Heterovalent and A-atom effects in A (B ' B '')O-3 perovskite alloys

Using first-principles supercell calculations, we have investigated energet ic, structural, and dielectric properties of three different A(B'B ")O-3 pe rovskite alloys: Ba(Zn1/3Nb2/3) O-3 (BZN), Pb(Zn1/3Nb2/3) O-3 (PZN), and Pb (Zr1/3Ti2/3) O-3 (PZT). In the homovalent alloy PZT, the energetics are fou nd to be mainly driven by atomic relaxations. In the heterovalent alloys BZ N and PZN, however, electrostatic interactions among B' and B " atoms are f ound to be very important. These electrostatic interactions are responsible for the stabilization of the observed compositional long-range order in BZ N. On the other hand, cell relaxations and the formation of short Pb-O bond s could lead to a destabilization of the same ordered structure in PZN. Fin ally, comparing the dielectric properties of homovalent and heterovalent al loys, the most dramatic difference arises in connection with the effective charges of the B' atom. We find that the effective charge of Zr in PZT is a nomalous, while in BZN and PZN the effective charge of Zn is close to its n ominal ionic value. [S0163-1829(99)02603-X].