On the nature of ferroelectricity in Sr1−x A xTiO3 and KTa1−x NbxO3 solid solutions

Abstract

Cluster calculations of the local adiabatic potential for an impurity atom in position A in Sr1−xAxTiO3 (A=Mg, Ca, Ba, Pb, Cd, Zn), as well as for the Nb and O atoms in the Ta-O-Nb chain in KTa1−xNbxO3, were carried out in the nonempirical Hartree-Fock-Roothaan MO-LCAO formalism. For comparison, similar calculations of the local adiabatic potential were performed for a sublattice-A atom in the ATiO3 cubic perovskites (A=Ca, Sr, Ba, Pb), for K and Ta atoms in KTaO3, and for Li in K1−xLixTaO3. The calculations revealed that in all the cases considered, except the Zn, Mg, and Li impurities, the impurity atoms move in single-well potentials and that the corresponding solid solutions are displacive ferroelectrics. Zn in Sr1−xZnxTiO3 and Mg in Sr1−xMgxTiO3 were found to occupy off-center positions, as does the Li atom in K1−xLixTaO3; i.e., they move in a multiwell local potential. An explanation is proposed for the first-order Raman scattering observed in the paraelectric phase of the above solid solutions with central impurities. The critical concentration xc for the displacive KTa1−xNbxO3 and Sr1−xAxTiO3 solid solutions was calculated in the virtual-crystal approximation within the soft ferroelectric mode theory. The values of xc thus obtained agree with the available experimental data.