Structural phase transitions in the Ag2Nb4O11–Na2Nb4O11 solid solution

The phase transitions between various structural modifications of the natrotantite-structured system x Ag2Nb4O11–(1−x )Na2Nb4O11 have been investigated and a phase diagram constructed as a function of temperature and composition. This shows three separate phase transition types: (1) paraelectric–ferroelectric, (2) rhombohedral–monoclinic and (3) a phase transition within the ferroelectric rhombohedral zone between space groups R 3c and R 3. The parent structure for the entire series has space group View the MathML sourceR3¯c. Compositions with x>0.75 are rhombohedral at all temperatures whereas compositions with x<0.75 are all monoclinic at room temperature and below. At x=0.75, rhombohedral and monoclinic phases coexist with the phase boundary below room temperature being virtually temperature-independent. The ferroelectric phase boundary extends into the monoclinic phase field. No evidence was found for the R3–R3c phase boundary extending into the monoclinic phase field and it is concluded that a triple point is formed.