Crystal-chemical effect of the thermodynamic history on the composition-temperature phase diagrams of (Na,Li)NbO3 and (Na,K)NbO3 solid solutions

Composition (x)-temperature (T) phase diagrams obtained for the first time for single-crystalline (Na,Li)NbO₃ (system I) and (Na,K)NbO₃ (system II) niobates are compared with the known phase diagrams for ferroelectric niobate ceramics prepared by various techniques (standard firing and hot pressing). In system I, the thermodynamic history (process conditions) is shown to have a pronounced effect on the diagrams throughout the range of perovskite stability, while in system II, the effect is the most significant at low potassium concentration (x≲0.15). These data are associated with a small value of the tolerance factor t of the solid solutions, whose properties are sensitive to the thermodynamic history. Small (≲0.9–0.93) values of t indicate the “porosity” of the structure of solid solutions I and II in the above concentration ranges. Under these condi-tions, polarization-and rotation-related structure instabilities compete on equal terms and substitutional and interstitial solid solutions may coexist.