Cluster ab initio modeling of local lattice instability in relaxor ferroelectrics

The possibility of a zigzag-type instability occurring for oxygen atoms in B-O-B, B-O-Nb, and Nb-O-Nb linear chains is examined in disordered mixed perovskite compounds Pb(B1/3, Nb2/3)O3 (B = Mg, Zn, Cd). Local adiabatic potentials for oxygen atoms are studied using total energy calculations by the ab initio Hartree-Fock + MP2 method for many-atomic clusters with different oxygen surroundings of lead atoms. The effect of lattice relaxation along the chain on the shape of the local potential in the transverse direction for the central oxygen atom is considered. © 2004 MAIK "Nauka/Interperiodica".Russian Academy of Medical Sciences North Atlantic Treaty Organization ?????????? ???? ??????????????? ???????????? (????): 03-02-17557, 01-02-17801ACKNOWLEDGMENTS We are indebted to L.S. Kamzina and N.T. Krainik for many stimulating and useful discussions. This work was supported by TUBITAK (NATO PC Advanced Fellowship Program), the Russian Foundation for Basic Research (project nos. 01-02-17801 and 03-02-17557), and the Russian Academy of Sciences program for physical sciences

Cluster ab initio calculations of the shape of local potential well and of positions for oxygen atoms in PCN (C=Cd)

Potential relief and equilibrium positions for oxygen atoms in Cd - O - Nb, Cd - O - Cd and Nb - O - Nb chains are studied in Pb(Cd 1/3 Nb 2/3 )O 3 (PCN) and Ba(Cd 1/3 Nb 2/3 )O 3 (BCN). Total energy cluster calculations are performed using ab initio restricted Hartree-Fock method. The electron correlation contribution is calculated within Møller-Plesset method. It is revealed that in PCN the oxygen atoms in Cd - O - Cd and Cd - O - Nb chains move in multi-well potentials with four minima shifted transversely to fourfold [001] axis. Whereas the oxygen atom in Nb - O - Nb chain in PCN and all oxygen atoms in BCN move in single-well potentials. © 2003 Taylor & Francis.?????????? ???? ??????????????? ???????????? (????): 00-02-16919, 01-02-17801We are indebted to L. S. Kamzina and N. T. Krainik for many stimulating and useful discussions. Supported by the TUBITAK (UNISTAR Umbrella Project) and by Russian Foundation for Basic Research (Grants No. 00-02-16919 and No. 01-02-17801)