Structural phase transitions in the Ag2Nb4O11–Na2Nb4O11 solid solution

The phase transitions between various structural modifications of the natrotantite-structured system xAg2Nb4O11–(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 R3c and R  3. The parent structure for the entire series has space group R3¯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.

Phase diagram for xAg2Nb4O11–(1−x)Na2Nb4O11 solid solution showing changes in crystal symmetry as a function of temperature and composition. The crystal structure is depicted.Figure optionsDownload as PowerPoint slideHighlights▶ Ferroelectric, rhombohedral Ag2Nb4O11 in solid solution with monoclinic Na2Nb4O11. ▶ Three phase boundaries were studied as a function of composition and temperature. ▶ Both rhombohedral and monoclinic variants exhibit ferroelectricity. The parent phase of the series has space group R3¯c.