Verification of the Thomas theoretical framework for A-substituted PbBnNbmO3 relaxor ferroelectrics

A Thomas theoretical framework (Butcher J and Thomas N W J. Phys. Chern Sol ids. 1991 52 595) for PbBnNbmO3 relaxors is verified by analysing the rate of the fall-off in the dielectric permittivity maximum, epsilon (m), with t he concentration x for several A-substituted PbMg1/3Nb2/3O3 and PbB0.53+Nb0 .5O3 (B3+ = SC, In, Yb)-based solid solutions. For the solid solutions with isovalent A-substitution (Ba --> Pb) the experimental dependencies of the epsilon (m)(x)/epsilon (m)(x = 0) ratio on x were found to be scaled to a u nique curve, (1-x)(alpha (x)), where alpha (x) is a sharply decreasing func tion at small x. At intermediate values of x alpha values are close to eigh t. Only this value corresponds to the assumption of Thomas that a Nb ion is ferroelectrically active only if it is surrounded by eight Pb ions. Reason s for the deviations of alpha from eight at small and large x are discussed .