Domain structure and lattice strains in FeNbO4

The monoclinic wolframite-type structure FeNbO4-III with complete Fe- and Nb-order on two different crystallographic sites is the polymorph stable at room temperature. Cation ordering is activated by annealing, different domain structures are formed depending on the annealing temperature. Three different stages could be distinguished in combined HRTEM and HRSD studies. In a first stage, well below the transition temperature into polymorph II with an orthorhombic α-PbO2-type structure and a random distribution of Fe and Nb on one site, the degree of cation order and the averaged size of the cation ordered domains increase with annealing temperature for constant annealing time due to higher Fe and Nb mobility. In a second stage with annealing temperatures close to the III → II transition temperature a proceeding relaxation of lattice stress accompanied with expanding antiphase boundaries is observed for increasing temperature. In a third stage the sample is annealed at temperatures in the stability region of FeNbO4-II, samples with low stress and a reduced degree of cation order result. The evolution of real structure in FeNbO4 with annealing temperature can be explained by the competition between thermal activation of Fe and Nb diffusion for cation order with accompanied stress at lower temperatures on the one hand, and relaxation of stress and cation disorder in FeNbO4-II at higher temperature on the other.