful information on Cu precipitates in Fe. Actually, by comparing the calculated and measured CDB spectra, we confirm the previous experimental conclusion: In an Fe 1.0 wt % Cu alloy after thermal aging at $550°\mathrm{C}$ for 2 h, the observed signals originate from the completely confined positrons in bcc Cu precipitates, which annihilate with valence electrons of Cu atoms.

• Received 25 October 2001

DOI:https://doi.org/10.1103/PhysRevB.65.195108