The information gathered so far indicates that in the family of ferroelectric (FE) ceramic materials, perhaps the most complex are niobium-containing systems, and among them the compounds, based on the presence of alkali metals. On the one hand, the mismatch between the radii of the alkali cations and the parameters of the interoctahedral cavitiesInteroctahedral cavities occupied by them in the perovskite structurePerovskite structure causes its “looseness” and, consequently, the easy loss of stability, associated with the occurrence of a series of successive phase transformations and the formation of states that “respond” differently to external influences. To some extent, this is related to the manifestation of three properties of complex niobium oxides: compound polymorphismPolymorphism, morphotropy and non-stoichiometryNon-stoichiometry of composition ( Sadykov et al. in Proceedings of the fourth international youth symposium on physics of lead-free piezoactive and related materials (analysis of the current state and development prospects) (LFPM-2015), 2–6 Sept 2015, Rostov-on-Don—Tuapse, vol. 2, pp. 184–189 (2015) (in Russian);Wang and Li in J. Adv. Ceram. 1:24–37, 2012).